Express/ Liverpool Cold Clipping

[Amwatts]

Hi Folks

I've built myself a Two Knob Express preamp and married it to a 5E3 deluxe cathodyne PI and output section,
housed in Swamp Mahogany TV Front style cabinet.

I was looking for something smaller in wattage and volume to take to small gigs / rehearsals etc , my Express clone is a little too much for that

I started out with rfgordons TNT schematic which is great outta the box, but the tweaker in me wanted some more adjustable gain, both more and less.

I played with the V2A section of the Express/Liverpool , did the adjustable pot on the 150k to ground resistor but decided to got back to fixed settings, I ended up with Mark Abbotts suggestion on one of his Liverpool drawings of 0.0022 and 68k ...

Next I decided to play with the cathode resistor value on V2A. KF has it at 10k which induces the cold clipping into the PI, so I decided on a 4 way rotary switch to give me some variations , the 4 resistors I decided on were 1.5K (Leo Fenders go to) 4.7k, 10k , and 22k .... I went as far as the Soldano 39k in my experiment but with the 5E3 phase inverter it was too much

I'm pretty happy with the mod , just trying to work out how to stop the popping when you switch between settings ... any ideas ?

Here's the schematic if anyone wants to try it

Express Cold Clipping Mod.jpg

tempImagelAZQYe.gif

[LOUDthud]

The cold clipper is always going to pop because you are changing the bias even if you arrange the switch and resistors to eliminate the dead zone where cathode current falls to zero. (Hook the resistors in series and short the ones you don't want to select.)

One long way around it is to have four separate stages and select the one you want after the coupling caps to the next stage.

Another trick is a muting circuit like Peavey uses in the EVH type amps. A second section of the switch could be arranged to create trigger pulses in the dead zone between poles, the muting circuit could stretch the pulses to cover the pop.

[Roe]

A pot is better - I have that in a k50ish preamp. you need not go lower than 2k2 or 1k8 really

[Amwatts]

A pot is better - I have that in a k50ish preamp. you need not go lower than 2k2 or 1k8 really

Great Idea .... I've found a 20kB pot in my stash to try, I turned it into a variable resistor and put a 1.5k in series , I'm getting 1.5k off and 22k on full , More than enough gain and sustain than I'll ever need

Thanks for the advice

here's the updated schematic

Express Cold Clipping Mod V2.jpg

[Roe]

A pot is better - I have that in a k50ish preamp. you need not go lower than 2k2 or 1k8 really

Great Idea .... I've found a 20kB pot in my stash to try, I turned it into a variable resistor and put a 1.5k in series , I'm getting 1.5k off and 22k on full , More than enough gain and sustain than I'll ever need

Thanks for the advice

here's the updated schematic
Express Cold Clipping Mod V2.jpg

yes, anything from 820R to 39k is worth trying

[Mark]

Sensational looking cab. You build that cab?

I imagine the gain really increases the closer it gets to 1.5K, how stable is the amp with increase in gain?

[Amwatts]

Sensational looking cab. You build that cab?

I imagine the gain really increases the closer it gets to 1.5K, how stable is the amp with increase in gain?

Yes I built the cab, It's Queensland Swamp Mahogany like the Barwatts cabs I've built ... I build the whole thing , Cab , chassis, boards etc all in my workshop. Have been building amps for 20+ years
The amp sounds fantastic , I've played with the circuit and fine tuned the components , I ended up using a 3w 20Kb pot for the V2a bias adjust , the Alpha I had in there was very scratchy after a while so I upped the wattage and it works great , The gain is very Express like , the sweet spot on the pot is around 10k as Ken used , but it can go over the top if needed ..I put the 470k PI balancing resistor on the V2b grid of the PI, and went with your suggestion of 0.002 / 68k feeding the V2a grid .... I'm very happy with it , sounds amazing , blows a flabby 5E3 out of the water ... I took out the vintage Jensen Alnico and put in a New Jensen N12K Neo , Very impressed , the higher db rating made it much louder now and it can stand on it's own with a band ..
I just finished another one marrying it to a paraphase PI . It's 40w cathode biased , 2 x 6L6GC's and a 5U4GB rectifier ... It took a lot more tweaking than the first one. I had to play around with the bias caps on the preamp and PI to get that one stable , but I'm almost there , There's a small oscillation happening that sounds a little like motor boating , it's very faint and only noticeable when not playing ... Once I sort that out I think it will be killer too ...

[sluckey]

I really like the way the top piece grain seems to flow into the joints and just cascade over the edge like a waterfall. Very unique and beautiful.

[Reeltarded]

I like your amp, but your experiments are bossy!

The 68k there is vital to the general voice. That is my choice as well.

[Amwatts]

but your experiments are bossy!

What does that mean ? Who have I been bossy with ? Examples please

[Colossal]

but your experiments are bossy!

What does that mean ? Who have I been bossy with ? Examples please

I think he is implying your efforts are (as the kids these days say) "like a boss", meaning, in full command of. It's Reeltarded for a compliment!

Great looking amp!

[Amwatts]

but your experiments are bossy!

What does that mean ? Who have I been bossy with ? Examples please

I think he is implying your efforts are (as the kids these days say) "like a boss", meaning, in full command of. It's Reeltarded for a compliment!

Great looking amp!

Never heard that saying before .... not here in Oz

Thanks for explaining

[Amwatts]

Just finished the 5U4GB / 2 x 6L6GC 1x 15 Alnico version .... this one has the 50's tweed 5D5 Paraphase PI , It had to change the NFB from 1M (original Fender value) to 100k . Also I used a 15k bias resistor on the V2A Cathode and added a 2.2uf bias cap where as Mr F used 10k and no cap, The more efficient PI didn't break up as I had hoped like the Cathodyne PI did , So I played around and got what I believe is a good combination, I started with 0.68uf and 39k - but that was WAY too much , not a clean sound to be had even with a strats volume knob on 3 ... Over the top ... clipped many combinations in there until I was happy with 15k 2.2uf ... Also with the spare hole on the top of the chassis I put a 100kL pot in series with the 68k Grid leak resistor on V2A ... it's not really a gain knob, it's more of a fatness knob, It's hard to describe it just adds some dimension and fullness to the tone , single coils I wind it up to full (I Read 155K) and with Buckers I wind it back off (68K) it's subtle but I like it .. I'm not sure if this is a mod that's been discussed here before ,I sort of stumbled upon it while try to change the gain dynamics in that part of the circuit .
I also individually cathode biased each 6L6GC instead of joining them, I used an old set I had lying around and they were very mismatched , so I thought , why not bias separately , they both have 470ohm 10w resistors and 50uf/100v caps ... One is giving me 21 watts the other 19 watts , so 40 watts cathode biased is pretty loud

the TV Front cab is the same size as the early Pro 1 x 15 cabs , it has genuine 1940's tweed I got a roll of about 10 years ago , I believe it's the same cloth used on the first Dual Professional in 1947 but I can't be sure.

IMG_6155 2.jpeg

[Reeltarded]

but your experiments are bossy!

What does that mean ? Who have I been bossy with ? Examples please

Awesome and thoughtful. Rockin'!

[Amwatts]

yes, anything from 820R to 39k is worth trying

Always experimenting , I went down to 820R to see what it would do ... it induced microphonics that were just too annoying and couldn't be tamed ... I think 1.5k is about the limit