Tube Amp EL 84

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Tube Amp EL 84

Post by JDL-TAIWAN on Thu Jan 16, 2014 11:12 pm

Hello colleagues, and friends of tube technology.
I have tried to build a good amplifier with 2 x EL 84 and a 5K 20W James output transformers.
After this circuit:
Drawing
at about 2Watt the sinus has deformed bottom round, and I had no phase rotation on the ECC 83 only two times positive signals at the grid of 84 EL.
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I have R4 changed to 100 K to produce a Symmetrix,
However, I had also positive and negative Signals
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at 2.5 watts my distortion was back. This is the Output Signal at 4 ohm 2,5Watts. The Sinus is in the bottom very round.

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I put for the Measurement view the two signals together so we can see the defect.

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Does anyone have experience with this type of circuit, or is it better to build another circuit.
thanks for your Help and to share you expiries.
mfg JDL-Taiwan.

This is the Testing Amp 2 x EL84 and one ECC 83 Jian Shin Transformer.

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Re: Tube Amp EL 84

Post by Dr. Radio on Fri Jan 17, 2014 7:02 pm

To be honest, I don't have a lot of engineering experience with tube audio circuits. The one thing that caught my attention from the schematic you referenced was all of the variable resistors incorporated into that circuit. Seems to be a lot of areas for "fine tuning" ? Not sure if the author anticipated problems with various components used or wanted to provide a way for more symmetrical operation (as you are investigating). Another thought (just throwing ideas out), is maybe the output transformer you are using (Jian Shin ?) isn't ideal for the circuit even if it does supposedly have the proper characteristics. Since this is a push-pull circuit, it is very important that the output transformer is properly matched AND the tubes are "matched" in their characteristics --the two EL84's. I suspect that is what the "fine tuning" variable resistors are for, but you can only have some much to work with it if the tubes vary greatly.

You probably are well aware of the theory, but here's some background. Don't want to insult you if you way beyond the realm of the basics. Surprised 

http://www.pearl-hifi.com/06_Lit_Archive/07_Misc_Downloads/Push_Pull_Theory_MIT.pdf
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Re: Tube Amp EL 84

Post by Ron Pond on Sat Jan 18, 2014 1:40 am

Looks like a noisy tube to me.

Can you get access to a spectrum analyzer?

Ron.


Last edited by Ron Pond on Sat Jan 18, 2014 1:43 am; edited 1 time in total (Reason for editing : typo)
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Re: Tube Amp EL 84

Post by Brian Hill on Tue Jan 28, 2014 5:34 pm

Your layout could be funky too. You got wires going everywhere so you could be introducing noise into your signal path from PS. Grounding layout is very important too in these designs.
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Re: Tube Amp EL 84

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