Help with power transformer for guitar amplifier!

Factual Questions
1

I’ve had an amp sitting in the closet for awhile with a blown power transformer. I wanted to replace it before, but I never knew what to replace it with - I’m not all that well versed in electronics.

Here’s the schematic.

I know the amp is 30 watts and the speaker is 16ohm. There also has to be 12.6V powering the preamp and a regular 12v line powering everything else, but I have no idea how to apply this knowledge to selecting a transformer. Primary voltage? Seconday voltage? 250 0 250? I have no idea!

Is there anything at Mouser that would work?

Thanks so much!

2

The schematic link gives me a 403 forbidden.

What is the make and model of the amp? Does the schematic list a part number for the transformer?

I doubt that mouser will carry a suitable replacement.

3

Try this link: http://www.freeinfosociety.com/electronics/schemview.php?id=370

Or, you can Google for it. It’s an Electar 30DR and should be one of the first links. It’s fairly rare - I think it was a weird project from Gibson that didn’t take off too well - so there’s not much information on it out there.

4

Let me show my ignorance for a second. Since the guitar is rated as 30 watts, doesn’t that mean it’s power requirement is 115/30 = .2 amps? That seems ridiculously small for some reason.

And since it requires +/- 12V, doesn’t that mean that one of the secondary windings should be 24 VAC center tapped?

I guess the tube is the most confusing. I’m assuming the 12.6V goes to the heater filament thing and it can be +/-10%. So would running 12V from the same lines as the others be fine or should it be on a separate loop? Should it be 12.6VAC center tapped, +/-6.3V?

Here’s a pinout for the 12AX7: Guitar & Tube Amp Repair / Chattanooga / Rossville - 12AX7 Tube Pinout On the schematic, it looks like 150V goes to pins 1 and 7. But where is that 150V coming from?

Thanks again for your help!

5

Looks like it’s a specialty transformer. The LM1875 output power IC has a maximum of 60V allowed on its rails (±30V), so you need a 42V 2A center tap power transformer to supply the LM1875. Then for the 12AX7, you need a 150VAC output plus a 12V filament supply. You could cheat by connecting the 12VAC secondary of a 110VAC/12VAC filament transformer to the secondary of a 220VAC/18VAC transformer, giving you approximately 150VAC from the primary. You would need to find room for three transformers, but that’s probably an easier problem to solve than trying to find a 110VAC/150VAC,42VAC,12VAC specialty transformer.

6

You can get your old transformer rewound. A lot of times that’s the only option with vintage tube equipment.

This place specializes in antique radio & amplifier transformers. They list a lot of Gibson transformers by part #.
I haven’t used them. But, they offer a 90 day warranty, Might be worth it to fix a vintage amp.
http://members.tripod.com/tubes_tubes_tubes/transformerrewindingservice/

7

According to the rep I talked to at Gibson customer service (1-800-4Gibson), replacement parts for Electar amps are “reserved for warranty repair.” The amp specialist was out until Monday. You might want to call him, even if he can’t sell you a transformer, he may be able to provide you with specs or point you in the right direction.

Besides OEM, there are a lot of people making aftermarket transformers for tube amps, and I don’t see why they couldn’t make them for hybrids as well. The parts that I’ve bought came from Antique Electronic Supply; you might want to call them in the morning and see if they can help.

8

Oh, I think I get it now! Essentially it’s a solid state amp with a tube preamp built in. So, in theory, I could just get a power transformer they use for LM1875 builds and a transformer they build for 12ax7s, right? Would I have to deal with any interference issues with two transformers?

ETA: Thanks so much cornflakes!

9

There’s no interference.

If you wanted to be cheap-ass, you could take one 24VAC 2A transformer, half wave rectify it into + and - rails, replace C44/C45 with 4700uF/35V caps, run the + rail to C55 for the filament voltage, and build a 6x voltage multiplier to bump your 24VAC into 200VDC for the 12AX7 plates.

The LM1875 has good ripple rejection, so you wouldn’t have to worry about an increase in hum from the half rectified rails, though you probably would want to double C52 to 470uF and C51 to 100uF for increased ripple rejection on the tube if you do the voltage multiplier thingy.