Re: Spirt Harp Pro

[The Bernadettes <mdestefano@xxxxxxxx>]

Hambone Hamilton wrote:

> Scooter,
> Can you clue us in on what mods were made to make it harp-friendlier?
> Thanks.
> Hambone
>

I'm not Scooter, but since I recently serviced a Spirit Harp Pro amp I can clue you in on
what's under the hood.  First it's basically a BF Champ circuit, single ended, class A, 6V6
power tube.  The differences are a 5AR4 rectifier instead of a 5Y3, this will push up the
voltages and power a bit, also a good idea since the 5AR4 rectifier has a slow warm up, it
will protect the tubes from cathode stripping since this amp doesn't have a stand by switch.
It also has an adjustable mid range control instead of the fixed mid range resistor of the
Champ.  It also has a "Sparkle" switch which is the same thing as a "Bright" switch on a
Fender amp, this simply shunts a small value cap across the volume pot to restore upper mids
that are lost by attenuating a volume pot.  It also has a "Boost" switch, this switch
disconnects the negative feedback loop, reducing the speaker dampening effect of the NFL, this
adds a little more gain to the output stage.  The plate resistor on V2/2 pre-amp tube is
bypassed with a cap to reduce high end response.  A big difference is the addition of a gain
stage (1/2 of a 12AX7) with it's own volume pot on the input, this allows you to dial in some
pre-amp distortion prior to sending the signal further into the amp.  It does have a center
tapped filament supply to help reduce filament supply hum...
    This is the problem with the amp and why I got it for servicing, they hum pretty bad.
This is something I've heard from other owners of them.  Single ended class A amps are known
for hum problems but after recieving the amp, I too felt the hum was excessive.  It took me
awhile to dope it out but the culprit is a improperly implemented star grounding system.  Star
grounding is used to reduce ground loop hum in amps but if it isn't done right, it can make
the problem worse.  I was able to rewire a different ground scheme and use some shielded
wiring to reduce most of the hum and make the amp fairly quiet.  There is some slight residual
hum due to the transformers not being shielded very well, but it's not too noticeable, I
recommended to the owner that transformer replacement wouldn't be very cost effective.
    Other than that, the amp is fairly well made, nice cab, neat point to point wiring, good
solid chassis (nice, easy amp to service, should be very reliable).  The cascading gain stage
does make it a bit gainy and feedback prone.  I personally wouldn't have used that design for
a harp amp (great idea for screaming little guitar amp though), but would have used one less
tube to reduce cost.

Hope this answers your questions...
Don D.