Selmer Corvette vs. Champ?

[Steve Conner]

Hi all

I'm in the middle of modding/restoring an old Selmer Corvette. This was a 3 watt SE amp, a combo with an 8" speaker like the Champ.

But how like the Champ really? I've drawn out the schematic, and I wonder if any of the old hands (MWJB etc) could comment. If I rewired the guts as a Champ, would it sound better?

Some things I noticed:

The cathode resistors on the preamp tubes are very large: 5k. My feeling is that Selmer did this deliberately to make these stages generate lots of 2nd harmonic distortion, and give the illusion of more bass. When I had the amp working, it certainly seemed to have a super-dirty tone with no clean headroom at all.

The OT was minuscule. If I put a larger one in, I liked the tone better, but the tremolo made the speaker cone flap in and out violently.

The speaker was a 3 ohm Celestion similar to ones I've seen in table radios. It sounded good at low volume but would flab out very easily.

The power tube is an EL84. I'd like to stick with the EL84 because I have a half dozen good quality ones to use up. I've only got two 6V6s and they're Russian.

Or should I get a new OT and rewire the whole thing as an 18 watt Marshall?

[Bruce / Mission Amps]

I wonder how much attenuation of the signal there is from the trem circuit when it is not engaged.
The cathode bias voltage of the EL84 is probably very low, (7vdc-10vdc) compared to a 6V6 and the preamp stage topology looks like it has enough gain to bang the bejeezus out of the EL84 grid.
I suspect the 5k cathode biasing resistors might also be to keep gain down a bit
I have always found that a larger OT in any SE 6V6 amp sounds better then the usual stock weanie OT used.
I also have never heard a SE 6V6 Champ that could make more then about 1.9w to 2.5 watts unless it was a tweed when they used larger OTs in them.
Even then they rarely would go more then 3.2 to <4 watts output.
The black and silver face Champs are a joke when it comes to clean output power.
The same 6V6 amp with a good 10-15 watt SE 5K-6K OT will hit about 4.7w to 5.7w cleanish.
I had an old AC-4 here a couple years ago and the little stinker would do an honest 3 watts... it sounded killer into a 1x12 cabinet

[Steve Conner]

Hi Bruce

The tremolo signal has a 470k resistor in series, so the worst-case loading is about 300k with the intensity knob full and the tremolo turned off.

It does indeed "bang the bejeezus" out of the EL84, making some pretty full-on distortion. It's fun to play, but I wondered if a 6V6 might make it a bit more subtle.

The AC4 looks like a nice circuit, I have a couple of old EF86s so I could try it out.

As for the power output, I've never measured it. It was always too loud to crank at home and not loud enough for a gig.

[MWJB]

It's only really like a champ because it's SE & lo power.

That 250vdc B+ is a bit low for a 6V6...I'd stick with the EL84.

"If I put a larger one (OT) in, I liked the tone better, but the tremolo made the speaker cone flap in and out violently." that might not be too unusual (I have couple of Triumph Leos, they do this noticably, if not violently)...a smaller coupling cap to the power tube might reduce this, as long as it doesn't detrimentally affect tone. If this flapping only happens at extreme intensity settings, tweak the intensity control...but you may have to live with some visible movement.

If you reduce the value of the 12AX7 cathode resistors, you may need to also reduce the value of the 47K power supply dropping resistor, to restore decent plate voltage. As Bruce says, I think these values were used to keep gain down...not that it worked.

I think that the super dirty tone & no clean headroom may be more down to the lack of attenuation between the 2 halves of the 12AX7 & lowish preamp voltages (what do you get on the preamp plates?), lack of a bright cap feeding the tone pot, around the vol control - maybe try a voltage divider after the input triode's coupling cap, followed by another coupling cap, so as not to load the intensity control...I'm not keen on having the tone & volume both located after the 12AX7 (if any headroom at all is desired), so I'd probably rewire them to go between the 2 halves of the 12AX7, like 5F2A.

I'd also maybe try removing the 2nd preamp stage cathode bypass cap (4700pf)? Add an NFB loop?

