1967 Fender Bassman Head AB165 early Break-up. Help.

[cluster]

Hello All, I have a 67 Bassman head that I converted to blackface specs. That is,

1) I rewired the PI to blackface specs (followed the AA864 schematic),
2) I converted the bias to an adjustable bias,
3) Replaced the electrolytic cap in the power section with F+T,
4) Replaced the 100k bias feed resistors at the output tubes with 220k,
5) Swapped out the tone caps in the Bass channel with .02uf orange drops, also swapped the 100k for a 56k slope resistor and took out the deep switch cap.
6) got rid of the 220k suppressor caps coming of the plates of the output tubes.

After all these mods I find that the amp has an early break up which I would like to fix. Now the amp did come with cathode bais in the output 150 ohm bypassed with a 5uf cap. I left that in since I've read that a hybrid bias could be a cool thing but now I'm wondering if that is the cause of the early breakup. The weird thing I noticed when I was checking voltages is that the cathode pin on output tube one was 3.5volts and on the output tube two it was 38.7volts. I would think that they should be the same. That got me thinking that maybe that asymmetrical voltage between cathodes on tube 1 and 2 could be the cause of the early breakup. Both plates had the same voltage of 449DCV. All other voltages seemed correct. -43volts at the grids of the output tubes. Can anyone help me with what might be causing the early breakup? I'm thinking of taking out the cathode resistor and bypass cap on the output but I thought I should ask here first to see if there is something I missed. Also, is there any need for the 470k resistor the joins to the 220k mixing resistors just before the PI. It's not there in the AA864 circuit. Thanks in advance.

joseph

[g1]

Yes you need to get both cathode voltages the same.
Now you removed the schematic with the mods, but it had the cathode bypass cap connected each end to a cathode. Did you do it like that, or maybe one end is grounded? Or touching ground or chassis?

And have you tried swapping the power tubes to see if the fault follows the tube, or stays with the socket?

[Justin Thomas]

Define early breakup. I've got one stock (minus the feedback on 6L6 plates) and it starts crunching at 4. If you left that 3rd shared "gain" stage from the AB165 as is with it's feedback loop, well, it's kind of a "compressor." Crunch city. None of these Bassman heads are known for sparkly clean headroom... not saying there's nothing wrong with your amp, but that particular stage is peculiar to the AB165 & later Bassmans; don't expect much sparkly clean! A Twin it ain't, and even they crunch on 10.

Justin

[cluster]

Yes you need to get both cathode voltages the same.
Now you removed the schematic with the mods, but it had the cathode bypass cap connected each end to a cathode. Did you do it like that, or maybe one end is grounded? Or touching ground or chassis?

And have you tried swapping the power tubes to see if the fault follows the tube, or stays with the socket?

Thanks for the response. I'll try that when I get back to the studio. The reason I removed the schematic was that I didn't follow every detail and didn't want any confusion.

Here is the schematic again: http://hasserl.com/Amp_Mod_Docs/Fend..._mod_schem.pdf

The details I didn't follow on this schematic include:

1) no cathode follower on bass channel or extra gain stage, and
2) I didn't follow his PI layout. I used the stock aa864 PI instead.

However the cathode bias scheme is the same as mine. I thought I would leave it in and hear how it sounds. I'm wondering if there might be a bad connection at on of the tube shocket pins? But your right, cathode voltages should be the same on each tube. I'll check the connections to those sockets when I get back to the studio to day and report back.

[cluster]

Define early breakup. I've got one stock (minus the feedback on 6L6 plates) and it starts crunching at 4. If you left that 3rd shared "gain" stage from the AB165 as is with it's feedback loop, well, it's kind of a "compressor." Crunch city. None of these Bassman heads are known for sparkly clean headroom... not saying there's nothing wrong with your amp, but that particular stage is peculiar to the AB165 & later Bassmans; don't expect much sparkly clean! A Twin it ain't, and even they crunch on 10.

Justin

Thanks for the reply. I'm using a standard Tele. The amp is loud enough for a Bassman and it crunches like a full on Marshall at full volume but my issue is that even at 1/3 volume I still need to pick very lightly NOT to get any dirt on the notes being played and to me that sort of defeats the purpose of picking dynamics. Usually, with the amp volume on full I can turn my guitar volume down to get a nice loud(ish) clean tone. To my ear it sounds like too much asymmetrical clipping giving rise to the early breakup. I'll have to poke around those output tube sockets to see if I can get the cathode voltage the same and go from there.

[pdf64]

...The weird thing I noticed when I was checking voltages is that the cathode pin on output tube one was 3.5volts and on the output tube two it was 38.7volts..

I suspect that something is causing one of the 6L6 not to conduct properly; either a cathode resistor, a 6L6, a tube screen grid resistor, etc is bad.
Most likely the cathode resistor with 38.7V across it.

