MegaPlex - Plexi meets ODS?

[llemtt]

Hi I am back again on amp building with a new project.

Actually it's a Plexi project so there should be poor relationship with dumbles but I would like to incorporate some ideas into it.

I would like to insert a switchable OD section between pre and poweramp, a la ODS, to add high gain sounds without cranking the power tubes, although with a different flavor.

So topology will be like something like that:

- plexi pre with cathode follower and tone stack
- switchable OD section
- fx loop? (maybe passive)
- master volume?
- plexi power amp (same as bluesmaster...)

Then came to mind that HAD builds something called MegaPlex, does anyone have info about it? Is it similar to what I would like to build?

Any help on the Plexi meets ODS subject would be appreciated.

thanks
Teo

[10thTx]

Is the type of idea you are considering? I haven't built this and don't intend to. Just drawing up what I think you might be saying?

I don't remember who it was but I think a guy on the Hoffman forum built something like this?

With respect, 10thtx

[Bob-I]

Cool idea. The only thing I'd change is that I'd have a bright switch instead of 2 inputs and the 2 triodes.

[llemtt]

Yes 10thtx, exactly that's the idea!!

However I think it's going to require some workout to behave as I would, especially the OD section. Plus some other ideas have to be incorporated as well.

@Bob

I am planning to have just one input with both channels jumpered, that's the key to a lot of plexi sounds obtained by carefully mixing the two inputs.

Ideally my ultimate amp should go like this
- fender/dumble clean
- plexi crunch
- ODS overdrive
- EJ plexi lead tone

how close can we get with 3 preamp tubes and a simple switching circuit?

[da Geezer]

However I think it's going to require some workout to behave as I would, especially the OD section.

Of course, it will "work"......I think you'll just have to build it & then tweak it on the fly....that's what I do. That's where the fun is!

Geezer

[Drumslinger]

why would someone want to get on a fly and tweak it?

I"M KIDDING!!

[Billion81]

Yes 10thtx, exactly that's the idea!!

However I think it's going to require some workout to behave as I would, especially the OD section. Plus some other ideas have to be incorporated as well.

@Bob

I am planning to have just one input with both channels jumpered, that's the key to a lot of plexi sounds obtained by carefully mixing the two inputs.

Ideally my ultimate amp should go like this
- fender/dumble clean
- plexi crunch
- ODS overdrive
- EJ plexi lead tone

how close can we get with 3 preamp tubes and a simple switching circuit?

This is exactly how I describe the tones I get from the 90s HRM EL34 amp that Tone Merc built. It has the Marshall Iron which I am not sure if they are custom wound trannies to Mark's specs or COTS PT/OT.

One of the cool things is the clean channel in to the FET with the gain cranked and a good 'bucker gets the Plexi crunch and then the OD side gets into the ODS Overdrive.

with the PAB and a strat it gets well into the EJ area particularly with the Strymon El Capistan dTape pedal.

I always struggled with my Marshalls to get the cleans I wanted and have all the OD and rather than go to a multi amp set up I use this amp.

Don't want to cool your jets on the Plexi/ODS build but this circuit gets and has a great clean platform for straight ahead jazz tones as well.

[10thTx]

Updated DRAFT idea

With respect, 10thtx

[FUCHSAUDIO]

Since you now have an extra triode section parallel them, halve the plate and cathode resistors and double the cathode cap value. Same gain, half the noise, a little more warmth....just sayin'

[renshen1957]

Since you now have an extra triode section parallel them, halve the plate and cathode resistors and double the cathode cap value. Same gain, half the noise, a little more warmth....just sayin'

I would recommend eliminating the Cathode follower and drive the EQ stack from the plate and be more effective. The EQ would be a bit more dynamic and you could use this 1/2 tube as one of the gain stages. Cathode followers help to reduce noise, but the Power Amp after the EQ is already in a feedback loop anyway however the OD section will have add more tube noise and the late EQ after the Cathode follower doesn't present much of a noise problem to quash. Cathode driven EQs are more typical of some Stereo Hi-Fi circuits of the 1950's and 1960's.

A better place for the Cathode Follower would be at the input of the feedback loop.

Best Regards,

Steve

[talbany]

AFAIK.. The tone stack is a filter network that is designed to ideally be driven from a zero source impedance to achieve it's proper frequency response....
The cathode follower provides a very low source impedance that allows the tone stack to work as designed. If the tone stack is driven from too large a source impedance, not only will there be a loss of gain, but there will be a different frequency response to the network, typically quite a few dB loss of the highs... The cathode follower prevents this loss, allowing the tone stack to retain more of it's theoretical frequency response...

So with this being said CF driven stacks are IMHO more dynamic..

A good example of this would be the Bluesmaster TS (very close to the Marshall stack) but without the follower.. As you will see in this post

https://tubeamparchive.com/viewtopic.php?t=13323

Most complain of the loss of high end and or very bass-y sounding.. This IMO is because the Bluesmaster is plate driven and the Marshall style is CF driven..BIG Difference!!..
The Downside
IMO Cathode follower driven tone stacks with preceding gain stages don't sound very good to me bright and grainy (2nd order harmonics).. Definitely not in the Dumble vein..

Tony

[10thTx]

Since you now have an extra triode section parallel them, halve the plate and cathode resistors and double the cathode cap value. Same gain, half the noise, a little more warmth....just sayin'

Couple more options to consider.

EDIT: Schematic needs a ground on the DPDT clean/OD switch

With respect, 10thtx

[talbany]

10hTX

If I were you I would think about adding a treble bleeder on V2B
As well as the 180k grid stopper V2B as well..

Just a thought..

Tony

[10thTx]

If I were you I would think about adding a treble bleeder on V2B
As well as the 180k grid stopper V2B as well..

That's a good idea. I do think that would be worthwhile.

I actually don't plan to build this amp. I was simply posting a schematic for discussion purposes so others could add insight and ideas. I was trying to put a picture with the comments people were making.

Thanks & respect, 10thtx

[talbany]

Cool!!
yeah I was thinking with the 56K/10k presence that thing will be pretty bright and open and in your face...The bleeder will give you some control in OD..

Looks Cool!!

Tony