From john_g00–(at)–otmail.com Wed Aug 19 19:08:38 CDT 1998
Article: 122474 of alt.guitar.amps
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From: “johng”
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Subject: Re: More than one way to blackface an amp?
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Jim Kroger wrote in message …

yadda…yadda…yadda….

>My very knowledgeable friend John Greene said he clipped
>the power tube grid resistors (in his DR), changed four
>resistor values in the phase inverter, and did something
>with one of the caps in the PI. He also noted the increase
>in bass response. I’m a bit mystified as to whether the techs
>are doing something else in addition, or what.

Well, you almost got it right… 😉

I clipped the 1200pF snubbing caps on the grids of the power tubes. I
changed the 47K plate resistors on the phase inverted to 82K/100K. I changed
the 330K resistors on the grids of the phase inverter to 1Megs, and finally
I changed the coupling capacitor on the input to the phase inverter to a
.001uF.

The 1200pF caps on the power tubes suck tone big time and are definitely
worth taking off. IMHO, I think the 330K/1Meg & .01uF/.001uF change was done
by Fender to compensate for the lack of low frequency response in the Oxford
and later Utah speakers. It gives these speakers back some of the bottom end
but cause the amp to start distorting at a much lower level. Again, that’s
just my take on it.

(a generous helping of personal opinion follows…)

I think the change to 47K plate resistors was to try and compensate for the
loss of high frequency response the 1200pF capacitors caused. They didn’t
want to get rid of the caps because it would require more careful assembly
work to keep the output stage from oscillating, more careful work == higher
manufacturing cost. I tend to think whoever put the 1200pF caps in there to
begin with figured that with the series 1500 resistors the rolloff frequency
would be up around 88KHz, no harm-no foul. However, what wasn’t taken into
account was the impedance of the 1200pF being in parallel with the 220K bias
resistors and how it loads down the phase inverter. With the caps installed,
the gain of the phase inverter has a significant drop in gain at just over
1KHz. By reducing the value of the plate resistors to 47k, this corner
frequency moves up to around 2KHz with an overall gain reduction of a little
more than 3dB, better but not good enough.

>Is there a correct approach to blackfacing? Why, if
>they are all emulating the same schematic, is there
>disagreement?

There really shouldn’t. I think the disagreement comes in because of
conversions that are only done part way, i.e. the components are changed but
the speaker isn’t. When I first Blackfaced my 74 DR, I left the original
Utah speaker installed. The amp sounded thin and weak after the change. I
didn’t like it at all. So I changed the 1Megs back to 330K and the .001uF
coupling cap back to a .01uF. However, once I put a Naylor in there I
changed them back again and tone city!

The only thing left to make it a *True* BF conversion would be to replace
the power transformer with a BF transformer so I could use a GZ34 rectifier
tube. I suspect that the major difference this makes would be apparent more
when the amp is cranked, something I rarely do so I haven’t bothered with
it.

The best part about Blackfacing a DR is that it is just component value
changes, very easy to change back if you don’t like the results.

–john greene