Enhancing microphonics in capacitors?

[JayBe]

Hello,

I was experimenting with shared vs. split cathode on my '72 superbass and, although I like both, noticed that I lost a lot of good feedback and sustain with a split cathode V1, at lowers volumes anyway.

I used an 820ohm carbon comp and a big 0.68 Chicklet cap because that's what I had. Really liked it on channel 1, not too different from ch.2 where I usually just have a 500pf on the volume. A bit tighter, bit more gain.

I often push the amp with a nice Xtc overdrive and can get really good tones at low volumes with a lot of sustain and controlled feedback. With the spilt cathode setup I had to work harder to achieve this.

That got me thinking. Since capacitors can get microphonic, and I can appreciate a slightly microphonic pre-amp tube, could this effect be used in a controlled way? How to make capacitor slightly microphonic or find one that is?

cheers

[danman]

The only type of caps that I have heard to be microphonic are ceramic disc caps. Maybe others will chime in with other suggestions though.

[JayBe]

Yes, that's what I read too, ceramics on bendy pcb's. Not quite applicable as such I guess. But condenser mics are basically microphonic caps, right? So should be possible, at least in theory.
Suppose it would be easier to try to tune in the feedback frequency with the right bandwidth. Higher value? But, I still wonder if the loss of feedback and sustain has to do with the split cathode circuit, the specific parts or just their values.

[danman]

If you dont have a .68uf cap on the cathode of v2, give that a try and see if it helps.

[Carbia]

I've found microphonic Mustards, but had to replace because was unusable.

Anyway, mustards use to be a little bit microphonic (only a little bit) and you can notice that if you hit one with a stick (specially the V1 anode cap) you'll hear it on the speakers. Nothing remarkable really.

But the ones I've found microphonic were very noisy and you hear a big CLONK if you hit it.

I haven't found nothing in between. It seems that's an on/off matter.

[JayBe]

Ok, so it might be an on/off thing in normal signal caps. Or a million to one shot in practice to get one that's like a slightly microphonic tube.

Did try a 0.68 across V2 briefly and didn't notice much difference, which is a bit strange. The amp has a 100 to 4 ohm NFB setup so it's quite gainy when turned up anyway.

I'll experiment with different caps on V1B and V2 next time I open it up, see if there is a difference concerning feedback. Different makes and values. I actually don't want to mess around too much since it's a nice all original Superbass. Would like to push it gently towards the early plexi lead sound, which it already is close to, I think.

[Carbia]

I prefer 2k7/680nF on V1's bright channel. Less gain but more focused.

the 680nF on V2 cathode is a subtle change, don't expect a big change. I like it, but I could live without it.

I like to lower a little bit the preamp voltage. Search the two resistors in series of 10k and 8k2 near the bias caps and change the 8k2 one for a 10k. This will lower a little bit the voltage on V1, V2 and V3 getting more gain and more "brown"

[JayBe]

Thanks for the tip on lowering voltage! Going to try it next time, see how it feels.

I'm not actually looking for a specifically gainy or brown sound, just getting the amp to sing at lower volumes. So far the absolutely best has been an XTS Precision overdrive into stock bass specs (with 500 cap on channel 2). Would be cool to get the same kind of tone without a pedal.

[JayBe]

I like to lower a little bit the preamp voltage. Search the two resistors in series of 10k and 8k2 near the bias caps and change the 8k2 one for a 10k. This will lower a little bit the voltage on V1, V2 and V3 getting more gain and more "brown"

Mine seems to have 2x10k stock. Is that normal for a '72 Superbass?

[danman]

That is normal. Throughout the years, Marshall has used various resistor combinations in that spot. I believe it also boils down to whether your amp has the higher voltage PT or the lower voltage models found in some of the late 70's amps. You can try some different combinations of resistor values to fine tune the preamp voltage more to your liking. Sometimes I prefer to lower the voltage to offer a touch more distortion and other times I have run one 10k only to raise the voltage slightly.

[JayBe]

That is normal. Throughout the years, Marshall has used various resistor combinations in that spot. I believe it also boils down to whether your amp has the higher voltage PT or the lower voltage models found in some of the late 70's amps. You can try some different combinations of resistor values to fine tune the preamp voltage more to your liking. Sometimes I prefer to lower the voltage to offer a touch more distortion and other times I have run one 10k only to raise the voltage slightly.

Ok, thanks. High voltage PT I would imagine. It was 530 volts but I now run it in the 250v setting at 230 from the wall. Lowers voltages to 470v and makes it a bit less loud.

I'm actually more interested in filter cap values. The amp was recapped with Tad 50+50's throughout when I bought it. What was stock on these, and could it be an idea to try other values? It's pretty stiff and brutal now, rattles the walls.

Just changed it to split cathode with V1 820/0.47 Bianchi I found in my box of goodies. It was the most crisp from what I tried. Going to play this a few days and see if I like it.

[JayBe]

Split cathode is out now, shared is just so much better. Split sounded cardboardy. Instead, I changed PI and plate filter caps to 32+32's. The amp lost some of the excessive dynamics and boomyness but also some immediacy and pick attack. An 0.68 bypass cap on V2, in shared cathode, seemed to restore playing dynamics. Not so much 'more gain' as 'more articulation'. I like it, left it in (a chicklet).

[danman]

Sounds like you have found the right combination for your amp!

[JayBe]

Sounds like you have found the right combination for your amp!

Yep, thanks! This way it works both with pedals and on it's own. (Still quite loud that way, might look into the Pec pot ppi master, just don't wan't to turn this into a 'more gain' project.)

Only problem now is that it's developed a hum/buzz. Can't locate the source. Worsens with the pedalboard but isn't caused by it. Hmm..