Deville bias question

[Leo_Gnardo]

Instead of buying more tubes, would a linear pot flatten out the rapid volume gain?

The reason so-called audio or log taper pots are made is that linear taper pots bring the volume up way too fast. Ah but that's the problem we already have with this series of amps. So again NO, a linear pot would make it even worse. If the following item existed, it would be good: a pot that has an even more "bent" volume curve. Even better if it physically fits the circuit board so we could swap it in with no major surgery. There ain't no such thing so the solutions left are 1: use a lower gain preamp tube and B) modify the existing circuitry. To expand on B, the volume pot is the lower arm of an already existing voltage divider. You (or a tech who is willing to suffer the PIA job of unassing the amp's circuit board, for which you will pay him a princely sum!) can either parallel a fixed resistor with the volume pot, or exchange a larger value resistor for the upper arm of the voltage divider. Either way will reduce the max volume you can dial on your volume control, and allow you to fine tune your volume over a wider arc of the volume control. Success at last - for a price.

Would a change of resistor value on the input stage do something similar?

Yes the one described in bold in the above paragraph. Sounds easy, and in principle it is, but it is a time consuming task to r&r the circus board.

[g1]

Your pickups are too hot.
Someone should market a little pad that goes between guitar and amp so you can set amp volume at 5. Done.
(because no one wants to play with guitar volume low )

[Tom Phillips]

...Someone should market a little pad that goes between guitar and amp so you can set amp volume at 5. Done.
(because no one wants to play with guitar volume low )

The #2 input of the classic Fender pre-amp will get you part way there. I just spent some hours going through my stock of 1 Meg Ohm audio taper pots to find a set that solved the problem for a customer's Weber 5E3 Clone. It was surprising how much the pot curves varied. Especially at the low end of the rotation. There were some pots that had a nice Audio taper curve but they had a fatal flaw that the first ~15% of rotation produced almost no useful volume and then the response jumped up to where it should have been. I ended up finding and using a pair of NOS Clarostat pots from 1963. Unfortunately, they were the only ones I had left. Sad to see them go but that's why I saved them all these years. Rambling on...that's also why I don't sell from my stash of parts to builders and hoarders. Those parts are to support the work I personally do for my customers.

There was once I guy in my shop who was known as a big collector. Never even heard him play a guitar. He saw the labels on some of my parts drawers such as "Vintage Fender Foot switches" etc and he took it upon himself to start looking through the drawers and asking what was for sale. He had no immediate "need" he just wanted the stuff. Still pisses me off to think about it. Kind of felt like a guy you didn't know opening your guitar case to see what was inside as you were setting up the stage for a show.

[Dave H]

Someone should market a little pad that goes between guitar and amp so you can set amp volume at 5.

I use an EQ pedal for that, set the sliders for flat EQ and turn the output level down. For a boost stomp on the pedal to bypass it.

[arcman]

HRDeville Bias Revisited... Also, I need some electronics educatin'.

Hey guys,

After thinking about this for a while, I've got a couple more questions.

As to Leo's reply. Would increasing the resistor on the upper leg or adding a parallel resistor to the pot change the taper I'm looking for or just reduce the overall volume? I'm guessing the latter..

I don't understand the 'taper' on these pots. For the heck of it I checked the resistance of the volume pot (in circuit) from the input leg to the wiper (rated 250k).

Position Value
1 288k (off, no sound)
2 285k (At 2-1/2, I have no need to go any louder)
3 280k
4 274k
5 260k
6 246k
7 233k
8 220k
9 149k
10 61k
11 4.5k
12 1.7 ohm

This is a audio taper pot. (Here's where I get dumb) Why is it at position one there is no sound and at position two it's already too loud? It acts like the opposite of the values I get.
How can there be such a drastic change in volume when the resistance is a fraction of the pots value? I'm starting to think a 'voltage dividing network' is some kind of magic. Though I've fixed dozens and dozens of electronic problems with common sense, even three Youtube videos on voltage dividers can't get me to figure this out. You'd think there'd be a simple way to change the taper without changing the overall volume.

It turns out there are different percentage tapers on some of the amps controls. The treble control is a 30% taper while the volume is a 15% taper. I would try swapping the two but the volume control has much more physical resistance to turning so if you accidently bump the volume knob it won't go sky high. I emailed Fender asking if a 15% volume pot was available and it looks like they could care less (no reply).

