First fire-up voltage problem [SOLVED]

[benoit]

Just looking at the pictures...

What is the 150k resistor on the Dale plate resistors doing? Looks like it is just going from B+ to ground. Is that correct?

This problem has to be something simple....were just missing it.

Can you post the schematic and layout that you used?

That resistor is supposed to approximate the load of the fet input which I did not include. The circuit is bone stock ODS-100-HRM except that the relay circuitry isn't included in that one. Relays work fine though so should be an issue. As for the layout, I did the best I could to lay the thing out according to pictures. My preamp board is almost a dead copy, only it's a little bigger the short way. I did do a drawing in visio but really if you have gutshots of the real thing, that's basically what I went by.

[moj067]

What that resistor coming off your plate resistors for V1.Does that go to a ground on the strip? I don't know what going on underneath,but I've only seen one slope resistor there before,may be a variation for you or something, just asking.

[benoit]

What that resistor coming off your plate resistors for V1.Does that go to a ground on the strip? I don't know what going on underneath,but I've only seen one slope resistor there before,may be a variation for you or something, just asking.

See the post above yours. FET simulator resistor since I did not build the FET board. It's just a 150k to ground. I could try lifting it once and see what happens I guess.

[benoit]

I lifted the fet sim resistor just for grins and it didn't make a real big difference. No surprise there but anyway...

The real interesting revelation came when I soldered in a 500k trimpot (initially set at about 200k). By adjusting the trimpot while tracking voltages on v1a and v1b plates, I discovered that 68k gives me about 195V on v1a and v1b plates.

This is just plain wrong. Any idea why it would happen like that?

[moj067]

Oh sorry, yeah I guess that makes sense. Just not thinking ths a.m..
Maybe resolder your joints and check the cathode grounds.
Good luck!

[danotron]

OK..try this.....disconnect the input resistor (22k), disconnect the cathod cap(5uF), disconnect the coupling cap to the next stage. Now the circuit is in it's most simple form....just two resistors and a tube....take some measurements...I hope it measures good...if so, add the components that you just disconnected one at a time....

[benoit]

I tried measuring voltages with cathode cap disconnected, input resistor disconnected, and coupling cap to next stage disconnected. Plate went up about 5V. So what then? The socket? I guess there could be something goofy under-board but that would be a HUGE pain to deal with at this point.

[benoit]

I tried measuring voltages with cathode cap disconnected, input resistor disconnected, and coupling cap to next stage disconnected. Plate went up about 5V. So what then? The socket? I guess there could be something goofy under-board but that would be a HUGE pain to deal with at this point.

[danotron]

now turn the power off, pull the tube out and measure the resistance of the plate resistor and the cathode resistor.....if they read correct it must be the tube socket thats at fault

[benoit]

now turn the power off, pull the tube out and measure the resistance of the plate resistor and the cathode resistor.....if they read correct it must be the tube socket thats at fault

The plate resistor should measure correctly in circuit, right? When I measure in circuit I get 110k. WHen I desolder one end and measure I get full value. The cathode resistor measures fine in circuit, as do the v1b plate and cathode resistors. So maybe it IS something under the board?

[Bob-I]

The plate resistor should measure correctly in circuit, right? When I measure in circuit I get 110k. WHen I desolder one end and measure I get full value. The cathode resistor measures fine in circuit, as do the v1b plate and cathode resistors. So maybe it IS something under the board?

It sounds like there's something else touching that plate resistor. With the tube out and power off it should measure the full value.

[kleinm]

I've caught goofy mistakes by looking underneath the board with a mechanic's telescoping mirror and a flashlight. You could try that before you lift the board.

[danotron]

correct me if I'm wrong but the plate should read 220k...well 220k in parallel with another 220k = 110k...so someware you have another 220k resistor parallel with it...

Where else do you have a 220k....another stage or eveb the power supply...check around for any little ends touchimg where they shouldn't....check continuity (with power off) to other 220k resistors,is there a connection?

[dogears]

I don't think this is so. If the plate load were 110K, the stage would be biased such that the voltage would be high and not low.....

correct me if I'm wrong but the plate should read 220k...well 220k in parallel with another 220k = 110k...so someware you have another 220k resistor parallel with it...

Where else do you have a 220k....another stage or eveb the power supply...check around for any little ends touchimg where they shouldn't....check continuity (with power off) to other 220k resistors,is there a connection?

[benoit]

I pulled the board out today and realized there were several leads of various components that were still at their full length, sticking out at various angles under the board. I believe one was connecting v1a cathode to part of the audio circuit. That was the only thing obviously wrong and I'm really hoping this fixes the problem.