jamesmking
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Some small changes I personally would like to see to the mini T2, based on my build style.
1. space for screw terminals for the transformer to el34 heater wires - I like to be able to remove boards easily for testing/inpsection etc without having to desolder anything.
2. more space between the 6922 heater screw terminal and the valve sockets. - its a bit tight.
3. the ground plane not connected to the screw holes for the heatsink L bracket. I like to mains earth my casework and leave the amp ground foating.
4. the screw terminal holes grouped like the blue hawaii, so I can use terminal blocks without having to cut a 2 way into a 1 way.. plus the one way can twist easily if you dont hold it when you tighten the screw.
5. a second ground screw terminal, - so I can have one connect to the psu pcbs and the other connect to the input signal ground.
6. have the 6922s at the front... more valves visible more orange glow 🙂
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My Initial impressions of the mini T2. Listening on my stax 007a mk2, normal caveats apply. There is no point comparing it to the Stax srm007 its like comparing a Bentley to a slug. Compared to my hi-amp alpha centauri there is also no competition. The mini T2 out does it in every department as does my Blue Hawaii. I will compare the mini T2 to the Blue Hawaii
(mini t2 (golden ref HV and LV, kevin gilmore synchronous rectifiers in the LV tkd volume, cardas hookup wire, groove tubes el34s, electro harmonix gold pin 6922s, simulated single box - transformers piled on top of each other 😞)
(blue hawaii, (golden ref HV and LV, kevin gilmore synchronous rectifiers in the LV tkd volume, cardas hookup wire, groove tubes el34s single box)
The t2 has more smoothness, and yet more detailed and faster treble with a little more sparkle than the BH. The treble speed is not done via a bright hardness but rather its just fast and articulate and is very musical and "valve like". Mid range is equally strong with more smoothness and more detail than the hawaii. Bass is good, not necessarily much better than the blue hawaii, perhaps the BH goes deeper but the mini T2 is again more rounded and smooth. I think the upper bass is better on the mini t2 which partly makes up for the slight lack of depth. The mini T2 does not shout or dazzle, its just fast and natural. It makes you want to turn the volume up, not because its missing something but because it sounds like its not straining and just wants to sing.
Micro dynamics are much better than my BH, Macro dynamics are perhaps a bit more restrained than the BH but the compromise works for me because the micro is so much better then the BH. The mini T2 loves strings and acoustic music. I can’t comment much beyond this since I only listen to Jazz and classical. Image placement is like a blanket around the listener, the Mini has the ability to present the sound stage close to the listener when the recording is close. With the BH everything seems about the same distance regardless of the recording. When the recording has the sound stage further back the mini T2 presents a very wide, detailed soundstage but does not hyper focus on individual instruments, there is detail and yet the acoustic of the venue comes through. I feel the mini T 2is less analytical than the BH, interplay between instruments is better presented, its more musical and makes you want to play at whatever volume you want. there is much less of a volume sweet spot than the BH.
Some suggested listening for what this amp can do
Rodrigo guitar concertos academy of st martin in the fields philips - beautiful guitar sounds with detail and fantastic plucked strings.
Hank Thompson live at the golden nugget. - the mini t2 pulls off the orange blossom special without brightness and the level of detail shows you it’s a packed venue with gambling noises everywhere.
j c Bach trio sonatas on chandos - mini t2 shows the mid range warmth that the blue hawaii missing, the recording is on original instruments and can sound a little thin... no so with the mini t2
paul desmond bossa antigua, tape hiss is separate from the instruments, centre drums are articulate, it’s a warm sound but not dull or muddy. The sound is beautiful there is detail but it’s not thrown at you.
I am very glad I started this project - even though I had very little smd soldering experience. Everything worked first time no smoke no drama. Now all I need is some cnc machining for the case. the mini T2 consumes about 25W less than the BH and runs cooler too.
Conclusion, both the BH and mini T2 are very very good. Building a blue hawaii is easier - there is almost no smd parts and will be cheaper: one less psu board, less transformer windings, less valves and can be built in one box. But the mini T2 is better sounding..... If you have the time, money, skill and equipment give the mini T2 some serious consideration. Now I have to build a full T2 with modern components....
and finally a very large thank you to JoaMat for providing me the mini t2 boards and for his help and support during this build.
