Next up a PSE Flea powered amp - the Mighty 71-A

analog addict

Glory or Death!
After the usual long hiatus, I'm embarking on the next proof of concept, a parallel single ended 71-A amp featuring a stout power supply with an oil filled cap and two chokes, a voltage regulated plate supply for the dual 6SN7 front end, independent center tapped filament supply for each output channel, and three, count 'em three 71-A output tubes per channel, combining for a mighty 2 to 2.5 WPC.

Why? you ask. Well, first off, because. Secondly, to prove something I've discovered with my last build, a PSE 45 featuring two 45s per channel. IMO, the 3.5 watts generated by the PSE amp has more oomph and bottom end than a similar single 2A3 per channel. Thirdly, because the rumor is that as you go down the old 4 pin triode list, the lower the output of the tube, the sweeter the sound. So far, I believe this is true, so this experiment to see if the trend continues. Anyway, here goes...

A mock up of the layout....

71 A mock up by Analog Addict, on Flickr

The mostly cut chassis, deburred.

71-A build by Analog Addict, on Flickr

The corner welds filed down to provide a seamless surface for painting.

71-A build by Analog Addict, on Flickr


I had to cut the power switch and power light holes myself, which I hate doing, since I can't drill a hole to save my life. Here's what I came up with...

71-A build by Analog Addict, on Flickr

These bulb holders are cool, because as I understand it, they were originally developed for WWII planes, so that the light can be closed off to help avoid detection. Stealth lighting to go with the missile switch.

The painted chassis, inside and outside, featuring a hammered black all in one paint.

71-A build by Analog Addict, on Flickr

71-A build by Analog Addict, on Flickr

And some of the components painted a gloss black...

71-A build by Analog Addict, on Flickr

71-A build by Analog Addict, on Flickr

Next up, populating the chassis....
 
Very interesting! And a very purdy chassis too.......

Dave

Thanks Dave, High praise coming from you. I'll post the power supply schematic when I lay my hands on it, but since I've used some of Dennis Had's circuit ideas, he's asked me not to post the actual schematic. Perhaps I can post a redacted version....
 
We likes it, preciousss. Very nice indeed.
I just finished a PSE EL84 with Edcor transformers. I was somewhat surprised that the paralleled output tube circuit sounds so excellent. Some believe that the paralleling of output tubes would create a blurring of the sound since the outputs are split then combined again. I found that this was not true at all. In fact the sound is the same if not better than the single output tube just twice as powerful.
What kind of OPT will you be using? Happy soldering.
 
Last edited:
Excellent plan! 71A is an outstanding tube. If you like 45s, you might like this tube even more.

Try some globe 71As for extra psychedelic sound!
 
I had a pair of LaScalas that I cobbled together years ago. I know everyone has their preference, but the only tube amp circuit that I preferred with the LaScala was a SRPP 6SL7/71a circuit I pilfered off the internet and made a bread-board. The filaments were DC heated with old Xentek regulated power supplies, with James universal OPTs.

My AC heated 6EA7/bridged Allied IT 2A3 SET amp had a good sound with the LaScalas, but there was a organic tonality about the 71A circuit that just lashed up better with those speakers.
SEP 6BQ5 amplifiers sounded good, but in kind of a "meh" way...

At 800 milliwatts, the circuit would only get so loud, and ran out steam pretty quick, but the sound/tonality was amazing.71a breadbored.jpg
 
I had a pair of LaScalas that I cobbled together years ago. I know everyone has their preference, but the only tube amp circuit that I preferred with the LaScala was a SRPP 6SL7/71a circuit I pilfered off the internet and made a bread-board. The filaments were DC heated with old Xentek regulated power supplies, with James universal OPTs.

My AC heated 6EA7/bridged Allied IT 2A3 SET amp had a good sound with the LaScalas, but there was a organic tonality about the 71A circuit that just lashed up better with those speakers.
SEP 6BQ5 amplifiers sounded good, but in kind of a "meh" way...

At 800 milliwatts, the circuit would only get so loud, and ran out steam pretty quick, but the sound/tonality was amazing.View attachment 1119453

Again, that's the point of the parallel outputs. All the 71-A goodness with the power of a 45...:rockon:

I've used a SRPP front end in the first iteration of my 2A3 amp. I decided it really was lacking something I couldn't quite put my finger on. Took it out when I rebuilt it and went with parallel 7308s, i.e. one tube with both sides in parallel for each channel. It was a proof of concept for this amp, which will feature a parallel/parallel design. One 6SN7 in parallel for the front end, and three 71-As in parallel for the amp section.

Excellent plan! 71A is an outstanding tube. If you like 45s, you might like this tube even more.

Try some globe 71As for extra psychedelic sound!

Heh, I'm all for full blown psychedelic sound. Unfortunately, I don't have three pairs of the globe 71-As yet. I think currently I have two pair. But I'm still looking.....
 
very nice will be watching. Globe 71-A are not that hard to find, just good ones are $$,duds have worth if globe shape. at lease you do not need 71 not A those are very hard to find good
 
Back
Top Bottom