833A (GU-48) Amp

[Mike H]

Tempted to say transistor ear but...

[pre65]

I'm accumulating some parts to complete the 833a (GU-48) project.

James D has offered to design a special buffer for me, and I've chosen to use a 12A (112A) triode with a Hammond 126C interstage to drive said buffer. I'm also thinking of using a Mike H "E choke" in the 12A ht supply.

Last thing will be the "cash in an envelope" mains toroids, I've got the secs upped to 900v now, just got to be brave enough to post the lolly.

[izzy wizzy]

Last thing will be the "cash in an envelope" mains toroids,

I have to ask, what's one of those and why?

Cheers,
Stephen

[pre65]

Normally I would use Antek in the USA, but the poor exchange rate + VAT, import duty and Parcelthief charges make them a tad expensive.

I found, purely by chance, a transformer winder in a rural area, who gave me a quote for two 400va toroids on 500va cores, originally 400v +400v but now 450v + 450v, and a good price including VAT and carriage but on a pound notes basis.

Now I have some cash (sold my caravan) I can take the plunge.

[pre65]

I have a feeling that the two E chokes I have were "fried" in an unfortunate misconnection event many years ago, So I've purchased two more from our Greg, who had some unused ones in hand.

[Mike H]

I've just noticed this topic is now > 1100 replies

[pre65]

As I cant find another Danbury DB1191, I'll experiment with back to back toroids for the 12A HT.

I'll try two 12v first, then 12v into a 15v to see what I get. I've got enough bits to lash up a practical experiment.

[pre65]

Last thing will be the "cash in an envelope" mains toroids, I've got the secs upped to 900v now, just got to be brave enough to post the lolly.

The "lolly" has now been posted.

[IslandPink]

Watch out it doesn't melt !

[pre65]

Watch out it doesn't melt !

Seems "it" arrived in perfect condition.

[pre65]

Some of the new bits for the rebuild.
1) 400va 0-450v + 0-450v toroid.
2) 130uf @ 1900v and 40A capacitor
3) 5uf @1200v (1680v max) and 64A MKP capacitor (2 off)
4) Russian filament choke for 833a (can't remember specs)
5) AA battery for size comparison

I'll detail the driving stage/buffer later when the design is finished.

[JamesD]

Nice bits and pieces. Always like seeing the components people are using.

[pre65]

Since the last picture I've received,

An A3 sheet of FR4 3mm board,

Two filament bias module kits,

Four 36v SMPS power supplies,

My Mouser order, eight transistors (4+2+2), various diodes and one hundred turret tags.

I was going to get the heat sinks from Mouser, but changed my mind, but they should be here tomorrow.

I will need to get some resistors from HFC, but I'm still compiling the list.

One thing still to be decided is how I'm going to integrate these driver assemblies to the existing amp, they must be separate because of the weight. If I can get brain cells A & B together for a while a plan (maybe cunning plan) might emerge.

[pre65]

I cleared a space on the operating table last night, and one of the amps is ready for some tinkering.

First step is to fit the Russian filament choke, and then run up the filaments to check the voltage, and give them a soak test. They have lain dormant for a few years.

Second step, will be to rebuild the PSU with the new transformer and caps. I may well run them under the old configuration with the step down interstage and driven by the KT66 (trioded) SET amps for the time being.

[pre65]

OK, filament choke fitted and wired in, with thick wire from my box of bits left over from the Chevrolet 6.5 V8 diesel ambulance I once had. Filament choke gets warm, but not hot.

I fired up the filament, just needing a tiny adjustment on the SMPS to regain 10V (@ 10A)

So if anyone had a dip in their electricity supply about 5:30 pm, it was me.

I've left the filament on, and after an hour I'll switch it off, and then prepare the other unit for the same treatment.