High gain pentodes

[AmpKiller66]

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

 

[Geek]

Hi,

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

Cheers!

 

[Brett]

6DJ8 cascoded.

 

[Miles Prower]

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.

 

[ray_moth]

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.

 

[SY]

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.

 

[Bandersnatch]

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

 

[Merlinb]

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.

 

[Geek]

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*

 

[AmpKiller66]

Thanks for all of the input. I have some time to kill, so I think I will get several of these tubes and experiment.

 

[Miles Prower]

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*

Until someone invents a plastic resin that is conductive, and can stand up to red heat without deterioration, and is clear (dunwanna lose the glowey bottle coolness) that one can use to fill VTs instead of filling 'em with vacuum, microphonics are (more or less) going to be a problem.

Of course, you see something like "VHF amp" in the spec sheet, look out. Since these are intended to operate at RF, all due care to keep 'em from making noise at AF wasn't taken.

 

[ray_moth]

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.

I rate it highly too, but quality was variable and you have to find a good example. Both my GEs are very microphonic, whereas my RCAs are not. On paper, 6AU6 is superior to EF86 in many respects, aqltohugh it is rather noisier. It has very flat plate curves at low current, making it a good candidate as a CCS provided that you keep its plate-cathode voltage above 100v.

 

[Boris_The_Blade]

I've used a 6SJ7 with success, though they are microphonic as well, so it may not be suitable depending on you tubes' location. Two 50V zeners stacked create a nice screen supply, but bypass it with a high and a low val capacitance for noise issues. Lots of gain with this, super high input impedance, you can usually get these NOS cheap off ebay, plus the metal shell is good shielding.

 

[coresta]

I rate it highly too, but quality was variable and you have to find a good example. Both my GEs are very microphonic, whereas my RCAs are not. On paper, 6AU6 is superior to EF86 in many respects, aqltohugh it is rather noisier. It has very flat plate curves at low current, making it a good candidate as a CCS provided that you keep its plate-cathode voltage above 100v.

or have a try to the real 6AU6 for AF : the 7543 , NOS can be found for cheap on ipay

 

[sparrott]

pentode gain stage.

I have good luck with using triode strapped 6JC6A pentode with CCS source as an initial gain stage in a couple of amps (not guitar amps).

I believe it is has a GM of 15.

Scott