SCA-35 Dynaco

[rhing]

Wow, that looks great, excellent work! I'm sure it sounds great too, the EFB mod really transforms it. One question: Should those PECs (packaged electronic circuit) be on the phono inputs after the phono stage mod? I read those instructions, but it's not clear to me...

I retained the PECs on the phono inputs. All I modified was the phono stage board. It was fun project, but it did take a lot of time to clean the chassis and recondition all the pots, wire it neatly, and then solder everything in place. It definitely is not a stock unit, but it sure sounds a whole lot better.

I just started playing a mint condition Parlophone mono pressing of The Beatles' "Rubber Soul," and I have to say this just sounds fantastic. Playing a 1960's album on a vintage sound system is something magical.

 

[mr2racer]

Hey All,
Got the EFB working and set. But the amp sounds awful. Must have done something really wrong. But about the PEC in the phono section. Is that just to allow for different input levels or is it some kind of equalization as well?

 

[SHyde]

....reviving an old post....also first time posting.

I wonder if someone might guide me in the right direction to balance out me bias readings? I installed the EFB yesterday and while adjusting bias I got .193 on left and .350 on right. I tried swapping tubes back and forth with no change leading me to believe that it may be something else.
a few questions...

Am I hurting anything by using it as is?

Where should I look first to correct it?....(what is correct anyway?)

I think I saw a 10% variance was acceptable between left and right, is that right?

Thanks!

By the way, I am constantly in awe of what this little amp can do...love it.

 

[SHyde]

No reply yet.... was hoping some EFB fans would chime in....

I replaced the last set of caps on the the driver boards and tried again, it helped some. I now have .239 on left and .301 on right. Still not as close as I would like. Tried switching tubes around again and no change.

Anyone have to just replace all (PC10A?) components to get them better balanced?

These new numbers are just a smidge out of the 10% range but I'd like to see them near even.

Is there any chance I just need to break in the matched quad Gold Lion EL84's? I wouldn't think so since the difference didn't follow after swap.

Again, any help would be great.

 

[jgf]

Are you using new boards or original?

Double-check your 5ohm resistors at the test points. Did you add the 100 ohm screen stopper resistors? Do you have new or old coupling caps installed?

 

[SHyde]

original boards with all original resistors, all new caps.

I can check the 5 ohm test points

I did add the screen stoppers and I checked those, all good.

It sounds great and I don't have any balance issues, but I still think they should be closer matched

 

[jgf]

I agree, the match should be better. If you swapped the output tubes from side to side, and the mismatch stayed with the channels then it seems the tubes are not the problem.

Next I would check the grid-to-ground resistors on the output tubes, all 4 should be close to the same. Also check the resistance of the output transformers from the plate and screen taps to the center tap. Each side will be a little different, but they should match closely between channels. Also check that you have good solder joints on your output tubes. If it's old solder, you might want to remove it and re-solder.

Do you have an SCA-35 or a stereo 35?

 

[ArtG]

Do the output tube plate and screen voltages measure approximately the same on each channel?
What are you using as 5 ohm output tube cathode resistors? Did you purchase close tolerance resistors, or did you match them yourself?

 

[SHyde]

So I got a reply from Dave and it turns out I'm not that far off. He said the "safe" range is .22 to .32. I understand that the right channel may burn the tubes up a bit faster...but the originals sounded great...so is tube life really a concern?

I will check everything suggested more closely the next time I get it on the bench. Thanks for the help guys.