Low gain vta board - the new one with LM334 voltage regulators

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

by the time you swap those around again, they may be heat damaged.
I hope to put my own amp on the bench this weekend but I've got a couple of customer preamps to get built and finished first.

This is what i've been told to do:


With your generator plugged into yourpreamp.



The feedback disconnected on the amp.



Using your pre to balance the output onpin 1 and 6 of the center tube with your scope.



--



Then check the output at the 8ohm taps tosee if it’s even.



If it’s even you know it’sstill in the first section if you had to balance it with your pre.



If it’s not even then check pin 5 onthe output tubes to see if it’s even output between

channels, if it’s higher on 1 thenit’s either the CCS or the tubes in there as well.



----------------------



Disconnecting the feedback an making iteven at the center tube with your pre(checking with the scope on pins 1 and 6)



Then checking the rest of the amp willtell you where any issues are.

if you try that, obviously you'll want to be using test loads, not speakers, so 8-10 ohm resistors rated at 50 watts.
Also don't forget you'll have about 4-5X as much gain (12-15db) of gain with the NFB disconnected.

Won't the preamp prevent speaker damage?

NO!! Remember....the signal is still going through your output transformers!! They need to be hooked up to a load.....and doing this with dummy loads is a good idea, since you eliminate the reverse emf and impedance fluctuations from a working speaker. So, if you don't have dummy loads......get some! As I think 'Roy' said, an 8 to 10 ohm resistor at 50 watts is good......but two 15 to 20 ohm resistors at 50 watts each, then in Parallel is better! Or, a good 8 ohm Dummy Load from Parts Express is the best way to go. I think they sell non-inductive 100 Watt dummy loads wrapped in heat-sink aluminum for about $10 each. I think I bought a pair of 8 ohm versions about 15 years ago....and 4 ohm versions a few years back. Money well spent.

Tom

I think I have it figured it out. A back and forth with Kegger over the phone lead to the discovery. With the feedback reconnected, preamp, scope and function generator running I balanced the VTA tubes with the balance pot on the preamp and scope so the output was equal on both sides- 8 ohm taps still showed a large difference between output signal which lead to investigation of the outputs. Screen and plate to center tap ohmed out even on both sides. The signal on the plate on one set of tubes was off the map, the other was normal. Screen to plate on one side reads ~180 ohms, the other ~80 ohms. Issue is the screen to plate winding on the one transformer, it is 100ohms higher than it should be, in mine and Kegger's mind, it's a manufacture defect. I need to call edcor and give em hell!!!

yeah, not at all related to the board itself, you've either got the transformers connected wrong, or more likely,
Edcor sent you a messed up pair

The transformers aren't connected to the tube sockets anymore, they are measuring this way stand alone.

Gee.....if ever there was a time for an 'Eytolyaso' maybe this would be it?......lol

Funny how the things ya just don't think could possible be so....turn out to be so!

Anyway.....glad you got it sorted out. Like I said, I've been through this kind of bugger.....and it took me nearly 2 weeks to finally do the obvious and check the primary windings on the Fisher amp. After some back and forth with Heyboer, I finally got a new matched pair for that amp....but in the interim I had acquired a 'used' set that, although un-matched with one being from an X-100 and the other being from a 400, both work beautifully in the amp. I think I mentioned that I also had to shoe-horn a 400 power transformer into that amp as well. So, it's a few pounds of iron heavier than it should be.....but certainly sounds good!

Anyway....I'll be interested to hear how Edcor treats you about 'fixing' this transformer. I think you mentioned that didn't buy them 'new'.....so perhaps the guarantee is a 'first owner' kind of thing? Wouldn't surprise me! But, perhaps they'll be 'big' about it and help you out....even if its just partially. I think you'd have to agree that even that would be 'fair'.

Glad you've made some progress!

Tom

It's all fixed, edcor miswired the pair of primaries for the tubes, it's wired blue brown stripe - brown blue stripe and is perfect now.