Salvaging an amp...

[Herec]

Hey guys,

I've got this old reel-to-reel that's died (Wollensak T-1500). It won't record or play tape, but it outputs its loudspeaker fine. I've been using it to play mono vinyls and reamping my recording interface signal, but I've grown tired of it. Got some good stuff to work with though.

1.) A multi-tap OT, but I'm just looking at using the 8.8k primary to 8 ohm secondary

2.) Two 12AB5s (6v6s), a 12AX7, a 12AT7

3.) A PT which I believe is 200 - 0 - 200, but I'm not sure. The power supply in this thing is verrrry weird. There is just one diode for rectification, with the initial voltage after the diode being 200VDC, and then a ladder of caps and resistors after that. It also uses some straight rectified wall voltage (!) for other things.

I'm thinking a 6G2 or possibly a 5E3 with the existing chassis (using a simulated tube rectifier). I'm not sure how much real estate there is, but it might be possible to add another noval socket, probably doing a lo-watt Express deal.

Do you think that a 6G2 or 5E3 is doable with a 200 - 0 - 200 PT and solid state rectification?

[Structo]

That's pretty low voltage for a PT and the current capacity is probably pretty low as well.

[Phil_S]

200-200 will give you about 280VDC. This, IMO, is low for a 5E3. For a lo-watt express (not sure what that is exactly) I think I'd try very hard to keep the preamp voltages up as high as possible. I think it is a better choice than the Deluxe. 6G2 is probably a better choice for this PT than 5E3, but, again, I'd modify the dropping resistors on the B+ to feed as much as I could to the preamp.

I love an old salvage project. The results tend to surprise in a nice kind of way. Go for it. I will be wonderful.

[Herec]

That's pretty low voltage for a PT and the current capacity is probably pretty low as well.

I was thinking that. The way the unit stands now there's 250V on the plates to the 12AB5s, which draw about half as much heater current as normal 6V6s IIRC. Something does drive the record motor/the cooling fan, but I'm not sure if its the rectified wall voltage or not, its not clear on the schematic I have.

I love an old salvage project. The results tend to surprise in a nice kind of way. Go for it. I will be wonderful.

I got the entire unit for free, so the worst case scenario is all I get is a couple of RCA preamp tubes.

[David Root]

280V is too low for a 5E3 or a 6G4. 5E3 is 365V, 6G4 450V. Unless you run it as a 400-0 with a full wave bridge, then you'd have 560V, which you would have to tame a bit, and then you wouldn't have enough current for two 6V6s I think.

OTOH, that "ladder of caps & resistors" might actually be a voltage doubler. I found a service manual for it on the web, but believe it or not it did not have a schematic in it, just a bill of material.

EDIT: I see you said 6G2, not 6G4, my mistake. Maybe a little light for a 6G2 at 280 or so, schematic is 315, but could be worth a try. Might work better with the earlier tweed 5*2, but they are single ended, more like a Champ.

[Herec]

OTOH, that "ladder of caps & resistors" might actually be a voltage doubler. I found a service manual for it on the web, but believe it or not it did not have a schematic in it, just a bill of material.

Aye, it's not a doubler, the voltages at the nodes show it decreasing by approximately 20VDC each resistor/cap stage, like a normal power supply I believe. I really have to get this schematic into a jpeg haha.

The 12AB5s don't draw as much heater current as the 6v6s, but are otherwise the same (on the datasheet). I think 0.2A per 12AB5 according to the RCA datasheet is what they draw.

[Herec]

For a lo-watt express (not sure what that is exactly) I think I'd try very hard to keep the preamp voltages up as high as possible. .

When I say "lo-watt express" I mean a normal express pre/PI with a push-pull 12AB5 output section which is approximately 10 watts.

[sliberty]

I am all for salvage projects, and have done several myself. But when it comes to the PT, I usually buy new so that I don't have to fight over getting the voltages i need. My moto is use what you can, replace the rest.

[Phil_S]

For a lo-watt express (not sure what that is exactly) I think I'd try very hard to keep the preamp voltages up as high as possible.

When I say "lo-watt express" I mean a normal express pre/PI with a push-pull 12AB5 output section which is approximately 10 watts.

That's what I had guessed. If the 12AB5's are usable, I think you might actually have a nice project on your hands. The one caveat is that the Express has a finicky preamp. If space is tight, think about your lead dress issues carefully before building.

[Herec]

Some pictures...

Stripped the core amp of its mechanical shell and was greeted by a dusty rat's nest.

[img:816:488]https://lh5.googleusercontent.com/-M_9N ... 525204.jpg[/img]

[img:816:488]https://lh6.googleusercontent.com/-KuCf ... 525203.jpg[/img]

I started to remove the old filter caps, when I noticed a couple of them, along with the transformers and tube sockets seem to be riveted into place. It looks like on the backside (inside the chassis) there's an indentation that might be for some hex-screwdriver, but it really just looks circular.

[img:816:488]https://lh3.googleusercontent.com/-JLG1 ... 525201.jpg[/img]

[img:816:488]https://lh3.googleusercontent.com/-LMzA ... 525202.jpg[/img]

Any way I can just pop those off? If I have to reuse everything in the chassis, I'm not gonna have too much control over short lead dress, so express is out of the question.

The extra shit on the chassis (old filter caps, a driver transformer) will definitely bother me, not even thinking about the real estate its using.

[Herec]

Oh yeah, the 7025 is a weird brand, but the 12AT7 is a GE.

The 12AB5s are both RCA.

[Structo]

I just drill out that kind of rivet.

That amp is pretty typical 50's point to point wiring.

How would you like to spend your days wiring those things up?

Take a look at a Hammond organ amp, more room but a lot of parts.

[Herec]

I would definitely go cross-eyed.

I'm thinking basically the 6G2 without the tremolo (I KNOW, but I really don't think I have the space), and maybe using the extra triode to have it more of a 6G2 input/TS, then two triodes like the 2nd and 3rd in a TW, then cathodyne PI to cathode biased push-pull 6v6s.

I'm going to test the PT to see how much unloaded DC it puts out.

EDIT: Actually, should probably keep it fixed bias instead. Don't know where to put the negative feedback though.

[Herec]

Got a question on the best way to hook up this OT...

If I'm doing a simple push-pull arrangement, would blue and brown each go to a 12AB5, the red go to B+?

And then on the speaker side, is it just green and black to the speaker?

[Herec]

Just FYI, that schematic is how the OT was hooked up in the reel-to-reel I took it from, not how it will be wired up in my amp. Mine's a 6G2 copy.