Bias problem

[M Fowler]

JCM900 dual reverb head schematic page 4 with EL34's 100w.

Found broken white wire pin 3 V6 EL34 fixed. R30 22k/1w burnt replaced with 22k metal oxide connects to junction of R22 (82k) & R23 (91k).

Replaced dead .500mA fuse tube protection circuit.

Using bias probe only get 17mA but I want 32mA plate voltage is 483vdc 25w EL34's

Schematic page 4 of 5 attached notes -38v to -40v

Not sure if I decrease R28 (47k) to increase mA bias?

[martin manning]

Yes make R28 smaller to increase idle current. Are you getting the -38 to -40V on the grids now?

[M Fowler]

Thanks Martin,

Getting -46v on grids now.

[Phil_S]

The clear image of the bias circuit is a the bottom of page 3. Not sure whether -50V is the relevant number as PT's may vary a bit?

Dumb question, maybe, but...before you fiddle with R28, is VR2 present and have you set for max and min (both ends) and what does the combination of VR2+R28 read at max and min. Like Martin, would like to know what you are seeing at the grids, now, with the pot at both ends.

Instead, you might consider reducing R7 56K. I don't know if it's easy to do, but you can tack something (220K?) in parallel to R7 and see what happens.

[M Fowler]

Current at VR2 goes from basically 0 or below 8mA to max 17mA

voltage is -46 to -88

Photo is a different JCM900 I worked on last year but same circuit for reference.

[martin manning]

Seems reasonable to modify the bias circuit to get voltage in the range shown on the schematic.

[M Fowler]

reduced to 22k and now get max 21mA bias.

Negative voltage at grids is now 38v-63v range using VR2

Still got a long ways to go to get 32mA

Yeah if I decrease R7 (56k) I should get an increased negative voltage swing to the bias circuit increasing the current so I can get to 32-35mA range, right?

[Phil_S]

I think you will get more of a range change by modifying R7, which affects the source. I am thinking those EL34's are pretty well spent if 38-63V isn't in the range. Have you got a way to test those tubes or a spare set of EL34's?

[M Fowler]

Already tested the old and new tubes.

I will insert another new set and set how the bias changes.

[martin manning]

Makes me wonder when it takes unusually high (less negative) voltage to get the desired idle current, especially with 483V on the plates. I agree your target current is in the right range. Could be worn or unusually low emission tubes, or possibly low screen voltage. Here you should have screens very close to plates with only a 100R dropping resistor between those nodes. Check the triode-pentode switch to make sure it is doing what it's supposed to do too. If all that looks right, smaller R28 or larger R29.

[M Fowler]

Thanks guys, I'm confused as to why I even have to change the resistors it should already be in range, something else seems off to me.

[pdf64]

Was the amp originally intended for 5881 power tubes, as they tend to require a bigger bias voltage relative to plate current?

[Colossal]

Thanks guys, I'm confused as to why I even have to change the resistors it should already be in range, something else seems off to me.

Completely agree Mark...something is amiss. Have you checked the bias circuit caps and that the individual resistors are still giving their stated value?

[M Fowler]

Dave, yes I have checked the resistors and caps.

The amp has 1k screen resistors so I am sure it is EL34 based and I last serviced this amp 3 years ago with new EL34's and he has been gigging weekly.

I might get back to it tonight but ran out of energy today.

Mark

[M Fowler]

Well I did have to decrease resistor values to get the range I wanted and all is well now, thanks guys.

Replaced both 10uf/100v bias circuit caps.
Replaced R7 from 57K to 47K voltage drop
Replaced R28 from 47K to 33K bias range
Replaced R30 because it was burnt with 22k/2W
Replaced blown .500mA fuse
Fixed broken white wire pin 3 V6 EL34 (tube bad)
Quad of JJ EL34's