JCM 800 Bias Problem

wattsup · Post by **wattsup** » Tue Feb 09, 2016 7:44 am

I have an 80's JCM 800 that has bias issues. When I try to set the bias around 35 or so milliamps, it will seem to settle for a minute then start creeping up pretty high. It gets as high as 98ma before I shut it down. Tubes are brand new, but it will do the same with older tubes also. Anyone have an idea as to what might cause this?

sluckey · Post by **sluckey** » Tue Feb 09, 2016 12:04 pm

Was it stable before you put new tubes in? Is the negative voltage on pin 5 drifting down as the current drifts up?

Stevem · Post by **Stevem** » Tue Feb 09, 2016 12:15 pm

If you have not replaced the two bias caps do so first, but replace them with 100uf caps rated at 75 volts!

wattsup · Post by **wattsup** » Tue Feb 09, 2016 3:37 pm

sluckey wrote:Was it stable before you put new tubes in?

It wasn't stable before. Had a set in already and thought those might be bad. Put new tubes in, same thing.

sluckey wrote:Is the negative voltage on pin 5 drifting down as the current drifts up?

No. Pin 5 voltage stays stable.

Littlewyan · Post by **Littlewyan** » Tue Feb 09, 2016 6:18 pm

No don't change the caps for 100uF, they'll take longer to charge up which means it'll take longer for your bias voltage to get up to the right level. Stick with 10uF. Do replace them though if you have some spare capacitors.

Also remove the PI valve to make sure there is no signal going to the power valves as well.

How did you notice the issue? Did a set of valves blow?

wattsup · Post by **wattsup** » Tue Feb 09, 2016 7:20 pm

Littlewyan wrote:No don't change the caps for 100uF, they'll take longer to charge up which means it'll take longer for your bias voltage to get up to the right level. Stick with 10uF. Do replace them though if you have some spare capacitors.

Also remove the PI valve to make sure there is no signal going to the power valves as well.

How did you notice the issue? Did a set of valves blow?

Thanks for the advice. I'll do that.

Putting in a new set of power tubes. Checked the bias and it's all over the place.

I'm kind of suspecting the bias trim pot too. It has a tough spot it catches really hard at about a 1/4 turn. Once it hits that spot the bias goes crazy. Could that also be a culprit?

Littlewyan · Post by **Littlewyan** » Tue Feb 09, 2016 7:23 pm

Hmm maybe. So the amp was absolutely fine with the previous valves? Could be the trim pot. Maybe bias it real low and watch to see if it does the same thing. Resistors can sometimes be bad in that they'll change resistance as voltage is applied so I imagine trim pots can do the same.

JazzGuitarGimp · Post by **JazzGuitarGimp** » Tue Feb 09, 2016 7:27 pm

The trim pot definitely sounds suspect.

wattsup · Post by **wattsup** » Tue Feb 09, 2016 8:00 pm

Littlewyan wrote:Hmm maybe. So the amp was absolutely fine with the previous valves?

No it was doing this with the old tubes also.

Littlewyan · Post by **Littlewyan** » Tue Feb 09, 2016 9:08 pm

Ok start with removing the PI valve and seeing if the valves bias stays steady. Then try changing the bias caps.

jaysg · Post by **jaysg** » Tue Feb 09, 2016 10:27 pm

JazzGuitarGimp wrote:The trim pot definitely sounds suspect.

I can't tell what you what is wrong, but basically trim pots and electrolytic caps are cheap and far less reliable than a carbon film resistor. Look a those components and make sure that bias ground is really ground. You don't see nylok nuts in production amps, but I like them. A dirty slightly loose screw & nut can create real havoc.

Stevem · Post by **Stevem** » Wed Feb 10, 2016 12:14 pm

In my test you need to keep the amp in standby all of 5 seconds longer for a100 uf cap to charge up as compared to a 8 to 10 uf filter , and the bias is much more stable when the amp gets pushed!
If this added time is a problem for some, so be it !

Littlewyan · Post by **Littlewyan** » Wed Feb 10, 2016 1:24 pm

It just makes the amp a bit more fool proof. I often flick the standby switch off on my Express after 10 seconds. On my JTM50 I don't use it to save the GZ34.

cbass · Post by **cbass** » Wed Feb 10, 2016 7:00 pm

Coupling caps can be an issue. With bias problems also. Yes elctros and trim pot are more likely to fail. I like to use f&t 33uf 160v caps in the bias circuit if they'll fit. A bit more filtering no issues with a start up.

pdf64 · Post by **pdf64** » Wed Feb 10, 2016 7:04 pm

Stevem wrote:In my test you need to keep the amp in standby all of 5 seconds longer for a100 uf cap to charge up as compared to a 8 to 10 uf filter , and the bias is much more stable when the amp gets pushed!

Bear in mind that the T of the reservoir RC feeds the T of the next RC, so the total time time reach its end V will be rather longer than that.

Also prolonged overdrive act to charge up the bias caps; a long T will clearly take longer to charge, and so yes, more resistant to brief moments of overdrive. But prolonged overdrive will still charge it up and the time to recover (perhaps moving out of cut off back to its intended operating point) will be commensurately longer too.

So for bias supplies, fairly short T with a reasonably low source impedance may be seen to make for better performance when overdriven.

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