Is transformer box cover safe to use?

[Bwaze]

Recieved my Bottlehead Crack kit, and I'm designing my custom case. I work in a company that makes stainless steel kitchen equipment, so fabrication shouldn't be a big problem. But what exactly to build is another matter. 



I've seen several builds that use transformer covers, and I like the look of that. I have searched on information about potential downsides (higher transformer temperature), but I haven't found much, a couple of threads on old Bottlehead forum.

As I understand transformer on Crack gets pretty hot. Would perforated or vented cover help? I've seen such feature on several commercial tube amps, and Lundahl transformer covers also have this feature.

So, would an oversized stainless steel cover, vented on the back be OK? I'd also add holes around transformer on the amplifier case, and the bottom plate will also be perforated.


Reason for transformer cover is mostly visual. I doubt electrostatic or magnetic shielding is needed.

 

[Doc B]

Temperature rise is the potential issue. Since we have not experimented with covers, we have no data to share. You will just have to try a design and see what happens. I would suggest designing a new bell end rather than a cover for the entire lam stack. That will remove the potential of the transformer overheating from lack of airflow. Years ago we kicked around the idea of a bell end that had cooling fins.

 

[Bwaze]

Thank you for the quick reply.

I will experiment with perforated cover and aluminium heatsinks I found at work (76mm wide 60mm high and 20mm thick), almost ideal for attaching directly to the irons.

Is there a downside directly attaching heatsinks to laminations? Electrical or mechanical?

I will attach them to custom brackets on three sides of transformer, and use thermal grease between laminations and heatsink. Would silicone pads be better / safer, so I don't accidentally make shorts between laminations?

And sorry for the questions unrelated to the build of standard kit. I promise I will share my findings for all who would like to do the transformer cover.

 

[Doc B]

It would not be very effective to attach heat sinks and then cover them with a perforated cover. The heat sink relies upon unrestricted air flow to cool it. Probably ought to try the cover alone.

 

[Bwaze]

There will be plenty of perforation around transformer - I intend to put the power entry and RCA sockets on the back, so there will be plenty of space to increase the cover and insure there will be no big obstructions for convection to work.

But I plan to first assemble it with stock transformer, and then try all the combinations (cover without heatsinks, heatsinks without cover, both installed).

 

[Bwaze]

Would increasing the distance between power transformer and power tube be a bad idea?

I was thinking about increasing it from 65mm (2.5 inch) to 100mm (4 inch, from centre of tube to centre of transformer), due to oversized transformer cover and added distance so that tube is one diameter away from cover).

I know this brings greater voltage drop, more noise from AC heater wiring and greater distance from B+ supply...

 

[Paul Birkeland]

Would increasing the distance between power transformer and power tube be a bad idea?

The Speedball won't fit and the 3K resistors won't fit. 

I know this brings greater voltage drop, more noise from AC heater wiring and greater distance from B+ supply...

I don't think it will bring any of those problems.

You'll need a bigger top plate for something like this, maybe an 8"x12".

 

[Bwaze]

Thank you for your reply!

The bigger plate is not a problem, I'm making a completely new case (ordered the Crack without wood base). I was thinking of extending the back part of the amplifier, not moving power tube forward. Since I don't intend to haul the amplifier around much I don't have to cram it as small as possible, and by making the case bigger it might run cooler and have lots of space for later upgrades (film caps, Valab stepped attenuator, choke...).

I expected to have problems with duplicating non-metric measurements of the top plate, but to my surprise most of the distances and holes seem to be metric (Noval hole diameter is 22mm, octal hole diameter is 27mm, distance between them is 100mm...). You guys secretly converted to metric and never told anyone? 

I plan to put the volume pot and headphone jack on the front and inputs on the back, sort of like this Crack from MoatsArt from HeadFi:

Oh, and I'm working as one of designers and the only CNC machine operator in a small company that makes stainless steel kitchen equipment like this, so fabricating really shouldn't be that much of a problem:

 

[Paul Birkeland]

The key dimension is the spacing between the octal and 9 pin socket, as this is where the big Speedball board mounts, and is also spaced appropriately for the 10 Watt resistors in the stock circuit.

 

[Bwaze]

Yep, measured 66.5mm between holes for stand-offs on big Speedball plate. Shouldn't be a problem hitting that.