Hi guys,

I'm considering building my first small tube guitar amplifier ( the
Sonic MA1 from Tino Zottola's book "Building Vacuum Tube Guitar & Bass
Amplifiers Vol 1").

The schematic calls for an output transformer with a 2K primary and
8ohm secondary. This matches the load resistance figure I found in
the data sheet for the 50L6 power tube (also used in this circuit).
http://www.pmillett.addr.com/tubedat...02/50L6-GT.PDF

I'm having trouble finding a transformer with a 2K primary, but I have
found one with a 2K5 primary. I've read that using a primary
impedance greater than the recommended load resistance affects the
output quality - reducing power and distortion if I understand
correctly.

I'm wondering if there is a generally acceptable tolerance for the
matching of the load resistance and primary impedance values, and if
the 2K5 primary would be an acceptable choice in this case?

I also have an old output transformer with a 10K primary, which I
would expect to be quite incompatible in this case.

Any comments, suggestions, or recommended reading material is most
appreciated.

Thanks,
Joe

"Joe" wrote in message
...
Hi guys,

I'm considering building my first small tube guitar amplifier ( the
Sonic MA1 from Tino Zottola's book "Building Vacuum Tube Guitar & Bass
Amplifiers Vol 1").

The schematic calls for an output transformer with a 2K primary and
8ohm secondary. This matches the load resistance figure I found in
the data sheet for the 50L6 power tube (also used in this circuit).
http://www.pmillett.addr.com/tubedat...02/50L6-GT.PDF

I'm having trouble finding a transformer with a 2K primary, but I have
found one with a 2K5 primary. I've read that using a primary
impedance greater than the recommended load resistance affects the
output quality - reducing power and distortion if I understand
correctly.

I'm wondering if there is a generally acceptable tolerance for the
matching of the load resistance and primary impedance values, and if
the 2K5 primary would be an acceptable choice in this case?

I also have an old output transformer with a 10K primary, which I
would expect to be quite incompatible in this case.

Any comments, suggestions, or recommended reading material is most
appreciated.

Thanks,
Joe




Not going to make that much difference for the tubes.
Especially in a guitar amp.
Tone might end up different though.

Doug

Joe wrote:

Hi guys,

I'm considering building my first small tube guitar amplifier ( the
Sonic MA1 from Tino Zottola's book "Building Vacuum Tube Guitar & Bass
Amplifiers Vol 1").

The schematic calls for an output transformer with a 2K primary and
8ohm secondary. This matches the load resistance figure I found in
the data sheet for the 50L6 power tube (also used in this circuit).
http://www.pmillett.addr.com/tubedat...02/50L6-GT.PDF

I'm having trouble finding a transformer with a 2K primary, but I have
found one with a 2K5 primary. I've read that using a primary
impedance greater than the recommended load resistance affects the
output quality - reducing power and distortion if I understand
correctly.

I'm wondering if there is a generally acceptable tolerance for the
matching of the load resistance and primary impedance values, and if
the 2K5 primary would be an acceptable choice in this case?

I also have an old output transformer with a 10K primary, which I
would expect to be quite incompatible in this case.

Any comments, suggestions, or recommended reading material is most
appreciated.

Pretty minimal difference likely with your 2k5 transformer.

Just ensure it's adequately rated power wise.

It is of course not necessary to have *just* an 8 ohm winding on the secondary.


Graham

"Joe" wrote
...
The schematic calls for an output transformer with a 2K primary
and
8ohm secondary. This matches the load resistance figure I found
in
the data sheet for the 50L6 power tube (also used in this
circuit).

http://www.pmillett.addr.com/tubedat...0Tubes%20Part%
202/50L6-GT.PDF

I'm having trouble finding a transformer with a 2K primary, but I
have
found one with a 2K5 primary. I've read that using a primary
impedance greater than the recommended load resistance affects the
output quality - reducing power and distortion if I understand
correctly.

I'm wondering if there is a generally acceptable tolerance for the
matching of the load resistance and primary impedance values, and
if
the 2K5 primary would be an acceptable choice in this case?


class A2 triode with around 100V on the anode looks nice...but
you're not going that way.

If you look at the second page of your link you will see that it
shows how power output and distortion vary with load. At the
voltages and signal level shown, you can see that power output rises
and distortion falls as you increase the load from 2k to around
3.5k. It doesn't show the individual harmonics, just the total,
which is a shame because it's the detail that matters in your case.
You'll just have to use your ears.

The third page shows a set of loadlines. Assuming the screen voltage
for these is the same as you intend to use, you can estimate the
power output and 2nd and 3rd harmonics by drawing a loadline on the
curves. If you don't know how to do this, there are several sites
that can help, but I have no links. Someone?

I stared at the curves for a while wondering what Ef = 50V means.
Then I realised it's the heater voltage. Never seen it that high
before.

There is more to consider about your transformer and operating
point. One characteristic sound of an overdriven guitar amp is
produced when the output transformer begins to saturate. That
depends on core size as well as the windings. Again, you are going
to have to experiment.

I also have an old output transformer with a 10K primary, which I
would expect to be quite incompatible in this case.

Lots of distortion and very little power, probably. But it depends
on the load you put on the secondary.

