Music Man RD50 distorted output

[Leo_Gnardo]

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.

In spite of failing to meet 6L6 min specs, these are what was offered for sale widely in the US from mid 70's through the 90's. Groove Tube, Peavey, other outfits. Some were relabeled and sold with major tube manufacturer's logos. Others sold through "jobbers" supplies, IC brand for instance. Many were paint marked with "West Germany" or "England". One fiasco of note, and it's been reported in MEF, Groove Tube sold them as a "miracle" 6V6 substitute, without warning buyers they had better set bias as if they were 6L6. So much monkey business!

[g1]

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?

How hot is the idle current? Measure the DC across the 6.8 ohm resistors. Typically these MM amps are biased very cold, around 6mA, so you should set for about 40mV across each of those 6.8 ohms.
You mentioned not seeing a crossover notch, and signs of tube failure, so idle current may be high (for this model).

[vintagekiki]

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.

Bravo. Welcome to repairman club.

Who knows and does not know that he knows - wake him ^Confucius)

[xtian]

Thanks, G1, but amp is gone.