Lowering B+ on Single ended amp with Choke input

[Guy77]

I came across this interesting idea for lowering B+ on a Single Ended amp that has a choke input.
It was used by a gentleman named Gordon Rankin in his Cardinal amps. I would use it for guitar amps.

DropB.JPG

He used a small input cap before a choke, you have to experiment to get the right voltage drop I have read that you need to use a (.22 to 1.5uF 630v cap) to get the output voltage to drop by .9 to 1.4 times the VAC primary: example 300VAC primary could vary from 270VDC to 420VDC for your B+. The smaller the cap the greater the voltage drop.

The only change I would make is that I would have a 47uf electrolytic cap between the smaller .68uf cap and the choke.

G.

[trobbins]

Yes that is a reasonably well known power supply 'technique'. The other advantage of that input capacitor is that it alleviates solid-state diode switching noise by lowering dV/dt at that node. Perhaps better to use UF4007, instead of 1N4007, for that application too. Not such an issue with valve rectifiers, which weren't as abrupt in switching current from one half of the secondary winding to the other half.

That input capacitor also has to have a large DC and AC voltage rating, as the voltage across it is onerous.

[sluckey]

The only change I would make is that I would have a 47uf electrolytic cap between the smaller .68uf cap and the choke.

If you do that you will no longer have a choke input filter. It becomes a cap input filter and the voltage will rise to 1.414 x RMS AC input.

[Roe]

This solution can be used on the screens together with an ordinary capacitor input on the plates but that requires two rectifiers

[TUBEDUDE]

A choke input filter should have a very small cap at the input to keep a small voltage on the inductor between cycles to avoid a complete collapse of the magnetic field, and the attendant noise from the current dump.

[sluckey]

A choke input filter should have a very small cap at the input to keep a small voltage on the inductor between cycles to avoid a complete collapse of the magnetic field, and the attendant noise from the current dump.

This Hammond AO-63 amp does not have any cap before the choke.

https://sluckeyamps.com/hammond_2/AO-63.pdf

[mhuss]

This Hammond AO-63 amp does not have any cap before the choke.

It also has a tube rectifier. I agree with the previous poster, the small value cap in the newer design is really only useful there to reduce HF noise.

OTOH, as you get up to 1uF, you start to have a (really weak) cap input filter. I guess this /is/ a way to vary the net DC voltage.

[didit]

Hello Guy --

Recommend you get set up to run simulation using http://www.duncanamps.com/psud2/. A bit of learning effort. However, in my experience quite accurate and much easier to learn than Spice or other similar general tools.

And yes, it's possible to both reduce voltage and improve B+ stability with a choke input power design. A small film capacitor between rectifier and choke input is a worthwhile addition, particularly if using silicon diodes. It will reduce diode noise and can by choice of capacitance help "set" your idle voltage. A few Henries at sensible current is about the cost of a tube rectifier. Excellent substitute unless you're truly committed to some modest sag, which isn't really a property in a single-ended amp.

Best .. Ian

[Guy77]

Thanks everyone for all the great info here on this subject.
I have grown very found of Single Ended amps lately and have built several ODS style circuits in single ended format, both high plate and low plate and really enjoyed the sound. I was pleasantly surprised!
Its interesting how less seems to be more when it comes to amp circuits!

On my last one I had a 6L6 that delivered 10 watts of clean head room before the sine wave started distorting and was impressed with how loud 10 watts really is.

I will definitely experiment with the smaller .22uf - 1.5uf 630v film caps before the choke in my next single ended build later this summer. I may even place it on a heavy duty switch so I can try different cap values. Having it on a double switch so that the power tubes cathode resistor can also be switch at the same time that the voltage is dropped ( with the small cap) would also be convenient. Especially if I am dropping around 80v on the B+ rail.

I will definitely use that Duncan site to make my calculations

Thanks
G

[trobbins]

If you notice some hum/ripple getting through the SE amp to your speakers then one way to suppress B+ ripple is to parallel a 'tuned' capacitor across the choke to form a ripple trap. The parallel LC forms a brick wall for 2nd harmonic to mains frequency, which is the major ripple component.