London pro by 65 amps what makes them sound different ?

[chopstuck]

I played a London Pro at a local GC and I have to say it did sound great.
I have built similar amps in the 15 watt 2 channel variety.
I have built pentode channels and tweed champ channels into a voxy PI but this one sounds different.

It sounds more authoritative in the bottom like it has 6V6s but does not get muddy when you turn it up, like many amps that sound good clean.

I wonder if anyone knows some of the details as I've heard that the designer was a frequent poster here or at 18 watt.

[Colossal]

I don't have many specifics, but I believe he used to post regularly on 18watt. They use Mercury transformers wound to their spec. In the 65 Soho signal path they use standard wound Sozos, carbon comps, silver micas, and Vishay/Dale RN65Ds in select locations.

Hope that helps.

[chopstuck]

So do you think it's a standard tweed champ, and AC 15-EF86 circuit into a standard Vox 2x EL84 output ?

Or more Marshally values in the PI ?

I was looking at "paralleling" the channels but thought I would need to also pad the PI input when doing so.

I have tried a few different cap and resistor brand choices but that doesn't really get me where I want to go. Values yes, but brand names don't impress me much. I have tried to use carbon comp in the plate loads but I don't hear the magic. Now putting in a small ceramic cap may add some nice "noise" . But a little goes a long way. He he.

[LeftyStrat]

The tube complement has only one 12ax7. So it has to be using a cathodyne PI.

Better bass response might be playing with the negative feedback or perhaps a conjunctive/corrective filter? Dr. Z used something like that on the Carmen Ghia.

Good discussion here:

http://ampgarage.com/forum/viewtopic.ph ... 1ae7b210c8

[chopstuck]

Lefty, thanks for that. I was thinking there were 2 x 12ax7s up until now. That does simplify topology a bit. I will have to play with some values to get the gain where I want.

I had already built a 6BM8 amp in this configuration with an extra triode before the PI ala tweed deluxe and it's too much drive.
Maybe I will remove it and see how it sounds for starters.
Leaving that poor triode all alone.

Maybe a CF ?

Well I can't just leave it there dangling !!!!

[Gaz]

Buy it, and reverse it

[chopstuck]

Well I eyeballed the amp and it (AFAICT) has 3 preamp tubes so I'm back to thinking it has a LTP inverter.

It also seems to have more gain than I had been able to get out of a pentode feeding a paraphase but less than a pentode feeding a triode gain stage and then the paraphase Like a 5E3.

I suppose it could have only the first triode/pentode stage separate and the second stage shared by both channels.

Doesn't anyone have a better idea of the layout ?
Maybe if I went to GC with a screwdriver and a spy camera....

[chopstuck]

OK , it wasn't a spy camera but it was a a high power LED flashlight. There are three preamp tubes including an ECC8* in the middle position and 2x 6bq5 and an noval recto. That's all I'm sure of.

OK Mr Phelps. balls in your court.

Amp not as advertised.

[Leo_Gnardo]

OK , it wasn't a spy camera but it was a a high power LED flashlight. There are three preamp tubes including an ECC8* in the middle position and 2x 6bq5 and an noval recto. That's all I'm sure of.

Lookin' more like a sort of AC15 variant sort of thing. There's a lot to like about THAT tone. Good spyin' agent 86! Or maybe that's EF86. We know 65 likes those.

[dorrisant]

Look here... 5th post down:

http://forum.metroamp.com/viewtopic.php?f=7&t=30368

Tony

[Leo_Gnardo]

Look here... 5th post down:

http://forum.metroamp.com/viewtopic.php?f=7&t=30368

Tony

I'm just watching this but thank you Tony! I went. I read. I laughed!

from MetroAmp:

Kind of funny how a guy who knew nothing about amps a few years ago is now considered one of the top builders

Exactly what I thought of Gerald Weber when he popped up as the New Big Thing in 1988.

Still not quite sure what the London Pro is, but I'll bet it's a gnat's hair away from a Vox AC 15. And that's a very good tone IMHO.

[LeftyStrat]

Got to agree with Lou. Start with an AC15. The EF86 channel uses what is used in the Matchless DC30. A rotary switch to change the coupling cap.

Maybe tweak the PI/PA to Marshall 18 watt values.

Low pass filter for tone between triodes on the 12ax7 channel.

The output of the 12ax7 channel will be in-phase with the input. The output of the EF86 is out of phase with the input. Each channel goes to a different side of the PI. No negative feedback.

Maybe up the PS filtering to get better bass response.

Also experiment with using a voltage divider on the screens. There's a thread somewhere on here, think Andy started it. Found it:

http://ampgarage.com/forum/viewtopic.ph ... ge+divider

[rhinson]

Look here... 5th post down:

http://forum.metroamp.com/viewtopic.php?f=7&t=30368

Tony

I'm just watching this but thank you Tony! I went. I read. I laughed!

from MetroAmp:

Kind of funny how a guy who knew nothing about amps a few years ago is now considered one of the top builders

Exactly what I thought of Gerald Weber when he popped up as the New Big Thing in 1988.

Still not quite sure what the London Pro is, but I'll bet it's a gnat's hair away from a Vox AC 15. And that's a very good tone IMHO.

that pic is of the regular london---just a standard 18w ch and ef86 ch. the london pro is a much different amp inside-----built on terminal strips, pretty complicated wiring and setup, relays switching different parts of the circuit in and out, a funky dual pot with a tone network, etc. i looked in one but did not care to back engineer it. the guy sold it and i built him an amp he liked better. rh

[dorrisant]

Just curious, what did you build for him that he liked better?

Tony

[jaysg]

From having been there at the time, Dan built an 18W and showed it to Peter, who liked it but wanted changes. Essentially, you're paying for Peter's ear and sensibilities. Anyone may or may not agree with his ideas of what it should be. They did iterate iron several times for the production version of the London. The layout was created by someone here who is fond of V2 rocketry...and yes, Dan knew jack about electronics. He was a guy who realized one day that he wasn't going to make it in music and went back to school to learn IT.

The London Pro appears to have what Reason Amps calls Stacked Mode.