Jump to content

Recommended Posts

Just an idea since your circuit is already 555 based .... how about feeding the heaters from a CCS  ....and have the 555 coupled as a Schmidt Trigger ...starting the HV when ever the voltage drop across the ´heater is less than some voltage?

Then the heaters will not pull excessive current and warm up gently ...and when ever warmed up the 555 will trigger the HV...

Edited by sorenb
Link to post
Share on other sites

 ....is it a bad Idea to feed the heaters from CCS and trigger on the voltage across the heaters? ....I measured the voltage drop across the heaters in the KGST ....(I have .47 ohm in series) it jumped right up at 1,6V and then slowly dropped to 0,5V and stays there ...and it seems to translate into the outputs swinging up to +rail ...and then begins to drop to zero after a while

I guess feeding the tubes from a CCS feeding 1amp will make the voltage across the heaters go from ~2V to ~ 6V .... then use a simple latch that triggers at ~6V to turn on the HV 

 

if several heaters of different tubes are at play one might use a master trigger controlled by some AND-gates making sure that all heaters has reached working state

 

....is it just a bad idea from some reason?

Link to post
Share on other sites

duly noted. I guess I need to implement the HV mains delay then? without the delay there's some swishing sound that takes around 5 minutes to settle out

does it take five minutes for the heaters to reach a steady state? 

Link to post
Share on other sites

does it take five minutes for the heaters to reach a steady state? 

that's the implication but it could just be the effects of the startup sequence. I don't think others with the Megatron are delaying the HV by 5 minutes

Edited by nopants
Link to post
Share on other sites

that's the implication but it could just be the effects of the startup sequence. I don't think others with the Megatron are delaying the HV by 5 minutes

Just had a go at the KGST ....it swings to +rails at turn on, after 8-10s it drops quickly to some 20-40V ....and reaching 0V in additional 10s

have you measured the behavior of your (magnificent Megaton btw)  from startup until it settles?

Link to post
Share on other sites

You could try this. 2 holes to secure the base. You could select the module for the timing range and the actual time can be set manually.

http://www.ebay.com.sg/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=h3y-2&_sacat=0

They come in ac or dc. You will need the ac for mains delay.

m

do they accept mains by default, independent of the dc rating?

Link to post
Share on other sites

So I have a similar issue with my SRX Plus, which uses a hybrid tube/solid state rectifier bridge.  The positive side is done by the tube and the negative side is solid state with a delay and soft start circuit to roughly mimic the rectifier tube turn-on time.  The output voltages go though some gyrations on turn- on, however the voltages on each channel track each other closely so there isn't a big differential voltage (which is what the headphones see as signal), and there isn't a big bang on turn-on - the gyration is subsonic anyway.

 

Tube filaments generally warm up in 10-30 seconds so that degree of delay should be enough - tubers actually reach steady state faster than transistors, which turn on quickly but then take awhile to heat up their heatsinks to operating  temperature.  That is exactly what sorenb is seeing with is KGST.  When it turns on, the output is connected to the positive rail through the current sources, but the output tubes aren't conducting because the filament is cold.  Once the filament warms up enough the output tubes start conducting and the output voltage drops and after another 10 seconds the tube is fully operational and the output drops to zero.  If you disconnect the servo, you can see that there is a relatively fast voltage change as the tubes warm up, followed by a slow drift as the solid state heatsinks reach operating temperature.

 

Now, if you don't want to be worried about voltage gyrations at turn-on, one way to do it is to have all the filament supplies on one transformer which turns on immediately, and the HV supply from a second transformer with an AC delay so it doesn't turn on until the filaments warm up, then all the high voltages turn on at the same time.  Does need two transformers, though.  Stax did it manually with two power switches in the T1S, one for heater warm-up and one for HV, which was labeled as the power switch.

Link to post
Share on other sites

I don't really see why it shouldn't work feeding with a CSS (that should even protect the tubes) and trigger on the voltage across the tube heater ... rather than using a timer delay???

Link to post
Share on other sites

I don't think there is anything wrong with the idea of feeding the filaments with a constant current source and using the voltage generated across the filaments to trigger the HV - actually as the filaments heat up their resistance increases, and hence the voltage across them increases, so the idea would be to trigger the HV when the filament voltage rises to a certain value.  But, it is probably significantly more complicated to implement than a simple time delay, for which reliable, time-tested circuits are already available.  Why go with a more complex method when a simple one does the job, aka KISS?

Link to post
Share on other sites

Just had a go at the KGST ....it swings to +rails at turn on, after 8-10s it drops quickly to some 20-40V ....and reaching 0V in additional 10s

have you measured the behavior of your (magnificent Megaton btw) from startup until it settles?

.

The 6S4A used in the KGST has a controlled delayed warmup for the heater. The A revision was released with this engineered in, to lower the turn on power surge in TVs.

Should be kept in mind when considered as a reference for conduction in other tubes/circuits.

Link to post
Share on other sites

nopants said:

 

"the real kiss would probably be a separate switch"

 

Yup, that's what the old Stax T1 had, a pre-heat switch and a power switch.  Now what you want with that system is an interlock so that the power switch won't turn on the HV unless the heater switch is on.  Since I've never seen a T1 in the flesh I don't know if it had that built in or not, but it shouldn't be difficult to do, just have an AC relay on the primary of the HV transformer, in series with the power switch, that is controlled by the heater switch.

 

Laowei said:

 

"The 6S4A used in the KGST has a controlled delayed warmup for the heater. The A revision was released with this engineered in, to lower the turn on power surge in TVs."

 

Shows how important tubes were in TVs in the 50s - versions were designed specifically for them.  For example, the 6SN7GTA was redesigned to the GTB version which had the controlled warmup.  Actually I believe the other big reason for controlled warmup was so that the heaters could be wired in series, e.g. use a 24 volt filament supply and wire 4 6V heaters in series - only works if they all warm up at the same time, otherwise the voltage isn't distributed evenly between the filaments in the string.

Edited by JimL
Link to post
Share on other sites

On my Atmasphere OTL amp, there are two power switches. One powers the first stage tubes (6SN7) fully and only the filament of the output section tubes (6AS7). The second one supplies the B+ to the output section 6AS7. The procedure is to turn on the first switch and wait at least 60 seconds before turning on the second switch. The switches are wired such that the second switch is only effective when the first switch is on. Turning off the first switch cuts off the power to the second switch. Seems very similar to what JimL described about the T1.

It was implemented this way to preserve/extend the life of the 6AS7.

It's manual but it works very well for me.

Edited by mwl168
Link to post
Share on other sites

Yes, its just an ac timer. Wire the mains to it and when the time is up it turns on automatically. Eggil, Ristar and myself are using it.

As I understand the sheet I need to supply 12V to get it to switch properly. how are you running yours?

Edited by nopants
Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...