Guys,
I'm pondering an amp design for a guy, and I'm thinking of having one channel be a 5879 pentode >> 12AX7 Cath. Follower >> tone stack sort of thing. I'm thinking that the mondo gain and richness of the pentode would accentuate the crunchiness of the CF and be a good fit for a very wide range of gain.
What do y'all think? Anybody else run a pentode into a CF?
5879 pentode into 12AX7 cathode follower?
Moderators: pompeiisneaks, Colossal
5879 pentode into 12AX7 cathode follower?
Rich Gordon
www.myspace.com/bigboyamplifiers
"The takers get the honey, the givers get the blues." --Robin Trower
www.myspace.com/bigboyamplifiers
"The takers get the honey, the givers get the blues." --Robin Trower
Re: 5879 pentode into 12AX7 cathode follower?
hi Rich--
I've built a couple of experimental amps with a 5879 and although it has a good pentode tone, I wouldn't say it has "mondo" gain. Unfortunately, I've not built one with a CF (though it's on the list), so I can't help ya there. I can tell you that experimenting with different plate resistors, say from 150k to 470k, will yield quite a wide variety of distortion/headroom. Have fun, man -- I look forward to hearing about your tonal adventures.
I've built a couple of experimental amps with a 5879 and although it has a good pentode tone, I wouldn't say it has "mondo" gain. Unfortunately, I've not built one with a CF (though it's on the list), so I can't help ya there. I can tell you that experimenting with different plate resistors, say from 150k to 470k, will yield quite a wide variety of distortion/headroom. Have fun, man -- I look forward to hearing about your tonal adventures.
Re: 5879 pentode into 12AX7 cathode follower?
jimipage,
Thanks for your input. I may end up building the pentode amp for myself, as an experiment, and make my friend's amp with fendery and marshally preamps. My goal is to keep the two preamps in phase (same number of inversions) so he can have either or both using an A/B/Y switch. He likes the idea of "no moving parts" inside the amp. He's had a couple of channel switchers that have had relay failures.
I'm drawn to the 5879 for its hi mu (2000) and fat tone. With one pentode I can get lots of potential gain (and only one inversion) instead of cascading two triodes.
Cheers
Thanks for your input. I may end up building the pentode amp for myself, as an experiment, and make my friend's amp with fendery and marshally preamps. My goal is to keep the two preamps in phase (same number of inversions) so he can have either or both using an A/B/Y switch. He likes the idea of "no moving parts" inside the amp. He's had a couple of channel switchers that have had relay failures.
I'm drawn to the 5879 for its hi mu (2000) and fat tone. With one pentode I can get lots of potential gain (and only one inversion) instead of cascading two triodes.
Cheers
Rich Gordon
www.myspace.com/bigboyamplifiers
"The takers get the honey, the givers get the blues." --Robin Trower
www.myspace.com/bigboyamplifiers
"The takers get the honey, the givers get the blues." --Robin Trower
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Cool idea
Mount the 5879 inside your chassis, on a socket that's mechanically "floating" on grommets or somehow isolated from the rest of the amp. I built one a few years back with the tube in it's own fully shielded enclosure. They can be microphonic indeed ! Not a bad pentode as they go.
The RCA tube manual had a nice mic preamp circuit that could be adapted to guitar input use easily. Use a 12AX7 or 12AT7 (for less gain), use one half as a cathode follower (DC couple off the plate of the 5879), and then a tone stack, followed by a typical voltage amp to finish off the channel.
Using a 12AT7 will give you reduced gain in the output amp, and a lower impedance cathode follower to the tone stack. With the pentode, you may not need the gain available from a 12AX7.
You might even get away with no cathode bypass cap on the output stage actually, or use one, and attenuate the output with a master volume or a couple of resistors. The end result is a channel that is in-phase with a standard Fender 2-stage channel and can sound quite excellent. [/b]
The RCA tube manual had a nice mic preamp circuit that could be adapted to guitar input use easily. Use a 12AX7 or 12AT7 (for less gain), use one half as a cathode follower (DC couple off the plate of the 5879), and then a tone stack, followed by a typical voltage amp to finish off the channel.
Using a 12AT7 will give you reduced gain in the output amp, and a lower impedance cathode follower to the tone stack. With the pentode, you may not need the gain available from a 12AX7.
You might even get away with no cathode bypass cap on the output stage actually, or use one, and attenuate the output with a master volume or a couple of resistors. The end result is a channel that is in-phase with a standard Fender 2-stage channel and can sound quite excellent. [/b]
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