How to soften the 'attack' of tube amp?

[bhuether]

I have a Koch Powertone II. Great sounding amp but on the lead channel I am hoping to soften the attack. Not sure if that is even the right word. Right now when I pick notes, there is almost no response time - the note just jumps out. I tend to prefer a softer attack which I also think makes for better dynamics. I am hoping to do a mod where I can adjust the attack.

Anyway, I have a schematic if anyone is interested.

thanks,

brian

[black_labb]

attack tends to come from the highs, so reducing the highs could be a method. do you know what type of tone stack is in the amp? could be that the mid control is stuck on 0? (boom and attack that is exentuated by the speaker being a flatter response than usual instrument amps)

[bhuether]

If I reduce the highs the attack is still there though muted of course. What I am talking about is a dynamic sort of response which suggests that there is a circuit likely with resistors/capacitors/inductors that can be adjusted to speed up or slow down the response time. But I would have no clue where to look...

Here is the schematic:

www.guitar-dreams.com/misc/KochPowertoneII.pdf

Maybe the place to look is page 1 lower left in the gain/ultra gain section?

thanks,

brian

[Robert M. Martinelli]

If I reduce the highs the attack is still there though muted of course. What I am talking about is a dynamic sort of response which suggests that there is a circuit likely with resistors/capacitors/inductors that can be adjusted to speed up or slow down the response time. But I would have no clue where to look...

Maybe the place to look is page 1 lower left in the gain/ultra gain section?

thanks,

brian

Nope, there' s nothing to be adjusted, your amp's "dynamic" behavior comes from : ( just what I have in mind right now, the list could be incomplete )

1 - solid state rectifiers - when the current demand rises, tube rectifiers tend to drop more voltage, so the amp's headroom is reduced by this phenomenon ( called "rectifier sag" ). Solid state rectifiers drop a very little voltage in comparison, so the amp's headroom stays unchanged even under loud transients ( notes' attack ).

2 - "big" ( value ) filter capacitors - These are a sign of a careful design and a sign of quality as well, ( and they're quite expensive for the builder to throw in ), in your case they act like a big current reservoir that releases its "potential" improving the amp's dynamic behavior.

3 - external bias ( non cathode bias ). Cathode bias tends to vary under dynamic conditions ( playing ) because the voltage drop at the cathode resistor(s) increases with current, so, when playing, less voltage drops across the tube ( as more voltage drops across the cathode resistor(s) ) and this limits the amp's dynamics, softening and sweetening the attack. External bias does not change when playing, so the amp headroom stays unchanged, and the response to loud transients is improved.

All the above factors indicate that even a well designed-well built amp can
"fail" to meet your expectations, because, as usual, there's not a "universal recipe" for amp building, it's all about taste, "what sounds good is good", and this teaches us that the "mojo factor " we find in playing old amps often comes out from something that some today's designers would call "imperfections".

If plenty of "attack" is your problem, you could try a compressor/limiter between the guitar and the amp, softening the attack to your liking.

Hope this helps

Best regards

Bob

[bhuether]

So you don't think a change in one of the areas you describe (maybe bias)would alter the dynamics? If I did change something I would want it to be easily reversible.

thanks,

brian

[Chuck H]

Unless your cranking the amp for gain the things that soften attack in vintage type amps, like tube rectifiers, smaller filters and cathode bias won't help all that much. Since you indicated the lead channel as the problem I think it could be adjusted more to your liking. But it takes time and patience to tweak high gain circuits. Lack of experience will make it more difficult. But since your not happy with it now...

I don't know what it's like inside that amp. If it's hard to work on or not. But I have some suggestions if your interested in taking a soldering iron to it. All reversable of course.

Chuck

[Robert M. Martinelli]

Hi Chuck,
well, telling the truth, when bhuether said that the notes were "jumping out" out the amp I figured he was playing with the amp cranked or at least at high volumes, so I tried to give him a rough idea about some of the the factors involved in "dynamics", and to help him understand what makes an old amp "sweeter".

OTOH, as you rightfully say, if he doesn't like the amp, then it's probably time for him to "smell some solder fume" ( thanks Enzo )

Best regards

Bob

[Chuck H]

I wasn't thinking anything contrary to your advice Bob. I saw that he indicated the "lead" channel and took it for a clue. So I looked at the schem and saw the way the gain/ultra gain channel was wired. I have some experience with the same problem. One of the first amps I ever modded was a Marshall 1959 RI. It was around 1990 and I was after that really high gain sound. I stacked the channels and added a master volume but I couldn't get that "sweetness" from the amp without some experimentation. Actually a lot of experimentation. But I've done it for myself so I thought I may be able to help.

