adding an OD section to an amp

[darefugee]

I'm considering adding a Dumble-esqe single-tube OD to a Plexi 6V6. Anyone tried anything similar before, maybe can discuss pitfalls I should beware of? My plan is to put it in between the tone stack and the P/I, make it switchable in-out with footswitch and/or switched OD LEVEL pot. I'm wondering if three stages (two gain plus the cathode follower) before the OD is already too much, considering the ODS has only two leading stages.

[romberg]

If two gain stages are good three must be even better and four better still. The ODS has four gain stages. Two in the normal channel which is then fed into the OD channel which has two additional gain stages.

The marshall plexi has two gain stages and a cathode follower (gain = 1.0) driven tone stack. The 2203 bumps this up to three gain stages feeding the cathode follower. And a hot-rod marshall (such as the famous S.I.R. amps) used four gain stages before the cathode follower.

A few years back I built my first marshall with help from others on this site. It is relay switchable between a 1959 (plexi), 2204 and S.I.R. #39.

http://ampgarage.com/forum/viewtopic.php?f=9&t=25084

https://github.com/mromberg/cerberus

The tricky bits are having enough real estate to fit the extra components and tube sockets. Relays also take room. Personally, unless you have an existing amp with lots of room to add more gain stages, I would do this as part of a whole new amp you can design to fit everything you will need.

It might be a whole lot easier to convert a plexi type circuit into a 2204 (two to three gain stages). Here you don't add any new tubes. But recycle the dark channel into a cascaded gain stage after the bright channel. This could even be made switchable.

Mike

[darefugee]

It's a new build in a Weber5E3X2 chassis. Plenty of room for what I want to do. I'm really fishing for information about whether the OD section is known to play nice in amps other than the ODS - which amps sound good this way, which amps don't play nice with the OD, that sort of thing.

[RJ Guitars]

I am in the middle of a project that involves adding a Dumble style overdrive to a Trainwreck Songwriter amp. This adds a nice switchable overdrive to one of the best sounding clean amps and includes a footswitched relay to go back and fort between the two channels. I am about 90% done with the design work and hope to build the amp before too long...

Attached is the prototype overdrive design...

[10thTx]

I've built a number of different versions of the D'Mars ODS amp. There is a schematic and layout on the Hoffman forum for it along with a bill of materials and a turret board available in his parts store.

It's sort of a Marshallish topology with a Dumblish overdrive .......... leading to the D'Mars name. I built it cathode biased but it would be easy to build it fixed biased.

http://el34world.com/Forum/index.php?topic=14302.0

Using a 300-0-300 150ma PT, I can use a 5Y3GT for 330v on 6V6's, or 5V4 for 360v on 6v6 or 6L6/5881, or GZ34 for 390v on 6v6 or 6L6/5881 or solid state rectifier plug in for 420v on 6L6/5881 tubes.

While this one uses a mosfet cathode follower and a 5879 tube in the OD channel, I've drawn up other versions without the mosfet CF and using 12A_7 tubes instead. Since I've drawn those in ExpressSCH, the schematics and layouts are editable and you can change the component values to what you want.

with respect, 10thtx

[10thTx]

Here are a couple more schematics along with ExpressSCH editable versions. You can use a 5751 in V1 with paralleled triodes for 30% more gain with no increase in floor noise.

With respect, 10thtx

[pompeiisneaks]

Interesting stuff, I'm thinking of doing a very similar thing to a Vox build I'm frankencobbling together with a top boost channel and an ef86 preamp channel that I'll allow to be footswitch jumpered together (I'm adding another half stage to the ef86 to drive a tone stack there so as to also get it back in phase with the Top Boost channel) and then I'll be adding in a dumble style switchable OD as well, I've got the first draft of my schematic done, and I'll then attempt to create a layout for it, but this gives me hope

~Phil

[10thTx]

I've heard very mixed reviews about trying to use EL84's in amps with the d-style overdrive. The EL84's overdrive fairly easily as is. I've heard more favorable reviews with 6V6's if you're looking for a lower wattage amp like that. You could consider two EL84's and two 6V6's working together instead of a quad of EL84's.

With respect, 10thtx

[pompeiisneaks]

Interesting, are you saying that the combination of the OD from the 12AX7's doesn't mesh well with the el84's? Does half and have on the power stage tend to blend it better? Sounds like a very interesting idea. Would you just keep the resistors and such identical for the tubes based upon their expected use and pair up one 6v6 with one el84 per half of the transformer?

~Phil

[10thTx]

Most of the dumblish inspired amps seem to get their overdrive in the preamp section. In contrast, many of the 18 watt EL84 amps get their overdrive from the overdriven EL84 power tubes. Combining and overdriven preamp AND an overdriven power amp may not give you the clarity you want in the overdrive?

I'm not advocating a 6V6 and EL84 quad power section, just mentioning that as a consideration. I'd have it cathode biased for the EL84 pair AND cathode biased for the 6V6 pair. You could consider "turning off" one pair or combining both.

I think the Egnater Rebel 20 does something similar to this? And maybe the Mesa Boogie Blue Angel also does something like this?

With respect, 10thtx

[TUBEDUDE]

At any rate, I highly recommend overdriving a 5879 for a preamp dist tone over a 12ax7. The overdriven pentode sounds much better than a triode.

[pompeiisneaks]

All very cool ideas and now I'm trying to figure out the best way to deal with it, thanks! (And sorry for the threadjack, although I think it's still completely in line with the original thread idea, I'm using it myself too

~Phil

[MakerDP]

At any rate, I highly recommend overdriving a 5879 for a preamp dist tone over a 12ax7. The overdriven pentode sounds much better than a triode.

An _easy_ way to pentode preamp overdrive to an existing amp without drilling for a new tube is to use a 6U8A triode/pentode. But yeah I like the 5879 sound as well.

[MakerDP]

Back to specifics on the OP... replacing the cathode follower with an IRF820 and using the triode for the extra gain stage is easy. You may want to add a gain pot in front of it so you can "dial it in" and make it switchable so you can have original and extra gain in the same amp.

[ChopSauce]

Yet a poster already noted that adding power amp distorsion to a Dumble-style OD might not suit:

- has anyone already attempted to mate an OD section to a cathode biased 6v6 push-pull power section "à la" 5e3