Valve amp - placement of dual chokes

Mikey R

Member
2013-04-21 10:59 am
Hi everyone, Im new here, this is my first post. EDIT: Just realised I may have neglected the site etiquate and failed to introduce myself properly - Ive just done that on the Intructions section. Cheers! :) Anyway, on with the story...

Im putting together a large 6 x 6550 bass amplifier. Im using stacked dual HT lines at 300V and 600V, for smoothing Im using CLC filters on both lines.

I just wanted to ask about the placement and orientation of the chokes. Ive have a long look online but I cant find an answer that Im happy with. Very few amps seem to use a choke on the upper HT supplying the power valve anodes, only on the lower HT supplying the screens and the preamp valves, so not many people have hit this problem.

From what I can tell, the chokes are the greatest offenders for stray magnetic fields, since they are dropping the entire AC component of a rectified sine wave, which has alot of harmonics at 100Hz, 200Hz, 300Hz etc. I need to place them as far from the input valve as possible.

[IMGDEAD]https://fbcdn-sphotos-d-a.akamaihd.net/hphotos-ak-ash3/558068_10151635765646340_195900044_n.jpg[/IMGDEAD]

But, can I place them side by side, or will their magnetic interraction with each other effect their operation as inductors? Will placing them side by side either increase or decrease their inductance, since they will either have the magnetic field generated from the AC component of the rectified signal in phase or antiphase with each other?

Is this important, or can I just place them side by side without any adverse effects?

Cheers for any help! Mike
 
Last edited:
Hi Mike,
Some quick "off the cuff" comments.
It is only leakage magnetic felds which can couple noise. These seem far enough from audio circuits to not cause a problem. I assume that the big transformer next to the chokes is the power tranny and not the output tranny. I would probably try to rotate the chokes 90 degrees so that their laminations are at 90 degrees to the power tranny lams.

I have other concerns, however.
Those chokes seem very small. I strongly doubt that they are ideal for the main B+. They may be suitable for the 1/2 voltage supply which I am assuming will power the output tube screen grids and the pre-amp phase splitter. Your post suggests that you want to build 2 x 300V CLC supplies and stack them.
It might be better to stack 2 ordinary 300V supplies and use CLC filtering on just the "critical" supply which will be the +300V.

To stack 2 off 300V CLC supplies then the chokes would need to be rated for 400mA.

To stack ordinary 300V supplies and then LC filter the bottom 300V would only require an 80mA to 100mA rating.

Would you care to post your proposed amp / power supply schematic, that would enable more informed comment.

Cheers,
Ian
 

Mikey R

Member
2013-04-21 10:59 am
Thanks Ian! Sorry for the delay in replying, life gets in the way as ever. The transformers and chokes were designed and built by John Wood, the chokes were specified for an amplifier this big, I believe they are rated at a whole amp each but I need to check with John when hes available.

From memory, I believe they are 5H, the caps are all 1000uf giving a very stiff supply.

This was how I was planning on arrangin the two HT supplies:

[IMGDEAD]https://fbcdn-sphotos-f-a.akamaihd.net/hphotos-ak-ash4/317391_10151656244671340_248035191_n.jpg[/IMGDEAD]

Im not totally sure of the position of the HT fuse, if it blows then it will protect the power valves by dropping the screen supply to zero, the preamp valves will also be fed from the lower HT supply so they will be protected too. I am a little worried that in a case of a fault and the fuse HT blows, we'll have to wait for the caps to discharge, in that time some expensive damage may be done. I could move the HT fuse to after the second cap - will need to check on Merlin Blencoes book.