Blueman2

Sounds like a battle between the Mechanical Engineer and the Electrical Engineer inside of all of us.

Look what arrived today! Boards look wonderful. @mwl168, I did get an extra PSU board, I think from the 1.6mm batch? Is that OK? I am sure I can use it!

I recall some threads on people upgrading the trafos on SRD-7s, but not sure how much of a difference it made. But wow, I am amazed to see how much these SRD-7s are going for on eBay these days. I bought 4 of them a couple years ago for $100 each. Now going for 3x that or more.

Great news. Definitely love PCBnet. Top notch organization.

@mwl168, I would not think that would be an issue for the PSU main boards. It would definitely be an issue for the mini-T2s, but it sounds like those came in as 2mm. So speaking for myself, I am fine with the 1.6mm PSU main boards.

Sent! Thanks.

Yup. Do you want the GRHV partially assembled? Might be worth it if you have not done a lot of SMD work.

boinger, You will need to specify which boards and how many. Also, if you want bare or partially assembled GRHVxxx boards. That way, there will be no confusion. Good luck!

Right now, my SRX is the most stable amp I own in terms of L- to L+, R- to R+, L-L+R-R+ to ground. All rock steady less than 1V. But before I did the better tubes and heater elevation, I had your issues. Sounds like the offset issue follows your tubes? So I would definitely try another set of tubes. As far as hurting your earspeakers, 20V between L- to L+ or R- to R+ does seem a bit high but I doubt it will cause any permanent damage. But others can chime in on how dangerous this is. I know I used my first SRX build at 20-30V offsets between -/+ and my SR-007 survived that. It took about 2 weeks before a new set of tubes arrived, which solved the offset issue for me. What is your L- to GND and R- to GND voltage? Are they OK?

The current draw is minimal. 1/4W will do fine, though I recall using 1/2W.

@mwl168, can you adjust my order to move from 4 pre-assembled GVHVxxx to 2 pre-assembed and 2 bare? on your spreadsheet: 2, 2, 2, 2, 2, 1 Thanks

I like the top transformer design as well. Reminds me of my first SRX build (on the right). Left one was my second where put trafo in the chassis.

@tomislavkufrin, how about some pics?

Or remedy for boredom. I do have 3 weeks of 'shelter in place' here in the SF Bay Area. Hmmmm.

Just for the heck of it, try tying Output to B- rail using 100K resistor and see if that does anything. I think leaving them floating is probably OK since it is more likely that input tubes are source of hum. But what the heck, worth a try. And as @jose and @mwl168 mention, you might want to still try cleaning your tub connectors or switching tubes around to see how that impacts things. And readjust the trim pots to null everything out. All this talk about hum in the SRX-Plus really makes me feel nostalgic about my first build!!

Correct, you just need separate windings, not necessarily separate trafos. And the lower diagram would work fine. For me, with 370V B+ rail, I have R1=470K and R2=100K. That gave me 65V for filament elevation. I did not use C1, but that could only help.

Two independent windings. And since Input uses 12.6V and Output uses 6.3V heaters for this SRX-Plus board layout, there is really no way to screw that up. BUT, I modified the input tubes wiring to be able to use 6.3V (rather than 12.6V) since I had a trafo with 2x6.3V windings already. I just wanted to make sure that if someone else did this, that they would not try to get away with just one 6.3V winding for both Input and Output. That would be very bad due to different cathode voltages.

MLA showed me a great article here: http://www.valvewizard.co.uk/heater.html Basically, there can be leakage between the Heater and the Cathode. Since the heater is usually driven by AC at 60hz, some of this hum can come into the signal path. Elevating the heater to a voltage near the Cathode can reduce the leakage and the hum. If you have an isolated transformer for the heater, then it will allow the heater to eventually elevate itself as voltage on the Cathode drives to heater to 'float' to near its own voltage. But this still results in hum in some cases. So forcing the heater to a voltage that is near to the cathode can help. Here is the way I constructed my divider: And, as stated before in this thread, do NOT use the same transformer to power both the Input and Output stage tubes. You must you 2 totally independent windings, one for input stage, other for output stage heaters.

And welcome, @tomislavkufrin, to the elite group of 10. I updated the first post to include you in the list of 10 builds so far for the 'simple-is-beautiful' SRX-Plus. JimL congo5 MLA Sorrodje mwl168 luca jose blueman2 Juggernaut1101 tomislavkufrin

@tomislavkufrin, Congratulations!! My SRX-Plus remains one of my favorite builds, even with my Carbon and Blue Hawaii. There is beauty in simplicity, and the SRX-Plus is all of that. As for the hum, I too have had issues with hum in the past, but was always able to get rid of it by 1) using different tubes and 2) elevating the filament voltages, and 3) fine tuning the trim pot settings. As for elevating the filaments, see my old post earlier in this thread: Definitely do not give up and accept the hum. I have 2 SRX+ builds, and both had hum at first but after doing some tube swapping and endless fine tuning of the trim pots, both are hum free. Enjoy!!

@mwl168, Please update my request: 4x GRHVxxx partially-assembled 2x GRLV79xx 2x GRLV78xx 2x Main Board 1x miniT2 (pair-right&left) Cheers, Bob

I remain committed to the buy, but definitely prefer as much pre-assembly as possible. Willing to pay for more assembly, even if there has to be a sub-buy for those wanting it.

Thanks for all your contributions to the community, mwl168. No rush here!

From an earlier post, but worth repeating for @jose

Now you guys are just getting nasty. ? Aren't you the same 3 guys who made me build the Carbon, and then the Blue Hawaii BJT after that? Come to think of it, more than 1 of you got me to build my original SRX-Plus too. Your taunts will no longer work on me. ?