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

[SoulFetish]

so, this is a fairly simple one, but want to hear some opinions about it.
I have a Pro Reverb on the bench that was modded to remove the hum balance and install fixed resistors, and the "output tubes matching" circuit was removed and the pot was repurposed as a bias control.
While I can appreciate the desire to have an adjustable bias control, removing or disabling the OTM feature does the amp a disservice.
IMO, the output tubes matching circuit included in most of those 70's Fenders with the UL taps is one of the best things Fender did in there amps post CBS acquisition. If you know how to set the OTM control, you can go a long way to making these amps have remarkably quiet.
The circuit is relatively simple, however the way it uses the center tapped pot to create the voltage divider makes it a bit awkward to include a bias control. Here is the stock schematic of the amp.

Fender-Super-Reverb-Ultralinear-1978-1981-Schematic.pdf


After looking at the circuit, and restoring the amp back to it's original configuration, the simplest, and most efficient way of implementing a bias adjust was as follows (changes indicated in red):



This is a fairly obvious alteration, but the clear drawback is the time delay in the bias voltage stabilizing due to the capacitors to charging and discharging. (Plus, I don't love using one resistance as a rheostat to change the divider ratio. But that not really a big deal, probably a bit of neurosis on my part).
Is there a better solution, which any of you have used?

[Enzo]

I have no issue with the rheostat approach, I dislike relying on the wiper for bias, as some amps do. However, in your example I see it as a problem. I always fear the wiper will fail. That is way more likely than the element going open. But in your scheme, the open wiper puts the entire 10k of the pot into the circuit, That results in the minimum bias voltage, doesn't it? In my view, better to make the 33k up by the balance pot the adjustable one. That way a failed wiper results in max resistance there, which puts the amp into coldest setting.

I also do not share the concern over caps taking too long to charge. If you are staring cold, it takes the tubes longer to start conducting than it takes that cap to charge. The 70uf charges up at power on, and assuming one uses the standby switch, it becomes moot. The 80uf@75 cap only has to come up or down when you are adjusting, otherwise it charges up at power on along with the other. Bias ought not be adjusted until the amp has been running 15-20 minutes anyway.

[SoulFetish]

I also do not share the concern over caps taking too long to charge. If you are staring cold, it takes the tubes longer to start conducting than it takes that cap to charge. The 70uf charges up at power on, and assuming one uses the standby switch, it becomes moot. The 80uf@75 cap only has to come up or down when you are adjusting, otherwise it charges up at power on along with the other. Bias ought not be adjusted until the amp has been running 15-20 minutes anyway.

Yeah, that’s not what I mean. Im talking about while adjusting the pot, the combination of changing the loading on the winding (presumably) and the cap having to charge or discharge to catch up to the voltage change, causes a little “latency” in the bias voltage to settle. Nothing like the warmup time from a cold start.
look, im just splitting hairs and bitching that I can’t have it all in this example.

[vintagekiki]

Set the OTM potentiometer (10k) to the middle position.
With the set bias potentiometer (22k) set the desired bias.
When the bias is set, with the OTM potentiometer (10k) adjust the output tubes balance to minimum hum.

[SoulFetish]

Yeah I like that one too. I like the location of the adjust and it moves it too the better location in case of failure as Enzo suggested.
i was a good point.

[Helmholtz]

Not sure if I understand correctly, but I don't see a risk with the OP's wiring. Provided it is exactly done as shown, meaning that both ends of the 10k pot are wired to the circuit.
If the wiper goes open, there is still the 10k track resistance ensuring max. bias.

[pdf64]

Yes, I’ve used the vintagekiki arrangement for my 75 and any other models from that range that land on my bench.
An increase of the max available bias supply voltage can sometimes be beneficial too, eg by reducing the value of the 1k2 current limiting resistor.

It’s been covered here a few times before, but it’s worth pointing out that these aren’t, as per its usual definition, UL; as the screen grid taps are at about 12.5%. Previous threads have kinda reached a consensus that the tapped primary arrangement was used to protect the screen grids from excessive dissipation at high power output levels, which, given the increased HT voltage used, they would otherwise be liable to.
Tube operation will still be closer to regular pentode than to true UL, and as improving linearity was seemingly not the purpose of the arrangement, and I’m not aware that they were marketed as such, referring to them as ‘UL’ seems a bit of a misnomer, which may give to a misleading impression of their designer’s intent. Which was probably more along the lines of higher power output but reduced tendency to farty bias shift / blocking distortion when pushed into overdrive.

