Marshall-Vox-erized with some Fender on top....?

[dehughes]

So, what are your thoughts on these ideas for a build?

1) EF86 preamp (essentially an AC15 preamp) into a long-tail PI into a cathode biased 5881 pair.

2) JTM-45 preamp into a split-load PI into a cathode biased pair of 5881s.

or?

3) EF86 preamp into a full tone stack then into one 12AX7 triode (gain recovery?) then into a split load PI and off to a cathode biased 5881 pair.

I'm still knocking around ideas given my 2nd hand bare chassis (cut by someone else for an Orange Retro 50 clone) and while I want to go straight 6G6-B or JTM-45, I keep wanting to experiment...but I don't have enough experience with amps to really get a good idea of what this would sound like a priori. I don't need or want headroom so much as I want a smooth, earlier transition from clean to overdrive all the while having a "big" sound.

What do you think?

[Andy Le Blanc]

I've tried "1". I used the simple vol. and tone for its impedance and utilized
a presence and resonance in the feedback loop as extra controls, worked nice.
You'll want to start on the power side and bias the 6l6 HOT!!! and use a large
value bypass cap, gets the phat mushies out with a lot of color.

[dehughes]

I've tried "1". I used the simple vol. and tone for its impedance and utilized
a presence and resonance in the feedback loop as extra controls, worked nice.
You'll want to start on the power side and bias the 6l6 HOT!!! and use a large
value bypass cap, gets the phat mushies out with a lot of color.

Cool. I figured someone had to have tried at least one of those options.

Two questions:

1) How hot on the 5881s?

2) What do you think a split-load PI would do to a JTM-45 circuit?

[Andy Le Blanc]

1) How hot on the 5881s?

HOT

max plate plus max grid #2 dissipation ratings for whatever type.

I've seen several old and working amps, alamo, wards, military all run at max ratings.
Don't be shy with it, its the most reliable way to run a power side.


2) What do you think a split-load PI would do to a JTM-45 circuit?

Fender tried a design close to that but used feedback around the follower.
5E5-A "pro-amp" ..... 5F4 "super-amp".... 5E8-A "twin-amp"
I've seen the same in other makers, but with a 12au7 in the PI.
The usable headroom is an issue that PI type only gives so much.

[dehughes]

Thanks Andy. Three more:

1) What I meant in using a split load instead of a long tail PI in a JTM-45 was that one would run the tone stack right into the PI. So, the Fender schematics you listed aren't quite what I was thinking. I was thinking triode -> triode -> cathode follower -> tone stack -> split load PI -> power tubes. Is that doable?

2) Why is running a power section super hot "the most reliable way to run a power side"?

3) I suppose my question regarding the split-load PI in a JTM-45 circuit had to do with the tone/vibe of it in place of a long tailed pair. So, the split-load would not drive the power tubes as hard, but would itself distort earlier than a LTPI? Would there be any tonal difference between the two or is it only a question of gain/drive, etc.?

[skeezbo]

dehughes

I built something similar to number 2 a few years ago.
I copied the tweed Bassman preamp and the tweed Super PI. Channel 2 has a Marshallish 2.7k Cathode resistor bypassed with a .68 cap.
It has Mercury Magnetics Super Reverb or Bassman iron, I can't remember, and no negative feedback. It has an Eminence Legend ceramic 12" speaker.
I have a 12DW7 as V3, with the 12au7 side as the split load and KT66 power tubes, but fixed bias. It sounds great and has plenty of clean headroom with a 6072 or 12ay7 in V1. I play mostly clean, and It works great with the few OD pedals I have tried in front of it. With a 12ax7 in V1 it does lean more toward early breakup. It rages if you turn it up.
I'd like to try some of those reissue Tung-Sol 5881s; cathode bias would probably sound great, too.

Skeezbo

[dehughes]

Very cool. Do you think the split-load PI messed with things much, compared to if you'd thrown in a LTPI?

[skeezbo]

dehughes

I would need to build it both ways to know for sure what the difference is. The mids seem less focused, somehow, than on some amps I have owned, and a Sunn Solarus that I had back in high school (also a split-loader) had a similar softer, sweeter midrange, if I remember correctly, so maybe that is it. The Solarus was different in a lot of ways, though, so who knows.... The '59 5E3 that I used to own, also a split-load, did not have the silky mids of the other two, but the tone control on that amp is very different, too, so maybe it is not a good comparison.
I bet LTPs drive the power tubes harder if you want that kind of sound, and I know some have complained the split-loads sound "fizzy" when they break up.
I like it the way it is, but I have read about other guys who have replaced split-loads with LTPs and been happy they made the change. I guess the only way for you to know for sure is to build one.

Skeezbo

[dehughes]

Thanks skeezbo. I appreciate the info. I suppose what I need to do is hear from someone who's built an amp both ways.....LT pair and split-load. The only way to really speak to the difference would be to keep most other things equal but the PI...

I like the idea of a sweeter midrange, as that's the thing I'm always fighting, it seems. Harsh mids.... I can get the airy top end down, and the low end thing as well....but finding that rich, cutting yet not harsh midrange tonality right before breakup is beyond my skill level at present.

[skeezbo]

dehughes

A buddy was over last night with his '60 ES 345, and he plugged into the Fender channel in that amp. It sounded, well, very much like you describe in your last post.

Have you ever spent much time playing a Princeton Reverb? If you like that vibe, but need to be louder, you should build yourself a plan #2 pronto!

Skeezbo

[dehughes]

dehughes

A buddy was over last night with his '60 ES 345, and he plugged into the Fender channel in that amp. It sounded, well, very much like you describe in your last post.

Have you ever spent much time playing a Princeton Reverb? If you like that vibe, but need to be louder, you should build yourself a plan #2 pronto!

Skeezbo

OOOH! Yes, a Princeton Reverb is basically my ideal SG amp....much like my Top Hat Club Royale is my ideal Tele amp.

I was playing my SG through my AC30 creation, and the EF86 side wasn't doing it at all for me...but the Top Boost side was just killing it! I think the EF86 rocks with my Tele, but for sure, I'm thinking my SG just plain works better with either octals (6SN7) or 12AX-something tubes.

I suppose my only fear is that in building a JTM-45 with cathode biased power tubes and a split-load PI, I'd be getting half of something instead of a cool synthesis of two great amps, you know? But, really, only one way to find out...