a little Power Brake and PI gain reduction EXP

[dai h.]

tried adding a 50k rev. log pot in series with the 470ohm in the PI in my 50W guinea pig #2 Marshall (apparently the "power dampening" cct. used by mojave amps--this is also mentioned in Kevin O'Connor's TUT1) and adjust.WW R across the 1mH in my PB to 22ohms, and another adjust.WW across the LC(12.5mH/160uF) pair set at 16 ohms. With PB set to min.("load" position), and signal taken direct through a Palmer PDI-03, listening via headphones, it sounds pretty good, better then expected. I'm not quite sure which part is responsible for the perceived improvement over the internal load in the PDI-03 (which consists of an R and L only). Can't use the spk. since it's late now. Will have to rewire the 50k pot since I think I got the wires to the lugs backwards.

[dai h.]

tried some "100uF film caps" (made of 15 6u8F 100V boxed metal polyester cellotaped together) across the LC pair and it was interesting. Changes the resonant freq. and it seemed more like a mic'ed speaker sound. Doing the "woman tone" (rolling back the tone pot all the way while on the treble pickup) and those "harmonic chimes"(??)--pull offs while rubbing your palm across the wound string--seemed more realistic. Maybe the load should be tailored to emulate (or at least attempted to emulate more closely) the response of the speaker that sounds best for the specific amp?

[Daver]

How well does the 50K pot in the PI work to reduce volume? What's the tone like? Thanks.
Dave

[dai h.]

works well but I really can't say how well it performs through an actual spk. at low volume. I tried it with the Power Brake one click up past "load" position, and it's still too loud (so I ended up raising the gain since it's through headphones). I have it hooked up like this (see pic). Wiper goes to cathodes, right lug to top of 470ohm. This gives a gradual change in gain with the 50k reverse log(audio) taper pot.

[Daver]

Cool. So, it has potential. I guess I'll have to experiment with it. It might make an interesting "Master Volume". Thanks for the info.
Dave

[dai h.]

keep in mind it works as a "gain" and not a "volume" (IOW it won't go to zero volume fully counter-clockwise). There's a thread on the Metroamps bbs where I saw it and got this from (this is apparently used on some Mojave amps). It's also mentioned in Kevin O'Connor's TUT vol. 1 and he says you can use a value from 10 to 100 times the value of the R (the 470 ohm) IIRC but goes on to say it won't work because of the DC shift (but it actually does if you use a reverse log taper--the idea of which I obtained from Larry/"novosibir" of Larry Amps).

[Daver]

Well, I gave it a try. I used a 50K Linear pot because that's all I had. The amp I tried it on is the only one I have open right now. It's sort of a Vox AC30 kind of thing I built. The PI cathode resistor is 820 ohms. Not a completely fair, accurate or completely tweaked out situation.
At the maximum settings the amp sounded a bit farty and congested. It did reduce gain and volume. Mojave claims that you can use it to reduce output and retain the same tone. I didn't get that. At higher settings the distortion was reduced and it worked like another gain/drive control. There isn't a ton of drive in the Vox preamp, so a lot of the overdrive comes from the phase inverter. This control changes the tonal characteristics a bunch. Remember that I did not try to optimize anything. I still don't think it would work as well as Mojave says it will. Anyone actually play with one of those?
Dave

[dai h.]

maybe it works differently depending on the amp. AC30 type so (guessing) something to do with no feedback? I haven't really thought that through however.

Yesterday, tried the amp out some more with the modded load with the spk. using various combinations of the settings (i.e. click positions), amp volume, tone controls, PI gain (in the lead ch.) but wasn't really happy with it. Was about to turn it off and on a whim tried the bass(normal) ch. and this was a lot better (DI'ed signal). Sort of Deep Purple, AC/DC sounding. Fat, round, a bit mushy. Had a thought, then tried putting the 250uF (with gator clips on ends) I had removed from V2a cathode some time ago back on, and this time the lead ch. was much much better, maybe even pretty good. Very Van Halen I-sounding which is what I was going for on that ch. Tried taping a bit on my old Tascam Portastudio 488 mkII, and it still seemed pretty good even on playback, but...I'm not 100% on my hearing since getting some high freq. tinnitus. If I could get the highs to be smoother it would be an improvement.

[dai h.]

last night moved the Palmer and PB off to the side of the amp using shorter spk. wires (instead of across from it) to deal with squealing which I gathered was coming from the powerful spk. signal radiating and interacting with the pickups exacerbated by the length of the spk. wire (greater area of radiation). Did that, works and confirmed that was the cause by being able to induce/eliminate the squeal by moving nearer/away from the Palmer+PB100. Tried putting another 1mH in parallel with the stock 1mH, plus shorting, plus reducing the value of the parallel R across the series L but couldn't quite get what I wanted. There is still a bit of buzziness in the top end I don't like. Tried recording the signal on tape and on playback the buzz doesn't seem to be as bad, so I'd say it's about good enough for a scratch track or late night jamming where it's impossible to use a spk. and to the average guitarist (probably) far more preferable to a SansAmp esp. for high gain.

[dai h.]

played for a longer period, maybe 30mins(?), and noticed that the R across the series L got kind of warm. The other one across the LC pair wasn't warm at all. The one across the series L now is about 25W IIRC. The one in that position in the Marshall SE100 is also 25W(alu-housed WW). Not sure how to calculate the correct wattage.

[dai h.]

just thinking that the best place to put the high freq. impedance limiting R is right on the alu heat sink where the other two alu housed wirewound Rs are. The new R (another alu housed WWnd 15W?) would go right after the big 10ohm 100W and move the 1mH from the PCB to there across the new R and short the 1mH spot out on the PCB. Seems to be the best solution since the heat emitting R would be moved away from the heat sensitive BP alu electro, and it would free up a little bit of space if you wanted to add capacitance and change the resonant freq. Kept trying to think of a good way to put the new R inside, but that wouldn't have been very smart. Just wonder how sensitive the 1mH is to heat, although there is some forced air cooling from the fan.