Bassman 50 power tube socket problem

[bluesguitar]

I have a '74 Bassman, circuit AA371 with a bias balance pot. One tube is reading 42Ma and the other is 64Ma. Trying different 6L6GC tubes it seems to be consistent with the tube socket not the tube. Any suggestions on where to troubleshoot the socket or bias circuit?

[sluckey]

Disconnect the .047 coupling caps first. Any better?

[Stevem]

I would also check the 100k resistors that the bias voltage dumps into and off of pin 8 of each output tube I would lift the end of the .0002 caps if they are still there incase one has gone leaky.

I would not reinstall the output tubes until you confirm a equal - voltage on each tubes pin 5.

[bluesguitar]

Disconnect the .047 coupling caps first. Any better?

Don't know what you mean by disconnect? Unsolder leg? If so which leg, near the tube socket, or the bias pot?

[sluckey]

First, pull the 6L6s and set aside. Now measure the negative voltage on pin 5 of each 6L6. What have you? Looking for two numbers. If they are not about equal then...

I like to unsolder the cap leg that connects to the 6L6s. Then recheck the negative voltage on pin 5 of each tube. If about equal, then the coupling cap is leaking and needs to be replaced. If not, unsolder one leg of each of those 2000pf caps. Etc.

[bluesguitar]

First, pull the 6L6s and set aside. Now measure the negative voltage on pin 5 of each 6L6. What have you? Looking for two numbers. If they are not about equal then...

I like to unsolder the cap leg that connects to the 6L6s. Then recheck the negative voltage on pin 5 of each tube. If about equal, then the coupling cap is leaking and needs to be replaced. If not, unsolder one leg of each of those 2000pf caps. Etc.

With no tubes in I read -46.9 and -41.2 on pin 5. The questionable socket is the -41.2 reading. I don't know if that is close enough to be considered a similar voltage to each other.

[xtian]

I don't know if that is close enough to be considered a similar voltage to each other.

No, that's a malfunction.

[pompeiisneaks]

yeah that means your bias isn't right on one of them. follow the bias voltage backwards until it's the same and some component between those two points is bad. That means either the balance pot is way off to one side OR the resistors after it have gone out of tolerance the 1.5k or the 100k resistors. Thus why I indicated to work your way backwards until the bias voltages are the same. That should point to what's failed. (or just balance the bias via the bias balance pot)

~Phil

[bluesguitar]

First, pull the 6L6s and set aside. Now measure the negative voltage on pin 5 of each 6L6. What have you? Looking for two numbers. If they are not about equal then...

I like to unsolder the cap leg that connects to the 6L6s. Then recheck the negative voltage on pin 5 of each tube. If about equal, then the coupling cap is leaking and needs to be replaced. If not, unsolder one leg of each of those 2000pf caps. Etc.

Funny thing is every possible schematic of my amp, AA270, AA371, and Bassman 50 all show the 2000 pf caps coming off the grid to ground. My amp doesn't have these caps. I am nearly certain it has never been modded, and no trace of solder on the chassis where these caps would have been according to layout diagrams. The amp has played fine for years.

[Stevem]

I have seen that also on some of these amps!
I think that at some point Fender / CBS engineers realized that the oscillation issue these caps where installed to shunt out really only took place in the 4 output tubes amps that had far longer wires needed off the PI section to get to the tubes next to the power transformer, and also in the guitar amps it was needed due to more of the preamp stuff like the reverb pot and the master volume pot being way over on that side of the chassis.

The bottom line as always comes down to layout and build .

[Ten Over]

First, pull the 6L6s and set aside. Now measure the negative voltage on pin 5 of each 6L6. What have you? Looking for two numbers. If they are not about equal then...

I like to unsolder the cap leg that connects to the 6L6s. Then recheck the negative voltage on pin 5 of each tube. If about equal, then the coupling cap is leaking and needs to be replaced. If not, unsolder one leg of each of those 2000pf caps. Etc.

With no tubes in I read -46.9 and -41.2 on pin 5. The questionable socket is the -41.2 reading. I don't know if that is close enough to be considered a similar voltage to each other.

Measure pin 5 with the balance pot at one extreme and then measure it again with the balance pot at the other extreme. Now do the same thing with the other socket. These are the two numbers per socket that sluckey requested.

[sluckey]

These are the two numbers per socket that sluckey requested.

No. He had it right. I wanted the grid voltage for each tube. Just waiting to see what happens to those two numbers when he lifts the coupling caps.

[Ten Over]

No. He had it right. I wanted the grid voltage for each tube. Just waiting to see what happens to those two numbers when he lifts the coupling caps.

Oh. OK.

Wouldn't it be better to make sure that he doesn't have the bias pot cranked and really there is no problem before he starts tearing things apart?

[sluckey]

Wouldn't it be better to make sure that he doesn't have the bias pot cranked and really there is no problem before he starts tearing things apart?

That's something to check for sure. He has not mentioned messing with the balance pot. But he did say, " One tube is reading 42Ma and the other is 64Ma." I doubt the pot has that much range. I'm still betting on a leaky coupling cap.

[bluesguitar]

Thanks to all for your help. I've order some caps and will update when I change one out. All resistors are in spec.