6v6 plexi-ish build questions

[donzoid]

I did this off of three layout diagrams but primarily off a Dockery 18 watt. I can build my own schematic but it would take me several hrs. (that's actually a good idea was going to do that anyway).

I took Loy's diagram and inserted a 7H choke in place of his 2.2K resistor after the first smoothing cap. I didn't set up el84 sockets as he did, specifically wanted the 6v6.

[donzoid]

Note that Loys updates on this diagram changed the PT sec. to 290 not 390.
Mine is actually 380-0-380 with a resounding 485 DC after the 5u4GB...ummm yeah a bit much.

[martin manning]

Replace the 100R screen dropper with something bigger (220-470R), or put 470R (or larger) screen R's on the sockets; that will fix your screen voltage issue. Switch to 5Y3 to lose another 25V or so.

[donzoid]

OK, I will put in a bigger screen dropper but I'm going to hold off on the 5y3 (even though so far in lotsa other amps I really like them with 6v6's) 'cause I have a dozen 5u4GB to try to get rid of! Hmmm...I have a few 5u4G's too...


Went to the local guy and (chuckle) of course he didn't have the thermistor anyway. Structo could tell ya (lives in the same town) this guy sells what he has, hardly ever re-stocks and his focus is on commercial accounts (yeah, I get it). So it was a long shot anyway at least for now. I'm going to swap the screen dropper, then re-do my heater resistors onto a term. strip so I can play with the values if needed and make it neater. Thanks!

[donzoid]

I've re-done the resistor for the screens and installed a term. strip for the heater resistors/cleaned them up. That was last night and I had to fly to the gig but I had a chance to flip the switch and its still got that obnoxious hum. So after I swill a couple more java's I'm going to pull the heater resistors back out and see if that's it. If not I'll have to start investigating...oh and checking volts.

[donzoid]

I tried jumpers across the heater resistors and the hum is still there...so that isn't the issue. Just for kicks I pulled the first and second pre tubes (they are just my startup tubes and none too good). No diff. So I think it's back to the drawing board to review what's up. Maybe i blew up a cap yesterday when I took that 300 or so volts across my forearm...

[martin manning]

I guess the hum happened to appear when you added the filament voltage dropping resistors, but the cause is somewhere else. Try removing preamp tubes one at a time and see if you can isolate the source.

[Colossal]

Note that Loys updates on this diagram changed the PT sec. to 290 not 390.
Mine is actually 380-0-380 with a resounding 485 DC after the 5u4GB...ummm yeah a bit much.

Donzoid,

Your voltages are in the ballpark of Mark Huss' original 6V6 Plexi (actually hotter by about 42V). The one I built was based on the original spec and used a 325-0-325 PT. Mark said his design was a "parts on hand" build. He later told me that to do it again, he would likely use 290-300VAC for the PT, but the original design sounded really, really good. To manage the screen voltages, a 5k 5W resistor was used in series with the choke and this resulted in a 40V drop between plates and screens. 1k screen resistors were also used and the amp had great compression. If you are getting hum, consider elevating the heater filament string to DC. This will certainly help the cathode follower which will hum if the heater-to-cathode voltage is greatly exceeded. The grid voltage on the cathode follower on my amp was 132V and heater elvation voltage was +67V. JJ 6V6s took the punishment in stride and lost their sparkle after quite a bit of abuse, but never failed. Bias was set at about 70-75%.

HTH.

[donzoid]

Phil, lotsa good tips there. I'll be tracking down the hum issue hopefully today! Then I think I might add that resistor in series with the choke. Would it be better after the choke or before?

Martin, I will try swapping / pulling some tubes to see if I can at least localize where that's coming from. If I pull the phase invert and it still does it, I'll start looking at supply caps and to see if (somehow) I knocked something loose. I need to suss this out before I add any more variables into the equation.

Thanks guys!

[donzoid]

Arrrrright! Re-touched all my solder points on the tag board and left the resistors on the heaters. Hum is now gone back to the level 'twas at prior to the "dynamo hum" session.
The "sag" resistor is now at 360R, and the screen resistor per Martin's recommendation I increased to 200R, but looks like that wasn't enough (yes Martin, you said 470 and I should have rifled around and found something closer....because...)

Plates are at 318
Screens 324

Heaters are at 6.2 (just about perfect) using .2R's.

So following Colossal's tips, If I threw a resistor in series with the choke it should bring the screens down some more, how far below the plates should I be percentage-wise? (I'll be running old stock and Russian 6p6s so even though I get his point I don't want to buy a set of JJ's when I have 10 tested pairs of 'em already.)

I think I'm pretty close to where I should be on plate volts. Any advantage to raising them higher? I know the old Fenders ran them really high (abt 400 from what I've read). But it's all about squeezing as much tone /compression as I can get. If lowering the plates is better, I can do that too.

[xtian]

So I have a Tube Amp 101 question do add to Donzoid's about screens vs. plates in pentodes. I thought the general idea was to have the screens lower in voltage than the plates so the current is attracted to the plates, and zips right by the screens. But I've seen a few amps lately where the screens have higher voltage, even with 1k screen resistors. And these amps work fine. What's the big picture?

[donzoid]

+1 on xtian's Q - I've read and seen that too, but don't want a tube eater on my hands.

[Colossal]

Donzoid,

Your plate voltage is quite low now so I'm not sure if you are still needing a power resistor in front of your choke. To answer your question, it shouldn't really matter the order they go in, but in my amp, I placed it in front of the choke. Keep in mind, I built mine very close to Mark Huss' original spec so that resistor was a good bit of insurance to drop the screens down significantly, about 40V in fact. Tonally it adds a good bit of compression which was reinforced by the 1k screen resistors.

I guess now you need to get to the bottom of the issue of your screen voltage being higher than the plates, unless that is a measurement error, or your plate and screen leads are reversed (?).

[donzoid]

Plates are pin 3 (OT primary) and pin 4 are screens, correct? There must be something going on there and it's prolly related to:

I spoke too soon, for the sake of consistency I plugged into the same cabinet I gig with (1 G12H and one G1265) and the hum "isn't" gone. Frikken Alnico was "hiding" some of it. So I'm going to have to go back through it.

I started out with 273 on the plates, and without this annoying hum it sounded better (tone-wise) but I'll attribute that to "something" else being wrong. Dang! I gotta figure this out.

[Colossal]

Plates are pin 3 (OT primary) and pin 4 are screens, correct? There must be something going on there and it's prolly related to:

I spoke too soon, for the sake of consistency I plugged into the same cabinet I gig with (1 G12H and one G1265) and the hum "isn't" gone. Frikken Alnico was "hiding" some of it. So I'm going to have to go back through it.

I started out with 273 on the plates, and without this annoying hum it sounded better (tone-wise) but I'll attribute that to "something" else being wrong. Dang! I gotta figure this out.

Can you check your cathode follower (V2b) voltage? Do you have a datasheet for the 6P6S? This comes up for the 6P6S, but yes, based on that drawing Pin 3 should be the plates and Pin 4 should be the screens.

http://oldradio.qrz.ru/tubes/russian/short/6p6s.shtml