Tube amp distortion

[theschlepp]

I have a completely restored 1959 Motorola three channel console stereo with the 6BQ5/12AX7/5U4 tube lineup. the 6BQ5's test OK but are definitely not matching. the 12AX7's and the 5U4 test good. I am hearing a distortion (muddled sound) at all volumes, even very low. When I turn it up load, it sounds good, but you can still notice the distortion. I hear it using the tuner or the phonograph, so it must be in the amp. Any ideas?

 

[gadget73]

The only tubes that sort of need to match, at least for current draw, are the pair for the center channel. The others are single ended and don't matter all that much.

Have you tried feeding the amp directly off a CD player or something else, completely bypassing everything in the tuner/pre ? I'm reasonably sure the phono feeds through some of the same tubes in the preamp as the tuner. I don't know much about the Motorola setup but I think they have RCA jacks on the amp for input. You should be able to connect something directly there.

 

[battradio]

I f you replaced all the electolytic caps and paper caps , check the resistors to see if they have drifted . Also check to make sure the speakers are good .
Some time thery are bad / other times i have seen mud dobber build nests that are hard to see and the speaker cones will rub on them .
There is acrossover in the amp make sure all the caps are good in that are .

 

[Kegger]

Plus what others have said, check all the voltages in the amp, mainly plate an cathode
on all the tubes if the supply looks good, that'll usually lead you to a bad/wrong circuit.

 

[jaymanaa]

Look at the output tubes in a dark room for any signs of redplating.