Tone control

[Mats]

Hi
I think I have seen this on some old Fender Deluxe amp.
Strange Tone control, to later tweed Deluxes.
2M Pot, but what about how many watts ?
(across the two 6V6)

Mats

[HotBluePlates]

I think I have seen this on some old Fender Deluxe amp.
...
2M Pot, but what about how many watts ?
(across the two 6V6)

That variation of a tone control was not found in any Fender amp ever produced. However, there may have been something roughly similar in small radios, but they would likely have been single-ended.

To me, this looks like someone's idea of a variable conjunctive filter, or variable corrective filter (the latter term is what RCA called this in the opening section of their Receiving Tube Manuals). Normally, the resistance is a fixed value, and RCA recommends the resistor be 1.3 times the plate-to-plate impedance of the output transformer. So for your 6V6's and a likely 8kΩ OT, the resistor would be 10.4kΩ or the closest standard value of 10kΩ.

Instead, you may have seen a similar circuit with a closer resistance in Vox amps. This is the Top Cut control, and is generally a 250k-1M pot and a cap placed between the outputs of the phase inverter. You could use any typical 1/2w pot for this circuit.

[tubeswell]

That tone control was on the Fender Model 26 (Woodie Deluxe).



As to the wattage, I would think the beefier it was the better

[HotBluePlates]

Good catch Tubeswell! I'll admit this is the first time I've looked at the Model 26 schematic.

Okay, then what should be the power rating of that 2M pot? After some researching and tinkering with some RC circuit math, I believe you could have a common 1/2w pot for this tone control. Although there could be as much as 335v RMS from one side of the tone circuit to the other, when the resistance of the pot is reduced two opposing voltages are brought together which should cancel each other.

There will also be no d.c. across the pot due to both the cap and the fact that the circuit bridges two points at the same d.c. potential.

Someone correct me if I made an error in my reasoning.

[jjasilli]

Might be quite a display if that conjunctive cap failed by going short.  Fabulous find.  Wonder what it sounds like.

[PRR]

> what should be the power rating of that 2M pot?

The speaker load for two 6V6 will be about 10K at the OT primary.

So a "2 Meg" seems like it would do nothing at all.

Such a plan was common on table radios. Usually a 5K pot across a 2K load. At full 5K, little effect. At half-way, 2.5K, output is reduced. At "1", about 500 ohms, output is very reduced. Only in treble; the cap blocks bass.

IIRC for a 1W 2K radio the cap was often 0.05uFd. Taking the 2K impedance, 2K against 0.05uFd is 1.6KHz. That's a bit brutal top-cut, but for AM radio or 78s you might need brutal.

If your signal had LARGE power above 1.6KHz, the pot must absorb the *entire* 1 Watt.

This 'could' happen in testing.

But speech/music systems like radios (and guitar amps), playing for people's ears, generally will NOT have huge power above 1KHz. A tweeter crossed-over at 800Hz takes less than a third of the total power; at 2KHz, less than a tenth. On those grounds, the 1 Watt radio could get away with a 0.1W pot. Since cheap pots are 1/4W, it's not a problem.

Now the model 26. That's about a 10 Watt. The pot should be rated at least 1 Watt. Perhaps more for modern bright/fuzzed guitar sounds. And the resistance should be somewhere above 10K, so full-up is full-bright, but part-up is part bright. 20K or 25K makes a lot more sense than 2 Meg.

I think that's why we don't see this type tone control on anything bigger than a Kay or Kent (cheap junk derived from table-radio parts). Why use a costly several-Watt pot in the output stage when a cheap pot in the preamp will work? (Guitar amp has more audio preamp than a radio, therefore more places to hang your tone control.)

> if that conjunctive cap failed by going short

Then the pot burns up. Small smell, no big deal.

If the pot happened to be turned to dead-zero ohms, the sound would stop. For a while. I suspect any reasonable power pot on a two-6V6 amp would burn-up even when turned to "zero" ohms (there's always a little parasitic resistance under the wiper).

[Mats]

Thanks for answers !
So it was Model 26, I was shure it was from an early Fender.
Yes it looks like an variable corrective filter.
I have the tubes, but have to get hold of PT and OT,
to try this.
Once again thanks and also for the schematic
Summer greetings from Sweden,
Mats

[Mats]

Hi
I have done some search and found one 25k Lin Pot rated at 4 Watt.
Thanks for your help.