So does any1 know how hot tubes should and shouldnt get, got a 73' twin reverb 5881 biased at 60 percent, when i am done playing and i go to put my standby on I can feel the heat coming from the tubes not like egg frying hot but hotter than warm, just want to make sure something isnt wrong and are these fenders made to take the heat from those power tubes, i know the main transformer is right by those 5881's, dont want to burn anything up.....

Regarding airflow, would this be a practical setup (for Showman style):
One fan blowing in (opposite side of tubes), another fan (near tubes) blowing out?
What would be ideal?

Bob_Kennedy wrote:

Also, keep in mind that silverface twins were designed to use 6L6GC tubes. 5881's should be fine, but they are probably running a little hotter than 6L6's would. 5881 are rated for 23w each, 6L6GC are rated for 30w each. Your twin is rated @ 100w. Four 5881's would be rated @ 92w, whereas four 6L6GC's would be 120w. Again, probably not a huge deal, but that could be why they're running hot.

Those 23w/30w ratings refer to their plate heat dissipation capacity, not their power output. The two aren't technically correlated - e.g. El34s are rated at 25w dissipation but will output more than 6L6s when push with high plate voltages. If an amp is running hotter than "normal" with a particular set of tubes, they may be biased too hot, causing them to exceed their plate dissipation ability. There are other possible reasons too, but it's a good idea to check that out first (are they running at 60% of 23w, or of 30w?).

Last edited: Feb 03, 2014 14:36:38

I use one of those clip-on fans to circulate air in my Twin Reverb. It's more than enough air and the fan noise doesn't come through the amp. Maybe the fan could be picked up by a microphone but I use the fan for practice and live gigs. I'm more concerned with heat-sensitive components like the electrolytics in the amp.

If it ain't broke, fix it until it is.

Hot is normal for tube amps. As long as the amps not making funny noises or having the plates glowing ruby red, its okay.

He who dies with the most tubes... wins

Surf Daddies

tubeswell wrote:

Hot is normal for tube amps. As long as the amps not making funny noises or having the plates glowing ruby red, its okay.

+1 If they're glowing they are happy. They are gonna get hot; that's what they do.

Wes
SoCal ex-pat with a snow shovel

DISCLAIMER: The above is opinion/suggestion only & should not be used for mission planning/navigation, tweaking of instruments, beverage selection, or wardrobe choices.

I've used small boxer fans to cool my piggy back heads (that are really biased on the hot side) for years. I glue a piece of foam rubber to two sides of the fan and wedge it under the cap pan pointed in the direction of the tubes. This really cools them as well as the transformers. The foam keeps the fan from sliding around and helps to acoustically isolate the fan's background noise. Solder an AC pigtail to the fan and plug it in to the Aux outlet when needed.

Modern boxer fans are electrically shielded and have generated no issues with RF in any of my amps including my super high gain modified one, the '68 Dual Showman Reverb chassis that is modified on the front end to the hilt. Standing close to the amp you can hear the fan running but it really isn't an issue. These fans also come in a variety of CFM ratings and it doesn't take much circulation to make a big difference, so consider one with a low CFM rating. It will be more quiet.

I like pushing my tubes to the brink of disaster electronically so the fans help to keep them working. I have very little tube issues.

ed

Traditional........speak softly and play through a big blonde amp. Did I mention that I still like big blonde amps?

BTW 6L6 or EL34 output tubes idle at about 400 degrees

He who dies with the most tubes... wins

Surf Daddies

Sarcasm! No I practice what I preach. I've used a fan pointed at my amps to keep the amp cool on several occasions in the past And if the fan is big enough, it also blows a little breeze around the playing area, which is beneficial when there are hot stage lights drawing the sweat out of you.

I know all that stuff about tube heat and I do design my amps so that the output tubes stay 'coolish' (by convection), and part of that is leaving enough space between the big bottles for heat to dissipate as much as possible and part of it is having plenty of ventilation space in the cab. I wasn't intending to be sarcastic at all.

E.g.:

Tube spacing (at least 1 x the tube diameter)

Cab ventilation (about 1/3 of the back of the cab)

These 6L6s are biased at 24W and the amp runs happily for hours.

Another example with about 2 x the tube diameter spacing

He who dies with the most tubes... wins

Surf Daddies

Last edited: Feb 05, 2014 18:35:25

If sarcasm is a problem, I'm glad I didn't suggest a FEM simulation of the heat transfer situation with COMSOL Multiphysics ...