Building the Push Pull Parallel Triode Amps

Push Pull Parallel Trioded 6L6 / EL34

Mono Amps using ElectraPrint OPT and Hammond Power Transformer

Pete Millet's Universal "UNIAMP" boards from 2001 with Sowter 3575c input transformers used for signal splitting.  Pete's Universal Driver Board

 From Tube Cad Journal:   https://tubecad.com/2021/03/blog0531.htm

Output tube bias from cathode bias with the TCJ Harder Still circuit using 8 of these circuits.

Tube Audio

PP KT88 Triode

-
LL1660S/AM/PP 

Lundahl Amorphous Core Transformer Phase Splitter

Now all Lundahl

With surplus Output Transformers

Much tweaking to get the final product.
6V6GT phase splitter driver replaced with 6SN7GT

Lundahl Phase Splitter Transformer w/ Amorphous Core

No power supply electrolytics (only 2 electrolytic cathode bypass caps)

Amphenol MilSpec Power Connectors

Neutrik Speakon & PowerCon

Started with different phase splitter transformers

Bias/VU Meter

Solen 100uf Metal Can films. Dual Mono power supply.

 

Heathkit AA-32- Archeology

 

The Heathkit AA-32

Good ol' Ebay

About 20 years ago I bought a sealed Heathkit AA-23 for about $200 and stuck it in a closet.  I felt an affinity to it because at that time I was working in a building Benton Harbor, MI that some of the Heathkit operations had used.  The last copyright date on the materials I could find was 1969.

Huge respect for people back then that could assemble this as a beginner with limited experience.  Society must have been better and more practical back in the day.

When I opened it everything looked brand new.

I replaced the electrolytic caps with modern equivalents.

The tubes are what you would expect from a 1960's budget kit amp, 6EU7s and the venerable 6GW8/ECL86 combo output tube.

The next pictures show the buid-up of the parts.  I have been building things for quite a while and this thing was a challenge. Part of it was the 34 page step by step instructions with non-intuitive 

"Connect a 4-1/2" wire from lug 1 of terminal strip D (S-3) to lug 1 of terminal strip E (S-2)" 

Looking up pictures of which terminal strip/lugs etc was headache inducing.  Ended up with just using the pictures and the schematic.

I uprated the voltage doubler from the supplied 30uf caps to 200uf caps and UF rectifier diodes.  I also used the Sprague Orange Drop 716p instead of the supplied caps as they looked rather powdery.  I also added a 100uf cap at the end of the chain to keep everything quiet.  The amp has virtually no hum despite the tiny 1" shallow chassis.  Full phono stage and bass/treble controls too.  It sounds like an amp, decent highs and bass for its 8 watt output.  

(One of the 6GW8 tubes did not survive hibernation so waiting on a replacement.)

With potentiometer upgrade.  The original volume control was all over the place channel to channel.

A build based on Metasonix TX-1 guitar noisemaker with a
SE 6V6 output

It has a life of its own.
 

Maximum 300b (KR 842 VHD)

Pete Millet A2 Driver

Rod Coleman filament supplies

Buddha Camille Constant Current Source

Camille Cascode Constant Current Source (C4S) on the 6SL7

Three Toroidal Transformers 

Four Chokes

MOSFET B+ Regulators

Amphenol Umbilical Connectors

 

304tl - Finished!!!

Finished - Eimac 304tl Single Ended Mono Amps

They have a certain appeal

Construction was as modular as possible

Safety is critical in these amps as there is 1,100 volts on the heat dissipating plate cap

They also put out considerable heat.  Like sitting by a fireplace!

Stereo 6550 - The Dorm Amp

Building an SE 6550 amp with my Daughter

The little Output Transformers just would not do.  These are the full sized Edcors.  Now we have Bass

 Edcor 10 watt SE OPTs are fine as this amp is not asked to perform bass duties

Currently used to drive Klipsch Heresy speakers 80Hz and above.
Low end handled by a NAD2200 and a 12 inch passive subwoofer

It can provide a very high output

Building the 304tl

304tl

The tube cage is one of the most important aspects of this design. Without a safe cage, the amp is completely impractical with over 1000 volts on the heat sink plate connector. This cage uses custom cut glass and aluminum L brackets for construction. Sides and top use a metal screen from the big bin at Lowes.

Sides and top

These amps cover 4 chassis, each weighing over 50 pounds.  Being modular helped with construction but care had to be taken to ensure that the connections were safe and flexible.

Iron

Iron

Eimac - Scaling Up

"TL" or low mu tubes are much easier to drive than "TH" or high mu tubes.  Driving a TH tube takes a power amplifier circuit.  This is where MOSFETs shine.  The advantage of TH tubes like the 100TH below is that by the ver design, they have to be run in A2.  This makes them capable of producing a lot of power.  And because they are very difficult to work with, they are usually a lot cheaper.

Where do you go from there?  The 304TL of course.  Here we were able to find some 304TL tubes at a moderate price which allowed for good old fashioned A1 driven through a interstage transformer.

These are currently being built with 4 chassis.

2A3 Push Pull

A 2A3 Push Pull amp is as perfect of a 10 watt amp as you can get.

This amp was designed to be a throwback to the start of higher power audio reproduction.

Inside there are no electrolytic caps, only film used with a MOSFET capacitance multiplier circuit.  It employs 6SL7 to 6SN7 to 2A3 with fixed bias.  Latest version uses high capacity chokes. 

This amp is built like a tank.  We had it temporarily on a shelf for storage and the shelf gave way.  The amp landed on the floor and gouged the wall.  The only damage was a slightly bent tube pin on one of the 2A3s

Building with Eimac tubes - 35T

Eimac tubes make great amplifiers as they can produce a lot of power from a relatively small size, employ the hight of 1940's military technology and are relatively affordable.

The downside is that they require very high voltage, most need A2 drive and they are very mildly radioactive.  ☢  👀

 The power requirements can be extensive.

Buttoned up, what is not shown is the 20 pound power transformer sitting below.
(Yes that is Elvis and hard to see but a St. Christopher medal)

They also make great nightlights and space heaters