Best regards and happy building
James
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My transformers have arrived! and I have begun initial amp board testing.
A few more mini T2 build notes/observations.
Once the amp boards are completed using diode test on a multimeter across the leds will not make them light.
The 2 red leds on the underside of the amp boards will not light if just heaters and + and -15V are applied... you need the high voltages too.
The el34s heaters run at about -400V DC with respect to ground plane on the amp board, the 6922 heaters run just about 0V with respect to ground.
The attachment holes in the pcb for the heatsink L bracket are plated and connected to the amp boards ground plane. So the L bracket, and therefore the heatsink and ultimately the entire case is connected to the pcb ground plane.
I have decideed to turn the amp boards around so the 6922 are at the front... to me it looks more visually pleasing... 🙃
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Well spotted Mirko, you are correct. I will update the schematic.
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I added the red anode mark myself when I tested the leds with a multimeter.
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My mini T2 amp boards. Note the screw terminals on the top to make troubleshooting easier I know JoaMat does not believe in screw terminals but I do. 😉. If you are careful its even possible to orientate the terminals so the silkscreen voltage markings are still visible 🙂 . I take a 2 terminal and cut it in half and put the original smooth side parallel to the silkscreen markings. There is not space for screw terminals for the heaters of the el34s so I (possibly temporarily) have soldered in a header pin at 45 dgrees
I have used nylon m3 standoffs for the first layer so that there is no chance of the standoff shorting against anything and brass standoffs for the remaining layers to get to the required height.
SMD soldering was done in a three step process.
1 apply solder paste to the pads.
2. melt the solder using a hakko 888D station at 330 degree C using a fine tip until the paste has migrated to only be on the pads.
3 Hold the component with twisers onto the pads and with my other hand use a Quick 861DA hot air station using the smallest supplied nozzle with airflow setting all the way down to 5 (even then its possible to send one of the small diodes into orbit with that air flow)... to reflow the solder onto the component.
the reason I soldered this way was,
1. I could collect any excess solder with the hakko, since I dont have an electronic solder paste dispenser... 😞
2. I could be reasonably certain there was no solder paste left under a component.
I use the same method for soldering Kevins' synchronous rectifiers.
this is the schematic of the v0.2 pcb with valves from the silkscreen. The red components have changed since the last published schematic.
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Pars, here is my reverse engineering of the kgsshvpssicfetdual2new I have added in red the extra parts for the delayed turn on but there are no pads present on the gerbers for them. As far as I can tell the dual hv boards stopped at version 1.7 just before the cpc117n was added. the individual "left" and ""right" boards went to version 1.8 - which included the cpc117n and the 600ohm resistor.
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Pars for the golden ref HV on both my blue hawaii builds and my current mini T2 build I used the gerbers from the file kgsshvpssicfetdual2new This board does not have the cpc117n for the delayed start - but you can implement it via external relays. I made my own bom for it. Note you will need to change a couple of resistors if you dont want +-400V also if you are going golden reference dual LV you dont need the parts for the simple +-15V, the voltage regs, this are marked on the bom as for low voltage section. Hope this helps. On the silkscreen the caps are 0.047uF 1KV not 0.47uF, So you might be looking at a different board from the golden ref HV I used.
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End of Life: Scheduled for obsolescence and will be discontinued by the manufacturer.
- - Add new for low voltage secti
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7 hours ago, Pars said:
- He sent along some of the left/right GRHV boards (which I asked about earlier). I would prefer to use one of the dual GRHV boards in order to do a mwl168-style universal PSU, which could accommodate a GG or other tube stat amp for him in the future. Anyone have a spare they would be willing to sell? I would like to do a 2 transformer build, with GRLV for the +/-15V supply, so 2 PSU boards and 2 transformers.
pars,
If you are interested I have up to 3 spare dual LV (goldenreference6d) boards from my blue hawaii build they are I believe the latest release colour Blue, hot air leveled for leaded solder. Im in the UK. I dont have any spare Dual HV boards but I could get some more made....
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Unfortunately my electronic dc load only goes up to 100V, otherwise I would gladly do some measurements and plot some curves.
