Pentodes in the Preamps

[Decko]

Hi All,

I was day dreaming about amps this morning.

Over driven power tubes create harmonically rich distortion. Over driving a 12ax7 (triode) creates a less pleasing distortion.

If I can use a pentode in a preamp circuit, can one theorectically conclude that in a properly design circuit, the gain/ distortion would be more pleasing?

I know that pentodes have been designed in preamp circuits. IE Gibson, Matchless.

What pentodes are most suitable for this application?

Thanks!
Danny

[M Fowler]

The 6SQ7 or 6SL7 and more.

[Andy Le Blanc]

Its not really the tube type you need to worry about.
You have to really respect the high impedance nature of the pentode
in your design choices, the loading that the circuit after pentode creates.

[M Fowler]

A good example is the new BC 7 amp he did a fantastic job designing his amp it uses 5AR4/GZ34 rectifier, 5881 power tubes and 6SL7 cctal preamp tubes.


Normal volume, bright Volume and presence.

[Decko]

Thanks for these examples.
These will give me a good starting point to study.

The 6SL7 tube, are they in production? Easy to find?

This may be a dumb question but can one use an EL84? If not why?

Danny

[Andy Le Blanc]

different pentode type, they're a sharp-cutoff pentode verses a beam power tube.

[UR12]

different pentode type, they're a sharp-cutoff pentode verses a beam power tube.

The 6SL7 is a dual triode, kind of a octal 12ax7. The 6SJ7 is a good choice for an octal pentode pre and both the 6SL7 and 6SJ7 are being produced by Sovtek.

[Decko]

Andy,

You wrote"different pentode type, they're a sharp-cutoff pentode verses a beam power tube."

How do these types of pentodes affect the circuit? Sharp cut-off versus beam power?

Has an EL84 been used in the preamp?

When designing a pentode circuit, does one keep in mind the "sound" of a tube? For example, the 6SN7 has specific characteristics versus an EL84 tube therefore one would use tube "X" to accentuate the desired characteristic??

I have used the 6SN7 which is a lower gain than the 6SL7 in the power section of my latest build.

Regards,
Danny

[M Fowler]

I have been eye balling this schematic for some time wondering if it would a good one to build.

[M Fowler]

I built a fun little Fender Bass amp the 5b6 octal and gathering the components for the Gibson EH150 type II with choke instead of field coil speaker.

[Andy Le Blanc]

el84 are a power tube and say a 6sj7 is high gain amp, but they are both pentodes.
They both have a suppressor grid. In a power pentode the this enables a
higher power out put with less driving voltage on the grid.
In the radio and audio frequency amplifier this enables
a high voltage amplification with less plate voltage, the plate voltage swing
can be made very large.

In a triode there is a practical limit to the size of the plate resistor and gain
you can get, The transconductance of pentode amplifier enables a much
higher amplification factor, gains of over 300.

[Andy Le Blanc]

I haven't tried a 6sn7 in a power side yet. All my 6sn7 are old stock and more
valuable as HI FI pre, wouldn't take the risk.

If you respect the impedance of a pentode pre they are wonderful.
But... it means choosing a tone controls other than a deep carving high
insertion loss stack like the fender style. The tone color is different.
Matchless used a low gain 12ax7 stage in front of a pentode to drive a
tone stack before it to avoid impedance issues.

[Phil_S]

I have been eye balling this schematic for some time wondering if it would a good one to build.

Whoa! It's hard to believe anyone would take one of my schematics that seriously. That was a wild hair, just so I could build a low power all octal.

I don't have any voltages posted except for the PT secondary. I had to do a little bit of "shoehoring" to keep plate voltage on the finals below 450 and on the preamps below 300. I think I'd go with a PT that's maybe ~425-0-425 or something like that. The idea of the high voltage was to give the amp plenty of headroom, as I was thinking it is pretty easy to drive triode finals into clipping.

I tried building it with a floating paraphase inverter, but it drove the finals way too hard and ended up going with my original notion that the concertina was the right choice.

I used the 4 triode sections for the finals because I had the 9K output transformer. If I had one ~22K, one tube (2 triodes) would work fine. I think the one they use for the Firefly is good for that.

This was, in nearly all respects, a bone pile build, except that I spent a bit of time trolling eBait to buy cheap tubes for it and I did very well with that, all USA made old stock, some appear to be NOS.

This amp has a tone all its own. IMO, worth building. Not loud at all. Use a pedal to produce some grit.

I occurs to me, you could do this design with four tubes in the final section (8 triodes) into a 5K or 6.6K OT, for about 20W. I think it would take on some real character at that level, particularly when you consider an OT with that sort of primary is probably rated for 30-50W.

Regards,

Phil

[M Fowler]

Phil,

I didn't realize that was your creation man Maybe driving 8 triodes might be the answer

It still is interesting and something different to build for fun. I have a bunch of odd ball tubes I want to go through again and see if I can make them useful.

[Phil_S]

Phil,
I didn't realize that was your creation man Maybe driving 8 triodes might be the answer

Yeah, it needs to be named after one of those cars from the 60's when they were all 8 cylinder gas guzzlers and we though 18mpg was off the charts. (Buick) Wildcat maybe?