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Thespeakerdude8

Thespeakerdude8

Super Member
Woke up this morning to go to school and thought since today is my last day of finals I would bring my dremel screwdriver and some tools with me, on the off chance that I would stop by one of the GW's and find something.

Didn't have much luck at the area stores, but I found something neat at the dist. center. I didn't feel bad about parting her out, the old Baldwin Orgasonic was beat to hell on the outside.

Look what I got for $10....:thmbsp:
40 Sylvania 6SN7GTA's. They test alright on my B&K 700, around 60 out of 100.. not sure what new is supposed to be for a 6SN7. Wish I had a readout in mircomhos. :sigh:
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Nice Monoblock! Large tx next to the power tranny is the choke, OT is next to the filter cans. It's got two 6L6G's and a 5U4GB. All tubes are Sylvania.
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I had to save the heater supply for bragging rights- thats one HUGE 6.3v transformer!
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I'm still trying to make sense of the power supply for the amp. I traced it out and drew up a schematic in paint. Notice how the filter choke is between the HV center tap and ground, with the first filter cap being grounded to the top of the choke, like they wanted to float the ground for some reason :scratch2:. The center tap output on the 5V winding I have seen before, so, while odd its, not new to me.
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Cool, you got the early octal version. Mine had about 30 some each of 12AX7 and 12AU7. IIRC mine had a more standard PS with the filter choke between the plate supply and screen supply of the output tubes.

mike
 
Forgot to mention if it means anything, the 6SN7's are all tall black "T" plates, not the standard wafer style grey plates. I started sticking them in my guitar amp and none seem to be very microphonic! :thmbsp:
 
Hi speakerdude,

I parted one of these about 5-6 years ago. Mine had all Raytheon 6SN7's. The amp power supply uses the choke in the return leg to minimize the voltage on it. It doesn't matter which side of the power supply the choke is in, the current still passes through it. Are you sure the 4uF isn't on the rectifier side of the choke? I made the amp into a guitar amp. Since then I've also parted out an old Allen church organ and got 150 6SN7's (many Sylvania's) from that.

John
 
The power supply isn't that unusual of a design. Lots of field coil setups ran the field as a filter choke on the B- side of life, pretty much like your schematic has.
 
I never found a schematic for mine (model 61P) but it also had a tapped inductor wired as an oscillator with one of the (I think it was) 12AU7s on the main amplifier board. There were also oscillators on each of the tone generator chassis as well so I am not sure what the one on the amp chassis was for.

mike
 
Amazing what a little dusting and proper care can do for your tube amp :thmbsp:



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Just kidding! :D I stripped the chassis today and painted it silver hammertone. I stripped the transformer bells and painted them with red epoxy, and freshened up the laminations with flat black. I replaced all the hardware as well. The skirt needs to be filled and stained yet.

Next up is rewiring it! It only has two coupling caps since it uses a paraphase splitter, so I will use Solen .22uf caps. Not sure why the filter caps had to be huge and rated for 600v, but I imagine it was for a reason. I'll be stacking two 8uf 500v capacitors to make one 4uf 1000v capacitor for each 4uf cap I have to replace in the power supply. I don't know why I typed 30mf originally, as it was really two 4mf caps.

Nerdorama: I thought the same thing about reducing the voltage on the choke. Doesn't this allow it to pass more current and have greater inductance than it would at higher voltages?
Gadget: That would make sense to reduce the voltage the wires traveling out of the chassis to the speaker have to carry.
 
Hi speakerdude,

You're quick off the mark to have the amp this far so fast. It looks nice. The inductance won't change in the return leg because of the voltage. It's the voltage drop across the inductor that it sees which won't change in the return leg. I think it's more an issue of the insulation rating of the magnet wire. Modern products probably have higher ratings. Someone else can correct me if I'm wrong. It also let them isolate the ripple current in the first capacitor and keep it out of the chassis ground since ground is the other side of the inductor.

Regarding the power supply caps, if it were me I'd check the voltage of the transformer secondary and get a properly rated first cap of 30-40uF to increase the smoothing and not exceed the rectifier's rating. The 4uF caps were there (I suppose) strictly based on availability and cost in the day. You could get a motor run cap with a 440-480vac rating and I suspect you'd be fine. If I remember, my amp's B+ was in the high 400's or maybe 500V. If you measure the no load secondary AC voltage, you can make a worst case voltage estimate.

John
 
Quite a nice resurection there!

Yep as others have said quite often these organ amps have the chokes in the "negative"
side of things, also as mentioned if you measure your high voltage windings you can see
what rated caps you really need, then you can try an model your supply in PSUDII also.
 
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