Modifying Blackheart BH5H for more gain - diyAudio
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Old 29th September 2008, 08:26 PM   #1
pjpoes is offline pjpoes  United States
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Default Modifying Blackheart BH5H for more gain

While I've read from some that the Blackheart BH5H has lots of gain, maybe too much, I can not get it to distort on its own. Using any of my guitars (Including my Phiga with 3 humbuckers), so I don't agree that it has too much gain. Turned all the way up I would say I get a little bit of overdrive, but not even as much as the Fender tweed deluxe or Bassman could get turned all the way up. I'm wondering if there is a way to increase the gain from the 12ax7 to get more overdrive sooner.

As it stands I'm using it with my 2x12 cab with Celestion Vintage 30 speakers. I'm also using the amp in triode mode (3w) and not UL (5w). The tubes are inexpensive used ones I had, including a Hodges 12ax7 and Tungsol El84. I'm thinking about doing the turret board PTP wiring modification, which would allow easier modifications, greater space for parts like PIO caps, and easily add the tube rectifier.
Schematic
Any help on what parts need to be changed to increase gain and in what way would be great. I've found some sights on modifying the amp, but none for increasing gain. There is a discussion of the designers own boost mod, but I'm not sure what that is, I think its a treble boost.
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Old 29th September 2008, 09:09 PM   #2
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I would get more gain if I replace R6 in the above schematic with something like a 1K instead of the 1.5K right? Also changing R2 for something less than 33k would increase gain. I'm guessing those are the best places to target, but it seems like R6 and R10 are already pretty low. I'm not actually sure if R10 should be touched, but I know some guys are raising R2 and R10 to increase clean headroom for using with their pods, so I figure, the opposite would give me less clean headroom, which is what I'm after.
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Old 30th September 2008, 03:59 PM   #3
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None of the three resistors you mention would alter the gain at all, they are 'grid stoppers', simply there to help prevent instability.

There really doesn't look to be a lot of gain there at all, an extra stage would be a good idea.
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Old 30th September 2008, 04:37 PM   #4
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Changing R2 to something smaller WILL give you more output out of that 1st stage.

2 AX7 stages should be enough to put you into clipping with an EL84 with hot pickups for sure.

I'd check if either C1 or C5 are open or unconnected in some way.
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Old 1st October 2008, 03:29 PM   #5
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Quote:
Originally posted by leadbelly
Changing R2 to something smaller WILL give you more output out of that 1st stage.
Perhaps you would care to explain how?.
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Old 1st October 2008, 03:58 PM   #6
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Quote:
Originally posted by Nigel Goodwin
Perhaps you would care to explain how?.
Less voltage drop across the resistor. Check commercial amp schematics, invariably the only difference between the "HI" input and "LO" input is the value of the series resistor.
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Old 2nd October 2008, 11:43 AM   #7
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Quote:
Originally posted by leadbelly


Less voltage drop across the resistor. Check commercial amp schematics, invariably the only difference between the "HI" input and "LO" input is the value of the series resistor.

I suggest you check the schematics yourself - those are part of an attenuator, well before the valve - NOT the grid stopper which in no way affects the gain.
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Old 2nd October 2008, 03:23 PM   #8
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Quote:
Originally posted by Nigel Goodwin
I suggest you check the schematics yourself - those are part of an attenuator, well before the valve - NOT the grid stopper which in no way affects the gain.
R2 is NOT just a grid stopper, if it was, it would be 1.5K like the other grid stoppers.
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Old 2nd October 2008, 05:02 PM   #9
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Quote:
Originally posted by leadbelly


R2 is NOT just a grid stopper, if it was, it would be 1.5K like the other grid stoppers.
Grid stoppers don't need to be any specific value - it won't affect the gain at all, how do you imagine it could?.
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Old 2nd October 2008, 07:09 PM   #10
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Quote:
Originally posted by Nigel Goodwin
Grid stoppers don't need to be any specific value - it won't affect the gain at all, how do you imagine it could?.
By voltage drop through the resistor itself. You are conveniently defining gain at the tube pins for your purposes; step back and include the input circuitry and gain is not the same.
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