18 watt tremolo cascading

[Bieworm]

hey.. I wonder something.
Suppose I want to give up the normal channel in my 18 watt tremolo for I don't use it...
Cascading V1 into the preamp half of the tremolo channel is a no go because it would make the oscillation have a lot of trouble. Is it OTOH possible to cascade the tremolo preamp triode into a triode of V1 to give it more drive at lower volumes? I'm looking for some more overdrive..not high gain.

[sluckey]

Is it OTOH possible to cascade the tremolo preamp triode into a triode of V1 to give it more drive at lower volumes? I'm looking for some more overdrive..not high gain.

That's possible.

But I would leave the tremolo channel alone and...

     ...rewire the normal channel as a TMB circuit. Lot more drive.

or

     ...replace the normal channel with an EF86. More drive, Voxy sound. (My choice)

[Bieworm]

Is it OTOH possible to cascade the tremolo preamp triode into a triode of V1 to give it more drive at lower volumes? I'm looking for some more overdrive..not high gain.

That's possible.

But I would leave the tremolo channel alone and...

     ...rewire the normal channel as a TMB circuit. Lot more drive.

or

     ...replace the normal channel with an EF86. More drive, Voxy sound. (My choice)

I understand, but I very frequently use tremolo. So I would like to have that on the switch. I have a single channel tremolo TMB already, but I wonder if I could gave a switch-in cascade on the classic...

[sluckey]

I did say that's possible.

[Bieworm]

I did say that's possible.

Could you hand me some clues or things I'd definitely should not overlook?

[sluckey]

I modified my 18W schematic to cascade the tremolo channel into the normal channel. You can see my original schematic on my website. I eliminated the input jacks and paralleled the triodes, connected R35 wiper to the grids of the triodes, removed the normal tone control, and grounded C18 (lower PI input).

If your amp is built on a turret board it will probably take less than 1 hour to do this mod. Gator clip leads allowed. This will give you the basis for your cascade idea. Tinker away, You may want to remove C11 and increase the size of R2 (maybe even to 10K cold clipper size). Heck, it may be just what you want without doing anything else.

[Bieworm]

I modified my 18W schematic to cascade the tremolo channel into the normal channel. You can see my original schematic on my website. I eliminated the input jacks and paralleled the triodes, connected R35 wiper to the grids of the triodes, removed the normal tone control, and grounded C18 (lower PI input).

If your amp is built on a turret board it will probably take less than 1 hour to do this mod. Gator clip leads allowed. This will give you the basis for your cascade idea. Tinker away, You may want to remove C11 and increase the size of R2 (maybe even to 10K cold clipper size). Heck, it may be just what you want without doing anything else.

Thx Steve. I'm going to give this a shot on the existing turretboard.  When it's tweaked to satisfaction I might build another (larger) board on which I might add the lamington reverb. For that I was thinking about using only 1 triode for the cascade mod and the other triode for the reverb preamp. For the recovery I can use the triode half of the 6bx6 I guess?
Noisewise.. should I put the 6bx6 betweet V2 and the PI tube? The PI tube on mine is V3. I will make a layout to make clear what I'm planning to do...

[sluckey]

Nice layout. What software? How does the mod sound?

[Bieworm]

Nice layout. What software? How does the mod sound?

no software; I borrowed the layout and edited it with MS paint ;)
the mod will be performed tomorrow. This evening I have practice.. so it will be late when I get home.

I have a question about the caps C1 an C18... what do they do exactly? Mainly the one to ground I'm wondering about. I connected the signal to the .022uf cap on the layout, because it's connected on my amp that way for the trem channel. I can not switch them easily, because I have a passive FX loop between C18 and the control grid on the EL84. On my Tremolo TMB it is a .1uf to ground.. so I wonder what the size of that cap affects.

Also.. is there a good 1-tube reverb you'd recommend? I like surfy reverb... I don't trust them 1-tube reverbs enough to give it a try. But maybe you are more acquainted with them?

[sluckey]

C1 and C18 are just coupling caps. They block dc voltage from the grids of the PI from getting on your pots. More important though, they prevent the pots from interfering with the bootstrap bias on the PI grids. If you decide to keep the mod I suggest changing C18 to .1µF. The cap to ground keeps the bottom grid of the PI at AC ground, preventing noise from getting to that grid. .1µF is most commonly used when the bottom input to the PI is not used.

I have a passive FX loop between C18 and the control grid on the EL84.

That's an odd place to put an FX loop. Are you sure?

I like surfy reverb...

Then you won't like any one tube reverb that I've heard. Personally, I don't think reverb is suited to the Marshall sound. Think about your favorite songs that used Marshall amps. Ever hear any verb?

I suggest to do the cascade mod and tweak it to your liking ***before*** moving on to reverb.

