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shonen 24th March 2012 02:13 AM

My first DIY schematic... need eagle eyes.!
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So this is just a sketch for an idea, might not be possible, that's why i'm posting first. I want to create a Fender 5E3 preamp section, with one 12AY7 and one 12AX7 and the PI from that period.. but power it with a 12AU7 and appropriate OT.

I've gone with the Hammond Universal 125A so I can match different Impedances and because it looked like the amps to the plate matched better than the 5Watt OT they offer.

My main question is, how will this work with the standard Deluxe Tweed 290AX Power Transformer, giving roughly 360V DC to the Primary B+ of the 125A Output Transformer.

Is this going to blow up???? I couldn't find any Volt infp for the B+ line of the OT on Hammond's website.

Any advice is much appreciated...


edited w/ updated layout below, ignore attachment, forgot to show B+ to the Output Transformer....

avp1 24th March 2012 02:25 AM

Can you redraw it using conventional symbols? Nobody has time to decode assembly diagram of yours.

shonen 24th March 2012 02:39 AM

I can give it a shot... I'm not an electrical engineer but it should be pretty easy. I've updated the photo of the layout but the thread is not showing up now.

shonen 24th March 2012 03:06 AM

How's this?

speakerfritz 24th March 2012 04:44 AM

your proposal would put a tube with an MU of 47 in V1, MU of 100 in V2 and MU of 17 as V3. whats the purpose? MU of 17 is not significant and the noise exposure is not worth it. You might want to put in tubes with a closer unity gain. 12AY7=47, 12AU7=17, 12ax7=100, 12aT7=70, 5751=70, etc.

Chris Hornbeck 24th March 2012 05:11 AM

You'll have *way* too much B+ with the light loading envisioned. Somewhere just south of 300 volts is more like it. Why did you choose this circuit?

All good fortune,

shonen 24th March 2012 05:13 AM

the reason i wanted to go with a 12AY7 was just to stick to the original characteristics of the 5E3 design... I have a Vox Lil Nightrain head that uses 2 12AX7's, as do later Fender designs. I've ordered a couple of 12AX7's in case I test this and find the noise is not worth it.

But for the 12AU7.. I don't know of a better low watt option for output, i'm trying to keep it under 5 Watts and the 12AU7 has a low MU but higher amps on the plate, so it can be used as an output tube.

shonen 24th March 2012 05:19 AM


Originally Posted by Chris Hornbeck (
You'll have *way* too much B+ with the light loading envisioned. Somewhere just south of 300 volts is more like it. Why did you choose this circuit?

All good fortune,

that's what i was wondering... thanks for the info. my goal is to not get evicted.. and to get this around 3 to 5 watts output with a vintage tweed 5e3 preamp, tone and PI circut.

i know the driving of the 6V6's are a big part of the tweed sound but i really love the tone of my Vox Lil Nightrain and it's using a 12AU7 as the output tube. Hopefully I can push them a little more and by using tube rectification and the 12AY7, it will sound more like an old tweed.

Chris Hornbeck 24th March 2012 05:42 AM

It's certainly an interesting idea. My worries would be matching the 12AU7's output impedance requirements (20K Ohm? or so - dunno) and the relative importance of the speaker to the sound you're wanting to get. One good way to tell for sure: make one and try it! Then season to taste.

All the best,

shonen 24th March 2012 05:50 AM

Thanks... so how do you know how many Volts to deliver to the OT's B+? Is there a formula that I can use? Hammond's 125A has "universal" impedance matching chart, so I can match the 22k? impedance of the 12AU7 plate with an 8Ohm voice coil, but i'm not sure the amps i'm putting on the 12AU7's plates or the voltage to the B+ will be correct... and also don't know correct resistor for the cathode bias. ha. i'm really winging it.

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