Recommended Filter cap size

[dude]

I'm building a single channel 6L6s head, Super Reverb topology. I'm finding it hard to find an 80uf filter at 500v. Best I see available is 450v, 100uf...?  I plan on making this amp take 6L6s, 6v6s and El34's with bias change and maybe a few switches to change voltages, 5AR4. PT is a Magic Parts TF120, lost the specs but around 350-0-350. What first filter size and is 450V too low?


al


 

[sluckey]

Super Reverb topology. I'm finding it hard to find an 80uf filter at 500v.

Look at the SR schematic. There are no 80µF @ 500v caps! SR uses a series pair of 70µF/350V for the first node and 20µF/525v. Hoffman sells 100µF/350v caps to replace those 70s.

I would not use 80µF for the first node cap with a tube rectifier.

[dude]

It's not a true BF era SR, But an Allen Old Flame. I guess it doesn't make a big difference if the first filter node is 80uf or 22uf..? I see in his first schematic, he used an 80uf, 22uf 22uf @500v but since changed his schematic to three 22uf @500.
 
I hear ya, original BFSR is 35uf first filter node @700v, and Allen changed that to 22uf. I was looking at his 1998 schematic he sent me in an older kit, since then he changed to 22uf.


Bottom line, your opinion, any big difference if I use a 33uf @500v, first filter node? Or do I need in series caps to up the voltage.., keeping around 40uf?


I plan on using a 5AR4, PT 350-0-350 (assume B+ 425 ish?) and want to use 6l6s mostly but be able to switch to 6V6s or El84's (I know I'll have to tie pin 1 to the cathode). Just want to get the best first filter for what I plan on doing.


al

[92Volts]

350v RMS "should" equal 495v peak, therefore 495v DC minus rectifier and load losses.

If that PT was built for 117v mains and you have 121v, and it's meant to produce 350v loaded, but you want to prepare for unloaded use... that's sketchy. Even with a handful of volts lost from a tube rectifier, it would be sketchy, as those losses will also be lower without load.

You should measure the actual voltage of the transformer, otherwise assume you need a higher voltage rating.

[sluckey]

I would feel better using a series stack of two 100µF @ 350V. Unloaded B+ will be about 500V with that PT. The 5AR4/GZ34 can handle up to about 60µF for the first filter cap.

One of my favorite caps is the F&T dual can 50/50µF@500v for the plate and screen nodes. Makes for a neat layout. Then spread the other filter caps along the board as needed.

[dude]

Good info, thanks. I'll measure the "unloaded" voltage on the table to really see what the voltage is but as you say the series 100uf@350v is the best bet if unloaded voltage is 450/500v.


Rule of thumb:


What "unloaded" PT voltage is acceptable with a single filter rated at 450v and one at 500v, something good to know?


al

[sluckey]

Unloaded DC voltage will equal 1.414 times PT AC voltage. Or, transposed, PT AC voltage will equal Unloaded DC voltage divided by 1.414.

So, a 450VDC cap can be used with a PT AC voltage of 450/1.414 = 318vac.
Or, a 500VDC cap can be used with a PT AC voltage of 500/1.414 = 353vac.

These are theoretical numbers. It's always a good idea to increase the cap voltage for a warm comfortable piece of mind.