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Old 2nd February 2006, 01:22 AM   #1
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Default 6V6 vs EL84 SE UL

Want to do something different..

so SE Class A UL output stage using EL84's or 6V6's.

looking to top 20W or so.

Of course i will go really different and use a E80F or EF86 input tube...

do i need another tube between this one and output stage...

some global feedback too!!

So, which way should i go??? 6V6's or EL84's??

Thanks!
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Old 2nd February 2006, 06:48 AM   #2
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Default Re: 6V6 vs EL84 SE UL

Quote:
Originally posted by AudioGeek
Want to do something different..

so SE Class A UL output stage using EL84's or 6V6's.

looking to top 20W or so.

Of course i will go really different and use a E80F or EF86 input tube...

do i need another tube between this one and output stage...

some global feedback too!!

So, which way should i go??? 6V6's or EL84's??

Thanks!
No way are you going to get 20+ watts from a 6V6 Class A(1) unless you really bust the specs: red glow of death spec busting. The most that the spec sheet says is 5.5W. Even in PP, it doesn't go over 8W.
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Old 2nd February 2006, 07:56 AM   #3
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Typical SE amps are 20 to 30% efficient. For 20W SE, you need to think about running 100W of plate dissipation. Paralleling matched 6V6, 6L6, etc. will work. However it would be easier to consider BIG Russian tubes that consider 100W Pd a week at the spa

But then you trade circuit simplicity for a PS nightmare.
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Old 2nd February 2006, 03:43 PM   #4
SHiFTY is offline SHiFTY  New Zealand
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SE EL84 will give you quite a loud, very nice sounding 5 Watts or so. They sound a lot more powerful than they are- don't wory about the watts.

Definitely enough for a decent amp, I would recommend using pentode or UL mode. EF86 should be good as a driver, maybe wired in triode mode for less gain.

Have a look around for 'SE EL84 schematic' on the web, I have seen a few around.

If you wanted more power, you could have 2 EL84s in parallel, but then you would need a better driver stage too... Also I don't know about UL tubes in parallel?!
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Old 2nd February 2006, 10:34 PM   #5
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Quote:
If you wanted more power, you could have 2 EL84s in parallel, but then you would need a better driver stage too... Also I don't know about UL tubes in parallel?!
The matching requirements for parallel "finals" are TIGHT. Even PP finals with a shared cathode bias network are somewhat more forgiving. Start with well matched tubes. Then use separate coupling caps. to each control grid and "fixed" bias with individual bias setting pots. Dial the idle current in exactly the same in both tubes.

The EL84 is an easy tube to drive, even in parallel. A 5965 section will do the job, loop NFB and all.
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Old 2nd February 2006, 11:39 PM   #6
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Default how about...

well i need enough in parallel for at least 20W UL SE connected...

so is it easier to use easy to work with tubes like 6V6 or EL84 or is it better to use one single tube in UL and build a difficult power supplY?

Thanks!
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Old 3rd February 2006, 12:04 AM   #7
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Quote:
so is it easier to use easy to work with tubes like 6V6 or EL84 or is it better to use one single tube in UL and build a difficult power supplY?
You win some and you lose some. The drive voltage requirements of a KT88 are higher than that of the EL84 and a KT88 is what you will have to use for a single "final".

The EL84 is easier to drive than the 6V6 and that's what I'd use. Obtaining 20 W. from 4X EL84s in UL PSE is a reasonable goal. Are you going to use a line stage with gain? If so, an ECC99 section should be enough in front of the "finals".
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Old 3rd February 2006, 12:58 AM   #8
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actually i have a passive line stage.

so need at least 0.5v for full output on el84's.

actually thinking of a ef86 input pentode with global feedback.

never done feedback... so unsure how to do this.
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Old 3rd February 2006, 01:53 AM   #9
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Steve Eddy to the rescue.

Wire an ECC99 section as a cathode follower, which drives a step up microphone trafo. The trafo's secondary drives a common cathode wired ECC99 section voltage gain block. Use individual coupling caps. to connect the voltage gain triode to the EL84 "finals".

BTW, if you use HIGH quality O/P trafos, you can avoid global NFB by using the "exolinear" topology of "Bandersnatch". Connect the voltage gain triode's load resistor to the screen tap of the O/P trafo, instead of connecting it directly to B+.
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Old 3rd February 2006, 08:20 AM   #10
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Heh sounds interesting, but too complex for my liking, a KT88 would be way easier.

I say go the brute force method: 6SN7 into KT88 in UL. Power supply can be reasonably simple diode bridge or use a GZ34. Google for SE KT88 and see what turns up.

By the way, why do you "need" 20 watts? Thats quite a lot to ask from a single-ended amp...
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