Reducing gain in a wreck?

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

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DaveMon
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Reducing gain in a wreck?

Post by DaveMon »

Hello all,

I have a wreck that I added a 33k resistor to VI from the input, as well as a 680 ohm resistor from V1 to the wiper of the volume pot. This has reduced the gain somewhat, but I am interested in further reducing the gain and break up...... to give me more of the awesome cleans I am hearing at extremely low volumes.....

My goal is to make the wreck more "pedal friendly" .......the above mods have helped tremendously ( I got the idea here at TAG ) but I need a little more reduction.....should I up the values on the resistors?

Ultimately, I would love to switch these resistors in and out of the circuit via a switch......and go from normal Uber Gain mode.....to something much more PA amplike for the purpose of a pedal platform.......

thanks in advance
strelok
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Re: Reducing gain in a wreck?

Post by strelok »

You could try lowering the value of the 150k grid leak resistor for the third gain stage. In my experience that resistor has a significant impact on how much gain the amp has, assuming you're talking about the express.
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DaveMon
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Re: Reducing gain in a wreck?

Post by DaveMon »

Sorry, yes this is an express, we are talking about.
Thanks for.the.tip, I will.try it tonight!
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martin manning
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Re: Reducing gain in a wreck?

Post by martin manning »

Mine has a gain control at that location, a 100k trimmer with a 47k to ground, and the wiper feeding signal intoV2a's grid. It's best to maintain the 150k from the coupling cap to ground so that the frequency response and AC load on the second gain stage are unchanged as you reduce the drive into the PI.
tictac
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Re: Reducing gain in a wreck?

Post by tictac »

12AU7?
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DaveWell
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Re: Reducing gain in a wreck?

Post by DaveWell »

Leave the amp circuit as it is.
Play around tube in V1 and V3

Here's a Gain Factor Chart:
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Last edited by DaveWell on Sun Apr 09, 2017 2:21 pm, edited 2 times in total.
andresound
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Re: Reducing gain in a wreck?

Post by andresound »

5751 in PI... my 2c
If it sounds good, it is good! Trust your ears
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sliberty
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Re: Reducing gain in a wreck?

Post by sliberty »

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.
Bob S
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Re: Reducing gain in a wreck?

Post by Bob S »

All the above are great ways to adjust where the clean to mean happens.
The stock circuit clips from back to front - power/PI/pre. Worth keeping in mind.
Skill with the guitar is a must to bring out the best.
The Express is a humbling experience for a hack like me.
:oops:
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Mark
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Re: Reducing gain in a wreck?

Post by Mark »

Dave, I think ultimately you have the wrong amp. I would leave the amp stock, and do what Ken Fischer did and put a 12AY7 as V1. Perhaps RJ's Rocket with KT-66's would be more to your liking?

Should this not work for you and you need to modify the amp, then implement the switch in the Komet for reducing gain. Only implement switch and the plate resistors.

http://ampgarage.com/forum/files/Expres ... iation.jpg
Last edited by Mark on Mon Apr 10, 2017 9:17 am, edited 1 time in total.
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rp
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Re: Reducing gain in a wreck?

Post by rp »

Can always add a low input :shock: You can progressively emasculate it back into a BF Bassman, but I agree that means you got the wrong amp. Sell the express at cost to someone and build a rocket for pedal friendliness. But if you insist try 5751 & 12AY7 (real ones) in V1 first. For more cleans maybe a 12AT7 for V3 with later SF values? The latter would be a fun experiment.
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M Fowler
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Re: Reducing gain in a wreck?

Post by M Fowler »

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
shipwreck
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Re: Reducing gain in a wreck?

Post by shipwreck »

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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geetarpicker
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Re: Reducing gain in a wreck?

Post by geetarpicker »

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
Mark
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Re: Reducing gain in a wreck?

Post by Mark »

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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Mark Abbott
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