Designing and Building
Audio Amplifiers.

Contest Winning Amplifier.

It began in the early fall of 2012 when someone on the Fun with
Tubes email list suggested we have an amplifier building contest. As list
moderator I should have taken control at that point and set the rules but I
didn't. They just kind of grew. Some things I would have done differently
but what happened has happened and we can't change the past. Below is the
text of the official announcement of the contest which was sent by email on
September 13, 2012.

OK. As list moderator it is my job to set the rules once and
for all and debate over. Here they are.

Specification for Defining Guidelines for a Contest to Design a
Tube-based two channel Hi-Fi Power Amplifier (PA) that Meets Minimum
Performance and Budget Constraints.

Scope:
Develop a tube-based power amplifier (PA) that meets a
minimum set of guidelines for both performance and cost. The design is for
a tube-based two channel hi-fi PA and power supply only and does not
include pre-amp or tone control functions. Public recognition will result
from achieving this goal better than other participants.

Submissions must meet the following Specifications:

The PA must drive a minimum of 25W into an 8 ohm resistive load
while being driven by no more than 1V at the input.

The total cost of all parts (power supply and two channels) must be
$300 or less. If the power of 25 watts can not be achieved within
budget the power should be reduced rather than going over budget.

Frequency response should be -3dB or better from 35Hz to 20KHz.

The signal path of the PA must contain only tubes, no solid-state
components are allowed. An exception is for voltage regulators and
rectifier diodes.

THD (total harmonic distortion) must be less than 1% when measured
at 25W into a resistive load of 8 ohms at a frequency of 1000 Hz.

The power supply should be robust enough to supply power to two PA
channels with both channels driven to full power.

All parts must be obtained from on-line suppliers. All parts must
be available in large quantities, no one of a kind surplus items will be
accepted. After some consideration I have decided that this will not
include shipping charges. This is to avoid putting persons who reside
outside of the United States at a disadvantage.

The amplifier must not present a hazard to the user when operated as
intended.

Submissions:

A schematic of one channel of the PA and the power supply must be
attached to a message posted to the list.

A complete Bill of Materials (BOM) with components from commercial
supplier(s) with part numbers and current single-unit prices must be a
part of the message body.

If you actually build the amplifier include test results, photos,
spreadsheets, etc.

If you don't construct include SPICE simulations showing compliance
to the technical specs above.

After the deadline a complete list of entrants will be posted to the
list in a single message.

After we know who the contestants are we can select the judges. I
will hear (read) nominations and I will select 3 judges from the list of
nominees.

Deadline:

The contest deadline for submissions is 15-Dec-12. A winner will
probably not be selected until the Christmas and new-years holidays are
over. Decision of the judges is final.

Prize:

The winner will receive the admiration of all FWT members and have
exclusive bragging rights. The winner can also insist that the other
members of FWT refer to them as the "Grand PABu" [Power Amp Builder] for
not less than one year from receipt of this illustrious recognition.

Much of the above was taken from a message posted by
John DiFilippo. Thanks john for the work you contributed.

Regards.

Max. K 4 O DS.

And the winner is:

Tim E. Smith.

Tim built his amplifier and then shipped it to me for evaluation. He had
wanted to use 7591A tubes as the outputs but that put him over budget. He
used four 6P3S-E instead. Tim advises to anyone who wants to duplicate the
amplifier not to use the less expensive 6P3S tubes which are not reliable.

I tuned up the cathode currents of the output tubes and the AC balance
controls. Then I installed it in my main listening setup. I noticed right
away that it sounded better than my EL34 amplifier that it replaced. The
difference in sound was eventually attributed to the fact that I had not
adjusted the EL34 for 7 years. That story has been added to the EL34
pages. Scroll down to the heading "Testing the Amplifier 7 years later.
May, 2013" Near the bottom of the page.

I liked the sound of the amplifier so much that I asked Tim if I could
buy it from him. He agreed so it is now mine.

Figure 1 Tim's Design, one channel shown.

Figure 2 Power Supply.

Introduction, Circuit, and Specifications. (You are here)
Distortion
Testing.
Changing
Output Tubes.

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This page last updated July 11, 2013.