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Old 23rd February 2012, 08:27 PM   #1
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Default Stuck... Output tube for Single ended?

Hey everyone.

I'm currently thinking of designing bulding a SE amplifier, mostly with what I've got laying around. I don't have any iron for this at hand, but I have a chassis I want to stuff everything needed into, and a load of tubes. What I've got, and have considered, are 6550 Winged C:s, EL34's, some NOS 807's, and some old Westinghouse 6B4G's. Valve rectification with GZ30 or 5V4GT (have some NOS Sylvanias around doing nothing...) was what I thought...

Here's what I'm looking for:
  • At least five watts power out with less than 2% distortion
  • Has to fit into a Hammond 20cm x 30cm x 5cm chassis
  • Hammond's 125xSE iron is cheap and easy to get, so that's what I'll be using for OPT:s
The aforementioned rules out the 6B4G; not enough power and I've heard they need regulated power supplies, thus the thing wouldn't fit in my chassis very easily.

This morning I got the idea of using the 807. But then I read the bugger does a measly watt with a truckload of distortion in SE designs. Is this true? I have'nt dug myself into the curves yet, but I thought a 25W tube could give more power even in triode SE?

I was thinking of using cathode feedback on the OP stage, also. That does'nt give a whole lot of NFB, I know, but I have an angst towards gNFB, based on experimentation. Local feedback I think is OK. Actually, at first I was thinking a 6550 pentode connected with local cathode feedback, but that does'nt bring the output impedance down enough with the Hammond OPT:s, basically regardless how you connect them.

Basic question: which output tube to go with? I already have a pair of KT88 PP monoblocks, so the sound of an EL34 would of course be welcome to my house. Then again, the 6550 would be more linear in triode. An 807 I've never heard... And I don't want to use tubes I haven't got.

Oh, whatever the output tube, I was kind of planning to feed them with a NOS 6SL7 dc-connected to a 6J5 cathode follower...
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Old 23rd February 2012, 08:59 PM   #2
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RH807?
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Old 23rd February 2012, 09:52 PM   #3
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Originally Posted by aardvarkash10 View Post
RH807?
Doesn't solve the problem. Pentode connected with plate to grid feedback wasn't what I was looking for. Albeit an interesting approach. I was thinking triode mode and perhaps cathode feedback if needed. And still, the selection of the output tube lies unanswered...
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Old 23rd February 2012, 11:40 PM   #4
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Originally Posted by Adolf Corkscrew View Post
Doesn't solve the problem. Pentode connected with plate to grid feedback wasn't what I was looking for. Albeit an interesting approach. I was thinking triode mode and perhaps cathode feedback if needed. And still, the selection of the output tube lies unanswered...
How about the 6550s in UL mode, one gain stage, and no gNFB?
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Old 24th February 2012, 12:01 AM   #5
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parallel 6B4G.

Or push pull, why not?

The other tubes you mentioned I would not consider for SE operation in most cases.

Consider a "parafeed" arrangement if output iron is a cost problem...

I'd take a class A PP 6B4G over a SE pentode, even strapped for "triode" any day.

My opinions are just opinions.

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Old 24th February 2012, 12:08 AM   #6
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Hi,
You are right about the 807 in triode mode. When you study the very good STC Datasheet you will notice that there is little use not use it in pentode connection.

I used the 807/1625 with local cathode feedback with good results. Here is more

Furthermore you don't need a cathode follower to drive a typical pentode like 807 etc.
And...don't expect too much with those OPTs
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Old 24th February 2012, 12:52 AM   #7
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I can't imagine anything better than 4P1L in triode for SE amp. Bunch of them, if more of power is needed. 2.5W / 5K output per tube. 12.5W / 1K from 5.
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Old 24th February 2012, 01:38 PM   #8
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Originally Posted by Wavebourn View Post
I can't imagine anything better than 4P1L in triode for SE amp. Bunch of them, if more of power is needed. 2.5W / 5K output per tube. 12.5W / 1K from 5.
It's quite microphonic though , went through my stock of 20 of these and all were unuseable as a linestage tube . If using as an output stage with sensitive speakers (98dB/1w) the same applies , even with damping and viton rings on the tube I can hit the pause button during a loud passge and hear 'cowbells' for a good few seconds . Reducing filament voltage helps but the issue does not really seem to go away completely . I found there is a distinct sweetspot in terms of operating point and I could not get anywhere near the 2.5w you quoted , more like 1.2w with 5k optx to get best sound . I am running a push pull 4P1L amp at work with an old pair of Kef Cadenza and it sounds very nice , albeit at lowish SPLs

2A3 is what I would recommend as an SE output stage . easy to light up , easy drive requirements and very forgiving Triode connected EL34 may be the best of the bunch on the posters list of available types...

316A
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Old 24th February 2012, 02:35 PM   #9
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The Hammond 125 series have quite low primary inductance so you do need a tube with low Rp or some sort of feedback arrangement..
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Old 24th February 2012, 03:25 PM   #10
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Quote:
Originally Posted by 316a View Post
It's quite microphonic though , went through my stock of 20 of these and all were unuseable as a linestage tube . If using as an output stage with sensitive speakers (98dB/1w) the same applies , even with damping and viton rings on the tube I can hit the pause button during a loud passge and hear 'cowbells' for a good few seconds . Reducing filament voltage helps but the issue does not really seem to go away completely . I found there is a distinct sweetspot in terms of operating point and I could not get anywhere near the 2.5w you quoted , more like 1.2w with 5k optx to get best sound . I am running a push pull 4P1L amp at work with an old pair of Kef Cadenza and it sounds very nice , albeit at lowish SPLs
Thank you for correcting my mistake. I wanted to delete my message yesterday, but it was too late. Then I calmed down myself thinking that people who are going to compete against my commercial products don't know how to cook them, to get 2.5W of nice clean power from a tube, with no microphonics.
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