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john kinder mod

11/20/2005 3:16 PM
mark h
john kinder mod
Anyone familiar with the "john kinder" harp mod for the tweed bassman? I know he performs the mod for a fee and I don't want to steal anyone's secret formula, but if this mod is commonly known.... I'd like to know what it is. Thanks...M
11/21/2005 3:56 AM
Mark Burness
I've never dealt with Kinder personally (tried once, but never got a response to my request for a final price), but I know a couple of guys who have. Kinder keeps his tricks close to his chest and verges on paranoia when faced with the possibility that an amp may fall into the 'wrong' hands.  
It is therefore extremely unlikely that you will ever see his mod posted on public forum like this.
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11/21/2005 5:24 PM
Rick Erickson

I can tell you this much - his mod sounds terrible with a guitar but pretty cool with a harp. Completely different approach to amp tone than a guitarist would take.  
11/21/2005 7:50 PM
mark h

Yes I agree. I work often on a harp player's gear and I'm never quite sure what he is looking for in terms of tone. It doesn't usualy make a lot of sense to me in guitar tone language.  
Thanks for the replies...M
11/22/2005 2:01 AM
Chris/CMW amps

I can hear you.  
Most of my customers are guitarplayers, and me too so about all amps I do modify and/or build are voiced for guitar. When a harp-player does want to try them with harp I just say no (but in a polite way, ha ha) and explain about the differences in voicing.  
When modding an amp for a harp-player it does sound wayyy more bassy/mushy when played with a guitar but great with a harp.  
In the past I did prototype an amp for guitar AND harp; 2 channels, pretty ok but still a compromise inho without installing an extra harp/guitar-switch.  
In general:  
- pretty low plate- and/or pre/pi-node-voltages  
- big couplingscaps  
- big cathodebypasscaps  
- not too much highs  
- 8" or 10" speakers  
11/22/2005 3:07 AM
Mark Burness

As far as I know the Kinder Bassman mod runs stock voltages and coupling caps. However, Chris has some good suggestions.  
This is what I do to RI Bassman (plenty of happy customers)...  
I actually go the other way with plate voltages than most and fit a solid state rectifier (occasionally GZ34), usually ending up with around 500 on the plates (475v for an early RI). More power, low end & touch sensitivity. The lower preamp voltages stop things from sounding too glassy.  
Stock Sovtek 5881WXT - Drier tone than some sweeter tubes that may be favoured for guitar, but this works well for harp live, stops the tone from getting too wet & garbled.  
Replace R40 (22K in RI, 15K on original/repro)with 6.8K resistor in series with a 10K single turn, top adjust, cermet pot, wired as a variable resistor. Bias can be set to taste, anything from 8mA up to 30mA (especially if they want good fidelity, though most don't). Most people seem happier with lower currents (<15mA), less feedback, more grind. Not good for guitar as you need the greater harmonic content of high current to make the note, most of which is decay, sing. Harp doesn't have decay, so it isn't really a problem with a 4x10. I wouldn't recommend it accross the board for other speaker compliments though.  
Upgrade screen supply filter caps to 2x100uf, giving 50uf @ 700v or better.  
Replace 10K preamp dropping resistor with 100K 3W.  
Replace .022 preamp coupling caps with .1uf.  
Go up on input grid load resistor (V1) to 5-10Meg. Theory suggests that this favours crystal & ceramic mics, it actually seems to improve dynamic response of any mic I've tried except the Shure 520DX, which seems happier with 1Meg. I would suggest that this mod is more instrument sensitive than mic sensitive.  
Preamp tubes - I don't actually like to go too low on preamp gain, due to losses in low end & dynamics. I always keep a 12AX7 in the PI. On a later re-issues I keep another 12AX7 in the tone stack, or sometimes a 12AT7. 12AY7 in the preamp. Some early RIs & reproductions run lower B+ (say under 475v), in these I tend to keep a 12AX7 in the preamp and sub out the tube in the tone stack for a 12AY or 12AT. Of course, the mic you use will have a big bearing on this, but most crystal mics seem happy with a combination of 2x12AX & 1x12AY. Use the Sovtek 12AY, I prefer JJ 12AX7s.  
I prefer the speakers running reverse polarity for more crunch and my own amps have 470K bias feedback resistors rather than 220K.  
This isn't aimed at those looking for a sweet, chimey LW "Thunderbird" tone, more a fat sounding, useful stage amp.
12/2/2005 1:00 PM
Rick Erickson

Speaking of Harp mods and such - I recently had a "Harp Modded" RI Bassman amp in my shop which had 12AU7 preamp tubes installed in it. The result of this was that the 1/4W plate resistors had burned and become out of tolerance & noisy. Something to be aware of when blindly swapping tubes - the voltage drop across the plate resistors and cf cathode resistors can become excessive and cause damage to your amp.  
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