pentode in triode mode SE vs a DHT SE like 2a3/300b - Page 7 - diyAudio
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Old 1st January 2013, 02:53 AM   #61
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The EL34 is the only pentode that can be run as a true triode.
Not true! Check the 6CH6/EL821 out. It's another of those fine "12" W. multi-grid power types.

On the OT, LOTS of folks have had success with triode wired 6V6 family members. Say what you will about the curves; the sound is excellent.
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Old 1st January 2013, 08:10 PM   #62
dhieber is offline dhieber  United States
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How do you run a pentode as a true triode? By leaving g2 and g3 unconnected?
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Old 1st January 2013, 09:14 PM   #63
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Originally Posted by dhieber View Post
How do you run a pentode as a true triode? By leaving g2 and g3 unconnected?
No, you tie g2 and g3 to the plate. Resistors are the usual choice for the connections, to ensure that grid dissipation is not excessive.

A small difference can be observed between both g2 and g3 tied to the plate and only g2 tied to the plate, with g3 grounded.

The 6V6, 1 of the best sounding "strapped" triodes, is a beam power tetrode and has only 2 grids. Make what you will of that fact.
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Old 6th July 2014, 02:13 PM   #64
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Hey All,

I did a search for 417a and AD1 and this was the only thread that came up. I want to build a push pull amp using using 417a and AD1 outputs. It will have an input transformer and a differential circuit. Would this be a good combination? It seems like it would to me from the data?

Kevin
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Old 7th July 2014, 09:55 AM   #65
disco is offline disco  Netherlands
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Push pull topology and a classical DHT without proper power (15-20W) doesn't agree in my book. Unless your speakers are very efficient the dynamics will suffer like a teethless tiger. Why not stick to penthodes for that goal?
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Old 7th July 2014, 11:38 PM   #66
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Check Steve Bench's web page. He measured the distortion for DHT vs strapped Pentodes without feedback ( very important as eliminating feedback is one DHT's biggest advantages) and the DHT had lower distortion and straighter tube curves. Strapped pentode is pseudo-triode because the distortion is higher. The best sound is going to be from the most linear tube. That would be the 300B and the 2A3.

If you add feedback, then the distortions all get lower and they will all measure very well but, you will be listening to a feedback amp. Crowhurst and others have made the arguments for zero feedback so I don't have to IMO.

Richard
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Old 8th July 2014, 01:35 AM   #67
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The old DHTs were made with low gm, while the later pentodes were optimised for higher gm. The higher gm provides an advantage for use with neg. feedback, but for no feedback designs these have higher distortion due to grid wire proximity (to cathode) effects.

The grid 2 of the later pentodes however is similar in transconductance to the DHT grid 1 gm levels, so an option is still available. That is to use g2 drive (typically on TV sweep tubes). The Rp however will be high in that mode, although not so high for TV sweep tubes, which are often in the 5K to 6K Rp range. So some local feedback is called for to bring that down some. The drive levels will be a challenge too, but no more than for the DHTs. No reason a TV sweep can't perform as well as a 300B with the proper circuit design.
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Last edited by smoking-amp; 8th July 2014 at 01:38 AM.
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Old 8th July 2014, 01:51 AM   #68
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Thanks guys, I didn't choose the topology a friend did. I built him a similar amp using 2A3 and he really liked it. The 2A3 had no feedback and sounded great. He was very happy.
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