Jcm800 2204 Bias Problem
renips
Well-Known Member
I have the same problem. I have a 1989 2204. I have been running 6550’s for years. I can’t recall if the amp came with El 34’s or not but back in the day I put 6550’s in it. When I measured it I could only get about 12ma max. Even the 6550’s at max would read about 52ma. Being that the difference between an el 34 (25 watts) and 6550 (35 watts) is 10 watts would there be a resistor that governs the readings?
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ampmadscientist
Well-Known Member
This is because your amp is set up for 6550 output tubes.
You cannot run EL34 without making changes.
To change over to EL34, you need to remove and replace 3 parts. Other than that: you can use 6550 instead.
Soldering the circuit board can cause damage. Therefore maybe you should have a professional change it, OR use 6550 instead.
renips
Well-Known Member
AMS,
Which 3 parts need to be changed? Trying to learn.
Thanks
Which 3 parts need to be changed? Trying to learn.
Thanks
ampmadscientist
Well-Known Member
NON- master volume model (does not have separate bias winding on PT)
Master Volume Models (has separate bias winding on PT)
WARNING
Soldering can damage the circuit board. If you are not qualified, hire somebody who is.
Please don't turn your amp into a paperweight.
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renips
Well-Known Member
Thanks AMS.
I appreciate the info and I hear your warning
I appreciate the info and I hear your warning
My amp has 220k resistor in the place that you mention, so it should work properly for el34s, but it dont.
I change the 56k (R26) resistor to 36k, and now it goes up to 12ma per tube, still not enough. I plug my guitar and it has the volume of a 10w amp.
I change the 56k (R26) resistor to 36k, and now it goes up to 12ma per tube, still not enough. I plug my guitar and it has the volume of a 10w amp.
I'm surprised you're jumping right into swapping resistors without giving out some basic info. For some reason you have a bigger bias voltage than normal (more negative).
You didn't mention the condition of your power tubes, how old are they? How are you measuring the bias current?
What's the AC voltage off that bias tap? What's the voltage on the pin5 of the power tube sockets?
You need to approach the issue systematically; start from the beginning and disconnect the bias tap from the board and measure the AC voltage. It should be around 60-70VAC.
Hi, I adjunt the schematic I use. I mark in blue the voltages that I get with all the knobs on 0, also the bias pot is all the way up and still get no more than 11ma.
Im using 220v at mains, and the el34s as well as the 6n2p are new (the 6,3v wires are wired to pin 4 and 5, pin 9 is NC).
Im using 220v at mains, and the el34s as well as the 6n2p are new (the 6,3v wires are wired to pin 4 and 5, pin 9 is NC).
Attachments
-21VDC on the pin5 would probably redplate the tubes if everything else was fine; for some reason, the tubes are not drawing enough current.
Also, that bias supply voltage coming off the diode is low, should be around -60VDC.
I recently serviced an amp with very low bias voltage, around -31VDC. Turned out the bias caps (2x10uF 100V) were dead. I rarely see that in Marshall amps, especially not that old.
Is that 135VAC between diode and RB2?
ampmadscientist
Well-Known Member
The voltages are high because there is no load on the power supply.
The bias is too cold, and the power tubes are shut off.
No load = no voltage drop.
When the bias is correct, the tubes will load the power supply and the voltage will drop down. More load = more voltage drop.
ampmadscientist
Well-Known Member
You measure voltage with the tubes out, it will always be too high. There is no load on the power supply, the voltage goes up.
Bias can't be too cold, pin5 voltage is -21VDC; at that voltage, EL34's are cooking. Something is preventing them from drawing current. Maybe he already cooked them with that voltage.
He needs to post more info, pictures of that bias resistor mod (maybe he didn't use a 1ohm resistor), bias network pictures etc.
I've never seen an EL34 in my life that was drawing 12mA with that kind of bias voltage, that's not right.
This is the first amp I make, and I check all the connection and resistor values several times and all is correct.
I use a classictone output transformer and choke:
O.T: 40-18025
Choke: 40-18032
The power transformer is localy made, but it a clone of the classictone (40-18023).
I use a classictone output transformer and choke:
O.T: 40-18025
Choke: 40-18032
The power transformer is localy made, but it a clone of the classictone (40-18023).
