8417 Power Tube Conversion

[thehero]

Put another capacitor in and got some readings, nothing at c51. 23v at c53 and c54, was 33 without the modification. Didn’t blow this time but was very hot when I turned it off.

[thehero]

With all modifications removed I’m getting about-16 to -21V. It will not run 6550s or EL34s as is. I tried each and could not get the 6550s current below 100mA, which is the limit of my probe. EL34s I got down to 67mA, which is still far too high. I have rechecked the modification several times and cannot get it to work. It always blows c51. Before it blows I get no reading across c51. Also no reading through the rest of the bias circuit. I must be misreading something in the altered schematic, but I’m not seeing it.

[martin manning]

Before it blows I get no reading across c51. Also no reading through the rest of the bias circuit.

Must be a wiring error somewhere. When you say you get no reading across C51, that's DC? Is your modification putting AC voltage on it?

[thehero]

Before it blows I get no reading across c51. Also no reading through the rest of the bias circuit.

Must be a wiring error somewhere. When you say you get no reading across C51, that's DC? Is your modification putting AC voltage on it?

Yes. I just checked and I’m getting 17VAC across c51. That explains why it’s blowing. This is the layout I have the modification in. I have severed the trace between R80 and C51, replacing it with the modification. Should I have left the trace intact?

[martin manning]

This is the layout I have the modification in. I have severed the trace between R80 and C51, replacing it with the modification. Should I have left the trace intact?

Your drawing looks correct, and you do have to disconnect R80 from C51. Maybe the new cap connected to AC is shorted?

[martin manning]

Disconnect your mod from AC and C51-, and verify that the positive and negative voltages are correct. Tack some new caps and diodes together off-board as shown below, then connect that to AC and C51- and see if you get -100V.

[thehero]

You were right, there was a short at the cap going to AC. I now have the tubes biased properly. Good measurements throughout the power section. Thank you so much for your help! Now I have the issue of no sound going to the speakers. I am going to probe the preamp section, but I’m hearing pinging noises coming from the output transformer and I’m guessing that’s not a good sign and it likely needs to be replaced.

[martin manning]

Good to hear that issue is fixed. Onward! You could run some basic tests on the OT, like DC resistance and voltage ratio.

[thehero]

I do not know how find the voltage ratio. I checked DC resistance and had about 44 between center and each plate. I also removed the 100 ohm resistors that were between the heater and cathode on the tubes and I can now hear it, but it is very quiet. Channel and master up to 10 and it’s quieter than the acoustic sound from the guitar. Both channels act the same, so if it is a preamp issue it’s in the master circuit. I also tried Different tubes and that has made no difference.

[martin manning]

Good sign on the OT primary resistance measurements, equal and not open. The 100Ω resistors are probably the ground reference for the heater circuit. You need those.

Do you have any AC signal on the power tube grids when you apply signal at the input? You should get a pop in the speaker when you touch your probe to the grids too. I'd probe at the junction of the 100k grid leaks and the 1k grid stoppers, just to stay well clear of the 600V on the plates.

[thehero]

Yes, AC at grids, leaks and stoppers. Pop and hum in the speaker when those are probed, much louder than the the guitar signal through the speaker.

[thehero]

I’m thinking there may be a problem in the PI circuit. I’m getting 19VDC on plate 1 (Pin 1) and 1VDC on plate 2(pin 6). I’m not sure what it is supposed to be, but I feel like it should maybe be higher than that?

[martin manning]

I’m thinking there may be a problem in the PI circuit. I’m getting 19VDC on plate 1 (Pin 1) and 1VDC on plate 2(pin 6). I’m not sure what it is supposed to be, but I feel like it should maybe be higher than that?

Definitely. If your PI is the same as the one on the guitar version, voltages should be as shown below.

[thehero]

The two amps are the same. Both SG-100. Mine just doesn't have the reverb or phase shift circuits. So as far as plate 1, the voltage drops at R82 down to 19V and is still that at the plate. The resistor seems fine so I'm suspecting an issue with c63, I'll check that out this afternoon. I haven't really explored plate 2 yet. I'll do that later today as well. Thank you for info! it's very helpful.

[thehero]

That was exactly it! Replaced c63 and I’ve got proper PI measurements. Most importantly, the amp now works! Thank you for all your help Martin!