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Old 23rd September 2007, 04:12 AM   #1
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Default High gain pentodes

I have seen this subject come up before, but I have never heard any definite answers, and not for my purposes, anyway. I am designing a guitar amplifier and need a lot of gain. For the input of the amp, I am looking for a small-signal pentode with a ton of gain to eliminate the need for several gain stages. Does anyone here know of such a tube? It would preferably be cheap and widely available. Linearity is not an issue, as distortion is desired.

Thanks in advance,
Matthew
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Old 23rd September 2007, 04:20 AM   #2
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Hi,

Standard input pentode for guitar amps is an EF86. Though 6EJ7 is catching on too

Cheers!
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Old 23rd September 2007, 05:44 AM   #3
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6DJ8 cascoded.
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Old 23rd September 2007, 08:32 AM   #4
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Default Re: High gain pentodes

Quote:
Originally posted by AmpKiller66
For the input of the amp, I am looking for a small-signal pentode with a ton of gain to eliminate the need for several gain stages. Does anyone here know of such a tube? It would preferably be cheap and widely available. Linearity is not an issue, as distortion is desired.

Thanks in advance,
Matthew
I can think of several:

12BY7A -- g(m)= 12mA/V
6AG7 -- g(m)= 11mA/V

These two may or may not be suitable since they like to pull rather big plate currents, being intended for driving CRTs.

6CB6 -- g(m)= 8.0mA/V -- VHF amp. Pretty decent audio characteristics.
6EW6 -- g(m)= 16mA/V -- Another VHF amp.
6JD6 -- g(m)= 14mA/V -- Ditto
6KE8 -- g(m)= 12mA/V (Triode/pentode type -- looks promising for audio)

These types don't require the largeish plate currents of the vid amps, and might be more useful as an audio front end.

Might also want to consider a cascoded dual triode such as the aforementioned 6DJ8, the 6BQ7A, or the 6SN7 (6FQ7 -- 9-pin mini version) all of which work quite well as cascodes. This might even be preferable "up front" since triodes don't have the partition noise problem.

Of course, there are lots more possibilities out there as well.
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Old 23rd September 2007, 11:05 AM   #5
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Triodes in cascode tend to be much less microphonic than pentodes. Also, pentodes with extreme gain can be problematic in terms of parasitics and instability.
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Old 23rd September 2007, 01:48 PM   #6
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The best, quietest pentode I've worked with is the EF86 and the American variant, 5879. I think they'd do well in this application.
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Old 23rd September 2007, 02:48 PM   #7
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hey-Hey!!!,
Pentode gain isn't a tube property, but a combination of tube characteristic, and loading circuit. Gain is about gm*plate load. This is *NOT* advertised gm, but the achieved in-circuit gm. Sooo you need a big load, and high gm. This combination usually requires a fairly high supply voltage. Since we're not going to worry about maximum voltage swing, we don't need to worry so much about the end-of-swing bunching up of the curves; a high load/near hirozontal load line can be employed.

Try a resistor in parallel with a CCS. Drive up the gm with current through the CCS, and get a near-horizontal load line with a big resistor. With this approach, some *VERY* interesting tricks are possible....
cheers,
Douglas
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Old 23rd September 2007, 06:23 PM   #8
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The EF94 / 6AU6 seems to be a a very underrated pentode- it's usually much less microphonic than most EF86s. I rate it VERY hightly.
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Old 23rd September 2007, 09:46 PM   #9
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Some of those pentodes are pretty noisy/microphonic for a V1 position though.... 6CB6 for example, only a few brands are suitable.

But that could be said for most though *shrug*
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Old 23rd September 2007, 10:18 PM   #10
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Thanks for all of the input. I have some time to kill, so I think I will get several of these tubes and experiment.
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