Schematic for review

[Howlin' Mad Mac]

Would someone knowledgable please review a schematic drawing I did recently before it goes into assembly? The basic design is a lite "Plexi" front end into an 18 watt back end however I've replaced the TMB controls with a basic tone control, added a master volume control & (I think) have the input switchable from normal to a "Jumped Plexi" kind of deal going on & many, many thanks for any assistance.

[RudeBoy]

You lost me there with the first 12ax7b cathode. Did you intend to tie those cathodes together?

[Howlin' Mad Mac]

You lost me there with the first 12ax7b cathode. Did you intend to tie those cathodes together?

Oops! Yes the tie between V1a & V1b cathodes is missing. Thanks, I'll revise right away. How's the rest of it look?

[Wilder Amplification]

The way you have the input switching done leaves the stage input on the "a" side of the first 12AX7 floating when un-jumped. I would relocate the switch so that instead it grounds out the wiper of the "Normal" channel volume pot when you don't want the "Normal" channel incorporated. This way your input on that stage never floats and your pickup loading remains consistent.

[d95err]

As Wilder says, leaving the input of one channel open is a bad idea. It will pick up a lot of noise, amplify it at mix it with your signal.

Another way of switching is to tie the inputs together, use separate cathode resistor/caps and then use a switch to lift the cathode of the input triode you're not using. An advantage of this is that you can voice the channels differently by using different cathode resistor/caps.

I think there's a thread somewhere in the Tweed (or 5e3) forum where Bruce Collins of Mission Amps describes how to do this for a Tweed Deluxe style amp.

[Wilder Amplification]

As Wilder says, leaving the input of one channel open is a bad idea. It will pick up a lot of noise, amplify it at mix it with your signal.

Not only that but you're removing the grid's ground reference, which is part of the biasing of that stage since it is cathode biased. Without the leak resistor from the grid to ground you'd end up leaving the grid undefined, where it can no longer be negative relative to the cathode. With a 100K plate resistor it wouldn't burn out the stage since it's nowhere near its max dissipation, but it's considered bad practice to do that.

Good idea to lift the cathode on that stage...however if the OP prefers the sound of a shared cathode stage this obviously wouldn't work for obvious reasons.

[tubeswell]

Otherwise you could just turn down the volume control on the channel you don't want to hear.

I did a similar type of input (to what you are proposing) with a tweed bassmanish build, where I used a single set of 'Hi-Lo' input jacks, but in my one they were permanently connected to both V1 grids. Each stage has its own coupling cap, vol pot and mixing resistor as you have shown. The only other thing I did to mine was have separate Crk combinations for each of the V1 stages, so that if I want to I can put a different ck combinations on them for different channel blending effects (altho with 22uF/1k5 on each side, there is a quite noticeable jump in gain when you open up both channels).

[d95err]

If you want to simplify the controls (single input, single volume control), but still keep most of the sound options of 2-channel "Plexi" amp, I would do the following:

Separate cathodes, and differently voiced channels. Tie the inputs together. Use a dual-pot controlling both channel volumes. Use an on-off-on switch to ground the volume wiper of either channel. The switch gives you: Channel1 - Both - Channel2

[Howlin' Mad Mac]

Howdy ya'll & thanks for all the great advice. I can be a bit dense at times but I believe I've got the gist of what ya'll are saying & with that in mind here's the ol' schematic with revisions so lemme know if ya'll think this 1 will fly. Thanks.Club Classic - Schematic v2.pdf

[tubeswell]

Yep but that still doesn't have separate Ck/Rk for each side of V1. FWIW I think the most recent schematic of my amp is in this post. Tweed bassman with verb and trem - Episode 90 million

[Howlin' Mad Mac]

I'm having trouble understanding what the difference between putting say a 1k5 resistor & a 100mfd cap on V1a & V1b cathode or tying them together and let both cathodes share an 820r resistor & 220mfd capacitor would be. Please explain.

[Howlin' Mad Mac]

Also, & please correct me if I'm way off base here, but I was under the impression that when switched to the "normal" input that V1b pot would be my channel volume, V1a pot would do nothing then V2b "master tone" & finally "master" volume & when switched to the "jumped" input V1b pot would act as channel volume, V1a pot as gain/drive then V2b "master" tone & "master" volume. That is what I desire it to do, am I wrong? Please someone explain & help me to understand. Thanks ya'll.

[Wilder Amplification]

I'm having trouble understanding what the difference between putting say a 1k5 resistor & a 100mfd cap on V1a & V1b cathode or tying them together and let both cathodes share an 820r resistor & 220mfd capacitor would be. Please explain.

For one when you use a split cathode arrangement, each stage has its own dedicated current flow path and thus you don't have one stage affecting the other. On top of this, you can voice the stages differently by using different value cathode resistors on each of the two stages. You can have one be a "Normal" channel with an 820R/250uF combo, which would make the stage itself full bandpass, while making the other side say a 2K7/680nF combo for a brighter voice on the other stage. This is the way the later plexis were set up.

Also...with split cathode arrangement you can disable one of the stages simply via ungrounding its cathode resistor without having to switch the inputs at all. With the cathode resistor disconnected, no current can flow through the stage regardless of how much signal is present at the stage's grid because the stage circuit is no longer complete.

Also, & please correct me if I'm way off base here, but I was under the impression that when switched to the "normal" input that V1b pot would be my channel volume, V1a pot would do nothing then V2b "master tone" & finally "master" volume & when switched to the "jumped" input V1b pot would act as channel volume, V1a pot as gain/drive then V2b "master" tone & "master" volume. That is what I desire it to do, am I wrong? Please someone explain & help me to understand. Thanks ya'll.

This circuit is not a cascaded circuit like you think it is (i.e. where one stage's output runs into the input of the downstream stage). It is a "mixed" circuit where the two stages are paralleled. The input signal gets split to the inputs of the two stages, then the outputs of each stage are "mixed" back together to a single output after their respective volume pots via the 270K mixer resistors. Thus one stage cannot overdrive the other one if that's what you're thinking it's gonna do. The plexi circuit was never set up like that.

If you're looking for a cascaded circuit, look at the schematic for a 2203.

[tubeswell]

Hi Howlin' Mad Mac

Maybe are you looking for a switchable cascaded/parallel V1 arrangement like this? (called 'dumb idea' not because your idea might be dumb - but because I devised it a while back without testing it, and I don't know if it will work very well, what with the DPDT switch'n'all over the signal path, although if may well work if wired carefully) JM2CW

[Wilder Amplification]

Hi Howlin' Mad Mac

Maybe are you looking for a switchable cascaded/parallel V1 arrangement like this? (called 'dumb idea' not because your idea might be dumb - but because I devised it a while back without testing it, and I don't know if it will work very well, what with the DPDT switch'n'all over the signal path, although if may well work if wired carefully) JM2CW

I'm thinkin' this would work a little better and keep the parallel config as close to "late plexi" spec's as possible -



On yours I would consider at least placing a 10M pull down resistor on the first stage's coupling cap. Otherwise you break the DC AND the AC path to the coupling cap which can cause popping while switching. The 10M pulldown resistor ensures that the DC path to the cap remains complete while switching the AC path.