Touch sensitive? Oh yeah...

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by JimmyR, Feb 8, 2008.


  1. JimmyR

    JimmyR Member

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    You'll have to forgive me - I'm pretty pumped right now. I've been modding the heck out of my Bad Cat Hotcat for a few months now. I have always loved the clean channel but felt that the dirt channel was too compressed. So being the sort that can't leave enough alone, I've benn in there changing stuff around.

    Well I finally have it where I always wanted it. The clean channel is same as always, but now the dirt channel is a cross between a DC30 first channel and a Bad Cat Two-tone pedal. So I no longer have an "edge" or "level" knob on there. I have used the second tube as a DC30 style V1 instead of a clipping tube. The tone stack remains untouched and the master vol works just on the dirt channel as always. But now I have the most amazing touch responsive, fat, clear, punchy, toneful dirty channel I have ever heard. It sounds a lot like the first channel on a Wildcat but fuller.

    With my 335 I can play softly and get piano-like clarity from the bass strings, then just dig in a bit and get plexi-like snarl from the trebles. It's pretty much my ideal amp. There's enough gain on tap to get into ACDC territory or a bit further and that's enough for me. Before I was getting too much treble or too much bass or the tone was a bit too grainy - now it's perfect.

    So I just had to vent somewhere! If anyone is mad enough to want to mod an expensive Bad Cat amp I can draw up a schematic of what I've done if you like! Sometimes I feel like I'm nuts to mod an amp like this but right now I'm glad I did. If you want to know any more just ask.
     
  2. conundrum

    conundrum Member

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    Did you leave the first stage grid-leak biased?
     
  3. David Robrecht

    David Robrecht Blue Fire Silver Supporting Member

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    I'd like to see the schematic if you want to spend the time to draw it.
    Dave
     
  4. JimmyR

    JimmyR Member

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    Probably the easiest way to describe the changes would be to look at a schematic of the DC30 Ch1 preamp. The layout is now identical and all the values are the same afrom the V2 cathodes on - ie the tone stack is identical and unchanged from the Hotcat's stock. All that I changed on V2 was the first cathode's bypass cap - from 25uF in a DC30 to 1uF now.

    The first tube is where all the changes are. By this I actually mean V2 of the Hotcat - but it's V1 of the dirty channel. I removed the "edge" and "level" pots completely and put "gain" where "level" was. I have used a 100K pot for gain with the 180pF bright cap.

    So now the input goes to the usual 68K input resistor with the 1meg to ground. The 68K goes to pins 2 + 7 as per DC30. The plates are parallel linked and go to B+ via pin 1 and a 220K resistor as per DC30. Pin 6 also links to the gain pot via a .0015 cap salvaged from the original circuit. Pin 3 cathodes are grounded via a 1k5 and .1uF while pin 8 cathode is grounded via a 1k5 and a .022.

    There may be a more elegant way to get the frequencies in the right areas but through a lot of trial and error this is what works for me.

    I found if I left it at regular DC30 spec it was too bassy and a bit raspy, hence the tiny cathode caps. Then it sounded a bit congested so I upped the cap going to the gain pot from 500pF to the .0015uF. The 100k gain pot has helped to dial out the low end - it is still too bassy with the 500k pot. But changing pot values didn't alter the amounts of gain as I expected it to. So I like it as it is right now.

    The tone is good now - with bass at 12.00 or lower it's just right. The biggest improvement for me is the lack of compression compared to the stock Hotcat. It seems a lot clearer and punchier now. I liked the Hotcat stock but found myself using the dirt channel less and less. If you like old plexi sounds with the punch etc then you may well like this version of the amp. If you prefer a lot more gain then leave it stock.

    David I could draw it out but then I have no way to email it to you - no scanner since it blew up sorry! I'll try to think of something.

    Conundrum - sorry - I don't know what grid-leak biased means, but hopefully what I have described will tell you! I think I got my DC30 schematic from schematicheaven.com. Thanks for being interested. I am stoked at how the amp sounds but have no-one here to tell!
     
  5. JimmyR

    JimmyR Member

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    Ok I've drawn up a crude schematic - if you PM me maybe I could fax it to you?
     
  6. David Robrecht

    David Robrecht Blue Fire Silver Supporting Member

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  7. JimmyR

    JimmyR Member

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    Ok David it's on its way! I changed it a little after playing my 6120 through it. Now it seems to like the Gretsch a bit better also. I worked out that the .1uF cap on the input of the original Hotcat circuit smooths out the top end a bit, so I've reinstated it.

    Compared with my '68 spec 50W plexi it's capable of similar tones with a 335 but at lower volumes and it's a bit more tactile and smoother. If you want a bit more raunch just remove the .1uF input cap.
     

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