AB165 Fender Bassman 50: multiple issues

[Chip tele]

My 1968-ish AB165 has the mod for adjustable fixed bias. It was working okay about 6 months ago when I last played it. When I plugged it in last week, the volume was normal but with some static on certain notes. I took all the tubes out and tested them on my Precision 912. One triode of the 12AT7 phase inverter tested bad, so I replaced it with one that tested good.. When I turned it on next, the static was gone. However, I lost that loud Fender clean so typical of guitar through these amps. In its place was a weak, unpleasantly distorted sound at less than half the normal volume. Plate voltage on pin 3 of both output tubes (Shuguang 6L6GC) is in spec at 450v. Bias voltage at pin 5 of both output tubes is also in spec at -45 to -50v. Pin 1 of the 12AT7 phase inverter (plate triode 2) is in spec at 210v. However, pin 6 of the 12AT7 (plate triode 1) is way high at 405v. Even with the bias and plate voltages in spec as above, the idle current at the plates on both output tubes is only 14-16 milliamps.

p.s. Pin 7 on the phase inverter has some heavy corrosion. The rest of the connections look okay.

Do ya'll have any suggestions as to what could be causing the high voltage on the first triode of the phase inverter and/or the low plate idle current? Is that errant voltage on pin 6 of the phase inverter related to the low plate current on the output tubes? I tried a brand new set of 6L6GC and got the same result.

Thanks in advance for your suggestions!

[Justin Thomas]

As others said, one side of your phase inverter isn't conducting; that explains the low volume & distortion.

I've had this happen in several of my builds and a couple times when replacing a socket. It's possible you have a second dud tube; if you've got noticeably dirty/corroded socket connections, that's a good place to start. Hard to tell from your pic but it looks like the solder connection between your cathodes might be cracked or broken? In which case one of your cathodes is ungrounded & won't conduct. I've never been a fan of the "bend the tabs over & solder" technique; I always use a short wire. You could have a break in a wire associated with that tube section, or the cathode resistor/cap may have lost its ground elsewhere. Unfortunately it could be a lot of things, but they're all pretty simple to check. I bet a thorough, methodical inspection will find your fault.

Jusrin

[Helmholtz]

https://el34world.com/charts/Schemat...b165_schem.pdf

[Chuck H]

Regarding the PI voltages and low, distorted output I think Justin has it. Check for continuity between the cathode pins on the PI socket and clean/retension all contacts (you can find instruction for this on this forum by searching). Otherwise a possible second dud tube.

Plate voltage on pin 3 of both output tubes (Shuguang 6L6GC) is in spec at 450v. Bias voltage at pin 5 of both output tubes is also in spec at -45 to -50v... Even with the bias and plate voltages in spec as above, the idle current at the plates on both output tubes is only 14-16 milliamps.

What method are you using to measure plate current?

Is that errant voltage on pin 6 of the phase inverter related to the low plate current on the output tubes?

Almost certainly no.

[Chip tele]

Thanks for your replies, guys! I'll keep testing continuity around pin 6 on the phase inverter, where the voltage is high. And also clean and tension those pins. That replacement 12AT7 tested good, but I have others I can try there. To measure plate voltage, I have a cheap Harbor Freight multimeter with an alligator clip soldered to the ground probe. I clip the ground to the amp chassis and carefully (with one hand behind my back) touch the other probe to pin 3 on the output tubes. For plate current, I have a bias meter probe that I connect to my multimeter and measure the plate current for each output tube individually. I looked at the schematic and amp for 4-5 hours yesterday and wanted some feedback on where to go next. Thanks for your input!

[Chip tele]

I used that bias probe method to set the bias after I initially installed the pot for adjustable fixed bias 15 or so years ago. The backstory is that I found that amp for $75 in a pawn shop around 2008. I haven't seen anything like that in a pawn shop for a long time! It was red plating the output tubes, and the bias adjust fixed the problem. I used it for a while and then got hooked on those inexpensive EL84 amps that Crate made back then. Fast forward to now, I decided it's too nice an amp to sit mostly unused--then this issue developed.

