Valco 510-11 Spectator Tone Circuit Question

[K Teacher]

Hello all,
I am trying to understand how the Valco chassis 510-11 (Spectator, I believe) tone control circuit works.
I did redraw the volume and tone controls portion of it for better visualization:

Attachment: Valco 510-11 Tone

I do not comprehend the function of that 100K resistor; it forms a volume out voltage divider with the tone pot. However, the reduction in volume is only 17% or -1.6 dB, which according to the specialized literature, cannot be perceived.

In my mind, this next circuit makes a lot more sense:

Attachment: Valco 510-11 Modded Tone 

Can someone help to explain this?

I think that, ideally, you want an audio (log) pot for volume and a linear pot for tone.
The Valco design, will cause a linear volume pot to have a response closer to an audio pot. The downside is, when volume is max., the load on the preceding amplification stage increases.

Additionally, if anyone has one of these amps, can you comment if the tone control works well all the way from zero to 100% or if there is any portion of it were, moving the tone knob does not produce any audible change in tone.

Any clarification would be much appreciated… Thanks.

[shooter]

try 'em both, 
surf up grid leak and grid stop resistors


run the circuit through software to see how the hardware responds

[PRR]

This is a cheap amp, isn't it? Isn't it one of those things which used the same pot-value everywhere?

We can perceive 1.6dB. Yes, it is extremely small, perhaps moot in live performance.

The mechanical idiot figures 4dB. I won't try to reconcile the math. It's still small.

At full-down the Tone makes a huge difference, by loading the vol pot wiper and the tube before it. But "all" the action happens in the last 2/10 of a turn, and probably less. Suddenly you "fall down into the padded cellar" and can only play mellow thumps.

My impression of Valco is that they were not trying to be the best amp in the joint.

[jjasilli]

If nothing else, the 100K R is a grid leak resistor to the next tube stage, in case both pots are dialed to -0-.

[K Teacher]

PRR,

Thanks for your analysis of the Valco 510-11 tone circuit. It shows that it is only a simple treble cut and its performance is basically the same with or without that 100K resistor.

I agree with your judgement about the Valco amps. It feels to me too, that they made great efforts to maintain their amp circuit's cost to a minimum. That's why I was intrigued about the addition of the 100K resistor to the circuit and its purpose.

Maybe, like jjasilli suggested, it's for circuit reliability, in case of bad contact from the pot's wiper, causing the triode to lose grid reference and resulting in excessive plate current.

Apart from that, another vintage, simple tone circuit that intrigues me, is the one used in the Magnatone 108 (Varsity) amp shown below:

Attachment: Magnatone 108 Tone
 
Just eye-balling it, seems that it will be too much interdependence between volume and tone controls.
How do you think this one compares to the Valco 510-11 tone?
Thanks again.

[PRR]

> suggested, it's for circuit reliability, in case of bad contact from the pot's wiper

I did not understand that theory. The pot -body- connects grid to ground. And Valco buyers were not looking for reliability, but price.

That 108 plan looks like several classic Fenders.

As to how all these things sound: I think $5 of loose parts and an hour with guitar and hot iron would tell you more than an hour of consulting the mechanical idiot.

[K Teacher]

PRR,
Thanks for your quick response.
 
I agree that the final answer will be obtained through some real-world testing, not simulation.
I was trying to accomplish two things:
   a) understand how a non-Fender tone control can contribute to the so called “Valco sound” and,
   b) get an educated guess to narrow down the tone control circuits to test instead of testing everything.

Thanks again and happy 4th!

[Willabe]

a) understand how a non-Fender tone control can contribute to the so called “Valco sound”

I don't think that that tone control has much to do with the 'Valco sound.' Probably has very little to do with that amps sound. And it's not in all the Valco amps, probably only in that 1 amp? 

Some of the main contributors to any amps sound are the choice of tubes, power tube class, their bias; power tubes/phase inverter(PI)/preamp tubes, -feedback loop or open loop, the iron set, PT/OT/choke, power supply, rectifier; solid state or tube, choice of rectifier tube, B+ dcv to the tubes, speaker(s); brand, type; alnico/ceramic/neodymium and size, cabinet/open/closed back and # of speakers, then the circuits, including; type of PI, and plate or cathode driven tone stack (TS), then parts, R's/C's.

