See the link below to my build thread. Having issues with the input tube plate voltage.
Long Overdue SE Build
Long Overdue SE Build
Couple things.
Went through all caps and visual inspection of semiconductors.
My b+ is 450v. B- is 260v.
I am using FDP5N50NZ.
Looking at U2/3 now.
Shouldn’t b- be around 150?
B+ seems high. Does adding power tubes drop b+ that much?
Went through all caps and visual inspection of semiconductors.
My b+ is 450v. B- is 260v.
I am using FDP5N50NZ.
Looking at U2/3 now.
Shouldn’t b- be around 150?
B+ seems high. Does adding power tubes drop b+ that much?
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So I think found the problem. I have bad cap values for c4, c2/3, and c6. I am gonna pull the board and check via the schematic. I am not sure it jives with the parts list for caps. Or I built this when I was not sober. Which could be possible. 🙂. C4 is my fault. I added a 0 to it so it’s 470uf.
Thanks for the input.
Thanks for the input.
Update
Replaced 1 cap and 2 resistors.
No input tubes
B+ is 442v
B- is -260
Grids are -104v & -120v
Input tubes
B+ 438
Grids are 175v & 172v
Can’t adjust the lower value any more. I run out of range but it is close enough for me.
Replaced 1 cap and 2 resistors.
No input tubes
B+ is 442v
B- is -260
Grids are -104v & -120v
Input tubes
B+ 438
Grids are 175v & 172v
Can’t adjust the lower value any more. I run out of range but it is close enough for me.
New issue popped up. Grid voltage of the output tubes went to almost zero and my b- is now defunct. Checking into d2/d3.
Any other advice?
Any other advice?
Swapped out diodes d3/d2 and now have 305v at R5 but nothing R7 so my B- voltage is still 0.
Looking for other areas to troubleshoot? Any suggestions?
Looking for other areas to troubleshoot? Any suggestions?
Check the 270 ohm resistor R5. Some of the smaller parts can't handle the initial turn on surge and burn out, often without looking bad. Use a 2 watt metal oxide or wirewound part and all will be good.
Ha. R5 is toasted. Time for my 20th mouser order in the last 2 weeks. 🙂
Thank you for the help. It is much appreciated.
Thank you for the help. It is much appreciated.
All through checkout. Need to order 300b tubes and had the rectifier tube go bad.
B+ Is 430v
B- is -290v
Grid V is -131 & -143v but pots ran out of room to adjust.
Input tube is @170v but pot ran out of headroom to turn.
Time to order output tubes and do some final assembly.
Thanks for the help.
B+ Is 430v
B- is -290v
Grid V is -131 & -143v but pots ran out of room to adjust.
Input tube is @170v but pot ran out of headroom to turn.
Time to order output tubes and do some final assembly.
Thanks for the help.
So new issues; been chipping away at this guy.
I can not adjust the voltage on 5842 over 165. My pots run out of throw.
The other issue is my 300b bias gets to over 65ma at start up with the lowest voltage.
The voltage goes to -165v for both channels.
I also found out 1 of my meters will not read mA even though it has the settings. I was using voltage instead.
I can not adjust the voltage on 5842 over 165. My pots run out of throw.
The other issue is my 300b bias gets to over 65ma at start up with the lowest voltage.
The voltage goes to -165v for both channels.
I also found out 1 of my meters will not read mA even though it has the settings. I was using voltage instead.
Some notes from testing.
Grid voltage on output tubes can adjust from -25 to -120v.
R5,6,7,18,29,33,34 all ohm out fine for values.
Still working through troubleshooting.
Grid voltage on output tubes can adjust from -25 to -120v.
R5,6,7,18,29,33,34 all ohm out fine for values.
Still working through troubleshooting.
We are dealing with 5842 tubes that are old and were pretty widely scattered parameter wise when built. It is possible that yours are a bit outside the range of the current board setup. The easiest way to fix this is increase the value of R10 and R21. If you have a 620 ohm in there now go to an 820 or even a 1K. This will give you more adjustment range.
Not quite sure what the grid voltage can go to, but -120 volts should run a true 300B to near cutoff (under 10 mA) even with 450 volts on it's plate.......but what is a true 300B? Many current production 300B's tubes are a bit different from the WE spec, so maybe your tubes need a bit more negative voltage.
This can be accomplished by increasing the value of R11 and R22, or reducing the value of R13 and R24, whichever is most convenient. Change the value about 10%.
Neither of these problems should prevent the amp from operating. Run some music through it and make sure everything works before changing lots of parts.
Thanks for the advice.
I measured voltage across R29/18 in the picture shown.
~.5v across both resistors with -130v bias voltage (measures w/o tubes).
Got sound from 1 channel. Got some awkward squawking from the amp so now I powered down and am working to find why only 1 channel works. Swapped all tubes and the problem didn’t follow.
Back to troubleshooting.
I measured voltage across R29/18 in the picture shown.
~.5v across both resistors with -130v bias voltage (measures w/o tubes).
Got sound from 1 channel. Got some awkward squawking from the amp so now I powered down and am working to find why only 1 channel works. Swapped all tubes and the problem didn’t follow.
Back to troubleshooting.
Attachments
Got sound to both channels now. 1 of the output trannies had an connection issue that is now fixed.
Back to biasing.
I have .7 and .5 volts through the resistors.
Shouldn’t that mean I have 70/50 mA of current?
Back to biasing.
I have .7 and .5 volts through the resistors.
Shouldn’t that mean I have 70/50 mA of current?