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Recommend an EL84 tube for HIFI

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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?

Thanks,

Jason.
 
U2,
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!

Aloha,

Poinz
 
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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.


OT:
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.
 
AndreasS said:
With this simple schematic (indepentdently of the tube) you don't get Hi-Fi.

Andreas

I must have done something wrong then, this amp is very pleasing :D 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.

Cheers,

Jason.
 
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.

Shoog
 
Originally posted by Shoog

... The CCS will turn it into a differential output stage

Yes, the balance will be better than with the common cathode resistor...

Originally posted by Shoog
- with possibly the best performance of any output stage.

No, the high distorsions of the left output tube will be further amplificated in the right output tube

Originally posted by Shoog
Of course it would sound better with a fully balanced drive.

That's right

Regards Andreas
 
Hi Andreas, I don't have a copy of the RDH, but I will try and hunt one down. Thanks for the reference.

My measurements show that the distortion is far from bad.

I was very skeptical when I started this project that it could work anywhere near as well as it does.

In general most phase spiltter stages make me feel as though there are compromises in play. The only one I really like is transformer coupling.

Jason.
 
EL84 tube rolling

I have used the JJ EL84, EI EL84 (now becoming scarce, their factory may have shut down or possibly sold) and a Russian mil spec EL84 that may in fact be your 6p14pi (6P14P-EB) in a Manley Stingray integrated amp. They are all fine. I may have a slight preference for the JJ followed by the EI followed by the Russian 6P14P-EB in that particular amp. I also got as much difference changing the input tubes in the amp (12AT7 in this case).

You could try the JJ EL84. They are quite cheap and are fairly highly regarded. I have no experience with the TAD. The 6P14P you have are probably quite good actually.

BTW I prefer my EL84 amp to my 300B amp which is actually quite a bit more expensive (again a commercial design, Parralel 300B 2 tubes per monoblock, 18W/Ch SET). The 300B is refined, polite and frankly boring. The Manley Stingray is a fairly lively amp, occasionally somewhat brash. My feet are tapping with the Stingray. They usually aren't with at least my 300B amp. One major caveat. This is only one amp vs another and things like speaker interaction, output transformers etc play a major role.

Sincerely,

Hany.
 
Shoog said:
I really think you are underestimating the potential of this circuit.

I tend to agree. For a very simple design, this ought to be a pretty good little amp, but only WITH the added CCS. Kevin's suggestions are good ones. Without the CCS, this amp is a rather pointless quasi-SE/quasi-PP amp. The CCS just makes it fully PP.

To be fair to DIYParadise, he does go on to suggest "Complicating the Simple EL84" with a CCS if you follow the links at the bottom. My choice of words would have been "Fixing the Simple EL84", but I'm trying not to be sarcastic here. :angel:

"Complicating the Simple EL84"
 
In general most phase spiltter stages make me feel as though there are compromises in play. The only one I really like is transformer coupling.

Now your talking.
I originally intended to make this amp but discovered my intended output tubes had to low a mu (2) so I went with an interstage transformer. Since I am not rich I used toroid power transformers. They take some driving - but its the simplest and cleanest sounding amp I have made so far. Component quality starts to become more of an issue. Very cheap to build.

I would consider this as your next upgrade. Interstage transformers work best when they are stepped down, so generating a bit of extra gain in the driver tube becomes an advantage.

Shoog
 
Consider the EL86 as well. They require around double the negative bias and drive - a good thing in my opinion for lowering noise and any half-way decent drive stage shouldn't have issues with the levels - but reward with a much more direct sound than than the EL84. Most brands, unlike the many EL84's I've tried, also show falling odd harmonics vs. evens under certain bias conditions, translating in p-p to reduced distortion.
 
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