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At what point in the circuit are the meter readings indicating? Dave

Kind of hard to see how that can be. To have -44 vdc on the output tube control grids, there must be plate voltage on the 12AX7 CF driver stage. But the 12AX7 driver gets its plate voltage from the plate winding of the OPT, so that means that the output tube screens (pin 4) must also have B+...

No purchases, but I’ve seen his work, and it is extremely neat and well done. Dave

One thing to consider specifically about the RCA 6L6GC is that over the life of this tube’s existence (60+ yrs), I’ve seen enough examples worth noting that would show color along the plate seam, even though actual plate dissipation was well within specification. Manufactured correctly, this...

Glad it’s running properly again as well! So did the output transformers also end up needing to be replaced? Dave

Thank-you for your suggestion regarding protecting the regulator. This was a subject I studied quite significantly early on when first developing the EFB™ concept. As a basic point of understanding, adding a reversed biased diode across the IN and OUT terminals of the regulator in traditional...

Wow! Quite a prize! Congrats! Dave

Do NOT change out any caps within the front-end RF box. They are specifically chosen for their thermal characteristics so as to minimize drift during operation. They rarely if ever go bad anyway. Dave

The rectifier circuit appears to be original. The item heat sunk to the chassis appears to be a power MOSFET that’s been added and is being used as the pass element of a B+ regulator that’s now supplying power to all of the small signal tubes. The reference voltage for the pass element appears...

Sat — Thanks for the version details. I didn’t realize there were that many versions before your definition of my own final-final version unit, which I had always thought was considered to be a 2nd gen unit. Dave

Nice save indeed! But wouldn’t that be a 1st- gen 240 with dual octal sockets and an AC operating voltage selector switch? Dave

It took a long time, but their work in reproducing a new dial glass for a Fisher receiver was perfect. Dave

The amplifier sounds tremendously unstable to me. Does it make the same sound through the speakers when they are connected? Is that without any input connected? Dave

Linear taper. Because of the spread tab mounting, the hole is 3/8 inch, so a full sized control should be used. Dave

04/2024 UPDATE If I've learned anything about the SCA-35, it's that just when you think every possible aspect of the amplifier has been tested, investigated, scrutinized, rethought, and otherwise laid bare for all to see, you find out that's not quite true. To that point then, yet another...

Thanks for the pics! I've never had the pleasure of listening to a MC3500 let alone have one come across my bench. The closest I've come was helping a fellow who came to AK looking for help in servicing one. Seems one bank of output tubes would virtually red plate shortly after turn on until...

The MC2300 certainly deserves its due, but for me it was the MC3500s that powered Woodstock. That amplifier forever cemented the relationship between Mac and the sound reinforcement industry for all types of music. It surely paved the way for all that followed. Dave

Could the transformer primary wiring to the voltage changing jumper terminals be all fouled up? Dave

Certainly clean the 1st 12AX7 tube socket, and move the input selector switch back and forth to see if that clears anything up. If so, some Deoxit is in order. Power transformer (back right transformer) will run the warmest, middle output transformer (right channel) will run notably cooler, and...

Remember not to confuse impedance ratios with turns ratios. If you're looking to use 6L6 class tubes as the authors did, the target screen tap location for the Ultralinear specification is 43% of the plate winding -- not 43% of the plate impedance. In the article then, relative to the UL...

Still best to check the quiescent output stage current draw after going to SS rectification — or even more efficient vacuum tube rectification. With significantly greater B+ voltage, output tube screen voltage will be increased by as much, and negative G1 voltage will be proportionately reduced...

I think the appearance of the choke is “normal”. I say normal, as I have seen a number of chokes from back in the day manufactured by Thordarson — that I know their full history of — that have that same appearance. Not sure what causes it. It’s not a particularly chard look by any means nor does...

Your unit is a very early model as evidenced by: 1. No tap on the power transformer HV winding for the bias supply (requires the higher rated 450 volt bias cap), 2. Using 1/2 of the input 12AX7 tube as a voltage dropping element for the other half of the tube, and 3. No cathode boot-strapping...

Nice find! Do you know if it is an original or later version? If you're looking for a mate, it's always nice if the versions match. Ideally, the 5AR4/GZ34 would make for a really excellent upgrade for the 60 with their slow warming low drop characteristics (and reduced heater current), but when...

Ooh! Battery power! Will definitely be following along! Dave