[Bieworm]

C1 and C18 are just coupling caps. They block dc voltage from the grids of the PI from getting on your pots. More important though, they prevent the pots from interfering with the bootstrap bias on the PI grids. If you decide to keep the mod I suggest changing C18 to .1µF. The cap to ground keeps the bottom grid of the PI at AC ground, preventing noise from getting to that grid. .1µF is most commonly used when the bottom input to the PI is not used.

I have a passive FX loop between C18 and the control grid on the EL84.

That's an odd place to put an FX loop. Are you sure?

I like surfy reverb...

Then you won't like any one tube reverb that I've heard. Personally, I don't think reverb is suited to the Marshall sound. Think about your favorite songs that used Marshall amps. Ever hear any verb?

I suggest to do the cascade mod and tweak it to your liking ***before*** moving on to reverb.

Ahhh silly me. Not the EL84... the PI tube ofcourse.

Big reverb is a must for my music. I have built that lamington reverb unit and run it through the effects loop. Sounds awesome.. even through a marshall. 
Here's a clip with that reverb through my classic tremolo (18watt)

https://www.dropbox.com/s/wxfty9ucjbj9gzr/18W%20modern%20classic%20with%20reverb%20unit.m4a?dl=0

[sluckey]

I've revised the schematic to include a better switch. The circuit now looks like Hoffman's Hot Switch mod on his Plexi50.

[Bieworm]

I've revised the schematic to include a better switch. The circuit now looks like Hoffman's Hot Switch mod on his Plexi50.

You used only 1 triode this time for the cascade.  I figured 2 would be too much.. is that your intention..to keep it a little smoother?

[sluckey]

My intention with the original drawing was to change as little as needed to give you a working cascade stage that you could quickly mod and play with to see if you even want to pursue this idea. That's why I left the triodes in parallel.

My intention with the revised drawing was to free up one triode since you mentioned reverb. Two parallel triodes will only have slightly more gain than one. And I suspect you will be trying to shed some gain, hence the voltage divider resistor. I also included a better switching option. Your switch circuit left the cascade triode grid connected to nothing when in low gain mode. That's a bad idea.

The revised circuit should work with no tweaking.

[Bieworm]

My intention with the original drawing was to change as little as needed to give you a working cascade stage that you could quickly mod and play with to see if you even want to pursue this idea. That's why I left the triodes in parallel.

My intention with the revised drawing was to free up one triode since you mentioned reverb. Two parallel triodes will only have slightly more gain than one. And I suspect you will be trying to shed some gain, hence the voltage divider resistor. I also included a better switching option. Your switch circuit left the cascade triode grid connected to nothing when in low gain mode. That's a bad idea.

The revised circuit should work with no tweaking.

I have a similar voltage divider in my tremolo TMB too. It's very useful!!! Thx for saving me a triode. With the 6bx6 's unused triode I might be able to implement that lamington reverb. I'm not sure if that triode is going to work like 1/2 12ax7 though...

[DummyLoad]

With the 6bx6 's unused triode I might be able to implement that lamington reverb.

say what? 6BX6 is a single high slope RF pentode.

--pete

[Bieworm]

With the 6bx6 's unused triode I might be able to implement that lamington reverb.

say what? 6BX6 is a single high slope RF pentode.

--pete

Right! I think  I've expressed myself wrong  ... but anyway,  I'm sure I've read about somebody used part of that pentode for the reverb recovery after the tank in a constant current driven reverb..a I dreaming? 😃

[Bieworm]

concerning the 1-tube reverb... I own a Marble Clubreverb. That's a Dutch custom built amp. 2x6L6 powersection. 3x 12AX7 preamp. I think V1 is preamp and tonestack driver. V2 reverb. V3 PI.
That is one kick@$$ good reverb!!! So that indicates that there is at least one 1-tube reverb I really like.
I opened her up and tracked down the circuit around the reverb tube, the reverb TX and the tank connections. Here is the schematic I came up with while tracing the circuit.
There is NO schematic of that amp available online.

Is what I drew right? Am I missing stuff here? Or can I use that schematic?

[Bieworm]

I've revised the schematic to include a better switch. The circuit now looks like Hoffman's Hot Switch mod on his Plexi50.

The cathode cap on V1 is obsolete? Shouldn't there be a .68uf cap parrallelled to the 2K2 resistor?

[sluckey]

The cathode cap on V1 is obsolete? Shouldn't there be a .68uf cap parrallelled to the 2K2 resistor?

No. Look at Hoffman's Plexi50...

     https://el34world.com/Hoffman/files/Hoffman_Plexi50.pdf

If you find you need a little more gain put in a cathode cap.

[Bieworm]

The cathode cap on V1 is obsolete? Shouldn't there be a .68uf cap parrallelled to the 2K2 resistor?

