Resistor types for 60’s fender.

[BrownIsound]

I have a drip edge late 1967/early 68 AA165 that is all original other than the electrolytic caps and added 3 prong power cord. Many of the resistors have drifted up out of spec. What is the current consensus these days on the best type of resistor to replace the carbon comps? Dependent on the specific part of the circuit?

[Helmholtz]

There's no need to replace a resistor that hasn't drifted by more than 20%.

[Colossal]

I have a drip edge late 1967/early 68 AA165 that is all original other than the electrolytic caps and added 3 prong power cord. Many of the resistors have drifted up out of spec. What is the current consensus these days on the best type of resistor to replace the carbon comps? Dependent on the specific part of the circuit?

Does the amp sound bad? Overly hissy or crispy frying bacon sound, indicating failing resistors? I would hesitate to modify an original amp any more than necessary, if you want to keep its value. However, if it's a mutt and you are compelled to mod it and/or it's not particularly good sounding, then have at it. I would say the choice of the resistor would be somewhat dependent on the specific part of the circuit.

[sluckey]

There's no need to replace a resistor that hasn't drifted by more than 20%.

Really? Fender used 10% resistors if not specified. And there are four 5% resistors specified in the power amp. I would replace any that are out of spec and I'd use same type resistors.

[BrownIsound]

The “5%” tolerance PI plate resistors have drifted up ~20%, the grid load resistors >10%. Some other miscellaneous resistors have drifted beyond 20%, including the PI resistor on the cathode (which I know will affect the bias of the PI).

The performance of this Pro…not noisy, but I would describe the tone and feel as “honky”. Not sure how much is due to the circuit, and how much is due to the stock Oxfords.

I would like to keep it as original as possible, being a genuine “blackface” drip edge, which are rarer than BF AA165 Pros. But I also want it to sound as best as it can as I want to play it, not sit and stare at the amp guts out of the cabinet.

I did the lazy vacation day task of measuring all the resistors in this amp and my early 68 drip edge AB763 Super Reverb. The SR has had several resistors changed before I got it, but all are in spec, and that amp sounds glorious. Speakers are apples and oranges between the amps, but I can’t help thinking it is also the resistor drift in the Pro.

Any good source of A-B CC resistors?

[Stevem]

I can’t see resistor drift making this amp sound honky especially since the tone circuit in these BF amps have a mid cut built in basically.
I would try the amp thru different speakers.

[BrownIsound]

I can’t see resistor drift making this amp sound honky especially since the tone circuit in these BF amps have a mid cut built in basically.
I would try the amp thru different speakers.

Yes that will be the easiest test over changing anything in the amp. Will report back my findings.

[BrownIsound]

It’s the speakers, for the most part. Plugged it into my Twins speakers, and was similar to my twins tone with the mid turned up a bit, though less bottom end. Still something a little off in the feel, like too much feedback or something. I didn’t isolate the circuit there to measure the FB resistor. Stays really clean as you turn it up, then hits a distortion wall. Doesn’t seem to have as pleasant a transition to distortion like my other fenders.

[pdf64]

I have a drip edge late 1967/early 68 AA165 ...

Are you sure?
It seems weird that an amp would be built to a design that had been superseded 2 years previously.
What leads you to think it isn't AB165?
https://el34world.com/charts/Schematics ... _schem.pdf

A concern is that the AA165 power amp is configured as a non inverting feedback amplifier, whereas the AB165 is configured as inverting.
So if things get mixed up, the feedback may be positive rather than negative.

[BrownIsound]

From what I can tell, the Pro Reverb was only the AA165, a different amp than an AA or AB165 bassman. Pretty much the same as a AB763 ____reverb amps.

https://schematicheaven.net/fenderamps/ ... _schem.pdf

[Smitty]

If you have an AB165 try lifting the two 220K feedback resistors from the plates of the output tubes to the plates of the phase inverter. Like you, I've always thought that was too much negative feedback. Alternatively you can increase the value...a lot.

https://schematicheaven.net/fenderamps/ ... _schem.pdf

[sluckey]

If you have an AB165 try lifting the two 220K feedback resistors from the plates of the output tubes to the plates of the phase inverter. Like you, I've always thought that was too much negative feedback. Alternatively you can increase the value...a lot.

https://schematicheaven.net/fenderamps/ ... _schem.pdf

He doesn't have an AB165 Bassman. He has an AA165 Pro Reverb.

