Power Cap Cans & Preamp Tubes

[jly56]

Hi...  When designing a custom chassis, does one need to be careful about the power caps (in cans) being too close to the 12A?7 tubes or the power tubes?  I know to keep the OT and PT away from them... Does this also include chokes?


Thanks - Jeff

[John]

You wouldn't want your cans too close to the power tubes on account of heat, I *think* leaving an inch of space gives enough room. For the rest of your question, someone who knows more will be along shortly. :)

[Ed_Chambley]

When designing it is always a good idea to keep everything away from the power tubes as much as possible.  John is correct about the caps and a choke can be damaged by heat as well.  If possible you can place the choke inside the chassis if it is small and you have planned for it.


An inch or more will do fine. 

[jjasilli]

Attached is a photo of a 100W Stromberg Carlson PA amp which I use for home theater. Sounds great.  Note the tight placement of components.  The far power tube has a vertical shield between it and an empty octal socket.  The socket is for an optional relay.  The relay has a coil like an inductor which would generate a magnetic field near the tube.

[John]

Yeah, but they didn't know any better. 

[jly56]

Thanks everyone...


The full plan is to build a 5f8a (Hoffman style board) on an 8x16 chassis.  I also can use a 10x17, but I would prefer the smaller chassis.  I'll be using KT66 power tubes (filament amps have been accounted for) and JJ caps for the power section (2 100uf/385v (for 1st stage) , 2 47uf/385V (for 2nd, 3rd, and 4th)) on Pete Millett snap cap boards.


I am also considering putting a cooling fan on the chassis, but know (from computers) the magnetic field that fans can produce.  Just put a fan close to you computer monitor!


I'm planning on putting the circuit board down the middle of the amp, between tubes and PT/OT (at the front). 


Here are a couple of layouts I am considering (in attachments.. if I do it right!)...  Any feed back is greatly appreciated


thanks - Jeff

[eleventeen]

Chokes are more often unshielded than power/output transformers. Yes, of course there are shielded and unshielded of all types. But chokes can certainly radiate hum every bit as much as power transformers. There should be no question about that. I don't think output trannies radiate much until you're talking about high power.



You want 12A_7 et al to be far away from the power supply/rectifier section. You will see that on I reckon 98%+ of any tube amp you look at. Anything else, more space is better. I would not want a can electrolytic right next to a power tube. The heat from the tube cooking the cap, the cap possibly radiating into the tube. Not good. 


And at some point, as we get older and our fingers get fatter, it can get physically difficult to get to tubes for replacement. Obviously you don't to do that very often. There are some WW2 aircraft radios where you actually need special tools to get tubes in/out!

[jly56]

Thanks 11teen...


So C or E would be a better choice, keeping the choke away from the 12A.7 ?  The rectifier was the far left octal tube (or below the 4 power tubes on E)' so that is far from the preamp tubes.  I can more the row of cap cans down lower towards the PT/choke/OT on C.  All iron is Hammond (272jx, 194b, 1760w).   All cap cans and tubes are at least an inch from each other in all designs..  Some are 1.2 to 1.75.   If 'stuff' is still too close, I can move to a 10x17 chassis for more space.


  My primary concern is functionality over size... Although size is important! 


Thanks

[eleventeen]

Offhand, I would reject B and D. I would take the entire cluster of caps in E and move them to the right and get the choke next to the PT. Those are your hum monsters. The caps themselves (eg; their metal cans) form a modest shield of sorts.


Do you have enough length to get your parts board kind of in parallel with the row of preamp tubes? If you are planning to use this long thin parts board designed for a long thin chassis, I believe you are destined to run yourself into problems with a more squarish chassis. In other words, do not only consider the big fat above-chassis parts you are planning on, take note of where that parts board is going to go. In the overall scheme of things, that board and its peculiar size and shape could be lots more influential than how big your trannies are and where you might mount them. A power trans for a 4 * 6L6 amp is about whatever size it is no matter what. Get my drift?


