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Re: 3-input LTP PI

Posted: Wed 08/12/20 8:31 am
by geoff 1965
yeah that's the difference with the couplers of the normal "lite" & TMB channels on my amp,the TMB has a 22n into the phase inverter and is a bit darker than the normal channel with 10n.
another thing i do on the 18W is replace the 4n7 feeding off the plate with a 10n which lets more breakup through "read the notes on ZP's parallel/cascade mod" but the you have to adjust the values in the tonestack to suit the extra bass coming thru.

Re: 3-input LTP PI

Posted: Thu 08/13/20 12:12 pm
by Tommy_D
crgfrench wrote:
Tue 08/11/20 10:41 am
If you look at my schematic you'll see it is just like what Aiken calls "3 input PI". Its great.
Craig, looking at your schematic in more details as I'm drawing mine:

Essentially your feedback loop is wired to the input which has the phase complications. You are sending feedback to both power tubes with the same phase, while the preamp signal is out of phase in each power tube. I'm not sure how that affects NFB.

It would be interesting to disconnect one of the channels and put the feedback in the current TMB channel input to see how that affects the sound...

Having read a bit more I'm erring on the side of using mixing resistors - will be closer to the original tone of the Reverb Deluxe which has these. I'm not sure what it does to the Bassman preamp tone but I won't get the true Bassman tone anyway. Feels like it will be easier to implement the Bassman-style presence pot too

Re: 3-input LTP PI

Posted: Thu 08/13/20 1:14 pm
by Tommy_D
Other idea for mixing signals, which I'm not sure I've seen before in a guitar amp build!

If I threw in one more tube to the build I suppose I could do that + have the trem after all. 1 stone..

Any thoughts?

Re: 3-input LTP PI

Posted: Thu 08/13/20 1:41 pm
by Tommy_D

Re: 3-input LTP PI

Posted: Thu 08/13/20 5:10 pm
by crgfrench
The Aiken discussion has an error. His input points 2 (lower end of C2) and 3 (lower end of R6) are the same point in the circuit. So it's a 2 input setup after all.

Re: 3-input LTP PI

Posted: Fri 08/14/20 7:45 am
by Tommy_D
That’s right

I think I’ve concluded on mixing AB763 and 5F6-A channels. Both actually have mixing resistors originally

So I would move the bassman mixing resistor after V1 to after v2 / tone stack and make it 220k rather than 270k

Just thinking whether moving the 5F6-A volume control after the tone stack would decrease the interaction of the 5F6-A tone stack and the AB763 channel. To be experimented with, I don’t think it would make a major difference