VTA ST-70 saga

I became a reader of this forum a few months back after I bought an ST-70 with the VTA board, triode-pentode switches and some Curcio mods. Not my first tube amp, but the first one that I've had open. I started noticing a little hum (through both channels, not affected by volume setting). After much reading and doing all of the isolation diagnostics that I could find, I decided to replace the quad cap. That's when things got interesting. On opening the amp, I see that the old quad-cap is not wired correctly - at least not according to the pictures on Tubes4hifi and a few Bob Latino posts. It's mounted backwards with the choke connected to the wrong terminals and a jumper from the 80 - 30 sections and a 2.6k ohm resistor from the 40 section to the jumper wire. That resistor is removed in the photo link. Aside from that, I also notice other things that look curious, but I satisfy myself with replacing the quad cap and firing the amp back up. The hum seemed much reduced and I listened to it for a few weeks - forgetting to re-check and adjust the bias on the tubes (Gold Lion KT77s). Then one day the left channel is producing nothing but low-level static.

The tubes won't seem to hold a steady bias, so I wonder if I cooked them. I'm taking them to be tested at a local shop, but in the meantime I start scrutinizing the other aforementioned "curiosities" (See picture links below).

1) An extra transformer is mounted at the right rear which is being used to supply power to the PCB instead of using the main input transformer. Huh? Anyone ever seen this? Should I remove this and revert to the wiring scheme recommended by VTA?

2) There are no ground wire connections from the PCB to the RCA inputs. Curious how big a problem this may be?

3) Note the wiring of the 10 ohm resistors between pins 1 and 8 of both output tubes, with a ground wire from between the resistors to the ground on the PCB. This doesn't seem quite right - this circuit adds 20 ohms of resistance between the 2 tubes.

4) The white wiring that was used to wire the pentode / triode switches is stiff solid-core wiring. Not sure of the gauge, I think that 20-22 gauge stranded teflon-coated wire is recommended. This is definitely heavier than 20 gauge Should I replace this?

I've tested all resistors in both channels and all test fine. This week I'll be running further tests but would love to have ideas from others on this great forum.

thanks in advance ...

pictures (inlcluding photobucket links as well as embedding them here)

Original quad-cap wiring:
http://s1305.beta.photobucket.com/user/panaseef/media/originalquad-capwiring_zps7e3f55c5.jpg.html?sort=3&o=4



complete insides:
http://s1305.beta.photobucket.com/user/panaseef/media/DSC00085_zps882c1c10.jpg.html?sort=3&o=1



Left channel close-up:
http://s1305.beta.photobucket.com/user/panaseef/media/DSC00081_zpsb75617b0.jpg.html?sort=3&o=3



top side:
http://s1305.beta.photobucket.com/user/panaseef/media/DSC00087_zps4ccb43f1.jpg.html?sort=3&o=0

Hi -- Welcome aboard!

In answer to some of your questions:

(1) The extra transformer looks to be a 6.3 volt supply (Green Wires) and is feeding the filaments of the 12An7 (can't tell whether you got 12AT7s or 12AU7s or equivalent tubes) instead of coming from the Output Tubes

(2) There has been lots of Quad Cap discussion here on the Forum; do an advanced search for "quad Cap" in quotes and you will see a lot of questions and replys.

(3) With the Stock Dynaco 30-20-20-20 Quad Cap and the Stock Dynaco PCB:
a) Pin #8 of the GZ34 Rectifier Tube and 1/2 of the Choke feeds the 30MF Quad Cap Tab;
b) the other 1/2 of the Choke and two RED wires from the Output Transformer feed the 1st 20MF Tab that typically faces the rear of the chassis;

The 2nd and 3rd 20MF Tabs are configured as follows:
c) a 6800 ohm resistor goes between the 1st and 2nd 20MF Tabs and then a jumper wire goes to Solder Pad #20 of the Dynaco PCB. (the 2nd 20MF Tab faces the right side of the chassis;
d) a 22000 ohm resistor goes between the 2nd and 3rd 20MF Tabs; the 3rd 20MF Tab (which faces the front of the Chassis) attaches via a jumper wire to Solder Pad #19 of the PCB.

***********************************************************************

If you replace the stock Quad cap with a 80-40-30-20 type Quad Cap which was apparently done in your case, then the wiring orientation of the cap is:

(1) the 40MF gets GZ34, Pin #8 and 1/2 of the choke and faces the GZ34;
(2) the 80MF gets the other 1/2 of the choke, the 2 RED wires from the OT and one side of the 6800 ohm resistor;
(3) the Quad Cap Tab that faces the right side of the chassis gets one half of each resistor (6800/22000) and the jumber wire to SP #20;
(4) the Quad Cap Tab that faces the front of the chassis gets the other half of the 22000 ohm resistor and the jumber wire to SP #20.

**************************************************************************

Check the Quad Cap Searches to see how the 80-40-30-20 Quad Cap hooks up to the VTA Board. Please note that all 80-40-30-20 caps are not the same and there is some wiring differences. I can't really tell from your pictures what Cap you have (CE or Dynakitparts or AuthentiCap or ??)

Hope this helps some!!
Stew

Hi Again -- I found the Quad Cap to VTA Board wiring info --

(a) 40MF Tab (faces GZ34) and is wired the same as above.

(b) 80MF Tab (faces rear of chassis) and is wired the same as above, but with a jumper wire in place of the 6800 ohm resistor.

