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Marshall DSL100 bias shifting
Marshall DSL100 bias shifting
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Old 18th May 2019, 10:58 AM   #11
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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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.
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Old 22nd May 2019, 11:47 AM   #12
Carlosraj is offline Carlosraj  United Kingdom
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Originally Posted by JonSnell Electronic View Post
Remove the EL34s and C6 and C7. What happens to the bias voltage.



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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Old 22nd May 2019, 12:38 PM   #13
dreamth is offline dreamth  Romania
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Did you check the el34 power valves anode and g2 voltage with no signal in?
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Old 24th May 2019, 11:41 AM   #14
Carlosraj is offline Carlosraj  United Kingdom
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Quote:
Originally Posted by Carlosraj View Post
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.




Hi


Today, I replaced both the bias pots, took two 47uf/63V caps and connected the negative terminals together back to back in series to form a 23uf non polar cap to replace C43 and C44. I set the bias to minimum and turned on the amp, the bias started around 42mV and started going up. Earlier I was only monitoring for about 30 to 40 mins, this time I decided to let it on longer. After an hour, the voltage has gone up to approx 56mV and seem to idle around it. I adjusted the bias pot to set it at 60mV and had to go somewhere and returned an hour later. Now I realised that the voltage has dropped down to 58mV and seem to idle around it, going up about 0.1 to 0.4mV and down by 0.1 to 0.3mV. I let it on for another more than an hour and realise that it still remains around 48mV. I think it has finally stablised. I turned off the amp, and about an hour later, I repeated the whole process again and realised that it only becomes stable after more than an hour. Any suggestions.


Thanks


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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Old 24th May 2019, 11:45 AM   #15
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Sounds like you have a leaking grid/cathode in one of the valves. V4 as you describe it.
The bias voltages are about right and the 1volt drop at standby on is correct.
Lesson learnt here; keep it simple and don't replace components on the off chance. ��
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Old 24th May 2019, 01:04 PM   #16
dreamth is offline dreamth  Romania
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replacing a leaky capacitor with another leaky capacitor...that is real bad luck
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Old 25th May 2019, 11:31 AM   #17
Carlosraj is offline Carlosraj  United Kingdom
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As of today, the situation remains the same, it appears to stablise after more than an hour. I will try to replace V4 and see what happens.


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