Huge sweep of current on Express V5 under load?

[rooster]

Yeah, something weird. Anybody else?

What happens is, as I plug in a bias probe type device, say an Alessandro dual meter device, and I begin pounding on my gtr, it becomes clear that V5 has a huge current fluctuation. As in pushing above 100ma sometimes. And let me add that the V4 power tube nearest V1 will actually reduce its current intake as this other power tube draws more - which migh be natural considering the current demands of V5, but not typical I think. It (V4) will fluctuate down to 17ma, for example, and never go above say 80ma. V5 does not go below 30ma if that. Which is to say that this pattern is unique to either socket. If you swap tubes, the pattern remains with the particular socket.

Is this the weirdest thing? The amp sounds great all the while, it never sputters or drops out, but this cannot be right I think. I have also tried the Weber dual bias probe type device and get the same results so it is not the particular device.

OK, and then when you pause the playing, the numbers come right back to 40ma for each tube, which is where I set the bias.

The OT is the TS Heyboer version that Moose sold me a few years back, and frankly, I am thinking there is something fubar with it. But could this be a normal function of the Express? Has anybody tried this on their Express, the dual meter device attached and gtr pounding going on?? It just seems odd, but then I have never tried this before either, it only recently came up when I loaned it to a friend. He was checking the bias on some new tubes and then just thought, OK, let me pound on this gtr and come back to see if the tubes are stable.

And I should add that all other voltages are stock, 400VDC on plates, etc., throughout the amp, and all the tubes were swapped with two other working sets. The results remained the same. Any thoughts or ideas, or results from your identical testing? Also, keep in mind, the volume pot is on 2 o'clock and the gtr is running pretty hot HBs. Thank you for looking.

[marcoloco961]

Once again I cannot find the recent thread that was discussing this, but I think it is normal. I personally have never used dual meters and watched as I played. But I do not think your PT is bad. I have played and watched both tubes (one at a time) and I seem to recall that one tube went over a hundred and the other wouldn't.

[gearhead]

It was a JTM-45 build, on this thread:

https://tubeamparchive.com/viewtopic.php?t=11760

Never did find out what was causing the difference cause I ran across another situation (resolved) that made it seem trivial.

If you do find out, would be greatly interested myself,

[PCollen]

I'd take a look at the AC signal input level at each tube's control grid (p5), and the DC bias voltage right at each tubes grid that the AC input is riding on. I'd also measure and compare the quiescent DC voltage drop across each side of the OT primary that the 40mA quiescent current is causing. Finally, I'd verify the cathode (and, if EL34s suppressor grid)grounds. A bad solder joint can really reak havoc, and be very hard to find in many cases.

[ampgeek]

EXACTLY like my recent Express'ish with VVR build!

I have external test points/1 ohm resistors and plug in two Flukes at the same time to see it.

I rewired the power and PI sections/replaced sockets/yaddaa...yaddaa..yaddaa....TWICE and saw absolutely no change whatsoever. All the while the amp sounded really nice.

Finally decided that it was normal and just got on to playing it like the "problem" didn't exist.

Well....I do rotate the power bottles (Siemens NOS) after every gig.

Cheers,
Dave O.

[rooster]

Wow, thanks for the help to all. Hm. OK, that long tailed PI might have the answer then, noting the JTM45 - identical PI circuits. And Fender moved away from the 10K value and 470 cathode resistor moving to the BF circuit....

So maybe it is unique to this PI. Which now leads me to say that if you are not swapping yur power tubes every so often, its for certain that one tube is really getting a workout while the other one is just having a good old time of it.

And that's what ampgeek suggested, too, so he is on to it. But wow, I missed this from day one. Thanks again for the feedback, this is what's great about this site, of course.

[PlinytheWelder]

Mine doesn't do that...(Goes to plug in the meter)

[rooster]

Gary - Understood. And let me add that after the issue was pointed out to me, and I then witnessed it for myself, I brought my Express home and changed the .1uf PI cap. I then plugged in my Weber dual meter and was convinced that I had 'cured' the problem. ....Wrong!

When I went back to try this again at my friend's house, he pulled out the gtr he was using before and preceeded to make a liar/fool out of me. Whew, I didn't see that coming and I think mostly because I have never heard of such a thing or spent any time looking for it. Like everybody else (I'm guessing), I put the amp at idle, bias the tubes, play lightly for a minute or two, unplug the meter and get on with things.

OK, one other note about this issue, it should be noted that Ken installed the pin 3 diodes on this amp. ?? Where before I thought it was a simple precautionary move that wasn't necessary, I am beginning to think otherwise. I now think Ken was aware of this PI issue and, wishing to prolong the life of the tubes in his amps, he elected to use the diodes as a way of somewhat limiting the extremes that were happening on V5. ?? Well, you can disagree about this, but then I am thinking that the ma numbers would really JUMP without those diodes in place. I have not tried the Express without the diodes in place so maybe I am wrong, but it is starting to make a lot of sense to me. And in fact, in the Groove Tubes book, he was reccomending the diode strings for Marshalls in particular I think. See what I mean?

OK, get back here to report, Gary.

[PlinytheWelder]

Nope, the idle current on both of my output tubes stays constant, one at 39 and the other at 37mA.

I'd make sure that your bias voltage is making it all the way to the tubes.

[rooster]

Gary - When I say the gtr has to pound the tubes, did you do this? As in HBs and 11's, volume on 2 o'clock? This is no baby steps testing going on, the amp is being pummeled. ??

Don't make me do a video...

[PlinytheWelder]

No I didn't do that, I live in a condo. I may get to try that tomorrow depending on who's here.
Right now it's hooked up to a 4x12 loaded with 2 EVM-12S's and 2 WGS ET-65's.... This might prove interesting....

[PlinytheWelder]

OK... Mine does it too. V5 went up to at least 80mA...
Next time I'll need earplugs.....
FWIW, mine has Pacific trannys.
I'd be curious to see the voltage on pin 5 when this happens?

[2tone]

that's not unusual when using the Allesandro bias unit.It pegs the needles with signal from the guitar..

[PlinytheWelder]

I guess since the output tubes are actually doing some work on what's probably an asymmetrical wave the current has to go up...

[ampgeek]

FWIW...in my express, the with-signal V5 current draw is ~2X that of V4 at all volume settings. So...I see the imbalance at all output levels. It is amazingly consistent.

My Dumble'sque and 5F6A'ish builds are both within ~5% of each other at all volume levels.

These observations all remain true with many different, well balanced output tube sets with flip-flopping between the sockets so I am convinced that it isn't a tube-quirk type thing.

I beat my head against the wall trying to make the Express balance (e.g., PI plate resistors, tail resistors, etc...etc...) but when I got a good balance it sure didn't sound like an Express by a long shot.

So..I am still convinced that it is a normal condition. But...I can't for the life of me explain it.

Dave O.