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New VTA ST-120 Build! Need Help! SOS

Analog Man: Posts : 56
Join date : 2013-09-01
Age : 62
Location : St. Louis, MO - On The Hill

Post n°1

New VTA ST-120 Build! Need Help! SOS

by Analog Man Sat Sep 14, 2013 10:06 pm

I have just completed my VTA ST-120 and have run into problem with V6, "Front right output tube". I have built many tube amps in the past and pretty much know what I am doing, at least I think so.
My problem is that the tube gets red hot and I am not getting any bias voltage readings, all 0v. I have checked all of my wiring and all of the voltage measurments as listed in the assembly manual. I have checked all connections and looked for ground short from the driver board to the chassis. I cant seem to find the problem, I'm perplexed!
Can anyone out there give me something to go on? I am so frustrated with this that I had to call it quits for the night. I hope I didn't damage the output tube, Very $$$.
Running new KT-120 Tung-Sols. I even did a tube swap to see if it was a bad tube. even after swapping tubes, no difference. I am leaning towards a transformer problem but I don't know for sure.
Any help you can give would be greatly appreciated! Crying or Very sad

Crying or Very sad

hawaii.ken: Posts : 157
Join date : 2012-01-31

Post n°2

Re: New VTA ST-120 Build! Need Help! SOS

by hawaii.ken Sun Sep 15, 2013 12:36 am

Analog Man wrote:My problem is that the tube gets red hot and I am not getting any bias voltage readings, all 0v.

With no tubes installed and power off, check the bias diode to see if it's still good and verify proper orientation. Also verify proper orientation of the bias caps.

With no tubes installed, apply power and check to see if you have negative bias voltage at all power tubes at pin-5

Analog Man: Posts : 56
Join date : 2013-09-01
Age : 62
Location : St. Louis, MO - On The Hill

Post n°3

Re: New VTA ST-120 Build! Need Help! SOS

by Analog Man Sun Sep 15, 2013 6:53 am

Hi and thank you. I did as you suggested and measured for neg. bias voltage at pin5 on all output tube sockets, I'm getting around neg. 62 volts.

Analog Man: Posts : 56
Join date : 2013-09-01
Age : 62
Location : St. Louis, MO - On The Hill

Post n°4

Re: New VTA ST-120 Build! Need Help! SOS

by Analog Man Sun Sep 15, 2013 7:55 am

Here are some pics.
https://s756.photobucket.com/user/hilldwellerstl/library/?sort=3&page=1

Bob Latino: Admin; Posts : 3282
Join date : 2008-11-26
Location : Massachusetts

Post n°5

Re: New VTA ST-120 Build! Need Help! SOS

by Bob Latino Sun Sep 15, 2013 9:01 am

Hi James,

With the amp OFF > can you measure about 10 ohms at all bias measuring points?

With the amp ON > What is the DC voltage from pin 8 of the rectifier socket to chassis ground ?

A "no bias voltage" situation is usually a bad rectifier tube (or a Weber SS rectifier in your case). It could also be that the TDR is not working properly and not passing the secondary power transformer voltage on to the rectifier tube socket. Try bypassing the TDR

If you did a full voltage check, what readings did you get?

You had the chassis "hammer coated" .. Sometimes this can interfere with a ground inside the chassis.

Bob

Analog Man: Posts : 56
Join date : 2013-09-01
Age : 62
Location : St. Louis, MO - On The Hill

Post n°6

Re: New VTA ST-120 Build! Need Help! SOS

by Analog Man Sun Sep 15, 2013 9:35 am

Bob Latino wrote:Hi James,

With the amp OFF > can you measure about 10 ohms at all bias measuring points?

With the amp ON > What is the DC voltage from pin 8 of the rectifier socket to chassis ground ?

A "no bias voltage" situation is usually a bad rectifier tube (or a Weber SS rectifier in your case). It could also be that the TDR is not working properly and not passing the secondary power transformer voltage on to the rectifier tube socket. Try bypassing the TDR

If you did a full voltage check, what readings did you get?

You had the chassis "hammer coated" .. Sometimes this can interfere with a ground inside the chassis.

Bob

Getting 10 ohms at all bias points
Rectifier: pin 8 = 583vdc
pin 2 = 583vdc
pin 4 = 420vac
pin 6 = 420vac

Quad cap: square = 581 vdc
half circle = 581 vdc
circle = 581vdc
triangle = 568vdc

V2: pins 5 & 6 = -62.2vdc
V3: pins 5 & 6 = -61.3vdc
V6: pins 5 & 6 = -54.4vdc
V7: pins 5 & 6 = -60vdc
I can get all output tubes to bias except for V6. With tube in circuit I get no bias reading at any point. I swapped tubes around and still have the same problem. No ground faults.

