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In Reply to: RE: Here's a picture of an older one. posted by Tre' on March 13, 2017 at 12:11:49
Thanks for posting the pix of one of my old amps.
That one had very good bottom-end response, and
followed the musical groove with alacrity, although
further improvements have been made to date.
It exhibited an extended high-end response that
was down about 1.5 to 2.0 db at 40KHZ, depending
on the output tube chosen. Rolloff STARTED at about
15KHZ, but was down very little at that point, with
a Single-Plate 2A3..
Perhaps some comments on the Buddy Amp that you
post can be of some help to forum members:
Here goes-- I hope I don't leave anything out!:
See that long black A.C. wiring going into the
rectifier filament transformer? Why not locate the
transformer between the A.C. and the tube sockets,
greatly shortening the Line Power leads. This keeps
crap from the line out of the aluminum chassis.
Two rectifier tubes in parallel? No. Use ONE.
You can't use a 5R4. The filament has too small
a surface area, and can't deliver enough instantaneous
pulse current on top of the idling current..
Use a MODERN 5U4GB. (Example: JJ 5U4GB, or
I can only guess, but it looks like the filter
cans are probably 70uf or so. C1 and C2. Choke-
input is nice here, but those caps should be
about 35uf. Oils are OK here.
See that big Oil Can as C3? Better get THAT
out of there or your Highs are GONE!
Looks like you have a nice Plate Choke on the
driver tube-- watch its value carefully. Make tests.
What is that red cap? Perhaps 4uf Dynamicap?
Excellent! Needs a few more small bypassers to
extend H.F., or you'll lose some highs here....
Driver plate resistor. Get one that can pass
high-energy pulses accurately. I suggest the
best 2-watter that money can buy.
I don't see any form of voltage regulation on the
driver plate supply-- can't tell exactly from the
picture, but it looks like only a voltage-dropping
This CANNOT work with a high plate-resistance tube
with high gain and low plate current.. The power supply
there must be stiff, and have lots of current available.
A series voltage dropper resistor KILLS the driver stage
thru voltage sag, and current starvation. You won't get
enough power to properly drive the 2A3 grid. The amp will
be a "wimp"-- both power-wise, and bandwidth-wise.
Driver cathode bypass: Get that Solen out of
there and get something good-- another Dynamicap
with extensive, careful bypassing, or use an
Audionote copper foil, etc. Don't do it and kiss
your highs and musical swagger goodbye....
That ground on the cathode bypass is crap--
all that other junk on that ground rail before
it gets to the driver tube, which is the most important part
in the amp! Re-locate the whole layout/grounding scheme.
See the driver tube filament leads? Those run right
parallel to The cathode SIGNAL leads from the 2A3 filament
into the filament transformer. THIS layout has DESTROYED
H.F. response in the amp! Re-Do the whole layout!
2A3 cathode bypassing: Get that SOLEN OUT OF THERE!
Cheapo! Another place for a bypassed Dynmicap. Or kiss
some more highs goodbye...
2A3 cathode resistor: This is a high-current pulsed
application, so use either two exactly the same resistors
(Mills, etc.) in parallel or use one 50 watt heat-sinked
inductive wirewound power resistor. Smaller sizes WILL
result in reduced dynamics and bandwidth.
See the 2A3 filament transformer mounted right next to
the filament transformer for the driver tube? The 2A3
filament trans contains the cathode signal on the 2A3.
DO NOT mount ANYTHING near the 2A3 filament trans. Re-do
See how close that 120VAC pilot light is to those two
transformers? What do you think the 120V is doing to
that whole area? Re-do the layout! Put all 120VAC into
one end of the chassis, isolated from all else.
Grounding system as a whole: It can't be a series string.
It needs to spread-out in increments from a center point,
that is bolted to the chassis at a strategic location only.
Re-do the layout, starting at the A.C. line ground
2A3 and driver tube sockets: These will sound better when
they have some physical means of isolation from the chassis.
Aluminum chassis: This metal will sink high frequencies into
attenuation, but will also add a "whitish" (old audiophile term)
coloration to all music. This is probably a result of aluminum
reacting against the steel in transformers and chokes.
For this reason, wood is becoming popular as chassis material.
Steel is better-- much better. It is the same thing as the
transformers-- that is very good, but transformers have to be
elevated above the steel on non-steel mounts. This is to prevent
transformer vibrations from shaking the steel chassis, and
smearing/reducing high frequencies.
Look at my older amp picture. In it, you'll find all of these
things, and much more, done right.
There is one mistake in my old chassis-- much has been improved
since then, but there is one glaring mistake in it: the H.V.
output leads from the plate transformer are twisted together.
That mistake wasn't made in the Buddy Amp.
All of us can still learn... That includes myself.
Thanks for sharing your observations Dennis.
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