[BrownIsound]

He doesn't have an AB165 Bassman. He has an AA165 Pro Reverb.
[/quote]

Yes! Since the Pro reverb was new in 1965, this is the reason for the AA165 schematic for it, though it is identical (?) to the AB763 amp schematics, I believe.

I’m going to recap the PS, and check the one old electrolytic cap and replace if necessary (cathode on the trem bulb stage), so I’ll check the 2 resistors in the feedback circuit then, and replace those if they are indeed off. One can imagine if the 100 ohm PI tail-ground drifted way high, and the 820 had not drifted up significantly, that could account for the higher feedback symptom.

[BrownIsound]

The other experiment I want to try is plugging the amp into my QR speakers. In theory they are the same oxfords, other than the Swiss cheese basket and 16ohm impedance, but the QR sounds great with them.

[B Ingram]

... Many of the resistors have drifted up out of spec. ...

There's no need to replace a resistor that hasn't drifted by more than 20%.

Before you replace parts, I would focus on the sound. That seems to be the point of your post below:

The “5%” tolerance PI plate resistors have drifted up ~20%, the grid load resistors >10%. Some other miscellaneous resistors have drifted beyond 20%, including the PI resistor on the cathode (which I know will affect the bias of the PI).

The performance of this Pro…not noisy, but I would describe the tone and feel as “honky”. Not sure how much is due to the circuit, and how much is due to the stock Oxfords.

Interesting. I normally work pretty hard to get more mids from a blackface Fender amp. Below is a 1966 Pro Reverb I used to own, Bass on about 1, Treble on 4-5, Reverb around 3, with a Tele using both pickups. Speakers are Oxford 12L6s, but they were replacements from maybe the early 70s.
https://soundcloud.com/hotblueplates/ne ... -in-town-1


My Opinion is that "tighter tolerance" won't yield "Tone" the way some folks expect. In fact you could have a good sounding amp with -50%/+100% on resistor values.

Tight tolerance in the phase inverter was probably mostly about balance of the output signals (which won't be exact even using resistors exactly-on Fender's part-values). And perfect balance of the inverter & power tubes just gives you the loudest, cleanest signal possible. Most folks these days seem to wish their tube amp would distort more (and at lesser volume), and buy all kinds of attenuators, re-ampers, load boxes, reactive loads & use IRs to pursue this goal.

It’s the speakers, for the most part. Plugged it into my Twins speakers, and was similar to my twins tone with the mid turned up a bit, though less bottom end.

Smart thinking here! Speakers are supremely important, because they change the sound much more than most circuit-tweaks you'd make.

If you're still sure "honky mids" are the problem, then look to the 6.8kΩ resistor at the Bass pot. A smaller value scoops the mids more (than the stock -25 decibels), while a larger value scoops mids less.

Still something a little off in the feel, like too much feedback or something. I didn’t isolate the circuit there to measure the FB resistor. Stays really clean as you turn it up, then hits a distortion wall. Doesn’t seem to have as pleasant a transition to distortion like my other fenders.

Could be feedback then (though typically all blackface/silverface Fender amps have pretty mild feedback). From the speaker, a wire runs back to an 820Ω resistor, which connects to a 100Ω resistor, which then connects to ground.

It's not stock, but halving that 100Ω to 47Ω will tell you if the feedback is too much for your taste. Disconnecting the loop might be misleading, if you like other Fender amps (that still have their loop intact).

Meanwhile, negative feedback around the power section always makes the transition between Clean & Dirty less-gradual. The amp stays clean until you overwhelm the feedback, then the amp distorts abruptly. But you may only notice this when the feedback-level is very high, or when directly comparing to a no-negative-feedback amp.