Anyone know of a dual pentode suitable for a LTP. - diyAudio
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Old 29th April 2009, 08:17 PM   #1
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Default Anyone know of a dual pentode suitable for a LTP.

Hi all,
I am currently pulling together ideas for a two stage Push Pull 6080 amp. I intend to have the transformer in the cathodes so I need a fair bit of gain. I have this perverse desire to build it with two valves per channel. I originally thought of using the ECC91 which is a B7A valve with shared cathode. Unfortunately not enough gain to get the 60V PP I need. I was thinking is there a dual pentode with just a shared cathode connection which could serve as a LTP. I have found plenty of strange valves with common g1 and g2 but with access to the g3, and one or two compactrons, but I was hoping for a B9A valve.

Any ideas.

Shoog
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Old 29th April 2009, 08:50 PM   #2
Jeb-D. is offline Jeb-D.  United States
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Shoog,
I used the 6BN11 compactron dual pentode in my 6as7 amp. It does a pretty good job and is probably the sexiest small tube I've yet to see. The only thing is that you have to expect mis-match between halves. Where-as separated pentodes would allow for you to tube roll one side to get a match. If the cathodes and screens are tied together, all of the mis-match shows up on the plates. Using a trim pot on the screen(s) supply is probably a good idea. I just have to live with mis-match because the screens are being used for feedback/harmonic equalization.

It is a 2 stage, 2 bottle (3 bottle, if you include rectifier tube) PP class AB1 (recently changed from class-A). It puts out about 14Wrms @ 2% thd before the onset of clipping, which is soft. The first 6-7W are class A, so there is about 6dB of headroom class B. Input sensitivity is 2V and it uses 6dB of feedback.

6BN11 isn't the most linear Pentode, but it's not driving the most linear triode either, so the combo works out ok.

The 6BN11's in action
http://img.photobucket.com/albums/v6...a/DSC00327.jpg
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Old 29th April 2009, 09:55 PM   #3
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I had just done a bit of searching and come up with the 6BN11. Great minds think alike. Theres also the 6AR11.

If you were to force the Pentodes into a LTP with a silicone CCS in the tail then surely the mismatch would sort itself out and the linearity will be improved.
I want really low output impedence and that is why I am putting the transfomer in the cathodes. I want that SS kick in the bass. I found on my previous outing with the 6080 that gain was less than 1 in the finals so having the transformer in the cathodes shouldn't effect the gain unduly.

Fantastic job on the amp. The little Compactrons make a fine partner to the stubby 6080's - though I have some nice Russian coke bottle style.

Shoog

Just looked at the spec on the 6AR11, looks far worse on the curves than the 6BN11. By the way what gain are you getting from the 6BN11?
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Old 29th April 2009, 10:14 PM   #4
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Old 29th April 2009, 10:33 PM   #5
Jeb-D. is offline Jeb-D.  United States
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Quote:
By the way what gain are you getting from the 6BN11?
In my circuit without the feedback, 6bn11 gain is roughly 125

I don't recall off hand if it's using 12k or 15k on each plate.
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Old 30th April 2009, 02:28 AM   #6
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Dual 9 pin tetrode is type 5656. May work OK if given enough voltage to stay out of the tetrode kink zone. I have not tried them yet though.
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Old 30th April 2009, 04:18 PM   #7
Funker is offline Funker  Germany
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Hi Shoog,

the QQE03/12 is a suitable double tetrode with common cathode an screens. This tube is obtainable for a reasonable price.

Data sheet:
http://tubedata.tigahost.com/tubedat...q/QQE03-12.pdf
Original intended for mobile VHF radios, it is also a good powerful tube for audio applications.

regards from Hamburg
Wolfgang
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Old 30th April 2009, 04:47 PM   #8
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I had ampl. with QQE03-12. Nice sound and strong bass!
If You interest, I can send You a schematic QQE03-12 & ECC83.
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Old 30th April 2009, 05:10 PM   #9
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Quote:
the QQE03/12 is a suitable double tetrode with common cathode an screens. This tube is obtainable for a reasonable price.
Apart from the fact thats its an output tetrode it looks really sweet and quite linear. Unfortunately running it as a driver would probably take it out of its happy place
Still it would effortlessly push the 6080's into class B. However my prefered choice of CCS's in the tails of the output prevents Class B as an option.

Tubelab - can't find a datasheet on the 5656 which probably makes it to obscure to be easily obtainable. If you remember the discussion on cathode follower output stages - you never did say how much voltage the top MOSFET needed to stay happy.

Shoog
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Old 30th April 2009, 05:22 PM   #10
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Some other dual tetrodes: 6360, 6939. And wouldn't an 832 look nice in there?

And of course there are the unotanium ELL80, 6DZ7, 6DY7.
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