Bob Latino: Admin; Posts : 3282
Join date : 2008-11-26
Location : Massachusetts

Post n°7

Re: New VTA ST-120 Build! Need Help! SOS

by Bob Latino Sun Sep 15, 2013 10:42 am

James,

Your pin 8 rectifier DC voltage is way high ... That 583 VDC that you see on pin 8 of the rectiifier socket should be maybe 515 - 520 with a Weber SS rectifier. Your line voltage must be noticeably higher than 120 volts. Check your line voltage! Try using a 5U4 rectifier tube rectifier instead of the Weber WZ68. This will drop the rectifier pin 8 DC voltage maybe 40 volts. If your line voltage is anything over 122 VAC, you should run the amp off a variac set to about 117 to 118 VAC.

If any tube will not bias in V6 then there is an issue on the V6 tube socket OR that part of the driver circuit that feeds V6. Check the coupling cap that feeds V6 and all surrounding parts. The triangle section of the quad cap is way high. The triangle section of the quad cap should be maybe 380 to 420 volts. Did you have the driver tubes in when you made this measurement ?

Bob

Analog Man: Posts : 56
Join date : 2013-09-01
Age : 62
Location : St. Louis, MO - On The Hill

Post n°8

Re: New VTA ST-120 Build! Need Help! SOS

by Analog Man Sun Sep 15, 2013 11:06 am

No tubes we in the circuit when I made the voltage measurements but the weber was in the circuit.

Analog Man: Posts : 56
Join date : 2013-09-01
Age : 62
Location : St. Louis, MO - On The Hill

Post n°9

Re: New VTA ST-120 Build! Need Help! SOS

by Analog Man Sun Sep 15, 2013 11:17 am

Line voltage is 119vac unloaded. V6 socket is wired correctly, everything on the driver board looks ok.

Bob Latino: Admin; Posts : 3282
Join date : 2008-11-26
Location : Massachusetts

Post n°10

Re: New VTA ST-120 Build! Need Help! SOS

by Bob Latino Sun Sep 15, 2013 11:17 am

Analog Man wrote:No tubes we in the circuit when I made the voltage measurements but the weber was in the circuit.

DON'T EVER have JUST the rectifier in the amp with no other tubes when the amp is ON! If you ever have the RECTIFIER in the amp ALWAYS have the other tubes in there. Without the other tubes plugged in, there will be no LOAD on the high voltage system and the high voltage system WILL SHOW WAY HIGH DC voltages. Do another voltage check (especially the quad cap voltages) with all the tubes plugged in. The voltages should come down near normal levels with all the tubes plugged in.

Also - You will also not be able to measure bias voltage at the four bias measuring point without the output tubes plugged in.

Bob

Analog Man: Posts : 56
Join date : 2013-09-01
Age : 62
Location : St. Louis, MO - On The Hill

Post n°11

Re: New VTA ST-120 Build! Need Help! SOS

by Analog Man Sun Sep 15, 2013 11:35 am

With all tubes plugged in, I get no bias voltage at V6. I want to test voltage levels with all of the tubes plugged in but fear tube damage during the process.

Analog Man: Posts : 56
Join date : 2013-09-01
Age : 62
Location : St. Louis, MO - On The Hill

Post n°12

Re: New VTA ST-120 Build! Need Help! SOS

by Analog Man Sun Sep 15, 2013 11:50 am

I am too frustrated to go any further today. Will try tomorrow. THANKS FOR ALL OF YOUR HELP!

arledgsc: Posts : 504
Join date : 2012-11-30

Post n°13

Re: New VTA ST-120 Build! Need Help! SOS

by arledgsc Sun Sep 15, 2013 12:06 pm

Check to verify the 10 ohm resistors are connected to both pin 1 and pin 8. The resistor appears to be not connected to pin 1 on V7 (right rear). The photos are a bit grainy but it also appears V7 pin 8 is connected to pin 7 but could be optical illusion. Also verify on V6 just to be sure. The triode/ultralinear switch connections look OK.

After some more looking recheck the V7 pin 3 black wire back to the triode/ ultralinear 100 ohm resistor. Again probably optical effects but it appears the pin 3 wire connects to the quad cap and the triode/UL 100 ohm resistor to the output transformer. I would also suggest rerouting V7 pin 3 wire away from the high voltage stuff.

