Tube (?) Issue with Fender 1998 Tweed Hot Rod Deville 410

[GLJazz]

Hi All,

Over the past few months my HRD has been regressing in sound quality and I decided it was high time for new tubes (5 years). I guess i should mention that I have done tube swaps and biases for myself several times before, and always do it with care and technique from a local tube repair shop. Had been running 2 6L6WGB powers and switched back to JJ/Tesla 6L6GC.

Well, It made sense that it would be the tubes after this long, so i hate to say i didn't pay a whole lot of attention to the behavior of the old tubes, but swapping in the new ones i got the same moderately low output and very muddy distorted sound on all channels. The GC's were pulling up 20mV on the bias, so I rebiased to 60mV (as per Fender suggested), but no improvement.

Upon closer inspection BOTH the old tubes and new tubes would glow bright blue when standby was off, and one of the two (the left with the new tubes, right with the old, when facing back of amp) would flash bright blue before settling in to a mild glow all around the tube. All the solder joints to the tube sockets checked out OK.

From what i've heard, that means bad tubes? The package was open, so i suppose they could have been a return that someone messed up and stuck back in the drawer, but before i go exchanging and burning up another set, I figured i should see whether you guys think its the amp or the tubes.

To add to the mystery, there is a mechanical clicking, very fast almost a hum, coming from somewhere in the left of the main PCB, but not the bias pot.

Any help would be much appreciated, i hadn't been able to turn anything up in searches but even pointers to a more appropriate forum would be great if this one doesn't belong here. I'll be happy to toss up some pictures if that will help. Sorry for the long post, just stressful when your baby is not doing so hot!

Thanks!

David

[Enzo]

Don't "check" the tube socket solder, just resolder them all. I mean DO look, because if you see cracks that verifies what we might be thinking, but resolder them all anyway, even if they LOOK fine.

Measure plate voltage on pins 1 and 6 of the phase inverter tube. You get B+ voltage on both? Or is one missing due to open plate load resistor?

You have preamp out jack and power amp in jack. Plug the guitar into the power amp in, does that sound clear? Or does it have your problem? And send the preamp out to some other amp for a listen. Does the preamp signal sound OK or does it have the problem?

[GLJazz]

Thanks! I'll have to wait on the resoldering until i can find a source of high wattage resistors, radioshack is my only local option, believe it or not, to discharge the caps safely.

In the mean time, i did a test of the pre out/power in. The pre sounds great, but no sound at all from the power. Not really surprised because that right tube is glowing like crazy.

Just to check my sanity, is it okay to test plate voltage and voltage at other points (even across the filter caps) with a standard multimeter?

[txstrat]

Just to check my sanity, is it okay to test plate voltage and voltage at other points (even across the filter caps) with a standard multimeter?

It is okay, as long as the multimeter supports the given voltages.
There are multimeters which go up to 400 vdc.
Your HRD has a B+ of around 431 vdc. No more to say; just make sure the MM goes up to 500 or even 600 vdc.

[Enzo]

And make sure the speaker is plugged into the main speaker jack, and not the extension speaker jack.

Unplug the amp from the wall. measure the voltage across any of the larger filter caps in the amp. Your meter will tell you if they are discharged or holding voltage.

[boogietech1]

JJ 6l6GC Bias

I would check the screen Grid resistors. 60mv bias with open screen grids would now be the wrong bias.

[GLJazz]

Thanks guys, I'll post my findings in the morning. Lucked out and my MM peaks at 600vdc so ill get to it

[GLJazz]

Alright, now we're getting somewhere. I popped in the new tubes, biased properly, and they are both glowing happily.

However, after sitting and listening and looking for about half an hour i believe i have tracked the source of the hum to the hole in the PCB that leads to the output transformer...

And what causes problems with the OT? Improper speaker load.... It appears fender cheaped out on the wiring and somewhere in the rats nest of tied wires they left there is a break. Resistance check revealed all kinds of crazy values. Next step was to run the speaker out to half of my 1960A Cab. Sounded MUCH better, but the lows were still a bit muddy especially in chords, and every once in a while it would sound like it was losing a bit of power to the speakers and regaining.

Oddly, the main fuse was not blown or hot, etc. Could this humming be an overheated and permanently damaged OT? If so, would that cause muddy lows yet preserve highs?

And more importantly... if its the OT, is it worth shelling out the cash to repair? Thanks a ton for your help so far!

Gracias,
David

[GLJazz]

Any ideas at all? I'm getting ready to take it to a tech if I can't figure out exactly what is causing this. I rewired the speakers with good quality speaker wire but something is still causing TERRIBLE sound quality. Sounds like all the speakers are blown, or made out of plastic cups, or something. I did test them, its not the speakers.

Should i just bite the bullet and take it in? what should i expect to pay, if you guys have any experience in that department?

[Enzo]

I don't know, have you resoldered the output tube sockets? And I don;t recall your report on the voltages at pins 1 and 6 on the phase inverter tube.

[GLJazz]

Enzo - Voltages were as fender intended and I hit the solder on the tube sockets, still nothing. That hum is coming from the innards of the circuit and i just don't have time to track it down. Going in to the tech today. Thanks to you guys for the help, I'll keep you posted when I get the verdict just in case anyone else pops up an issue like this!

GLJ