Please help me figure out why my plate voltage is too high here

[mfaughn]

Working on a Magnavox 8601 from the schematic below. The voltage on one of the plates of the 6EU7 is fine, the other is way too high, ~185V, and that channel isn't working. On the supply side of each plate resistor I've got about 200V. The plate resistor and cathode resistor on that triode of the non-working channel are within spec. It isn't the tube (tried more than one). I've jumpered input from the working triode to the non-working channels power tube and that worked fine (though I didn't disconnect the non-working plate from that same tube when I did it). I still need to check C6/C7 and R22/R23. R22/23 looked fine but I think (in that I'm still a novice kind of thinking) that could be causing it if one were seriously out of spec. Could either of those be causing this? If not them then what could be doing this?

Thanks for any assistance.

[aardvarkash10]

thanks for the nice big diagram... for next time, most screens are set to 1200 pixels width - no point posting an image wider than that.

Measure and report these voltages please for both channels:

voltage between cathode and ground, 6BQ5
voltage between grid 1 and ground, 6BQ5
voltage across the opt primary

Cheers!

[mfaughn]

yea, I tried to post immediately after and apologize for the size but for some reason it didn't get posted. It is a link to somebody else's photobucket post and it didn't look that big in the page from whence I copied the link.

Will check those voltages when I'm less sleepy.

[Alain Poitras]

Originally Posted by mfaughn

yea, I tried to post immediately after and apologize for the size but for some reason it didn't get posted. It is a link to somebody else's photobucket post and it didn't look that big in the page from whence I copied the link.

Will check those voltages when I'm less sleepy.

If you get 185V at the plate, it is because the current in the triode is very low ... The only thing I can see is the 820 ohms autobias resistor of the not working channel is open or the soldering was cold.

You can measure the bias of both channels between the ground and the cathodes of the triodes to compare, the current in each triode suppose to be about 0.77ma and you got about 0,085ma ... You suppose to have about 1,654V bias ... Because the voltage divider made by the 150K resistor and the parallel 820 ohms and 6800 ohms feedback resistor.

If the 680 ohms is open like I think, you will get about 9,76V bias and the operation point of the triode will be very close to cutoff and this channel will not work at all ...

With the power off, if you measure the resistance between the cathode and the ground, you suppose to have about 731 ohms, if you have near 6800 ohms, it is because the 680 ohms is open ...

Cheers,

Alain.

[DF96]

Bad valve socket? If the anode pin is not connected, then you may still get a bit of voltage drop across the anode resistor due to leakage of the coupling capacitor to the next stage.

[mfaughn]

Originally Posted by aardvarkash10

thanks for the nice big diagram... for next time, most screens are set to 1200 pixels width - no point posting an image wider than that.

Measure and report these voltages please for both channels:

voltage between cathode and ground, 6BQ5
voltage between grid 1 and ground, 6BQ5
voltage across the opt primary

Cheers!

6BQ5 voltages:
Cathode to Ground = 5.6 both tubes
Primaries = 260V both tubes
Grid to ground on working channel = ~30-35mV (music playing)
Plate to ground on non-working channel = continually rising. got up to ~115mV after a few minutes

see other post for more details

[mfaughn]

Originally Posted by Alain Poitras

If you get 185V at the plate, it is because the current in the triode is very low ... The only thing I can see is the 820 ohms autobias resistor of the not working channel is open or the soldering was cold.

You can measure the bias of both channels between the ground and the cathodes of the triodes to compare, the current in each triode suppose to be about 0.77ma and you got about 0,085ma ... You suppose to have about 1,654V bias ... Because the voltage divider made by the 150K resistor and the parallel 820 ohms and 6800 ohms feedback resistor.

If the 680 ohms is open like I think, you will get about 9,76V bias and the operation point of the triode will be very close to cutoff and this channel will not work at all ...

With the power off, if you measure the resistance between the cathode and the ground, you suppose to have about 731 ohms, if you have near 6800 ohms, it is because the 680 ohms is open ...

Cheers,

Alain.

Cathode to ground current on both triode sections = ~1.1A
When I take the reading on the (what I thought was) working channel the volume increases noticeably (i.e. more than doubles).
The channel that doesn't make any sound at all doesn't change when taking the reading.

Also, on the channel that isn't making any sound at all. The voltage of the incoming signal is about -50mV while on the working channel it is ~50mV. I am sourcing from a digital audio player and I think this might be normal for them to do (wouldn't it change the phase of the headphones?), but I'm not certain of it.

Deoxit in the sockets and worked around a bit. I think they are clean. They look good, no loose pins or broken tabs.

It is definitely time for me to learn something new

[Frank Berry]

You can't measure cathode current by using an ammeter across the cathode resistor.
That is why the volume increased.
You must measure the voltage across the cathode resistor and calculate the cathode current from that voltage.

[mfaughn]

Originally Posted by Frank Berry

You can't measure cathode current by using an ammeter across the cathode resistor.
That is why the volume increased.
You must measure the voltage across the cathode resistor and calculate the cathode current from that voltage.

There, I did learn something. Thanks. Will get back to it tomorrow. Already started in on the adult beverages this fine and finally not so hot Saturday evening. Ethanol and HV don't mix well.

[kevinkr]

Originally Posted by mfaughn

<snip> Ethanol and HV don't mix well.

Well said, and I couldn't agree more.. Kudos for realizing that..