Your costs for all those can caps is going to be high. Those can be $15-20 each versus $1.50 each (times 2 to replace a dual-can) for PC-mount e-caps. The axial mount ones are quite a bit more. And they (the cans) sure take up a lot of real estate. And don't forget the $2-3 each for the clamps to mount those caps if those are called for. I can appreciate you may be looking for "a look" but you could get that with 2 or 4 of those cans above the chassis versus EIGHT. I myself would consider, at least, a cap board (or some physical structure producing a row of them, on terminal strips or separate turret/eyelet board) under the chassis. Something like the under-doghouse board in a Fender.


You're definitely doing the right thing playing around with possible layouts. Paper and ink is vastly cheaper and easier to deal with than metal and big holes that have to be neatly executed.


I have to tell you that I would consider a pre-punched 5F8 chassis from mojo or even some sort of adaptation of a Marshall chassis would very possibly make your life incredibly easier. Chromed, silkscreened numbers, right size, punched, done.

[jly56]

Offhand, I would reject B and D. I would take the entire cluster of caps in E and move them to the right and get the choke next to the PT. Those are your hum monsters. The caps themselves (eg; their metal cans) form a modest shield of sorts.

Do you have enough length to get your parts board kind of in parallel with the row of preamp tubes? If you are planning to use this long thin parts board designed for a long thin chassis, I believe you are destined to run yourself into problems with a more squarish chassis. In other words, do not only consider the big fat above-chassis parts you are planning on, take note of where that parts board is going to go. In the overall scheme of things, that board and its peculiar size and shape could be lots more influential than how big your trannies are and where you might mount them. A power trans for a 4 * 6L6 amp is about whatever size it is no matter what. Get my drift?

The board is 12x3 (one of our hosts 5f6a with additional turrets for the additional components for the extra power tubes) and 'should' fit below the cap can boards parallel to the tubes.  I still need to verify the fit.

Your costs for all those can caps is going to be high. Those can be $15-20 each versus $1.50 each (times 2 to replace a dual-can) for PC-mount e-caps. The axial mount ones are quite a bit more. And they (the cans) sure take up a lot of real estate. And don't forget the $2-3 each for the clamps to mount those caps if those are called for. I can appreciate you may be looking for "a look" but you could get that with 2 or 4 of those cans above the chassis versus EIGHT. I myself would consider, at least, a cap board (or some physical structure producing a row of them, on terminal strips or separate turret/eyelet board) under the chassis. Something like the under-doghouse board in a Fender.

The JJ cap cans will be about $15 to $18 per power stage, including P. Millett's snap in cap can pcb.  The pcb has room for the necessary resistors for caps in series.  His boards can be seen here http://www.pmillett.com/ppdrv.htm#the%20Capboard. They will be top mounted with stand offs .  No clamps needed!   Even so $80 for just the power caps IS high, but this amp will be a prototype bass amp for myself.   If I can pull it off, I'll look into less expensive methods, if I decide to try to retail it.

You're definitely doing the right thing playing around with possible layouts. Paper and ink is vastly cheaper and easier to deal with than metal and big holes that have to be neatly executed.

I have to tell you that I would consider a pre-punched 5F8 chassis from mojo or even some sort of adaptation of a Marshall chassis would very possibly make your life incredibly easier. Chromed, silkscreened numbers, right size, punched, done.

The 'plan' is for a cage over the top and no case.  Heavy duty side handles and all.  I wanted a square shape, top tube/component mounted instead of the long skinny, side tube configuration... Which forced me into DIY.  I have the necessary hole punches and drill press and in a previous life, polished steel and AL car parts to a chrome like finish.  It will be very labor intensive and plenty of chances to screw up... But I'll be retiring from 'real' work at the end of the year... And will gave plenty of available time!

Thanks for your valuable input!!!


Edit; I untangled the quotes, Brad

[SoundmasterG]

You can get discrete radial or axial caps for much cheaper than can caps, and in a larger variety too. They also perform better, especially if you locate them in your amp close to the stages that they supply. Mouser sells Panasonic, Xicon, Nichicon, and quite a few other brands and they are cheap to purchase and work great.


Greg