The PHILCO Phorum

We just finished electrically restoring a Philco 37-60 but it has no tone control. The original on/off switch with two tone position switch was replaced with a pot that has an on/off switch. The tone caps were never hooked up so there is no tone control at all. The owner has told us a replacement will be very hard to find. Instead, he wants to see if we can make the pot arrangement work. Before I start bread boarding a circuit for this, has anyone ever converted a fixed tone control to a variable one? Is there a circuit I can use?

Thanks.
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Update: After repairing another issue with this radio, I revisited the tone control. Thanks to Mike, his tone control design was adopted and it worked very well. The key thing that had to be determined was what revision of this 37-60 we had. This one relocated the bakelite line caps to a different location. The original tone control caps were located outside and inside a bakelite block. This radio was version Run #5. The #40 condenser block was stuffed with a 0.1 mfd cap. One end was connected the ground. The other end is connected to one side of the tone pot. Condenser #41 was removed and discarded. This was a very easy mod. Thanks Mike!

This wires in as a substitute for the switch type tone control, and offers more variability, it is actually an improvement to the original circuit. You can try several different condensers of various values until you find one that offers you the best range of tone. The power switch on the back of the pot hooks up as shown in the Philco schematic.

Mike,

This is much simpler than what I was considering, some complicated treble/bass circuit from a tube amp. We will give it a try Monday and try a few different caps to see which sounds best.

Thanks

Let me know how this works out for you, and the value you finally select for the condenser. I'm sure others will find this information useful if they need to replace the tone/on-off switch.

< I was considering, some complicated treble/bass circuit from a tube amp.
You may find that a circuit will give you the frequency response you desire but drop the overall gain lowing the volume level.

With the above circuit I would wire the pot the other way around so the the cap is wired so you get maximum tone control as soon as you turn the set on. Else wise when you go to turn the set off the tone will get brighter and brighter.
GL

A more-complicated tone control has to involve an additional stage of gain, ( I've tried otherwise and ended up killing too much gain).  You can find one on page 335 of RCA's 1956 vacuum tube manual, (http://www.nj7p.org/Manuals/PDFs/Tubes/R...8-1956.pdf).

But frankly, I wouldn't attempt such a modification in an older radio like that, and I am one NOT beyond modification! You'd have to find a place for an additional vacuum tube AND an additional potentiometer, not to mention all those resistors, caps and wiring.

I think MiketheDruid has the right idea, since you already have a place for a single tone control pot.

Yes, Jake, and it is basically just a continuously variable version of how the original Philco tone control worked. I got the circuit from Everybody's Radio Manual, a Popular Science publication from 1934, page 114. It has the advantage of small size, simplicity, and is a circuit that was used in many radios of the period, including the little no-name set I am currently working on. It is really very similar to the circuit Philco used, just using a potentiometer and one condenser, instead of a resistor, switch, and more than one condenser. It also has the advantage of being easy to wire right into the original circuitry.

Exactly! , . . .  very good bud.

Incidentally, I looked at your no-name.... and I would love to have one like it!

Yes, tried several caps a 0.01, 0.04, and 0.10 uF and the last one sounded best. Then we tried a 0.22uF and the range from trble to bass was the best and we stuck with that. The pot is a 100k ohm with wiper to ground and on side to the cap wihic is connected to the plate of 6F6.

We're in the middle of alignment and the IF is done but we're hearing some motorboating. Need to finish the alignment and hear what we got.

Update: The Rf alignment is incomplete since the signal from the generator is too garbled at 600khz. We're getting motor boating from 600 khz and below. Everywhere else, the signal is fine. I'll have to check the values of each new cap we replaced to make sure we're not off someplace. Worse comes to worse, I'll have to check the micas. If I can't resolve this, I'll open another thread.

Thanks

It was my belief that if you drive the output transformer and want to make the tone ctl as the bypass to its primary, you need lower capacitance and lower resistance. In other words, the impedance of the RC should be comparable to the primary's.
I suspect that no matter what cap you use, at the pot set to 10 kiloohms and up to the full scale resistance the effect is small. And when the cap is fully on (pot is shorted) 0.22uF is too much to be in parallel to the primary.

I remember one radio with such tone ctl, I think the pot was wirewound and fairly low in ohms. Maybe 2K or so.

This circuit actually is a circuit that reduces the treble, the higher frequencies from the audio output. The cleanest output would be with NO circuit at all, which would give you the full spectrum of what the amplifier is putting out. The larger the capacity in this circuit, the more the high frequencies will be reduced. This gives the impression of having more bass. With less resistance, the same thing happens. Think RC reactance circuits. You could put a .22mfd condenser directly in with no resistance at all, and it wouldn't harm the radio, it would just clamp off almost all the treble end, and sound like crap. I strongly suggest finishing with the RF and IF alignment work, and getting the radio receiving actual stations well before deciding what condenser to use. Do your evaluation listening to various stations with music and talk both. If your potentiometer is 100K, I would think a 0.1mfd condenser would do the trick pretty well, and not make things too mushy even when full on. It all depends on the speaker and your personal taste.

I will have to revisit the tone control after I resolve the alignment issue. It looks like the broadcast band antenna transformer has an open in the primary. What is stumping me right now is that I still gets the broadcast band pretty well except for some motorboating at 600khz and below. Also the police/sw band is not fully aligned. Trimmer #7 is maxed out and I can't align 6 mhz to the dial.

Once all that is resolved, I  will have to redo the whole tone control circuit. I realized why the 0.22uf sounded good. It's in parallel/series with the other tone caps that are still in the circuit. They need to be all removed and then a single tone cap to the 100k pot is added.