Big Tranny Project

[Andy Le Blanc]

Just finished a project with bone yard parts. The power transformer is a generic
replacement for a Twin, and the output transformer is a 100W Marshal.
The PT has a 690V/ct secondary rated at 310ma., quite a bit less than the current
Hammond drop in which is advertised with 450ma.. The OPT is a 1.7k primary
with 4, 8, and 16 ohm taps. I've chosen to use 6550 push-pull and purposely miss-matched
the 4 ohm tap with an 8 ohm driver to get an effective plate load of 3.4k to
more closely meet the GE and RCA published figure of 3.5k. The power supply
uses the twin choke (again a generic replacement) and 2.2k screen grid resistors.
The plate voltage is 483v with the 6550 biased to a static dissipation
of around 30w (29.95w), with zero-signal plate current for the pair at 124ma..
The bias is fixed and uses 100k resistors (2.2k grid stops).
6550 data is published with a note to use 50k grid resistors maximum, but
this is not reflected in any schematic of guitar amps built using this tube.

The pre is two parallel stages of a 12ax7 using the zero-bias configuration
with 220k plate resistors, followed by the old and commonly use volume and
tone arrangement. The driver is again old and commonly use cathodyne
affair using old Gibson values. The amp uses no feedback.

The tone is very open and engaging, and the breakup is surprisingly smooth.
I had heard in the past very negative reports about 6550's breakup in the past,
but I think not so now, great tone for blues and rock.
The only issue is the power and volume of this setup. It is very LOUD.
Much so more than a pair of 6l6 set up for around 50w.

I'm surprised so far, but need to gig it out, its nice to have a rig that can
use this family of tubes. It has old RCA 6550 (short coke bottle, st14?),
I'd like to try 6550A, the various KT88's out there, and even the KT90.
The KT 90's have a published max plate dissipation of 50w. The project
looks like a good bass amp with minor value changes. A good watt figure
in a simple amp, with only a pair of tubes.

[Alexo]

Hey Andy, I've posted here about my 100 watt B15 using 6550's, a similar setup to yours with 570 volts on the plates into a 5K load. I was surprised at how guitar-friendly the overdrive of these tubes was when I dialed out the nfb - you can crank them up and hit an open chord and feel the harmonics rippling through every organ in your body. Amazing tubes, those 6550's....

[Wayne]

Never owned anything with 6550s in it, but I like the look of 'em - they even look loud when they're not connected to anything!

W

[Andy Le Blanc]

A good friend of mine has an old B15 in a porta-flex cab, wonderful amp, at low to
medium volume requirement. The notion of a higher powered "B15" came
to mind for me as well. I tried another PT that gave me 600v at the plates,
but dropping the voltage for the screen grid supply was impractical. The distortion
with that setup was harsh although it did make a lot of watts. I'll keep looking
for a more appropriate tranny and supply configuration to pull it off without
having to resort to dropping the voltage for the screens supply. I did find a
a power supply in the 1937 radio amateur's handbook, where a bridge rectifier
is used and the center tap of the PT sec. provides a "half power" source for the screens.

The tone of these without feed back is really nice. A real surprise. I've had a
couple higher powered amps with 6550 but they were ultra-linear and didnt
sound good when pushed, sterile cleans, which was the point. I can see why
they were used in marshal's, but never heard good things about them in
that application. The head room is great. The ratty old things I've got in
it now are at 30w plate dissipation. They should go to 35w. 6550A's go
over 40w, and KT90's go to 50w. If the tone of a KT90 is close to the beat
old tubes I'm running now, it should make for one hell of an amp.

[v00d00blues79]

Andy,

I'm very interested in the concept and design you have going here. Would you mind sharing a schematic of what you have going here? I'm trying to understand what you have going on, but it would make more sense to me if I could see it on paper.

Thanks!

Andy

[Andy Le Blanc]

Here's a rough schematic of the pre. . The values are very generic and
should be tweaked to what ever is preferred. It's nothing new, this is a
zero-bias pre, very old fashioned, but it makes quite a bit of gain. There's no
feedback, degenerative feedback can be used critically where ever there is a
cathode resistor and by-pass cap. Try to use high impedance controls to
limit insertion losses.

The output is even less remarkable, its a SF Twin power supply, basically.
The output is configured for two tubes, with the sockets wired for 6550's etc..
And the OPT/spk pairing is used get the effective load for the tube type.
The big 100w trannys handle the current with no problem and let you use a
current reserve while still pushing the plate dissipation to get the most out of
the tube type. Its very simple and makes for a big, loud, and very sweet tone.