Heat issue ?

[dude]

Here's a picture of a head cab I made for one my AO-43 6v6 plexi conversions. I made a copy of the Marshall JTM-45 head to enclose the AO-43 chassis. After playing for 30 minutes I felt warm heat on top of the uncovered plywood head, over the power tubes. There's about 1 1/2 inch above the power tubes and 1 inch on the sides to the back. Both top and back, near the tubes get warm. Before I cover the cab with Tolex, I don't want to have the wood burn. My plan was to use metal tape, like use on hot furnace ducts around these area above the power tube and on the side back panel.
I would assume that Marshall had their tubes about as close as mine, maybe a little more distance, would the metal tape help reflect the heat. Marshall didn't use much ventilation in the Marshall head cab I copied, just the back top opening that they covered in gold metal grill which would inhibit air flow more...?


At the moment I have EL34's in the amp which are much taller than 6V6's, so with 6V6s 3" top clearance to wood and still about an 1" from the back panel. I plan on using 6V6's mostly.


Thanks, in advance,
 

[acheld]

Unlikely for heat from the tube to burn the wood.  Even up close and personal.

I doubt that metal foil tape will deflect much heat, but it can't hurt.

[PRR]

I've laid a tissue on a full-heat 6550 for a half hour and it didn't burn.

[dude]

Thanks, if a tissue didn’t 🔥 certainly the wood won’t. l’ll skip the reflective tape too,  probably reflect the heat back at the tube.

[ac427v]

You have a nice big slot for rising hot air to escape through. I don't see any cool air access in the lower part of the cabinet to create continuous air flow. Is there an attractive way to allow cool air in?

[Latole]

I'll put a piece of light metal sheet. At any good hardware store.

Duct seal tape may not stay on wood long time.

[dude]

Is there an attractive way to allow cool air in?

Marshall cabs had no openings on the front panel, just like mine. I copied the Marshall heads. I plan on using the gold grill over the back opening, just like a Marshall head cab. Anyone know if Marshall put any protection on the wood over the tube area? If PPR, put tissue on a hot 6550, and nothing, I’m not worried. Probably just bias the amp a little cooler. Besides a negative voltage reading on the bias, anyone know what the % of dissipation Marshall set their bias at when they left the factory?

[Latole]

IMO bias little cooler will not help.
Best sound is the only option... with right bias.

How to keep cab cool is so easy

[dude]

I always thought the hotter the bias the more heat generated by the power tubes. Correct me if l’m wrong, 70% dissipation is the norm for fixed bias amps, maybe 10%, more or less.  I never seen a Marshall head with any opening on the front panel, although some may have had a vent on the top and back, not sure. The 18 watt Marshall combo’s had a vent in the top with the back open but the tubes were horizontal inside the amp. I was just concerned about the closeness of the tubes to the wood, my Hammond AO-43 chassis has the tubes close to the edge and therefore close to the back panel of the cabinet l made to fit that chassis. I’m still thinking about the tissue on a hot 6550 for half an hour and nothing.

[sluckey]

If you are concerned about heat you could always put a small fan inside. Or just remove the back panel or redesign it to open up some more. Look at this Sunn back panel...

[Latole]

I always thought the hotter the bias the more heat generated by the power tubes. Correct me if l’m wrong, 70% dissipation is the norm for fixed bias amps, maybe 10%, more or less.  I never seen a Marshall head with any opening on the front panel, although some may have had a vent on the top and back, not sure. The 18 watt Marshall combo’s had a vent in the top with the back open but the tubes were horizontal inside the amp. I was just concerned about the closeness of the tubes to the wood, my Hammond AO-43 chassis has the tubes close to the edge and therefore close to the back panel of the cabinet l made to fit that chassis. I’m still thinking about the tissue on a hot 6550 for half an hour and nothing.

Class AB amp. I alway bias at 60%, at 70 % with driving your amp at high volume you may damage you tubes.
Heat are more produce with volume you play, not bias.

Why not a steel plate ? So cheap !