[cluster]

Ok. I think I figured it out. I yanked out the resistor/bypass cap on the output tubes that went to ground and the amp perked up immediately. Took a close look at the cap and noticed a bulge. It was hard to see in the amp but obvious once I removed it. here is a picture of it. i guess that's why I got the amp so cheap. Someone didn't like the tone and that was probably why. Go figure.



Next question for those who are familiar with modding bassmans, what does the 470k (just before the PI) and after the 2, 220k mixing resistors do? AA864 doesn't have it but the later ones did.

[Leo_Gnardo]

I yanked out the resistor/bypass cap on the output tubes that went to ground and the amp perked up immediately. Took a close look at the cap and noticed a bulge. It was hard to see in the amp but obvious once I removed it. here is a picture of it. i guess that's why I got the amp so cheap. Someone didn't like the tone and that was probably why. Go figure.

Never was much a fan of Fender's combination bias. What I do is sub 1 ohm current sensor resistors in the cathode to ground, or for cheapskates jump cathodes straight to ground, and adjust grid bias voltage accordingly as in other Fenders.

Those ol' white Mallorys, lookin' for trouble whether in old Fenders or anywhere else. Long time ago Crown service bulletin advised replacing them in their flagship SS amps of the 70's. Some Mallorys good: M150 film. Some bad: white plastic electrolytics.

[Justin Thomas]

In the AB165 it creates a negative feedback circuit, just for that stage. If you check Machine Gun Amps' site, he says it kind of ends up working like a compressor of sorts, and helps with some tasty distortion. It wasn't Fender.s intent, but... notice when they got into the Bassman 100 and more "serious" bass amps, they omitted that 3rd stage entirely. It's only the 50W heads that kept it, but that particular feature is one I really like. Can't comment on the AA165.

Justin

[Alex R]

For posterity, notwithstanding the bypass cap problem, it might be worth adding that the main effect of 'blackfacing' is to produce earlier breakup. Not being a smartass as your amp really did have an issue, just pointing it out.

[Justin Thomas]

Good point about black face early distortion, although my '75 Bassman 50 (basically same circuit, but w. different values & a MV) also started crunching about 4 on the volume knob... I would venture that the Bassman AB165 may have been the first "silverface" Fender... even with a black front panel. I keep toying with turning my Bassman 100 into an AB165 preamp.

Re: combination bias, I only ever saw it in an early 69 Dual Showman Reverb. I loved the sound of that amp, and it was dead stock but for one OD cap and a three-prong. Really warm, punchy, but enough bite to cut through. I always get any SF I meet running in top stock form before any mods, and I usually end up liking them as is.

Justin

[cluster]

yeah, like I said i got this amp cheap for the purpose of learning how to blackface fenders. i'll probably replace all those Mallorys cathode caps now. Any comments on the 220k vs 100k bias feed resistors to the output tubes? For or against. I know 220k is blackface spec but some prefer the 100k. thoughts?

[pdf64]

...Any comments on the 220k vs 100k bias feed resistors to the output tubes?..

I can't see any benefit in 220k rather than 100k, whereas there are several drawbacks, not least that the limiting max value for fixed bias is 100k.

[g1]

I'm not sure if you removed just the bypass cap, or the cathode resistors also?
If you did remove the resistors, I'd suggest adding the 1ohm resistors like Leo mentioned so you can check the idle current of the power tubes.
It seems odd that the cap would have only affected one tube, as it was connected between the 2 cathodes.
It's possible there is still some other problem like pdf64 mentioned in post #6.
Checking the idle current of the power tubes would confirm whether the problem is solved or not.

[cluster]

I'm not sure if you removed just the bypass cap, or the cathode resistors also?
If you did remove the resistors, I'd suggest adding the 1ohm resistors like Leo mentioned so you can check the idle current of the power tubes.
It seems odd that the cap would have only affected one tube, as it was connected between the 2 cathodes.
It's possible there is still some other problem like pdf64 mentioned in post #6.
Checking the idle current of the power tubes would confirm whether the problem is solved or not.

Yes I removed resistors and cap and replaced it with a ground wire. I have a weber bias-rite and I was able to confirm the idle current to be about 30mA per tube. That's the highest I was able to get it to with the bias pot and 27K bias range resistor (on the pot to ground). Might have to switch that resistor to put me in range. I might also switch back to 100k bias feed resistors and get rid of the 470k after the two 220k mixing resistors. I'll also add the 1ohm resistors on the cathode for future bias measurement-adjustment. I did check all the connections before removing the res/cap and found nothing wrong. The cap was soldered from cathode to cathode, so maybe an open connection in the bad cap could have effected one side and not the other. The only real way to check would be to wire up a fresh cap and then recheck the voltages.