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EL34 Baby Huey Amplifier
EL34 Baby Huey Amplifier
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Old 13th February 2019, 10:51 PM   #321
Grumpish is offline Grumpish  United Kingdom
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Join Date: Mar 2018
Got the first board built and connected, but I can't set the bias on the tubes. It comes on with B+ of 430v off the 270v winding (my mains voltage runs at close on 250v most of the time) which starts to drop as the tubes warm up and the bias starts to climb... and keeps on going up, until I bottle it and pull the plug at somewhere around 80ma (0.8v on a 2V meter setting) with the pots set around the mid-point - I start to get a increasingly loud buzz from the speaker at the same time. I have been over the board, no dry joints or shorts, the only thing I could find was that soldered D5 in the wrong way round - I have corrected that but it still does the same thing. Any clues where to start looking?
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Old 13th February 2019, 11:21 PM   #322
Bfpca is offline Bfpca  Canada
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Join Date: Feb 2007
Location: Nova Scotia
EL34 Baby Huey Amplifier
The first thing to do is to remove the tubes. Check your HV and +- bias voltages to make sure all is ok. Next you want to check the bias voltage adjustment range. Meter on DC volts and - lead on ground. Check the voltage on each el34 control grid. You should be able to adjust it to a high enough negative value to reduce current to the desired amount. With 400+ volts you will need at least -45 volts. Run the bias pots through their entire range to make sure you have a good smooth voltage adjustment. Then return them to the maximum - voltage position.
Keep in mind that the pots work backwards. The left one adjusts the right tube/test point and visa versa.. That confused me the first time I tried to bias the outputs.
If the bias voltage is not adjusting or won’t go to at least - 40 then there may be an issue with the circuitry. If you have say, -50v max. then set the pots to max. voltage, power down, then plug in the 12ax7 and adjust the balance pot for equal plate voltages on each side of the tube. After that is setup input a signal and check for equal ac drive voltages at the output tube grids ( as well as the -50v dc)
Power down, install the output tubes and you should have a low plate current which you can then adjust up to your desired amount. Those opposite bias pots take some getting used to and perhaps that was what was causing your adjustment problems to begin with. Anyway, good luck getting it running.
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Old 14th February 2019, 10:01 PM   #323
Grumpish is offline Grumpish  United Kingdom
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Join Date: Mar 2018
Thanks, that was helpful, I was able to get around 60ma on the tubes (KT888's so well within the limits) with that and dropping R39 down to 7.5k. Although I think I may have fried something by firing it up with D5 reversed, I only get 20v across one side of the 12AX7 and nothing across the other side and no adjustment, plus no adjustment to the bias on one of the output tubes. I will build up the second board a bit more carefully, and see how it goes.
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Old 14th February 2019, 11:50 PM   #324
Bfpca is offline Bfpca  Canada
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Join Date: Feb 2007
Location: Nova Scotia
EL34 Baby Huey Amplifier
Reversing D5 means that you put + voltage on C10 instead of - If c10 is still working you are lucky. You probably damaged the current source transistors Q1 and 2 and possibly the LED. Does the LED light up when you have D5 connected properly?
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Old 17th February 2019, 07:37 AM   #325
kp93300 is offline kp93300  Malaysia
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Join Date: Mar 2007
Marc,

I decide on regulated HV because my input ac ranges from 210 to 240V I refer to your TENA power supply schematic in post 299.

1) what is voltage rating for C17 and C19 for the HV power supply ? Can I use 10Uf or higher instead of the 1UF ?
2) What is the wattage rating for the 1N4742A?
3) Can i use IRF 740 in place of IRF 840 ? My input supply is about dc 430V

Thank you

Regards
kp93300

Last edited by kp93300; 17th February 2019 at 07:38 AM. Reason: formatting
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Old Today, 06:38 AM   #326
Hanze Khronye is offline Hanze Khronye  Australia
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Join Date: Jan 2018
PCB's arrived today. They are very sturdy and with good layout.

Thanks to Prasi (buy with confidence).

HK.
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Old Today, 08:04 AM   #327
bandol83 is offline bandol83  France
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Join Date: Nov 2004
Location: near Bandol, French Riviera
Hi kp93300,
I am in holidays with limited access to my files, but HV capacitors should be 500 V or 630 V. For polypropylene capacitors 1 or 2.2 uF should be enough, 10 uF will be very big...
The Zener diode is 12 V, see datasheet.
IRF740 or 840 will be OK.
Rgds
Marc
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