No. Look at Hoffman's Plexi50...

     https://el34world.com/Hoffman/files/Hoffman_Plexi50.pdf

If you find you need a little more gain put in a cathode cap.

thx.. I found it in the mean time.. so many schematics to look into. But I found the one you meant a few minutes ago..

Have you seen the last post on the reverb subject? Any remarks ?

[sluckey]

I saw your last reverb post. Not enough info to comment on.

But, one thing at a time. Have you done the cascade mod yet?

[Bieworm]

I saw your last reverb post. Not enough info to comment on.

But, one thing at a time. Have you done the cascade mod yet?

not yet. This evening I will do it. I'm still at work... it's 1:30 pm here in Belgium ;-)

[Bieworm]

Did the mod. But I don't like it...
I tried your schematic. Too low drive and more noise. Thin sounding...
Parallelled the triodes of V1. Same but more drive.. thin too
Added a .68 cap parallel with the 2k2 resistor.. naaah
Should I tweak further or put it back to stock?
I mean, was this to be the expected outcome?

[sluckey]

Jumper the 470K voltage divider resistor. Still don't like it? Put it back to stock and put an EF86 in the normal channel.

[Bieworm]

Jumper the 470K voltage divider resistor. Still don't like it? Put it back to stock and put an EF86 in the normal channel.

No... it's all right. I played with the pots some more. I didn't expect the volume pot on the trem channel to be so significant. When I turn that one on 8 and the gain on 2 it's fat and creamy...
What would lowering the 2k2 resistor to 1k5 do? Don't that add a little gain? I remember taming my tremolo TMB with going from 1k5 to 2k2 on the cathode. And is it bad to run the triodes of V1 in parallel without a cathode cap? When I put in a cap its z lot noisier...so I would want to skip that one...

Sorry for jumping to conclusions too soon. I don't mean to be ungrateful. You're helping me a lot Steve!!! Thx

[sluckey]

You're at the point we call tweaking. Play with the value of the cathode resistor. Play with different value cathode caps. Play with the value of the 470K voltage divider. Put a 50pF to 500pf smoothing cap across the plate resistor. It's all good. Have fun. You're not gonna hurt anything. And remember... you can always go back to stock if you don't find the overdriven sound you seek.

My 18W is LOUD. This mod may not give you a bedroom level overdrive sound. Might need to add VVR or a speaker power attenuator for that.

 

[Bieworm]

You're at the point we call tweaking. Play with the value of the cathode resistor. Play with different value cathode caps. Play with the value of the 470K voltage divider. Put a 50pF to 500pf smoothing cap across the plate resistor. It's all good. Have fun. You're not gonna hurt anything. And remember... you can always go back to stock if you don't find the overdriven sound you seek.

My 18W is LOUD. This mod may not give you a bedroom level overdrive sound. Might need to add VVR or a speaker power attenuator for that.

Yes thank you.. let the journey begin :)
It's not bedroom level I seek. But at practice I have to put the volume between 2 and 3. I'd like to have some smooth break up at that level.. it's already pretty loud.. but too clean for my style.
Did the mod without the switch for now.. I'll put in that switch when it's on target. I never use the normal channel anyway. So I won't miss it. Tremolo is something I use on 30% of my songs.. so I don't really have the choice playing that normal channel.

[sluckey]

When you get it tweaked let me know what you think. I may just modify mine. 

[sluckey]

BTW, Hoffman's Stout is based on the normal channel only 18W. It includes the optional hot switch mod. Take a look...

     https://el34world.com/Hoffman/files/Hoffman_Stout.pdf

[Bieworm]

Steve.. when I nearly dime the volumepot on the tremolo channel the sound gets really trebly. My guess is that the PI gets full blast from the tremolo preamp so the tonestack has little effect on that. I was thinking about reconnecting the tonepot on V1 to tweak the frequencies before the tremolo preamp. Is this logic thinking?

[sluckey]

yes

[Bieworm]

yes

It's after the tremolo preamp, gosh it's confusing due it being V1 ;)
Makes sense to shape the tone after preamp stages... with the volume pot on the first preamp stage dimed (tremolo) it nearly cancels out the tonepot so I have to shape tone right before the PI. At least that's what I think happens as far as I understand the amp's circuit...

[sluckey]

I like the tone control on the tremolo channel much better than the tone control on the normal channel. I would use the tremolo channel tone control on the cascade circuit.

[Bieworm]

Added the tone control per schematic.
I separated the cathode resistors of both triodes. So 1 goes to ground through the standard 820R resitor with a 47uf bypasscap. The other C goes to ground through only a 2k2 resistor. This amp rocks now.. even on low volume. So if I want clean I turn the trem preamp to vol 2 and cascade vol to 4. If i want crunch I turn the trem vol to 8 and the cascade preamp to 1.5. Nice thing is it still has all the touch sensitivity so with light picking it's also clean
Will make some sound clips later and post them. The amp is awesome Steve!!!
I'm gonna put it in the shell and play it for a while.. see if I keep liking it. It doesn't sound like I should be tweaking it more.. but who knows after practice I might think differently.