Not sure that the PT would take 2xEL84, as regards the 18W plan? If the OT is under designed I'd guess the PT isn't far behind (some old British SE amps have well underrated PTs that struggle to support the stock tube compliment!).

I think it's salvagable, with a little tweaking, my Leo's are more like Vibrochamps (treb & bass controls) running EL84 and no NFB, I wouldn't swap them for a champ (they don't give away anything in the volume stakes)...I love champs but trying to make an EL84 amp, running 250vdc, into a champ is too much like banging a square peg into a round hole for me.

[Steve Conner]

Hey, thanks for the input MWJB!

I suppose I'd better give a bit of background. I picked this amp up in a junk shop. The construction quality is pretty cheap, it has all the tubes on a PCB dangling off the front panel, and a cheesy hardboard back like you might find on a table radio. The cabinet is plywood, but it's thin. The PT is screwed to the bottom of the cabinet and connected to the rest of the circuitry with a long bundle of wires.

Selmer Futurama Corvette Amplifier - 2nd Series

Different cosmetics, smaller OT, but seems to be the same general idea as the Triumph Leo.

I used it as a practice amp for a while, recapped it, replaced the resistor in the power supply with a choke, and swapped the farty speaker out for an Eminence Beta 8 I had lying around. Along the way I accidentally put a huge rip in the ugly grill cloth, and replaced it with an equally ugly piece of wire mesh.

Then I used it as a guinea pig for an idea I was working on, a solid-state power booster for SE amps. This worked really well, putting out enough power to endanger the poor Eminence, but unfortunately once the experiments were done the amp was left completely dismantled. I cleaned up the empty cabinet, and that's where I am now! An empty cabinet that I could build anything into.

In other words, I've wrecked it! :-( I apologize to any vintage Selmer fans in advance.

The PT was small, but I have a bigger one, a Partridge 300-0-300 from a piece of lab equipment. Unfortunately it's a "table topper", with a tagboard on top covered in bare live terminals, so it has to go inside something for safety.

[Wakculloch]

I have a TV8/T, that has a very similar layout except the first valve was a EF86, so it has even more gain, which i love, and there was a three stage RC filter after the tremolo modulator.
As your model has no filter, perhaps they put in a wee OT to compensate and filter out the tremolo.
In my experience of old Selmers, none of them had much low end, some were adorned with stickers telling you not to use bass guitars; there seemed to be a general paranoia that bass would destroy they're speakers. Perhaps they had a rash of warranty claims or something.
Mine did get changed some years ago to the layout of a Zodiac, with the ECC83 in the first position, cos with all that gain the noise from the EF86 was a bit too annoying.
Hope your project goes well. Personally i love these little amps because they dont sound like Fenders.
Useless for playing blues or jazz, great in a punk band.

[Steve Conner]

Useless for playing blues or jazz, great in a punk band.

I think that just about sums it up! It was a real ankle-biter of an amp. Maybe it deserves to be put back that way rather than becoming yet another Fender clone?

The bass guitar thing is interesting. My old band shared a practice space in Glasgow with another band who had loads of old Selmer gear. Including a 1x 18" cabinet that sounded like a massive elephant fart. Maybe that was why they warned against using it for bass :-)

[MWJB]

"The construction quality is pretty cheap, it has all the tubes on a PCB dangling off the front panel, and a cheesy hardboard back like you might find on a table radio. The cabinet is plywood, but it's thin." You say that like it's a bad thing! :-)

I only move my Leos when I have to, otherwise something tears, splits, or snaps!

But it all adds to the mojo, the flimsy cabs really breathe (or wheeze?), like Wakculloch says, amps like this can be desirable, just as much for their offbeat, non Fendery vibe, as for anything else.

[dai h.]

you need to put one of those lightning bolt shooter things in it to deal with hecklers and sundry undesirables

of course you'll have to disguise it so no one ever suspects "the cheap olde selmer"

[plook]

Selmer Corvette Layout

Has anyone got a layout drawing of the Selmer Corvette, or could they give me some guidance on doing it myself.