Tone Control for Bass-Treble Cut & Lift

[w5hro]

Has anyone ever tried to add a simple tone control circuit to lift and cut the bass and treble in their tube speech amp or driver amp section for a modulator before?

I came across the article below and it has some pretty good detail on the do's and don’ts and why with some really simple configurations. I'm thinking of adding one in mine just to have some flexibility to adjust it externally.

I did find one idea using a crossover circuit that can be used for push-pull input, but it’s a bit more complicated.

[Pete, WA2CWA]

I added a tone control to my Apache years ago. The tone control is mounted where the old audio gain control(behind the key jack) is located. Added a metal coupling and a short phenolic rod which went through the actual key jack opening. Added a knob to the end. Seems to me I pulled the circuit from some old Fisher tube amp that use a pair of EL-34's. Seem to work reasonably well from the reports I remember getting on 75 and 40 meters. Can't remember if I mounted the RC components on an epoxy board or hung them in mid-air.

[Pete, WA2CWA]

I'm remember having to stick the old screwdriver thru the key jack on the Apache to adjust the clipper. That was until I yanked the stupid thing out

Clipper control was on the front panel next to the mike connector. The pot behind the key jack was the audio gain control. The thought presumably was that once you set your audio gain for any given mike, you really didn't have to play with it that often. However, the front panel clipper control might get more use depending on QRM and pileups you were trying to break.

[KM1H]

This is the tone control modification I made to my Scott 800B, pretty much copied from RCA. The green is new and pink is original that was removed. It made a huge difference in an otherwise flat response.

Carl

[KL7OF]

I built the speech amp from the 49(?) ARRL handbook that uses a 6SJ7 mic amp driving a 6SN7 that is transformer coupled to a pair of 6L6s..The design uses an interstage transformer with a split secondary to allow for negative feedback on the 6L6s...It works well...I then found a circuit in the Radiotron Designers Handbook that provides for bass and treble BOOST on a 6SJ7 tube..I applied that BOOST circuit to the 6SJ7 and after adjusting the plate and screen voltages upward for that tube, got some very good results...Lots of audio... and the bass and treble ranges are increased greatly...I use this speech amp to drive an 813 modulator....I've been very pleased with the results ....This setup provides lots of audio....The modulator is driving a pair of class C 813s...150% positive peaks are no problem and it can easily be adjusted for punchy communication audio or a more mellow audio with the big bottom by adjusting only 2 knobs...

[KL7OF]

I'm away from my shack right now so I can't give you details on the radiotron handbook circuit I used..The circuit uses a pot for overall gain and a pot for treble boost and a pot for bass boost...I do remember playing with the values of the caps that couple the boost circuits to the 6SJ7 to get the tone the way I liked it...If I remember correctly, The treble used a very small value (.0002?) I'll be back in the shack in a month or so and I can post a schematic if you are still interested...Good luck  Steve

[WBear2GCR]

There is a danger in using standard "tone controls" in a transmitter.
That is that these circuits tend to provide boost that has no cut off as you move away from the center frequency... that means that you have (for example) more boost at 10kHz than at 5 kHz if you are boosting.
Similarly on the low end, as you go lower the bass increases...

best case you find a "shelving" type arrangement where there is a boost that then flattens out - but even so that is NG unless you have done something to really drop the response on the HF at some reasonable frequency (maybe 7-8kHz or lower, given the boost) and also as a HP at some low frequency.

There are a number of ways of accomplishing this, but imo to keep from saturating iron on the bottom and maybe more importantly not eating up +/- 15kHz or more of bandwidth some relatively sharp roll off on the high end is pretty much required. A 4th order filter would not be too steep as a HF rolloff (Low pass)

              _-_-bear

[WBear2GCR]

An alternative might come in the form of a lowish Q series or parallel resonant circuit - more or less a notch filter circuit - that works in a similar manner by placing it gradually in or out of the signal path. This would provide a boost only in a range of interest, not outside that range.

The effect is more or less like the classic Pultec EQ, a wide band "graphic equalizer", or a single (actually two band) parametric equalizer... the use of small inductors, btw is 'legal"

                      _-_-bear

[WBear2GCR]

A moveable LP filter is not a bad thing... but hard to do with a simple circuit.
I'd probably opt for a switchable circuit where you can adjust the value of cap in a tuned
circuit at an appropriate point in the circuit.

I designed (using LTSpiceII) just such a circuit using the inductor already in the Apache - they use it for
the cliipper's filter - to do this job. It is pretty simple to implement. You can model it as a pi-filter to start.
This will give you 18dB/oct roll off...

As far as the "AVC" circuit you have there - what is your intention? As configured it might serve as a variable feedback... not compression or limiting. The Orr book has a limiter circuit, iirc, and there are compression circuits - I think the speach amp for the BC-610 has either compression or limiting, worth looking at, although I don't know how good it is...

Also, the long tail pair will not be balanced - the output signal will not be equal with just two identical resistors in the plate. You either have to alter the values there, trim the outputs, or better still put active current sources in the plate... a current mirror is sweet.

Using a driver xfmr is one way to both balance the drive and put the thing in either A2 or AB2 depending on ur bias setting...

                    _-_-bear

[WBear2GCR]

Ah ha... helps if one looks for more than a few seconds... I see... yeah. 

But a long tailed pair will not balance with equal plate resistors... not a big deal if there is feedback.

Depending on the power and the impedance ratio you want for the driver xfrmr they ought to be pretty easy to find, I would expect. A second winding might be more difficult, but depending on the xfrmr yo might be able to use an "unused" winding??

A 1:1 or thereabouts in the 600ohm variety would be pretty easy to find.
The other thing is that you don't want or need ultimate fidelity... of course if you are running a 600ohm xfmr you might have to drive it with a cathode...

       _-_-bear