Hey everyone,
Im revisiting the big monster amp, a 200W 6 x 6550 beast. Placement of those six big valves is causing me a problem, as I dont want a totally mahoosive chassis. Im thinking of a 2 x 3 arrangement, with forced fan cooling in an otherwise traditional Hiwatt / Orange style cabinet.
The KT88 datasheet suggests 4 inches / 100mm between valve centers, but this just seems excessive, as I would have to give up loads of my chassis real estate.
What kind of spacing would people recommend? Does 3 inch / 75mm seem sensible?
Cheers!
Im revisiting the big monster amp, a 200W 6 x 6550 beast. Placement of those six big valves is causing me a problem, as I dont want a totally mahoosive chassis. Im thinking of a 2 x 3 arrangement, with forced fan cooling in an otherwise traditional Hiwatt / Orange style cabinet.
The KT88 datasheet suggests 4 inches / 100mm between valve centers, but this just seems excessive, as I would have to give up loads of my chassis real estate.
What kind of spacing would people recommend? Does 3 inch / 75mm seem sensible?
Cheers!
According to guitar amp manufacturers: barely any spacing...but they also support one-year-tubelife...
4inches is overkill. I've seen one tube diameter as ok. (Not center to center, but tube edge to edge)
4inches is overkill. I've seen one tube diameter as ok. (Not center to center, but tube edge to edge)
If you are going to have a forced fan cooling, you can definitely get away with less than 4" CTC, so one tube diameter between tubes should work. Still gonna be a big chassis though... But how do you get 200W out of 6 6550s? Datasheet suggests 1 pair for 50W, 3x is 150W.
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The GE datasheet suggests a power output of 100W per pair if you run them fixed bias at 600v anode and 300v screens.
http://www.drtube.com/datasheets/6550a-ge1972.pdf
In practice, you probably wont get that much output from a modern valve, Im expecting around 70W a pair.
Apparently, if you run them with 400v on the screens you can get the full 100W a pair, but thats only an increase of 1.5dB for substantially shorter valve life - I dont think that decibel and a half is worth the extra cost.
The datasheet says they are 1.813" max diameter, so if I space them out by one tube diameter then thats still 3.626" between centers. Still, every little bit of space counts - thanks guys! 🙂
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Wouldn't go below 1 diameter spacing.
FWIW there's *vacuum* inside those tubes, a fan only cools *the glass* but does nothing for plates, which still rely 100% on radiation.
FWIW there's *vacuum* inside those tubes, a fan only cools *the glass* but does nothing for plates, which still rely 100% on radiation.
In Morgan Jones First Edition, "Valve Amplifiers", he recommends 1 1/2 envelope diameters spacing. Personally, I would tend to think you could "get away" with One diameters worth.
__________________________________________________Rick..............
__________________________________________________Rick..............
True, but hot glass will have its own blackbody radiation, heating (to some extent) the internals.
Removing that heat source can only make things run cooler.
So, I guess this means that any other components, such as the transformers or a heat shield, would need to be at least half a valve diameter from the edge of each valve envelope?
With plates between the tubes and forced air cooling, you may be able to get down to about an inch between envelopes, but you truly are testing your luck.
This might explain why Im a little confused:
Two Ampeg SVTs;
Trace V8;
Mesa something or other, maybe a 400+
All of these have the power tubes crammed in. Granted, the modern SVT, the Trace and the Mesa all have forced cooling, but it goes against all the advice here.
Anyway, I think Ive got a layout sorted that allows nearly a full envelope diameter between tubes. Im running them quite cool anyway, so I should get decent valve life.
Cheers everyone 🙂
Two Ampeg SVTs;
Trace V8;
Mesa something or other, maybe a 400+
All of these have the power tubes crammed in. Granted, the modern SVT, the Trace and the Mesa all have forced cooling, but it goes against all the advice here.
Anyway, I think Ive got a layout sorted that allows nearly a full envelope diameter between tubes. Im running them quite cool anyway, so I should get decent valve life.
Cheers everyone 🙂
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Yes, the modern SVT layout moves the output tubes into two staggered rows to improve separation and cooling.
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A pair could easily do 70-100w really wound up and yes they do run pretty damn hot regardless how they are run but you can still have them pretty close together no problem.... and man you gotta have NOS tung-sol or the GE 6550s....these new production Russian/Chinese just don't cut it on the 6550 tone.... they are far more simple sounding....The GEs are more aggressive and can handle some higher voltages better...while Tung-sol has the best smooth clear/hauntingly real/BIG 3D sound in my opinion.
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Unfortunately NOS tubes are way out of my price range! 🙁
Moving house soon, so this is on hold yet again. Hopefully I'll get this done before 2017.
Moving house soon, so this is on hold yet again. Hopefully I'll get this done before 2017.
Man you might be able to and get lucky on scoring a a strong/decently strong pair of at least the tung-sol 3 hole grey plates for around $100 on ebay. The Damn GEs though are usually going for $200+ a pair though... they are not worth that to me for one they sound inferior to the Tung-sols but they handle voltages a lot better... The Sylvania 6550s even are up there too and they also are on the lower end of sound quality to me too....
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This is exactly why I love sweep tubes so much. Big power and NOS tone for cheap! The GU50 is another interesting tube that can be had for cheap, I've always wanted to make a bass amp out of 6 of those bad boys. They look really cool as well. The point being, if it's a one-off project, why not think outside the box when it comes to tube selection?
Nothing wrong with that at all man, there is ALOT of oddball tubes out there...same can be said about very different/oddball circuits, If it is safe and works it can get very interesting for sure.
Forced air cooling adds a lot of options. The SVT at top, when assembled, essentially has the power tubes in a duct with air being blown through. The real rule for tube spacing is that the envelope temperature should not exceed its max rating, ever. How you get there is up to you.
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