My most recent creation is my best yet and employs aspects of everything you covered to get the response just right. It's a non master vol. amp and I hope to use some sort of built in attenuator system since I have an unnatural aversion to "power scaling". It is sooo good that I come away with a big stupid grin every time I play through it. I actually got to show it off at NAMM this year. Great experience.

So, truth be told, I think the best you can do is design the whole amp to respond the way you want. But I also know some tricks to circumvent the limitations of the spanky, big watt amps.

Chuck

[Robert M. Martinelli]

Don't worry Chuck!
We were only saying different things.... ( both true )

I just wanted to pinpoint ( and explain him in a simple way ) some of the main factors that contribute to make the amps "big and spanky", while you were trying to explain that there are some tricks to be tried to overcome his problem and make his amp sweeter ( and I'm sure you indeed have many rabbits inside your hat ).

BTW, I followed your "NAMM adventure" and I wish you once again all the best and a huge success for your amp ( though I'd have named it "Chuckinger" rather than "Challenger", to better honor its creator ).

Cheers

Bob

[Bruce / Mission Amps]

... BTW, I followed your "NAMM adventure" and I wish you once again all the best and a huge success for your amp ( though I'd have named it "Chuckinger" rather than "Challenger", to better honor its creator ).

Cheers

Bob

I'd change the name too Chuck.... there are a few other amps already named that. Even I have one that is called the Challenger R/T (Reverb and Tremolo).
I'm not even that much of a Chrysler fan but always liked that name.

[bhuether]

I guess maybe I should have described the amp as super tight. For instance, I have another super tight high gain amp (Diezel VH4S) and a 'looser' sounding high gain amp (Diezel Einstein). I am trying to get this Koch Powertone II amp to have some of the looseness of the Einstein.

thanks,

brian

[Chuck H]

Thanks Bob, Bruce.

Well... Now that I know there is already another amp with that name...

I don't think we're married to the name Challenger. It's funny you mention the automotive connotation. As one idea I wrote down a bunch of classic and muscle car names as a possible theme. Dean liked Challenger and Bel Air. Since Carvin is already using Bel Air (very visibly) we went with Challenger. I didn't find any current production amps with that name and the only one I knew of was the old Bogen P.A.

I would like to change the name and the cosmetics on the amp.

Brian, I do have some ideas for your circuit if your interested in soldering.

Chuck

[bhuether]

Hi Chuck,

I am definitely interested to hear your ideas.

thanks,

brian

[Chuck H]

Cool. The way the gain channels are structured now drives alot of treble through the stages and then dumps it at the end of the chain. This makes for a very attack forward feel. These changes will trim the top end through the circuit and push more upper mids. You'll also be increasing resistance in the power supply for the preamp. Overall this should give a less sharp and more compressed feel to the preamp. This will change the clean channel a tiny bit but probably not for the worse. These are simple mods that are very reversable so save your parts.

I don't know how much electronic experience you have so I have to start out with:

Never work on your amp with it turned on or even plugged in. Always drain the power supply before working on the amp. To drain the power supply you can use a piece of lead wire with an alligator clip on each end. with the amp unplugged and the standby switch in the "play" position clip one end to the chassis and the other to pin 3 of any power tube socket and leave it there for several minutes. Then use a volt meter set to DC to test the voltage between the chassis and the + end of C55. You should not read more than a couple of volts. With your amp it's actually easier but this is a foolproof method for most amps so just learn to do it this way for now.

Lift one end of C9

Change C10 to .022uf (22n) film 400V or better

Change C11 to 2.2uf electrolytic 10V or better (this is a polarized cap so be sure to observe correct polarity)

Change C12 to 1500pf film 200V or better

Lift one end of C13

Change R18 to 220k 1/2 watt

Change C15 to 2.2uf electrolytic 10V or better (this is a polarized cap so be sure to observe correct polarity)

Change R25 to 220k 1/2 watt

Change R121 to 15k 3watt

If these changes drop too much high end change C16 to 100p
If there is a loss of gain change R15 to 1.5k

Disclaimer: It can be tricky to adjust high gain preamps and I am attempting to do it remotely. If there are bugs we can work them out. If I die tomorrow you can always put it back stock.

Chuck

[bhuether]

Interesting! Should I get the parts from any electronics supplier? And will a 40W soldering pencil iron be sufficient?

thanks,

brian