6bm8/ECL82 vs EL84 - diyAudio
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Old 24th January 2007, 06:21 PM   #1
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Default 6bm8/ECL82 vs EL84

I have been building small amplifiers for a while, and I'm interested in other people's opinons of the difference in sound between the 6BM8 and the EL84.

I have several single ended and PP 6bm8 and EL84 amps. They all sound decent.

Of the 6bm8 amps, I have two motorolas. One was a 3 channel PP amp with 6bm8 high channels and an EL84 PP bass channel... The other was also 3 channels.. it had single ended high channels with 6bm8s and a PP bass channel which also used 6BM8s. The bass channels were pretty much useless to me in both amps, so I removed them and removed the high pass caps in each and replaced them with .02 caps which let the full sound through.

Of the EL84 amps, I have two zenith single ended amps. One of those was another 3 channel (which I have posted about before..) It derived the bass channel from the two single ended outputs... the other was a pretty standard SE stereo amplifier.

Of all these amps, the motorola SE amp (after I removed the bass channel) sounds the best. Does this make sense? Of all the amps, it has the smallest lowest quality looking output trannies, no NFB, and it has outstanding response with just about any speakers and it's loud.

My real question here, is how do these two tubes compare? Are the specs similar? or about the same? or completely different...

I'm very curious about this.

These were all integrated amplifiers, and I think I am going to build new chassis for each of them this winter and turn them into regular power amplifiers, becuase I have some pretty nice preamps.
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Old 24th January 2007, 08:27 PM   #2
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Default Re: 6bm8/ECL82 vs EL84

Hi,

Quote:
Originally posted by akimball442

My real question here, is how do these two tubes compare? Are the specs similar?
No, the ECL82/6BM8/PCL82/16A8 pentode section is quite different to EL84/6BQ5 in many regards. The closest thing to EL84/6BQ5 you can find in a noval based compound valve certainly is the pentode section of ECL86/6GW8/PCL86/14GW8, which can be viewed as a plate Pd derated (9W vs. 12W) EL84/6BQ5.

Tom
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Old 26th January 2007, 09:22 AM   #3
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I have used the ECL82 in a few projects. I also had a Leak Stereo 20 which used the EL84.
My experience of the Leak was that it has a very rich smooth sound, which I understand is common to most EL84 amps.
I have made a headphone amp using the ECL82 and also have used it to drive interstage transformers. I found that it is tremendously detailed, but a little zingy and bright. In the end I had to introduce a small amount of loop feedback into both to tame the top end. I suspect that its the triode stage of the ECL82 which is the real weak link, but I have no way of been certain.
If building from scratch I would guess that a better result could be achieved with the EL84. The ECL86 seems to be a superior brother of the ECL82.

ECL82 need handling with care.

Shoog
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Old 26th January 2007, 12:42 PM   #4
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Hi Shoog,

Quote:
I suspect that its the triode stage of the ECL82 which is the real weak link, but I have no way of been certain.
It is very similar to one section of 6SL7, and there is nothing wrong with that

Quote:
If building from scratch I would guess that a better result could be achieved with the EL84. The ECL86 seems to be a superior brother of the ECL82.
I agree with both. BTW, the triode section within the ECL86 is one section of ECC83, except being a little bit plate Pd derated (this is only due to the massive heat dissipation of the power pentode nearby).

Tom
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Old 26th January 2007, 04:53 PM   #5
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I think if you look at what the ECL82 was designed for it will be no surprise that its performance is a little compromised. Ultra cheap output transformers which probably benefitted from a bit of brightness in the output stage.

Shoog
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Old 26th January 2007, 11:09 PM   #6
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The triode in the 6BM8 strongly resembles the triodes in a 12AT7. It has high gm/low Rp. The 6SL7 is a low gm/high Rp type.

6BM8 data sheet here.

12AT7 data sheet here.

6SL7 data sheet here.
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Old 27th January 2007, 04:01 AM   #7
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I don't get-on with the triode in the ECL82.......I find the gain too low. It's tempting to use triode sections of two tubes as a floating paraphase p/splitter but the overall gain is too low and causes misbalance.
Anyone else tried sim ?

richj
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Old 27th January 2007, 09:08 AM   #8
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Quote:
I don't get-on with the triode in the ECL82.......I find the gain too low.
To low for what. It has a gm of 70 which I usually find way to high for normal preamp duties. Coupled with its very low plate current, and a useful bias range around -1v, and its a pain to use in modern situations.
Might be good for guitar amp use, which I intend to try.


Looking at the Triode curves, theirs no way it can be considered a 12AT7, it miles off.

The 6SL7 is a much better fit, but not perfect. Its also got the right gm.

Shoog
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Old 27th January 2007, 11:52 AM   #9
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Shoog,

You're mixing mu and gm up. Depending on operating conditions, the mu of the 'T7 has been quoted as 60 and 70. The mu of the 6SL7 is quoted at 70. The 6BM8's triode also has a mu of 70. The gm of the 6SL7 is quoted at 1.6 mA./V. The gm of the 6BM8's triode is quoted at 2.5 mA./V. The difference is significant.

Your point about curves is valid. Splitting the difference seems reasonable. The character of the 6BM8's triode lies between the 'SL7 and 'T7.

The few 6BM8 circuits I've perused use 2 bottles for PP "finals" and common cathode/"concertina" splitter.
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Old 27th January 2007, 04:04 PM   #10
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Quote:
Your point about curves is valid. Splitting the difference seems reasonable. The character of the 6BM8's triode lies between the 'SL7 and 'T7.
Good point.

I'am thinking of redesigning my 6080 amp to do away with the ECL82. It performs really well at moderate levels, but sounds strained in the top end when driven hard. I think I'am getting overload of the ECL82 triode before I achieve full output. I might either keep the pentode section as the interstage transformer driver and put a 5687 infront.
Seems a little waistful so I might just drop the ECL82 and replace it with PL84's and a 5687. Only one extra valve and should perform a lot better overall. Won't need a hell of a lot of modification, but I might struggle to find room under the hood for the neccisary changes.

Shoog
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