6V6 Red plates on Princeton Reverb clone.

[varaso]

Hi, I'm new here I see you've got a very nice community built.. So I've got a problem with my Princeton reverb clone that I built and 2 times I had issues with super hot power tube.

Warning.. Huge post below..

The first time that this happened, I thought it was the tube's fault because previously that day the tube had fallen down hard by accident. I replaced the tube and everything was fine for about a month. Some days ago while the amp was on for about an hour or more at very low volume I saw the new tube glowing red hot. The issue happened on the same socket and that's suspicious. Since then, I opened up the amp and took a lot of measurements to try to identify the cause of the problem.

So I bias the amp at 21mA @ 425V. The tubes are a matched set of Tungsols. They're not idealy matched so the other tube was running a bit colder. So t1 is the tube running hot and it's in socket s1 while t2 is the tube running colder in s2 and s2 is where the problem appeared twice.

The issue must be after the bias circuit, so I checked PI coupling cap for leackage, changed grid and screen resistors on the socket, changed the 220k resistor from the bias to the coupling cap. I also inspected the socket and it seemed fine (it's a belton socket btw). I measured the components in the bias circuit, the pot and everything seemed normal. The bias pot also is a 10k and has a nice usable range. It's schematic is uncle doug's in youtube and it's just an adjustable grid bias. The bias is somewhere near -41V.

Finally I got to the tremolo circuit and found out something odd. When I increased the intensity pot, the t1 and t2 plate currents shifted and boosted that's normal but, the t1 current swinging between 18.5mA to 26mA (it reached 26mA only when intensity cranked and speed at 0) while the t2 had a crazy range of 7mA to 37mA!! (up to 40mA). Then I recalled the day the tube was glowing I indeed had the tremolo on for a long time so I guess that might be the problem. I changed the tubes position and the crazy range continued on socket 2.

For a fix I put a diode in the outer lugs of the intensity pot and the t1 and t2 currents weren't boosted only decreased but the tremolo was weak and perhaps there was a little drop in volume and increase in the treble. It's an easy fix but I didn't liked it at all. The tremolo without the diode is nice and lush and rich I'd like it that way. Then I decreased the 1M resistor from lug 1 of the tremolo tube to a 500K and the tremolo depth increased but still it's a different sound.

After all these, I assembled the amp and it was working fine, but the plate current wasn't stable.. as the amp heated up the current increased.. not rapidly, it was on for a couple of hours because I wanted to be sure that it was stable.. I kept rotating the bias pot to keep the current at 21mA. Then I powered it down let it cool and fired it up again. The current started from 19mA and in an hour or more it went to 21mA. Then I stopped it because it was already too late at night.

Today I tried to measure if the bias was stable and it was but so was the current at 19,4mA. Assembled it again, increase in the current again from 19 to 21mA.

Something's messing with me..

Before the diode "fix" shouldn't the tremolo bias be symmetrical and equal to both sockets? What could cause that huge difference in current?

After the "fix" what could cause the current to increase? heated elements like resistors(btw old school carbon comp), changing value? duff tubes?

Sorry for the long post and the many question.. but I'd appreciate any answers.. thanks in advance!

[martin manning]

The drifting bias always stays with the same socket, and that’s the one thing you didn’t change? Belton sockets are generally good quality, but you could have a bad one, or even a counterfeit.

[varaso]

The drifting bias stays with the same socket when the amp is "stock"..

now with the diode in place the only issue is the rise of the plate current that happens to both tubes.. the numbers 19-21 mA is from the hottest biased tube of the two..

[martin manning]

This set up is putting a very high impedance between the grids and the bias voltage. The usual 220k on each grid, plus another shared 250k when the intensity pot is turned up. As the tubes heat up there is more grid current, which pushes grid bias voltage up, which causes the tubes to heat up more. Maybe try another set of tubes and see what happens. I’m still wondering about the socket leaking voltage too.

[varaso]

Can I measure the tube socket somehow ?? Sadly I have to order one from abroad as well as a new pair of tubes just in case.. So this may take some time..

[Stevem]

Pull the tube out of that socket in question and check to see that the coupling cap feeding that pin 5 is not passing any D.C. From the plate of the PI tube.

[martin manning]

The op says that was done per above, but it could be repeated. You need a high input impedance meter to detect a voltage resulting from a small leakage current. The typical DMM’s 10M should be sufficient, though. If you get something on pin 5, disconnect everything from that pin and check it again.

[varaso]

Well I tried measuring the voltage at while lifting the one end of the cap (the one that goes to the 1k5) and with the multimeter set in DC 2V I got something like 0.2 and then it was rising sometimes to 0.7 rarely to 1.2V then again 0, 0.2 sometimes it went to the minus same values.. No value was constant all of the were instant. BTW i use a cheap multimeter DT9205A.

[jjman]

Looks like your meter is 10M input so those readings imply a bad coupling cap. You can check the other one to verify it shows lower figures, to confirm the diagnosis.

[tubeswell]

There will always be changing dc on the 'ac-side' of a coupling cap if there is changing DC on the DC-side, and that is normal. What you shouldn't get is any DC on the AC-side of a coupling cap, if the voltage on the DC side is constant, because that would indicate a leaky cap.

[varaso]

I haven't really find the issue but the diode I connected to the outer lugs of the Intensity pot managed to keep the tubes biased correctly and only decrease their cathode current when the tremolo is engaged.

So the amp is stable now, I haven't a issue since then. It still have some flaws when cranked but hey! not my intention to keep it cranked, so I can live with that as it sounds wonderful at the settings I intend to use it!

Thanks again!

[Guy77]

On a side note I have noticed that when I use Tung Sol 6v6 tubes in various amps ( mainly cathode biased) they sound great but cannot take punishment like other tubes such as the sovteks and the JJs can.

Cheers
Guy

[TUBEDUDE]

I thought the "tungsols" were Sovteks..

[Lynxtrap]

I thought the "tungsols" were Sovteks..

They are manufactured in Russia by New Sensor, but not necessarily to the same specs, I guess.
They do EH, Tungsol, Sovtek, Svetlana, Genalex, Mullard and probably other brands as well. Are they all the same specs? I haven't investigated it, but I'm sure someone has.

[Guy77]

Hi, when I was referring to sovteks being able to take more punishment I was actually referring to the the Grooves tubes 6v6GT-R which according to Doug's Tubes is actually a re-labeled Electro Harmonic.

http://www.dougstubes.com/brands/groove ... 6v6-r.html

Link to Tung Sol 6v6 data sheet
https://s3.amazonaws.com/tubedepot-com- ... 1395671228


Link to EH 6v6 datasheet
https://s3.amazonaws.com/tubedepot-com- ... 1400187484

I haven't studied the data sheets but maybe there is something there that can explain the differences.

Cheers
Guy