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Designing a Single-Ended Triode EL34 Power Amp
Designing a Single-Ended Triode EL34 Power Amp
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Old 8th November 2017, 01:54 PM   #11
kstagger is offline kstagger  United States
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If you need more power for your speakers, consider a pentode output stage, but use the EF86 in pentode with some (plate-to-plate?) feedback.

One of my favorite amps was an EL156 (later 6550s) in Ultralinear driven by a EF86 in pentode. Loop feedback was used.
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Old 8th November 2017, 03:20 PM   #12
LeftHandFool is offline LeftHandFool  United Kingdom
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Quote:
Originally Posted by Ketje View Post
ght:250px" tabindex="1" dir="ltr"> It's not a stopper, but a path to ground for the grid to get it on zero voltage.
Mullard specifies 500k as a max. , with 330k we are on the safe side.
Mona
Thanks. I think I meant to say grid leak, but thanks for the explanation. Although it seems obvious now, I hadn't realised it was performing that function. I thought it just served as a path to ground for stray electrons which found themselves attracted to the grid, on thier way to the anode.

Matt.
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Old 8th November 2017, 06:29 PM   #13
scott17 is offline scott17
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Hello,

Post #12 here is for an easy and good sounding build.

Good schematic for SE EL34/KT88 amp | Audiokarma Home Audio Stereo Discussion Forums
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File Type: gif KT88_EL34_SE_UL_400V_M1_adj.GIF (14.4 KB, 473 views)
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Old 8th November 2017, 06:46 PM   #14
LeftHandFool is offline LeftHandFool  United Kingdom
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Originally Posted by scott17 View Post
Hello,

Post #12 here is for an easy and good sounding build.

Good schematic for SE EL34/KT88 amp | Audiokarma Home Audio Stereo Discussion Forums
Hi,

Thanks for the link, but having gone to the trouble of designing my own circuit, I'm at least going to give it the honour of a fair trial.

Matt.
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Old 8th November 2017, 06:54 PM   #15
LeftHandFool is offline LeftHandFool  United Kingdom
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Originally Posted by kstagger View Post
If you need more power for your speakers, consider a pentode output stage, but use the EF86 in pentode with some (plate-to-plate?) feedback.

One of my favorite amps was an EL156 (later 6550s) in Ultralinear driven by a EF86 in pentode. Loop feedback was used.
I had worried that 4W might not be enough to drive my current speakers. I made some enquiries to that effect in the speakers forum last week, and the general consensus was that whilst the frequency response may need tailoring, they'd be efficient enough to be driven from a mere 4W.

I also designed a circuit using the EL84 as a pentode, which put out a litle more power (around 6W), but I wasn't happy with my calculations for the screen grid voltage/current.
There also seems to be a certain positive opinion about SE triode amps more generally. Whether this is hype, fashion or b/s I won't pretend to know, but given the relative simplicity of a triode stage, I'm willing to buy into it, for now.

Matt.
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Old 9th November 2017, 07:37 AM   #16
Schmitz77 is offline Schmitz77  Germany
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The real Problem with EF86 pentode input is that the gain is much too high for any modern Pentode output to drive. Look at the old Mullard circuits, they have much less input voltage. Same with the Klangfilm. EF86 is intended to be used as an instrument amplifier tube with exceptional high gain or as phono preamp tube. There you can use that lot of gain. In an amp that needs only 8-15V to drive the output tube and a high input source voltage youll end up with a hard negative feedback circuit like all the last generation studio amps that used this tube. So your sound will be over-analytic with very low distortions (as the studio equipment was intended for), but hard sounding without any swing.

Last edited by Schmitz77; 9th November 2017 at 07:41 AM.
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Old 9th November 2017, 02:48 PM   #17
audiowize is offline audiowize  United States
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A triode strapped EF86 will work well, but they are a fairly expensive tube to use for such use.

The 12AT7 is a good choice. A 12AU7 with the output of your phono stage will fall a little short. (Though to be fair, you should post your cartridge output voltage and phono preamp gain)
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Old 9th November 2017, 04:05 PM   #18
LeftHandFool is offline LeftHandFool  United Kingdom
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Originally Posted by audiowize View Post
A triode strapped EF86 will work well, but they are a fairly expensive tube to use for such use.

The 12AT7 is a good choice. A 12AU7 with the output of your phono stage will fall a little short. (Though to be fair, you should post your cartridge output voltage and phono preamp gain)
The EF86 is indeed comparatively expensive, but has the advantage of being a valve I already have a small stockpile of. I suppose this is a short sighted outlook, as valves don't last forever although I'd estimate under normal usage I'll get a good few years from my supply before I need to replace them.

I've been playing around with some loadlines this afternoon, trying out the 12AT7 vs. the EF86 as a triode, but I haven't settled on a preference as yet.

A quick question - the 12AT7 is a dual triode, is it not? Could I use one half of the valve to drive the left channel and the other to drive the right, or would I be asking for trouble doing it that way?

I'm using an Ortofon VMS20 MkII, which puts out 5mV. The phono stage has a stated gain of 40 dB.

Matt.
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Old 9th November 2017, 04:22 PM   #19
scott17 is offline scott17
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One half of the 12at7 for each channel is how it's been done 1000's of times.
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Old 9th November 2017, 04:40 PM   #20
LeftHandFool is offline LeftHandFool  United Kingdom
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One half of the 12at7 for each channel is how it's been done 1000's of times.
Cool. Apologies for asking daft questions but I'm fairly new to this. Experience tells me you end up looking a bigger fool having not asked a stupid question, and cocked things up, than you do if you take the plunge and ask.
I was worried there might be some issues with cross talk between channels or any number of cruel and unusual tricks of the electron I'm yet to encounter.

Being able to drive both channels from a single bottle definitely swings the advantage in favour of the ECC81.

Matt.

Last edited by LeftHandFool; 9th November 2017 at 04:51 PM. Reason: spelling
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