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Only change R8 for positive rail and R9 for negative, don't change R7 and R10 unless you have to. If you keep R7 amd R10 as stock 1.5K then
+ output required value of R8
20V 1.5K
15V 750ohm
12V 300ohm
For negative rail output adjustment change R9.
The two resistors NOT to change (R7 and R10) are the ones directly connected to the ground rail.
with R7 and R10 fixed:
R8 in ohms = ( (voltage you want in volts / 10) - 1 ) * 1500
if R7 and R10 are not stock
R8 in ohms = ( (voltage you want in volts / 10) - 1 ) * R7
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For me, with the golden reference hv connected to my modern blue hawaii via a variac the golden ref hv needs an *absolute* minimum of 315.5VAC at its AC input to regulate at 404.3VDC output. In normal use I have 330VAC input to the golden ref for 404.3V output.
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3 hours ago, JoaMat said:
You don’t want to use same winding for small and output tubes. Separate windings for channels. So total four filament windings.Thank you JoaMat, I thought seperate heater supplies was the case looking at how the heater pins are connected up via resistors to other parts of the circuit....
I think your are right about going seperate LV and HV transformers. I'm going golden reference HV and LV boards and have made a golden reference HV 220V dedicated board. I want to use synchronous rectifiers for the +-15V on the golden reference LV board so I will go 2x15V 0.3A rather than centre tapped - Kevins sychchronous rectifiers dont like centre tapped transformers. Thank you for the specifications. Time to order some transformers....
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JoaMat,
Can I ask what are the specifications for the transformer you are using? also do I need seperate heater windings for the el34s and 6922s.?
thank you for your help
James
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sorry to hear that... I hope the opa197 works for you (and me)
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in Jose's post there was a mini T2 in vaders hand it was just too small to see..... where as the megatron is too large to ignore...
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Well my blue hawaii is working fine with golden ref lv and hv boards....
I want someone to convince me to build a mini t2 seems the next logical step for me without going into rare and expensive transistors....
if only i had the amp boards... or if someone would let me have the gerbers.....
<a few momments later......>
Me and my big mouth.... I tried to resist but looks like I might be building a mini T2 thanks to JoaMat....
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I’m afraid hifi in general in the uk (and probably elsewhere) has been going down the path of high price oblivion for sometime. The market fragments, upgrade paths vanish and less people start on the hobby and good quality kit becomes unaffordable for anyone except professionals/and the rich. I remember when I was a child the local hifi store (stereo only no multi-channel) had Quad, Heaffler, Cambridge audio, castle, Radford, hafler, leak, walker, lowther, b & w, kef, Kinshaw, ferrograph, Goldring Garrard, Decca, Hadcock, Sme, wharfedale, so many British manufacturers and some others like teac, yamaha, ortofon, luxman etc. Over the years one by one as the founders retired and they were replaced by their sons and accountants (c.f. Quad ) the prices went up dramatically, the skills were lost... Quad "lost" all the design work Sir Peter walker did on the esl 57 and 63 and had to reverse engineer them from scratch and rediscover what worked and what did not before they could make any major changes hense the "replacement" to the esl 63 the esl 988 which was a copy of the esl 63 with lower build quality (plastic top and bottoms)... I went to the Quad demo of the 988 at a hifi show, they were selling quad clothing and had pretty girls everywhere... the sound was terrible - I suspect more the hifi they matched with it then the speaker. But the message was obvious buy quad merchandise. The traditional manufacturers closed one by one or got taken over by large groups c.f. Quad etc. Audiolab got taken over by tag who sold the same mid to low budget audio lab kit in expensive looking chassis for 10 times the price.... the hifi magazines where full of their advertising, raved about their products and less than a couple of years later tag vanished.... a while later I believe the original audio lab staff brought the company back and started reselling their original designs back in the original casework and sensible prices.... I remember talking to the owner of my local hifi shop and he told me that in many cases a hifi manufacturer would not let him stock individual components from their range - he either had to stock everything or nothing. This was too much of an investment for him and in some cases he only thought some of a manufacturers range was good... but his ability to pick and choose was being erodded.
