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Found schematic 50W EL34 AB. Needs help...
Found schematic 50W EL34 AB. Needs help...
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Old 15th April 2018, 06:48 PM   #11
artosalo is offline artosalo  Finland
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I have tested different EL34 PP amplifiers with fixed bias and with 6F12P triode pentode front end. With 4k output transformer and 460 V +Ub the available output power is from 55 W (UL) to 72 W (pentode).

Attached the schematic and some test results.
Attached Images
File Type: gif EL34_6F12P_UL.GIF (34.6 KB, 358 views)
Attached Files
File Type: pdf EL34_6F12P.pdf (44.0 KB, 52 views)
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Old 15th April 2018, 10:40 PM   #12
Zekk is offline Zekk  Singapore
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Hi Arto...Can I use your GU-50 power supply for this schematic?
Thanks!

Zekk
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Old 16th April 2018, 05:59 AM   #13
CaterpilarSK is offline CaterpilarSK  Slovakia
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Quote:
Originally Posted by kodabmx View Post
You should be able to get full power with about 1V input.
Great. Thanks. I sent you a direct message. Please check.
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Old 16th April 2018, 06:26 AM   #14
artosalo is offline artosalo  Finland
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Quote:
Originally Posted by Zekk View Post
Hi Arto...Can I use your GU-50 power supply for this schematic?
Thanks!

Zekk
Basically you can, but that PS has regulated screen grid voltage too, that is not necessary required if +Ub is some 450 V and of course not with UL.

I have used this. The 68 V zener in this schematic can be replaced with a small electrolytic capacitor.
Then input voltage variations has the same effect on all voltages and less effect to idle current of output tubes.
Attached Files
File Type: pdf EL34_PP_UL_PS.pdf (30.8 KB, 92 views)

Last edited by artosalo; 16th April 2018 at 06:40 AM.
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Old 2nd September 2018, 11:03 PM   #15
Zekk is offline Zekk  Singapore
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Quote:
Originally Posted by artosalo View Post
I have tested different EL34 PP amplifiers with fixed bias and with 6F12P triode pentode front end. With 4k output transformer and 460 V +Ub the available output power is from 55 W (UL) to 72 W (pentode).

Attached the schematic and some test results.
Hi Arto !
I've decided to build ur design but can I use ECF 82 instead of 6F12P ? If possible, what'll be the required changes?

Thanks!
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Old 3rd September 2018, 06:00 AM   #16
artosalo is offline artosalo  Finland
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Quote:
If possible, what'll be the required changes?
Surely it is possible to use ECF82 instead of 6F12P. The main difference is the lower gm (=gain) of the pentode halve. Therefore the NFB-loop must be re-adjusted. Also the bias of both sections differ a lot from 6F12P. To put it briefly, the whole front end of the amplifier must re-designed for ECF82.
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Old 3rd September 2018, 09:22 AM   #17
Zekk is offline Zekk  Singapore
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Reason being that I've a cartoon of 5pcs. Philips ECF82 sitting on the shelf, waiting for inspiration to starts on the Radford ST25. Anyway, thanks for the response and the kind advices.

Zekk
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Old 3rd September 2018, 01:10 PM   #18
artosalo is offline artosalo  Finland
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If you want to proceed with ECF82, the first thing is to find correct operating point for the pentode section. You can start with 47k plate resistor and the same cathode resistors I have used. Then search suitable value for the G2-resistor to get some 150 V at the anode. This can be later fine tuned with a distortion meter.

For the cathodyne, find suitable value for the cathode resistor to get 85...100 V at the cathode.
Other cathodyne resistors as in my schematic.

The feedback resistor (1k5 in the schematic) must be re-adjusted to get some 12...15 dB of NFB. Parallel capacitor should be tuned to get clean transient response of 10 kHz square wave. This depends a lot on the OPT you use.

Last edited by artosalo; 3rd September 2018 at 01:15 PM.
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Old 3rd September 2018, 02:02 PM   #19
Zekk is offline Zekk  Singapore
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Am using Dynaco A-431.
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Old 3rd September 2018, 03:57 PM   #20
artosalo is offline artosalo  Finland
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I have no experience about that OPT. The NFB-loop must be adjusted as I described.
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