Cheaper output transformer

Can anyone suggest a low cost alternative to commercially made audio output transformers for a tube guitar amplifier? For example, 4-5K primary and 4-8 Ohm secondary, capable of 10-15 watts.

Perhaps some way to modify an off-the-shelf power or PA toroidal transformer.

Some compromise in performance would be acceptable, commiserate with cost savings.

Thanks for any advice.

Mark Robbins

Reply to
Mark Robins
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You can try using a power transformer backwards, but it will have poor high-end response.

The high-voltage center-tapped winding is kinda a match for push-pull plates, and the filament windings (12 or 24V preferably), kinda match a speaker.

The medium-fi is not a a problem at all for a bass amp. Try it and see.

Reply to
Ancient_Hacker

For a push-pull amplifier he would need a transformer with a 120-0-120 primary. Desired turns ratio would be SQR(4000/4) = 31.6. So the transformer output voltage is 240/31.6=7.6. I would go with the 240CT to 6.3 V or 12 V transformer. I have tried this with speech, and it sounds sort of mellow. A single ended amp probably will sound really bad because of the DC on the transformer primary.

I once measured the frequency response of a 60 Hz filament transformer. Based on a sample of one, the minimum loss accured at about 400 Hz.

Tam

Reply to
Tam/WB2TT

Mouser sells Hammond output transformers. About $30 for 10 - 15W. Probably cheaper ones exist.

Tam

Reply to
Tam/WB2TT

Quite some time ago I came across a site with information on using commercial line matching transformers as plate fed audio output transformers.

Line matching transformers are those things they use with public address systems to carry the sound via thin wires to distant locations with no loss of volume.

Power transformers? They are usually stacked without gaps in the laminations you'd run the risk of driving the transformer into saturation with the DC on the plate.

Real audio plate transformers are designed unlike power transformers to minimize inter winding capacity and sometimes with screen taps, and ultra linear cathode windings - so what you use may depend on what the amp schematic looks like.

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Try Edcor

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Reply to
PhattyMo

30 to 40 years ago, there was a scheme to use push-pull 6AQ5 driving two transformers, where the transformer plate impedance was half of the plate, and the speaker windings was double that of the speaker. The reason was that the circuit (in effect, due to coupling) put the plates in series, and the output windings were wired in parallel. The plate winding was tapped for a Williamson type configuration, and there were cathode windings for a MacIntosh configuration. Triad made and sold these 5W transformers for a few dollars $1.50 each then. Crowhurst circuit; hope i remembered the specifics correctly. Distortion was far lower than tube datasheet indications; beat the pants of a heavy feedback 50W tube amp, both running at 52W for comparison (the heavy feedback amp was into major clipping at that level).
Reply to
Robert Baer

My first thought, if quality of the sound is paramount, get a good transformer for you application. There is plenty of old stock around in the world.

What is cost worth to you?

From what I know and read - and hopefully remembered . . . . this guy (in a magazine like "Glass Audio") took a standard (remembering they go for differnt wattages) audio matching transformer, and took the output impedance as a base. From there he applied a small audio frequency voltage to the speaker windings.

TaDa . . . he gets the turns ratio from the output to the input.

He knows the output impedance of the transformer (8 ohms) and calculates back to the input impedance - He can measure the turns ratio! The transformer is standard (don't saturate it and you can get the turns ratio within 1%).

From there, turns ratio is the square of the impedance - - - have 1 turn giving you 1000 ohms 2 turns will give you 4000 ohms - he figured out his output transformer was 5K on the 70 volt (line side).

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