hey, so i have bought a Marshall jcm 900 sl x 100watt , re tube the amp head with Sovtek 6L6WXT , bought a bias probe kit, plugged in the probe, and the tubes are measuring 70mA! that's hot right?! i tried to adjust the bias trim pot, but the bias reading isn't moving! can anyone on these forums help?
Hi,
If this is a second hand amp you could have a leaking coupling cap to the power tubes! Or the bias supply may be at fault.
As Hi-Q says you need to start by measuring the bias supply!🙂
Are the tubes red plating inside "not the heater" the grey metal plate in side is it red?
As a second thought! What tubes are supposed to be fitted? 6L6? If this is a mod the bias supply may not be high enough for the 6L6. I would have thought 5881 or EL34 would have been the standard fit!
Regards
M. Gregg
If this is a second hand amp you could have a leaking coupling cap to the power tubes! Or the bias supply may be at fault.
As Hi-Q says you need to start by measuring the bias supply!🙂
Are the tubes red plating inside "not the heater" the grey metal plate in side is it red?
As a second thought! What tubes are supposed to be fitted? 6L6? If this is a mod the bias supply may not be high enough for the 6L6. I would have thought 5881 or EL34 would have been the standard fit!
Regards
M. Gregg
hey, the slx is a 5881 tube amp, but thought the 6l6wxt was a 5881 type/ new replacement of the 5881, as for the coupling cap what is that, i have an understanding of tube amps but might need help with some parts of the amps, as for the attachment i will measure the resistors thanks for the schematic, thanks
Do you have any of the original tubes? If you do do they bias correctly?
Sorry if you know about tube amps, however I need to make sure you are aware that 400V + may be inside the amp and that the amp may not have bleed resistors to drop the voltage so it could hold charge after power off for some time. It is lethal!
The coupling caps are C11 & 12 on the drawing from Hi-Q. If this has only happened since a re-tube of the amp then tube type is suspect! Linked to bias voltage.
Regards
M. Gregg
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i threw the other tubes away as one of them blew when i used it for 2 hours /the tubes where already in the amp, the amp was bought second hand but the original tubes where tube amp doctor 6l6wgc, as for the measurement of resisters 26 to 27 i could only get a measurement of 140k ohms i dont know what setting to put the multimeter on
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Its voltage DC that you need to measure.
It is the bias voltage between r26 & r27 to ground that you need to check.
Have you worked on tube amps before?
If you make a mistake and touch the 400v rail with your hand it may kill you!
Just for interest, if you can increase the current through the tube but not decrease it, then the -bias voltage is to low. This would require a mod such as a voltage doubler on the bias supply.
Also check Fs1 fuse is not blown!
Regards
M. Gregg
It is the bias voltage between r26 & r27 to ground that you need to check.
Have you worked on tube amps before?
If you make a mistake and touch the 400v rail with your hand it may kill you!
Just for interest, if you can increase the current through the tube but not decrease it, then the -bias voltage is to low. This would require a mod such as a voltage doubler on the bias supply.
Also check Fs1 fuse is not blown!
Regards
M. Gregg
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hey, no i haven't really tried to fix a tube amp, my friend is an electrician, i have done a little studying on electronics, i am guessing you shouldn't touch capacitors with your hand what else is there you should avoid touching inside an amp, i am just learning while trying to fix the amp,
is this bias pot fixable, what are the usualy reasons for an pot to not adjust the bias current, thanks
is this bias pot fixable, what are the usualy reasons for an pot to not adjust the bias current, thanks
mortie,
You need to learn with your electrician friend helping you. Show him this information and post what you find. Some one will pick up what you post and take it further.
Please do not go any further until your electrician is with you. I know you are interested and I want you to learn and be safe!
So please post what you find after you show this to your friend!
I cannot be confident without help with you so I will stop at this point!
Regards
M. Gregg
ok, thanks for your help, like i said i have an understanding of tube amps, and i will not touch the tube amp circuit only with the probes which are insulted, i have not touched the capacitors as they store electrical charge, but can you tel me which parts of the amp shouldn't be touched with a probe at all, i am just trying to see what needs fixing on the amp head need to play some tunes haha!
i measured the current of r26 and t27, both measured 42volts, ii put croc clips on the resisters and one on the ground so i did not have to put my hands in the amp, so the measurement of r26 and r 27 is fine right?
i have measured the right points? just each resistor
or is there a certain resister leg that should be measure, thanks
i have measured the right points? just each resistor
or is there a certain resister leg that should be measure, thanks
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All my Marshall JCM amp circuits I have indicate EL34 output valves. Your last post is confusing as you should be measuring Voltage and not Current. The correct voltage monitor point is shown on the bias circuit I attached, namely between chassis and the junction of the two 150k resistors. This voltage should be adjustable using the bias control and I would suggest you set this provisionally for maximum negative voltage. Then check that a similar voltage is obtainable at the junction of the 150k and 1k5 resistors i.e. Junction of R24 and R27 then junction of R25 and R26. Any large shift to a positive value could be caused by leaking coupling capacitors C11 and/or C12. Also a leak within either of the output valves or their bases may cause this.
Remember you are working with parallel pairs of valves, a good one with a bad one beside it will upset the good one!
Heed the warnings given about high voltages, the energy stored under certain fault conditions could very easily attract the attentions of your local undertaker.
Take care!
P.S. As there is only one control for biasing, it would suggest that a matched quad set of valves may be required because you ideally want all four valves to draw roughly the same current with the same common bias applied.
Remember you are working with parallel pairs of valves, a good one with a bad one beside it will upset the good one!
Heed the warnings given about high voltages, the energy stored under certain fault conditions could very easily attract the attentions of your local undertaker.
Take care!
P.S. As there is only one control for biasing, it would suggest that a matched quad set of valves may be required because you ideally want all four valves to draw roughly the same current with the same common bias applied.
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hey, the tubes are meant to be all matched according to the seller, is there a way of seeing if the tubes are at fault by measuring thrm with a multimeter?, also if if it is the resistors or capacitors, will they need replacing? thanks
Il just say again, im not recommending working on anything with no experience, and its very dangerous, but have you heard about "the one armed technician"?, it wont prevent possible death, thats always there, but it breaks any circuits that will go accross your chest. Theres plenty of high voltage safety guids, and dos and donts.
hey, i will just re tube the amp with new tubes, as some of you said it could just be the tubes right? but all the tubes are reading 70ma
also when i was measuring the r27 and turning the bias pot the current did change, does that show the pot and resistors are working?
also when i was measuring the r27 and turning the bias pot the current did change, does that show the pot and resistors are working?
it would seem its working, but it seems there isnt enough negative voltage for a much lower current
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