Music Man RD50 distorted output

[Helmholtz]

That's 93mA and 81mA...across the screen grids! WTH is happening here?

Sorry, but are you sure you installed 470 Ohm screen resistors? 47k resistors could be an explanation.

[xtian]

@Helmholtz, that was exactly the right question. Nope, 470K.

Excuse me while I was the shame off, and put in the proper screen resistors...

[xtian]

Yep, that was it! Amp Tech error. Thanks for taking this time wasting trip with me, gentlepeople. Amp is sounding good.


Is there a way to edit the title of this thread?

[Helmholtz]

Okay, this explains a lot: Low screen voltage, very low power tube gain and output. The distorted cathode signal was probably the result of heavily overdriving the grids.

[xtian]

This is the second time I've received this amp in the same condition: blown screen resistors and blown 30v bias voltage Zener D9. Is this a common failure, because of the 560v plate voltage? Can I prevent this from happening again?

[Helmholtz]

The plate voltage should be no problem. EL34s/6CA7s can take up to 800V plate voltage. Maybe use screen resistors and zener with higher power ratings.

[xtian]

This amp uses 6L6GC tubes, not EL34.

[Leo_Gnardo]

Yep, that was it! Amp Tech error. Thanks for taking this time wasting trip with me, gentlepeople. Amp is sounding good.

Now that it's working right, do you see/hear that little remnant crossover notch?

As for the error, had a similar one in a friend's Fender he had to have fixed by the local self proclaimed "hot hand" tech whilst I was out on tour. It came back, sounding like poopoo, and with a big bill besides. Had a pair of 4700 ohm screen grid R's. Got me to thinking, does this "genius" techy guy work in the dark? Or is he color blind? In either case, there is such a thing as an ohm meter. And his ears should have told him his "fix" was crapola. So much for the competition... Fooey!

[xtian]

Now that it's working right, do you see/hear that little remnant crossover notch?

After the fix, I gave it a quick workout with the 1KHz sine wave, saw that it was making 20vAC RMS against an 8R load (50 watts). The sine looked smooth, no notch.

[Enzo]

Can I prevent this from happening again?

Your screens sit at half the plate voltage, voltage was never a problem for the tubes. Tubes fail because tubes fail. They are the least reliable thing in an amp. The amp is 20-25 years old. If it takes another 20 years to blow up, you will be OK.

[Helmholtz]

This amp uses 6L6GC tubes, not EL34.

You're right. I mixed up MM models. Was too late for me last night.

But I can't imagine that the 560V plate voltage is the reason for the failures.

[Leo_Gnardo]

You're right. I mixed up MM models. Was too late for me last night.

But I can't imagine that the 560V plate voltage is the reason for the failures.

Depending on which 6L6 are installed. Those awful Russian cheapies and their Chinese copies, popular in decades past, don't stand up to much. Tubes better than "minimum quality" not so much of a problem.

[Helmholtz]

Don't forget that the cathodes are sitting at around 60V, making the tubes see only 500V at idle.

Actually the OP didn't mention blown power tubes - but only a tube malfunction could take out a screen resistor.
Still it's hard to imagine how defective tubes could damage the zener in the bias circuit. Would need something like short between grid and screen.

[Leo_Gnardo]

Don't forget that the cathodes are sitting at around 60V, making the tubes see only 500V at idle.

Actually the OP didn't mention blown power tubes - but only a tube malfunction could take out a screen resistor.
Still it's hard to imagine how defective tubes could damage the zener in the bias circuit. Would need something like short between grid and screen.

Here's a photo of the 6L6 type I mentioned, a Russian 6P3 from the tubes-store site. These were also made in straight sided bulbs. Chinese versions also. Note tubes store quotes plate power at a whopping 20.5 watts - rather weak. Also a max plate voltage of 375. Despite their poor reputation, I occasionally do see 6P3 functioning OK in amps. OTOH over the years I've had to heave hundreds into the bin with breakdowns including all sorts of internal shorts.

When Sovtek first started importing from Russia the tube they called 5881, I was cautious, considering the reputation of the previous 6P3. Instead, the so called 5881 turned out to be a hit, very tough and took high voltage well in spite of its published ratings. In fact I've used them in RD50 with no problem except for the small crossover notch I've mentioned. Perhaps 6L6GC or 5881 with a higher emissivity would solve that problem.

[Helmholtz]

Here's a photo of the 6L6 type I mentioned, a Russian 6P3 from the tubes-store site. These were also made in straight sided bulbs. Chinese versions also. Note tubes store quotes plate power at a whopping 20.5 watts - rather weak. Also a max plate voltage of 375. Despite their poor reputation, I occasionally do see 6P3 functioning OK in amps. OTOH over the years I've had to heave hundreds into the bin with breakdowns including all sorts of internal shorts.

Interesting, thanks.
To me a tube that doesn't meet minimum 6L6 specs is no 6L6 and shouldn't be labeled and sold as such.