Curtis Mathis console tube stereo distortion issues from FAILED COUPLATES AT POWER STAGE.

[wparks]

When I squint I can see this amplifier is a dual channel single ended design. There is no phase inverter. The two triodes of the 12AX7 are just pre-amps to drive the respective EL84 for each channel. It would be great if you could take a picture of just the power amp section.

 

[sean sweeney]

Ok, and here is another diagram, one of the likely centralab couplate that was used. I think this may get the job done with discrete components.

 

[sean sweeney]

When I squint I can see this amplifier is a dual channel single ended design. There is no phase inverter. The two triodes of the 12AX7 are just pre-amps to drive the respective EL84 for each channel. It would be great if you could take a picture of just the power amp section.

That's not the amp I'm working on, unfortunately i can't even find that schematic. Two el84's per side, and one 12ax7 using a cathodyne phase inversion for both sides, with the couplate balancing out the two phases.

 

[sean sweeney]

I think perhaps this may get the job done, though. That schematic was just there to hopefully help me unlock the couplate riddle. But didn't even do that!

 

[sean sweeney]

A.I. Says that the different chassis (TV and tuner) were made by different companies, so you want to look up the service literature based upon the particular chassis number rather than the furniture model. The amp/tuner chassis may have a tag or a stamp on it with "28A2" or "28B" and that would likely lead to the proper schematic. This vendor offers a Sams (622 folder 8) for those models, and you can also find those folders pretty cheap on Ebay, etc.

There were a ton of different chassis for these at the time. And finding the one that was in this one without a number just proved to be nearly impossible. So I decided to just try and engineer around that issue if possible. And these are all things I have from memory, which I figured earlier on that it was a Cathodyne phase split, and now I'm pretty sure that the c o u p l a t e s provided the balancing of the two phases to go into the power tubes, so that gives an indication or clue as to what would be in them and AI helped with the actual values some of which were taken from the centralab products available at that time. All in all, I wish they had just built a better amplifier. It wouldn't have had any of these issues.

 

[sean sweeney]

And they called these made in the US, BUT NOPE... under the hood, there is a maze of suminiature, sub par Japanese work, shortcuts, and compromises. No american hands could have created these, at the time, unless they were Edward Tweezerhands...

 

[sean sweeney]

Is there a part number on the couplates? I have several old Centralab catalogs that show the internal schematics.

That would help. I wish i had gotten the part #, but they are used to balance the two phases on a push pull, el84 stereo amp, using a single 12ax7 for both sides, using a cathodyne arrangement. Basically, they're there to provide the evening out of the levels of the two phases. So it doesn't sound distorted like a paraphase amp would... or like an early altec tube compressor with mismatched outputs! That's the issue, if they have any that are described like this. And if they list the components.

 

[sean sweeney]

Thanks for the help, everyone. Heading back to finish in a day or so, now i have to build a few leslies!

 

[dcgillespie]

Is there any chance that there is an inadvertent short on the output of the amplifiers — from possible extension speakers connected or something like that? A short on the output leads would definitely cause the signal at the output tube grids to be very distorted. If the unit can be safely operated with the output tubes removed, then remove them and see what the signal is like at the output tube grids then. If it’s clean, then the problem lies elsewhere.

Good luck with it!

Dave