Oldtimer question (valves/tubes)

A make and model would help considerably. But, I'm gonna generalize here, if it's cheap enough to only have two tubes, they certainly didn't waste any money on complications like switchable bias.

Keep in mind that a 2-tube SG is, was and ever will be a hobby device. With that in mind, a little, even a lot of distortion which is still "on frequency" hardly matters to a hobbyist aligning a consumer grade radio.

Peter Wieck Melrose Park, PA

Sorry, guys, it's a Taylor 68A/M. I can't find the exact model on line but this one is as close as it gets:

The grid/Cathode bias does change with range selection; I just measured it. There doesn't seem anything obviously amiss with the readings:

Range 8 (lowest frequency) -25V

Anode is +80 to +90VDC vs. Cathode over all ranges.

If you are troubleshooting this then be aware that plate resistors often fail high and are (after capacitors) the next suspects in repairing old tube/valve equipment. Schematics are a huge help of course to be able to verify meeting manufacturers BOM as older parts colours have usually faded to obscurity....antique radio societies will be an aid in tracking down schematics...

If you can't find a match check out EICO or Heathkit of the same vintage as chances are someone may have copied the schematics to their product.

John :-#)#

If you've eliminated other faults as possible then time to adjust the bias on the iffy range.

NT

It breaks my heart to do this, but I think it's a junker. On further inspection some screwdriver jockey's f***ed around with it and since I only paid a fiver for it, it's time to consign it to recycling.

The computer museum at Bletchley Park has (or had) a valve re-cycling exchange. I bequeathed (or dumped) my father's collection of valves and his (Taylor?) big wooden-cased valve tester to them and they made appropriate gratitude noises...

Mike.

Oh, I save the tubes alright; I'm not that much of a philistine. That valve tester of yours was probably an AVO. I've got a couple of those in my collection.

ECC81 is the same.

Yes, and fortunately I did have some spare new ones to try (on the suggestion of a pal of mine who I would have ignored were it not for the fact that he's 20 years my senior and has a doctorate in electronics). It didn't fix the problem - unlike him I didn't expect it to - but did tend to suggest that the issue was one of poor biasing (for whatever reason) as I'd expected. Sadly the thing has passed its prime by many years. It's not worth continuing with when you discover the many and various pot cores have nearly all been shattered by some dimwit who didn't know what he was doing.

There is NO adjusting the bias. It's NOT adjustable. The difference in readings is based on the difference in output for each range. It's entirely likely it was like that new from the factory.

Whist it's true there are no trim pots involved, the fixed resistor values will have changed over the last 65 years, which is quite possibly responsible for the fault. They would need to be hooked out individually and replaced, but it's not worth it.

Most likely the difference in the Q of the circuits and maybe the gain of the tube at different frequencies. That would change the power of the osicllator and voltages. One thing I would look at would be all the reisitors in the generator. Those old carbon ones often change value after a number of years, most seem to go up in value.

In the past some coil problems have been caused by the change in the cores. Not due to being shattered but just that the material ages.

Highly unlikely. I still have Dad's "Universal Avometer Model 7", so if the valve tester was from the same stable I would have linked the two. Unfortunately I can't find any record of my offer to Bletchley, to a chap who IIRC has since died.

Mike.

I think you'll find AVO took over Taylor in the late 1950s so it's all just about dates of manufacture.

One needs replacing. Sounds like a trivial job. Send it to me if you want.

NT

Oh, thanks, I might look that up. But the valve tester would certainly be older than that. It did have a lacquered wooden case, after all!

Mike.

I have a Radiosonde weather balloon transmitter which has a single valve. The valve is a 3A5 twin triode with one being the RF oscillator at 72 MHz and the other an audio oscillator which was controlled by the sensors.

I did have a one valve test oscillator which was powered by a nine volt grid bias battery.