Ampeg v4 tubes glowing red

[danno9999]

I am at my wits end on this one!!!
The bias circuit all rebuilt, I have -50v on the output of the circuit.
Now on pin 5 of the power tubes without the tubes in and the switch in standby I have 470v on pin 5, but when I turn on the standby switch I get
-50 volts on pin 5. I guess this seems ok. This amp is weird as their isn't a real ground and the ground of the dc bias is different than the ground of the high voltage B+.
Ok when I put in the power tubes and monitoring pin 5 , turn on the bypass
switch , neg bias at pin 5 drops to -11v , I get hum and I know the tubes a drawing a lot of current. All the power supply has new caps and new .33uf coupling caps from the 12au7 feeding the power tubes.
All I can think of is the tubes are drawing too much current??? why?? and then the voltage is dropping on the neg DC at the same time.
I have tried heating the solder joints on the board with no luck.
These are also new Svetlan 6L6 tubes.
Any ideas guys!!!

[Jazz P Bass]

Ampeg V4

You might want to check out this post.
Ampeg v4 bias supply issue

[g1]

One of the most frustrating things that can happen is spending hours on something only to find the problem is you got a bad new part.
Never rule out this possibility. I'm not saying it's definitely bad tubes but it is a possibility. Those amps run the power tubes at pretty high voltages and need really good 6L6's to handle it. Trying them in another amp might not even show the problem if it is running more conservative voltages.

[danno9999]

Ampeg glowing tubes

I did go back from the previous thread and read all that. My schematic is correct and the .047 cap does connect to the resistor and diode.

Here is another question? What is the resistance on the primary winding of the output transformer from centre tap to each plate winding.
I have disconnected both plate wires and have about 57 ohms on each winding to the centre tap.

I'm trying to rule out a bad transformer. After this it may be the tubes not handling the high voltages of the amp???

The only thing I can't figure out is why the neg volatge at pin 5 of the tubes drops to -11 from -52 when the tubes start drawing current and the bypass switch is turned on?

One of the most frustrating things that can happen is spending hours on something only to find the problem is you got a bad new part.
Never rule out this possibility. I'm not saying it's definitely bad tubes but it is a possibility. Those amps run the power tubes at pretty high voltages and need really good 6L6's to handle it. Trying them in another amp might not even show the problem if it is running more conservative voltages.

[Ronsonic]

I am at my wits end on this one!!!
The bias circuit all rebuilt, I have -50v on the output of the circuit.
Now on pin 5 of the power tubes without the tubes in and the switch in standby I have 470v on pin 5, but when I turn on the standby switch I get
-50 volts on pin 5. I guess this seems ok. This amp is weird as their isn't a real ground and the ground of the dc bias is different than the ground of the high voltage B+.
Ok when I put in the power tubes and monitoring pin 5 , turn on the bypass
switch , neg bias at pin 5 drops to -11v , I get hum and I know the tubes a drawing a lot of current. All the power supply has new caps and new .33uf coupling caps from the 12au7 feeding the power tubes.
All I can think of is the tubes are drawing too much current??? why?? and then the voltage is dropping on the neg DC at the same time.
I have tried heating the solder joints on the board with no luck.
These are also new Svetlan 6L6 tubes.
Any ideas guys!!!

Every Single Part Of The Bias Supply ... Replace it. Start with that .047 cap, yes, no matter how sure you are it is good. Replace it. It is not feasible to test it in the way that this amp tests it. Also do the goofy diode. Of course the other caps in the bias supply too.

Lift off, nuke from orbit. It's the only way to be sure. I've wasted a lot of time in V series Ampegs attempting to sharp shoot it. It isn't worth it. Just replace the parts, we can solder faster than we can figure this out. Or that's my conclusion.

[danno9999]

Every part is new in the bias supply. New .047, the resistors, the filter caps, the 100k after the bias supply to the split and the 47k feeding pin 5.
The only thing I haven't changed is the diode.
I'll pop in a 1n4007 and try that.
Another question..what about the diodes in series from each side of the output transformer winding? The schematic says they are 1 amp at 1400v.
Think I can use the 1n5408's they are 1000piv 3 amp?
Thanks for all your help!!

Every Single Part Of The Bias Supply ... Replace it. Start with that .047 cap, yes, no matter how sure you are it is good. Replace it. It is not feasible to test it in the way that this amp tests it. Also do the goofy diode. Of course the other caps in the bias supply too.

Lift off, nuke from orbit. It's the only way to be sure. I've wasted a lot of time in V series Ampegs attempting to sharp shoot it. It isn't worth it. Just replace the parts, we can solder faster than we can figure this out. Or that's my conclusion.

[Bruce / Mission Amps]

Every part is new in the bias supply. New .047, the resistors, the filter caps, the 100k after the bias supply to the split and the 47k feeding pin 5.
The only thing I haven't changed is the diode.
I'll pop in a 1n4007 and try that.
Another question..what about the diodes in series from each side of the output transformer winding? The schematic says they are 1 amp at 1400v.
Think I can use the 1n5408's they are 1000piv 3 amp?
Thanks for all your help!!

