Cascade Switch

[LedZepp007]

I'm planning to add a cascade switch on my GDS 18-watt TMB. I drilled for the switch last night (located it above the normal channel volume and tone controls).

My question is this: aside from simply sending the output of V1A/B to the input of the TMB channel, what other modifications do I need to make (e.g., increasing grid stopper values, changing cap values) in order to make this work and sound good? I know that blocking distortion can be a problem.

Thanks!

[Colossal]

So in 18W mode are you running V1a and b in parallel with the standard 820R/47uF cathode setup? Going into cascade mode, will you be switching with a relay or passive signal routing? What cathode and bypass cap(s) values will you be using in cascade mode?

[LedZepp007]

So in 18W mode are you running V1a and b in parallel with the standard 820R/47uF cathode setup? Going into cascade mode, will you be switching with a relay or passive signal routing? What cathode and bypass cap(s) values will you be using in cascade mode?

Passive routing (DPDT mini-toggle with shielded cable). 820R/50uF in parallel in 18 watt mode. What cathode and bypass caps should I use in cascade mode?

Thanks for the help!!

[Colossal]

You said your amp was Normal channel and TMB channel. Are you just looking to cascade the Normal channel? Or are you planning to cascade the Normal channel into the TMB channel? do you have a schematic you are working from?

[LedZepp007]

You said your amp was Normal channel and TMB channel. Are you just looking to cascade the Normal channel? Or are you planning to cascade the Normal channel into the TMB channel? do you have a schematic you are working from?

I was thinking:

Normal -> TMB (with a gain control somewhere in the mix). Not working from a schematic just yet. Have looked at Zaphod Phil's mod attached), but it seems like a lot of work for a somewhat simple cascade (and I'm not sure how I would lay all of the components out in limited space and keep noise down).

[LedZepp007]

OR I could cascade the normal V1A into V1B and then cascade that into the TMB channel lol. Super gain.

[pdf64]

Yes, it's bad practice for cascaded stages to share cathodes, as inadequate bypassing can result in a positive feedback path via the cathodes.

[Colossal]

I was thinking:

Normal -> TMB (with a gain control somewhere in the mix). Not working from a schematic just yet. Have looked at Zaphod Phil's mod attached), but it seems like a lot of work for a somewhat simple cascade (and I'm not sure how I would lay all of the components out in limited space and keep noise down).

I actually put that drawing together for 18watt.com. I worked with PCH18, the guy who did most of the work, to clarify the details. I think the old thread is still around. There were some photos and I think a layout of it might still be in the Downloads section. The values shown will give a Plexi kind of sound with the second stage biased quite warmly to give some soft clipping. If you cascade the paralleled single stage into the TMB, you would need a gain pot for sure. That would certainly be the easiest thing to try.

[LedZepp007]

I was thinking:

Normal -> TMB (with a gain control somewhere in the mix). Not working from a schematic just yet. Have looked at Zaphod Phil's mod attached), but it seems like a lot of work for a somewhat simple cascade (and I'm not sure how I would lay all of the components out in limited space and keep noise down).

I actually put that drawing together for 18watt.com. I worked with PCH18, the guy who did most of the work, to clarify the details. I think the old thread is still around. There were some photos and I think a layout of it might still be in the Downloads section. The values shown will give a Plexi kind of sound with the second stage biased quite warmly to give some soft clipping. If you cascade the paralleled single stage into the TMB, you would need a gain pot for sure. That would certainly be the easiest thing to try.

So if I started with the simple cascade, I could just run the output of V1A to the switch to a gain pot (500K Log) and then to the TMB input? Would I need to increase the value of the screen resistors?

[Colossal]

So if I started with the simple cascade, I could just run the output of V1A to the switch to a gain pot (500K Log) and then to the TMB input? Would I need to increase the value of the screen resistors?

Yes, you could start there, just to get a feel for it. One thing though, are you sure your Normal channel is paralleled? In a typical 18W TMB, there would be three preamp tubes. One for the phase inverter, three stages for the TMB channel and that leaves 1/2 of the remaining 12AX7 for the Normal channel. Please check on that.