I read that the entire circulation of all uk hifi magazines now is less than the circulation of one of the smallest ones back when I was young… In 2018 I purchased a hifi/absolute sound issue which stated on the front “ultimate guide to headphones” I read it and was appalled. There was no research, no discussion of the topology at all, One page per product, no internal photos and almost every single review was entirely positive, in most cases the paragraph on the company history was longer than the paragraph on listening experiences. If the review was not 100% positive they added things like “but the more I listened to them the more I liked them”… So its easy to think they you have to have golden ears to appreciate any hifi over a few £100….
Now it’s tough to get started in hifi, most people are happy listening to low fi headphones and compressed music on mobile phones or through laptop speakers.... The hifi magazines refuse to write a bad review of any piece of hifi since they are worried about not being given free review samples, or being sued, or the company pulling adverts from their pages...
Cameras are going the same way. Look at what happened to Olympus.... Canon changed the market with the 5D mk 1 - the first affordable full frame 35mm camera, now look at the price of the latest 5Ds and the L series lenses keep going up and up....
I see the same trend now starting with computers, only 2 cpu manufacturers, only 2 graphics cards manufacturers, only 3 memory and flash manufacturers. Graphics card prices have been sky rocketing so in general have cpu prices and especially motherboards ...
(sorry for the rant)
James
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Kevin,
I have just built 3 of the synchronous rectifiers, I built a little test setup with just the + side of a spare golden reference LV board and used a single non centre tap AC ionput. (LV board set to +14VDC output transformer known good). I found all 3 rectifiers to produce rectified output with some 100hz ripple by scoping the + terminal output of the synch rec and the scope ground on the ground of the LV board. Testing drawing from 0mA to 300mA using a DC electronic load from the LV board produced a stable, non drifting clean output - I expected 14V and got 14.014V. The output voltage dropped by about 3mV when comparing the 0mA draw to 300mA which seems reasonable. My conclusion was the sync rec boards where correctly soldered and working.
I then desoldered the traditional bridge rect from my well tested and fully working blue hawaii gold ref LV board (which uses a centre tap transformer for it AC input) I replaced the bridge with the synch rec, brought the amp up slowly on a variac and immediately noticed a buzzing sound coming from the golden ref board and only the + side of the board got to 15V and stable with both leds lit and the neg side could not get past about -10V and the second led did not light on the neg side... this was with the gold ref lv connected to the blue hawaii amp boards and drawing current but before the delay circuit activated the B+ lines. I immediately pulled the power before the B+ came up to protect the amp.
I repeated the experiment with no load i.e. the blue hawaii not connected to the gold ref LV and still got the same behaviour. I tried all 3 sync rec and got the same result. Thinking I had damaged the LV board I put back the original bridge rect and both the - and +15V both were fine and measured exactly as before I removed the original rect.
regards
James
Update,
Looking at the diy forum, https://www.diyaudio.com/forums/group-buys/333844-ideal-bridge-rectifier-gb-23.htmll they also say the LT4320 will not work correctly on a centre tapped transformer.... the two AC outputs of the transformer must be in phase with each other and most centre tapped transformers are out of phase...
I have successfully got the synch rectifier working with the golden reference LV power supply. but only using transformers that have dual secondary windings. I cant not get centre tapped to work correctly.
Update I am convinced the synchronous rectifiers only work with non centre tapped transformers.
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Im using LSK389A in my blue hawaii, with a 10K log pot I get good control over the volume and "normal" listening around 12 oclock. The source to the hawaii outputs 2.2V full scale....
I believe the LSK389B would also be fine, especially if you have a source with lower output or you like to listen at higher volumes or prefer to not have the volume knob around so far...
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Depending upon the price,
I would be interested in 2x GRLV78xx and 2x GRLV79xx
Thank you.
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Blue hawaii build notes based on kgbhver6sbipolarinc.
The amp board is fairly easy to build, but I do have a few observations
1. EITHER populate the LSK389 6 pin dual transistor OR the two single transistors each side NOT both
EITHER populate the STN9360 smd transistors OR the KSA1156 just below them NOT both
important and high voltage tracks run close to the screw holes for attaching the heatsink L brackets to the board on the top side of the pcb. I would be very wary about placing the L bracket on this side, if the solder mask is rubbed there will be a short to the L bracket which is probably electrically connected to the rest of the case and the case is probably connected to earth (certainly in countries with 3 pin mains leads) and that’s going to be bad....