[K Teacher]

Willabe,
I am aware that everything contributes to the so called "amp's sound".
However, I am not asking about everything. I am asking about this particular tone control circuit design.

The treble-cut tone, is used in many Valcos and other products. The difference is, that in this particular model, there is an addition of a 100K resistor (circled).

As pointed out earlier, Valco was a "pinch pennies" operation and I am wondering -- what is the improvement obtained by adding this 100K resistor to the tone circuit?

So far, I see little to none…

[Willabe]

I am aware that everything contributes to the so called "amp's sound".
However, I am not asking about everything. I am asking about this particular tone control circuit design.

No I'm not telling you everything contributes to the so called "amp's sound".

I saying that all those things, each by themself, make way more difference than that tone control and 100k R. Trying to re-focus you, missing the forest from the 100K resistor on a single knob tone control tree. I was trying to redirect your focus. That tone control is not the secret sauce in that amp. It's the last place to look for what makes that amp sound the way it does. I think your over analyzing it.

All single knob tone controls, like that 1, with or without the 100K R, have very little insertion loss and do very little to the amps overall sound.   

As pointed out earlier, Valco was a "pinch pennies" operation and I am wondering -- what is the improvement obtained by adding this 100K resistor to the tone circuit?

So far, I see little to none…

Right.

Why they put that 100K in, is anybody's guess. Where they trying to balance it a little? With the 100K in place, a little more signal is forced to go through the tone pot.     The more that tone pot is turned up, towards the grid connection, the more hi's go around that 100K. The more it's turned towards ground, the more the hi's get bled off to ground and more bottom end goes through the 100K to the tubes grid. So, there's some balancing/juggling going on. Is it to make it smoother as you turn the pot?   

You want to understand it better, do like PRR suggested;

"As to how all these things sound: I think $5 of loose parts and an hour with guitar and hot iron would tell you more than an hour of consulting the mechanical idiot."

Hook up 3, 4, 5, as many as you want, single tone controls and see how they sound and how effective/noneffective and how smooth they are.   

[K Teacher]

OK, thanks...

[Willabe]

I'm sorry, I could have written my reply's better, I don't want to squash your thinking and asking questions. That's always good. I think you are asking good questions. When I saw your post with the schematic I thought the same thing, why did they put that 100K in there. 

You can play around with different tone controls without soldering them up. Just use short double ended alligator clip cables and carefully clip in the different parts temporally. Fast, easy. 

Have you seen these web sites? There's some great reading in these, might be of interest to you?

Amplifier Tone Stacks - monster.party.hat

http://chasingtone.com/yourguitaramp/guide-to-single-knob-tone-controls/

Amp Stuff

How to design valve guitar amplifiers

https://www.aikenamps.com/index.php/white-papers
 

[sluckey]

K Teacher, do you actually have a Valco 510-11 amp?

[PRR]

Here's a plan from a whole different train of thought which reduces to the same thing:
https://postimg.cc/1frpfJMp

The series resistor allows a slight boost, as well as a severe cut. It does not look like much, but when a plan keeps coming back it *may* be more musically useful than it seems on paper.

[K Teacher]

Thank you all for your valuable input!

Willabe,
I've seen some of the sites you listed but not all of them. I will check them out, thanks.

sluckey,
I do not have a Valco 510-11 amp, but I have all the parts to build a clone on a Mojo 5F2-A chassis and cabinet (FYI, I already built a 5F2-A clone few years ago).

I was deciding between the Valco 510-11 and the Valco/Supro S6606 and chose the former. Did not like the grid-leak on the first stage of the S6606, because, according to previous testing, bias varies a lot from tube to tube.
 
The final detail I need to work out is the tone control… There are better designs but, the Valco needs all the voltage swing available to compensate for the un-bypassed 6V6 cathode resistor.

PRR,
Thanks for the additional info and schematics.

[sluckey]

I'm curious about your inspiration to build this amp. Would you share?

[K Teacher]

No particular reason… some say this is a good amp and others not so much… However, this schematic caught my eye and it seems a good use for my spare parts…