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832A power amp

I know what you mean. I can't take credit for the original design, I just adapted it for use with a tube rectifier and somewhat higher voltages. I'm looking forward to building another pair with premium parts and my Eico HF81 OPT's.
 
This should be on another thread, but I have a couple of Heathkit PS-23 power supplies (700 and ~300VDC). With a little tweaking on the 300VDC leg, they would be excellent for powering some 829B monoblocks. I do like the idea of remote power supplies....

Comments, suggestions, am I nuts?
 
I have a handful of these tubes, as well. I was planning on using them with the sections paralleled and strapped as triodes, though. These are tubes that have been on my project list since i first contracted whatever virus it is that makes be want to use lightbulbs for amps.
 
I'm not sure I would use them strapped as triodes. The maximum rating for the screen on the 832A and 829B is about 250VDC. This would limit your plate voltage to roughly the same and severely limit your power output. Maybe another forum guru knows more than I do about these tubes, but by all means do some research on them strapped as triodes.

Just my 2 cents....not that it amounts to a hill of beans.....been wrong many a time!
 
I know of someone who has run them strapped as triodes at 400v without issue. I'd most likely start much lower and gradually see what the can take. Nice Allen 20 in the avatar. :)
 
What the heck, give it a try. Mine get rather unhappy at much over 250 volts on the screens, but it could be due to the fact that they are not strapped as triodes.

I am impressed that you recognized the Allen 20. Great amp. I rebuilt a pair of them for a friend of mine. About the only thing original on them are the sockets and the transformers. Should be good for another 40 years. Fabulous sound.
 
Hi Guys,

Here's an 832 amp I built about a year ago. Same schematic. It works great and sounds very good.



Regards,
 
Nice workmanship! Is your version the original schematic (with solid state power supply) as shown on the website?
 
For the next pair of 832A (or 829B) amps, I'm thinking of using a pair of 6AU4 or 6CG3 damper diodes in the power supply, another choke for added filtering in the screen circuit, and maybe fixed bias.
 
Nice job!

I am right now in the midst of wiring an 832A stereo amp. The physical cutting, painting, and bolting together of the chassis, trannies, and sockets is done. Now I must tune and wire. I went with iron I already had, so I will be using a higher B+ compared to the starting schematic found elswhere on the web. Here's what I had in the garage, so here's what I have on my chassis: 300-0-300 - 200ma PT, 8K - 8R OTs, 5H-200ma choke, 5U4GB rectifier. So it looks like my B+ will be close to yours.

I have available as preamp/drivers a bunch of the 12AU7, 12AX7, 12AY7, & 6EU7 type tubes. Have you tried anything besides the 12AT7?

I wondering if the higher B+ necessitated any changes in your schematic. I see there are two differences that I would expect - a larger cathode bias resistor 330R vs 270R, and a larger grid resistor 27K vs 4K7.

What about 330uF cap at the 12AT7 cathode, and the coupling cap and resistor - .68uF/470K vs .22uF/390K - parts on hand or tuning? I plan on doing some experimenting or tuning myself, but I thought you may be able to share some experience.

Thank you so much in advance for any advice. I'm still a noobie at this (and may always be...).
 
Good morning and thank you.

I have since changed the 12AT7 to a 6CG7. Just change the filament connections, no other changes needed. To my ear, the 6CG7's sound a little bit better and are less microphonic. Your mileage may vary. If you don't want to use the 6CG7, I like the 6EU7 better than the 12AX7.

As far as the cathode bias resistor on the 832A, with a 270 ohm resistor, the plates would just start to glow red. Too much plate dissipation. With a 330 ohm resistor, all is well. If you can find a pair, try using the QQV07-50 tubes in place of the 832A. No changes needed, quite a different sound and very quiet. I'm also (at this time) using a vintage pair of Sylvania 5V4's for rectifiers. These have a bit less voltage drop with respect to the 6087 and hence a lower power supply impedance in presented. With these two changes, my power supply transformers still run only warm to the touch.

All other changes were strictly tuning. These are running my pair of Khorns that have the EV-T350 tweeter. Great tweeter, but ruthless with respect to hiss and noise. Good luck and post a thread, I'd love to see what you have come up with!

Cheers!
 
Thanks for the advice. Its always nice to talk with someone who's been there before turning something hot-n-red yourself. I was thinking of using 6EU7's but thought that they might actually be too high of a gain. I have a bunch of NOS 6EU7 on hand - Mullards, Amperex, RCA, Sylvania, etc - so at least there wouldn't be a quality or availability issue. Hmm...

I still have to figure out how I want to connect the top pins on the 832's. I could tightly wrap solid core wire around and then "clamp" it on with heat shrink tubing. I actually have some colored tubing that matches the colored wire which would look cool. Hard to roll tubes that way. If I could just find a readily available connector of some sort.

What do you think of the russian equivalent GU-32 tubes that are newer and readily available on ebay cheap. Might be worth playing with since they're only 25 bucks for a quad.

I was thinking that if I enjoyed the finished 832A amp, that I might try the 829. It seems as though it would work just fine in the same schematic.
 
No worries, the 6EU7 is basically the same tube as the 12AX7. The pin assignment was changed to reduce capacitance and the filament changed to a 6 volt.

For the top pins, I use one half of a female spade terminal, generously wrapped in heat shrink. Slightly crimp the terminal end (before you put it on the tube!) so it is a snug fit.

Ah, the 829 tube. It's on my list of projects. I don't know about installing it in the 832A amp. The 829 likes a plate voltage of around 600 volts. or so. Maybe this weekend I will try it in my monoblocks. As far as the Russian tubes go, I haven't tried them yet, but would like to.
 
TOP PINS FIND
I found a nice connector for the top pins. I got some "D-Sub connectors" from the local radio shack - p/n 276-1428. The radio shack package includes a housing (throw that part away) and several small female pins. The pins are just a little small so they provide a snug press-over fit on the 832 male pins. Solder on the OPT leads and add a little heat-shrink, bam! I like it.

CAP QUESTION
I see that you've used 100uF caps on the B+ supply to the 12AT7's and that the other schematic floating about the web had 33 and 47uF caps. I was wondering just what drives this value (within reason)? I have no 47 uF caps on hand and I am a bit tight on room in the chassis, but I do have a bunch 'o 22uf caps. I could use 22uF at each location or double them for 44 uF at each location. Any advice?

THANKS in advance for any help you can give.
I'm almost ready to put this thing on the variac. Can't wait!
 
Just Mikey,

Thanks for the tip on the top post plate connectors. I'm gonna have to try that!

You should be okay with the caps for the 12AT7 (or even better, a 6CG7) B+ that you mentioned. I am a little bit excessive-complusive about well filtered and tight power supplies for hi-fi tube amps, hence the values I choose. Make sure and use a 100uF for the screen supply. I tight voltage on the screens is pretty much essential with these tubes.

Let me know what you think of your amp when you're done. I was very pleasantly surprised with mine. I have a pair of them running my rear khorns at the moment. Very happy with them.
 
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