6v6 plexi-ish build questions

[Littlewyan]

donzoid I don't understand how your screens could be higher than the plates. If you've got the OT Primary connected to the first mains filter cap and then the screens after the first dropping resistor they should straight away be lower than the plates?

[Colossal]

Edit: My screens are still higher than the plates. So I am going to bump up the screen dropping resistor 'cause I want that compression Colossal spoke of. Voltage-wise, I think I'm right in the ballpark for what MHuss built out of his old Traynor donor. Depending on the tone though, I may drop the whole B+ string back down a bit but we shall see how things progress after correcting the screen volts, and scrounging around here to see if I have the parts on hand for the heater elevation. Thanks all I think this puppy is close to being playable.

Don,

I'm confused on a couple of things. With the Dockery schematic, that is really an 18W derivative and while the TMB channel is similar to a Plexi, the output section is different from Mark's 6V6 Plexi.

When you say screen dropping resistor, I am assuming you mean you have changed the 2k dropping resistor shown on the layout between the plate and screen filters to 4k. The layout does not show any additional screen resistors, which, on the 6V6 Plexi schematic are 1k. Do you have dedicated screen resistors in your amp? If so, what value are they?

Second, when you say your voltages are in the general ballpark of Mark's Traynor donor amp, your voltage chart shows 351VDC for the plate voltage, but Mark's amp had 440V on the plates. With your PT at 380-0-380, with say a 50V drop with a 5U4GB, this should yield around 380VAC * 1.4142 = 537VDC - 50VDC = 487VDC rectified. How are your plate voltages so low? Did you add series resistance to each leg of the PT's secondary, before the rectifier? There is a note on your voltage chart which says "480V before the resistor" so I'm guessing you added a sag resistor.

donzoid I don't understand how your screens could be higher than the plates. If you've got the OT Primary connected to the first mains filter cap and then the screens after the first dropping resistor they should straight away be lower than the plates?

I am wondering about that too.

[donzoid]

6k screen dropper now has 360v @ plates, 340v at screen. Moving on to elevate heaters, and then get a better set of tubes into it.

[donzoid]

Just saw your last two comments. So, here is how it's layed out so far:

No resistors on the AC side of the PT secondary.
Into the 5u4GB
then into the 100R / 20 watt "sag" resistor

-- The secondaries are putting out 485v before (at the top of) that resistor

Into the 22uF/450 1st smoothing cap
to the standby switch - and that's where my B+ rail starts
B+ from standby switch to CT of OT, choke input from that point to the next cap (40/450) and that's where my screen dropping resistor is. It started out as a 120R. Wyan, I didn't really get why/how the screens could be higher, either.

Have I kludged it beyond belief? Dave, no if you are asking if I have separate 1k resistors on the screens (between pins 3/4 are I think where I saw these on sev. more standard variants of the 18w build).

Let me know if I need to reconfig or add the screen resistors. I didn't see them on this layout? Perhaps its incomplete.

[donzoid]

graphic representation

[Littlewyan]

Ok have you double checked that the screens and OT CT are connected to the correct places?

Drain the caps then use your multimeter to measure resistance between the rectifier and the plates. Then do the same for the screens.

[donzoid]

Yep. OT CT coming off the standby switch (granted that 100R resistor happens before that...)

Screen resistor feeding pins 4 of 6v6.

OT primary legs are on pins 3 of 6v6

I'll take measurements and post shortly.

[donzoid]

Rect to plate v5 294R - v6 317R

Rect to Screen v5 and v6 - 6.1K

[Colossal]

Just saw your last two comments. So, here is how it's layed out so far:

No resistors on the AC side of the PT secondary.
Into the 5u4GB
then into the 100R / 20 watt "sag" resistor

-- The secondaries are putting out 485v before (at the top of) that resistor

Ok! That makes sense then. And you are keeping the plate voltages down to play nicer with your Russian tubes.

Have I kludged it beyond belief? Dave, no if you are asking if I have separate 1k resistors on the screens (between pins 3/4 are I think where I saw these on sev. more standard variants of the 18w build).

Yeah, the whole screen thing is odd, but I would be inclined to put a screen resistor right on the tube socket.

Let me know if I need to reconfig or add the screen resistors. I didn't see them on this layout? Perhaps its incomplete.

That particular layout did not have them but they're a good idea and part of the compression equation.

[donzoid]

I appreciate you guys! You have no idea how much...

Being slightly dyslexic, I sometimes have a tough time with writing. That's why the "graphic representation".

OK, so if I put resistors right on the sockets do I need to back off the 6K I have there (feeding both now), drop its value down, take it clear out, or just leave 'er alone (stays in there).

And yes, I didn't want to get much higher with my B+ so I could have the option of running the Rusky 6p6s, 'cause I can get those dirt cheep.

[Littlewyan]

Try 1K Screen Grid resistors on each valve first and leave the 6K as is. If you like how it sounds then leave it. If you want less compression or a sharper sound then lower the 6K. Experiment

Are you still having an issue with hum?

[donzoid]

Wyan, there is still some hum (after all, I'm standing under flourescents, startup set of tubes etc so I expect some), but much, much less so after swapping my PI plate feeds. I still plan on elevating the heaters, but wanted to work through these things first.

In other words, before even with volume all the way down I had a nasty hum, now I gotta turn the volume way up on the master so that issue is at least tolerable for now.

[Littlewyan]

Have you got a cover on the bottom of the amp? That can help a lot. I buy sticky back foil and stick it to the bottom of the head cabinet so the chassis rests on it and grounds it.

[donzoid]

I have a shield, but am doing all this w/ it off and the chassis flipped upside-down on a cradle, so I expect that will help "later".

1k's are installed I'll be firing it up shortly...

[donzoid]

Interesting.

After installing the 1K 5w screen resistors directly on pins 3/4 of the tubes, Now my screen volts are almost equal with my plate volts again.

350 plate
348 screen

The plates dropped, and the screens came up. Normal?