Gabby enterprises Fake Fender Showman project

[davohilts]

I've had this fake fender showman combo amp sitting around that I have been saving for a rainy fall project.
the amp guts are shown here:
http://s1191.photobucket.com/user/davoh ... e%20Fender
I've gone thru the amp and replaced bad caps/resistors and the tubes are all known good tubes.
My problem is I have no idea if the circuit is original of if it has been modified.
I spent a couple of days trying to draw out the schematic and have attached it here.
Any help or comments would be very much appreciated!
Thanks,
Dave

[davohilts]

Here is the correct link to the gut pics

Gabby Enterprises Fake Fender Photos by davohilts | Photobucket

[Justin Thomas]

Is that a recycled Solid-State Fender Showman chassis, or a flagrant clone w. a tube amp built in?

Anyway, a quick peek at you schem, on page 2... the Normal Channel has the first stage plate feeding the 2nd stage grid, but the output from the 2nd stage feeding the tone stack also seems to be taken from the grid? Might wanna check... I will never claim to know and understand all possible tube circuits, though! But pretty sure I never seen that before... It's late!

Justin

Edit: make that both channels! Same thing on Effects...

[52 Bill]

Is that a recycled Solid-State Fender Showman chassis, or a flagrant clone w. a tube amp built in?

A flagrant copy of the solid state chassis design with a tube amp built inside from a factory in the Far East. Fairly common back in the late 60s early 70s. A lot of Vietnam era GIs would bring then home.

It looks like 80% of the original parts have been replaced. No way to know what changes may have been made to the circuit through the years.

[Leo_Gnardo]

Had something like this thru the shop about 2 years ago. Single 12 combo labeled "Showman" with fake Fender badge, wrong grill cloth, dodgy looking Tolex. Although I expected a disaster, it turned out to be a point to point wired (no circuit board) copy of a brown Vibrolux. Put in some fresh filter caps and a good speaker & it sounded a treat.

[J M Fahey]

A tube amp which in order to be sold claimed to be solid state?

Sure !!!!

I also remember that era.
Also BIG "Solid State" labels on the front panel, even proud "7 transistors" ones.

What's funny is the fake ventilation grill printed top right but even more the Reverb label and pot hole marker .... unpunched.

As you say, only an uneducated GI might have fallen for it.
Or more probably he noticed it was fake but bought it cheap anyway, just to impress the guys back home.

Onstage it sure looked hotter than the (more expensive) Silvertone-of-the-day which his friends had.

That shop must have had plenty $20 Rolex watches, $1 Chanel and Dior perfumes and so on.

Of course, today nobody would fall for a Chinese $200 "original Gibson Les Paul"

[davohilts]

Is that a recycled Solid-State Fender Showman chassis, or a flagrant clone w. a tube amp built in?

Anyway, a quick peek at you schem, on page 2... the Normal Channel has the first stage plate feeding the 2nd stage grid, but the output from the 2nd stage feeding the tone stack also seems to be taken from the grid? Might wanna check... I will never claim to know and understand all possible tube circuits, though! But pretty sure I never seen that before... It's late!

Justin

Edit: make that both channels! Same thing on Effects...

Hi Justin,
Thanks for the reply. I am completely mystified by this circuit and wonder if it ever worked. I have never seen anything like it and it is very interesting to try and figure out. Unfortunately I don't know enough about design to troubleshoot the problem beside looking for bad connections or components. I see what you are saying about the grid to grid connections thru the tone stack. The chassis on this amp is very thin aluminum and is very hard to work on and must be carefully supported. The caps also hold their charge a long time so I have to be careful to discharge them before working on the amp. I keep coming back to this because it is such interesting project and I've learned a lot, but without much success. It has a closed back ported cabinet with a 12" speaker. I may build a showman circuit and install inside just to get it working.
Dave

[52 Bill]

I am completely mystified by this circuit and wonder if it ever worked. I have never seen anything like it and it is very interesting to try and figure out. Unfortunately I don't know enough about design to troubleshoot the problem beside looking for bad connections or components.

You never actually say what the problem is.

If you look at the schematic that you drew, the basic circuit elements are very similar to the basic Fender blackface circuits. The power amp and driver are pretty much is the same as say a Vibrolux. So are the tone stacks. The tremolo is different. It's more like a typical Japanese amp with the trem being applied to the cathode of the tone recovery tube.

The one thing that is different is the input stage. I don't understand what the second section of V1 does in this circuit.

[Justin Thomas]

Second Stages...

That was my question. I don't know if it was an error in drawing or an actual wiring error. I would think IN THEORY they just put two stages ine after the other before the tone stack (massive drive before losses) instead of one before and one after (drive and recovery). But as it's drawn, and if it IS wired that way, I'm not exactly sure what the effect of feeding a "Triode Stage to Nowhere" AND the tone stack would be... Hmmm. Good Saturday afternoon experiment.

Justin

[J M Fahey]

I think you have a couple drawing mistakes ... unless somebody messed with it in a bad way before.

If it were mine [tm] I'd strip the guts, hate those PTP mounting bridges anyway, and would make/get a standard Fender layout eyelet board and build a killer amp there.

You'll have the added help that Fender Layouts are very clear, well made, and widely known, so building and troubleshooting one should be the safest bet.

For now, forget the tremolo, you can add one later if you have any triode left although I guess I would use a 555 driving a Led/Ldr optocoupler (hey!!, even "tube purist" MB does that!!!)

[52 Bill]

I think you have a couple drawing mistakes ...

I did a close look at the photo of the input tube wiring, and the signal to the tone stack comes from the plate pin 1 not from the grid pin 2.

So the signal goes through two gain stages before the tone stack.

[davohilts]

I did a close look at the photo of the input tube wiring, and the signal to the tone stack comes from the plate pin 1 not from the grid pin 2.

So the signal goes through two gain stages before the tone stack.

you are correct, I made a mistake in my schematic and the signal does go from plate to grid. I agree that getting rid of the guts and building a known fender circuit in there is the smart way to go. The amp will be more stable, sound better and be more rewarding in the long run. I'm sorry about my mistake because it has wasted a lot of bandwidth.
I appreciate everyone's help on this, you have got my head out of my a#@ and pointed me down the road in the right direction. Thanks everyone!!!
Dave

[davohilts]

You never actually say what the problem is.

If you look at the schematic that you drew, the basic circuit elements are very similar to the basic Fender blackface circuits. The power amp and driver are pretty much is the same as say a Vibrolux. So are the tone stacks. The tremolo is different. It's more like a typical Japanese amp with the trem being applied to the cathode of the tone recovery tube.

The one thing that is different is the input stage. I don't understand what the second section of V1 does in this circuit.

The amp works on the normal channel but is really overdriven. The trem doesn't work and the effects channel has very little gain. Both channels are wired exactly the same but there is some must be some kind of cross over causing high gain on the normal channel and very low gain on the effects channel.

[davohilts]

I have one last question and that pertains to sizing and selecting the correct choke for use in converting this gabby into a vibrolux 6G11 clone. How does one go about selecting a choke for this circuit?
Thanks,
Dave

[J M Fahey]

FWIW I don't use chokes but I guess in this case you call one of the usual suspects and asks for "a Fender choke" .
The idea is that they are sized for screen+preamp current but not for main tubes consumption.