Technical Meeting?

[Guy]

We now have two members trying to debug electrical problems with amplifiers. Is anyone interested in supporting an afternoon meeting attended by people with troubles and people with test gear to try and debug them in real time?

I'd like to help, but I start on vacation next week. It may be as late as September before I can support this but if a specific date comes up, I'll address it.

[dberning]

Actually this sounds like a blast! Could do at my house. Folks could bring busted amps, beer and snacks, and I could help resolve issues. Can't promise busted amp resolution, but something will be resolved. Tube amps take priority, as SS amps are more time consuming and parts availability is more difficult. Next weekend is out, but perhaps the following weekend? Interest???

David Berning

[craigh]

I can bring my amp, beer & chips.

Craig

[Jim G]

Actually this sounds like a blast! Could do at my house. Folks could bring busted amps, beer and snacks, and I could help resolve issues. Can't promise busted amp resolution, but something will be resolved. Tube amps take priority, as SS amps are more time consuming and parts availability is more difficult. Next weekend is out, but perhaps the following weekend? Interest???

David Berning

Very gracious offer, Dave. Makes me wish I had a busted amp.

Jim

[dberning]

Very gracious offer, Dave. Makes me wish I had a busted amp.

Actually, Jim, you and anybody else could just be an observer. Perhaps you would have a valuable 2 cents to offer.

David

[TubeDriver]

Quick semi technical question. In a PP pentode amp that is setup to run either in triode mode or in UL mode, gain will be less and power output will be higher in the UL mode compared to triode mode?

[FerdinandII]

Quick semi technical question. In a PP pentode amp that is setup to run either in triode mode or in UL mode, gain will be less and power output will be higher in the UL mode compared to triode mode?

Gain in U/L will be similar to Pentode mode. A Triode mode output stage requires much higher input voltages to achieve full power output.

[TubeDriver]

Quick semi technical question. In a PP pentode amp that is setup to run either in triode mode or in UL mode, gain will be less and power output will be higher in the UL mode compared to triode mode?

Gain in U/L will be similar to Pentode mode. A Triode mode output stage requires much higher input voltages to achieve full power output.

That is what is odd. The UL has less gain than the triode version. I also like the UL sound a lot better which is a little bit of a surprise since I am SET guy.

Unless I am completely misreading the wiring on the switch?

It is a 3 pin, 2 position switch. Pin 1 has a UL tap going to it. Middle pin goes to Grid 2 (resistor in series). Pin 3 leads to the primary. Switch towards Pin 1 is UL and switch toward Pin 3 is triode mode?

Why does UL mode have less gain? Could this be a function of how many turns (%) on the primary that UL tap falls?

[Roscoe Primrose]

Why does UL mode have less gain?

UL operation is negative feedback applied to the screen grid. Negative feedback ALWAYS lowers gain, all else being equal.

Keep in mind that for most SPDT toggle switches, the center contact and the one opposite the position of the toggle are the ones connected together.

Roscoe

[TubeDriver]

Keep in mind that for most SPDT toggle switches, the center contact and the one opposite the position of the toggle are the ones connected together.

Roscoe

Oh, I understand the theory but could not understand what I was hearing. I was thinking that there was a something wrong with the triode mode setup and that it was actually running in pentode mode(which would explain the gain), despite the fact I could see no B+ connections to grids.

I think you answered the question with the info about the switch. So when the switch is switched to the position above the pin connected to the UL tap, it is actually running the other pin (triode mode) and vice versa.

Which means I prefer triode mode over UL mode with my horns. UL mode sounded a bit flat and had a certain harsness to it.

This McGown amp is using global feedback (4 ohm output tap is connected to driver). Probably not needed for UL operation but better for triode mode I guess? Can any PP guys here comment on what has sounded better in their opinion (UL with or without global feedback, triode with global feedback, triode with no global feedback etc). I'll probably play around a bit and see what I prefer but I would think that output stage output impedance would be really high with a pentode operated in triode without feedback? Too high?