Once you cascade that Normal stage into the TMB, I would most definitely reduce the Normal's cathode bypass cap from 47uF to 0.68uF. You are going to need to control the low end to keep it tight.

Regarding screen resistors...are you talking about the output tubes or the input grid resistors in front of each preamp channel?

[tictac]

I never get this thing where hobby builders want to put high voltage DC on a mini switch. The Trainwreck guys do the same thing and there's no need to.

It's really easy to switch a gain stage in and out of a circuit on the AC side of the circuit.....why not do that?

The idea of High Voltage DC a millimeter or so from a metal switch contact makes me nervous but maybe I'm just too paranoid....

TT

[LedZepp007]

I never get this thing where hobby builders want to put high voltage DC on a mini switch. The Trainwreck guys do the same thing and there's no need to.

It's really easy to switch a gain stage in and out of a circuit on the AC side of the circuit.....why not do that?

The idea of High Voltage DC a millimeter or so from a metal switch contact makes me nervous but maybe I'm just too paranoid....

TT

i wouldn't be putting high voltage on the switch would I? Just signal I think. I hope there's no HV since I think the switch is rated only for 20v.

[Colossal]

I never get this thing where hobby builders want to put high voltage DC on a mini switch. The Trainwreck guys do the same thing and there's no need to.

It's really easy to switch a gain stage in and out of a circuit on the AC side of the circuit.....why not do that?

The idea of High Voltage DC a millimeter or so from a metal switch contact makes me nervous but maybe I'm just too paranoid....

TT

i wouldn't be putting high voltage on the switch would I? Just signal I think. I hope there's no HV since I think the switch is rated only for 20v.

The way that mod was wired, yes, it is splitting a parallel triode into a cascade where HV is switched. Not particularly my cup of tea either for simplicity or safety. I was but the humble scribe. On the Trainwreck, when switching the Gradual/Fast mode, it's far easier to do it after a cap on both sides of each plate load. No HV switching.

My recommendation is to just tap the signal after the gain pot of the Normal stage, which is after the coupling cap. You may also need to lower that coupling cap from 0.0047 to 0.0022. That is going to be a lot of gain.

[LedZepp007]

I never get this thing where hobby builders want to put high voltage DC on a mini switch. The Trainwreck guys do the same thing and there's no need to.

It's really easy to switch a gain stage in and out of a circuit on the AC side of the circuit.....why not do that?

The idea of High Voltage DC a millimeter or so from a metal switch contact makes me nervous but maybe I'm just too paranoid....

TT

i wouldn't be putting high voltage on the switch would I? Just signal I think. I hope there's no HV since I think the switch is rated only for 20v.

The way that mod was wired, yes, it is splitting a parallel triode into a cascade where HV is switched. Not particularly my cup of tea either for simplicity or safety. I was but the humble scribe. On the Trainwreck, when switching the Gradual/Fast mode, it's far easier to do it after a cap on both sides of each plate load. No HV switching.

My recommendation is to just tap the signal after the gain pot of the Normal stage, which is after the coupling cap. You may also need to lower that coupling cap from 0.0047 to 0.0022. That is going to be a lot of gain.

Thanks for the help. I'm a serious noob when it comes to this stuff. I'm working with a GDS TMB kit that has instructions for the normal tremolo version and so there's a little guesswork (Graydon's TMB layout is different than most of the other layouts I've found). Additionally, my build is a good bit uglier than most builds I've seen. I guess that is due to my inexperience. For instance, the heater wiring was actually kind of a PITA. Hopefully, it doesn't mess with noise. Additionally, I am not 100% sure I wired my cliff jacks right, since I'm only using one input per channel. Right now, I have a dual 500k pot for a PPIMV the mini DPDT switch and a 1 M gain pot for the cascade mod.

I can try to post pictures later.

[Colossal]

Yeah, wiring heaters is never really all that fun. However, it's something you want to take your time on and get right.

I am familiar with Graydon's work, but it would be helpful to know exactly which schematic or version you are working from so as not to suggest something that could cause you duress.