What would a 500k volume pot do? Nothing?

If I were to go with the tube change approach and try the 12AY7, do I need to replace both AX7's or just the first one?

Thoughts?

Thanks again.

[TimmyP1955]

Maybe change R9 (220k that feeds the volume control) to 1M?

[g1]

When you are trying to analyze, look at the measurement from the wiper to ground. That is what has most control over the level of signal passed to the following stage.

[Leo_Gnardo]

As to Leo's reply. Would increasing the resistor on the upper leg or adding a parallel resistor to the pot change the taper I'm looking for or just reduce the overall volume? I'm guessing the latter..

Correctamundo! You're sure never going to need to turn that pot up to 12. Sacrificing some volume at the other end - the hot end - of the pot will let you open it up further. Now if you go to a 12AY7 preamp tube - or two - you'll find the level you need at 5 or 6 or 7 furthermore you can fine tune your level. Instead of being too loud at 2 1/2, and moving the pot 1/100 of a turn either way gets you too much or too little.

I emailed Fender asking if a 15% volume pot was available and it looks like they could care less (no reply).

What would a 500k volume pot do? Nothing?

Neither would get you a satisfying result. A steeper taper, you'll be at 3 instead of 2 1/2. The taper we need here, they don't make. And a 500K pot would give you more volume, which you don't need, in this circuit.

If I were to go with the tube change approach and try the 12AY7, do I need to replace both AX7's or just the first one?

Def. the first one, if you find it's a step in the right direction you could also change the second & see how that plays.

[arcman]

When you are trying to analyze, look at the measurement from the wiper to ground. That is what has most control over the level of signal passed to the following stage.

Now I'm really confused. The ground side to wiper only goes up to 43k at full volume. I would've thought it would be a mirror image of the other side. Are the other components up or downstream interacting with these values? Doesn't seem like they would.

Leo.

Another dumb question. I can try the 1meg on the upper leg in place of the 220k as mentioned by Timmy, but If I were to try the resistor addition to the pot, what value can you recommend and how would I attach it? Across both legs, from input leg to wiper or from wiper to ground?

(more dumbness) Why I can't put a pot in place of the 220k resistor and mount it on the chassis using shielded wire? Or would it need another 'voltage divider network' ? If the former would work, what value?

Thanks once again.

[Leo_Gnardo]

Now I'm really confused. The ground side to wiper only goes up to 43k at full volume. I would've thought it would be a mirror image of the other side. Are the other components up or downstream interacting with these values? Doesn't seem like they would.

You're measuring the combination resistances of volume, treble, bass & midrange pots unless you remove the volume pot from circuit.

Another dumb question. I can try the 1meg on the upper leg in place of the 220k as mentioned by Timmy, but If I were to try the resistor addition to the pot, what value can you recommend and how would I attach it? Across both legs, from input leg to wiper or from wiper to ground?

I've used 100K to 47K, from ground to hot. The lower values will allow you to turn your volume control up further.

(more dumbness) Why I can't put a pot in place of the 220k resistor and mount it on the chassis using shielded wire? Or would it need another 'voltage divider network' ? If the former would work, what value?

Ow! What a hassle, drill another hole in the dashboard, fish a pot thru while getting the rest of that dratted circus board mounted correctly. 1 Meg pot, that's the highest value easily found, if you must. The two pots would form a voltage divider. I cannot recommend it.

[arcman]

Leo,

May be a hassle but since the chassis is out of the cab for new tolex, I'm willing to give it a try. I would put the pot on the bottom near the speaker out jack, for occasional use. One position for bedroom volumes and another for stadiums.

[Leo_Gnardo]

Leo,

May be a hassle but since the chassis is out of the cab for new tolex, I'm willing to give it a try. I would put the pot on the bottom near the speaker out jack, for occasional use. One position for bedroom volumes and another for stadiums.

A sure fire formula for unplanned ultra high frequency oscillations = put input circuitry right next to output. As much as I don't like 'em you'd be better off making a speaker attenuator for your bedroom/stadium choice, and simply put a 1M series with your volume pot.

[arcman]

A sure fire formula for unplanned ultra high frequency oscillations = put input circuitry right next to output. As much as I don't like 'em you'd be better off making a speaker attenuator for your bedroom/stadium choice, and simply put a 1M series with your volume pot.

Drat! Shot down again. This ain't working out. I'll fiddle with it.