Repairing Blackface Bassman AB165

[MakerDP]

The bass player in my band pulled out this 1967 Blackface Bassman head tonight that belonged to her father. (She also showed me a nice 1963 Fender Jaguar that was his but that's another story.)

Anyways, she tried to turn it on and one of the power tubes was arcing blue like crazy and that "smell" came out strong. Here is what I found when I tore it open tonight...

That fried resistor is the 470R/1W between pins 4 and 6 of one of the 6L6's.

What would cause the resistor to do that? A bad 6L6? Bad filter caps? Nothing else in the amp looks bad. Even all the electrolytic caps look good... no signs of leaking at all. I'm thinking all the caps should be replaced anyways. I can post pics of the rest of the guts if you think you need to see them but all the wiring looks pretty good at first inspection.

[MakerDP]

Also, what is the "collectability" of this amp? If I put metal oxide resistors to replace that one and replaced the electrolytics with "modern" caps would I damage the appeal of this amp? I don't think she has a lot of money to put into repairs for this.

[AZJimC]

The bass player in my band pulled out this 1967 Blackface Bassman
That fried resistor is the 470R/1W between pins 4 and 6 of one of the 6L6's.

What would cause the resistor to do that?

The screen resistors are somewhat a fuse also. If the PT screen shorts to, usually the filament, or anything, they will burn before the transformer.

Pull the tubes, check all voltages, and replace screen resistors, and bias a fresh set of tubes. That should fix the issue you are seeing.

As far as original value, you're on your own. restore as best you can, but stay as original as you can, A working amp is worth much more than a non-working amp, whether it is the holy grail of collectors or not. You can always replace the CC resistors etc. after you get a solid amp.

[MakerDP]

Thanks. I'll just go ahead and get some carbon comps since I don't have a 470R on hand anyways.

I'm not sure this is the "holy grail" of amplifiers. From what I've been reading, this is one of the first ones that CBS got their hands on and made some changes that were not very popular. It looks like one in good condition goes from $700 to $850. Not chump-change, but not like that Jaguar she has which is worth somewhere in the neighborhood of $2000 to $3000! She does have the ginormous 2x cabinet for it though, so that helps the value out.

She actually wants to start using it for our shows, but I don't think it has enough power to keep up with the rest of the band.

[7string]

I think you should put in the 5 watt wirewounds. Doug sell them here for 75 cents.


It is not highly collectible and they are usually on Craigslist for 500-1000 US. Rough estimate. I've bought a broken one for 50 bucks a few years ago.


She can slave it into a SS power amp by using a dummy load with a line out for high volume use.

[eleventeen]

"I think you should put in the 5 watt wirewounds."


Isn't there a partial school of thought that we don't want "invincible" resistors in this position because if a 6L6 output tube shorts out internally, it can take out the OT if this resistor does not blow?


Kind of the same line of thinking that (and I am convinced on this one) that the "synthetic center tap" on the heaters of a tube amp (comprised of the 2 x 100 ohm R's to ground) is superior to the actual center tap coming from the PT.

[HotBluePlates]

I think you should put in the 5 watt wirewounds.

I agree with others on the fuse value of the screen resistor.

This amp doesn't use EL34's, and shouldn't need a resistor over the 1w rating of the original screen resistor.

Also, what is the "collectability" of this amp? If I put metal oxide resistors to replace that one and replaced the electrolytics with "modern" caps would I damage the appeal of this amp?

No. Most AB165 Bassman amps have been converted over to AA864 specs, and they're not hard to find. It's better to have solid, functioning caps and screen resistors than try to exactly match the original parts.

10-15 years ago, you could buy that amp in excellent to near-mint condition for $425 or less all day long. They were among the cheapest blackface amps, along with the blackface Champ and VibroChamp. Their value has gone up because all blackface amps are more scarce than they used to be, but the Bassman is on the low end of the price scale because nearly everyone knows you likely need to alter the circuit to get the best sound for guitar (though stock really isn't as bad as some writers will lead you to believe).

[MakerDP]

Thanks for that info HBP.

I'm going to do a 1W CC or MF resistor, change the tubes and see what happens from there. Obviously that resistor did it's job last night so I'm not going to change the rating on that.

After a little reading last night I came to the conclusion that this was kind of the "red-headed stepchild" in the Bassman line and not very collectible.

Actually, I'll pass all that info on to her before I do anything at all with it. She may not want to go through the expense unless she just wants to restore it for the sake of her father's memory, which would be cool too. I'd do all the labor for free but the parts can add up to $200 or more with the tubes and replacing all the caps.

