Harmony H430 Reverb Trouble

[Daver]

I'm working on a Harmony H430 combo that has no functional reverb. I can hear the springs in the mini tank if I hit them so I know the return circuit is working. The reverb transformer is driven by a 6973. I've tried subbing different tubes and transformer and tanks. The last two were Fender spec items I know don't exactly match the originals but I still got nothing. The odd part is the voltages on the 6973. The plate and screen are at 352 volts. The grid measures -0.850 and the cathode only measures 0.150 volts. That last one seems really low. The cathode resistor is two 330 ohm 1 watters in series bypassed with a 33uf cap. It measures 630 ohms. I have continuity through the tube socket and to all the components the socket goes to.
I think I'm getting tunnel vision. Any ideas? Thanks.
Dave

http://www.webphix.com/schematic%20h...armony_430.pdf

[Chuck H]

If you have the correct tank, tried a known working tube, appropriate transformer and you know the return circuit is working then there MUST be a fault in the input circuit. The voltages don't look right to me. Check the cathode bypass cap, it could be shorted though that would ususally have more profound symptoms. Check for ground faults in the circuit. It's possible that the input transducers on the tank are bad. Subbing an incorrect tank won't always give info because of extream impedance differences though I don't think thats the case here. Check to see if the 6973 is conducting when the reverb should be operating. If it is then the tank transducers are probably bad. If it isn't, and this is what I suspect, there is a fault in the circuit between the coupling cap that feeds the drive tube and the tank transducers.

[riz]

The reverb send cord?

[Daver]

riz, the send cable has continuity and is not shorted to ground.

Chuck, I don't think the cathode bypass cap is shorted. I can measure the cathode resistor's value just fine and it has been replaced recently. I can try taking it out and see what happens. I know subbing in incorrect components is iffy, but I don't know where you'd find the correct items these days. I did hook up the Harmony reverb tank to my Super Reverb. It works, but is a bit weak compared to the original. I assume this is due to a mis-match. So I think the tank is OK. I measure 270K to ground from the drive tube's grid, so that isn't grounded. I agree the voltages seem weird. I would have thought I'd have more than 0.150 volts on the cathode. Thanks for the help!
Dave

[Jazz P Bass]

That cathode voltage is way off.
I would think 3-6 volts would be more like it.
Did you sub another tube?
What voltage do you measure on either side of the plate resistor.
You mentioned 352Vdc on the plate, but what is the B+ voltage onthe other side.
It sounds to me like the tube is not conducting.

[Chuck H]

+++ Recheck the ground for that cathode, as I mentioned earlier.

[Daver]

Jazz,
I did try another tube. There is no plate resistor, it's driving a reverb transformer. The voltage feeding the transformer is the same as what's on the plate. I agree that it appears to be not conducting.

Chuck,
I removed the bypass cap with no change. I measure just the resistance of the meter leads when checking the ground for the cathode resistor, so the ground seems good. There must be something I'm overlooking. Thanks!

Dave

[Daver]

OK, now we're getting somewhere. Three used, but supposedly good vintage 6973's don't work, but a newish EH one does! But, it takes about 20 seconds for it to start to work. I'm a bit confused. The old stock tubes work in the power amp but not for driving the reverb and the newer EH will drive the reverb but needs an extra 20 seconds to do so. Ummmmmmm. D'oh! Any ideas? Thanks.
Dave

[Daver]

Aha! A forth old stock 6793 works for the reverb without any warm up delay. This amp has a separate power chassis on the bottom of the cab. The other 6793's don't like to work when they are mounted pin side up! When pin side down in the power amp or tube tester they seem to work fine. When pin side up in the main/preamp chassis like driving the reverb they don't work! Wild! I learn something new every day.
Dave

[Chuck H]

Found a schem for that amp. It looks like the HV for the reverb driver is seperated from the rest of the power supply (somewhat). I would check the integrity of the filter caps and resistors in that supply. Also, the schem shows a single 300 ohm resistor to ground for the cathode of the driver tube. No series resistors and no bypass cap. Is it possible (based on what you see) that there was a problem with this circuit before and someone has already worked on it? Make sure that power supply rail is capable of delivering the current needed by the circuit. Here's the schem I found.

http://harmony.demont.net/documents/...armony_430.pdf

[riz]

Maybe this isn't helpful, but I wonder if all this tube subbing has cleaned the contacts of the tube socket somewhat?

[Chuck H]

I thought the same thing. But I couldn't figure how that would warm up slowly as described. Still, there's only something to be gained by cleaning and retensioning the sockets.

[Daver]

Guys, I had already cleaned and retensioned the tube socket. The filter caps are new. They are some other slight deviations from the schematic, but the two 330 ohms resistors in series look to be factory stock. So, it appears to be the tubes themselves that were the problem. I can switch back and forth and the same ones fail and the same ones work. Strange but true. Thanks for all the help and suggestions. I'm going to finish cleaning all the sockets, pots and jacks and burn it in for a while. If it continues to work then it's done! Thanks!
Dave

[riz]

If it works, it works... Something to be said for that!