Fender Pro Reverb (Blackface, but w/Ultralinear OT). Output Tubes Matching with adjustable bias.

[vintagekiki]

Do I modify the chassis for an externally adjustable bias (as located in traditional fender amps?, Or, install a trimmer, either under the hood with the filter caps or inside the chassis?

Externally adjustable bias as located in Fender amps SF70. SF135 in fact it is output tube matching (OTM).
Bias at traditional fender amps is fixed and it defines resistance 33k (see schematic your post # 1)
Modification Fender amp for adjustable bias (see schematic post # 4)

Trimmer for adjust bias install under the chassis, not outside the chassis, because unprofessional handling can cause the amplifier to malfunction

[SoulFetish]

Externally adjustable bias as located in Fender amps SF70. SF135 in fact it is output tube matching (OTM).
Bias at traditional fender amps is fixed and it defines resistance 33k (see schematic your post # 1)
Modification Fender amp for adjustable bias (see schematic post # 4)

Trimmer for adjust bias install under the chassis, not outside the chassis, because unprofessional handling can cause the amplifier to malfunction

yes, I know that. But i disagree with your characterization of the OTM control.
It really does not in fact “set the bias”, its function serves as a noise/hum nulling circuit.

but i installed the external bias control, with a 10 turn Pot. The shaft is fixed in its place by a locking mechanism. It’s impossible to turn without a flathead screwdriver. A Tech will know what it is and what it’s for.

[Helmholtz]

It really does not in fact “set the bias”, its function serves as a noise/hum nulling circuit.

That's exactly what he said.

[SoulFetish]

That's exactly what he said.

That’s not what he said. Re-read his post.
but, I know English is not his first language, so I may have misunderstood what he meant

[Helmholtz]

That’s not what he said. Re-read his post.
but, I know English is not his first language, so I may have misunderstood what he meant

See his post #4.

[SoulFetish]

of course, probably a simple case of miscommunication.
Just so we are all on the same page here, and can move on.
I’m fairly confident that all of us posting on this thread have a pretty fundamental understanding of standard Fender bias circuits, and how the OTM works and how it differs.

[Helmholtz]

While the OTM pot doesn't set bias, it's meant to balance bias for equal gain on both sides of the OT primary at idle.
Balanced gain results in cancellation of common mode noise.

[g1]

Looks good.
If only one minor quibble I do like how the standard Fender bias pot is adjustable from both ends (inside and outside chassis). I don't think there is such a thing for locking type though.

[vintagekiki]

That’s not what he said. Re-read his post.
but, I know English is not his first language, so I may have misunderstood what he meant

Dear Colleague
don't base loose conclusions based on a superficially read post.
Carefully (engineering) read what is written in post # 4.

https://music-electronics-forum.com/forum/amplification/guitar-amps/mods-tweaks/917141-fender-pro-reverb-blackface-but-w-ultralinear-ot-output-tubes-matching-with-adjustable-bias?p=917146#post917146

For technical correspondence it is not obligatory to know English as first language, but a certain minimum of technical (engineering) knowledge.
Non-linguistic comprehension in correspondence arises from different language syntaxs, because each individual in correspondence assumes that his language is the first.

Let’s get back to your post.
With resistor (33k) adjusts plate current (operating point) for power tubes.
With potentiometer 010427 (10k) adjusts balance (symmetry) the output tubes in push pull amplifier on minimum hum.
The Noise / hum nulling which you mention is adjust with hum balace potentiometer 028421 (100) in the tube heaters circuit.

[vintagekiki]

Looks good.
If only one minor quibble I do like how the standard Fender bias pot is adjustable from both ends (inside and outside chassis). I don't think there is such a thing for locking type though.

Good solution, set bias inside chassis, output tubes matching (OTM) set outside.
Something between was done at Fender The Twin.

https://pupcoresearch.com/wp-content/uploads/2017/04/Fender-Red-Knob-Twin-Schematic.png

[g1]

Good solution, set bias inside chassis, output tubes matching (OTM) set outside.
Something between was done at Fender The Twin.

Fender The Twin (red knob) has both pots accessible to the user from outside the chassis (rear panel).
I like SoulFetish system with OTM pot accessible to user on rear panel, but bias pot hidden away under chassis (yet still accessible to user or tech).

[SoulFetish]

Vintagekiki, my intent was not to denigrate your technical knowledge, or your English. I was simply allowing for the possibility of miscommunication, and put the burden of misunderstanding on my part. So, if It came across as rude or disrespectful, I apologize.

in fact it was your edit you posted in your schematic changes, that I ended up using. It was clearly an improvement over my initial idea.

My comments earlier were in response to your post 16 (post 16 alone) about external bias in SF70 & SF135 (And it all seemed so benign back then).

So, to all (I implore you), can you please stop reposting the the same basic explanation of this circuit, unless you have some new insight to share? If you thought you were doing it for my benefit, I’m very clear on how it works and wasn’t asking.
Its already been covered adnauseam for other readers who may have entered this thread with any question.

maybe Im being overly pissy about it, but I thought I was respectful and clear in my previous post that I wanrted to move on

[vintagekiki]

Fender The Twin (red knob) has both pots accessible to the user from outside the chassis (rear panel).

No
At Fender The Twin with pot Bias (R212 25k) adjust plate current for tubes V6, V11 to the desired value (40mVDC on TP202, 203), and with pot Balance (R209 25k) adjust plate current V5, V10 to the same value as tubes V6, V11 (0VDC on TP201, 202)
With resistors R210, 213 (100k) adjusts plate current (operating point) for power tubes.

The OTM in the SF70 / 135 has balance matching pot with center tap (50%) and basically behaves like a 2 pots connected in series with a common slider.

Balance (symmetry) of the PP output tubes is performed with balance matching pot (OTM), by increasing the bias voltage on one the PP tube, while decreasing the bias voltage on the other of the PP tube.
Output tube matching (minimum hum) is achieved when both tubes of the PP have the same plate current, independent of the bias voltage.

[vintagekiki]

https://music-electronics-forum.com/forum/amplification/guitar-amps/mods-tweaks/917141-fender-pro-reverb-blackface-but-w-ultralinear-ot-output-tubes-matching-with-adjustable-bias?p=917943#post917943

I read somewhere

The road to hell is paved with good intentions.

[Helmholtz]

Output tube matching (minimum hum) is achieved when both tubes of the PP have the same plate current, independent of the bias voltage.

Not necessarily.
Minimum hum is achieved when both tubes/sides have same gain and preferably equal idle currents. But tube parameters vary, so with unmatched tubes best results may be obtained wirth somewhat unbalanced idle currents.