Taming an Expwess...

[Paul-in-KC]

All,

I love my Twainwek Expwess (new naming scheme) - but it doesn't have much "clean(ish)" range.

That is, when playing my LP through it - with my neck pickup selected (DiMarzio 36th Ann PAF) there is very little volume control before the Exp. transitions to the fire-breathing high-gain mode.

There is a little more "room" playng my Strat (with original Lace single-coil pickups) but still not that much.

I have tried swapping in lower gain tubes, which helps, but doesn't really tame things down to where I'd like.

I'm going to try changing the anode load resistor on the first input stage (R3 in the docs) from 100K down to 47K and put a 47K POT in series with that to try tweaking some of the distortion out of that first stage.

I'm guessing folks here have already experimented with this - but I couldn't find any previous posts using the search tool.

Anyone try something similar?

I was also thinking I might want to do this at a different gain stage.?.?

I am going to try it and see if it has the effect I'd like (which is pushing up the input signal threshold where it transitions to high gain) but would like to hear from anyone that has already experimented with this (or similar).

Thanks
-Paul

[Reeltarded]

Damned. It's your instrument volume pots. I bet.

The taper in a modern pot is awful.

[Paul-in-KC]

Damned. It's your instrument volume pots. I bet.

The taper in a modern pot is awful.

Could be, but I have a "vintage" wiring harness that I can plug in to my LP. It has CTS 500K XL pots (which actually measure 560K). The vintage harness uses the 50s to early 70's wiring scheme (for the tone caps and pots).

NOTE: I don't have that in the LP right now, but I have had it in there to use with the Exp. and the stock PCB and controls actually work a little better than the vintage arrangement. Which is why I have the stock controls in it now.

So, I don't think it's just a case of modern control section (volume pots) eliminating the amount of signal in the "Goldie Locks" range - but I could be wrong.

[sliberty]

I have tamed my Express by doing the following:

Replace PI with 12AT7. I also tried a 12AT7 on V1, but that was TOO tame for me.

Install a split plate load resistor on V2, I used 22k on B+ side and 82K on plate side, but if that is too much clean for you, you can shift the balance (33K an 68K, or 47K and 56k for example).

Between these two mods, you will see dramatic changes in the amp's sweet spot. My amp can now by dimed if I want to without getting over saturated. And I have nice cleans up to about 5 on the dial now. Huge difference.

[Paul-in-KC]

I have tamed my Express by doing the following:

Replace PI with 12AT7. I also tried a 12AT7 on V1, but that was TOO tame for me.

Install a split plate load resistor on V2, I used 22k on B+ side and 82K on plate side, but if that is too much clean for you, you can shift the balance (33K an 68K, or 47K and 56k for example).

Between these two mods, you will see dramatic changes in the amp's sweet spot. My amp can now by dimed if I want to without getting over saturated. And I have nice cleans up to about 5 on the dial now. Huge difference.

Well - I already have a 12AT7 in the PI (and 5751's int V1 and V2). So, I'm half way there.

Now, if you can help me out with the "split plate load resistor". you say that you used a 22K on the B+ side and 82K on the plate side.

Sorry for being so slow, but I can't imagine what that means. I think of a load resistor as having B+ on one side and the Plate on the other.

So, using two resistors with one on the B+ side and one on the plate side sounds like two resistors in series, but that can't be what you mean.

I'll assume the existing load resistor on V2 (R9 in Ron's docs) would be replaced with a 22K (in your case). Where are the leads of the 82K resistor connected?

Again, sorry to be so dense and thanks for the help.

-Paul

[Paul-in-KC]

okay - I think I have it now.

(a little Google goes a long way)

So, the split pair are in series, but the C9 capacitor (sticking with the labels from Ron's doc and the V2 anode load) is connected in between the pair.

Thanks, I'll play with that.

[fishy]

https://tubeamparchive.com/viewtopic.ph ... split+load

Richie has recommended reducing the feedback resistor to slow the onslaught
which might be an easier option for you.

Must admit, I'm not familiar with the firststage distorting without there being a problem.....that goes against the architechture of the amp.

[sliberty]

You'll notice that in the posting that fishy points to, the split is reversed. 82K on the B+ side, 22K on the plate side. This results in a much less dramatic change. But, the exact split you use should be based upon your own liking. So feel free to experiment. As long as the total is approx 100K, the tube will be operating at the same levels. The split in the resistors, and taking the signal for the next stage from the junction simply attenuates that signal as it heads on to the PI. The larger the "plate side" resistor is, the more attenuation occurs.

[jjman]

I have a 50k-50k split for tapping the output of the 3rd stage. But my voltages are not clone values. My PI was clipping pretty early and the 3rd stage much later. Now the PI clips around 4 and the 3rd stage still clips around 7. As mentioned, the split load reduces the output from that plate. It does not change the clipping of that stage. It will make the clipping of the following stage "later" on the volume dial.

The 1st stage is likely not clipping. I don't think the 2nd stage clips either but I know dimarzios can be very strong.

12at7 PI can sometimes drive outputs stronger than 12ax7, at the same position on the volume knob. 12at7 itself will sometimes clip later on that knob. A scope is needed to know for sure.

I suggest splitting the second stage assuming typical design and voltages.

[Paul-in-KC]

As always guys, you are the best.

[geetarpicker]

Damned. It's your instrument volume pots. I bet.

The taper in a modern pot is awful.

Yep I agree. Almost all of the modern 500k pots come on like light switches compared to the old Centralabs Gibson's came with in the 50s-60s. Even new custom pot kits with new CTS pots miss the boat taper wise... That said, the new Gibson Historic pots have a very nice taper, and even come on a little slower than an original Centralab which with a wreck is even better. Gibson finally got CTS to make the pot with the proper taper, and other after market guys that had tried before simply didn't get the specs even close. The one issue though it's hard to find them near 500k and even harder with values much over that. Still IF your strat (with 250k volume pot I bet) is fine with your Express but your humbucker guitar (with 500k) is not DON'T even mess with that amp until you get decent taper pots! Even if you mod the amp the pots will still make a world of difference.

GK

[Paul-in-KC]

Thanks GK - I reckon I'll get a couple of the Gibson Historic POTS and try those.

Sounds like they would be preferred with or without any mods.

Credit to Realtarded for pointing that out right away.

[Paul-in-KC]

I ordered 4 of the Gibson Historic 500K pots.

I'm a little concerned that the threaded part of the housing that surrounds the shaft may not be long enough.

Though, I can't imagine that Gibson would have gone to all the expese to get the vintage taper correct and have the "neck" (whatever it's called) be too short to use in newer Gibson guitars.

I guess I'll find out in a day or two.

[redshark]

If your Les Paul guitar is a Gibson USA and not Gibson custom shop your pots won't fit. Historic pots are short shaft and all USA use long shaft pots. That a is a great reason to buy Heritage instead of Gibson!

[Reeltarded]

Credit to Realtarded for pointing that out right away.

It is life altering to play a guitar where you roll back the volume and the sound is still loud, but clean. The tone roll-off just isn't there on a good rig, which is almost impossible to build these days because it's not in a spec sheet?

Grrrr