Garage Sale Amp Find... Suggestions?

[ampfab]

375v, no problem for el84s. IMO

[bcmatt]

Well, the current EL84s seem to be fine, and I suspect they are very old.

Anyways, in order to match the desired voltage of any particular circuit, am I mistaken in assuming that there is no down side to merely experimenting with one additional resister and it's value in a VVR circuit like this?

I understand that this would increase the overall pot value being seen in the circuit, so if I want the bottom voltage limit to stay at 10%, I would increase the Value of R1 and R2. I would probably start them at about 300K anyways, because I don't like the bottom end of the voltage anyways. You can tell me to turn down, but there is a limit!

[bcmatt]

So. I ripped apart this amp today so that I could get at everything and move along the process. I need to find an AC power supply so i can check my OT. I did notice that there are two other unused secondaries on the OT. Could actually have some impedance choices.

I'm leaning toward a very basic EF86->PI(12ax7)->EL84s type concept in a head. I'm still not sure whether I want to move the tube sockets in order to get some boards in the chassis. Maybe I could make some boards with holes for the preamp tube sockets.

I think I might ditch the choke. The wires are frayed and exposing metal as soon as they come out of the cover... and space is really tight.

They did the heaters that old way of just sending one wire and then grounding out the other heater connection at the socket.

I'm trying to make sure I have all the PT leads clear...
Black Cloth: 120V mains input
Black plastic: one goes to heater and other to ground
Yellow Plastic: One to part of switch that goes to the lamps, other to ground
Red cloth: to rectifier
Red/yellow plaid cloth: grounded only when amp is in play mode (What is this?)

I guess there is no centre tap.

It is fun/confusing to run into so many weird parts. I guess the metal block is like a little tunnel thing for photo-cell bulbs or something. There is a massive 1meg pot with an unused spst switch that opens when the pot is clicked past all the way down. Seems pretty rugged and high quality.
There is that one weird cap or something in the bottom middle of the picture that looks like a piece of chocolate. There seems to be no value written on it, so I might just try eating it.
And what is with those tiny yellow caps? they say 0.01, but that must be nF or something. I wish I had something to measure caps. I think I need a new DMM...

[ampfab]

Maybe this will help with some of the wire identification.

[billc]

That thing might have started life as some sort of Gregory, the tremelo,tube lineup, and xfmrs look about right...and Radio Shack did indeed sell them back in the day as "Realistic" amps. FWIW I was given a Realistic Carnaval 34 (Gregory Mark X) awhile back, needs work but does function.
Anyway, there is an excellent site with lots of good info on Gregory amps at

http://www.diyguitarist.com/GuitarAmps/GregoryAmps.htm

[bcmatt]

Thanks ampfab!

So, I'm assuming that the lamps are running off of what is intended for the rectifier filament (since this is SS rectified).

I guess I'll switch the heaters to a twisted pair with an artificial centre-tap.

From the chart you sent here, my best guess is that the two-color red/yellow cloth wire could be a B+ centre tap. However, I am really confused about why it would be lifted (ungrounded) when the amp is in Standby or Off. This (like many amp building ideas) is very unfamiliar to me. Does anyone know about this? I'm assuming I should let it remain like that?

[Phil_S]

Use the 6.3V or 5V secondary on your PT to test the OT.

[bcmatt]

That thing might have started life as some sort of Gregory, the tremelo,tube lineup, and xfmrs look about right...and Radio Shack did indeed sell them back in the day as "Realistic" amps. FWIW I was given a Realistic Carnaval 34 (Gregory Mark X) awhile back, needs work but does function.
Anyway, there is an excellent site with lots of good info on Gregory amps at

http://www.diyguitarist.com/GuitarAmps/GregoryAmps.htm

I scrolled through, and it does seem conceivable that it is in the same vein.
The cabs start looking cheaper and cheaper as you scroll down. My cab was made of tin or something with a faux-wood finish. The amp sat in the bottom and was only accessed from the rear. Everything looked un-professional like something someone thought to throw together. I am even beginning to doubt it was even a kit, because every part is just generic electronics equipment; not thought to faceplates or anything. Seems like someone knew electronics and just threw together a guitar amp for their kid or something. It had 2 6-8" speakers in the cab too.

[bcmatt]

Use the 6.3V or 5V secondary on your PT to test the OT.

great idea! How did I not think of that?

[bcmatt]

Use the 6.3V or 5V secondary on your PT to test the OT.

great idea! How did I not think of that? I guess that should have been obvious. Especially now that I have everything else stripped out of the way to make it easy.

[bcmatt]

I used the 5.7V yellow taps to figure out the Primary Impedances on the OT:

16 ohm Secondary : 7572 ohm Primary
8 ohm Secondary : 6786 ohm Primary
4 ohm Secondary : 7633 ohm Primary

So, I guess it is intended to be a 7600 ohm Primary OT with the 8 ohm tap being significantly low...

I am still trying to figure out the PT Secondaries exactly and I took some voltages:
Between Yellow Plastic Pair: 5.7V (Rectifier filament?)
Between Black Plastic Pair: 6.8V (filament supply?)
Between Red Cloth Pair: 662V
Between one (then other)Red Cloth + red/yellow cloth: 334V (331V)
Between Yellow Cloth + Red/Yellow Cloth : 105V
Between one (then other)Red Cloth + yellow cloth: 321V (112V)

So, It would appear that the Red/Yellow cloth is a centre tap for the HV Red wires. But, what is the Yellow cloth wire? Initially I assumed it was the centre tap for the Rectifier filament supply but it totally is not (giving over 100V between it and one of those).