2. the tube sockets and trim pots go on the other side of the pcb to the rest of the components. If you want you can also put the leds on the top side so you can see what is going on. The 3 leds will give you an indirect indication of the status of the board. All 3 leds should be lit for normal operation. The led by itself gives an indication of the -400V line, the two leds closer together indirectly show the state of the +-15VDc lines. There is no led for the +400V line.
3. to set the constant current to 20mA you need to measure 1V DC across the 50ohm resistor near the trimpot (one for each valve). Since the trim pot is on the top side of the pcb it makes sense to put the 50ohm resistor on the top side two. I raised the resistor off the circuit board so I could easily clip insulated multimeter probes to the leads.
4. I found mounting the screw terminals to the underside was a pain in the ****. it may look neater BUT to screw the terminals you have to get to the underside of the board. BUT the pcb is mounted to an L bracket mounted to a large heatsink. So, you either have to flip the entire thing over.... hope you have long enough wires to allow this OR put the entire thing on a desk and stand on your head and screw upwards from below.... I am sorely tempted to desolder the screw terminals and mount them on the top of the pcb.
5. if you are going to have a single ended input to the amp e.g. RCA/phono then the -input should be connected to the ground for minimal hum.
6. when you are doing the inter board wiring for the first time make sure the inputs are shorted to ground. I got some instability issues because my input wiring was too close to either the stax output socket wiring or the main transformer and had nothing connected to it and was not shorted to ground.
7 when you build the amps boards set all the trim pots for halfway and verify with a multimeter in ohms mode. This will save you from having a trimpot at one extreme of its travel and result in LARGE DC offsets or high constant current pull....
8. if you have a scope or two multimeters it’s much easier to set up the amp because changing the DC offset also makes the balance change and vice versa. With a 2-channel scope or 2 meters its far easier to chase the correct spot. Adjust the two trim pots near the 50ohm resistors have been adjusted for 1V across the resistor i.e. 20mA current, first. Next you can adjust the dc offset and balance between the valves, the audio inputs should be shorted to ground and no headphones attached for all setup. If you are using a scope make sure it’s set to DC coupling... AC coupling will ignore any DC offset and its DC offset we are trying to null out. Also, if you have a scope do no connect the ground terminals of the probes to anything only the centre pin of the probes is needed. (almost all scopes internally are grounded via the main input) almost all multimeters are NOT and require the ground lead connecting to the amp board or psu ground.
9. Im my experience if you are building a single box amp, even if the transformer has a magnetic shield foil, putting the audio input wires anywhere near the transformer will result in hummmmmmm..... the magnetic field is STRONG with this one...
10 The hottest transistors on the blue hawaii are the FQPF8N80Cs, measuring the mounting screw of the transistor - (which seems to be the hottest part I can find) I get a stable 65C after multiple hours of use, this is using arctic ceramique thermal paste, aluminium L bracket 200mm by 80mm heatsink with 40mm fins and aluminium oxide insulation pads.... 20mA constant current and +-400VDC B-+
11. double check which trimpot you are adjusting. I have adjusted the constant current trimpots by accident when trying to do DC offset and balance multiple times....
12 some labelling of the trimpots to say what does what on the silkscreen would be nice...
13 labelling the screw terminals on the top side of the board would be nice for checking wiring after the boards are in place.
I hope this is useful
James
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Blue hawaii build notes for the kgsshvpssicfetdual2new PSU
The kgsshvpssicfetdual2new is based on version 1.6 of the kgsshv psu single boards and does NOT have the cpc1117N or resistor required for simple B+,B- delayed start. (it’s still possible to implement delayed start but it requires a relay(s) which the cpc1117N avoids). In all other ways its similar to the v1.7 single boards. The single boards put the b+ on one board and the b- on another and split the -+15V and bias between them.
The kgsshv ps dual has all the dc voltages required to power the blue hawaii and apart from the notes above is topologically and componentry identical to the single psu boards. The notes bellow applies to BOTH the single and the dual psu boards.
