I am considering implementing the T Bridge tone control described in

http://www.valveradio.net/audio/t-bridge-tone-control.html

on my STC830.

This is because the present tone control is much the same as tossing a couple of blankets over the radio - sound muffled as the bass increases and the treble decreases.

This radio uses a 6J7 - 6F6 combination for audio amplification, with the tone control on the plate of the 6J7.

It looks easy to do as only a few resistors and capacitors are needed. However there are always traps and I am wondering if anyone else here has attempted this sort of modification.

It’s got the looks of an audio oscillator circuit.
I would be very interested to find what your results are.
I was only recently restoring a 3 valve radio I built as a boy some 60 years ago that really could have done with a tone/HIFI control.
JJ

It could be a while before I take the soldering iron to this.

Am currently painting the chassis which has required removal and re-installation of the tuning gang so the alignment will be out, though the radio seems to be working OK after the re-installation. Also looking at the changes to the voltage divider biasing arrangements that I have made and wondering if they are needed as the next radio in the line, the 831 did not use a voltage divider, and I am wondering if my radio did not either, despite the circuit given in 1938 AORSM. Once that is sorted out will have ago at the tone control.

Also the filament voltage has always been low at 6.0V which I have never investigated as it is within +/-10% of 6.3V. The rectifier voltage is OK at 5V so don't think that there is a problem with the primary. There is no sign of distress to the transformer (leaking wax or suspicious smell). But the only time this radio has let the smoke out has been a short in the dial light wiring - there are 4 altogether on the MW setting so am having a close look for other signs of distress.

In the mean time thought I would put out feelers on the tone control. It apparently works in conjunction with the output valve feedback circuit.

Perhaps the dial light short, which would have created the heating of the 6.3 volt winding has slightly damaged the copper wire to the effect of a slight increase of internal resistance. Causing the small voltage drop in the filaments.
Or maybe the mains transformer is underrated when used with numerous dial lights that may be the 600mA type, instead of 300mA.
JJ

The chassis was out when this short occurred so was able to turn off quickly, which fits in with lack of visible distress, but not proof of it.

Megger checks on the various transformer windings are suitably high so at least no inter-winding damage.

I have thought of adding some extra turns to filament winding, over the top of the existing windings, but haven't because if there is a short in the filament wiring, it might at some time "un-short" leaving me with a high filament voltage.

The original globes were 600mA, well blackened, and long ago replaced with 150mA. I have some 300mA which I will try so see how sensitive the voltage is to loading.

On the subject of the STC830, there doesn't appear to be any photos of that model on the Radiomuseum site. That ought to be remedied.

My radio is disassembled at the moment. When reassembled with a replacement grill cloth (the original was rotten and is long gone) will make an effort to make decent photographs in daylight. It lives upstairs so that will be a bit of an job getting it down 25 stairs, putting in the chassis and taking the photograph.

I do have a photo of a 1939 732 which I got off ebay, which has an identical cabinet to mine and has the same original grill cloth. Unfortunately it has a date stamp so might present copyright problems.

Am not a member of Radiomuseum - who could get these photos on the website?

Further to my post 5, I should emphasize that the smoke came from the dial light wiring short, not from the transformer.
Reading up on transformers of the time it was the practice to wind the filament in between the primary and HT windings to help separate these windings with what amounts to an earthed shield. So perhaps my idea of adding extra turns over the top of the existing windings is not a good idea.

The dial light wiring connects to the 6.3 volt transformer winding.
So your short would have been a short on that winding?
So, that’s the winding that would have got stressed.?

I agree it would have been stressed but for seconds only. If there had been a short waiting to happen in the winding it certainly wouldn't have helped. But I am not inclined to open up the transformer when strictly it is working to spec and there are no ups and downs in volume or dial light brightness.

But that said I doubt that the radio would have been designed to operate at a filament voltage at the bottom of the spec. To help conserve valve life in a radio with 9 valves?

Hi STC830,

Got an 404 Error when trying to Post earlier in the day and it has evaporated ....

As regards the Tone Control, I will be interested in how it performs ie is the 'boost' to higher frequencies significant?? My hearing is loosing the higher frequencies as I get less younger .... Lots of 'muffle' on radios particularly when people use phones and low quality microphones on Talk-back radio ... doesn't seem to matter much when listening to 'my music' ...

Am not a member of Radiomuseum - who could get these photos on the website?

I can arrange that. Just let us know when the photos are ready. Do you really need to drag it downstairs? Good flash photography (point the flash at the ceiling not the radio) should do the trick.

Just reading up on tone controls and

https://vintage-radio.net/forum/showthread.php?t=102185

says that the capacitor on the anode of an output valve can prevent parasitic oscillations in the output stage. This is the sort of trap I meant in my Post #1.

Presently am drawing up the circuit of this part of my radio - the AORSM circuit has the tag-boards in the circuits which makes it difficult to follow for this purpose - especially as there are errors in this part of the circuit eg wrong value for the 6F6 grid resistor and two of the resistors in the feedback arrangement transposed.

An explanation of the effect of over modulation now used on AM stations:

https://www.antiqueradios.com/forums/viewtopic.php?t=38130

Last post

"When these vintage radios were designed, AM transmission systems were required to pass an audio "proof" whereby the engineers had to document "flat" response (no boost or roll-off) in their audio signals. They also had to show their stations had reasonably low distortion. Today, a considerable boost is allowed in the high frequency region up to 10 kHz in an effort to overcome the narrow response designed into modern receivers. There is also no requirement to prove low distortion. Because of this, most AM stations sound "tinny" on vintage sets.
There are some statons, however, that still do make an effort to sound good."

I have got around to writing up the modifications to my STC 830 to implement a T-bridge tone control. Will send a pdf to Brad for his consideration for inclusion here. Then it will be a matter of getting on and actually doing it.

T Bridge Tone Control

Document uploaded to Post 14.