[Bieworm]

I like the tone control on the tremolo channel much better than the tone control on the normal channel. I would use the tremolo channel tone control on the cascade circuit.

Did that first. The interactive control with tone pot wiper to vol pot wiper with a 100R resistor. That was really no good!! The volume jumped from nothing to loud as hell.  Tonepot wiper via 100R to ground was the golden ticket!!!

[Bieworm]

Here's the adapted schematic for those who want to try it too. I'm pretty happy with the mod.

[pdf64]

Looks good!
Note that the fx loop needs to be the other side of C2, as LTP grids are at an elevated Vdc.
That standby arrangement is a hot switching type, which will put excessive stress on the rectifier. FYI even if cathode stripping (ie the usual justification for standby) was an issue (which it isn’t), a tube rectifier (especially an indirectly heated type) would mitigate for it.

[Bieworm]

Changed the tone circuit of the cascade preamp. It was too dark after all...

[sluckey]

I thought you would prefer the tone circuit from the tremolo channel better. I know I do.

[Bieworm]

Sound sample:

https://www.dropbox.com/s/czn6pj3s4hejith/Modern%20classic%20cascade%20mod.m4a?dl=0

Sorry for the people who think reverb is not compatible to marshall type amps.. I prove the opposite here!!!
Don't know what you guys think, but I think that's a pretty nice preamp overdrive here. In real life it sounds more overdriven than the recording. And more trebly for that matter..

[DummyLoad]

With the 6bx6 's unused triode I might be able to implement that lamington reverb.

say what? 6BX6 is a single high slope RF pentode.

--pete

Right! I think  I've expressed myself wrong  ... but anyway,  I'm sure I've read about somebody used part of that pentode for the reverb recovery after the tank in a constant current driven reverb..a I dreaming? 😃

not dreaming: 2deaf commented on this article in another thread long since buried.

http://www.channelroadamps.com/articles/reverb_driver/

article discusses use of a 6GH8 for the driver pentode, it's paired with a medium mu triode,  the 6CM8 and/or 6JW8/6LX8 might be better choice since they are also high rp high gm pentodes, however, they paired with a high mu triode, which is typically a better choice for a reverb recovery stage.

IMO this all moot: none of these types of pentodes are being manufactured any longer, so when the current NOS supply dries up, you'll be looking for a SS solution - may as well just use op-amps in that case.



--pete

[Bieworm]

With the 6bx6 's unused triode I might be able to implement that lamington reverb.

say what? 6BX6 is a single high slope RF pentode.

--pete

Right! I think  I've expressed myself wrong  ... but anyway,  I'm sure I've read about somebody used part of that pentode for the reverb recovery after the tank in a constant current driven reverb..a I dreaming? 😃

not dreaming: 2deaf commented on this article in another thread long since buried.

http://www.channelroadamps.com/articles/reverb_driver/

article discusses use of a 6GH8 for the driver pentode, it's paired with a medium mu triode,  the 6CM8 and/or 6JW8/6LX8 might be better choice since they are also high rp high gm pentodes, however, they paired with a high mu triode, which is typically a better choice for a reverb recovery stage.

IMO this all moot: none of these types of pentodes are being manufactured any longer, so when the current NOS supply dries up, you'll be looking for a SS solution - may as well just use op-amps in that case.



--pete

Thx pete! But I'm leaning more towards the 1 tube reverb from my marble clubreverb. It's a really good and lush reverb , transformer driven and a standard 12ax7 for preamp and recovery.  When I have the itch I will figure that one out for the scratch. For now the lamington will do.. I'm pretty satisfied with it. I built it in a matching 2x10 cab flr my classic 18W

[Joobacz]

Sound sample:

https://www.dropbox.com/s/czn6pj3s4hejith/Modern%20classic%20cascade%20mod.m4a?dl=0

Great sound! I am thinking about building this amp. I like the idea of ​​a single channel Marshall with tremolo. Could you share a sound sample with the tremolo on? I'd like to hear how it beahves when cascade stage is on/off. Thank you

[Bieworm]

Sound sample:

https://www.dropbox.com/s/czn6pj3s4hejith/Modern%20classic%20cascade%20mod.m4a?dl=0

Great sound! I am thinking about building this amp. I like the idea of ​​a single channel Marshall with tremolo. Could you share a sound sample with the tremolo on? I'd like to hear how it beahves when cascade stage is on/off. Thank you

I wouldn't go through the hassle anymore. There are way better single channel 18W marshall type amps. My favorite amp is the Tremolo TMB, an amp I've built like 6 or 7 times since. Waaaayyyy better amp.
Check my YT page for clips

(my most recent one)

(the prototype)