While making the last Woden amplifier I wondered what an experimenter would do about the power and output transformers.

The options are to buy stock units, find substitutes that will do the job or make them.

A search of the internet suppliers showed that suitable units could be bought but at some great cost, anything up to $1000 for the pair.
Alternatively there are X TV power transformers from the valve era that could be used and there is a suggestion that standard line speaker transformers could be used.
I used this approach and decided to make a chassis that could carry a number of Valves and plenty of room for any sorts of transformers.

The aim will be to show how a valve TV transformer can supply around 400 volt HT to drive pairs of output valves and investigate the claim that the 100 volt line transformers will work to match valves to speakers in push pull mode.

The first step will be to make a general purpose chassis and mount up some transformers and output valves and see how they work together.

I have completed a part 1 covering the making of a chassis.

Part 2 is underway and as I find time I'll work on the project.


Fred.

Woden Amplifier - Part 1
Woden Amplifier - Part 2
Woden Amplifier - Part 3
Woden Amplifier - Part 4
Woden Amplifier - Part 5

Part 1 uploaded.

I have a part 2 finished for this project.

This part covers bringing the amplifier chassis to working condition and load testing the Altronic Line transformers.
A web site 'cheap output transformers' suggests using M1130 as replacements in a Fender guitar amplifier.
I test this suggestion and come to a conclusion.
Also I set up the output valves using a Woden output transformer of my design to get a base line of performance of the Tripler power supply and 6CA7 valves.

A part 3 will cover testing the operation of a 'Hammond' 'fender replacement from Evatco.

Also I will do a survey of B/W TV power supplies that could be used as guitar power supplies as a part 4.

I'll send a PDF of part 3 to Brad to attach to the thread.
Part 4 in production.

Fred.

Unless I’m missing something Fred, I feel your proposed output power is a little conservative. 6x EL34s, fixed bias and plate voltages in-excess of 400V = a lot of clout.

Hi Dave, its not so much how many valves you have it's whether the power supply can deliver the wattage!
Valves don't make any power; they only modulate the power supply in sympathy with the input signal.
You will see the effect of this in part 2 as I scratch about trying to get the best efficiency out of the valves and use the available power to the best advantage.

I used to run 6CA7pairs with 800 volts on the plate as this was allowed by the maker.
If the power supply could deliver, then I could measure 90 watts per pair.
I wound transformers on grain-oriented strip wound "C" cores.
The HT power supplies were capable of 500 watt plate supply so I could make "300" watt amps with 3 pair.
The wattage output also depends on how much distortion you allowed!
Obviously if you allowed 30% distortion you could rate an amp far greater in size if you only allowed say 5%.
On hard square wave drive its all up to the power supply, the transformer size, choke and filters.
90 watts was with visible clipping and about 10% distortion.
The vlaves were happy at this level with careful biassing and lasted a long time.

Audio Hi Fi folk tremble at 10% distortion, a mind set created by theoretical designers and promoted by magazines like Radio and hobbies, EA, SC and so on.
But really how music consists of pure sine waves?
NONE.
Voices and instruments are harmonic devices, with say 30% of low order harmonic distortion.
It's not the amount of 'distortion' it's the TYPE of distortion that sounds melodic or not.

Anyway, this is not a music class, it is having fun with valves.
I'm rambling, enjoy the articles as I dig up the past
.Cheers, Fred.

Hi Fred,
Yes, I’m more than comfortable with the theory around the design of output stages. I just feel looking at your initial schematic, biasing, plate voltages and output impedance that this is 100W even in Hifi circles. With the only issue I can see is that the 100mA supply current being a bit light on? I look forward to reading #2 instalment. Thanks Fred.
Dave

Part 2 uploaded.

I will send a 'part 3' to Brad to post in this series.

In this I test the Hammond output transformer from Evatco.
I sent a copy of the test report only to Evatco for their information.

As I can fit it in, I'll pursue various topics to do with running 6CA7's as a guitar amplifier valve.
The purpose of all this is to try using old power transformers for the HT supply, the Tripler system work well so far.

Next I'll use a 'Henderson' radio transformer as used in some radiogram way back in the 50's.

I'll send the part 3 to Brad to attach to this thread.

Cheers, Fred.

Part 3 uploaded.

Re TV power supplies Fred, have a look at the power transformer in the HMV F series - made from 1957 to 1959.

With no good CRTs available, I don't feel so bad at losing one of these iconic, master-class in design TVs to such a project.

The transformer drives a CT FW rectifier using either a pair of 6N3s or 5AS4s to get 260 volts about 300mA input to filter. There are two filter chokes and most F series have PP audio with a pair of 6BM8s.

But the main thing is that transformer is HUGE! I reckon it could easily supply a diode bridge to get 600V plus 300V from the tap, with good regulation.

A couple of 6.3V 10 amp windings too. The F series had a LOT of valves.

Exactly Ian!

In a later part I will survey types of power supply in B/W TV's.
They vary from brutes like Ian suggests to much more smaller but more efficient doubler.
As Ian also points out those chassis also carry filter chokes, another expensive item nowadays.
The current draw of B/W TV's was between 200 to 350 ma exactly the same as a 50 or 100 watt audio amp.
Keep in mind the TV PT's were on near full load 100% of the time.
Hence the whopping size and current capability.

From these experiments I'm heading toward making another valve Woden Bassplayer using a centre tap brute PT, the Evatco OPT and just one pair of 6CA7. That will be in a separate head unit to split the weight with a 15" Bass driver in a vented box for best acoustic efficiency.

Yes, if you have a dud tube B/W TV chassis and may in future look at making a valve HI-FI or Music amplifier don't chuck out that chassis, the power transformer is a $300 to $400 item that could drive a twin 30 watt Hi-FI stereo or 50 to 100 watt music amp.

I'm fighting a losing battle with health, but will plug on in the workshop having fun.
Will I get back to Radios?
I think I've done the lot actually, scratch built superhets,TRF tuners, Reinartz positive reaction 1920's style set, used filament valves, cathode valves, glass and metal types, built radio grams, done work on zero frequency IF superhets, made an original 'Little General' and decade models, repaired or refurbished Bakelite and plastic cabinets, made 3" and 5" TV. Its a fair body of Work.

Back when I can, Cheers, Fred.

I have a part 4 where I do some experiments with a rough speaker box with a ported vent and different tone shaping circuits including a Baxandall cut/boost and an output valve variable bias and negative feedback control.
I do a bit of musing about the power transformer and using the types you will find in B/W TV's.
All of that should lead to the next part where I build a gig ready head and speaker cabinet.

I'll send on part 4 to Brad to add to the posts in this thread.

Cheers, Fred.

Part 4 uploaded.

I apologise for the first page of part 4.
I forget to finish it and correct obvious mistakes like the heading saying 'part3' !
The text should start at the top with the list below it.
Getting old I think!
Fred.

If you want to amend the document and resend, it will be easy enough to replace the old one.