Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by jcat5503, Jan 23, 2012.
Anyone know much about them?
I wanna build a single channel 5879 amp and use a 12ax7 pi
What would you like to know? Search for a Gibson GA40 schematic online which will show you how to wire a 5879.
a 5879 does not have enough gain unless you pair it with a 12AX7 before the PI tube.
I'm not sure about that.Maybe true in certain context. There is a fellow on here that recently built his own GA40 amp with 5879 preamp tubes. With a 6v6 push pull they can deliver decent cleans to light blues overdrive. This is just a single 5879 into a 12ax7 PI.
Like Blue Strat said, a GA40 would be the go to amp.
I imagine you could push a pair of el84s pretty good with a single 5879 and a well set up PI, you could get those el84's crunchin' since they overdrive easier than 6v6's. If your going for BIG clean and you use pedals then maybe el34's or 6L6's would be better.
I depends on what kind of circuit you're building...if you're going to use high-loss tone stack after the tube then you may have trouble having enough gain getting to the phase inverter and power tubes for any real overdrive.
Most of the schematics of vintage amps I've seen with pentode preamps have had only a single tone knob, or no tone knob at all driving directly into the phase inverter.
Some designers from today that use pentode preamps, but have more intricate tone options are Dr. Z and 65 Amps...Dr. Z uses a modified low-loss version of the Baxandall tone stack, and 65 Amps usually has a cathode follower directly after the pentode to give it a low output impedance and limit losses that a tonestack will incur.
Another no-loss tone option is a rotary switch with multiple capacitor values between the pentode and the phase inverter to be the "Tone" knob of the amp. You can get quite a few different tones, and have none of the losses a usual tonestack brings with it.
Yeah I for sure want to run a pair of 84's. I'm wondering if a more vox or marshall inspired PI would be better..
Vox phase inverter values (at least from the 1960 schematic I'm looking at) will emphasize the preamp more, and have better balance while the typical marshall values will emphasize the power section more and have worse balance. However, being somewhat out of balance can be a good thing because it can generate a bit of 2nd harmonic distortion. I wouldn't recommend going much over a total of 50k or much lower than 10k for the total tail resistance.
[QUOTEost of the schematics of vintage amps I've seen with pentode preamps have had only a single tone knob, or no tone knob at all driving directly into the phase inverter. ][/QUOTE]
Same here, have you thought about 2 5879's cascaded. They are cheap.
Agreed. The lower the "tail" resistance the more you can push the power amp, and the more unbalanced it is. The opposite is true. If you cascaded 5879's then you could have a very balanced PI and get most of the distortion from the 5879's and the el84's.
Same here, have you thought about 2 5879's cascaded. They are cheap.
Agreed. The lower the "tail" resistance the more you can push the power amp, and the more unbalanced it is. The opposite is true. If you cascaded 5879's then you could have a very balanced PI and get most of the distortion from the 5879's and the el84's.[/QUOTE]
Hmm two cascaded 5879's may be cool also.. Would it have too much gain though? I'd like it to have a decent breakup around noon on the dial. I'm thinking no tone control at all and maybe just use a Vox style PI and typical cut control.. Or do you think a tone control would be needed to tame the gain a little using cascaded pentodes?
Hmm two cascaded 5879's may be cool also.. Would it have too much gain though? I'd like it to have a decent breakup around noon on the dial. I'm thinking no tone control at all and maybe just use a Vox style PI and typical cut control.. Or do you think a tone control would be needed to tame the gain a little using cascaded pentodes?[/QUOTE]
You'd have to try it for yourself, most pentodes have a pretty high output impedance which is why tone stacks aren't often used, and they tend to have more headroom than triodes from my understanding, so you might not end up with too much distortion. However, a lot of the distortion you'll hear will be from the second 5879, that said it will probably have a pretty cool unique sound, and I'd recommend putting in a master volume so that you can get it at lower volumes, and when you want power tube overdrive as well you can turn up the master volume for some added thickness and crunchy el84 distortion.
Of course that's just a suggestion, if you design it right it'll sound great with or without an MV.
Most of the designs I have built have not had MV's. I did one with a PPIMV once that was pretty good. I'm assuming a PPIMV would be better
Jcat, I wouldn't bother with a 7-pin tube myself. I consider these completely defunct. If you want a pentode first stage use something like a 6GH8A tube. Its a pentode with a triode inside. You can run the pentode with a 500k plate resistor and use the triode section as a cathode follower if you like. That'll give you some gain with low output impedance.
Must be thinking of a different tube. 5879 are 9-pin tubes.
You dont have to worry about too much gain, stick a tone stack and volume in-between the 5879 gain stages. After the second you use a voltage divider to control the gain feeding the PI. However much you need to attenuate you'll be able to dial in with the voltage divider made up of two resistors after the 2nd stage coupling cap.
You could do a dual volume pot and control the volumes after both gain stages at the same time. So control the gain going into the second stage and the PI with one knob. If you hook it up like this and find you need more gain faster in the pot's rotation you then add a resistor to the lug you usually ground on the 2nd stage volume. So this way the 2nd stage volume always has some resistance between the 2nd gain stage signal and ground even if the volume is all the way down. I know this is a bit confusing, but if you have any questions or want me to throw a schematic together for you just send a PM. I really like the idea of the cascading 5879's. I've never done it or thought about it until now. After hearing what one 5879 sounds like on my amp I think cascading would work great.
PM sent! Anyone else built a 5879 based amp?
Are 5879 a very finicky tube? Or is it just my bad luck? They seem to die kinda fast in my D13 amp.
Generally speaking, I think most high gain pentodes are inherently finicky tubes..
The EF86 even more so than the 5879..
At least good NOS 5879's are still cheap!
But aren't 5879's low mu tubes? Like, 12au7 type low...
Only when used as a triode. Then it's 20mu.
As a pentode...at average preamp voltages...~180-200mu.
Ultimately, it don't believe there is a pentode preamp tube that is tour-rugged.