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This explanation is in the kit documentation. It is compiled from various sources.

Interactive Volume Controls – Fender Tweed Amps

The Deluxe has what’s referred to as “interactive volume controls” . Interactive volume controls are also on the Tweed Fenders (Bassman, Twin, Super, Bandmaster, etc.)

On the Twin for example, there are two volume controls each controlling one stage of a 12AY7 tube. They are very simple and economical circuits, only 5 parts per circuit. Each output comes from the volume control and passes through an “isolation” resistor – a resistor placed in the circuit to isolate the two volumes from each other, and at the same time to mix them for the next single input tube stage.

The “feature” is that the isolation resistors values (270K) do not isolate the stages from each other enough – one stage can still “see” the grid load resistance from the other. It appears to the channel you are plugged into, that there is another resistor in the circuit formed by the other unused volume control and its isolation resistor.
The real “magic” of it all is probably one of the absolute best “lessons” you can learn about vacuum tubes. You would guess that the best and loudest tone would be with the highest resistance, with the unused pot turned all the way up, but that isn’t so. It is somewhere near 5 or 6 (on old amps, a new amp may have different pot tapers and it could appear at different points.)

When you are turning the unused volume control you are “fine tuning” the grid load resistor to the next stage of the amp. An optimum (best sounding) value is not all the way up, but somewhere near the middle – the lesson? Tubes are NOT linear or mathematical devices – they do not respond to exact mathematical figures and have “hills and valleys” in their response – maximum performance can not be calculated directly – you have to hear it and you have to be aware that the “truth” may not be at some exact calculated place, but at some far more imprecise figure.


[edit] The Tweed Deluxe is unique in that the volume controls are not voltage dividers and work by loading down the signal from the plates of the pre-amp tube. Because of this, you get the max mids in the channel you are plugged into when the other volume control is at it's mid position.. Max midrange scoop ocurrs when the other control is turned to full.

Set one channel (say Normal one) to max and the other (Bright) to mid position. When you select the Bright channel, you should get nice clean tones and when you select the Normal channel, you will get nicely distorted tones.

The clean tones work because you are not overdriving the bright channel and you are scooping the midrange. On the Normal channel, you get the nice lead tones because you are overdriving the output stage and boosting the mids.

So, each of the Deluxe volume controls affects your tone to a noticeable degree. The interactive volume controls create a hidden voicing control, which allows the player to sculpt the tone in a way that no other regular multi-band tone controls will permit. The effect is somewhat like scooping out the mid-range and extending the bass and treble frequencies as you turn up the adjacent volume control. In other words, the Normal volume interacts with the Bright and vice versa. This allows the player to control the midrange of the channel used by turning the volume of the channel not being used.

Maximum midrange occurs when the volume control of the unused channel is set about half up (about 5). Maximum midrange scoop occurs when the volume control of the unused channel is almost all the way up. When plugged into the bright channel with the volume about half up and then turning the normal channel almost all the way up (around 10) will give a cleaner tone.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Stephen...I don't see that in the schematic of the tweed deluxe that you have posted. No mixing 270K resistors and the "out" is from the tone control not from the volume pots!?????
Stew

Truly. The description is generic to some Tweed models. I guess that's not clear.

Take a look at the schematic and it is clearer what is being discussed.



The Tweed Deluxe is unique in that the volume controls are not voltage dividers and work by loading down the signal from the plates of the pre-amp tube. Because of this, you get the max mids in the channel you are plugged into when the other volume control is at it's mid position.. Max midrange scoop ocurrs when the other control is turned to full.

Set one channel (say Normal one) to max and the other (Bright) to mid position. When you select the Bright channel, you should get nice clean tones and when you select the Normal channel, you will get nicely distorted tones.
The clean tones work because you are not overdriving the bright channel and you are scooping the midrange. On the Normal channel, you get the nice lead tones because you are overdriving the output stage and boosting the mids.

