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Old 5th May 2016, 03:42 AM   #1
wianieq is offline wianieq  United States
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Join Date: Dec 2008
Default Tubelab se 300b monoblocks

Hi everyone.

I have been piecing together a pair of monoblocks using 2 se boards. I finally had them comlpeted enough to start the checkout and biasing procedures. I followed the advice on another thread about doubling r3 an r6 and did that, otherwise all parts are derived from the parts list. I'm using the Edcor xpwr131 transformers and Transcendar 10w 5K outputs, using the c14 chokes. My b+ is a bit too high at around 450v, b- is at -277. I have 47uf caps for c4 but I have 15uf motor run caps that I was going to use when the clamps come in the mail. That will drop it a little.

Another problem I have is that the voltage on both of the 5842's will not rise above 155 and 150. I added a 1k to r17 to bring that down to 500ohms but that only raised it to 165. I can't wrap my head around the b+ being that high but then the 5842 plates being that low, unless there is some kind of interaction between components when both channels are in use. It seems to me that everything should be higher if anything.

Can anyone suggest other areas of the circuit that I should address for this issue? I'm almost thinking that I could live with that, but I have to bring the b+ down and I thought raising the plate voltage on the 5842's up to 175 will help in that regard.

Any advice is greatly appreciated.



Last edited by wianieq; 5th May 2016 at 02:45 PM.
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Old 5th May 2016, 12:20 PM   #2
wdecho is offline wdecho  United States
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Your B+ will drop when a load is placed on it. Not to worry. I am not sure about what is happening with the plate voltages on the 5842's. I am having some of the same problem myself. I keep burning out the CCS's and transistors. My B+ starts out about like yours without a load but now it drops way to low when the tubes are installed along with the plate voltages of the 5842. I have ordered some more and different Mosfets with a higher V rating plus some more 10M45S's I am in the process myself of trying to figure out exactly what is happening. Mine tested fine and played for a few days and then the semiconductors started failing. The amplifying part of the amp does not have that many components. I expect you are having problems with the semiconductors as well. I feel like it may be some interaction with the output tubes I have chosen. I am using the Genalex PX300B.

Last edited by wdecho; 5th May 2016 at 12:37 PM.
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Old 11th May 2016, 06:29 PM   #3
wianieq is offline wianieq  United States
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Join Date: Dec 2008
wdecho, thanks for the reply. I am using the EH gold grid 300b's.

I haven't been able to work on these until last night. I was able to fix the 5842 issue by replacing the 510ohm r21 resistor with a 1k. After trying multiple cap swaps for c4 I could not get the voltage below 450. It dawned on me finally to use the 260v taps on the power transformer. That set my voltage at about 350 and 362. I have 5k outputs and want that voltage higher. In the end I don't think the xpwr131 just wont work for monoblocks and I probably need something like a 275-0-275.
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