PPIMV on an AB763? Anyone tried one?

[rogb]

allen is PostPIMV, DR Z is Pre PIMV

Interesting. I had only ever seen inside an Allen, haven't seen inside any of the other companies I mentioned. Thx for info. I wonder about other companies - I had simply assumed they had used postPI, since it has good results.

to the OP, I have used PostPI in BF circuits and it works well for my needs, although I always think combining MV with an attenuator/etc is the only "good" way to get low enough for apartment. Using both in combination I think gives much more versatile possibilities. Of course, things never sound great when you're not moving the speaker

But if you don't care about low volume, and only versatile distortion at gig levels, then ppimv works well

I've got a Dumbelator that I can use as a global master, sounds great!

[Roe]

a good PPIMV should also compensate the reduced signal on the feedback loop

if you are going the LARMAR way you need a three-ganged pot, not easy to find...

cheers

Don't think so, this is from the Metroamp forum, where it was conceived.

yes, but adjusting NFB is preferable - I'm using a second pot, usually 100k

[llemtt]

yes, it depends a lot on personal tastes and how you plan to use your PPIMV

without compensation basses get looser and flabby, and your PI gets into clipping sooner

in my plexi 50 without compensation I can't go down to bedroom volumes with an acceptable sound...

cheers
teo

[rogb]

The PPIMV is in and working well as far as I can tell, it's night here so I was playing real quiet...

Tried an NFB pot but I preferred the lift switch so that went back. Also tried a resonance pot but that didn't seem to do much. The Deep switch is good and staying.

Its good not to have to rely on the D'lator as a master, so I can grab the amp and go

I must say though, the TC reverb though the Dlator and passive loop sounds really good to my ears. I got a nice vintage spring set up.

Just waiting on a eBay bid on a Marshall foot switch to build a simple ABY and vibrato switch box, so I can switch channel and stab that vibrato in for some organ trio double stops and general fun.

Thanks as usual for all the TAG help, advice and above all inspiration

[jjman]

Took the plunge and installed the Larmar on the SFDR. Dual 250k with 4.7meg “safety” resistors. Works good but I want to increase the potential PI clipping. Could use some more distortion with the channel volume dimed. I’ll probably reduce the 3.3meg going to the grid of the 3rd stage. I’ll definitely try a 12ax7 in the PI 1st since it should clip sooner than the 12at7. Still need to hook the reverb back up. The trem still sounded good with the PPIMV down and the channel cranked.

I don’t want the control in the back so I sacrificed the #2 input. (Non original faceplate.) The "knob" is a vacuum cap, at least for now.

[rogb]

Good work

My PPIMV is still in, and I still like the sound. Usually I try something and think AAAAGGGH and pull it out, but this is OK so far.

Made some other changes today too, added a Dale 100R 6.5w at the diode recto, as advised on here, drops the voltage a touch, looked like about 8V so should ease stiffness here.
Added 270pf snubbers to V1 a and b, the Normal channel, which is my modded channel. Liked the sound of that.
Added some O-ring dampers to V1-4 and an O ring damped tube shield to V1.

Added 2 x 220K 2w to the PS caps 1 and 5 either side of the choke as bleed resistors.

On testing with Normal volume 3/4 cranked and brushing the strings the open G string in an open G chord seems to feedback really fast. I wouldn't mind some other strings getting in on the act too

I notice on the D style PS that there are two 220K 3W as balance resistors on the totem pole first 2 caps. Has anyone ever done balancing on the parallel AB763 first pair of caps? Or is it not considered necessary?

Thanks
Rog

[Roe]

... want to increase the potential PI clipping. ....

increase mid pot to to 25-100KA. lift trem ground with push-pull pot

[jjman]

... want to increase the potential PI clipping. ....

increase mid pot to to 25-100KA. lift trem ground with push-pull pot

No mid pot on DRs. I already have a pull pot that switches in/out a 12k in line with the stock 6.8K for an optional 18.8k mid "setting." Works nice.

I'm a trem whore so I need it to be available in all modes. So I'm thinking of allowing some gain (signal strength increase) in the 3rd stage by lowering the 3.3meg so it doesn't divide the voltage as significantly. I've read it would also be appropriate to also increase the 10pf cap bypassing that 3.3meg to preserve EQ in that vicinity. Trial and error for both. Hopefully I can get more out from 3rd stage by putting more into it, and that it won't start to clip itself (too soon.) I'm thinking there is room since I think the 3rd stage would normally not be close to clipping. Based on my experiments with my ~Express~, I prefer PI clipping to 3rd stage clipping.

[Roe]

... want to increase the potential PI clipping. ....

increase mid pot to to 25-100KA. lift trem ground with push-pull pot

No mid pot on DRs. I already have a pull pot that switches in/out a 12k in line with the stock 6.8K for an optional 18.8k mid "setting." Works nice.

I'm a trem whore so I need it to be available in all modes. ...

Ok, try increasing the 50K pot with a 100k then or try a bias shift tremolo like the blonde amps had

[jjman]

I have the bias trem in my ~Express and it would sound better than the roach approach, but that's a line I don't know if I would cross in the DR. Maybe some day.

Just finished tinkering with the 3.3meg/10pf. With the mid "boost" on and 100mv on the amp's input @ 500hz:

3.3meg: 1.8vp-p in to 3rd stage > 55vp-p out from 3rd stage

1.6meg: 3.0vp-p in to 3rd stage > 80vp-p out from 3rd stage

1meg: 4.2vp-p in to 3rd stage > 82vp-p out from 3rd stage

The output is clipping at 82vac just after it's output cap and I'm sticking with the 1meg for now. Definitely more distortion potential from the PI now. With the 1meg, the high treble was reduced with the 10pf as expected. Going to 47pf restored it. Noise is now higher than before (with the PPIMV maxed) for any given Vib channel dial setting other than min. No surprise considering all the extra signal (and noise) coming thru that channel now. But dialing back the MV reduces that noise and it's all relative to the signal which is also now louder at setting "X" on the channel volume. Trem working fine but still need to hook up reverb. Hopefully the reverb will still have enough potential but if it doesn't I'll reduce the 470k from its wiper to the 3rd stage to re-harmonize.

I guess the Normal channel is now a "clean" channel and it's much cleaner than the Vib channel with either on 10. Previously they were very close to each other in that regard.

Hopefully the vintage gods don't lay any wrath on me.