What differences would one expect between the Express and Liverpool phase inverters? They seem to run at similar B+ voltages, but the tail and bias resistors are very different. What would happen if someone put the Liverpool values in the PI of an Express: less gain, +/- headroom? I'm very curious as my Express clone has too much gain and really never cleans up as nicely as I believe it should. Thanks for your input.....
Dobb
PI comparisons
Moderators: pompeiisneaks, Colossal
-
loverocker
- Posts: 104
- Joined: Thu Mar 03, 2005 11:55 am
- Location: England
- Contact:
Re: PI comparisons
My understanding is that the Liverpool PI is 'dirtier' than the Express PI. In fact, ISTR the Liverpool is very similar to the Dominator/18Wer PI - and that is famous/infamous for having fairly little clean headroom.
in the wilds of Borneo, and the vineyards of Bordeaux, Eskimo, Arapaho, move their bodies to and fro...
Re: PI comparisons
The liverpool PI is actually a buttload cleaner than the 18watter/dominator PI. I actually experimentally modded an 18watter to get more headroom and less fizzy tone and came up with almost the exact values of the liverpool, in fact.
Part of the differences in the PIs is the power tubes they're driving. The octals need a little more voltage swing to break up compared to EL84s. Plus, the liverpool is cathode biased, which tends to give a little squishier performance and slightly less headroom.
And, if you're using turrets, it's super simple to change a resistor value here and there to experiment for yourself. The only substantial difference is on the cathode and tail -- two resistors, only.
In fact, I've got a couple of trimpots in one of my PIs right now and I will do something like dial out some resistance on the tail and play, then dial in more and play, and more... until I decide I like the compromise I reach. Then I measure the value and swap in a resistor to match it. I've had great success in customizing the imbalance of the PI this way, and it's very easy.
One more thing to add. There's really no substitute to hearing and playing for yourself when it comes to this. The first time I started swapping out PI components, I discovered things like making a better top end by raising the BASS rolloff frequency (which turned out to be swamping the overdrive tone -- a lesson in intermodulation distortion), making the distortion richer by imbalancing the PI using the tail resistor, driving the poweramp harder with a cleaner PI and higher value resistors from grid to ground, and all the associated differences in player feel and perception that come with the changes. Dive in. It's an eye opener.
Part of the differences in the PIs is the power tubes they're driving. The octals need a little more voltage swing to break up compared to EL84s. Plus, the liverpool is cathode biased, which tends to give a little squishier performance and slightly less headroom.
And, if you're using turrets, it's super simple to change a resistor value here and there to experiment for yourself. The only substantial difference is on the cathode and tail -- two resistors, only.
In fact, I've got a couple of trimpots in one of my PIs right now and I will do something like dial out some resistance on the tail and play, then dial in more and play, and more... until I decide I like the compromise I reach. Then I measure the value and swap in a resistor to match it. I've had great success in customizing the imbalance of the PI this way, and it's very easy.
One more thing to add. There's really no substitute to hearing and playing for yourself when it comes to this. The first time I started swapping out PI components, I discovered things like making a better top end by raising the BASS rolloff frequency (which turned out to be swamping the overdrive tone -- a lesson in intermodulation distortion), making the distortion richer by imbalancing the PI using the tail resistor, driving the poweramp harder with a cleaner PI and higher value resistors from grid to ground, and all the associated differences in player feel and perception that come with the changes. Dive in. It's an eye opener.
Re: PI comparisons
Thanks for the input!! I used a perf-board without turrets, so it's not really easy to change out parts quickly. If I understand you correctly, the Liverpool PI doesn't have the gain that the Express PI has??? I really need to warm up the iron and try different values as you did and report my findings.
D
D
Re: PI comparisons
Liverpool should be slightly less, but there's a little more to it than that.
You can START by clipping in resistors in, you know. Make a 1K cathode R into a 470 by clipping another 1K in there (parallel, makes 500ohms) and then clip a 47K into the tail to make it less than 25K. There. You've now approximated the Liverpool and you don't even need to solder.
It's not perfect, but it's super simple. and will give you an idea of comparative sounds.
-Moose
You can START by clipping in resistors in, you know. Make a 1K cathode R into a 470 by clipping another 1K in there (parallel, makes 500ohms) and then clip a 47K into the tail to make it less than 25K. There. You've now approximated the Liverpool and you don't even need to solder.
It's not perfect, but it's super simple. and will give you an idea of comparative sounds.
-Moose