[HotBluePlates]

I'd yank the output tubes, replace the resistor and power on. Most likely, everything will hold up fine and you can decide on whether to replace output tubes without testing, or attempting to reinstall the tubes. You won't really know what caused the screen resistor to burn until you try powering up without the output tubes and seeing if the new resistor burns. As a result, unless there's a hard short inside the tube and you can measure it with an ohmmeter or tube tester out of the amp, you really don't know if the tube caused the resistor to burn.

Unless the amp hums or the filter caps have visible signs of failure, they might not need replacing. Obviously if it was your personal amp you'd replace them as preventive maintenance, but that adds to cost.

If it were my amp, I'd look at this as a $1 fix (for a new resistor, depending on your source and whether shipping is involved). Maybe an additional $30-50 for a pair of 6L6's, if replacing the old tubes burns out the new resistor. Everything else very likely falls in the "nice to have" category. Obviously, this doesn't account for you charging for your time.

[7string]

I didn't mean to spread mis-information. Just trying to be helpful. I thought that this was common practice. After some searching I found this thread that supports your opinion with math. Though I trust HBP's word on it. 
http://el34world.com/Forum/index.php?topic=11891.0

 

[MakerDP]

I didn't mean to spread mis-information. Just trying to be helpful. I thought that this was common practice. After some searching I found this thread that supports your opinion with math. Though I trust HBP's word on it. 
http://el34world.com/Forum/index.php?topic=11891.0

And I appreciate your efforts! 

[MakerDP]

I'd yank the output tubes, replace the resistor and power on. Most likely, everything will hold up fine and you can decide on whether to replace output tubes without testing, or attempting to reinstall the tubes. You won't really know what caused the screen resistor to burn until you try powering up without the output tubes and seeing if the new resistor burns. As a result, unless there's a hard short inside the tube and you can measure it with an ohmmeter or tube tester out of the amp, you really don't know if the tube caused the resistor to burn.

Unless the amp hums or the filter caps have visible signs of failure, they might not need replacing. Obviously if it was your personal amp you'd replace them as preventive maintenance, but that adds to cost.

If it were my amp, I'd look at this as a $1 fix (for a new resistor, depending on your source and whether shipping is involved). Maybe an additional $30-50 for a pair of 6L6's, if replacing the old tubes burns out the new resistor. Everything else very likely falls in the "nice to have" category. Obviously, this doesn't account for you charging for your time.

OK so to test the tube with my DMM (I don't have a tester handy and I don't know anybody with one) would I test for a short between the pins that resistor bridges? (pins 4 and 6)

I consider anything that teaches me something new and useful as "free labor" as I have a lot of spare time since I cannot work full-time anymore. I'd actually like to find someone who can teach me how to be an amp tech. There are not many in my area at all.

[eleventeen]

"OK so to test the tube with my DMM (I don't have a tester handy and I don't know anybody with one) would I test for a short between the pins that resistor bridges? (pins 4 and 6)"


No. Pin 6 has zero significance to the tube. It is a dead pin and in most cases, that pin will be physically missing from the tube base. Thus, pin 6 >>on the tube socket<< goes nowhere and can be used as a tie point. That's what's going on. So on pin 6 we make the junction between the incoming screen supply wire and the 470 ohm screen resistor. It is the FAR (or "cold) side of that resistor which connects to the tube pin 4 = the screen grid aka grid #2.
 
So you would check for a short between pin 4 (G2: Go look at "6L6 tube data" on google) and *anything* else. It's not absolutely clear that such a short would show up with the tube cold but it's certainty free to try.

[MakerDP]

Cool thanks.

[MakerDP]

I went ahead and ordered some 470/1W C.C. resistors, a new three-wire power cord and a new matched-set of power tubes. I replaced the blown resistor and rewired the power input to eliminate the "death cap" and bring it up to modern safety standards.

The amp works now. It doesn't sound horrible with my guitar but I don't think I'd rush out and duplicate the circuit for my own use or anything. I have NO IDEA how to bias it with that funky balance pot.

Now that it works, I also discovered it has two microphonic preamp tubes. Too bad, one of them is an old Telefunken 12AT7. The other three are old RCA 7025's. It also has some very scratchy pots which probably should just be replaced.

This thing has seen A LOT of smokey clubs me thinks. It smells pretty foul (not bad electrical smell) and has a white powdery substance all over it. She told me that her mother and father used to play in a band together and they used to support some pretty big name country stars in the late 60's-early 70's that would travel around without a band, using local talent to support them. I can't remember the names now, but I did recognize a lot of them. This amp has a really cool story behind it.

She also has an old Twin Reverb in her closet that she wants me to check out next... can't wait to get my hands on that one!