Early 1970 SVT Blueline (6146B) with Problem

[Outthewazoo]

I'm working to repair a very early (~3/1970) SVT Blueline with 6146B power tubes. The amp blew one of the 6146B tubes and took out most of the plate resistors (10 ohm), grid resistors (22 ohm), and the 2 bias test resistors (1 ohm). Traces on the board were also burnt along with one bank of the grid feed wires. I've repaired and replaced all the above. I also found bad resistors in the phase inverter and power tube bias circuit, which have been replaced.

Note that this amp is one of the rare versions with the Ampeg factory mod on the board that changed and lowered the B+ feed to the plates of the 12DW7. (tube tests good and also has been substituted with known good tube).

The problem is that one side or bank of the 6146B tubes negative bias voltage that is fed to the grids will "flip" to positive after all tubes are warmed up on startup. This is the bank that blew the one power tube. I can run the amp on a variac at lower "line" ac voltage and with a DMM watch the bias change on start up from ~-70 then drop to zero, then settle about +2.5 vDC. The other bank is working well and settles about -28 vDC. At full vAC line voltage, the bias voltage is supposed to be -47vDV.

I found this note on a 1969 Restoration thread, but I don't see this happening to my problem 6146B SVT:

The reason most 6146 SVT amps constantly blow-up tubes and resistors is due to front end 12BH7 voltage amplifier is being fed from the 430V node....during loud transients and overloads this will produce an AC signal that far exceed the 12BH7 follower that has 220V on the plates... So when the follower grid is driven far over it's own plate voltage it saturates on the positive half of the signal and thus takes over the BIAS voltage forcing it very positive...then the time for this voltage to come back to normal is based on the time constant of the 150K mixer resistors and the coupling cap...by this time it is too late, the BIAS got pushed way positive and the current gets slammed through the 6146 and they blow...
Simply re-wire the 12BH7 feed same as on the later 6550 heads... Add a 1K and filter cap feeding from the screen supply to the front voltage amp of the 12BH7... AMPEG actually did this mod by cutting traces but very few 6146 heads had this factory mod since they moved over to 6550 tubes at this time and this mod remained in place on the following board revs with 6550 tubes.. Based on drawing dates, this factory mod was initiated in July of 1970...while the 6550 update was sometime in Oct 1970....

Any help or advise is appreciated. Thanks!

 

[beans-on-toast]

This is a problem that is difficult to diagnose without seeing the amp in person. These sort of problems can be difficult to resolve, you have to sweat the details.

Attached is a clean 7-70 schematic.

Starting at node D on each side, I would check all the components and reflow the solder joints of the bias circuit through to the R21-R22 junction which is ground. This extends to pin-5 of the 6146B tubes where the bias is fed.

Check across each bias pot with the wiper unsoldered to ensure that one leg isn't open on the bad side. Also, while there, check the balance pot and ensure that the circuit is properly functioning. Check the resistance across each calibration pot, then through the full travel of the wiper.

Check the components with a meter to ensure that they are the correct value and up to spec. Sometimes color bands are not correct.

Check the reference voltages at pin-7 of the 12BH7's. An oxidized tube socket terminal or a tube socket that needs retensioning is a possibility. The same applies to the 6146B tube sockets at pin-5.

 

[Outthewazoo]

This is an odd problem that is difficult to diagnose without seeing the amp in person. These sort of problems can be difficult to reselve.

Attached is a clean 7-70 schematic.

Starting at node D on each side, I would check all the components and reflow the solder joints of the bias circuit through to the R21-R22 junction which is ground. This extends to pin-5 of the 6146A tubes where the bias is fed.

Check the bias pots to ensure that one leg isn't open on the bad side. Also, while there, check the balance pot and ensure that the circuit is properly functioning.

Check the resistance across each pot, then through the full travel of the wiper.

Check the components with a meter to ensure that they are the correct value and up to spec. Sometimes color bands are not correct.

Check the reference voltages at pin-7 of the 12BH7's. An oxidized tube socket terminal or a tube socket that needs retensioning is a possibility. The same applies to the 6146A tube sockets at pin-5.

Thank you for the advice!

The bad bank is the V4-V6 power side. At full line AC (allowed for a very short time), the 6146Bs on that bank will red plate and the amp will hum badly....it doesn't like positive bias voltage....obviously! I think this is what caused all of the issues, but why?

V5 has been replaced and I've replaced R22, R25, tested and substituted C6, replaced R18 & R19. R19 is supposed to have -77 vDC at Pin 7 of the 12BH7, which then provides -47 vDC via the cathode (Pin 8) to the grids of the three 6146Bs. At low line vAC after warm up, the 12BH7 Pin 7 (grid) and Pin 8 (cathode) voltage readings are both positive. The plate voltage at both 12BH7 Pin 1 plates is ~60v and at Pin 6 is ~106v at both, which are in range. Is maybe the signal going into or within the 12BH7 being inverted to cause the negative voltage to "flip" to positive?

I've done most of what you suggest including component swapping. I haven't checked the full travel of the bias pots and balance pot. I will also re-check it all again and report back. Thanks again.