Construction is straightforward other than
1. the silkscreen shows 15-0-15VAC input for the + and 15VDC outputs, Since the -+15V output is a simple DC supply with a bridge rectifier and voltage regulators 15-0-15VAC is NOT sufficient to provide -+15V output... I have verified this using a variac. There is about 0.6V drop across each diode in the bridge *plus* the voltage regs need between 1V and 1.5V more input DC than their output in order to regulate properly. so 18-0-18AC is actually required to get a properly regulated -+15VDC output... Any more VAC input will be converted into heat by the voltage regs. However, with 18VAC-0-18VAC centre tapped input the voltage regs run very cool with the small heatsink they are bolted to.
2. the blue hawaii amp boards -400VDC lines draw more current than the +400V lines. If you connect a single channel up to the psu board for testing there will be no problems. Connecting two channels results in the -400V line on the psu going into current limit and the voltage dropping to (im my case) about -175VDC. The fix is fortunately easy. The 5.1ohm 3W or 5W resistors are the current sense resistors for the over current protection. On the -400V psu side either replace the 5.1ohm resistor with about 2.6ohm or similar wattage and type or parallel another identical 5.1ohm around the existing one. The +400V does not need modification. the lower the resistors value the more current supplied before the current limit cicruit activates. I don’t have anyway to variable load a line that has such a high voltage but I would guesstimate that the 5.1ohm resistor current limits at about 100mA since two amp boards +400V together draw around 81mA and there is no problem with 5.1ohm and 2 -400V boards draw between them around 129mA and that causes current limiting with 5.1ohm sense resistors.
3 make sure none of the metals tabs on the transistors are shorted to the heatsink/L bracket that mounts to the heatsink. Use electrically insulated spacers between the back of the transistors with metal tabs and the L bracket/heatsink AND use a nonconductive gromet to insulate the bolt from the transistor. The psu transistors do not get that hot with a reasonable size heatsink attached.
4. if you are building in a u2 high case the largest main psu caps you can put in are Kemet 500V 470uF long life at 65mm tall. Beware they also do an 80mm tall 470uF cap - this will NOT fit in 2u. Similarly 680uF caps at 500V are 80mm high and will not fit.
5. if you want to check the bias is 580V use the bias test point. Dont measure at the bias screw terminal. Why, typical multimeters have a 10M ohm input impedance and assume whatever they are measuring across is much less than 10Mohm... this is NOT the case for the bias line which has a 4.7Mohm resistor. result the multimeter will NOT give the correct voltage reading between the ground screw and the bias screw. Measure from the bias pad nearby to the ground screw. Also, don’t measure the bias with headphones attached.
6. there is only one screw terminal for +400V, -400V, +15V and -15V so you will need to connect two wires to each screw terminal. This limits the gauge of the wire. Also make sure you are screwed the terminal down well, its easy to have one wire firmly affixed and the other one lose....
7. to adjust the B+ and B- lines the two 390K resistors (R8 and R9) in series with the 20K resistor (R7) just after the 1N007 diode that have the 0.047uF cap across them need to be changed. Approximately (for my psu) I got the following
442K for 450VDC (note requires a transformer with 360VAC output to have enough voltage input to get proper regulation)
390K for 404VDC (transformer 330VAC or more output)
365K for 375VDC (transformer 310VAC or more output)
the approximate equation for me was the total of the 2 resistors in ohms/(20,000-700)*10
(the 20,000 is the resistor in series with the resistors you are changing, the 10 is the output of the voltage reference LT1021-10), the 700 is the fudge factor to make the numbers accurate for me... (variation in voltage reference, variation in the 20K resistor etc etc...)
8. To test for proper regulation connect a volt meter to the nominally +-400V output and a ground screw terminal and either power the transformer from a variac OR if your variac can go high enough you could go from the variac output into the 300VAC input of the psu. There is no need to have amp boards connected at all and its safer if you dont. Slowly increase the AC and you should see the DC output rise. At some point increasing the variac by some volts will result in very small output increases (in the order of few milli volts) now the psu is regulating. Measure the 330VAC input of the psu and that’s the minimum transformer output voltage you need for regulation. Both the + and - 400V sides of the psu should have similarly behaviour and similar minimum VAC input requirements since they are almost identical in topology and are identical in components.
