FeathurAmp1958TweedNatl

1958 Tweed National amp

Here's a cool amp...A 1958 National Amp with two 6" X 11" Rola speakers, a somewhat rare and great sounding little combo. It came
with a tube line up of 1-12AX7, 1-6V6GT, and 1- 5Y3GT, and has 2-instrument inputs, 1-microphone input, Volume and Tone controls,
pilot light and 2 amp fuse. This one is serial # T8525. The two Rola eliptical speakers are date coded 285836 and are also stamped
10120 on the speaker frame. Both Alnico magnet covers have 550-20 on them, the 550 number on the speaker denotes the speaker
was made for Valco, and you will see this 550-# on most all Valco amp speakers that are still original to the amp. So, we have just a
simple little Champ/Princeton type circuit here, right? Not really, same tube line up, but a few twists to the circuit from the Valco
engineers. The first gain stage triode of the 12AX7 has the old style grid leak bias set up. Most every manufacturer had abandoned
that and went to the cathode bias circuit at this period in time....but not Valco. The grid leak bias circuit works by sending the input
signal thru a capacitor to the grid and using a high value resistor (6.8 meg in this amp) coupled from the grid to ground, causing a
negative voltage to be developed on the grid of the 12AX7. The cathode of the 12AX7 in the grid leak bias set up goes to ground. As
long as the Grid is negative with respect to the cathode of the 12AX7 and within operational parameters of the tube the tube is biased
properly. The amp also has no negative feedback like most Fender amps do, and in this circuit, there are no bypass caps on any of
the cathodes. Valco also went with a two speaker set up for more cool vibe and tone...way more beef and cone surface area than your
little 8" champ speaker could ever deliver. The eliptical Rola speakers are each 4 ohms, and wired in series for an 8 ohm load to the
output transformer, giving the amp plenty of reserve speaker power handling capacity, way more than Fender ever dreamed of using
on their budget amps. The cabinet is 3/4" finger jointed pine, and the tweed covering with the black strip, cool gold steel mesh and
black felt embeded grill separated by two verticle sections of the tweed cab give the amp a distinctive and stylish look. The National
logo is gone on this one, but a close look at the area where the logo was and you can still read National. This amp is one of my
personal favorites, it sounds nice and thick and overdrives with humbucking type pickups or p-90's in a mean manner, plug in a tele
and it still will rip it up. The grid leak bias gives the amp a bit of a nasty edge when pushed which is good and part of the Valco amp
character that I really like. The lower voltage circuit (unlike the Fender champ, and typical of most all Valco amps) still pushes the two
Rola's hard enough to get them to growl a bit, with the 6V6 adding it's overdrive character to the mix gives the amp a nice combination
of pre amp tube, output tube and speaker overdrive and break up. I would think a Harp player would love this amp as well, since it is
very touch sensitive, thick sounding and has the two Rola's to put it over the top. I'm not sure if there is a Supro version of this exact
amp, but an earlier version with the two eliptical Rola speakers and a 6SQ7 pre amp tube and a Supro logo was made for a time. I
drew up the schematic for this amp, look it over and feel free to download it. I will also post another schematic a little later of a 1968
"vibro Champ Type" amp I used to own, and you can see how the Valco Champ type circuit evolved as compared the the earlier
versions. Stay tuned for more featured amps, I will get more on the site as time permits, T Dobbs