blown ot?
Moderators: pompeiisneaks, Colossal
blown ot?
Hi, I have never had this problem and I want to be sure before I buy a new one. With a 275-0-275 SS rectified PT and Hammond 270FX PP I was getting about 350v on the 6v6s, 340v on the screens, 300v on the PI, B+4 292v, B+5 305v. Last night I would get a low volume distortion with several seconds of normal volume clean then back to distortion.Swapping out all tubes, giggling wires and tubes made no difference.Then it quit the normal clean sound . I isolated the PI and power tube section and I got the same results. I now get about 60v over what I used to get on every node. With the standby ON I get about 300v on the hot side of the switch. With the standby OFF I get 410v on the power tubes. Any advice please.
Re: blown ot?
How can B4 have lower voltage than B5? That defies Ohm's Law. The B+ voltage is measured at the + side of the cap and not somewhere else?
It is fairly easy to test an OT. The idea is to run low voltage on the primary and measure that comes out the secondary. You need a source for low A/C voltage. If you have nothing else, pull all the tubes and capture the 6.3VAC filament supply. Use a couple of jumpers with gator clips. Run from one leg of the filament supply to the plate pin of one power tube, and run the other jumper from the other leg of the filament supply to the other power tube plate. Power up and leave the standby in the open position (set to off). Put a meter on the secondary connection for the VAC output.
It is necessary to lift the center tap connection on the OT primary to do this test. Disconnect it from the B+ supply.
This is strictly a matter of the transformer's turns ratio. If this is 8K to 8 Ohms, the turns ratio is the square root of the impedance ratio. 8000/8 = 1000. Sqrt (1000) ~ 32. If your input to the primary is 7v, then the output should be 7/32 or about .2VAC.
If this is too low for your meter to resolve, then you need to run the test in the other direction. If you have a speaker jack, you can clip one jumper to the tip and one to the sleeve. In that case, you expect 7*32 or 224VAC.
If the turns ratio(s) check out (check all taps as one may be partially shorted) then your OT is probably OK. Of course, adjust all your calculations to the actual voltage supplied.
It is fairly easy to test an OT. The idea is to run low voltage on the primary and measure that comes out the secondary. You need a source for low A/C voltage. If you have nothing else, pull all the tubes and capture the 6.3VAC filament supply. Use a couple of jumpers with gator clips. Run from one leg of the filament supply to the plate pin of one power tube, and run the other jumper from the other leg of the filament supply to the other power tube plate. Power up and leave the standby in the open position (set to off). Put a meter on the secondary connection for the VAC output.
It is necessary to lift the center tap connection on the OT primary to do this test. Disconnect it from the B+ supply.
This is strictly a matter of the transformer's turns ratio. If this is 8K to 8 Ohms, the turns ratio is the square root of the impedance ratio. 8000/8 = 1000. Sqrt (1000) ~ 32. If your input to the primary is 7v, then the output should be 7/32 or about .2VAC.
If this is too low for your meter to resolve, then you need to run the test in the other direction. If you have a speaker jack, you can clip one jumper to the tip and one to the sleeve. In that case, you expect 7*32 or 224VAC.
If the turns ratio(s) check out (check all taps as one may be partially shorted) then your OT is probably OK. Of course, adjust all your calculations to the actual voltage supplied.
Re: blown ot?
Hi PhilS, On the B4-B5 voltages I had resistors to the preamp nodes coming off the screen supply Matchess style instead of the dropping style. It was easier for me to match the voltages on the schematic this way. Your OT test helped me find the problem in a roundabout way. I was taking the filament voltage from a heater post on the tube socket and the post broke off at the socket. I replaced it and the amp works fine now. Thanks you for the reply though it probably saved me from a sleepless night.
Re: blown ot?
Sure, glad to help even though I didn't do much. So glad it worked out to be a simple fix. I doubted there was a problem with the OT, but the test is fairly harmless, so I figured why not rule it out. What you described was more indicative of an intermittent connection. I didn't expect what you found, and I guess you didn't either!