A new style VTA board came in the mail, it's now in the amp. Some of you may have been tracking the build on the AK forums. This is the ed dantes amp. There is a balance issue with this vta board. I've got the feedback running to the 8 ohm taps on the transformer. I heard a noticeable difference between right and left channels. I swapped tubes from right to left channels, I've tried different 12au7 tubes to no avail. I've traced it with the scope to the center 12au7 tube. Both outer tubes exhibit the same output, it's the center tube that shows a difference. Not sure where to go from here, I just know something is wrong. I've checked the resistors back to the tube from the B+ on the board and from the input jacks. The resistance matches the diagram included with the board. I checked all 4 33uf caps, they all show properly on my cap meter.


I'm lost.....

Here's the left channel vs the right, an obvious difference in output. Pin 3 and pin 7 of the center tube showed a similar waveform.

Also there are some mistakes on the schematic. V2 and V1 on the schematic are reversed and some of the resistors don't match the parts list.

email sent to you, get back to me with voltages on each cathode and plate on each tube on the board, not much to go wrong here unless the resistors are incorrect or a bad tube.

Left tube Middle Tube Right Tube

3- 12.49V 3- 4.6V 3- 12.62V
8- 12.48V 8- 4.6V 8- 12.58V
1- 235V 1- 142V 1- 243V
6- 246V 6- 142V 6- 238V

Wow.....I'll be VERY interested to see how you end up with this Brad! Keep me posted. I'm about to embark on my first VTA Board excursion in the near future. I have 2, as I think I mentioned to you on AK. One is built and was 'used' but very little (according to it's prior owner). The other is still in 'KIT' form as received from Mr. Latino.....what Bob.....2 or 3 years ago? Shame on me for not getting to this sooner......but I HAVE been busy! Anyway.....I'm now running one of my ST-70s with my new C-20 McIntosh pre-amp....and other than finding out that my speaker box is blowing apart, I'm in heaven. I started a thread on AK about that problem.....and in process of repair right now. I'll silently curse Paul K for HOT GLUING cabinets!! Anyway.....later! Tom D.

Not sure these are the same boards as what I have, these ones are the latest version built this year. Sorry to hear about your speaker woes, I haven't had speakers go south on me yet, fingers crossed. I'm pulling my hair out with this one, it's going to be something silly I just know it. I moved the feeback taps back to the 16 ohm ones without success. I also ohmed out the speaker taps on the transformers, they matched.


Here are the pics roy requested.

all those voltages are perfect, just like the schematic.
So have you tried swapping the tubes around?
Seems kinda obvious the tube sections aren't matched, that happens sometimes, manufacturing tolerances and all.
If you can't get the gains matched closer, I'd suggest buying a tube with matching gain sections.
TubeDepot.com and others sell tubes pretested for matched section gain.

I have a second st-70 with different tubes, I can try them but IIRC I did without scoping it and there was noticeable difference. As far as the feedback , can a bad 220pf cap cause this issue?

a shorted feedback cap would give you ALOT less gain, and a open feedback cap would give you ALOT more gain, so not likely.
It appears the gain is about double on one side? I've seen tubes do that before, but usually within 20%
Your center tube voltages are an exact match, so strange that the gains are so much different, obviously the tube current is identical.
Your left tube plate voltages are 5% different, within normal range, but you COULD try swapping R19 and R21 to see if that makes any difference
(even though they are the same value)

ok, i'll try other tubes and report back.

Brad,

I just sent you a PM on this, but I might suggest also that you consider swapping the leads from the OTs to the tubes.....AND doing your 'ohm' test on the primary side of the OTs as well. I once had a Fisher that had an OT go south.....but it did so in a way that mimicked what you're experiencing. Weaker output on one side! Turns out that the OT had gone 'open' on one side of the primary circuit from center, giving me in effect, a Push-pull amp on one channel, and a Single Ended amp on the other. And, even if this is NOT your problem....at least by swapping OTs you can assure that OT performance is 'equal'...and not part of the problem. I'm not pointing a finger at Edcor......but that would at least rule out the possibility and let you focus on the driver board! For what it's worth!! Tom

Swapped to the set in my other stuff, same result. Now I did initially install both 334's reversed, then I removed and resoldered them back properly, could I have damaged one? The edcors were sold to me from an ak'r from a working amp.

Swap the 334s from side to side and see what happens! TSD