Is it a single ended amp, or PP?

cheers, Ian

Thanks a lot Ian! I really appreciate your explanation of the curves.
I wasn't quite sure how to read them until you stepped through it. I
picked up a great book titled "Valve Amplifiers" by Morgan Jones that
has an excellent section on load lines, so I'll be studying that and
taking a closer look at these curves.

I believe this amp is single-ended. The schematic shows the output
transformer connected to the B+ supply on one side, and to the plate
of a single output tube on the other. No center tap is used. There
is also a .005mfd capacitor across the output transformer, which is
supposed to prevent high frequency oscillations according to the
description.

Can you elaborate a bit on what you mean by not going the way of the
class A2 triode?


On Fri, 19 Nov 2004 03:51:28 GMT, "Ian Iveson"
wrote:

"Joe" wrote
...
The schematic calls for an output transformer with a 2K primary
and
8ohm secondary. This matches the load resistance figure I found
in
the data sheet for the 50L6 power tube (also used in this
circuit).

http://www.pmillett.addr.com/tubedat...0Tubes%20Part%
202/50L6-GT.PDF

I'm having trouble finding a transformer with a 2K primary, but I
have
found one with a 2K5 primary. I've read that using a primary
impedance greater than the recommended load resistance affects the
output quality - reducing power and distortion if I understand
correctly.

I'm wondering if there is a generally acceptable tolerance for the
matching of the load resistance and primary impedance values, and
if
the 2K5 primary would be an acceptable choice in this case?


class A2 triode with around 100V on the anode looks nice...but
you're not going that way.

If you look at the second page of your link you will see that it
shows how power output and distortion vary with load. At the
voltages and signal level shown, you can see that power output rises
and distortion falls as you increase the load from 2k to around
3.5k. It doesn't show the individual harmonics, just the total,
which is a shame because it's the detail that matters in your case.
You'll just have to use your ears.

The third page shows a set of loadlines. Assuming the screen voltage
for these is the same as you intend to use, you can estimate the
power output and 2nd and 3rd harmonics by drawing a loadline on the
curves. If you don't know how to do this, there are several sites
that can help, but I have no links. Someone?

I stared at the curves for a while wondering what Ef = 50V means.
Then I realised it's the heater voltage. Never seen it that high
before.

There is more to consider about your transformer and operating
point. One characteristic sound of an overdriven guitar amp is
produced when the output transformer begins to saturate. That
depends on core size as well as the windings. Again, you are going
to have to experiment.

I also have an old output transformer with a 10K primary, which I
would expect to be quite incompatible in this case.

Lots of distortion and very little power, probably. But it depends
on the load you put on the secondary.

Is it a single ended amp, or PP?

cheers, Ian

"Joe" wrote

Thanks a lot Ian! I really appreciate your explanation of the
curves.
I wasn't quite sure how to read them until you stepped through it.
I
picked up a great book titled "Valve Amplifiers" by Morgan Jones
that
has an excellent section on load lines, so I'll be studying that
and
taking a closer look at these curves.

I believe this amp is single-ended. The schematic shows the
output
transformer connected to the B+ supply on one side, and to the
plate
of a single output tube on the other. No center tap is used.
There
is also a .005mfd capacitor across the output transformer, which
is
supposed to prevent high frequency oscillations according to the
description.

Can you elaborate a bit on what you mean by not going the way of
the
class A2 triode?

delay...wrote something beautiful and lost it in the ether...this'll
have to do.

Check http://members.aol.com/sbench101/

First table, last column, there are links to several pages on load
lines. Lots of other good stuff there too!

In class A2 the valve is biased with a voltage less than the max.
signal level, so for part of the time at full power its grid is +ve
with respect to its cathode.

If you look at the last page of your link, you will see that a few
curves are shown with +ve grid bias, and these bend the other way to
those for -ve grid. So the curves form an "eye" shape. If you pick
an operating point in the middle of the eye, and a load line along
the eye's minor axis, then the pattern of intersections either side
of the operating point is symmetrical, with more or less equal
compression of the signal for +ve and -ve signal peaks. This
indicates a high level of even harmonics, mostly 2H. Odd harmonics
tend to result from asymmetrical compression.

There is a heap of problems associated with A2 operation, however.
As bias voltage approaches zero, the grid begins to pass current
and, as it becomes +ve, this current increases dramatically. The
previous stage, or driver, must therefore have a very low output
impedance at peak signal level, otherwise its output voltage would
drop as soon as it tried to provide the necessary current, causing
distortion. Some guitar amps make use of this by pushing the grid
+ve but starving it, but I assume that this method of creating
distortion requires much experimentation and experience of smoke to
perfect.

Most valves won't tolerate much grid current for long, because the
grid gets hot and can burn out like a fuse. Running with such a high
bias current does not assist longevity either, but that may not
deter a tone-hungry guitarist.

Yes your amp is SE and so it must be class A1 or A2.

The cap across the output counters the effect of leakage inductance
in the output transformer, which otherwise can cause ringing or
oscillation, depending on how feedback is arranged. Morgan Jones has
a section on components including transformers, and also discusses
parasitic oscillation and output stage ringing in several places.

cheers, Ian