New amp build - pics and progress so far.........

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MikeR670
Posts: 85
Joined: Sun Jul 10, 2016 7:56 pm
Location: B.C. Canada

New amp build - pics and progress so far.........

Post by MikeR670 »

Hi all
Been working on a new build from a big old Garnet pa, with excellent help and advice from members here.
The old pa was a 65 lbs five channel unit with 12ax7 preamps, 12au7 pi and four 6550C push pull power tubes. I also have a couple jj 6v6gt tubes available, and I like the sound of those.

I've wandered around various ideas for a build, chasing a single ended clean headroom 8 watts practice amp or a low watt push pull with good cleans and nice creamy breakup when pushed. Eventually realising that I've only garnered enough knowledge to keep it very basic. I've concluded that building an amp to produce the sonic textures in my head is some way away from me at this point.

So I thought the best step would be to build a small single channel push pull unit, from the parts I have - modelling it on being similar to the old layout until I fully understand what everything does. Maybe with the only addition being a switch to idle one power tube, as a low/ high output deal.

I really wanted something compact in size, I'm not worried about weight as using the old transformers will make it heavy anyway. I also wanted only tubes and transformers visible outside the chassis.

The original chassis:
rps20170409_120010_200.jpg
rps20170409_115933_518.jpg

I hacked it up to use as a prototype. I'll make an aluminium one to put the guts into once it all works. This is what I have so far, with the big four filter caps now mounted inside and a small board I made to hold the electrolytic caps:
rps20170409_120130.jpg
rps20170409_120224_570.jpg
It's now 14" long, to go into a 16" case I'll make. It'll still end up weighing 30lbs but what the hell.

I thought I'd go with one 12ax7 preamp tube used both sides and one 12au7 used one side for maybe first preamp and the other side for pi. Then onto two 6v6s or two 6550C s with the idle switch to run one only if that works out. I re mounted those sockets on the front of the chassis. Using the original power structure from the pa offers me 400-0-400 from the pt, and five dc voltages after caps and resistor circuit: 590, 515, 390, 315 and 290 - so I figured I could abandon the first two higher ones and use the lower three for powering preamps and 6v6s. Or perhaps use the 515 and 390 into the 6550s, to run them a little lower? A little confused in this area.....my main questions are:

Can I just build a straight forward 12au7/ 12ax preamp circuit from a web schematic, and pi?
Could I then figure a 6550 power circuit from a web schematic and work out bias with my dmm which measures amps? Like cathode resistor bias type?

Also, the old unit had a ss rectifier circuit on a little board which I'll reuse, shown here:
rps20170409_122329_836.jpg
It has a reversed diode wired to the 63v cap and a yellow wire coming off it, which went to the power tubes - is that a bias circuit and do I need it?

Any help and suggestions at this point would be hugely appreciated. Thanks!
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MikeR670
Posts: 85
Joined: Sun Jul 10, 2016 7:56 pm
Location: B.C. Canada

Re: New amp build - pics and progress so far.........

Post by MikeR670 »

This is the original pa schematic, thanks to David Root for finding it:

http://ampgarage.com/forum/download/fil ... &mode=view
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