Recent content by dcgillespie

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

Check which secondary end of winding lead lets the 8 Ohm lead actually represent 8 Ohms. Unlike the 4 Ohm lead, only one end of winding lead will. Once that lead is determined, it must then become the Common output lead, regardless of its color. If its color is correct, then it’s the primary...

Either the transformer was manufactured wrong (reversed leads to a winding, which could be the primary or secondary), or, the drive leads from the previous stage to the two output tubes in question have been reversed. I hope this helps! Dave

Also, be very careful when using plate-to-plate feedback. On a schematic, it may appear as (say) just a 100K resistor, but what the driver stage sees in reality is the value of that resistor divided by the AC gain of the output stage. That might typically be a value of (say) 9 or so, so instead...