1967 AB165 Bassman Head

[JSB474]

Hello,
I have a 67 AB 165 Fender Bassman head that is blowing tubes. I recently replaced all the electrolitic capacitors, and biased the tubes. Any suggestions for testing this amp? This is my first amp project and I am wondering why the amp is blowing power tubes. The first tube that blew was the second power tube. I rebiased the amp as cold as possible and the problem seemed to go away, but then just last week the first power tube blew up on me. So both power tube sockets have blown...

Any help is greatly appreciated...
Jon

[dehughes]

And by "blow", he means a crazy arcing plate...worse with the first tube than the second, and both times it was different sockets. No smoking anything, voltages seem to be normal. Standard AB165 setup for bias, etc...

[Structo]

How are the screen voltages?

[JSB474]

This is my first amp project so you will have to excuse my limited knowledge. by screen voltage do you mean the voltage at pin 3 of the power tube socket?

[Structo]

No pin 3 is the plates or output to the output transformer.

The screen grid is pin 4.

You should have less voltage on the screen than the plate or it can stress the tube.

I think the voltage there may be pretty close to the plate since it uses a choke before the screens.

How was the recap job?
Has all the trouble started after you did that?
Are you certain you did it right?

What were the symptoms before you recapped it?

Perhaps the OT is bad. That could cause arcing in the power tubes.

So the problem is in both sockets? Right?
Not the same tube in the same socket blowing?

That is a strange one, if the voltages are within spec.
Is the B+ at the right voltage? That would be the voltage at the first filter cap or at the standby switch.

Could it be a simple coincidence that the tubes you bought were marginal or bad from the get go?

Hopefully somebody with more experience with Bassmans will chime in.

[dehughes]

Interesting...as the OT in this amp has been replaced. I noticed that once Jon took the amp out of the chassis, as the leads were clearly not stock material.

Hey Jon, if you need to borrow my multimeter and take some readings, just let me know and I'll bring it by Wednesday.

[JSB474]

No pin 3 is the plates or output to the output transformer.

The screen grid is pin 4.

You should have less voltage on the screen than the plate or it can stress the tube.

I think the voltage there may be pretty close to the plate since it uses a choke before the screens.

How was the recap job?
Has all the trouble started after you did that?
Are you certain you did it right?

What were the symptoms before you recapped it?

Perhaps the OT is bad. That could cause arcing in the power tubes.

So the problem is in both sockets? Right?
Not the same tube in the same socket blowing?

That is a strange one, if the voltages are within spec.
Is the B+ at the right voltage? That would be the voltage at the first filter cap or at the standby switch.

Could it be a simple coincidence that the tubes you bought were marginal or bad from the get go?

Hopefully somebody with more experience with Bassmans will chime in.

Yeah I actually bought a new set of matched power tubes and different tubes in different sockets arched and blew. As far as the amp goes I acquired it as a fixer upper, so I don't know if the problem was there before I got it. I immediatley did a recap job on it as the electrolitics were original to the amp... I will measure both voltages wednesday (screen/plate) and get back to you. (dehughes has been kind enough to help me with this project) I discovered my multimeter isn't working properly so I will see if he can help me take some measurements wednesday when I see him next. I am not sure if the OT is original or a replacement, it does have plastic covered wires which makes me suspect it isn't original (should be cloth covered?). Anyway thanks for you input...

jon

[Firestorm]

In 1965, you should be seeing plastic-insulated wires on the transformers. Cloth-covered transfomer leads (a really bad idea) was a '50s thing. See if you can find any numbers stamped on the transformer housings and post them so we'll know what we're dealing with. Did you notice if the tubes were red-plating before they blew?

[John_P_WI]

I think I would be looking at the screen resistors.... Are they there? Correct values?? It is hard to argue against 1K 5w resistors here.

John

[JSB474]

I think I would be looking at the screen resistors.... Are they there? Correct values?? It is hard to argue against 1K 5w resistors here.

John

Are you talking about the resistors mounted on the tube sockets (pin 4?) If so I replaced those with new metal film ones. I think with just the same values that are on the schematic. I will double check the values of those also...

thanks Jon

[John_P_WI]

Yes that is correct. I was referring to those mounted to pin 4.

Are there any grid stops on pin 5?

How about the the grid leaks to ground (bias ref)?

Is the bias set up as a "balance" pot between tubes or "variable resistor"?

The reason I ask is it seems the OP tubes are going unstable, whether from losing ground ref, or the bias is intermittant etc. I would focus on pins 4 (previous post) , pin 5 and the bias / grid leak circuit (r's, diodes, caps, pot etc). You could always try subbing the 220k grid leaks for a lower value to try to gain some stability....

How about the sockets, do the tube pins fit tightly?

Just a few more things to think about.

John

[M Fowler]

I would go back and look at the filter cap replacement job to make sure that is proper as well as the incoming AC section for shorting.

Mark

[Firestorm]

As long as we're checking everything -- you did replace the bias cap, right? (Hopefully one with a voltage rating higher than 50 volts, which was marginal when the wall voltage was 110). And, if so, you did remember to connect it positive side to ground, right? Just a guess, but it sounds like it coulb be an intermittant bias fault. If all else is good, you might want to shoot some pot cleaner into the bias pot; these get full of gunk just like everything else and sometimes don't make good contact.

[ckpop]

1. Check your screen grid resistor and make sure you have the correct voltages on both sides and the are in good shape. Alot of times when a tube go's out the resistor may crack or go bad and everthing looks OK

2. Check tube sockets for any arcing that may be burned in to socket.

3. Anything to do with the bias, bias pot, grid stoppers, ect.

4. Bad ground connection or solder connection on your output jack. Just double check eveything then do it again.

This is where I would start then filter caps and transformer. In my experience with OT transformers they either go out or the arcing happens when the amp is turned up and you dig in. Sometimes after the amp heats up this is when the problem occurs also.

This is a tough one because unless you have a way to test certain component voltages don't always tell you what you need to fix the problem.

Good luck

[Jimmyt]

Hey Jon,

Did you ever figure out what the problem was? I have an ab165 doing the same thing. I almost wonder if it's the same amp.

James