Merlin's LTPI idea

[uneumann]

Just heard Merlin talk about the LTPI on Kendrick's webinar yesterday. He gives a good talk, btw, with nice presentation slides and an easy to understand speaking style. Anyway, he mentioned an idea he also describes in his new book, which is to vary the bias resistor (not the current source) to effectively control the the clipping amplitude of the PI output. It's basically a way to get a master-volume-like behavior. I'm curious if anyone out there has tried this or seen an amp with this kind of control. I'm sure there can be lots of variations in exactly how it is implemented, but I'd appreciate any comments on how any variation you ran into sounded and worked in practice. Here is the circuit from his book...

[dai h.]

it's been discussed before. You need a reverse audio taper (for some reason I forget--something to do with "cutoff"). Mojave Amps does this on an amp (but with audio taper pot that attenuates clockwise). I've tried with (the 470 ohm stock R in series with) a 50k rev. audio (on a Marshall) but for me personally (while it functioned fine) it was more practical not to use it (since I was mainly using a spk. emulator to go direct which enabled me to "go louder" (using headphones)). It was just more satisfying with a certain amount of volume.

[uneumann]

it's been discussed before. You need a reverse audio taper (for some reason I forget--something to do with "cutoff"). Mojave Amps does this on an amp (but with audio taper pot that attenuates clockwise). I've tried with (the 470 ohm stock R in series with) a 50k rev. audio (on a Marshall) but for me personally (while it functioned fine) it was more practical not to use it (since I was mainly using a spk. emulator to go direct which enabled me to "go louder" (using headphones)). It was just more satisfying with a certain amount of volume.

Sorry for repeating the topic...
I'll do some more searching - I must have missed the threads about this.
Thanks for the comments.

[tedmich]

Merlin used to hang out here until some OCD guy went off on him about parallel triodes in a preamp and noise... he's more common over at ax84 I think now.

[daz]

Just try it. It'll take 2 seconds to implement it temp with a trimmer. I've tried as much as 2k there and it's a fairly radical tone change. It takes the brightness and edge off in a way that's nice at first, but in the end it for me at least turned out to be more of a bandaid for other issues and it IMO is best left with the 470R. I've tried a few master volume methods with the PI too. Again, in the end i always end up back to the standard PI. It just works.

[teemuk]

The first amps where I've seen it are Carlsbro TC-series tube amps from the early 1970's and GMT Gallien-Krueger transistor amps (those output transistor coupled ones) from the same time period. In Carlsbro the "bias resistor" was variable, in GMT/GK the whole LTP current. The idea works just as well with both technologies for achieving the very same purpose.

Then of course in the early 1980's you had Mesa/Boogie amps, like the D-180, with variable current source on PI.

As said, Mojave amps utilise the variable "bias resistor" trick, as well as some Egnater amps. The latest development is actually in the 3rd Power amps where variable bias resistor pot is actually ganged dual -type and the other half works as a master volume.

[teemuk]

[eschertron]

Merlin used to hang out here until some OCD guy went off on him about parallel triodes in a preamp and noise... he's more common over at ax84 I think now.

I've tried to forget that thread... not enough alcohol left in the lower 48, unfortunately

[dai h.]

this?

Kendrick Tube Amp Webinars

interesting. Not the first guy I'd personally go to for authoritative tech info (or quality tube amp repairs), but different strokes I guess.

[uneumann]

this?

Kendrick Tube Amp Webinars

interesting. Not the first guy I'd personally go to for authoritative tech info (or quality tube amp repairs), but different strokes I guess.

As you say different strokes...
I have done some guest talks there over the past few months and in return I get invited to attend for other guests (like Merlin). I think it's actually pretty cool in that you get some 30-40 people from all over listening and asking questions and making comments. It's just another form of amp discussion that compliments forums like this. Gerald handles all the details for hosting the sessions (like the people managing this forum) so it's easy to participate and it works pretty well. I can't speak for Gerald, but if any of you've got an interesting story to share about some amp-topic, you may want to ping him and see if he's up for scheduling a session. Spreading info and experience is what its all about. I have not sensed any commercial angle or axe to grind.

thanks for all the responses ...

[stormbringer]

at $50 per month, I think I'll pass.

[uneumann]

at $50 per month, I think I'll pass.

Sorry - I should have been clear. Gerald doesn't charge anything for people who give talks. I did two talks and in return he keeps me posted about other speakers and I've had free access to any sessions I've asked to attend. Experienced people on this forum would likely be speakers, not attendees. The point is that smart experienced people have knowledge and info to offer others and Gerald's webinar is a good way to get that info out there in a nice hour long format, rather than a 10-20 line blurb on a forum. We're all giving away info (and time) on postings like this. Forums and webinars are just different ways to teach and build community.

True, the people who benefit do have to pay something, but that's a choice a number of people seem to make. Getting a good info "package" is worth something. Just check out tuition fees for a good EE program anywhere.

Gerald does have to pay the bills on the teleconference software he uses so a fee for regular attendees makes sense - nothing is free out there. Even forums have adds all over them to pay the bills.

Anyway - I just want to clear that up - I don't think Gerald is doing webinars for $$ and he's making his tools available (free) for anyone who wants to share what they know with a group of interested people.