Re: T2p "Plus Zero" push-pull potato amp progress report


John Byrns wrote:
This is a report on the progress of my push-pull potato amp project.
There is only one major design question that remains to be resolved, I
will come to that at the end of this report.

First, I have given the amplifier a name which is the T2p "Plus Zero"
potato amp, in honor of Andre Jute whose work on his 417A T68bis "Minus
Zero" potato amp was the inspiration for my push-pull potato amp design
effort.

My dear fellow, I am so flattered.

What does "T2p" stand for?

Second, with the exception of a few small items of hardware, and the all
important tube, I have all the required parts to construct the amplifier
in my junk box. With the submission of the winning bid for a quartet of
E182CC tubes on eBay yesterday, I hopefully will also have the tube
situation in hand shortly.

Just as well you're laying in a stock. I seem to remember that there
was talk of the Slovakians making the E182CC again but I don't think it
came to anything; the talk was years ago and by now we would have
heard.

To recap the design, the T2p "Plus Zero" potato amp is a push-pull mono
block amplifier using a single E182CC tube. Phase splitting will be done
with a tapped high impedance choke between the two grids with the input
taken directly to one of the grids. I plan to use a line to push-pull
grids input transformer as the choke, with the primary left flapping in
the breeze as it were. This input/phase splitting scheme is a compromise
to fit the parts I have available in my junk box. My original plan was to
use an input transformer with a high impedance primary winding, and a
turns ratio of 1:3 from the primary to the total secondary, but these
transformers are hard to find on the second hand market and expensive when
they are found. So I will use the choke approach until I can find a
suitable transformer, which leaves me about 3 dB short of the desired
input sensitivity.

The output transformer is a Hammond unit with a 10k center tapped primary
and 4-8-16 Ohm secondary taps.

The power transformer will be a Stancor unit that has a 6.3 volt heater
winding and a 125 volt 50 mA B+ winding. With a full wave bridge
semiconductor rectifier this should provide the required 162 volts of B+
plate to cathode at 40 mA, even with some allowance for the use of cathode
bias, although it isn't clear how much voltage drop will be available for
a pi filter.

I would prefer doing a stereo amplifier in the first go around, but I only
have one output transformer, and the power transformer isn't large enough
for a stereo amplifier, so I guess I will have to consider this a
breadboard and if I like it do a stereo version, or try to find matching
parts to do a second mono block.

Small amplifiers like this cry out for point source speakers. There is
something about point source speakers that is not often mentioned,
which is that they are superb mono sources. Very often I use mono,
either natural (a single horn) or artificial (two ESL63 stacked one
behind the other). In fact, at the moment I have only one horn since
the complete pair walked out of the door. It's the sort of sound that
with properly sensitive speakers surround you anyway, rather than
approach you from the front, so "soundstaging" becomes even more of an
artificial concept than it usually is.

The question that remains to be resolved is what sort of bias scheme to
use? The three choices are cathode or self bias, fixed bias provided by a
rectifier off the heater supply, or back bias which is a variant of
cathode bias which has the advantage of eliminating the impedance in the
cathode circuit which is common to the grid and plate circuits.

Fixed bias will give you deeper bass. However, one of the great joys of
an amp like this, the immediacy and tunability of the sound with each
of the tiny number of components, can very easily turn into a
disadvantage. I despise the sound of regulated filaments, for instance.
I can easily hear the difference between cathode and fixed bias. There
is a danger, generally speaking, of pinning the voltage down too hard
and thereby making the sound bland. If you can hear the component being
put in this simple circuit, you can also hear it being removed. You
should seriously consider making provision for trying all three
negative bias modes.

If
cathode bias is to be used, there are a couple of secondary questions,
should a single cathode resistor be used, or should each tube have its own
cathode resistor? And if a single cathode resistor is used should it be
bypassed with a capacitor?

Gordon Rankin once made a point of telling me that in multitube power
stages each cathode should have its own resistor. Bob Danielak, on the
other hand, a guy with good ears and good taste, and at least as much
test equipment as Gordon, routinely builds amps (including with the
7044 that you earlier considered) with shared cathode resistors. Again,
it's a small change; try it both ways.

I would welcome any thoughts or opinions on what bias scheme would be best.

A potato amp is almost by definition an experimenter's amp.

Regards,

John Byrns

Surf my web pages at, http://users.rcn.com/jbyrns/

I hope you won't take these modest (1) remarks as wishy-washy, on the
one hand and the other; I believe that you will be able to hear and
judge for yourself, which of course makes the effort of trying the
permutations worthwhile.

Good luck.

Andre Jute

(1) Note that I don't say humble. I actually know this stuff, but the
real knowledge in potato amps is always what you can discover for
yourself, which is why I made a potato to study the driver stage of the
KISS 300B "Ultrafi".

.

Relevant Pages

  • T2p "Plus Zero" push-pull potato amp progress report
    ... This is a report on the progress of my push-pull potato amp project. ... grids input transformer as the choke, with the primary left flapping in ... plate to cathode at 40 mA, even with some allowance for the use of cathode ... The question that remains to be resolved is what sort of bias scheme to ...
    (rec.audio.tubes)
  • Re: IGNORE this DAMN FOOL
    ... You need a resistor from ground to cathode to serve as a shunt, ... charges for a schematic with factory recommended bias set procedure is ... CS400 power amp power transformers in a certain range of production. ... Suppose the tube are a bit old. ...
    (rec.audio.tubes)
  • Re: Another one for the Setties...
    ... plate, but there's no way of telling from this circuit ... There is a knob for adjusting bias and a meter which I believe reads in ... short of pulling the amps apart and prodding around has anybody any ... would have prefered more like 1k in the cathode. ...
    (uk.rec.audio)
  • Re: time-delay turn-on with tube rectification
    ... cathode stripping is when B+ is applied before the cathode is ... If you are building an amp for grins, do not worry about cathode ... If you are building an amp to seriously listen to, ... That leaves bias voltage stability an interesting problem ... ...
    (rec.audio.tubes)
  • Re: Intermittent fault in Eizo 19" T766 CRT
    ... as winding an extra turn on the flyback, ... between the original filament power supply+gnd and filament? ... the cathode signal is not DC, ... No, it doesn't work like that, the transformer isolates it. ...
    (sci.electronics.repair)