STE AA12 Tube Modulator - Questions about Guitarization

[kagliostro]

Many years ago I opened a thread about this modulator that I've









(The old thread) https://el34world.com/Forum/index.php?topic=10392.msg95195#msg95195

At that time my knowledge was poor and confused (not that today things are much better  ) and I didn't focalized well the situation (if someone has read the old thread he knows that I confused ECC81 (12AT7) gain with ECC82 (12AU7) gain)

Whell I have a bit of questions

The spec of the modulator claims 300Hz - 8500Hz of bandwidth (plus / minus 3db)

So, I think I must do something to enlarge the bandwidth to go up with bass abilities ...... but how ??

Other thing, the modulator has a volume control but no Tone Control and I would like to add one

My first idea was to insert a Tone Control between V1 and the Volume control

Nowaday I would like to here your opinions

better  to go V1 > Tone Control > Volume > V2a (V2 is an ECC81/12AT7)

or        to go V1 > Tone Control > Volume > V2a (V2 is an ECC83/12AX7)

(but this way ... with which values of components around V2 ?) (56K - 56K+1.5K - 1M like in 5e3 ???)

Till now things are simple, because I can put the Tone Controls components on the potentiometers and 12AT7 & 12AX7 has the same pinout

Buth there is one other option, to put the Tone control between V2a & V2b and also here there is the option to swap the ECC81 (12AT7) with an ECC83 (12AX7), the components of the Tone Control can be placed on the pots

What do you think ?

How to enlarge the bandwidth and insert a Tone Control ?

Use the 12AT7 or swap to 12AX7 ?

BTW why that 16uF in parallel with the 2.2K resistor at V2 cathode ??

Many Thanks

Franco

[PRR]

> claims 300Hz - 8500Hz of bandwidth (plus / minus 3db)

It is a lot better than that.

Put an OT on it and measure.

[sluckey]

I would not change anything to begin with. Connect an OT and speaker and play guitar through it just as it is now. Maybe use a 12AX7 rather than a 12AU7 for V2. Evaluate the sound. Then start thinking about mods. Remember the EF86 is not fond of tone stacks. Might be better to put a simple series pot and cap (treble bleed) between the grid of the PI and ground.

All three 16µF caps are just typical cathode bypass caps.

[tubeswell]

You could try some variable bypass controls (in Merlin’s Preamps book) on the EF86 cathode and/or screen. (To alter the 1/2-boost frequency)

[kagliostro]

Thanks PRR - Sluckey - Tubeswell

@ PRR

It is a lot better than that.

OK I belive you

@ Sluckey

Maybe use a 12AX7 rather than a 12AU7 for V2. Evaluate the sound. Then start thinking about mods.

As mods I think you intend components around V2, correct ?

Remember the EF86 is not fond of tone stacks

Yes, I remember well, but if I compare this schematic with your VOX AC15, here there is a gain stage between V1 and PI, so I was wondering ......

All three 16µF caps are just typical cathode bypass caps.

OK, I understand, but there is a reason to put one on the PI ? Usually I don't see them there....

@ Tubeswell

You could try some variable bypass controls (in Merlin’s Preamps book) on the EF86 cathode and/or screen

I know you experimented with the option in Fig. 3.29 but I think you refer to a or b in Fig. 3.27

To which option do you refer ?

---

Thanks Again

Franco

[sluckey]

The LTP-PI in the AC-15 has gain of it's own, but your cathodyne PI has a gain of less than 1. That's why you see the gain stage just prior to your PI.

Better check out that unusual filament wiring.

[kagliostro]

The LTP-PI in the AC-15 has gain of it's own, but your cathodyne PI has a gain of less than 1.

Ah, OK, didn't considered that

Better check out that unusual filament wiring.

I'll take a careful look

Thanks

Franco

[octal]

The 220pF caps to ground by the first tube and particularly the 2nd one after the wiper of the volume control are going to make for a really dull amp. I'd remove them.

[tubeswell]

@ Tubeswell

You could try some variable bypass controls (in Merlin’s Preamps book) on the EF86 cathode and/or screen

... I think you refer to a or b in Fig. 3.27

To which option do you refer ?

B is more of a tone control. You could experiment with different Ck values, or try that in parallel with a permanent HF cathode bypass arrangement (so that it has HF fixed bypass, but so that the pot adds more LF boost as you rotate it, or some goosed-up thing with a double-ganged pot)

[kagliostro]

Many Thanks Octal & Tubeswell

Franco