• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

first post, PS for 6em7 SET amp


2008-01-30 4:29 am
HI all,
This is my first post here, but have read the forum on and off for a few years. I have built many kits and circuits on pcb, recapped a few guitar amps, but have not built an amp from scratch.

I am starting with Tome McNally's 13emt SET amp. I like the simple one tube per channel design and it looks simple enough.
I am looking for a standard plate transformer for power - Tom's design use a 240/12 transformer backward to get the B+. I amlooking for the least expansive option - would the hammond 270ex have enough current to drive the tubes?

open to any suggestions or other ideas

Jeff Foster

link to 13em7 amp

I have my doubts, the 270 has a current limit of about 40ma, and the AX version 50ma.

I suspect that you will need more current, sorry I don't have time to work out the current demand, however, the transformer is rated at 2A 12V which is about 100ma on the 240 volt side, so it does not look like it, the authors solution is quite neat for getting the appropriate voltages from off the shelf components



2007-10-09 4:16 am
Hi Jeff.

The em7 tube sounds great biased at -20 on the grid at about
50ma. it also lends itself to dc coupling because the power side
only needs about about 175 volts from plate to cathode.

Anyway, supply should be good for 125 ma. minimum. I used Edcor xpwr106 power tranny. and Edcor xse15-8-5k outputs to build this amp. The circuit you show would work with the xpwr104
transformer. consider Garry's amp, it sounds very nice and is even simpler.


I had to use a separate 5v to heat the rectifier tube.

I also built this (my own design)

I used Hammond 272fx Plus anaother 6.3 volt tranny because
of wildly different cathode voltage needed two separate heater supplies.


  • accordion 6em7 (small).jpg
    accordion 6em7 (small).jpg
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2008-01-30 4:29 am
thank rman for the info.
yeah, it seem 100 + ma for calculation of PS is what most folks are doing - I guess the 6em7 like amps more than volts....

looking at the edcore tranny, I have model the following -

180 - SS bridge - 47uf - 8H choke - 100u - 100ma load

I get 208vdc - This is about 10% les than the voltage on
the 6emt3 DC amp PS. should be ok ????

I have the parts of the McNally amp but might reorder to get the parts for the DC amp.

again thanks for the info and the pointer to edcor power transformer....I only looked for them for OT. another order I guess



2008-01-30 4:29 am
oops ...

sorry for the miss informed post.
I was looking at the wrong schmeatic. duh!!

the 6em7 dc amp required 325 volts....not 225...

well I'll put the wine away and try again....

using the 275v edcor ( or hammond) and this PS I get about 329v.

275-SS bridge-47uf-8h-100uf-100R-100uf-100ma.

I hope this is close enogh .... the 8h choke is 259ohms,

anyway, I 'll start a list of parts and go from there.