(c) Tab that faces the right side of the chassis, other end of the 80MF's jumper wire and one end of a 2200 Ohm resistor (NOTE-there is no conection to VTA Board from this TAB and there are no 6800 or 22000 ohm resistors)

(d) Tab that faces the front of the chassis gets the other end of the 2200 Ohm resistor and a jumper wire that goes to the B+ TAB on the VTA Board.

====>>>what Quad Cap do you have?


TRY the following LINK:

http://www.tubes4hifi.com/VTA70-diagrams.htm

Hope this helps,

Stew

Stew,

Thanks for your post. My quad cap is a 525V CP Manufacturing 80-40-30-20 purchased from Tubes4hifi. I now have that wired correctly - exactly as you stated. I think I've been to every thread on this forum about the quad cap. I'm new to the ST-70 and trying to learn all that I can.

I'm using ECC 81 tubes (12AT7 equivalent) and you're correct that the extra transformer is feeding those filaments. In all of my reading I've never seen anyone recommend or talk about mounting a separate transformer and using it to feed the input tubes. Any potential merit to this? downside? Anyone?

thanks,
Paul

Hi Paul --

Re the extra 6.3volt filament transformer -- I don't think it hurts anything because the ST70 Power Transformer feeds the filaments of the output tubes (EL34s) and the Rectifier tube and has always been thought of being wimpy or underpowered, so the extra transformer is taking some of the power tranny's load off.

On the matter of the way the two 10 ohms resistors are wired. Yes, it is strange and not the normal or correct way. Look at the wiring diagram at the LINK that I gave you and wire the 10 ohmers as per the diagram.

The white wire is OK. It is proably 16 or 18 gauge. I normally use 20 or 22 and occassionally 18, but even 16 is not going to hurt anything.

Let us know how your changes/corrections work out.

Stew

Thanks again guys. RCA inputs were definitely not grounded correctly (thanks Bob). New set on the way. I'll report back.

the 10 ohm resistor connections are OK, it's not usually done this way, but even I have done them this way before, when the sockets don't have a ground tab on them. Pin 1 and 8 of each output tube are tied together, and then go thru the 10 ohm resistors to ground.
The way is was done is correct, just different than usually seen.

So I replaced the input RCA jacks (thanks Bob) and grounded them correctly. Pulled out my slightly used quad of JJ EL34 tubes and fired up the amp and now I can't get the bias below .90v on all 4 tubes uniformly. Also, the bias adjustments are working backwards - turning them CW raises the bias - CCS lowers it. Very strange. I checked the main bias resistor at R39 and it reads 10k. Checked pins 1 and 8 to chassis ground and they all read 10.5 ohms.

Everything was working fine until I changed the quad cap, wiring the new one correctly. I'm at a loss. Any more ideas?

Hi,
can you give us the bias voltage measured on pin 5 of your tube sockets (without tubes), CW and CCW ? CW will raises the bias voltage, thus lowers idle current.
Possible problem source: bad bias rectifier, bad ground, bad bias filter cap, etc...
Are you sure about the power tubes grid resistors (value, continuity) ?
Hope this can help you,
BR,
Danskman

This VTA board is pre-April 2012, so the bias is working backwards for some reason. I've searched other posts and found other people who have experienced this. With all tubes removed and my multimeter on the 200v setting, I get readings of -37.5v on the right channel at pin 5 and -38.6 on the left channel

My last post wasn’t quite clear enough – this is a pre-2012 board, so I understand that the bias voltage should increase as the pot is rotated counter-clockwise, and decrease as it is rotated clockwise. It used to work this way, but after my quad-cap replacement, it is operating “backwards” from the way it should. The range from fully CCW to fully CW is about .9 to 1.3.

So this morning I finally got a block of time and decided to re-install the previous quad-cap. I did this with the theory that the original hum problem was probably due to the improper grounding of the input jacks and that the new problems were due either to the new quad cap or my installation of it.

After completing the installation, I turn the bias controls fully clockwise, hook up speakers, have nothing connected to the inputs, pentode / triode switches in triode position and turn it on, Bias reads .02v on all 4 points. As the amp warms up, I start to hear ugly popping and static and immediately shut it down. Out with the ECC81 driver tubes and in with a brand new set of EH 12AT7 tubes. I turn it on again and let the amp warm up. No noise. After maybe 10 minutes I adjust the bias pots and am able to get a steady .4v - and they work the correct, counter-intuitive way. Progress.

Now for the acid test. I re-install it into my system and play an album. Beautiful music, and the hum is gone.

The saga may not be over – I’ve only had time for a very short listen, but this is promising. Thanks very much to all who posted. I’ve got a few lessons learned …

I went through a few 12AT7's in that center position on the VTA board before I got the hum eliminated. The right and left tubes aren't nearly as critical. Glad to hear you are up and running!

OK, so I have to ask nmchiefsfan - what 12AT7 did you settle on?

Panaseef,

I ended up with a Mullard CV4024 (12AT7) from Upscale Audio. I got the platinum grade which has the lowest noise and microphonics.

Good luck!

Dave

So I have to ask, mmchiefsfan - which 12AT7 is your favorite?

I stopped looking after the Mullard CV4024. I have not done any tube rolling on the pre-amp tubes but since I have the VTA ST-120 I upgraded to the KT-120 output tubes. I am very happy with this combination.