Plate / Cathode component switching

[OrganicEffects]

Hola,

Looking at Rob’s mod, turning the normal channel of an AB763 circuit and modifying to more of a lead channel sound:

https://robrobinette.com/AB763_Modifications.htm#Lead_Channel_Mod

Is it possible to switch out the components on the cathode and plate?

Plate I’d assume not, as it’s a high voltage and would be akin to turning the amp to standby between switching. So I thought it might be best to parallel two resistors and put the switch between the plate and one of the resistors, opening or closing the circuit. A 220k and a 183k should yield the 220k and the 100k specified. Thoughts?

For the cathode, I don’t think there’s a lot of voltage going on down there, and it would be super simpler to just switch out a pair of cap/resistor for the other set. Is this going to cause loud pops? Is there a better way?

Thanks! Matt.

[MFowler]

Plate load switching is very loud as on Komet 60 and Komet Concorde (50) amps unless you use a switching circuit like the one Dana Hall was selling.


Switching cathode resistor or more popular switching between different bypass caps for boosting is easily done.


Mark

[OrganicEffects]

Thanks for the reply! Attached is what I had originally envisioned.

I looked at the Komet 60, and it looks somewhat similar to what I tried to draw  I looked about for the Dana Hall switching you mentioned, but can only find the Dana Hall VRR topics? Any clues on where to go for what you mentioned?

What do you think about the cathode switching?

Thanks!

[shooter]

FWI, this circuit worked pretty well on the breadboard, never implemented in an amp

[brewdude]

Regarding the plate resistor, I would be inclined to try two resistors in series with a switch to bypass one.

[92Volts]

In both cases, use resistors in series/parallel with a switch to bypass or disconnect some of them. Otherwise there will be no connection while the switch is between positions, which will cause a loud pop.

This is easy for the values your show in your cathode example. On the right side the cap is larger and resistor smaller, both those can be achieved by keeping the left-side components and adding others in parallel. Keep the 22u cap (22.68 is close enough) but use a 3k3 resistor on the right. That will be close to 1k5 when paralleled with the 2k7.

Also, you might want to set up a high-value bypass resistor to keep the cap charged to cathode voltage to reduce switching noise, big enough to have little effect on sound. You'd need to cut out the low-value cathode resistor or it would drain the cap anyways. I can't wrap my head around the wiring right now but I believe it can be done with a SPDT switch?

FWI, this circuit worked pretty well on the breadboard, never implemented in an amp

I think Orange amps use that system, and it has a lot of fans

[66Strat]

FWIW, you can get the same result in gain difference by switching different values of the following stage grid resistor. Not worth the effort IMO. The circuit that Shooter suggested offers a lot of voicing options with the turn of a switch. This combined with a Cut Control would cover a lot of ground.

I think Orange amps use that system, and it has a lot of fans

+1. That and their Mid Boost control. This might be a good experiment type project for some of those inductors out of the AO30 Organ amps.

[OrganicEffects]

Shooter - The variable cathode resistor is interesting! I'll have to keep that tucked away for another project, but thank you for posting that!

brewdude / 92Volts - The series/parallel switching makes a lot of sense. I implemented it on the plate resistor in the first post, but the description of the cathode components make a lot of sense. I've attached a schematic that I think reflects the description?

As far as the large resistor to keep the cap charged - I don't quite understand. If I put a (1M?) resistor from the cathode to ground, it doesn't accomplish anything that the 2k7 does? If I put the 1M across the switch, I'd have 1M+ resistance in parallel with the 2k7, getting a total resistance of 2k6 or so. 

66Strat - the tonestack follows this stage immediately. Are you saying turning the volume pot would yield the same results as switching between 220k and 100k?

[66Strat]

66Strat - the tonestack follows this stage immediately. Are you saying turning the volume pot would yield the same results as switching between 220k and 100k?
[/quote]


No. The volume control is a voltage divider that apportions the amount of signal that is passed on to the subsequent stage. The voltage gain of the triode is determined by the amplification factor of the tube times the plate load. The plate load is comprised of the plate resistor and the grid resistor of the following stage. Switching grid resistors as reflected in the attached would change the gain structure much the same as switching plate resistors. Switch grid resistors as shown would also change the roll off frequency similar to switching the coupling capacitors as reflected in shooter's schematic.

[PRR]

> Is this going to cause loud pops?

Yes.

Me, I doubt the change of "tone" is worth the sore ear if you switch while on.

[shooter]

thank you for posting that

I stole it from somewhere, but can't find the source, enjoy 

[92Volts]

brewdude / 92Volts - The series/parallel switching makes a lot of sense. I implemented it on the plate resistor in the first post, but the description of the cathode components make a lot of sense. I've attached a schematic that I think reflects the description?

Yes, that looks like what I was suggesting

As far as the large resistor to keep the cap charged - I don't quite understand. If I put a (1M?) resistor from the cathode to ground, it doesn't accomplish anything that the 2k7 does? If I put the 1M across the switch, I'd have 1M+ resistance in parallel with the 2k7, getting a total resistance of 2k6 or so. 

Rather than a 1M from 22u cap positive to ground, I was suggesting a 1M from cathode to 22u cap positive. In other words you bypass the switch, not the cap.

You don't want the cap to be equal to ground voltage, or "floating" (no ground connection) with 0v across it. In those cases, it will pull the cathode to 0v when connected (until it recharges), causing a pop.

The problem is, the 3k3 resistor connected to that cap is will discharge it. Any resistor small enough to stop that from happening would basically mean the resistor & cap are still in the circuit.

I think a double-pole switch which separately disconnects the cap/resistor from the cathode and from each other would let this work.