Hi
A friend of mine brought this amp to me complaining that it is blowing tubes and fuses. I checked all components on the main board, they are all fine. He brought along 4 new Electro Harmonics power tubes. I put them in, connected the speakers, turned ON the power switch. Turned the Bias all the way down on both sides. Then turned ON the standby, and set the bias to about 50mV on both sides. Then I realised that the bias is not stablising on both sides. it starts to slowing increase by 0.1mV to 0.2mV. It kept increasing constantly and at around 100mV, I turned off the standby fearing that the tube might blow. After about 15mins, I turned back the standby and the same thing starts to happen. I tried this a few times, and no change. I have read about the JCM2-60-00 and JCM2-60-02 PCB board problem on the net. It is suggested to change the 220K resistors to 5.6K resistors on the bias circuit. Does this help. Did anyone tried this successfully, or is there any other solution. Please help.
Thanks
Carlos
A friend of mine brought this amp to me complaining that it is blowing tubes and fuses. I checked all components on the main board, they are all fine. He brought along 4 new Electro Harmonics power tubes. I put them in, connected the speakers, turned ON the power switch. Turned the Bias all the way down on both sides. Then turned ON the standby, and set the bias to about 50mV on both sides. Then I realised that the bias is not stablising on both sides. it starts to slowing increase by 0.1mV to 0.2mV. It kept increasing constantly and at around 100mV, I turned off the standby fearing that the tube might blow. After about 15mins, I turned back the standby and the same thing starts to happen. I tried this a few times, and no change. I have read about the JCM2-60-00 and JCM2-60-02 PCB board problem on the net. It is suggested to change the 220K resistors to 5.6K resistors on the bias circuit. Does this help. Did anyone tried this successfully, or is there any other solution. Please help.
Thanks
Carlos
That is a bodge.
Check the capacitors feeding the bias voltage have not dried up and check the coupling; C43 and C44
Today, I tried out certain suggestions that were posted on the website. I isolated all the 4 pin, 5s(control grid) from the board by drilling out a hole around them and connected 5.6K metal film resistors (R7,R70,R66,R10) by jumping them from the board. I changed R7,R70,R66,R10 to 5.6K metal. Changed R67,R69(220K) to metal film and lifted all the resistors off the board. Replaced C36,C37,C42(47uf 63V) on the bias circuit and C6,C7 on the PI circuit. I could not get the 22uf 50V (C43,C44) NP caps as they are not available, can I use 47uf NP caps as a replacement. After doing all this, I find that the bias is still shifting up. When I removed V4 (Phase Inverter), then the voltage moves up to a certain value and remains constant. I replaced V4, but no change. Can there be a problem in the PI circuit.
Thanks
(If the voltage is incorrect, check C42 and pin1 of CON11 for -50volts ish).
If the voltage can now be set, replace C6. Test and monitor.
If OK replace C7. Test and monitor.
If OK replace individual EL34s, EL34s are notorious for grid cathode leaks when over driven in class AB2 for maximum 'dirty' power.
If the voltage can now be set, replace C6. Test and monitor.
If OK replace C7. Test and monitor.
If OK replace individual EL34s, EL34s are notorious for grid cathode leaks when over driven in class AB2 for maximum 'dirty' power.
That's expensive.I heard of an identical problem in one of my friend's tsl100.One valve died first, it was replaced and the output transformer was fried later. I would suggest to introduce a cathode resistor to ground of 100...220 ohms paralleled with 330uf in each valve's cathode .With fixed bias you need at least 10...22 ohms in each cathode.
Not absolutely sure about that but if you remove two push pull valves of all four you might see that the bias gets a bit more stabilized because the idle current inrush of the impaired valves can cause the change in b+ power supply which in turn will cause additional bias change.I think that Marshall never looked to match the valves in the final stage.You don't have this problem in any other professional amp that's at least twice the price of that Marshall.
Otherwise there's a full bias calibration procedure in the service manual:
MARSHALL DSL100 Service Manual download, schematics, eeprom, repair info for electronics experts
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Replacing the valves that are faulty, is not expensive. It is of course good practice to replace the quad mathed valves, if one has failed.
Removing the EL34s and then monitoring the grid voltage when replacing the EL34s one at a time will prove the faulty valve, if that indeed is what it is.
A complete set of Marshall (Chinese) EL34/6CA7s is £48 including free postage from that well known online store.
From RS Components around £55.00 with redress if there is an issue.
RS PRO Pentode Thermionic Valve, Octal Base, 25W, 6.3V, 36.5 (Dia.) x 115mm | RS Components
To add cathode components is not good practice. R6 and R9 are already there to monitor the current drawn.
Removing the EL34s and then monitoring the grid voltage when replacing the EL34s one at a time will prove the faulty valve, if that indeed is what it is.
A complete set of Marshall (Chinese) EL34/6CA7s is £48 including free postage from that well known online store.
From RS Components around £55.00 with redress if there is an issue.
RS PRO Pentode Thermionic Valve, Octal Base, 25W, 6.3V, 36.5 (Dia.) x 115mm | RS Components
To add cathode components is not good practice. R6 and R9 are already there to monitor the current drawn.
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Well...you can change the cheap valves with other cheap valves then you might need to change the fried output transformer which is another 100 quids.I know that because i got for free the fried Dagnall! I can tell you that that is exactly what happened to a friend of mine with a TSL100...I'd double c34/c35 to have a more sustained anode voltage.I suspect it's about unmatched valves in the final stage.That is why i suspect that using 20...220 ohms in the valves cathode might cure this problem.
If there's a pot doing the bias that should be replaced too with a good one.
If there's a pot doing the bias that should be replaced too with a good one.
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If an EL34 flashes over, the HT fuse will blow.
I have been repairing Marshall, Orange, Messa, Vox and many more makes of valve amplifiers since 1968 and have changed only one output transformer in all that time, due solely to the wrong HT fuse being fitted.
That is why the fuse is a critical part of the amplifiers protection.
Same applies to the mains transformer.
I have been repairing Marshall, Orange, Messa, Vox and many more makes of valve amplifiers since 1968 and have changed only one output transformer in all that time, due solely to the wrong HT fuse being fitted.
That is why the fuse is a critical part of the amplifiers protection.
Same applies to the mains transformer.
Hi
I remove C6, C7 and all the 4 EL34s, before removing, I set PR1,PR2 for minimum bias voltage. I turned on the amp and checked the bias voltage at Pin 1 on CONN 1, it reads approx -48V to -49V. When I on the "standby", the voltage drops about 1V to approx -48V. With the valves removed, I checked the voltages to all Pin 5 (control grid) to ground. With the standy on, they read approx -36V with the bias pots all the way down and they seem to remand stable. I put in just one EL34, turned on the standby and monitored the bias voltage at Pin 3 (out to valve). They bias voltage starts around -36V and keeps going up but the bias voltage at Pin 1( in to bias pot) seem to remain stable.
Earlier I have replaced C42,C36,C37,C6,C7, replaced the R7,R10,R70,R66,R6,R9 to metal film. I have not replaced the non polarised C43 and C44 because not available. The problem seem to remain. One thing I noticed is that if I remove V4, then the bias seem to remain stable.
What's your opinion.
Thanks
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