What Power Transformer for R30?

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solson1041
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What Power Transformer for R30?

Post by solson1041 » Tue Jan 01, 2019 1:36 pm

Hi all, I spent most of yesterday lurking this forum and the rest of the internet, planning an R30 dirty-channel-only build. I've been repairing an old radio from the 40's, and it's given me the itch to build an EL34-based amp. At first I was trying to modify the JCM800 circuit to what I wanted, and then I realized I really just wanted an R30. I've always lusted after its tone, and I'm looking for a project.

I cannot for the life of me find out what power transformer goes with this amp. The schematics provided by a user on here (jondog) have a much higher B+ than the other 30w Oranges, and I found one forum comment saying the R30 uses the same PT as the RV50, but I can't really find any info on that transformer either.

I'm not eager to pay for a Mercury Magnetics transformer if I can avoid it, but I'm not trying to do anything stupid in the name of saving money, either.

What secondary voltage should I be looking for here? I'm in the USA, so it'll be 120v coming in, obviously.

Thanks very much for any help.

Jondog
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Location: Ontario, Canada

Re: What Power Transformer for R30?

Post by Jondog » Tue Jan 01, 2019 1:59 pm

You need a transformer capable of providing enough voltage and current for two EL34’s, and three 12AX7’s. Along with their filament circuits. The preamp uses DC filament circuits and the power section uses AC. The RV50 transformer meets these specs. I used an MM Rockerverb 50 transformer in my Rocker. They are the same. You don’t need to go DC, but it makes for less noise in the preamp. You could get away with a Marshall type PT. The voltages are what I measured in my amp plugged into my wall voltage which can vary and therefore changing the voltage readings inside the amp.
Another thing of note, is that since you need two 12ax7’s for the preamp and only 3 gain stages for the dirty channel, without the natural channel there, half of one valve is not getting used. To me it’s worth putting it in, or use that extra gain stage as a cathode follower, OR50 like.
Below are the specs from Mercurys Rockerverb 50 transformer. You can expect this transformer in all the 50 watt amps. Rocker would be 50 watts if it wasn’t cathode biased, just like the Rockerverb would be 30 watts if it was cathode biased. The tube compliment still have the same power requirements. The 6.7 v tap is for the AC filments in the power section, phase inverter included, and the 6.3 v tap is used for the DC filament circuit, which needs to be rectified into DC. That all can be done off one 6.7v tap, but you need to do it properly.
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solson1041
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Re: What Power Transformer for R30?

Post by solson1041 » Tue Jan 01, 2019 4:44 pm

Thanks so much JonDog. It might be a while until I actually pull the trigger on this project, but the fact that you've been so helpful on this forum for so many years is outstanding. When I finish it, I'll let you know and send an extra measure of gratitude.

Good point about the clean channel, I might just include it since there's no reason not to, as you point out. It really wouldn't give me any benefit aside 15 mins of wiring to exclude it, and it'll probably take me longer to verify I've got my bastardized circuit correct than to just follow the ones you've posted. Good call.

As a last question, Where exactly are you pulling your A+, B+, C+ voltages from? The Heater supply circuits don't explicitly call it out, or show a violet wire for that matter. Is there a separate power rectifier section implied?

Jondog
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Joined: Thu Aug 21, 2008 12:16 pm
Location: Ontario, Canada

Re: What Power Transformer for R30?

Post by Jondog » Tue Jan 01, 2019 5:11 pm

Those schematics were ones I traced and drew up for myself and shared on this forum, then they spread like wildfire. It’s not properly formatted and has a couple minor errors, but generally correct and an experienced amp builder would be able to recognize and correct any errors, like the elevtrolytic caps are all drawn upside down because I copied and pasted before realizing and never bothered going through and fixing them all.
Jondog wrote:
Mon Feb 18, 2013 2:38 pm
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solson1041
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Posts: 3
Joined: Tue Jan 01, 2019 1:25 pm

Re: What Power Transformer for R30?

Post by solson1041 » Wed Jan 02, 2019 1:32 am

Aha, it looks like the post I saw before didn't have that first power supply schematic. Thanks, this sets me on the right path, and I'll give it a shot from here.

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