Transformer usage ??

[lugnut]

I have some peerless 16458 transformers I'd like to use in a 6B4 push pull amp. The primary is tapped at 50%. Can I connect the 6B4 plates to the 50% taps and just tape off the primary taps so I can get a better match to load the 6B4's? Would this do any harm to the output transformers?

[Alain Poitras]

Originally Posted by lugnut

I have some peerless 16458 transformers I'd like to use in a 6B4 push pull amp. The primary is tapped at 50%. Can I connect the 6B4 plates to the 50% taps and just tape off the primary taps so I can get a better match to load the 6B4's? Would this do any harm to the output transformers?

This will not harm the transformers but the inductance will be lower and it can have an effect at low frequency, just try it !

[Alain Poitras]

Originally Posted by Alain Poitras

This will not harm the transformers but the inductance will be lower and it can have an effect at low frequency, just try it !

If the impedance you get is correct for the amplifier, it will not harm your transformers but ...

If the impedance is too low, you can harm the transformers and the tubes because the average plate current will be too high ...

Don't forget the impedance ratio is the square of the voltage ratio and at 50% of the winding, you will not get one half the impedance but just one fouth ...

There is a problem with those very good transformers, some people said they have a 10K primary impedance, other say 9K and some say 6,6K ... At the 50% taps, it is respectively 2,5K, 2,25K and 1,65K ...

For the 6B4G power triode, in my olds tubes databooks they said for push-pull class AB1 at 300V plate supply, the transformer impedance have to be 3K with a -68V fixed bias to get 15W at 2,5% THD and 5K with a common 850 ohms cathode bias resistor for both tubes to get 10W at 5% THD. Even if the transformer impedance is 10K, at the 50% taps, it will be too low for your 6B4G ...

In push-pull "pure class A" at 250V supply, the impedance can be 10k ohms plate to plate ( then 2500 ohms per side ), but you will get just about 3,5W output power ...

Those Peerless transformer are made for a 45W output power, you should not use them for a low power triode amplifier using 6B4G ... This is a real wasting ... You should consider using other's kind of tubes and use them like it suppose to, not with the plate at the 50% ultralinear taps ...

Why not using superbs power pentodes 7591A ... In class AB1 with a 450V plate supply and 400V screen grid supply, with a 6,6K load and a -21V fixed bias, you can get 44W at 5% THD ... With the same supply voltages and 10K load and a common 170 ohms cathode bias resistor, you can get 28W at 2% THD ... And with a 450V plate supply and 350V screen grid supply, with a 6,6K load and a -16,5V fixed bias, you can get 38W at 2,5% THD ... All that without a global and locals negative feedbacks and ultralinear screen grids connexions to the 50% taps to get a much lower distortion ...

In a single end triode amplifier, you have almost only second harmonic distortion not unpleasant to ear, the third harmonic distortion and over are very low ... It you use them in a push-pull circuit, it is different, you got almost only third harmonic distortion just like with a pentode or tetrode but very low second harmonic distortion and other pair harmonics because they nullify themself in the output transformer ( if the tubes are well matched and their bias preciselly adjusted ) ...

So the only way to get naturally ( with no feedback ) a very low third harmonic distortion is power triodes in a single end amplifier, not in a push-pull amplifier ...

The first thing you should do is not asking everybody on the net what is the primary impedance of your Peerless 16458 transformer but measure it yourself ... It is very easy ...

Just hookup the "total plate to plate" primary of the output transformer ( with no load on the secondary ) to the output of a small low voltage transformer, any low voltage ones will do, like 6 to 30 VAC ... You can just use any cheap "AC wallwart" ... Then measure the voltage across the primary and the voltage across the 8 ohms secondary winding of your output transformer ... Divide the voltage at the primary by the voltage at the secondary, it will give you the voltage ratio ... The square of this ratio is the impedance ratio, multiply it by 8 ohms and you will get "the exact" primary plate to plate impedance.

Example : You measure 12 VAC at the primary and 0,418 VAC at the 8 ohm secondary, then 12 / 0,418 = 28,7 ... The voltage ratio is 28,7 : 1 ... Then 28,7 x 28,7 = 823,7 which is the impedance ratio, you multiply by 8 and you get 6590 ohms ( about 6,6K ) ...

Cheers,

Alain.