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In the Forum: Melquiades Amplifier
In the Thread: Valve Preamplifier for Macondo/DSET
Post Subject: A preliminary valve listPosted by anthony on: 4/25/2019
So, I have been looking into suitable valves to base a preamplifer around that are suitable for driving the DSET's.  The pre must have a broad bandwidth, low output impedance, some current drive, and not be prone too easily to microphony.  Oh, and preferably DHT...but that one is not set in stone.  I have also decided that I would like to chase 6x or more gain in the preamp.  Simple!

Ale Bartolas site has been particularly useful.  He has spent an inordinate amount of time refining designs over time and openly shares his experience.  Initially I was attracted by the 71a and 45 because of their relatively low plate resistance but at 3x and 3.5x gain I would need two stages and I don't want two stages.  DHT's such as 26, 01a, 10y are around that 8x gain or a little more so are suitable from the gain perspective, but they vary a lot in plate resistance and therefore output impedance.  Driven with Ale's "Hybrid Mu-Follower" aka gyrator load, these DHT's should have the following output impedances:



So, if I knock out those valves with insufficient gain and unsuitable output impedance the only DHT that I am left is 10Y/VTA25/801a.  Or the excellent Russian pentode.  Apparently the 2P29L is just not microphonic at all and the 10Y is manageable, similar again.  Both tubes have high transconductance and can push more current than usual for triodes;  both have a bandwidth out to near MHz range;  both use a similar B+.  7.5V filament vs 2.2V heater.

So there are two.  Nothing in between them regarding my initial design brief.  One is DHT though...the other is just not microphonic. 

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