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In Reply to: RE: 1959 Ampex 6973 Monoblocks posted by rlschow on April 28, 2010 at 22:33:59
K40y-9 couplers for the .1s and .068s and .047s depending on the ones you have. I have a pair that belongs to a friend in Ca and I used VIt Qs in his but k40s are great also.
For filter caps,I like using a polyprolyene coming off the 6ca4 and a 15 to 20uf at 450v will do nicely.
On the other side of the resistor coming off the rectifier,use 100uf and down stream use another 100uf. I can post a photo later.
Also,you have to change some wiring that erodes over time.
If the power supply waveform isn't pretty,neither is the sound in most cases.
Follow Ups:
> > I can post a photo later. Also,you have to change some wiring that erodes over time.
Michael,
I'd really like to see any photos you have.
I now have a copy of the correct schematic thanks to Tom Bavis.
This is encouraging! Thank you.
If I can't find 6973's I'll try the 6CZ5's. I have yet to hear anybody say good things about the EH's.
I've got some VitQ's in my stash, this might be just the use for them! Very interesting: Mundorfs or Solens for the first filter, then big electrolytics downstream. Did you try subbing the first resistor with a choke?
I did not sub the resistor with a choke.There isn't enough current in those amps to justify the use a choke especially if you use good polys.
If the power supply waveform isn't pretty,neither is the sound in most cases.
Nice score! Be careful using the 6CZ5. The schematic has 345V on the plate (B+ = 375), but when I measure mine the plate voltage is actually ~360V. That's OK for the 6973 since it is rated for a 440V max plate voltage. But the 6CZ5 has a max plate voltage of 350V, so you will be pushing the limit there. I don't know why but the EH 6973 tubes look much smaller than NOS. I use K40-9y caps in mine. They are killer amps for sure.
Dan
And how does it Sound Michael?
The Mind has No Firewall~ U.S. Army War College.
FABULOUSSSSOOOO!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
If the power supply waveform isn't pretty,neither is the sound in most cases.
Looking at the schematic and the inside of my chassis, I see the cathode resistor connects to a bias balance circuit. I've seen this before on the Heathkit W-3AM (see attached). This could be nice given that real 6973's are so hard to come by and if they don't match well, this compensates. But then if I did have well matched outputs it might sound better to bypass this network and take the big cathode resistor directly to ground. Any opinions on that?
No
leave the cathode bias balancing circuit in because its a fantastic way to match the tube currents. Look at the actual circuit and you will see by the time you measure the cathode to ground,your resistance will pretty close to just putting in a resistor and this way you can get almost perfect current draw from each tube. We know tubes usually don't stay matched over time and being this is a PP circuit,matching is more critical with the output stage.
If the power supply waveform isn't pretty,neither is the sound in most cases.
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