• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

on the toroids again

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D

DF96

R.I.P.
Joined 2007
myweb.tiscali.co.uk/g8hqp/
Globulator said:
how can I find out a little more about how to distinguish the two?
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Start by telling yourself that, despite what you may see others say, output impedance and optimum load are completely different concepts. Try to separate them in your mind. To a first approximation, optimum load for a power stage is set by DC supply voltage and current. Output impedance is set by the active device technology and circuit, and feedback. People who write manuals/spec sheets often get the two confused: they write 'output impedance' when they actually mean 'optimum load' or 'minimum load'.

Then read a good book.
 
316a

316a

Member
Joined 2002
Toroids are limited by their primary inductance , better to use lower turns ratios such as 240V:6+6 with secondaries in series to achieve approx 3k into 8 ohms SE rather than with secondaries in parallel . My LCR meter reads 25H at 125Hz primary inductance for a 50VA Talema unit , ideal as a 2A3 parafeed output transformer but not too good for push pull pentode EL84 . For push pull low Ra types such as 6080 provide better performance . I measure -1dB at 10Hz and 20khz with the Talema which I consider to be very respectable

316A
 
Shoog

Shoog

Member
Joined 2002
316a said:
Toroids are limited by their primary inductance , better to use lower turns ratios such as 240V:6+6 with secondaries in series to achieve approx 3k into 8 ohms SE rather than with secondaries in parallel . My LCR meter reads 25H at 125Hz primary inductance for a 50VA Talema unit , ideal as a 2A3 parafeed output transformer but not too good for push pull pentode EL84 . For push pull low Ra types such as 6080 provide better performance . I measure -1dB at 10Hz and 20khz with the Talema which I consider to be very respectable

316A
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I would agree whole heartedly with all these observations.
They are not ideal for your typical output tubes. I have improved the situation a great deal though by applying Schade feedback to the output which helps lower output impedance and gives more bite on the low inductance.

However some toroidals have been reported to have higher primary inductances than the Telema's.

Shoog
 
D

dave slagle

Member
Joined 2003
LinuksGuru said:
Flux density should not exceed 15000 G
Stacking factor 0.92 .. 0.95

IMHO you should look at 250 - 300VA toroid at least.
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I agree with the 15KG limit but you cannot tell much by the VA rating. All of these transformers are designed to work at just under saturation so a 25VA unit will have the same flux on the core as a 300VA unit when energized with the same wall voltage.

The inductance of the unit sets the magnetizing current and smaller VA units need to have smaller magnetizing currents so they actually have more inductance. The downside of the smaller units is the resistances are higher so the copper losses are greater.

dave
 
Q

quikie22

Member
Joined 2009
DF96 said:
240V to 6V is 40:1, so the impedance ratio is 1600:1. 8R becomes 12.8k. Whether it will make a decent audio transformer is a separate question. My guess is OK for a guitar, no good for hi-fi. You will need to ensure DC balance.
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If in Push-pull, he will be using the centre tap of the 0-230... 115V tap....
Ratio would be 115:6 = 20:1
so Impedance ratio is 400:1, 8R becomes 3.2K
 
D

dave slagle

Member
Joined 2003
quikie22 said:
Sorry.Can you please explain that step by step?

I think my understanding may be a little off :)
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For Class B operation, one tube is conducting and the other is shut off. This means the conduction tube sees only 1/2 of the coil making the turns ratio 20:1 or 3K2:8.

When you move to class A operation the other tube and winding come into play and the entire winding is energized making the 40:1 ratio or 12K8:8 anode to anode resistance. Since in class A each tube sees 1/2 of the total A-A load each tube gets a 6K4 load.

dave
 
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