Fender Red Knob Dual Showman is Killing Me

[doctorvoltz]

Hi guys:

This one has really got me stumped. The Clean channel works fine but the high gain channel is only very faint on high volume. I have checked everything I could think of and I believe I have tracked down the trouble spot to preamp tube V2 (which is mistakenly marked V3 on the attached schematic) It is the tube coupled to V1. I'm finding high plate voltage on pin 1 at 325 volts where schematic indicate 211 V. The same reading as if the tube is pulled from the socket. I get no clicks when I touch pin 2 grid and pin 3 cathode. I have checked and subbed the tube, socket, cathode resistor coupling cap, surrounding resistors, traces etc and can find nothing wrong, yet this half of the tube does not conduct. Help!!!!!!

[52 Bill]

What about the heater wiring for that side of the tube?

[doctorvoltz]

Thanks Bill. I am not near the chassis right now but I had the same thought. I will check later.

[doctorvoltz]

I checked heater connections, heater voltage, the socket and the tube. All good. I am more stumped than ever

[g1]

Does it measure 1.5K from the cathode to ground? What is the voltage on the cathode?

[km6xz]

It sounds as if you are missing something fundamental in the stage. Are you sure the print you have matches the chassis? The draftsman would not have made such an error in mislabeling a tube because schematics are signed off by several people.

If you have anode voltage, measured right at the pin, and a cathode return, measured right from the pin, and no grid, as you say in the first comment, there has to be some current flowing if the filament is heating the cathode. Measure right on the pins, not the socket, to bypass a possible corroded or open socket receiving contact.

In other versions of the D. Showman, the tube designations, V1, V2, etc are as you say your version represents V2 and V3.

[doctorvoltz]

If you look at the attached print, you will see that 2 different tubes are marked as V3 so mistakes are definitely made on schematics. Nevertheless, the tube in question is the one that is coupled to V1. I can only read plate voltage and I get no reading on cathode or grid. The Cathode resistor is good, however. When I get a chance to check the chassis again tonight I will see if I am getting negative voltage on the grid.

[kenda47]

Hi. doctorvoltz. I may be missing something here but I thought you said that the tube in question isn't conducting. From your description and the fact that you have renewed all components and that the voltage at the anode is the same as the DC supply voltage there is no way the tube can conduct as it is not drawing any current. Sounds dum I know but are you sure the tube is lighting up at both heaters(best checked in the dark as can be quite difficult to see). Also check from the ground end of the cathode resistor and bypass cap to chasis as you may have a ground break somewhere which will certainly prevent the tube from conducting.

[doctorvoltz]

Thanks Kenda47. But I have already verified that heater is lighting to both sides and that cathode resistor and bypass cap are good and grounded to chassis. Voltage on cathode and grid on this half of tube are essentially 0 at 2.5mv What about LDR3? if this is somehow always on and shorting the grids to ground, could it cause the same symptoms?

[kenda47]

In a word yes because if grid shorted to ground the tube would have no bias ie would be switched off which you have confirmed with your measurements and the fact the pop test did nothing. You should be reading nearer 2.5v at pin 3 not 2.5mv. OK lets see if we can identify the culprit. Couple of checks for you. 1) What measurements do you get for pins 7 & 8 ie 3b grid and cathode? 2) What is voltage reading across R39 47k grid bias resistor for 3a and R44 the 47k grid bias resistor for 3b? Also check their restance values. Please post back the readings.

[doctorvoltz]

I know cathode pin 8 is about 1.7 volts and if I remember correctly grid pin 7 is about 0v. The resistance values of R39 and R44 are correct. I am at work and not near the chassis so I will need to measure voltage across R39 and R44 either tonight or tomorrow.

[doctorvoltz]

Hi Guys:

Thanks to all. I finally found the problem. Pin 5 heater of the socket was spread just a little too wide and not making contact with the tube so half the heater was not operating. I never even suspected it as all other socket holes were tight and when pulling the tube in and out of the socket it felt tight and like it was making good contact on all pins.

[kenda47]

Well done doctorvoltz. I thought all along that was your problem which is why a couple of comments back I suggested you check the tube in the dark even though you said you had confirmed both heaters were working. It's also the reason for the voltage checks I suggested.