Reducing gain in a wreck?

Express, Liverpool, Rocket, Dirty Little Monster, etc.

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sunnydaze
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Re: Reducing gain in a wreck?

Post by sunnydaze »

sliberty wrote: Sat Apr 08, 2017 12:18 pm I did this by installing a split plate load on the third (I think) stage. You can modify the split to taste (50k/50k, 22k/82k, 82k/22k, etc). I love the results. This can also be switchable, whee one position engages the split, and the other position goes back to stock.
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M Fowler
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Re: Reducing gain in a wreck?

Post by M Fowler »

sunnydaze wrote: Mon Apr 10, 2017 8:56 am
sliberty wrote: Sat Apr 08, 2017 12:18 pm I did this by installing a split plate load on the third (I think) stage. You can modify the split to taste (50k/50k, 22k/82k, 82k/22k, etc). I love the results. This can also be switchable, whee one position engages the split, and the other position goes back to stock.
Plus 1 - Like this one.
Mike I haven't tried this mod yet but should work well.
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M Fowler
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Re: Reducing gain in a wreck?

Post by M Fowler »

shipwreck wrote: Sun Apr 09, 2017 9:45 pm How is the treble cut pot circuit implemented, specifically (a diagram would be nice)?

And what exactly does the original precence circuit do? It's always been such a vague term to me, precence...


M Fowler wrote:Good suggestions.

Reduce the 150k grid leak resistor in 3rd stage gain circuit as mentioned to reduce gain.

5751 tubes in either V1 or V2 and 12AT7 in V3.

I replace the presence with treble cut pot circuit and ground the PI tail resistor.

Presence circuit reduce the 100k to 68k to tighten up amp.

PI tail resistor 1ok can be increased to 27k or even 47k for less gain.

Mark


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Look at the layout and schematic for the Trainwreck Rocket it uses the treble cut pot.

Presence is a potentiometer controling the amount of negative feed back (NFB) from the output of the amplifier back to the phase inverter (PI). The amp will be cleaner with more NFB..
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DaveMon
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Re: Reducing gain in a wreck?

Post by DaveMon »

Huge thanks to all who responded!!! I managed to scrounge up a 12AT7 that was branded "Randall" of all things, from my junk box. This alone got me in the ballpark...... Additionally, I am inclined to agree that the express is probably the wrong amp for this.......... however I plan to experiment, with the other suggestions mentioned......
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Re: Reducing gain in a wreck?

Post by Roe »

geetarpicker wrote: Sun Apr 09, 2017 10:23 pm To me the point of a Express is that it doesn't need any pedals in the gain and overdrive department. Still you need to have the smoothest taper guitar pots available, and have the patience to learn how to use the guitar knobs a LOT to find the cleans. Also it takes a very proper Express build to make one that will truly clean up well with little volume drop. Still, I could see how one would miss ambient effects like delays and reverbs, but a wet/dry setup will work great with an Express for that. GK
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bcmatt
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Re: Reducing gain in a wreck?

Post by bcmatt »

M Fowler wrote: Sun Apr 09, 2017 5:49 pm Good suggestions.

Reduce the 150k grid leak resistor in 3rd stage gain circuit as mentioned to reduce gain.

5751 tubes in either V1 or V2 and 12AT7 in V3.

I replace the presence with treble cut pot circuit and ground the PI tail resistor.

Presence circuit reduce the 100k to 68k to tighten up amp.

PI tail resistor 1ok can be increased to 27k or even 47k for less gain.

Mark
Hi Mark,
You've mentioned going to a cut control and ditching the presence before. So do you not use any NFB at all?
I'm a bit intrigued to try this. I suppose one could play with that PI tail resister value to balance out the increase in gain resulting from eliminating the NFB? Is that what you did?
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M Fowler
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Re: Reducing gain in a wreck?

Post by M Fowler »

On two of my Express combo amps and one head I removed the presence pot and used a treble cut pot and .0047 cap.

The tail resistor goes to ground.

No NFB.

Two of the amps are gigged weekly. An attenuator is used and had asked about VVR install which I don't do, never used a VVR though I think I have a cathode bias VVR from Dana.

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sunnydaze
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Re: Reducing gain in a wreck?

Post by sunnydaze »

M Fowler wrote: Mon Apr 10, 2017 4:26 pm
sunnydaze wrote: Mon Apr 10, 2017 8:56 am
sliberty wrote: Sat Apr 08, 2017 12:18 pm I did this by installing a split plate load on the third (I think) stage. You can modify the split to taste (50k/50k, 22k/82k, 82k/22k, etc). I love the results. This can also be switchable, whee one position engages the split, and the other position goes back to stock.
Plus 1 - Like this one.
Mike I haven't tried this mod yet but should work well.
Hello Mark, This one works great. Have mine on a mechanical switch. Better to switch while in standby.
Mike
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bcmatt
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Re: Reducing gain in a wreck?

Post by bcmatt »

M Fowler wrote: Fri Apr 14, 2017 2:57 am On two of my Express combo amps and one head I removed the presence pot and used a treble cut pot and .0047 cap.

The tail resistor goes to ground.

No NFB.

Two of the amps are gigged weekly. An attenuator is used and had asked about VVR install which I don't do, never used a VVR though I think I have a cathode bias VVR from Dana.

Mark
Thanks Mark. I guess you were completely clear from the start. I guess I just didn't believe it was that simple. I'm gonna try this on my Liverpool tomorrow for fun.
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geetarpicker
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Re: Reducing gain in a wreck?

