Straight question:What's the optimal rating for power tubes?

Tried to search here for some direct answers, but just found a lot things depending on matter of taste or technical disagreements about cold or hot and so on...

I want to use other tubes than Mesa, but it would be nice to know to best ratings for 6L6s and EL34s. Just looking for longlivety and the best for the amp.
It is a Road King I.


Thanks

If you define "best" as "longevity", then you may want tubes that are "cold".

Someone will be along shortly to tell you that "cold" tubes will result in "bad" sound.

The opposite opinion, of course, is that "hotter" tubes will give a "better" sound.

The trade-off being that "hot" tubes will not last as long as "cold" tubes.

Most of the time, a tube somewhere in the middle of the extremes of "hot" and "cold" will result in the "best" balance between "best" tone and "best" longevity.

I would buy a "hot" set of tubes and a "cold" set of tubes, then let my ear be the judge.

Thanks MarkIII ! Well, somewhere in the middle between hot good sound and cold long longelivity. Buying 4x6l6+2xEL34 frequently will become expensive..


But: I was really looking for numbers, bias values, so I can ask specifically at a tube seller

Just tell the seller which amp you have, they'll fix you up.
There are usually a range of values, and tubes are picked to fall within that range. If you want exact, you will need to modify your amp to be able to test the bias on your amp, then adjust the bias with a trimpot to fit whatever tubes you buy.

norskmetall said:

Thanks MarkIII ! Well, somewhere in the middle between hot good sound and cold long longelivity. Buying 4x6l6+2xEL34 frequently will become expensive..


But: I was really looking for numbers, bias values, so I can ask specifically at a tube seller

Click to expand...

6l6: 26 - 28
El34: 38

I usually use =C='s in my Mesa heads, and buy from Watford Valves or Hot Rox in the UK. Order online and confirm they are for Mesa ratings, and you'll be set. Both will ship to Europe

I have two Mark lll's, one blue stripe simul-class that has Mesa 420 6l6's and =C= el34's and a black dot early 100/60 with the 105 transformer and =C= 6l6's rated @ 21.5 ma. When I bought the =C= matched quad I told the vender, Antique Electronic Supply, what amp they were for and that is what they sent. I thought 21.5 ma wouldn't sound all that great but they really sing compared to the tubes that came with it and the groove tubes I bought when I first bought the amp. The blue stipe came with Mesa STR 416 el34's and Groove tube 6l6's so the Mesa 420's and the =C= el34's did wonders giving it much needed horse-power. I also tried a bunch of pre-amp tubes which also made a huge difference. I play both amps about equally but I really like the Black dot so much more you'll probably see the blue stripe for sale soon. I did notice my favorite 12ax7 in the black dot sounded terrible in the blue stripe, and vice versa, whether that has more to do with the amp or power tubes, I can't say.

6l6: 26 - 28 and El34: 38 ? mA ? For each single tube I guess?

I have been buying TAD 6L6GC's and Tung Sol EL34B's for a while from eBay. They sound great but sometimes the volume just drops and comes back..

Bumpedybump!

Preface: I apologize if you already know this info.

Simple answer: The general rule of thumb is "optimal" tube performance (longevity vs tone) is when tubes idle at ~ 70% of maximum plate dissipation.

The ratings of the tubes you mention should be safe, ASSUMING these numbers are derived when the tubes are tested at the same plate voltages as YOUR amp. Power tube pairings should be matched to within 5% of each other with respect to idle current draw. I've seen two problems when buying tubes. First, many on-line sellers (eBay, etc) only "match" the tubes with respect to mutual transconductance, which tells nothing about how much current the tube draws. Transconductance is a measure of how much life the tube has remaining. Second, when tubes are matched with current draw, the plate voltages of the testing unit don't match my amp. But when you buy from a reputable, knowledgeable dealer, they know your amp and can supply tubes that are matched TO YOUR SPECIFIC AMP.

The general rule of thumb is "optimal" tube performance (longevity vs tone) is when tubes idle at ~ 70% of maximum plate dissipation. For instance, EL34s are rated at 25 watts. If your amp draws 475 plate volts at idle, the max dissipation is 53 ma, and 70% would be 37 ma, so that is the max you'd want the tubes to draw when idling at 475 plate volts. In an adjustable bias amp, one would simply adjust the bias until the tubes are drawing the 70% number, like this:http://www.eurotubes.com/eurotubes-Generic-How-To-Bias-Video-Marshall.htm

Using the above example, in a Mesa FIXED BIAS, the tubes become the variable because there is no bias adjustment. You'll want tubes that idle =< 37ma. Anything below that is cold; above that is hot. Above 53 (100%) can be dangerous or will burn through tubes quickly. For instance, DC-3s have EL84s, which are rated at 12 watts. The DC-3 runs 400 plate volts, so the dissipation is calculated as 12 / 400 = 30 ma, at 70% = 21. So ideally I'd look for tubes that draw 21 ma at idle because I cant adjust the bias unless I have an adjustable bias installed. But EL84s don't come that "cold", and 25 is about the coldest I can get, so the DC-3 runs at ~ 83% dissipation (25/30 = 83%), meaning it is hard on tubes, but sounds very good!

I hope this helps.

Dodger - just came across your post. Best explanation yet I've read on this subject. I have a pair of el34s in my Mark IV (460 plate voltage I believe) at 29ma. Also have a pair at 42ma. The ones at 42ma are about 79%, they run hot but sound great.

Do you know what 6L6s are rated at, is it 30 watts or 19?

Anyway, I'm glad I came across your post.

Thanks again.

"The voltage and power rating of the 6L6 series were gradually pushed upwards by adding features such as a Micanol base, thicker plates, thicker grid wires, grid cooling fins, and special ultra-black plate coatings. The original metal version was rated for 19 watts dissipation while the later 6L6GC is usually rated for 30 watts." From Wikipedia.