Gerald BBQ explains it in his book, Tube Amp Talk. A worthwhile read.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

I tried this...at least the clean tone part. Like GW said, Set the bright channel to 12:00 o'clock and the normal channel volume just a little less than full. Tone set to taste. The result is an amazing clean tone that, with very minor adjustments, will work with any guitar even humbuckers. I tried to replicate this sparkley clean tone using just one channel and I couldn't get it...too much mids.

Nice tip...you get very blackface like cleans.

LB

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Guitologist at Large
"How does that feel?"

Hey, Coco, just a quick question here re the Deluxe volume controls. I understand (and use) the interactive nature of the the two volume pots. Very cool and useful feature! But I've also noticed that on both normal and bright channels there is a very quick jump to maybe 50% of full volume at the very front end of each pot. It is virtually impossible to get a really low volume, even by using the "lo" rather than "hi" input jack, and rolling off midrange by increasing the unused volume pot (the other channel) to near max. I would expect such behavior with linear taper pots, but these supplied in your kit are logarithmic - designed to "level out" the volume increase more evenly across the range of the pot.

Have you noticed this? Is it intrinsic to the Deluxe circuit, even with log taper pots? Or could it be possible I might somehow have gotten linear taper by accident? Can the resistance taper be measured accurately with a meter to verify, without unsoldering? Or if not accurately, then at least meaningfully, relative to a linear taper? Also, aren't there two or three log tapers manufactured, instead of just one? - i.e., perhaps a steeper slope would provide for higher resistance and thus lower volume along a little more of the control's front end travel?

Just curious... This is not a problem playing with other musicians, where one would always want more volume; but in the living room, sometimes more quiet is helpful. I suppose the VRM is another way to go to solve this - can that fit into the Deluxe circuit easily enough?

Thanks for any wisdom you have to offer on this.

Jim

Symptomatic of the circuit. Roll back on the guitar, use low input. We dont stock any 1M linear so it's very doubtful you got a linear one.

Yes, a VRM could help here!

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

OK, Stephen, you've got my curiosity aroused now about the VRM. I haven't tried one yet with my Deluxe or TC-15. I would like to install one on the Deluxe. I notice your schematics for installation don't include the Deluxe - do you have a sketch yet for that application? Or could you send me simple installation directions for the Deluxe? Also, do you still have some, and at $25? Would Paypal be good to order (how much for shipping)?

Do you know any Trinity folks that have installed one on a Deluxe, so I could check with them to get ideas for possible installation location (control pot, etc.)?

Thanks,
Jim

Had to pipe up on the VRM!

I put a push-pull pot in the Power hole on the front of my TC-15. To me, that makes sense. You pull the pot to turn the amp on, then vary the voltage by turning the pot. Makes perfect sense to me!

JP

Hi, J-Rock.

Thanks for the idea. I've looked more carefully at my Deluxe, and I agree that your installation will be best for my application. I think I can fit the VRM board between the PT and the turret board, but there's not much room for the control pot - looks to me like I'd either need to replace the power SPST toggle on the front of the chassis (as you did), or replace the aux speaker jack on the rear of the chassis. I can do without that aux speaker jack, but that location would involve some longer wiring runs, plus more difficulty avoiding the heater wiring - neither being a good idea.

What rating is your push-pull pot? I can thus far only find 1/2W - 500V in linear, and 1/4W - 250V in audio. I think audio taper is a better choice, but am not sure if that 1/4W rating is sufficient for this circuit.

Thanks,
Jim

You're quite creative; I appreciate the suggestion. After giving my Deluxe a closer inspection, I've come to the conclusion that your installation is the greatest option for my needs. It seems like I could squeeze the VRM board in between the PT and the turret board, but there wouldn't be much room for the control pot, so I'd have to either move the power SPST toggle to the front of the chassis (like you did) or move the aux speaker jack to the back. The lengthier cable runs and the increased difficulty of avoiding the heater wiring mean that I can do without the aux speaker jack. drift boss

Both volume controls and the tone control all interact with one another even when only one channel is in use. Smaller tweed amps have simpler tone controls than the larger amps, and many have interactive volume controls. retro bowl college