Having used toroids as O/P transformers in PP Audio Amps there is always the problem of keeping the quiescent currents through the two sides of the ouput transformer primary balanced such it does not saturate. This usually entails measuring the Ia (Valve O/P stages) and balancing every 6 months or so. A simple compass on top of the transformer will indicate an imbalance of 1.5 mA on a 350 VA
transformer. An automatic system would be ideal however this gets a bit complicated with DH valves. There appears to be a lot of bull written about leakage inductance re toroids, when C cores are employed this appears to be swept under
the carpet, another example of snobism?????
Ideas on automating the balancing of the winding currents would be appreciated.
Khalif
i know that W.A.D used toroids as pp output tx in a few of there designs ,i will see if i can find one and see how they overcame the problem.
rob t
I'll be very interested to see that.
boiss
Don't they sometimes use two cathode resistors?
My amp uses EL34s, one triode strapped and the other as a pentode with the g1 grounded and the 2 cathodes coupled with caps. Each cathode has a CCS to ensure that the Ias are the same. Note Ia not Ik. O/P transformers are RS 150VA toroids.
Will have to check and see how the Ias are now after almost a year.
KRs Mike
i have now found the artical hope it is usefull
rob t
oh dear to big to post i will try and email it
could you pm me your email address?
rob t
Hi Rob,
These just have the normal 2 Rs in the cathodes, as the valves age so the Ia's will change for better or worse.
If it rains here next week as predicted I'll knock up a sig gen with an RS chip that I've had for 20yrs and try unbalancing the toroids by varying amounts, just to see what effect it has. I hand wound a 350VA toroid for PP use (2 x 813) and found that if a small compass was placed on the toroid then a 1mA imbalance could be seen.
What effect this has on the sound I really don't know. However the cct that you sent me doesn't seem to worry about this. I had some small toroids cut with a 1mm slot and then wound, I thought about using these for small class A amps but the L is so low that I'm sure that LF would suffer.
My present amp with EL34s runs about 110mA (per bottle) in class A balanced to +/-1mA quiescent after warm-up.
BRs
mike