Garage Sale Amp Find... Suggestions?

[bcmatt]

This was in a crappy metal cabinet with dual 8 inch speakers that I threw out. I got it for $20 at a garage sale from a friend of a friend. I figured it would be worth at least the knobs, jewel lights, and the 5751s to me. It works but is sounds entirely underwhelming. It is sort of neat that it is all point-to-point with some decent caps. My first thought was to 3-prong power cord it, and re-cap it.... now I'm thinking I might as well just build it into something else and hopefully salvage some of the components and transformers. I think my first step would be to measure the transformers to see if they are suitable for some new project, or if that is the source of disappointment.

Does anyone know the story of what this could be? Some sort of Radio Shack kit from the 70s?
Tube compliment (the lettering I can read):
GE "Made in Canada" 5751 62-??
6C4
GE "Made in USA" JAN-5751 67-41
2x DIXIE "Made in Japan" 6BQ5 8-3

Any suggestions?
The only thing that I can think I might like to build might be a sort of half-power Rocket in a 1x12 combo. Not sure what the PT could support, or the voltages it produces yet. Maybe a spitfire would be more appropriate.

As it is, it's sounds super boxy and doesn't break up in any sort of interesting or pleasing manner... not harsh, just not at all magical. I can't see anyone being pleased with it as is. It has a bit of hum too.

If I made it into a combo, I imagine I would move the tube sockets to the back side and put an eyelet board on the chassis floor.

I suppose I could go through the trouble of sketching a schematic of what it presently is... not totally sure if that is worth it...

[ampgeek]

My bet is that you can gut it and coax back out a very passable 18 W Lite, Stout, Spitfire or other 18W variant on the cheap.

Is that a 6.3V filament tranformer inside the chassis?

Good luck!
Dave O.

[bcmatt]

My bet is that you can gut it and coax back out a very passable 18 W Lite, Stout, Spitfire or other 18W variant on the cheap.

Is that a 6.3V filament tranformer inside the chassis?

Good luck!
Dave O.

No, I believe it is a choke. The filaments are fed from the PT.

[Structo]

What is that metal enclosure on the right side of the chassis?

[Phil_S]

You've got enough knobs in the front to build a full 18W-TMB. It's a classic. How can you not like one of those. I'm assuming the voltages will be OK. if not, there are many other good projects you can do with a pair of 6BQ5's. Tell us what you find when you fire it up for the voltages.

[bcmatt]

What is that metal enclosure on the right side of the chassis?

I really don't know, but I think it is for the trem circuit. It is connected to two points in the circuit; a plate of one 5751 and a grid of the other 5751.

[bcmatt]

You've got enough knobs in the front to build a full 18W-TMB. It's a classic. How can you not like one of those. I'm assuming the voltages will be OK. if not, there are many other good projects you can do with a pair of 6BQ5's. Tell us what you find when you fire it up for the voltages.

OK, I got 375 Volts on the Plates of the Power tubes.

Any other readings significant? 6.2 Volts for the heaters (pin 5) (0 V on pin 4)
379V on the Power Tube Screens.

Well, here are all the plate voltages:
6BQ5x2: 375V and 375V
5751: 260V and 59V
6C4: 232V
5751: 141V and 176V

I guess what we really care about is the voltage going from the PT to the Rectifier is 316VAC and 318VAC. After the diode rectifier assembly it is 390VDC. Thoughts?
I don't have a library of circuits in my mind with any sort of required voltages for them.

I have no idea the heater current this thing produces and whether it could support any more tubes ... but, if I use this 6C4, I suppose I have enough triodes for some sort of half-power rocket or liverpoolish thing...

I am super curious about the OT... this may be a stupid question, but is there a safe way to measure it before removing it? Can I just read voltages before and after it while it is plugged into a load? When I thought of this, flags were thrown in my brain, but I don't know the call...

[Phil_S]

375V on the plates? Ouch. That's too high. Seeing 6.2V at the heaters tells me the PT is probably rated for 120V primary, so plate voltage is probably very close to spec. If this were mine, I'd probably remove the noval sockets and drill it for octal so I could use a pair of 6V6. Either that, or you'll need to build a B+ supply ladder that reduces the screen voltage considerably before you cook those EL84's.

I wonder if that choke is good for the whole load...I'm thinking choke input will reduce your plate voltage considerably.

To test the OT, first check for continuity. Then test it for turns ratio. Once you know the turns ratio, simple assumptions can be made for impedance ratio. Ordinarily EL84's use an 8K primary. Read about it here. http://www.radioremembered.org/outimp.htm I usually take the low AC voltage from the filament supply. If you get a result that doesn't comport with common speaker impedance (4-8-16) then post here and someone is bound to chime in.

I'd be happy with that $20 find. Eventually, you'll figure what you can do with it.

[Teleguy61]

Before you go ripping things apart, assuming it works, you might want to sub 12ax7s in for the 5751s, to see if a gain boost livens things up a bit.

[David Root]

A pair of old Soviet military EL84 types would handle 375V no problem. 6P14EV?? Cheap too.

[statorvane]

Looks like the voltage is too high for a 15 w Rocket or Liverpool. Definitely enough umphh in there for an 18 watt, or 2061X.

I agree with Phil, you may want to consider a PP 6V6 amplifier before you decide to dump some HT. And Teleguy's suggestion to try some 12AX7s is a really good idea too.

[Phil_S]

A pair of old Soviet military EL84 types would handle 375V no problem. 6P14EV?? Cheap too.

Good idea! I'm thinking maybe that high voltage will really wake up those tubes, too. And it beats boogering up the chassis to install octal sockets.

[bcmatt]

Hey guys, been thinking about this some more.
I've been looking around for 6P14P-EV and only really finding them on Ebay. They actually seem pretty reasonable, assuming they are matched close enough based upon being military.
But then I got thinking, I would most likely give this thing VVR regardless, seeing as a still have a couple left from ordering several from Dana a few years ago, and I figure it is a no-brainer for any sub-30 watt amp.

So, maybe I should just have an upper limit on the voltage of the VVR that is less than full, but still safe and appropriate for the circuit I want. Is there a down-side to this? A rarely run any of my amps on full voltage anyways, but I would just not make it possible to go to full voltage.
Thoughts?

I'm also thinking maybe I should use one of my two extra EF86 tubes in this thing, and forget about the 6C4... just an EF86 and a phase inverter.

[gui_tarzan]

I need to find one of those deals to gut and finish my Alamo.

[Phil_S]

You will probably find them as EL84M, and should be about $15/pair from someplace like Tube Depot or Parts Express.