Fender Showman High Bias on one pair of tubes

[stratomaster]

Hello,

I have a head scratcher. A Fender Showman AA763, has a very high bias (200+ mA!) on only one pair of power tubes and it will often pop fuses when taken off standby. I've tried all the obvious things like checking for opens, shorts, failing caps in the bias supply, trying a new quad of tubes, checking the phase inverter, etc. This led me to disconnect the negative feedback and swap the primary leads (blue, brown) on the output transformer. The problem traveled with the leads to the other pair--so I tried a new output transformer. However, the problem still persists. So I'm at a loss.

When using a lightbulb limiter the amp fires up and the disparity in bias current almost vanishes between the two pairs. Therefore the lower power seems to be masking symptoms. The bulb remains lit, however, indicating a possible short that I've yet to find.

My question is... where else should I look? The PT and choke should not affect only one side of the push-pull tube set--and it certainly shouldn't travel with the primary leads.

One additional piece of info: there is an audible hum coming from the transformers when the amp is powered on but in standby. I can reduce the severity of the hum by putting my finger on any transformer and pushing down slightly. I've heard transformer hum like this before on other amps, but figured I'd provide this additional information since the hum seemed unusually loud.

Thanks in advance.

Omar

[Enzo]

How about other obvious things, like pulling the power tubes and verifying good B+ on both pins 3 and 4 of each socket. More important, good bias voltage on all pins 5.

Did you measure 200ma on each tube? Or only on the pair of tubes? Install only ONE power tube in the end socket. How much current does it draw? DO NOT TOUCH the bias, but move the tube to the next socket, what current? Do all four sockets read about the same?

Now pick one of those tubes, and install it in the end socket and measure again. Pull it and try the next tube in the same socket, measure it. Do not adjust anything bwteeen tubes. Do all four tubes read about the same in that socket?

We just verified both the individual tubes, but the individual sockets as well. This will make evident if any one thing is bad.


By the way, "bias" is the voltage applied to the grid. or more correctly, the voltage difference applied to the grid versus cathode. The 200ma is the tube "idle current."

[stratomaster]

Enzo! I was hoping you'd chime in. Learned lots from reading your other posts and your comments on my conundrums in the past.

I measured 200mA on both tubes in the pair, each socketed with a 'bias probe' one at a time. I did not perform a transformer shunt current measurement for the pair.

No tube voltage (each socket)

Pin 3: ~ 493V
Pin 4: ~ 492V
Pin 5: ~-55.5V

This is with no lightbulb limiter or variac.

Unfortunately the single tube test will have to wait until tomorrow evening. But I will attempt and report back.

Thanks.

[stratomaster]

Alright. Enzo's systematic approach is proving fruitful. Numbered each tube 1-4 and each socket V5-V8.

(mA)	Tube 1	Tube 2	Tube 3	Tube 4
V5	29.0	31.4	30.4	236
V6	29.1	32.2	30.9	235
V7	30.2	32	29.5	242
V8	29.5	32.5	30.8	233

Tube 4 stick out like a sore thumb.

Looking at just this data and disregarding the primary lead swap this looks like simply a bad tube.However, since this is my 2nd set of tubes since the amp started doing this, I'm wondering if the bad tube is a result of the problem or the cause.

In other words, will simply replacing this tube fix my problem? Or is there something in the amp that fried this tube and will fry the next one as well? The reason I'm skeptical is because the way the high idle current seemed to follow when OT primary leads were swapped. Unless I'm just unlucky enough to have moved the bum tube with the primary leads.

Any suggestions?

[Enzo]

Remove tube 4, and run the amp with just the other 3. Does it settle to more reasonable current? Does the amp sound OK?

[g1]

The bad tube was probably pulling it's mate down with it.
Try a pair from your good 3. In the inner sockets, then in the outer sockets.

edit: I was posting at the same time as Enzo. No problem trying with the 3 good ones like he said.

[stratomaster]

With a pair of tubes (one per side) it stabilizes pretty well. A lot of low level hissing noise and few sporatic pops (rather loud), and some arrhythmic ticking or putting. Amp has had a cap job in the last year or so. Did some preamp tube swapping. Only pulling the PI makes the noise go away entirely. Other configuration of preamp tubes made the noise more prevalent. Put the noisiest one back in the trem oscillator (V3). Found a crack in the PI tube socket between pins 3 and 4, see image. Don't know if it is a cause of these noises. The pop has me the most concerned, since a pop usually accompanied red plating before removing the bad tube.

[Tom Phillips]

... Found a crack in the PI tube socket between pins 3 and 4, see image. Don't know if it is a cause of these noises...

Good photo work. It just looks like a chip in the socket body and it is not anything that will cause the problems you are troubleshooting. I would not worry about it and I certainly would not change the socket because of it. The socket's contact pins are still being held in place just fine. When looking for the cause of problems one often finds things like that. We have to keep in mind that you also find the same kinds of things in perfectly functioning equipment if you look carefully. Thing is, we don't often notice them and the amp lives out a long life with the little defect causing no problem. The chip was probably there from the day the amp was built. When troubleshooting we need to resist the temptation of getting distracted by things like this.

You did a good job by noticing this. With such attention to detail I'm sure you will sort out the problems with your amp.

Tom

[Mick Bailey]

The chip is between pins 3 & 4 so no high voltage stress at that point. I'm with Tom on leaving it alone. Pops can be down to pretty much anything - a bad coupling cap (or any other cap), a resistor stressing under voltage, a bad solder joint, slack tube socket contacts etc. If you still get noise with the PI in place but with the preamp tubes removed, and it's silent with the PI removed, then I'd check around the PI and its supply. Make sure there's no DC leakage on the coupling caps. If one intermittently leaks high DC levels it can skew the bias on a pair of tubes and cause them to red-plate.

Your transformer hum is likely to be mechanical - loose covers, mounting bolts, laminations or windings. Other than ensuring all the bolts are secure, there's not much you can do outside of getting the tranny vacuum impregnated.

[stratomaster]

Thank you all for your help. Turns out the Volume pot in the Normal channel was the culprit. I tried cleaning it, but replacement was the only cure. Checked for DC on preamp grids and saw 0.1 mV on pin 2 of V1 but 0.0 everywhere else. I think she's ready to button up.

Thanks again to everyone who responded. Learned a lot and now have more troubleshooting tools n the toolbox.