That's fine... use two or three 1N4007s or two or three 5408s (kinda big) but those diodes are grounded at the anodes of the FWB... which is held above chassis ground by your possibly defective standby switch.
Why there is two letter As at possible ground potential I can see but why is the hard core B+ marked A also?
The bias supply is held up from chassis ground by the standby switch too.
Maybe I'd replace the diode (D5) that holds the B+ rail ground up too.

[Enzo]

SO did we try DIFFERENT power tubes? A shorted power tube can easily kill the bias.

Or at least try this. Pick ONE power tube and ONE output socket. Try the tube there. Does the bias drop or not. Now try the next tube in the same socket. Try every output tube you have in that same socket. DO they all work the same or does one kill the bias while the others do not?

Once you have weeded out any bad tubes, now go to the second socket and try tubes there. Does the bias hold up or does it sink?

Then try a tube in the third socket, and after that the fourth.

BY doing this one tube one socket sort of test, we can determine if we have any odd tubes and if any of the sockets are acting up.

I don;t care if they are new, new tubes can come bad in the box. test them.

[danno9999]

I will try the tubes one at at time as you suggested.
I redid the bias supply again and it is rock solid, like I mention the bias drops when the tubes start to conduct. The more I think about it it may be bad tubes or a couple. They are all new Svet 6L6GC which should handle the 540v of this V4.
I'll let you know..thanks agin for all your help.

SO did we try DIFFERENT power tubes? A shorted power tube can easily kill the bias.

Or at least try this. Pick ONE power tube and ONE output socket. Try the tube there. Does the bias drop or not. Now try the next tube in the same socket. Try every output tube you have in that same socket. DO they all work the same or does one kill the bias while the others do not?

Once you have weeded out any bad tubes, now go to the second socket and try tubes there. Does the bias hold up or does it sink?

Then try a tube in the third socket, and after that the fourth.

BY doing this one tube one socket sort of test, we can determine if we have any odd tubes and if any of the sockets are acting up.

I don;t care if they are new, new tubes can come bad in the box. test them.

[danno9999]

It appears it was a pair of new tubes drawing too much current, loading down the neg bias.
I installed 1 tube at a time and monitored the neg bias go down to -11 voltys at pin 5 with the 2 bad tubes. Then I tried theother tubes and as I installed one at a time I monitored the neg bais and had -50v steady with 4 tubes running and no glowing plates.
It might of even been one of the other bias circuitry which I replaced a second time..anyway it seems rock solid for now..we will run it and see what happens.
Thanks for all your help on this one!!!

[Ronsonic]

Here's the somewhat twisted test for these. Turn on power, wait 20 seconds and then hit the standby - do the tubes run away.

When I've run into problems with these, the amp would respond normally if allowed to warm up for a minute before taking off s'by OR if the standby was up when hitting power. It was when the tubes hadn't settled in before hitting standby that the problem emerged.

Yeah, that was grim.

[Bruce / Mission Amps]

It appears it was a pair of new tubes drawing too much current, loading down the neg bias.
I installed 1 tube at a time and monitored the neg bias go down to -11 voltys at pin 5 with the 2 bad tubes. Then I tried theother tubes and as I installed one at a time I monitored the neg bais and had -50v steady with 4 tubes running and no glowing plates.
It might of even been one of the other bias circuitry which I replaced a second time..anyway it seems rock solid for now..we will run it and see what happens.
Thanks for all your help on this one!!!

Many times the most obvious answer is the one we refuse to accept for some reason.

[danno9999]

Thanks for everyones help!! The amp in now running and no red tubes!!
I did check the bias and was around 42-43ma per tube.
The last tech I guess changed the 75k resistor in the bias to a pair of resisitors around 60k. I tried the original value per the schematic and dropped the bias just below 20ma..too cold.
So I went for a 68k and now they around 30ma -32ma. Does anyone think that might be too cold and remove some of the sound these amps produced?
I think I feel better at 32ma . These newer russian tubes just aren't like the old mullards and rca's.
Thanks....

[g1]

Thanks for everyones help!! The amp in now running and no red tubes!!
I did check the bias and was around 42-43ma per tube.
The last tech I guess changed the 75k resistor in the bias to a pair of resisitors around 60k. I tried the original value per the schematic and dropped the bias just below 20ma..too cold.
So I went for a 68k and now they around 30ma -32ma. Does anyone think that might be too cold and remove some of the sound these amps produced?
I think I feel better at 32ma . These newer russian tubes just aren't like the old mullards and rca's.
Thanks....

The stock values for bias were quite cold. This is why some of these amps around today still have the original tubes in them. Also helps them withstand such high plate voltages.
If you want the "sound these amps produced" then you want the factory bias setting.
If you "feel better at 32ma" than 42ma, then you should feel even better at 20.
Not saying I'm a big fan of cold bias but you have to choose whether you want tone or tube life.
Anyway, sounds like you have found a happy medium. Why not put in a trim pot for adjustable bias rather than playing with resistance values?