Last edited by arledgsc on Sun Sep 15, 2013 1:32 pm; edited 1 time in total

Analog Man: Posts : 56
Join date : 2013-09-01
Age : 62
Location : St. Louis, MO - On The Hill

Post n°14

Re: New VTA ST-120 Build! Need Help! SOS

by Analog Man Sun Sep 15, 2013 1:00 pm

10 ohm resistors are connected correctly! Only problem is with V6 and it can only be an issue with the driver board. I have eliminated all other possibilities. I will focus more closely with the driver board and see what I can find there.

sailor: Posts : 269
Join date : 2011-04-04

Post n°15

Re: New VTA ST-120 Build! Need Help! SOS

by sailor Sun Sep 15, 2013 2:39 pm

If your wiring is correct and the correct parts in the correct place then Check all of the electrolytic caps and diode for correct polarity direction. If you are not getting any bios voltage I would think one of the caps in the bios is in backwards or the diode. If so it may now be blown and need to be replaced. I would pull all tubes and rectifier and then check the voltage of the bios. If it is still 0 then you have something wrong in the bios circuit.

hawaii.ken: Posts : 157
Join date : 2012-01-31

Post n°16

Re: New VTA ST-120 Build! Need Help! SOS

by hawaii.ken Sun Sep 15, 2013 3:11 pm

Analog Man wrote:
Getting 10 ohms at all bias points
Rectifier: pin 8 = 583vdc
pin 2 = 583vdc
pin 4 = 420vac
pin 6 = 420vac

Quad cap: square = 581 vdc
half circle = 581 vdc
circle = 581vdc
triangle = 568vdc

V2: pins 5 & 6 = -62.2vdc
V3: pins 5 & 6 = -61.3vdc
V6: pins 5 & 6 = -54.4vdc
V7: pins 5 & 6 = -60vdc
I can get all output tubes to bias except for V6. With tube in circuit I get no bias reading at any point. I swapped tubes around and still have the same problem. No ground faults.

The 'V6: pins 5 & 6 = -54.4vdc' points to a faulty coupling cap. Replace it temporarily with a known good (or new) cap with at least 450V rating. The capacity (uF) is not important.

Analog Man: Posts : 56
Join date : 2013-09-01
Age : 62
Location : St. Louis, MO - On The Hill

Post n°17

Re: New VTA ST-120 Build! Need Help! SOS

by Analog Man Mon Sep 16, 2013 8:05 pm

hawaii.ken wrote:

Analog Man wrote:
Getting 10 ohms at all bias points
Rectifier: pin 8 = 583vdc
pin 2 = 583vdc
pin 4 = 420vac
pin 6 = 420vac

Quad cap: square = 581 vdc
half circle = 581 vdc
circle = 581vdc
triangle = 568vdc

V2: pins 5 & 6 = -62.2vdc
V3: pins 5 & 6 = -61.3vdc
V6: pins 5 & 6 = -54.4vdc
V7: pins 5 & 6 = -60vdc
I can get all output tubes to bias except for V6. With tube in circuit I get no bias reading at any point. I swapped tubes around and still have the same problem. No ground faults.

The 'V6: pins 5 & 6 = -54.4vdc' points to a faulty coupling cap. Replace it temporarily with a known good (or new) cap with at least 450V rating. The capacity (uF) is not important.

Faulty coupling cap? There brand new and test ok!

hawaii.ken: Posts : 157
Join date : 2012-01-31

Post n°18

Re: New VTA ST-120 Build! Need Help! SOS

by hawaii.ken Tue Sep 17, 2013 2:08 am

Analog Man wrote:Faulty coupling cap? There brand new and test ok!

How did you test it/them? How much voltage was applied during the test?

Maybe you got the 1 in 10,000 that fail?

It's easiest just replace it temporarily with a standard cap just to troubleshoot your bias problem.

Analog Man: Posts : 56
Join date : 2013-09-01
Age : 62
Location : St. Louis, MO - On The Hill

Post n°19

Re: New VTA ST-120 Build! Need Help! SOS

by Analog Man Tue Sep 17, 2013 10:24 pm

Success! Problem was a solder blob that collected under the far right tube socket on the driver board. After this was cleared, everything worked fine.
I would recommend that any wires coming off of the driver board be soldered in first before installing the driver board.
You would then have to splice into the chassis wiring.

Analog Man: Posts : 56
Join date : 2013-09-01
Age : 62
Location : St. Louis, MO - On The Hill

Post n°20

Re: New VTA ST-120 Build! Need Help! SOS

by Analog Man Wed Sep 18, 2013 8:42 pm

Bob Latino wrote:

Analog Man wrote:No tubes we in the circuit when I made the voltage measurements but the weber was in the circuit.

DON'T EVER have JUST the rectifier in the amp with no other tubes when the amp is ON! If you ever have the RECTIFIER in the amp ALWAYS have the other tubes in there. Without the other tubes plugged in, there will be no LOAD on the high voltage system and the high voltage system WILL SHOW WAY HIGH DC voltages. Do another voltage check (especially the quad cap voltages) with all the tubes plugged in. The voltages should come down near normal levels with all the tubes plugged in.

