Boat anchor OT

[Marc]

I thought surely someone will out bid me, after all I only bid $15. So now it's mine and I'm wondering what I can do with the OT from a Webster PA. It is driven by a pair of 7027's and the secondary is 70v-25v-0-25v-70v. Yep, it's a pager pa, TP 85 with 85 watts rated output... Any suggestions for this OT?

I think the Pt HV is shorted to the filaments, at least partially, as it seems to have a very high dc bias on the filaments, like 110V or so. It's an interesting circuit with an OC3 to regulate the screen voltage.


Marc

[tboy]

...it seems to have a very high dc bias on the filaments...

Have you checked to see if maybe that was done on purpose, as a filament hum minimization scheme?

[Marc]

When I first looked at the amp the pin 2 and 3 on one of the power tube sockets were bent together. So I hope I have jumped the gun on the bad PT diagosis. I was just looking at it again and I think the pt seems be OK. there was a ground wire missing from one of the can cap lugs and the other lug on the cap has the CT for the filaments on it, so it was floating well above ground... Now that it is attached to ground the filaments seem to be OK. That's the good news.
the bad thing is it's a rats nest and the cap seems to have no B+ on either section so it's back to tracing the leads......

Thanks,
Marc

[Enzo]

I expect the 70 and 25v windings, but doesn;t it also have low impedance outputs like 4 or 8 ohms?

[daz]

I think you already answered your own question....instead of an amp, build a boat !

[tommy d]

"build a boat !"

good one!

[Marc]

Enzo,
No it only has the 70 & 25V windings. I've seen some pa schematics that show the 25V with a 4 ohm designation and a 70V with a 64 ohm. Not real useful for a guitar amp unless I put a bunch of speakers in series.
Sure wold be a shame to let that iron go to no use.

ow, Tommy and Daz you're just tryin' to make more work for me and it's Friday. So forget building a boat at least until the end of next week......

Marc

[Enzo]

Well if you still want to use it, just treat the thing like any other 70v system. Put a 70v transformer on the speaker. Doesn't matter that it might be the only speaker.

[Marc]

Well, I got it working last night. after tracing out the power supply I found previous tinkering left one of the power tube's screens disconnected. Once that was hooked back in the amp worked just fine.
It has combination of a fixed bias scheme and a 200 ohm cathode resistor with a 100 mfd bypass cap.
The 25v tap works ok with a 4 ohm load, I haven't measured the output.
The plates are at 604V and the screens are at about 490 being fed by the OC3 regulator.
The best part of this amp are the 4 bugle boy 6267 preamp tubes... all in all a decent investment for my shelf collection.

Marc