Is this a screen resistor problem?

[wcrankshaw]

I'm trying to figure out why my amp is producing no sound, but having normal voltages everywhere. The only thing fishy that I've found so far is that the voltage is the same on both sides of the 6V6 screen resistors, but the 470R on both tubes still meter within tolerance. Shouldn't there be a voltage drop? And if there isn't a drop shouldn't the screen resistors be shot? Thanks in advance.

[cagamp1]

It's not a screen resistor issue. Make sure your power tubes are operating properly. Are the tubes good? Heaters lit? Tubes biased correctly? Try another set of 6V6's...anything?

[Enzo]

Ohm's Law. If the resistors measure OK, and there is no voltage drop across them, then there is no current flowing through them. And when you are measuring 400VDC, it can be easy to overlook the half a volt that a milliamp of current would cause.


Are the tube heaters glowing? Bias is what type? If fixed bias, what voltage is on pins 5? And verify that there is continuity from pin 8 of each tube socket to ground. If cathode bias, then what resistance do you see between pin 8 and ground? And I mean from the socket pin to the chassis, not just reading the resistor at its leads.

You've tried different speaker? Different speaker wires?

Is the speaker absolutely silent, or is there background noise like a little hum or other sign of life?

Apply a steady signal of some sort to the input and trace it through the amp, don;'t focus on those screens, just find where the signal disappears. I mean you coulod have a bad phase inverter stage or something.

[wcrankshaw]

Thanks for all the info. I'm going to stop staring at it tonight, and get back to it later. I'll try to follow the leads you've given and get back.

[wcrankshaw]

I'm at school right now, so I haven't had a chance to test anything yet, but I wanted to get this stuff written down while I was thinking about it...

It's a 5G9, and fixed bias. Was working beautifully, and then it sounded like a couple ticking noises (I think actually within the chassis rather than through the speaker), and no sound. Each time I turn it on now there are a couple quiet electrical sounding ticks from inside the chassis.

Tube heaters are glowing with just under 3V on each side. Fixed bias, and measured about -15 to -16 V at the tube (on both). The bias supply was supplying a correct voltage around -65V. Am I looking at a failed bias cap since the bias at each tube is about 10-12V too high (should be more like -27V)?

I think the speaker is absolutely silent, but I'll have to check on this, and check with a different speaker.

The voltage on the screen was ~480 when I checked it. The spec sheet for a JJ 6V6 says 450 is all it can handle. Is it reasonable to think the screen in at least one of the tubes shorted (or burned up?)?

If this is the case, the question becomes why are the voltages so high? Have they always been so high and it just now caught up with me (after a couple weeks of intermittent use)? Or did something else fail that would allow the screen voltages to be really high?

I'm full of questions, but the help I get on here really helps me learn more.

[cagamp1]

Check everything out, Like Enzo said. That way you can see what is going on around the 6V6's. If you have a high bias voltage as said, it will cause your 6V6's to shut down (cutoff). Check your bias network from your power supply to your tremolo tube. Man, I don't care for that bias/tremolo tie-in.

Fender put higher than recommended voltages on tubes all the time. 480V is way too high though for a Fender 5G9 design...should be under 405V.

Are you sure you have a Fender Tremolux 5G9 with a 8160 power transformer? Gotta ask.

[wcrankshaw]

well guys...another novice problem. Bad tube socket. I swapped the tubes, discovered that it worked, and put the old tubes back in. Still didn't work. But noticed it was intermittent when i touched things around the power tubes and output jack. Jiggled the power tube around and it works now.

However, the bias still seems too hot at -16V instead of -25V. Not sure what a 6V6 can handle, but I feel like I'm pushing my luck. The plate voltage is a tad on the low side if that has any effect.

thanks for all the help and making me think about this logically.

[MWJB]

-16V sounds scary low, but even -25v is pretty meaningless, we need to know how much current the power tubes are drawing, do you have any bias probes? If not, can you add 1ohm 2W resistors between pin 8 of each tube socket & ground?

If your tubes are drawing too much current (too low a negative voltage), this will pull down the plate voltage. Bias should be read/set with vibrato switched off (not just with the pots at lowest setting).

[wcrankshaw]

Amazingly, living in Nashville it's difficult to get decent electronic components despite all the studios, and I haven't made an effort to order the resistors. I'll swing by a place this afternoon to see if they have any. The schematic shows the screens having 368V or so, and right now they're at 323V. It would make sense that getting the bias in check would fix that.

[MWJB]

If your transformers are original, then the plate, screen & negative grid voltages should all be high by 10-15%. The schem shows the neg bia voltage at the junction of the 220K bias feed resistors as 8% of the main B+ voltage, your amp should mirror that. Where does this low bias voltage start? If you're getting -69vdc off the bias diode & low neg voltage after the 82K resistor check that resistor value. 8uf/150v caps are new?

[Chuck H]

Not sure if you want a "working" amp or an "original" amp. Assuming this is a Fender and not a clone. It sounds like you want to fix it. The bias is too hot. Especially for those voltages. Old amps very often need to have the tube sockets cleaned and electrolytic capacitors replaced. Carbon comp resistors should be checked for value drift and film caps should be checked for leakage. If it has a two prong power cord it should be changed for a proper three prong grounded cord. I would see to these things before proceding otherwise you could go through other components and power tubes in this process of fixing the amp.

Armed with this information you could take the amp to a tech. Most any tech that services old tube amps is very aware of this process and should be able to make short work of it saving you a lot of time possibly even money before it's over. It sounds like your a busy student so...

Of course your in the right place if you want to service and repair the amp yourself. But it will take you longer than it has to and you wont have the word of a good tech that your classic amp is all snuffed up.

[wcrankshaw]

transformers are new, and bias caps are new. I get -65vdc at the diode, and -25 after the 82K. Just at pin 5 of the power tubes is where I'm seeing the lower bias voltages.

[MWJB]

What do you get at each end of the 220K bias feed resistors?

[wcrankshaw]

Chuck - thanks. It's actually a clone so it is three prong, new caps, new resistors, etc. But, I assume that doesn't change what you said about the relationship between the voltages.

MWJB - Everything is -20vdc after the depth pot. after the 82K resistor it's -25vdc. Maybe I was measuring it earlier before the bias supply was fully charged up? Still seems a little hot though.

[MWJB]

Firstly list all loaded dc voltages (trem switched off), are your negative voltage & B+ in proportion?

If the B+ is high & the neg bias voltage low, either reduc the value of the 82K, or increase the value of the 56K (either will increase neg bias voltage) & see if that cures the problem. It would still be good to know plate current in mAs.