Here are a few images. Reason I'm using the field coil is I already had it and used my other field coil in my Supro Amp build. It does a nice job at reproducing low bass frequencies...maybe not super low, but good for recording purposes. I'd like to try and use it. Obviously if it doesn't sound good, I'll get a more suitable speaker. I'll disconnect the NFB and ground the speaker as in my other amp.
Here are a few other voltage readings...with the amp on, 225vdc at the (red wire) primary of the output transformer and -145vdc at the input of the field coil. My Supro amp had a -53vdc at the field coil input and a 395vdc at the primary of the output transformer. This is with the output tubes and rectifier tubes installed in both amps.
Without power tubes installed, just rectifier: (Resistor Not Hot)
Filter Capacitors:
33uf = 525vdc
22uf = 525vdc
22uf = 515vdc
22uf = 507vdc
Well, found the culprit. One of the 7591 octal sockets that I had used (which was a previously used socket) had pin 8 grounded to one of the four ground points on the socket holder...just a little bitty bus bar that I totally missed.
Here are my new voltages and the resistor isn't hot anymore. The NFB network has been disconnected and the black wire from the OT is now connected to ground.
These are measurements taken after the hot resistor episode? If so I would check your wiring carefully, as something was drawing a huge current from the screen node. Are you sure that there is no short in either of the 7591's?
Edit: Simul-posting... Well, there it is.
Not to contradict Steve, but I see no reason not to ignore the black CT wire on he OT sec and ground one side.
This 13.65mA current was measured by shunting across the OT winding? Yes at 450V, you would divide 19W by 450 to get 42.2mA, then 70% of that is 29.5. The voltage will drop as you dial up the current, so you'll likely have to iterate once or twice.
Yes, in milliamp mode across the OT center tap and one of the 7591 pin 3 connections. The voltage drop was 3.89vdc and 3.71vdc at the different sides. 302 ohms and 265 ohms winding resistance.
So I'll dial up the current using the bias pot? Never done this before...that's why I'm asking. The bias put is at it's maximum negative voltage reading so I'll need to decrease the negative voltage to increase the current, is that correct?
So if one side is 302 ohms times .0295A = 8.91vdc and the other side is 265 ohms times .0295A = 7.82vdc...I would be looking to increase the voltage drop across the transformer sides. Should I split the difference or go for the lower voltage drop? And will the current reading also stabilize at 29.5mA?
The voltage drop on each side goes with the resistance on that side. One tube has a bit more current than the other, so use that side. You'll come up short on the dissipation anyway because the plate voltage will drop.
So I put the preamp tubes in and one of the 12ax7's (V4) heater's quickly glows and then trickles down to a normal glow giving a slight constant buzz in the speaker. This is with no instruments plugged in. After some trial and error, I turned the treble pot to full on (clockwise), powered the amp up and the tube heaters don't quickly glow on that tube. There is still a buzz in the speaker though. With the tube out of the circuit, speaker doesn't buzz.
I don't know how the heater could be affected by the treble pot. Try a different tube there and see if you still get the hum. If so I suspect it's a lead dress issue. Get voltages on all tubes too.
dawsonaudio wrote:So I put the preamp tubes in and one of the 12ax7's (V4) heater's quickly glows and then trickles down to a normal glow giving a slight constant buzz in the speaker. This is with no instruments plugged in. After some trial and error, I turned the treble pot to full on (clockwise), powered the amp up and the tube heaters don't quickly glow on that tube. There is still a buzz in the speaker though. With the tube out of the circuit, speaker doesn't buzz.
Any ideas where to check here?
If it's an old Philips tube, they're supposed to glow brightly at start up. I suspect the buzz is caused elsewhere and the tube is just amplifying it.
1141]http://www.serviceairsocal.com/amp/1.jpg[/img]