A downside of the tapped primary arrangement is that the screen grids are fed a more ripply HT, so making the bias balance more important to keeping idle hum levels low. Almost all well regarded ‘UL’ amps use a CLC / pi filtered HT feed to their OT, which acts to stops issues from HT ripple at all power levels, not just idle. But sufficiently beefy chokes have weight and cost downsides.

[vintagekiki]

The bias setting is not done by changing the series resistor which is an integral part of the RC filter, the bias adjust by change by the resistor according the ground "behind" the RC filter.
Bias in Fender amps is defined by a resistor 33k because implies use original Fender (part number) tubes, such no need adjust the bias except adjust output tubes balance to minimum hum.
If not use the original Fender (PN) output tubes, it is necessary to adjust the bias voltage to the required value idle current by changing the resistor 33k.

[Helmholtz]

The bias setting is not done by changing the series resistor which is an integral part of the RC filter, the bias adjust by change by the resistor according the ground "behind" the RC filter.

I also prefer the method you showed.

And it's true that lowering the 2.2k shunt resistance significantly will increase bias hum.
But in reality it is hardly necessary to reduce resistance by more than 10% (increasing resistance will even improve filtering), which shouldn't have a noticeable effect.

(My comment above only referred to the reliability concerns.)

[vintagekiki]

It’s been covered here a few times before, but it’s worth pointing out that these aren’t, as per its usual definition, UL; as the screen grid taps are at about 12.5%.

When I was younger, amp service include rewinding transformers. Initially, this include counting the number of windings, to later include mathematics and calculation.
For UL Fender OT, winding for g2 (blu-yel ili brn-yel) is the same as secondary winding for highest speaker resistance (blk-grn).

When rewinding Fender transformers the calculation is for 60Hz as opposed to the European transformers which is calculation for 50Hz.
This explains why PT at Fender amps warmer during operation in Europe than in the USA (difference in winding 60/50 Hz)

[Enzo]

Not sure if I understand correctly, but I don't see a risk with the OP's wiring. Provided it is exactly done as shown, meaning that both ends of the 10k pot are wired to the circuit.
If the wiper goes open, there is still the 10k track resistance ensuring max. bias.

Assume the balance pot is centered. The bias feed is a -62v supply, so the bias feed is a 10k over 33k voltage divider when the 10k pot is turned to max (or the wiper opens.) That puts -44v on the grids. That means the default is the hottest setting. And other end, turn the 10k to zero and -62v on the grids, the coldest setting.

Hence my suggestion that we make the 33k variable instead of the 10k. With the 33k variable, an open wiper results in the highest bias voltage rather than the lowest. I consider that safer.

[Helmholtz]

Thanks for explanation, Enzo.

You're absolutely right.
I didn't consider the complete bias circuit, wrongly assumed that high resistance meant more negative bias.

So the only reliable mod is making the 33k shunt resistor variable - as already recommended by you and others.

[pdf64]

When I was younger, amp service include rewinding transformers....

Wow, just wow! From today's perspective, that seems next to impossible, especially those with epoxy potting. Even if just wax potted, it must have been an enormous, tedious task.
I guess if there's no alternative, then you've gotta do what you've gotta do!

[SoulFetish]

Thanks for explanation, Enzo.

You're absolutely right.
I didn't consider the complete bias circuit, wrongly assumed that high resistance meant more negative bias.

So the only reliable mod is making the 33k shunt resistor variable - as already recommended by you and others.

Yeah, I totally agree. (And I made a similar wrong assumption).
well, Im glad we had this talk. Appreciate the helpful insight, and comments from you all.

so bere’s the final question:
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?
im leaning towards drilling for the external bias control.

[Leo_Gnardo]

[QUOTE=SoulFetish;n917884
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?
im leaning towards drilling for the external bias control.[/QUOTE]

What I do in these cases is replace that 33K resistor with a trimpot + resistor combination right there on the back of the balance pot. FWIW I use the Bourns "blue cube" single turn trimpots.

If you want to make your new bias adjustment approachable from outside the chassis, find a good spot to park it and drill a mounting hole for it there.