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Mosfet for Bias control in ARC VT200
Mosfet for Bias control in ARC VT200
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Old 26th July 2019, 09:44 AM   #21
audiomania is offline audiomania  Germany
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Hello,
Yesterday I was at a friend's house,he has a VT100mkII.
Anode resistor R44 of 1ohm died. Put in a new one,and this tube actually works fine,but the V12 has too much bias.(there are some traces of voltage-sparking [?] near V12) Measuring the inputs bias without all 6550, does not give good results. Shortly after Q1 died and now I need to replace it.
Have any of you come to a conclusion.
Does that DN2530 really work; should I order DN2535 also, just in case?

Hope we will get these amps working again and find the right Fet for all the Audio Research amps...Mosfet for Bias control in ARC VT200

Regards, audiomania
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Old 26th July 2019, 10:52 AM   #22
savu is offline savu  Ireland
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Quote:
Originally Posted by Ketje View Post
Better replace the fet circuit with bipolar transistors, much more reliable current setting.
And put 1M grid leak for the 6922, 2M2 is asking for trouble.
Mona
If it's fixed bias CF then you can go as high as 10Meg

Silviu
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Old 26th July 2019, 03:02 PM   #23
audiodoc is offline audiodoc
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Quote:
Originally Posted by audiomania View Post
Hello,
Yesterday I was at a friend's house,he has a VT100mkII.
Anode resistor R44 of 1ohm died. Put in a new one,and this tube actually works fine,but the V12 has too much bias.(there are some traces of voltage-sparking [?] near V12) Measuring the inputs bias without all 6550, does not give good results. Shortly after Q1 died and now I need to replace it.
Have any of you come to a conclusion.
Does that DN2530 really work; should I order DN2535 also, just in case?

Hope we will get these amps working again and find the right Fet for all the Audio Research amps...Mosfet for Bias control in ARC VT200

Regards, audiomania
I couldn't get it to work with the DN253 nor will ARC send /sell you the parts.The MOSFET's in the VT200 are a little different.I don't think there is much difference though.
BluesTM got his amp to work.I broke down and had a local tube amp specialist look it over. Cultrit still seems to be the mosfets.Give it a shot but don't put the 6550's in until your bias voltages are correct or your toast a bank of 6550's.
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Old 28th July 2019, 02:48 PM   #24
audiomania is offline audiomania  Germany
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Thx audiodoc for your quick reply.

Reading a lot and trying to understand. The Q1-2 are just normal CCS with 3,4mA as I see it.
It shouldn't be a problem to adjust a depletion mode MOSFET to give 3,4mA on around 36V
I have to get into depletion mode more to understand and find a solution.
After getting the frontend working good again, I will have to look at why R44 got shot and even the mainsfuse got blown. Looks like a problem there also.

Audiodoc, did your local tube specialist fix it and how?

Regards, audiomania
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