Post by geetarpicker »

Somehow I tried to quote the original question, but it got separated so I'll cut and paste so that it makes sense!


"geetarpicker wrote:
Also it takes a very proper Express build to make one that will truly clean up well with little volume drop.

The Express is an amp of few parts, and a common complaint of the Express is that it doesn't clean up well. As far as I can tell transformers and valves are important, but what else should the builder be looking at when looking at making a "proper Express build" Glen?

I did ask you about pot taper a while ago, what was Ken's preference. I'd imagine largely 10% taper apart from the treble pot and perhaps the mid pot.

Look forward to your reply.



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Yours Sincerely

Mark Abbott"

_________________________________________________________________________________________


To be honest I only really know it when I hear it!

That said, one issue is the modern analog taper pots these days come on really quickly. This means the volume and treble "analog" taper pots will come on more quickly than normal, and the traditional "all knobs on 1/2" may not apply very well. Set a clone with modern analog taper pots all on 1/2 may actually sound more like the knobs are 3/4 of the way up vs an original KF built Express... So one trick to simply get the gain down is don't turn the volume and treble up as far. To demonstrate a point, I have an original NOS spec Express pot here on bench to measure. It's marked "1M" but reads a bit high at about 1.25m. Also if you turn the control to the 1/2 way point the resistance is only ramped up to about 90k. That is even less than 10% of the full value. It seems most modern "analog" taper pots will ramp up more quickly than that, so one might need to be more conservative with the amp control settings.

The same issues goes for the guitar pots, especially with humbucking guitars. The old Gibson pots in vintage 50s-60s guitars had a quite smooth gradual taper in the first half of the rotation, then they really ramped up toward the 3/4 to full up range. My old Centralab pots are around 10% taper pots, meaning a 500k it only up to 50k maybe 60k when on half. This spreads out your control range on the lower end of pot, making the sweet spot for cleans (on the guitar) easier to dial in typically in the 3-6 range. A modern pot may come on so quickly that you have to back the guitar down into the 1-3 range, and it will be a touchier and less spread out range for the cleans. For some reason it seems many modern day 250k analog taper pots actually have a pretty smooth range, so there not as tricky to control. The issue is more the modern 500k pots that humbuckers usually like. That said, the new Gibson 500k Historic pots (that you can easily find separately online) actually have a pretty decent taper, best I've tried in a modern day pot. Shame they don't make a 1M Historic pot.

I'll mention that I run 50s wiring in my Gibson guitars, and it makes a difference in clarity when you roll back the controls. Also though my Gibsons will get muddy when first turned down a number or two, if you continue to roll the volume down further the high end actually nicely reappears. So, the trick is to use enough gain that to get cleans you have to roll the volume back passed the initial muddy point. Get the gain right on the amp, so you can ramp the guitar down to the 3-6 range where the clarity is great but with enough gain it's still loud enough to be usuable.

Another thing I noticed in my Express amps is they seem to clean up better running the amp actually on the bright side. I have found the best clean to mean range is running the full (500pf) bright switch setting. This can actually be a bit too bright with some tubes and speakers but it seems to be where the amp wants to be. The trick then is to darken it back down in the rest of the signal chain. For my '89 Express I often use 40' of cable on the input (Mogami or Canare), darker sounding preamp tubes (old Sylvanias) and a somewhat darker sounding old 4x12 with G12Ms that tend to be darker than say G12H speakers. These speakers also have a bit more than typical natural compression, which helps the clean to mean range stay more even volume wise. The trick though is you have to crank the rig and push the speakers to get there.

Hope that helps for now!
GK
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Re: Reducing gain in a wreck?

Post by Mark »

Hi Glen thanks for your reply.

I don't have a problem getting 1M audio pots with a 10% taper. I have CTS pots, PEC pots, and Clarostat pots that have a 10% taper. I'm not big on the treble pot having a 10% taper as the treble control does nothing from 0 to 50% on amps that use this taper. I tried a clone that used the Clarostat pots with the 10% taper throughout the amp and I couldn't get over how dull it sounded. I was expecting the amp to be quite bright, the amp didn't sound awful but it didn't have that Express magic.


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bcmatt
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Re: Reducing gain in a wreck?

Post by bcmatt »

I'm currently using Gibson Historic 500K pots on not only my humbucker guitars, but also as the volume control on my Express and Liverpool (in series with a 470K resister).

I just finished some of these mods on my Liverpool. I really wanted to try Martin's trimpot idea, but my Liverpool is already using a 68k resister there, and from what I've read, people seem to prefer that. I might experiment with that one in the future.
I did go to a split plate load on a switch 82K/22K on the second gain stage. It definitely works well for cleaning things up, but I like the option of reversing it for gainier tones.
I removed the presence and replaced it with the rocket cut control, that I find WAY MORE useful. This is also a push/pull pot for my plate load switch.
I did like the sound/feel of the no NFB, but not as a full time commitment. It would be great if it were only played with single coils. Humbuckers can't get entirely clean without a bit of help from NFB. So for now I have a 33k Resister over to a 500K pot on the top of the chassis near the back that feeds over to the tail resister before the 4.7K to ground. I think I'll figure out a few favourite settings and replace it with a 3 position switch. 33K is a little too much NFB, making it a little too sterile. I might end up with something like the choices of 56K, 100K(stock), and 470K (no NFB). For now I'll just keep playing it. I'll mark and measure my frequent setting in the near future for the transition to a more user-friendly switch.
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