pentodes and triodes in preamp

[macula56]

i know a lot of you have been messing around with pentodes and triodes in your preamp sections and i was wondering what setup you prefer. do you prefer the triode into the pentode or pentode into the triode? i have seen a few schematics with a 5879 into a 12ax7 CF. i also saw a schematic somewhere that had a triode stage feeding a pentode as well. what do you guys recommend and why? thanks as always, JMac

[firemedic]

My own limited experience & a little research (Merlin Blencowe) tell me that a pentode sounds pretty much like a triode when used as the input stage, due to the tiny voltage guitars usually produce. To get that pentode "chime" it should be fed by an amplified signal, i.e., a triode stage.

More experienced & intelligent members here can most likely provide a more coherent and comprehensive treatment of this subject. Definitely check out Merlin's website, the name escapes me at the moment but he expands on preamp pentodes in depth, and he is highly regarded as a no BS expert.

[tubenit]

Having taken a Little Wing amp using 12AX7 paralleled in V1 and converting it to a 5879  HoSo56 ............... I heard a dramatic difference in tone. Much fatter and with alot more chime and overdriven tone. I could not get that fat of a tone with a paralleled  12A_7.

I did the same thing with SoLow Watt in reverse using ECL84's. The 5879 had too much chime, too overdriven and too fat ............ so I changed to a 12AX7 and that sounded better in that particulalr amp.  The ECL84's are very very easily overdriven, IMO.

Having said all that, I prefer the 5879 further down in the signal line and with a CF triode following it such as on the D'Mars ODS.

With respect, Tubenit

[macula56]

thanks firemedic.   i've been thinking that i should get Merlin's book for sure.
and tubenit, you have said that your 5879 Blues amp is one of your favorites. that is a 5879 straight into the CF. do you like that better than say the Nightclub Guy/MatchVox style where the 5879 is fed by a 12ax7?

[firemedic]

tubenit, you are as always better informed & much more experienced on the subject.

In my build, I didn't notice a big difference in tone between 5879 & 12A_7 in V1. But a lot of variables apply such as plate, screen & bias voltage. My screen as I recall was fairly high voltage compared to yours, for instance.

Hope this discussion helps, macula.

[tubenit]

your 5879 Blues amp is one of your favorites. that is a 5879 straight into the CF.

It's a 5879 into a 12AX7 triode followed by a CF. It's essentially just Geezer's HoSo56 with 5881's.

I can't remember if I've tried the Matchless style with 12AX7 into a pentode in the preamp?   Seems like I did try it and then vetoed it, but I'm not sure?  Sorry.

And Firemedic is absolutely right in that how those tubes sound in V1 depends on all kinds of variables such as those that he already mentioned and the general topology,  etc........     Very well said.

With respect, Tubenit

[SoundmasterG]

I've tried several amps with an EF86 at the input followed by a CF using a 12AY7 and a 12AX7 respectively. I've also tried a paralleled 12AX7 at the input followed by a 5879 and then into the phase inverter. I ran into oscillation issues quicker with the EF86 at the input, but it also has more gain than a 5879 so that could be a factor. I have not tried the 5879 at the input, or the EF86 as the second stage yet. I have a feeling that when you run the gain lower as Merlin recommends, in all cases it sounds better and works better, though that is just an educated guess on my part. I think especially with a pentode, the voltages it runs at make a huge difference in the tone and stability...YMMV.

Greg

[printer2]

I messed around with a 6AU6 with separate pots to adjust the value of the cathode, plate resistor. and screen resistor. Found you got to get the three in the right spot in relation to each other to get it to sound its best. I also ran the same tube as a triode in the same amp to compare the sound of a pentode to triode. In pentode the sound was fuller, mind you, you have to get the signal up enough to drive either configuration in its nonlinear zone. Otherwise one clean signal sounds more less like another clean signal.

[macula56]

what's the purpose of the mosfet in the D'Mars amp? is that so you can have the CF without having to add another tube?