6v6 plexi-ish build questions

[donzoid]

I'll check v2b but right now there are old rca philco 6v6 in there. they test pretty good n hot but are slightly microphonic so I use them for startups.

[donzoid]

Hmmmm. Better check for something wrong there.

v2
plate pin 6 - 248v
Grid pin 7 - 132v
Cathode - 133v

[ChrisM]

Missing ground?

[sluckey]

V2B voltages look typical for a cathode follower. Pin 6 which is tied to a B+ node is lower than I usually see, but I believe your entire B+ string is low?

[donzoid]

Thanks Chris..missing my mind at the moment, but sure seems like its possible. Still tracing wires...so far not seeing anything obvious.

[donzoid]

Yes sluckey my whole B+ string starts about 325 which I think I can adjust up a bit.

[donzoid]

I'm going to re-measure and re-post my volts chart and the dockery pdf version that I worked off of this afternoon so you guys can have a more comprehensive look. It's gotta be right here under my nose (but my nose is pretty big so that might hide a lot).

[Colossal]

I'm going to re-measure and re-post my volts chart and the dockery pdf version that I worked off of this afternoon so you guys can have a more comprehensive look. It's gotta be right here under my nose (but my nose is pretty big so that might hide a lot).

I would consider elevating your heaters to DC. This very much helps with cathode follower hum, if that is indeed the source of your misery. And if not, it's still useful and easy to implement.

[donzoid]

OK Colossal, and that is the diagram from Merlin's site (earlier in the post)? Diodes on one side of the heater string? I just wasn't sure based on his diagram if that worked for a CT heater.

Edit: I was also thinking about swapping the 1st cap in the CLC string since I have one to see if that made any difference. I have a 22/450 in there now and have a 30/500 on hand.

[Colossal]

Nope, the diagram above shows a method for reducing the heater voltage.

Heater elevation is injecting DC via your heater winding centertap. This places DC on top of the 6.3VAC. The heaters still find AC ground via the DC filter cap.

To do this, you can tap off your screen supply node. Use a 180k resistor, then a 10-100uF 100-250VDC cap, then a 47k resistor to ground. The 180k/47k divider should provide 40-80VDC, depending on your screen supply voltage. The cap adds additional filtering and decoupling. You then connect your heater winding's center tap to the middle of the voltage divider. Doing this provides a DC offset. This is very useful for new production tubes used in applications like cathode followers were the heater-to-cathode voltage limit may be exceeded. Marshalls run over 100VDC and Boogies sometimes over 200VDC. Offsetting that with DC lowers the ratio. I do it on most amps as it is only a few parts that helps ensure hum free operation.

It may not be the source of the hum in your amp, but I think it's worth mentioning and a useful addition to any amp with a CF. My $0.02.

[ampgeek]

Here is an example of the heater circuit elevation circuit that might be helpful in visualizing the technique.

Slide it over one B+ node to the screen supply and tweak the voltage divider resistor values and it will very closely match Dave's example.

Good luck!
Dave O.

[donzoid]

Awesome, Thanks!

OK so here is the latest update:
1. Found the PI plate connections were reversed per my diagram. Corrected that.
2. Changed the "sag" resistor to 100R / 20watt and here are my new volts.
3. re-touched the solder on all the output tube pins.

NOTE: I am back to "normal" hum but plan on elevating the heaters AFTER I get this last issue ironed out. (Thanks Dave and DaveO)

I Noticed a strange behavior (by coincidence, or happenstance if you will) on the grids of the output tubes. One tube slowly comes up then is "somewhat steady" and the other goes up, and up and up. My thinking is, that its a tube problem (tube not the amp), but throwing it out here.

Here is an example of what I read in mV over about ten min. of warmup.
V4 / V5
38/24
42/28
46/40
49/50
53/63
56/70
60/78
80/115
75/117
72/120

Finally, I measured about 226mA from Cathode of power tubes to ground, and am using a 250R with 470cap. Of course that seems too high. IF the numbers I just plugged into Webers calc are correct that makes 19watts per tube.

Summary: That really nasty hum is gone (I think because my PI plates were reversed, but...) am curious why my grids are drifting on one output tube like this. Bad resistor? Bad tube? I'll try a different pair of tubes and see if it behaves similar. I guess then I'll know. Interesting that I am getting a -0.5 volt reading on v2 grid, pin 2...

[donzoid]

Here is the Dockery version I have been using. I inserted a choke instead of the 2K resistor after the first cap, used a 220R/470uF cathode follower and a 22K tail resistor since I wasn't going to have the el84's.

EDIT: that's actually a 250R cathode resistor not 220, sorry.

[donzoid]

Oh, and it's fair to mention that at the moment, the amp (to me, not the bosslady) sounds like it has a blanket thrown over it. But that's half the fun-tuning the tone stack...

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.

[donzoid]

Put in a 4k for the screen dropper, now have 10 less volts than the plates.



Looking for a 6k to see what that gets me.