Triode mode

[K-MAN]

I've read through the forum but still have a few questions. In my SCA-35 I want to try running it in triode mode. But I'm not sure about what resistor to use, I've read either 100 or 200 ohm 1W resistor from pin 1-2. And do I disconnect the UL taps?

[TomMcNally]

Yep - disconnect and safely store the UL leads from the transformers ... a wirenut, some heatshrink or even electrical tape is fine. most guys use a 100 ohm resistor. mesherm likes to put a diode inline ... you can read his posts for more info.

[EWBrown]

The resistor value isn't critical, anything from 100 0hms to 1K will work. use at least 1 watt, or 2 watt resistors, especially for the 1K value.

The resistor can connect on the terminal boards from "plate" to "UL" and just tape off, or otherwise insulate the unused UL taps from the output transformers.

There has been discussion of adding a diode in series with the resistor (anote towards B+, cathode towards G2). Apparently this improves the pseudo-triode operation.

/ed B in transition between states

[K-MAN]

Are any other mods useful for triode operation? Can the negative feedback be disconnected?

[Geek]

If the amp is stable enough and the OPT good enough, NFB can be disconnected with varying results.

A buddy of mine, YvesM has the best saying about NFB - "NFB is like salt - a pinch will enhance an already good meal, a spadeful can turn anything [bad]"

Cheers!

[K-MAN]

So pin #2 is the grid where the signal comes in correct? On my amp schematics the UL taps go to pin 9 but I'm not sure if the numbering system is different for the diytube35.

Never mind I got it figured out. Wired up the plate to screen and sounds pretty good to me. Still has enough power to drive my speakers to a decent volume and it does sound different but I can't decide if I like it or not. The sound is warmer and especially when I listen to music from my PC it takes the harshness out of the music, but on the other hand it seems that some of the higher vocals and instruments are less defined than before.

[K-MAN]

I've switched to listening with my Dynaco A-25 speakers and find there's not enough power in triode mode. I'm going to replace my single cathode resistor with one for each channel while I'm in there. I'm not sure about which resistors to use though, I got a pair of 180ohm and a pair of 200ohm. I was thinking of trying the 180ohm to raise the current a little, I just want to know if biasing them a little hotter will damage my amp or just shorten the tube life?

[Ty_Bower]

The stock circuit uses a 95 ohm resistor, shared between all four. You would want to use 190 ohms common to each pair of output tubes. You could also use a separate 380 ohm resistor for each individual output tube.

Remember to bypass each cathode resistor with an appropriately sized capacitor, rated for about 35V.

The stock circuit ran them pretty hard. Today's line voltage is higher than it used to be. I'd probably start with the 200 ohm that you have, and work down from there.

[Hi-Fi Low-Life]

My DIYtube ST-35 Rev-D is strapped into triode mode. I'm acquainted with a tube guru who mentioned something about lowering the high voltage down to the Mullard specification for triode strapped EL84s. If this can be accomplished by inserting resistors between the PT and J1 position 1 & 2, I would like to try it (I don't want to mess with the PCB at all.) What values of resistors should I use? Thanks!

[EWBrown]

If 380 ohm resistors are hard to obtain, then just use commonly available 390 ohm, 2 watt, 5% resistors (bypassed with 100 to 470 uF, 35VDC caps) for each individual cathode (pin 3), this works quite well. In fact any common value between 390 and 430 ohms should be fine.

/ed B in transition between states