[Chuck H]

I (and Justin) think the problem with your PI isn't on pin 6. It's more likely on (or between) pins 3 and 8. Those would be the cathode pins and they should be connected together and then to the board circuits. Test for continuity between those pins and test for continuity from those pins to where the lead connects to the board.

Oxidation on unsoldered contacts isn't uncommon when amps sit for a time. You should clean the socket where the pins insert with a deoxit type product. You would probably do well to just clean all the sockets and jacks and their switches too.

Since your bias is cold there's no danger to the tubes so I'm focusing on fixing the PI problem for the moment.

[Chip tele]

Thanks Chuck and Justin! I will focus on continuity around pins 3 and 8 as you suggested. It makes sense that the issue isn't around pin 6 on the PI, as I tested continuity around it pretty thoroughly yesterday.

[g1]

As to the cold bias, you did not mention the pin 4 voltage for the power tubes.

[Chip tele]

hello Joker--I didn't think to measure pin 4--thanks for the reminder! Pin 4 on both output tubes is 445v; so a littler higher than the schematic reference of 425v. I am as puzzled about the cold bias as I am about the PI issue at this point--not that I have that much of a knowledge base to draw from!

[Chip tele]

Voila guys--there it is! It's working again now, quieter and louder than it's ever been! Chuck was right--it was a second bad 12AT7. That's what I get for buying cheap used tubes on Ebay (they're getting harder to find). Also, I had an over reliance on my tube tester results. I didn't even check this one, just installed it in place of the old one. The bias adjustment was also low--I think I moved the adjustment pot sometime over the past week when I was working on it. I adjusted it back up to 26ma plate idle current using my bias probe. Does that sound about right? It sounds good at that level!

[Chip tele]

These Shuguang 6L6GC are only 19 watts plate dissipation, not 30 watts like a real 6L6GC-. They're more equivalent to a 6L6, not a 6L6GC. So 26ma idle current is just over 60% or average bias on Rob Rob's bias calculator. I was wondering because when I tried to take them up to 40ma like a real 6L6GC they were spitting at me! Tone-lizard says Leo Fender used to bias his 6L6 amps at 50-55%, so I think I'm in the ballpark. I'm more of a traditionalist on tone anyway.

Almost forgot to mention that pin 6 on the PI went down to 280v after I put the replacement 12AT7 in. So that 12AT7 is in much better shape than the other one but still not what a NOS tube would be.

Thanks again for all your help! I'll have to check in on ya'll since you like a Fender Bassman as much as I do.

[Chuck H]

Buying used tubes on *bay is a careful dance. I've done it to get the RCA 12ax7's I adore but I always look for some indication of "working", age and a decent picture of the tube itself to see that it maybe hasn't spent time in a bad environment (both operating conditions and physical). So far it's been worth it if not always 100%

It sounds like you're on point now but I want to reitterate that cleaning any non soldered contacts in the amp would be a good idea BECAUSE there's still the possibility that the tube you had in there was ok but wasn't making good contact in the socket. The extra abrasion of installing yet another tube could have been enough to scrape the contact clean for a working connection.

Cleaning contacts, checking for drifted resistors (carbon comp type) and testing for leaky caps is all run of the mill maintenance for Fender amps of this age. If this is done with some knowledge these amps continue to function as "working amps" for musicians. If you don't want to pay a tech to do it you should absolutely add relative info and skills to your repertoire so you can enjoy this cool, old amp.

JM2C

Uncle Chuck

[Chip tele]

hey Chuck
You got a good point on that abrasion. I had to really work to get that replacement tube in there. And I fished the old one out of the trash can after you mentioned that point. I replaced those old .01uf round carbon caps that D-Labs on Youtube talks about replacing, along with a few resistors that tested out of speck. It's always been missing the faceplate ever since I got it.
Here it is in all its ragged glory! Complete with my home made cabinet from an old Marshall solid state amp that died--it's got a good 4 ohm speaker that is VERY loud with this amp.


take care,
Chip

[Chuck H]

FWIW I'd roll with it looking like that just to throw other players off If it sounds good it IS good. (<period)