9. I would recommend initially testing the psu board without amp boards connected in case voltages are way off. However, with no load on the psu the b+ and b- lines, they will take some minutes to fall to 0v and with no headphones the bias line will stay high for a long time. Fortunately the bias line can be discharged just by connecting a multimeter to it and ground and set to DC volts. Even a meter with 10Mohm input impedance will drain the bias line in a few minutes. the -+15V lines will be drained by the voltage regulators.
10 I would recommend a variac for initial testing it allows you to bring voltages up slowly and make sure that voltages are going up in symmetry (-+400V, -+15V) and becoming stable.
11. If the outputs look good with no amp boards attached, connect one board and repeat the tests. Better to put in channel in danger than both... especially if the amp boards have not been tested.
12. if test with the other amp board only. You don’t want to blame the psu if one amp board only is miss behaving...
13. test with both amp boards. Remember if the -400V line goes down in voltage significantly only with 2 amp boards attached and is fine with one you probably have a current limit problem and did not implement item 2 here!
14 don’t plug in headphones unless 1. you have a spare pair you don’t mind frying. 2. you have tested voltages, dc offsets are nulled and you are happy. ideally look at the outputs on a scope and put some sine waves in....
15 Before you plug in anything visually inspect for solder bridges... I had a partial bridge it only started conducting when the input AC to the psu went above about 26VAC... thank you variac...
16 use probes with insulation, there is lots of high voltages don’t short out things with you probes....
17 Before you power up anything double check the wiring the - and + on the silkscreen are quite small and you dont want +400V going to a place expecting -400V....
18 temperatures are low, with the psu attached at a 200mm by 80mm heatsink with 40mm fins, (both amp boards running constant current 20mA and -+400V B-+), the centre of the case of the +400V side transistors are about 40C, the -400V transistors are around 3C higher at around 43C. The cases on the C2M1000170D transistors take some time to become as hot as the mounting screws. The 10M90S cases reach the mounting screw temperatures far faster.
19 I know the board has been made as small as possible and around the screw terminals its rather crowded. But it would be nice if there was sepertate screw terminals for the -+400V and -+15V for each channel, this would allow for the use of thicker wires and reduce the possibility of a wire comming out. (I have already had the situation where I though both wires where screwed in tightly to a single terminal and then one came out)
Hope this is useful
James
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For anyone building a blue hawaii, here are the current draws for my build after 5 minutes of power on, rounded up to the nearest mA. These figures provide no margin for variation, to headroom for inrush on power up and should not be considered minimum specs for the transformer...
PSU board set to 400V output (actual around 404V)
constant current on the amp boards set to 20mA per valve
inputs shorted to ground
no headphones attached
groove tubes el34
kemet long life 470uF caps @ 550V for B+ and B-
Boards used:
amp Blue Hawaii kgbhver6sbipolarinc
PSU kgsshvpssicfetdual2new (single board version 1.6 with no delayed start cpc1117n that appeared in ver 1.7)total for both channels:
-15VDC line draws 10mA
+15VDC line draws 11mA
+400VDC line draws 81mA
-400VDC line draws 128mA
EACH 6.3VAC line draws 3.1A
Primary side current draw 0.67A @230VAC
All Measurements made on a Brymen bm869s multimeter. NOTE With @230V primary the inrush current is significant and will blow a 3A fast blow fuse on power up, and blow a 4A fast blow after half a dozen power ups (tested with no B+- delay circuit). With a delay circuit 3.15A Timed fuse looks good.
transformer I used (which is overkill, but as a result the transformer generates very little heat. )
330VAC 300mA
330Vac 300mA
36V AC centre tapped 200mA 18v-0-18v
6.3VAC 4.5A
6.3VAC 4.5A
Now I have measurements I would say that the transformer could be scaled down to somthing like
330V 250mA
330V 150mA
+18V centre tap -18V 50mA
2x 6.3V 4.5A
I hope this post will help someone.
regards
James
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DIY mini T2 Build Thread
in Do It Yourself
Posted · Edited by jamesmking
I did the flip 🙂 the small signal tubes are at the front but the labels on the pcbs for left and right and now wrong 🙃 I have the input wires going straight from the back to the tkd pot in the middle of the front panel and then nice short wires from the tkd to the boards inputs... heater wires route around the edge of the pcb close to the heatsinks. So far with the volume all the way up zero listenable hum.