AB763 Single Channel Troubleshooting

[purpletele]

Looking for advice;

I had recently installed 6L6 tubes into both of my Deluxe Reverb amps.  I am running the exact transformers in each amp, the FDP 20 from Mercury.

A colleague that has been using the head amp took it to a show and it failed 3/4 through.

When I got the amp back today I checked it out and found the V6 power tube to be out.  The fuse was good so I replaced the rugged Mesa 6L6's with new 6L6's.

I had to consider that there is a chance that a power tube failed.

I ran the amp and played it for 1/2 hour and let it idle for another 45 minutes, then it redplated!

I had ran the amp for about a year with 6 v 6's without an issue. 

Any suggestions or theories of what might be going on.  I would expect my other amp to do the same soon.

Thanks,

BV

[st]

Assuming bias is set within safe range, start by looking for bad connections and failing components in the power amp section, especially bias. Visually inspect solder connections, reflow if necessary. Coupling caps are also suspect. Try and found out if the one for the redplating tube is leaky.

[sluckey]

Yes, you need to reset the bias for 6L6s. May require changing the bias range resistor and/or R66 if bias adjustment range is not enough to accommodate 6L6s.

In addition, ask yourself (or the guys at MM) if that PT can supply enough current for 6L6s. And be aware that the speaker impedances for your OT will be half value when using 6L6s, ie, 8Ω tap becomes 4Ω, 16Ω tap becomes 8Ω, etc.

[st]

Yes, you need to reset the bias for 6L6s.

I had assumed the power supply and output transformer were up to the task... I wish people realized that you can't just go around plugging in differend tubes just to try them out. The whole amp is designed around the tubes...
If you want to try a diesel engined car, you don't just fill your gasoline car up with diesel, do you?

[pdf64]

Assuming bias is set within safe range...

Yes, the following point may not be related to the redplating issue raised in post #1; the safe range here should be for the tube and the iron. Idling each power tube up at 23 watts (the 6L6GC 70% limit) will surely stress the iron.

[purpletele]

Thanks guys.

I am going to re-check with Patrick at MM and verify I have the right transformer for the task (X2).

I might have fried my tube and I'm out of 6L6's, so my testing will be delayed.

Coupling Cap theory: I had my share of interesting component failures so I will definitely roll through that cap during this process.

Getting late

Thanks for the advice

BV

[purpletele]

I double checked with Patrick at MM and the FDP 20 Fat Stack is designed for 6L6's.

When I originally replaced the tubes with 6L6's I indeed re-set the bias.  Yesterday with the new tubes I had 446 V on the plate so I set the bias at 40.8 mV and it ran really smooth for approximately 45 minutes as mentioned previously.

I started over again with the same tubes that redplated.  I get 446 V on the plate again, so I set the bias at 40.8 mV and I am into the test about 15 minutes so far.  The amp is quiet and plays really well.

I had no idea about the speaker impedance impact when using 6L6's.

To clarify, if I have a 16 ohm speaker I should be feeding it from my 8 ohm tap on the amp?

I am just going to burn it in, I'll wait and see what happens while I work.

[sluckey]

To clarify, if I have a 16 ohm speaker I should be feeding it from my 8 ohm tap on the amp?

No, that's bass ackwards! 

[purpletele]

To clarify, if I have a 16 ohm speaker I should be feeding it from my 8 ohm tap on the amp?

No, that's bass ackwards! 

So if I had a 16 ohm speaker hooked into the 16 ohm tap on the amp would that impedance mismatch create detrimental heat or cause enough strain on the OT to cause a Redplate? 

[shooter]

go pull half your plug wires outta your motor n drive it like it has 'em in 

[purpletele]

I have played the amp for well over an hour and I haven't had any issues.

I set the bias a little cold but I also found that I had originally used data for a 6L6 tube and not the 6L6WGB which I have in there. 

Seems to be good, we'll see.

Thanks for the advice

[sluckey]

So if I had a 16 ohm speaker hooked into the 16 ohm tap on the amp would that impedance mismatch create detrimental heat or cause enough strain on the OT to cause a Redplate?

Not likely. Well, maybe if a wild man was playing the amp dimed for 4 hours. 

But before we jump to big collusions, tell us about your OT. We already learned that the PT was rated for 6L6s. What about the OT? Let's see the data sheet.

[purpletele]

So if I had a 16 ohm speaker hooked into the 16 ohm tap on the amp would that impedance mismatch create detrimental heat or cause enough strain on the OT to cause a Redplate?

Not likely. Well, maybe if a wild man was playing the amp dimed for 4 hours. 

But before we jump to big collusions, tell us about your OT. We already learned that the PT was rated for 6L6s. What about the OT? Let's see the data sheet.

I am using the Classic Tone 40-18006

I attached a cut sheet for review

[sluckey]

Seems like you don't have a Deluxe Reverb at all. You have a PT that's made for 6L6s and you have a OT that's made for 6L6s. So, everything should work fine with 6L6s. And 6V6s would not be happy, or at least, would not be working at optimum...

I don't see a problem. Well, except, don't loan your gear to colleagues.

[purpletele]

Seems like you don't have a Deluxe Reverb at all. You have a PT that's made for 6L6s and you have a OT that's made for 6L6s. So, everything should work fine with 6L6s. And 6V6s would not be happy, or at least, would not be working at optimum...

I don't see a problem. Well, except, don't loan your gear to colleagues.

I played it for another two hours last night without issue.  Then I played my combo Deluxe for another hour, so I played a lot of guitar yesterday.

I think it is fine now that bias is right on the money!

Thanks for the advice!!