Recommend an EL84 tube for HIFI - diyAudio
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Old 12th May 2007, 01:55 PM   #1
ME2 is offline ME2  Australia
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Default Recommend an EL84 tube for HIFI

Hello, I have built the 'simple EL84' amp. Simple EL84

The power tubes are the Rusian 6n14pi I bought from Ebay very cheap some years ago. I am starting to really like this amp, but am wondering about better EL84 tubes.

have heard that the TAD EL84 tubes are very good, do they live up to the rep ?

What do you like and how do they compare to other tubes?


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Old 12th May 2007, 02:07 PM   #2
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With this simple schematic (indepentdently of the tube) you don't get Hi-Fi.

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Old 12th May 2007, 06:48 PM   #3
Zibi is offline Zibi  Poland
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TAD is not a tube manufacturer.
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Old 12th May 2007, 07:31 PM   #4
Colt45 is offline Colt45  Serbia
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i like 6p14p...
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Old 12th May 2007, 08:37 PM   #5
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These are from the Tube Amp Doctor, in Germany. He has two or three different ones on his own site, and I found one on Tube Depot, all four with different suffix series (M, W, RFT, can't remember right now), and at least two or three seem from the photographs to have different construction. He's very coy about where they come from and who manufactures them, and so I would suggest that asking him is not going to get an answer. I would say the chance is about 90% or more that they're either Russian or Chinese.

Sorry, but this is just a sort of elaborate way of saying that you're just gonna have to get some and try 'em out. Also, these gentlemen are correct; your amp is a quirky circuit, which I learned about as the 'compact'. It has some rather large technical issues, but I've talked to folks who like its sound quite a lot. Its reaction to differing tube brands and types, though, is thus not predictable from other amps anyway.

Roll on!


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Old 12th May 2007, 10:45 PM   #6
kevinkr is offline kevinkr  United States
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I like the Russian 6P14P and the JJ EL84 is quite nice as well. Vintage Amperex and Mullards are hard to beat, but extremely expensive.

One possible improvement I can see for this design is to replace the 135 ohm resistor with a CCS (constant current source) set up for about 85mA total - this should somewhat improve ac balance in the output stage.

You could use an LM317 configured as a CCS for this application, but to maintain sufficient voltage compliance for proper operation at maximum signal swing requires a small negative supply. You could just rectify and filter the filament supply for this or use a separate 6.3V transformer. If you want to go this route connect the input pin of the LM317 to the EL84 cathodes in place of the 135 ohm resistor. Connect a 14.7 ohm (15 ohm ok) from the reference pin to the output pin, and connect the - 6.3V to -7.5V (or slightly higher) supply to the reference pin. The LM317 must be isolated from chassis which may be used as a heatsink. Dissipation will be under 2W.
"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine
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Old 12th May 2007, 11:03 PM   #7
cotdt is offline cotdt  United States
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The GE 6BQ5 is the best one. Those Russian military ones are decent, but not as good as the GE.
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Old 13th May 2007, 01:17 AM   #8
ME2 is offline ME2  Australia
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Originally posted by AndreasS
With this simple schematic (indepentdently of the tube) you don't get Hi-Fi.

I must have done something wrong then, this amp is very pleasing As it stand now I have heard nothing in the HIFI shops which sounds better, but then the SEL84 is being compared to EL34 and KT88 PP type amps available at my shop, not 2A3 300B etc.

There are issues with this design for sure, but they are not apparent to me just by listening. The rest of my system is the Lynn Olson ME2 speakers and a Rega Apollo CD player. Until now the ME2s just seem to have punished all the other gear I have tried.

Should probably have given more details to start with. I built this amp so I could play with it and learn as much as I can.

It started off with 100V line transformers for the outputs and the taps just happened to suit Ultra linear mode. Driver tubes were Ruby 12AX7 I had and the output tubes of course are the 6n14pi.

I added the LM317 CCS and found a good improvement, but bass was still lacking due to the nasty transformers. Next I added a form of negative feedback (from the output back to the drivers cathode) this helped greatly to flatten the frequency response.

The SEL84 now sounded OK, if not a little dull. A sine wave test showed some 2nd, 3rd and 5th harmonics around 60 to 70dB down. My anaylser only goes to -80dB so I could not really see the full picture.

A switch was added to change from ultra linear to triode mode and I found I liked triode so much that it just about permanently stays in triode mode.

The output transformers were swapped for Hammond 1608 iron and the negative feedback was removed since it was not needed with the better output iron.

I tried the EL84 drives EL84 scheme as described on the SEL84 page, but didn't like that much. Too much colouration added. JJ ECC99 drivers have just been added and I am very pleased with these.

That experiance with the EL84 driver tubes is what made me wonder if I could improve the output stage also with better tubes. In the future I would like to try a normal phase spilt driver stage with and without feedback to see if I can better the current state.

Thanks all for your comments so far, alll advice is taken gratefully.


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Old 13th May 2007, 06:33 AM   #9
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Originally posted by ME2

I must have done something wrong then, this amp is very pleasing ....
Hi Jason,

please read Chapter 13.6 (viii) in RDH about phase inversion in the power stage...

Regards Andreas
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Old 13th May 2007, 09:03 AM   #10
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please read Chapter 13.6 (viii) in RDH about phase inversion in the power stage...
I doubt that the RDH book talks about this technique using a CCS. The CCS will turn it into a differential output stage - with possibly the best performance of any output stage. Of course it would sound better with a fully balanced drive.
I really think you are underestimating the potential of this circuit.

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