Can anyone identify these parts I find in my 575 curve tracer. Is this a mod that tek made?
The dc balance doesn’t adjust properly, and the dc adjust pots appear to have been replaced with much higher resistances. This is in vertical and horizontal amplifiers.
Can I check the heater voltages of the remaining 12au6s in the string by identifying the cathode pins and measuring the voltage across them?
Please help me with these questions
Sincerely,
Jack

Jack,

I do not know if the FETs are factory or not. The screwdriver adjustment for d-c balance is in the screen circuits which is likely not connected to anything in these adapters. That will cause it not to work right. The value of this pot depends upon the s/n. That could explain the difference you see.

12AU6/As are not hard to find or are expensive. The heaters of these are in series to the 100V regulated supply. If any are removed, no heaters will come on. To check for the other 12AU6s coming on, look to see if they light up when the tracer is on. Heater pins are 3 and 4. If no heaters of these are lit, the tracer will not work right.

If you get 12AU6/As, check them for gm then get the ones that are close together in gm for the pairs in each circuit. This makes balancing easier. No need to buy 'matched pair/quad'. A seller on eBay, 3d_electrical, has two quad sets of GE 12AU6s. I have dealt with him in the past and is great to deal with. There are others who have them as well. Do NOT use 12BA6! These are super-control types, not sharp cutoff. The BA6 type is used for r-f/i-f amplifiers in a-m/s-w radios. Getting the 12AU6/As should get your tracer working.

Mark

Hi Anita,

While you are at it, if you haven't already done them, you might want to take a look at the Tek list of "factory updates/mods" for the 575. There are a BUNCH of them. I'm wading my way through them now on my 575. I'm about half-way there.

Here's the link to the 575 TekWiki, where quite a few of the critical mods are listed individually, there in the "Manuals" Section. FWIW, there is also a complete assembled "all mods" document floating around somewhere, but I didn't see it here. I might have missed it. It has every mod documentation assembled into one PDF

Anyway, the link; https://w140.com/tekwiki/wiki/575

Several of these mods did make a measurable improvement in my unit's performance. Some were reliability improvements.

Hope this helps.

Keith

Any possibility that a HV MosFet might be substituted ?

LND150 Mosfets have been substituted for a different tube ( the 12ax7 ; which is Not the 12au6 discussed herein for the Tek 575)
See: https://www.youtube.com/watch?v=5-pLkgAKv1w

And likely, one might have to modify the circuit or bias somewhat, depending on the particular tube circuit.

I have successfully directly replaced triodes and dual-triodes using HV Fets in some old WWII military surplus equipment ( such as all of the triodes in the Rx section in the URC-4 , etc. ) with good results and no frequency shift nor distortion. Crazy but it worked very well. Also had success doing such with the BC-1335 Rx section which has a large compliment of tubes.

With such a modern diverse selection of IRF type HV MosFets there could be some interesting experimental work to be tried...

And other examples of replacing a 6AK8 tube ( triple diode/triode tube ), with a solid state replacement
in HF Rx receiver using HV MosFets.

https://www.youtube.com/watch?v=KlXMnNx7ZDg
https://hackaday.com/2024/07/22/making-a-solid-state-6ak8-tube/
http://amfone.net/Amforum/index.php?topic=30654.0;wap2

Clearly, the simple substitution with HV Fets in radio Rx circuits
will Not be the same as specific circuits in a precision Tek measurement instrument....

am simply providing "food for thought" ideas for intrepid experimenter; use at your own risk !!

There were actually serious attempts at replacing some tubes with JFET alternatives in very-high-volume applications like telephony (1960s-70s). I think the most well known is Teledyne's "FETRON," which was JFET circuits packaged up on a tube base for direct replacement in certain spots. I recall discussions over the years about these and home-made equivalents. Whatever you have in there is probably a similar approach, whether figured out by Tek or someone else. Don't toss it, but study up and figure out if it's legit, then maybe fix it up to work properly. If it doesn't work out, then as others have said, you can always go back to the original tube circuits. Good luck.

Ed

You can use 6AU6 in place of 12AU6. They have identical characteristic, the only difference being the heater voltage, 12.6 V @ 150 mA for the 12AU6 vs 6.3 V @ 300 mA for the 6AU6.
You can compensate the difference by adding a resistor in series with the heater. Sure, it's an inefficient solution, but it works seamlessly.

Max

Ed

Thanks for that info about Fetrons.
I had never heard of them, but I went and read up about them.
Pretty ingenious solution.

Menahem

A 6AU6 is identical to a 12AU6 so why not sub in the 6 volt version and drop 6v across a zener or a resistor. It being a constant current load, either will work.

Bill,

The heaters of the (8) 12AU6s are in series to the 100V supply. A 6V type will not work in this case.

Mark

I'm away from home on a laptop and couldn't look at the schematic first (I have a cherry 575 BTW) but even so there must be some brainiac kludge to circumvent the problem, it's simply not that difficult.

The 12AU6 is a 150mA filament while the 6AU6 is 300mA and 8 is series drop 50.4v. So if the supply will handle that current it's a pretty simple problem; considered with no schematic that is :-)

Bill,
The heaters of the (8) 12AU6s are in series to the 100V supply. A 6V type will not work in this case.
Mark

Bill @ PEARL, Inc.

Bill,

Anita said someone had put in a power resistor to compensate for the reduction of the number 12V heaters. Voltage checks are/have will/been done to confirm the correct heater voltage across the remaining 12AU6s.

Mark

The bigger deal is the doubled current draw. I reckon it -highly- unlikely that a 300mA draw might kill a secondary but I'd be checking things to be sure.

Bill @ PEARL, Inc.