 

[beans-on-toast]

Check if the D node is reading around the expected negative -150 vdc at both the R24-R20 junction and R25-R23 junction. The D node capacitor needs to be installed with [+] to ground to obtain the negative voltage. That should be ok since you are seeing some negative voltages. Trace from the two resistor junctions R24-R20 and R25-R23 through each component of the bias circuits and see how the voltage is reduced.

The voltage at D is divided by the resistors and comes out the wiper to provide the -77VDC. That’s why the VR-1 pot needs to be carefully checked.

Have you checked the bias voltages with the power tubes pulled from the amp?

The power tubes are supposed to be held at cutoff while the amp is in standby, this allows the bias voltages to stabilize. I assume that the standby switch is working properly.

 

[Outthewazoo]

So after re-tracing the circuit with low line ac, I found that the bias pot on the problem bank (which was showing a decent sweep range) is shorting the bias current to ground, which "flipped" it from negative to positive. That appears to be the problem! All components are ok. Replaced the bias pot with a 30k (all I could find). Amp is working now, with proper negative bias voltages on both banks and bias voltages of 0.072 vDC at the bias test points. Letting it burn in on the bench now...

Thank you Mr. Toast for your assistance!

 

[BassmanPaul]

I’m happy that you fixed the amp and even happier that you didn’t kill yourself doing so!

 

[beans-on-toast]

So after re-tracing the circuit with low line ac, I found that the bias pot on the problem bank (which was showing a decent sweep range) is shorting the bias current to ground, which "flipped" it from negative to positive. That appears to be the problem! All components are ok. Replaced the bias pot with a 30k (all I could find). Amp is working now, with proper negative bias voltages on both banks and bias voltages of 0.072 vDC at the bias test points. Letting it burn in on the bench now...

Thank you Mr. Toast for your assistance!

Good news!

This is what I use for the SVT bias pot: Invalid Link Removed

It's a bit expensive at $10 each but I like that you can lock down the setting which adds a bit of roadworthiness.

 

[Outthewazoo]

Good news!

This is what I use for the SVT bias pot: Invalid Link Removed

It's a bit expensive at $10 each but I like that you can lock down the setting which adds a bit of roadworthiness.

I agree that those pots would good, particularly to lock down the bias settings! Just a touch of a screwdriver can make a big change in the bias voltage (current) to the power tubes. Do you usually set a SVT with 6146Bs at 0.72 v bias per the Ampeg schematic?

 

[Outthewazoo]

Sorry meant 0.072 vDC bias.....not 0.72 vDC.

 

[beans-on-toast]

I agree that those pots would good, particularly to lock down the bias settings! Just a touch of a screwdriver can make a big change in the bias voltage (current) to the power tubes. Do you usually set a SVT with 6146Bs at 0.72 v bias per the Ampeg schematic?

Yes I do mostly. I set it as per the specs, then I tweak it by ear to see what sounds best and recheck the bias to ensure that it is within a safe range for the tubes. Balance is set to minimize THD with a meter.

 

[Outthewazoo]

Thanks again.

 

[Serge Hamel]

I have an early SVT that still uses 6146b's. I had a similar problem as you are described. As I recall, the schematic I was using called for 660v at the plates of the 6146b. I had about 710v and I thought that was causing the tubes to overheat and internally short. I measured the voltage out of my wall socket and I was getting about 123 volts. I figured that if the amp ran on 110v, my plate voltage would be around the desired 660v. I used a variac and sure enough, the plate voltage was 660v and the amp was fine. To rectify the problem, I did not do anything to the amp. I bought a buck-boost transformer and I wired it to reduce the wall voltage from 123v to 108-110v. Whenever I use the amp, I plug the amp in the buck-boost transformer and I have never had any problem with the amp since.

 

[Wasnex]

I have an early SVT that still uses 6146b's. I had a similar problem as you are described. As I recall, the schematic I was using called for 660v at the plates of the 6146b. I had about 710v and I thought that was causing the tubes to overheat and internally short. I measured the voltage out of my wall socket and I was getting about 123 volts. I figured that if the amp ran on 110v, my plate voltage would be around the desired 660v. I used a variac and sure enough, the plate voltage was 660v and the amp was fine. To rectify the problem, I did not do anything to the amp. I bought a buck-boost transformer and I wired it to reduce the wall voltage from 123v to 108-110v. Whenever I use the amp, I plug the amp in the buck-boost transformer and I have never had any problem with the amp since.

The schematic shows the PT is spec'ed for 120V so this seems a bit odd, but maybe your PT is just extremely aggro.

I believe a normal PT should produce a bit over 730V at node A if you pull all the tubes out of the amp. In addition to the output tubes pulling current at Node A, the preamp pulls a bit of current at Node B and the PI pulls a bit of current from Node C, which helps pull Node A down.

Keep in mind that the first driver section, which should be connected to the screen supply at Node H, was originally fed from Node B. Because of this, change the voltage at Node A will be a bit higher than 660V. When the amp is biased and idling, I believe Node A should fall to about 685V when everything is working properly. Despite what the schematics say, this seems to be fairly normal for SVTs...at least from what I have seen.