Also - You will also not be able to measure bias voltage at the four bias measuring point without the output tubes plugged in.

Bob

Hi Bob,
I think I found the problem with not getting bias voltage reading ant V6. I discovered a short from pin 4 to pin 5 on the driver board at V3. I can not clear this because the pads on the board are too close together due to the filament jumper. I have tried a de soldering pump and de soldering wick but I still can't clear this short. Do I need a new board? Have you been presented with this problem before?
I would really appreciate your help with this as I am very frustrated with this. Neutral

Neutral

Bob Latino: Admin; Posts : 3282
Join date : 2008-11-26
Location : Massachusetts

Post n°21

Re: New VTA ST-120 Build! Need Help! SOS

by Bob Latino Wed Sep 18, 2013 9:04 pm

"V3" is the left front output tube. The three driver tubes are not given any numbers.

On the noval tube sockets on the driver board pins 4 and 5 are wired together internally on the board to accept one leg of the 6.3 volt AC filament current. There IS a short between pins 4 and 5 - that is not a board error. All three of the noval sockets on the VTA driver board are wired that way. The pads on the board are not too close together - BUT - someone doing sloppy soldering can sometimes run solder across eyelets that are not supposed to be connected ...

Bob

Analog Man: Posts : 56
Join date : 2013-09-01
Age : 62
Location : St. Louis, MO - On The Hill

Post n°22

Re: New VTA ST-120 Build! Need Help! SOS

by Analog Man Wed Sep 18, 2013 9:13 pm

Bob Latino wrote:"V3" is the left front output tube. The three driver tubes are not given any numbers.

On the noval tube sockets on the driver board pins 4 and 5 are wired together internally on the board to accept one leg of the 6.3 volt AC filament current. There IS a short between pins 4 and 5 - that is not a board error. All three of the noval sockets on the VTA driver board are wired that way. The pads on the board are not too close together - BUT - someone doing sloppy soldering can sometimes run solder across eyelets that are not supposed to be connected ...

Bob

Ok guys,
I have tried everything you have suggested and to no avail. I still can't get a bias voltage at V6. I am so aggravated with this, I don't know what else to do. Please note that I am no novice and have built many amps in the past with no problems.

Bob Latino: Admin; Posts : 3282
Join date : 2008-11-26
Location : Massachusetts

Post n°23

Re: New VTA ST-120 Build! Need Help! SOS

by Bob Latino Wed Sep 18, 2013 9:44 pm

With the triode/pentode switch set to pentode ... Do you get high voltage DC at both pins 3 and 4 on V6? You should be getting 500+ VDC on both pins 3 and 4. See if you do ? If you don't check the triode/pentode switch wiring for that tube socket ..

Bob

Analog Man: Posts : 56
Join date : 2013-09-01
Age : 62
Location : St. Louis, MO - On The Hill

Post n°24

Re: New VTA ST-120 Build! Need Help! SOS

by Analog Man Thu Sep 19, 2013 7:34 pm

All is well! The problem the whole time was a break in a piece of hookup wire coming from the driver board to V6. Who would have thought something so simple would have caused these headaches. And no, I did not over bend or stretch the wire!

How long before the bias voltage stabilizes ? Should I run this amp with dummy loads and shorted input plugs for a few hours?

Thank you all for your help and support!!! Very Happy

Very Happy

Bob Latino: Admin; Posts : 3282
Join date : 2008-11-26
Location : Massachusetts

Post n°25

Re: New VTA ST-120 Build! Need Help! SOS

by Bob Latino Thu Sep 19, 2013 7:59 pm

The bias voltage will never totally stabilize if you live in an area in which the LINE voltage goes up and down. Even one volt up or down will cause the bias to change. When you start the amp up COLD the bias will be a low. On start up maybe initially set all the tubes at .520 VDC or so. As the amp warms up the bias will rise slightly. You then make your final bias setting with at least 1/2 hour warm up with the amp at idle (no signal running through the amp). As long as all the tubes are between .530 and .560 VDC, you are OK. Slight bias differences between the tubes have no affect on the sound of the amp. Check the bias once a month. As the output tubes wear a little bias will change slightly.

The thing is not to try to get all four output tubes precisely at .550 VDC (on the VTA ST-120). The 50K bias pots give great range to allow biasing many different types of output tubes > BUT they don't allow you to set each output tube at precisely .550 VDC. Getting the bias "perfect" is not really necessary .. As long as all tubes are close to the recommended setting - you are OK.

Re: > "Should I run this amp with dummy loads and shorted input plugs for a few hours?" This is not really necessary .. Just play the amp and listen to music ..

Bob

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Re: New VTA ST-120 Build! Need Help! SOS

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