Russian TrainWreck - Expresso Lite(Liverpool), Soviet tubes

[TheGimp]

When I got the amp back from Roger yesterday I asked him what commercial amp it sounded like.

At the time he couldn't say for sure, said that it had it's own sound. However, this morning he came by and told me he used to own a VOX AC-30, and when played at the begining of overdrive it sounded very much like it and it was loud like an AC-30, just not as loud.

I stripped the FET front out and am in the process of changing V1 from 6N2P to 6J9P pentode. I think I can get an AV of about 70 so I can leave V2 down at not quite half the original gain. Driving the tonestack will be a bit of an isue as it is half the recommended load impedance (111K vs 200K). That will drop the gain slightly. The simulation looks good.

If the 6J9P is noisy (hiss) this won't work but is worth a try.

I also found I do have one EF86, so I may try it as well.

[TheGimp]

The j-FET solution seems to work fine but I'd rather have a tube based solution if possible.

With that in mind I removed the first dual triode and replaced it with a smalls signal pentode. Gain was out the roof (126) with a bias point of 1.55mA, cathode bias.

In order to tame the gain I used Schade feedback by capacitively coupling a large resistor (4.7M) from the plate to the grid, and added an input resistor of 39K. This allowed me to adjust the gain to achieve the same as the original design.

Hiss is down to 10.4mV at full gain. It is still quite loud and tone seems to be the same although it is difficult to tell for sure.

I'll take it in to work tomorrow and see how it goes.

Oh well, Roger is on vacation all week so I'll have to wait a while.

Mean time I've picked up a cheap guitar (Had to return the Strat) and dug out the guitar lesson guide my wife got me last year. I see why I never learned in the past. I was ADLD/ADHD as a child (and probably still am to some degree). The caterpillar is not exactly fun.

[TheGimp]

I've been playing around with gain and distoriton.

With all controls set at noon, and the guitar set at 5 everything is clean.

When I roll the guitar volume up between 7 and 8 it breaks up on G Major, but not on C major.

Is this due to power supply sag effecting lower notes more than higher notes since G major includes the low E string as well as EBGDA strings? while C major only includes EGGDA and not low E (top) string?

[TheGimp]

I think I finally have the design done. I need to go back through it and verify all component values and document them. Then I need to strip all the wiring and redo it as it has been hacked pretty badly with all the changes I tried.

I'm still waiting on the OPTs Edcor is designing for me. I hope they don't require me to go back and re-tune it.

I started a head, and made it way out of proportion (9" is way too tall) and had to cut it down. It should end up 7-1/2" high, 15-1/8 wide and 6-5/8" deep. It is Red Oak, with a Poplar band between the front of the box section and the round-over section of Red Oak that will be added.

the hand miter box is not going to be accurate enough, or at least I'm not accurate enough with it for the round-over corner cuts. I'll have to visit a friend with more woodworking tools to see it a power miter cutter works better.

[TheGimp]

Current Schematic:

Oops. C18 is 2.2uF,not 22uF.

[RJ Guitars]

I've been lurking on this thread for a while. Cool project! I bought a whole bunch of Russian tubes based on some input from a HiFi guru but haven't managed to put them to work yet except for the 6P1P Saratovs. I've also got a hand full of octals that are supposed to be great guitar amp tubes, just need more time to put them to use.

rj

[TheGimp]

I've been using 6P1P-EV with LED cathode bias at 1.65V and a current source as a plate load and it seems to give the least distiortion. I don't remember what the current was set to but the plate voltage sweet spot ended up being between 110 and 125V. I've built two stereo amps using this as the input stage.

[M Fowler]

I have been following along as well but didn't have anything to add so keep quiet and listened.

Nice project

Mark

[TheGimp]

Thanks for the comments. It is nice once in a while hear that someone is watching.

I've got a couple more things I want to try on the front end (like a switchable option for 12A_7 vs 6N_P) , but I think I need to stop tweeking it and get the head finished. Then I'm going to build a second one before doing any more mods so I have one with the complete pentode input design finished to compare changes to.

The Edcor transformers ought to be in tomorrow as well, so there is something else to experiment with over the holidays.

[TheGimp]

Transformers never came in. Lost in Christmas shipping I guess.

Head is almost done, but the Bezel is too thick. The only material I could find locally was 1/4" not 1/8". I need to contact a friend who has a planer to see if he can plane it down to 1/8".

[M Fowler]

The cab is looking good.

Yeah either 1/16 faceplate or counter sink a larger diameter hole around the existing pass through hole enough that the pot nut/jack nut fights diameter wise and counter sunk deep enough that the threads can be caught to tighten.

Mark

[TheGimp]

I went ahead and counter-sinked this Bezel.

I took some blank wood to a friend with a planer to have him plane it to 1/8". He told me he couldn't do the Bezel that is finished as he chips the ends when he starts planing.

I still need feet and a handle, but otherwise it is done.

[M Fowler]

Looks good and many guys have done it that way.

A drum sander rather than planer is used for thin it down.

Mark

[TheGimp]

This is Victoria.

Got the planed slats back and started the next build. I got the Hammond transformers in and am going to go for fixed bias at 375V B+ this time to see if I can get above 12W out. Unloaded the B+ is 385V and the bias is adjustable from -5 to -17V. I think I need to tweak it to a range of -10 to -20V just to be safe.

I don't like the Hammond output impedance configuration. Too many wires to switch to be able to have selectable output impedance. This build will be 8 ohms nominal.

[RJ Guitars]

Looks awesome - I like it!