Ampeg R15-R

[Ted]

I have an Ampeg R15-R with a burnt-out mains transformer. I'm unlikely to locate an Ampeg part, so I need to know the B+ (HT) voltage of these amps to select a suitable replacement.

Anyone know? - The R12-R has the same amp in it so the figure for that will do.

Cheers
TW

[stokes]

Try www.fliptops.net they have original parts for Ampegs.

[Ted]

Try www.fliptops.net they have original parts for Ampegs.

Sorry, should have mentioned I'm in England.

Yes they have them, but by the time I've paid for carriage, import duty to the UK etc it will cost nearly as much as the amp is worth.

Cheers
TW

[Ted]

From the schematic and 7591A data I'd reckon the HT voltage was 350 to 400V. Can anyone confirm that?

Just discovered that the transformer it had in it was for a B-15N or similar and of a type that probably wasn't originally fiited to R-15Rs. Probably made its way across the pond as a 110V model and some bright spark fitted the only 230V Ampeg mains transformer they had. Which produced enough voltage for a pair of 6L6s (420V +) which probably explains the large number of scorched and non-functioning components around the output stage.

Original RCA 7591As still test strong though.

Cheers
TW

[J Martin]

I can't find the PT-113 specifically, but the PT-113-A, pt-113-A-240 is either 117v or 240v primary with 720v/135Ma and 6.3v/3.5A secondarys. It is used in the 7591 based Gemini series. The 6L6 based Geminis used the PT-113-B.

[Ted]

Thanks for that.

That 720V - would that be peak - i.e. rectified DC is 360V - or rms?

Joe Piazza's website shows the HT voltage as 360V for a G-12 7591 based Gemini and 430V for a G-15 Gemini II.

With 430V HT and the standard 140R cathode resistor I reckon the idle anode dissipation of the 7591As will be about
16.5 W - which sounds a little on the high side for a 19W valve in AB1...

I'm guessing that my 1964 R-15R is closer to the one with 360V HT. Does that sound reasonable?

Cheers
TW







Cheers
TW

[Enzo]

RMS DC?

Transformer ratings are given in RMS, I am not aware of any listed by their peaks. 1.414 gets that anyway. Once rectified the DC wil be close to that peak. So 720CT will get you about 500VDC.

The R15R used PT # PT113. That was in the USA with 117VAC primary. But 230v primary wouldn't change the secondary ratings. There is a listing for Autotransformer SD-12 rated 230/240 in and 115 out @ 250 watts. I assume that was for export, so perhaps there never was a true 230v PT for this.

PT-113 was rated 720VAC CT @ 120ma, also 6.3VAC @ 3.5A, and also ( apparently unused) 5VAC @ 3A.

[Ted]

The smoothing electrolytics are all rated 450V.
Is it likely they would have used them on a 500V (ish) supply?

TW

[J Martin]

Those amps didn't run that high a voltage. Maybe the diff between the 113 abd 113-A was the rectifier winding? I'm sure someone like Sowter could make one for you. Closer to home, excellent work.

[Ted]

Those amps didn't run that high a voltage. Maybe the diff between the 113 abd 113-A was the rectifier winding? I'm sure someone like Sowter could make one for you. Closer to home, excellent work.

Its not a problem getting a transformer wound (and you're right, Sowter make excellent transformers). I could even get the PT108 rewound, if someone could just give me an idea of the HT voltage...


Cheers
TW

[hasserl]

I had one of those in for repairs last summer. I just looked thru my notebook, but it looks like I didn't write down any notes on it, which is rare for me. I've got notes on about every other amp I've touched for the last few years but that one.

I know I replaced the filter caps (had to go with individual caps as I couldn't find a can style with the correct values). The caps were rated at 450 volts and I remember the voltage at startup getting uncomfortably close to that rating. Operating voltage once warmed up was probably in the low 400's.

Not very definitive, but I hope it helps a little. Nice sounding amp, by the way, once I got rid of a nasty hum. Beautiful Tremelo and Reverb.

[Enzo]

For what it is worth, I took those numbers from the AMpeg manual for that model.

[Ted]

From Joe Piazza's schematics, the Ampegs nearest in age and design (G12, GS12R) seem to have 360-370V HT.

http://home.rochester.rr.com/piazza/

So I'm going to aim for 370V.


Thanks for your help.

TW

[Enzo]

From the AMpeg factory drawings that sounds right, the GS12R uses PT110 and has 365V on the plate supply. And my horrible copy of G12 agrees.

I think it is great Piazza copied all the schematics and did them up neat, but he drives me crazy leaving out the part numbers. I want R43, C12, etc on my prints.

[Ted]

I think it is great Piazza copied all the schematics and did them up neat, but he drives me crazy leaving out the part numbers. I want R43, C12, etc on my prints.

Yes, big respect to the guy.
There are a couple of mistakes in the R15-R schematics as well - pin numbers swapped, a missing dropper resistor. I've mailed him to point them out.


TW