HELP!!! with tube please

[Enzo]

The bias is not likely an issue here.

You need to replace that tube. That may well cure all the other ancillary issues, but even if it doesn't, it still needs to be done. it is like your car with a flat tire. The thing you may be concerned about is that the car shimmies over 40 miles an hour. It may be the result of the flat tire, but even if not, you cannot successfully troubleshoot the car until you fix the tire.

[wiggs13]

So I followed the channels from the bias jumpers to the pots and that is where I measured the voltage is that pot 9, it was the pin second counterclockwise from the wide opening in the pot, would #2 be second clockwise from opening that is with the opening towards me? Measurements were all taken with power tubes out

[wiggs13]

I'm sorry wrong termenology I was measuring the voltage at the tube sockets

[wiggs13]

I should be able to measure bias at the socket with tube out correct or no

[DRH1958]

Yes you can with the tubes out.

[wiggs13]

so following the layout and schematic here are some measurements and questions, back tracked from the #2 pin i find the jumper, at the jumper i read 14.7 volts at the pots i read between high -10's- low-11's. routing follows as this Jumper splits to two branches each containing a 220k resistor both of those branches then split into two more branches each containing a 47k resistor then they go to their respective sockets. somewhere Im loosing roughly 4 volts does that seem a little large for a 220k and 47k run in series?

Resistance as read from jumper through the 4 circuits to sockets pin#2 240,231,234,250

Resistance of 220K resistors 204, 194

Resistance of 47k resistors 46,45,46,46

[g1]

That's with the power tubes removed?

[wiggs13]

yes voltages are tubes removed, and resistance is also tubes removed and across jumpers that lead directly to resistors. I could add a jumper wire to the tube pin and insert to check voltage with tube installed if that would help.

the voltage at jumper is 14.7 while the voltage at after 220k resistor is 11.1 and 12.1 this is checking off a jumper that splits off after the 220k and connects to C32 & C35 caps then one to preamp

the tubes aren't seeming to be red plating anymore rather spuratticly on startup....

[J M Fahey]

a tube becomes more red than the others, after 30-40 seconds or so the hum fades and the tube dims.

Given the short time constants, are we talking red plating or just normal filament glow?

Filaments in parallel sort of self-adjust, if one has marginally less DC resistance than others. will pull more current, heat more, rise resistance and so dissipate less after a few seconds.

Filaments in series behave the opposite way, a slightly higher resistance or with less thermal inertia one will pass same current as others (remember: series connection) , heat up more or faster, increase resistance further, heat even more, and so on.

Eventually other filaments catch up and voltages equalize.

The problem is so important in certain cases, as in tube TVs that used long strings of filaments in series, that special "controlled heating filament" tubes were developed for that use.

Even today some user worries and writes in Forums about "a 12AX7 which flashes very bright, then gets same colour as others".

I have not discarded that possibility yet, so please check where in the tube does that "red colour" appear.
Turn amp on in a darkened room, look from close by: does the glow appear as apple seed sized bright dots at the top and bottom of tube or as a duller fingernail or stamp sized area in the middle of the plate? (large grey tube, the largest element inside the glass envelope).

A(flashless) picture would be great.
Hold the camera steady with books or firmly against the chassis edge, exposure time might be a couple seconds so hold your breath and stand still until the camera finishes its job.

[wiggs13]

so this situation was just created by me firing up the amp with transformer unplugged :/ going to move tubes around now and see what happens, but that was the look i was getting but not so bright... yep pretty sure i just toasted something in socket 4....

Okay that issue is corrected it seems that while inserting meter probes into socked I opened the terminal a bit got it tightened up and not overheating, now i still have the hum though and will post new picture

Could this possible just be a dirty terminal issue and just appear to follow a tube it seems after in and out so many times they are heating up less..., although Im still concerned about the voltages and resistance being 20K under spec but to me that is exactly opposite what the voltages say.

[DRH1958]

Never run a tube amp without a transformer or some dummy load hooked up. The first pic is definitely red plating and the second definitely things are OK. If the socket was bad, this position would red plate and now it doesn't. There could be a possibility of a loose socket that needs to be tightened. The different tube in the socket could be making proper contact because the pins are slanted differently or something else. Usually, it is sockets needing tightening not the reverse.
I wonder if the hum is due to the red plated tube being ruined. This happened to me once. I agree with Enzo on this one. Read post #14. Can you replace it temporarily with a different one? If the hum goes away, replace all four or match the new one to the old ones, but all four might be better.
One thing bothers me. I have an EL84 amp with these plate and screen voltages and I have -11 volts bias. It seems for these voltages, -14 is kind of high. Anyone else have input on this?

[eschertron]

I have an EL84 amp with these plate and screen voltages and I have -11 volts bias. It seems for these voltages, -14 is kind of high. Anyone else have input on this?

The -11v (I have about -12 on my 2xEL84 amp, different brand and model) is a hotter bias than -14. The bigger the negative number (more negative) the bigger the dampening effect of the bias.

-14v is what's printed on the schem for this amp. What seems weird to me is the voltage measured at the tube pins is reported to be -11 or -12v. Although that's closer to to what you think the bias aught to be

edit: Is there something else besides C32 or C35 leaking that could cause the voltage to change? Especially w/ tubes pulled.

[DRH1958]

Yes, I understand about the bias. I guess I should have said too high a negative voltage. For 332v, 328v for plate and screen, -14 seems too negative. But the problem still is why does it drop 3 volts? It really shouldn't. I still say replace the tube, it is probably damaged from red plating. This could cause the hum. If he has -11 or so volts, like you and I agree, this is more normal for these voltages and shouldn't worry about it. This is enough bias to keep them from red plating under normal conditions. A lot of these companies cold bias amps for extended tube life. Another example is the Fender Hot Rod DeVilles, 30mA per tube for 6L6s.

[wiggs13]

Thank you for understanding what I'm trying to achieve, so my question would be to you if one of those caps would be leaking could that cause my total resistance in that circuit to show weak it reads at 20 ohms less than it should , the 47k's in the series are reading within 1 ohm, wouldn't this cause voltage to go up closer to transformer output rather than drop?

If c32 or 35 were both leaking would this lower voltage?

[Jazz P Bass]

"Quote: Another example is the Fender Hot Rod DeVilles, 30mA per tube for 6L6s. "

That's not cold.