I received a personal message from a forum member, but when I attempted to reply, the site was already down. Unfortunately, I lost the original sender's name so I am "in the dark" here. In order to TRY to reply to that message, I'm posting it here, *just* in case the sender might see it. Here's hoping and here's the message with an "integrated reply."
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Hi, Patrick!
Sorry for the delay. I'm working on a project to set up a streaming server over the Internet and I missed getting my messages.
Let me take the questions "in line" if you don't mind. Here we go!
First off, one of the tone control knobs on the amplifier is seized to the shaft. Any neat tricks to remove it without causing a lot of damage?
Remove the set screw. Squirt a little deoxIT into the hole, wait about 15 minutes and then work the control back and forth to the stops. Don't use a LOT of force, but try to give it a little "nudge" at each end. Eventually, it will begin to "wiggle" just a *teensy* bit. Add a bit more deoxIT and wait. Then do it some more! EVENTUALLY, it should free up enough for you to "twist" the knob off the shaft without harming the control (we hope.)
#2: I'm willing and able to go "all out" and replace just about everything in the amplifier. Obviously new caps, resistors, tubes and whatever.
This is a GOOD idea. Modern components are MUCH more reliable than the originals.
Also, I am capable of doing the artwork to make replacement PC boards, and making them. I don't know if this would be necessary, but from your advice, and others, it is seems as though the Dynaco original boards are fragile. (They are VERY fragile!) I can make MUCH more robust boards. I am willing to provide this artwork to the community, and also have a run of boards made. Do you think making new boards is a worthwhile effort.
If you CAN make them, I would definitely DO it! Amplifier boards are usually cooked pretty badly and will fall apart if you sneeze too loudly.
Also, if I do make them, do you think replacing the 7199 sockets with solder lug types would make sense, so a user could wire in either 7199s or the other possible more available replacements at will.
I wouldn't go that far, but I have nothing against 7199's (and I have a large stock of them - probably at least 40 left in stock and maybe more.)
#3: If the FM-3 does have the Telefunken tubes, would these be a good swap for the preamp in the SCA-35?
(YES!!!!!!!!!)
#4: Should I just retube the whole amplifier, or should I try to salvage the two 6BQ5s?
You will probably have to re-tube the 6BQ5's and the 7199's. The 6BQ5's really should be a matched quad. The better the match, the better the amplifier will sound.
are the non-matching 7199s OK to use, or should they be matched?
They should be made by the same manufacturer. As far as electrically matching them, it isn't as important as most people think. The amp will work PERFECTLY with 7199's that are made by the same manufacturer, but not critically matched against each other. That said, a good match is ALWAYS preferable!
#5: Would an all-in-one replacement PC board be desirable (like the one for the FM-3), including the power supply filter cap replacement circuit?
That is totally up to you. It is certainly more convenient to use a replacement Power Supply board and you can specify MUCH better capacitors.
#6: Are the pots audio taper, or linear, or some of each?
The Volume Control is an Audio taper control. The tone controls are linear taper. The BALANCE control is special. It consists of a log (audio) taper control and a REVERSE log (audio) taper control, but there's more. One-half of each resistive element has had a conductive "paint" applied. When the control is centered, the wiper contacts the two "conductive traces" in the middle. A standard 750 K pot won't work correctly in this application. Of course, this pot is now made of "unobtainium." I have known people to buy two pots, combine them into one for the correct taper, and then to pull-em apart and paint the conductive portion of the elements using either Circuit Board Repair paint, or auto de-fogger trace repair "paint". There is no guarantee, but it *usually* works "ok". OR, the balance control can be "electrically" removed. I'd keep it mounted for "looks."
I apologize if any of these questions seem naive, or silly. (They don't) This is my first tube project. I have a pretty fair amount of experience with custom circuits, digital electronics, etc., but not with older circuit designs. While I do want to retain the charm and function of these units, if the amplifier seems like it is too far gone to mess with, I'm willing to just salvage the good stuff (transformers, etc) and just build a custom "rip off" amplifier too.
Just curious what your thoughts on this are and I certainly appreciate any and all feedback you may have!
BTW, the chronicles of your work on this site are a joy and VERY informative.
That is EXCEPTIONALLY kind. I just hope folks can get an idea that repairing these things ISN'T "rocket science." While it might not be possible to restore them to PERFECT cosmetic condition, they can almost ALWAYS be made "electrically workable,"
If I can help in any other way, just e-mail me at
nostubedforyourdynaco@gmail.com and I'll try to assist.
Sincerely,
Bill Thomas
