2x12AX7 + 2xEL84, any TW designs?

[Fischerman]

I have a 1x10" brown leatherette Gibson GA15 (Trace Elliot Velocette in slightly different clothing)...it has 2 each 12AX7 and 2 each EL84 in cathode-bias. There is only one input jack, one Tone, and one Volume plus a bright switch. The PT puts out a bit too high of a voltage...about 375vdc when idling. I'm thinking of gutting it and rebuilding it into something else (yet similar). Right now I'm considering either the Weber Smokin Joe II (1 each 12AX7 is in cascode then a Fender-type PI then the EL84s in cathode-bias) or the Marshall 20W circuit.

Did KF have any designs like this? Should I replace the PT from the get go? Any designs suitable for this that you could suggest? What about a DLM with parallel SE EL84s? Anybody ever find a schemo for the DLM?

Note: It's a very small combo so an EF86 is probably out due to potential microphonics issues.

[benoit]

There's the TNT, or "Two-knob trainwreck" which I think is the work of rfgordon.

https://tubeamparchive.com/viewtopic.ph ... hlight=tnt

Your HV tap on that PT might be a little hot for that though - he used one that was 345-0-345.

[Fischerman]

Thanks benoit. It looks like a Blues Express with that tone circuit (some sort of T-filter thingy looks like) instead of the regular tone stack. Maybe I haven't had enough experience with cathodyne PIs but I've never been able to get that great of an overdrive with them. I had considered basically exactly what that amp is except for the EL84s instead of 6V6s and using the dual-pot full tone stack like the GA15/Velocette have (schemo attached...it's basically a FMV (or Express) tone stack with a dual-pot controlling the treble and mids simultaneously (adds treble and scoops/cuts mids as you turn it up) with a fixed bass resistor (btw - the 470K fixed bass resistor is essentially 'bass on 10' and is too big...100K or so is about right IMO).

EDIT: Just remembered that schemo doesn't show the bass resistor or slope resistor values; bass resistor was 470K and slope was 100K.

[Fischerman]

And just in case somebody wants to mod their Velocette/GA15...they will probably find this very helpful. Note the incorrect cap values I found in my amp...that coupled with the 'bass on 10' resistor mentioned above and is it any surprise this thing was UBER-bassy when played through a 4x12? I only changed the PI input cap and left the other @ .22uF.

[Doug H]

You could build a Rocket and sub a MOSFET for the tone stack buffer. Check R.G. Keen's "Mosfet Follies" article at his GEO site for details,

[Fischerman]

Hi Doug,
Small world...it's Matt from the old Ampage board...we met at a gig of mine (you brought an early Pentode Driver) and then again at an EJ concert @ HOB. Hope you're doing well.

That sounds interesting...I'm not hung up on "100% pure all-tube signal path"...whatever works and sounds good.

[dartanion]

I like the TNT idea the best here, using EL84s. The concertina PI is pretty much pulled straight from a 5E3. The big difference here is that the PI will have no net gain, but the preamp will have plenty to drive the power tubes. You could also use a plexi style pre which would swap the clipping stage with a cathode follower and move the tone stack and volume after the CF.

[Doug H]

Hey Matt! How's it going? I didn't realize that was you.

The reason I suggested it is because I'm doing this kind of thing right now, only with a 12au7 instead of el84 output stage. I got a little tired of my Firefly and decided to redo it into something different. I wanted to keep the same tube/transformer complement and the same number of controls. That way I didn't have to drill the chassis any more and it could be a simple rewiring project. I haven't tried the MOSFET thing before, but I figured what the hell? The Rocket fit the bill and I had been curious to try it anyway. I still have to finish the PI and power sections. I'll let you know how it sounds when I try it out.

[Fischerman]

dartanion,
I still might do something like that...I'm a little familiar with the concertina. I've also tried running that PI fixed-bias instead of boot-strapped (my early SF Princeton is set-up like that now). But when it comes to OD I usually seem to prefer long-tail PIs. Maybe I just haven't heard a good example (very possible). However, these are EL84s which are more easily overdriven than just about any typical guitar amp tube I can think of so maybe it works better here.

Doug,
Thanks for the MOSFET tip (and I'm doing well). Just quickly scanning that article...there's a few places in this amp I could potentially use MOSFETs. To drop the B+, to drive the TS like you said...or to drive the power tubes better.

Since EL84s distort so easily...I wonder if using MOSFETs to drive them would be a good idea? Looks like they help prevent some of the bad stuff when we drive power tubes really hard.

Maybe a Smokin Joe II with MOSFETs driving the power tubes? Lots of options...I can see where this is going already.

[Doug H]

Yeah, I don't think you need any more drive for EL84's. I'm just using it as a replacement for the tone stack buffer, to drive the lossy voxy tone stack.

I have used them for setting B+ in two amps now and they work fantastic for that. I didn't use a zener like R.G. suggested, you don't really need strict regulation of the amt of voltage dropped since the PS isn't regulated anyway. I just used a simple voltage divider (suggested by Carl B. at ax84). I just sub a 1M pot for the big resistor and dial in B+ exactly where I want it. Then sub a fixed resistor for the pot and it's a piece of cake.

Check this schematic for an example:

http://www.aronnelson.com/gallery/d/181 ... st_sch.GIF

[Fischerman]

Doug,
I wasn't really trying to get more drive but rather to allow those MOSFETs to 'eat' the grid current (using terminology from the article) and to prevent the bias shift and grid current limiting. In short...to allow the EL84s to overdrive 'better'. Is that a potential here?

[roger]

This is my first post here so hello fellow tone addicts.

I drew up this for 6V6 as a start for further modification, it should work even better with EL84.
No, I dont think its a TW but one of the secrets with TWs has to be the balance between gain stages.

[Doug H]

Doug,
I wasn't really trying to get more drive but rather to allow those MOSFETs to 'eat' the grid current (using terminology from the article) and to prevent the bias shift and grid current limiting. In short...to allow the EL84s to overdrive 'better'. Is that a potential here?

Matt, I haven't read that far into the article in a while. I just looked up the part about the buffers. It almost sounds like a Class C thing, where the grids are driven positive, etc. I have no experience with that. I know some people have played with Class C in guitar amps and really like it. I remember back in the good days at Ampage, people were experimenting with Class C, ultralinear, etc. Time flies...