Single Ended Supro Thunderbolt?

[passaloutre]

Having finished one amp project and wanting more, I saw my Vox AC4TV across the room and said "well there's a chassis and two transformers!"

So let's say we marry the preamp from a Supro Thunderbolt with a single-ended EL84 power amp and a 10" speaker. Does it lose the magic? Does it become something else that's still cool?

It's not many resistors/caps to change if I keep the same PCB, but it'd be a lot more fun to come up with a turret board for it.

[sluckey]

Sounds like a fun little project. I say go for it. We want to see/hear what you come up with.

[HotBluePlates]

So let's say we marry the preamp from a Supro Thunderbolt with a single-ended EL84 power amp and a 10" speaker. Does it lose the magic? Does it become something else that's still cool?

It might become something else cool, but it won't be a Thunderbolt.  Not trying to be a downer, but that's my opinion having owned a '65 Thunderbolt up until last year.

The Thunderbolt's "preamp" is only a single triode plus Volume & single-knob tone control.  After that is a paraphase inverter.  You can keep the 1st triode of the inverter as a driver for your single-ended output stage, but at that point you have an altered tweed Princeton with some different component values in the preamp gain stages (which still might be worth a try).

The Thunderbolt doesn't display its character unless you're playing the loud onstage.  It's mostly about being a design intended to be clean, but that falls apart in a specific way.

In fact, my reason for selling (other than to have the cash for a 60s AC30) is that I'm rarely able to play that amp loud enough for its particular character to come out.

[shooter]

fwiw;


I married a "Fenderish" pre to a PSE EL-84, was liked as a "crunchy, Marshally" at home alone volume

[passaloutre]

Yeah I'm definitely not expecting the full Thunderbolt experience, but I figured it might be a more interesting preamp than the basic Champ, and with a single EL84 it won't be hard to overdrive the power section.

I'm attracted to the Big-Muff-style tone control and with the paraphase-type inverter, I don't imagine I'm losing any tone magic by dropping the second triode. But yeah, an EL84 and 10" speaker ain't gonna give me that push-pull 6L6 into 15" tone.

[bmccowan]

The Thunderbolt doesn't display its character unless you're playing the loud onstage.

I agree with this 100%. I have both a Thunderbolt and a Harmony 420 which is the Valco Thunderbolt in a different cabinet. They sound great cranked - not so much at lower volumes. Many years ago I built a Weber Smokin Joe 1. Its a SE EL84. I never liked it much, but you might. Worth a look. I converted mine to a SE 6L6 amp with a Framus mid control. Big improvement in my opinion.

[passaloutre]

What I'm getting from the replies is that the sound of the Thunderbolt is the cranked push-pull 6L6 power amp. Needless to say, I won't be getting that.

Much in the same way, the Vibro Champ doesn't sound like the AB763, though they share the same preamp.

But the Vibro Champ still sounds good.

I guess what I'm asking is, is the preamp of the Thunderbolt (yes, I'm including the first stage of the inverter, since I have the triode available) worth exploring?

I harbor no expectations that this will sound like a Thunderbolt, just wondering whether it would make an interesting preamp for a single-ended EL84.

Posed another way, the question becomes: if you had a set of transformers to support a single 12AX7 and a single EL84 in a 1x10 combo, what would you build?

[HotBluePlates]

I guess what I'm asking is, is the preamp of the Thunderbolt (yes, I'm including the first stage of the inverter, since I have the triode available) worth exploring? ...

Sure!  Give it a shot with an open mind.  Worst-case, you can change the value of cathode resistors to convert it into something more conventional.

[66Strat]

I agree with HBP that the sonics will be different. But, I do think that it's a good project. The Thunderbolt is louder because the output section is a pair of cathode biased 6L6GCs running in Class AB1 (about 20 watts before clipping), vs a single end EL84 in Class A (about 5 watts before clipping). The 6L6 output section also takes a lot more signal to push into overdrive. It would be an easy circuit to implement just by eliminating the inverting triode of the paraphase and the output tube fed by the inverting triode. You might consider a single end 6973 or 6CZ5 in lieu of the EL84. You might also look at the  Supro Model S6616 for inspiration.
https://el34world.com/charts/Schematics/files/Supro/Supro_s6616.pdf

[PRR]

Why not run a pair of 6Y6 at 150V for about 3 Watts? Or a pair of 6AU6 at 150V for about 3/4 Watt?

Yes, this may complicate your power supply: high voltage at low current for preamp, low voltage at high current for power amp.

[bmccowan]

I like PRR's 3 watt suggestion and will add that to my notes about a future low watt build.
But to your question as to what to build with the transformers you have (from an AC-4); I converted a single ended Bogen PA - CH8. I did not want another champ circuit, and I wanted to keep as much of the Bogen circuit as I could. Sockets are V1 - 7pin; V2, 3 and rectifier are octals. I used clues from the Matchless Clubman preamp, and some of Tubenit's projects, and Merlin's preamp book and used a 6AT6 to drive a 6SJ7, with a 6L6 power tube. 6AV6 can be used in V1 for more (too much IMO) gain and 6V6, or 6K6, can be used with slight adjustment. Jack Hester drew up the Bogen CH-8 original schematic which helped me figure things out. It definitely sounds different than a champ.
Of course you could do this same thing with an EL84 (I've tried but do not like this tube in guitar amps) driven by a 12_7 and a 9 pin pentode (5879 or EF86)

[shooter]

Is the Vox working?  if so, sell it for tranny money, start fresh 

[HotBluePlates]

... I saw my Vox AC4TV across the room and said "well there's a chassis and two transformers!" ...

Is the Vox working?  if so, sell it for tranny money, start fresh 

Even used, the AC4TV brings more money than an Epiphone Valve Jr, which has the same 2 transformers & chassis.  If you plug that Epi into a real speaker, it only gets better.