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Old 14th November 2005, 07:31 PM   #1
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Default Tube power ratings

I've been lurking in the tube area for a while, and I've started a project loosely based on this http://headwize.com/projects/showfil...ammer2_prj.htm

But I've changed the output tubes to 2-5687 med mu dual triodes to increase output current for my 64 ohm HD 570's. I've also added pots to adjust output bias and neg feedback to the grounded grid in the LTP (I can't help it, I cut my teeth on SS. I can turn it down to zero if I don't like it)

The question is.... How conservative are the plate dissapation ratings. If I'm running class A, do I need to derate things 50% like with transistors or can I safely run a tube that's rated 4.2w each section, 7.5W total for both sections at 3.5W per sec, 7W total in class A continuous and expect 5000 hrs out of a tube, or what would you say is a safe continuous dissapation. I'de rather not learn by burning up $50 worth of tubes 3 or 4 times.

I've also bought a couple of 833 's I hope to make a big *** SET out of, but I thought I should try something a little more forgiving and less costly for my first glass amp.

I'll take some pics of the board I etched last night and post after I hear some music. The tubes should be on the front porch when I get home this evening, and I may get to fire it up tonight.
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Old 14th November 2005, 07:35 PM   #2
SY is offline SY  United States
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If you don't exceeed other maximums, you can indeed run the tubes full throttle. But that means not exceeding maximum current, voltage, or (very important!) bulb temperature. The last is often a problem in tight constructions with no forced air cooling.
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Old 14th November 2005, 07:38 PM   #3
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Thanks SY That's the answer I was hoping for. It's going to be in the open proudly displaying itself, so temp shouldn't be a problem.


Brian/ at it again with the extra output devices
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Old 14th November 2005, 08:31 PM   #4
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You can generally run tubes at their maximum ratings with no adverse effects. They will usually live longer if operated below their maximums. Many of the guitar amp makers used to run the tubes well above their published ratings, that is why the guitar players are always changing tubes.

I have been collecting the parts for my big *** SET amp using 833A's. I put together a prototype for the purpose of testing some transformers. Pictures are on my web site. I would definitely NOT recommend this kind of stuff for a first amp. The power supply has enough juice to kill you and then cook you to a crisp. Hopefully I will have this done well before you get to building yours. I will post the schematics and construction details when it is finished. There will be two amps, a HiFi amp with about 100 watts per channel. The output transformer is the limiting factor here. There will be a bad to the bone guitar amp that can put out over 200 watts (it only has to go down to 80Hz).

http://www.tubelab.com/833SE.htm
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Old 14th November 2005, 08:53 PM   #5
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Good to hear you made it through the huricane OK. I've checked out your site a while back.

I've came up empty handed on a SE output transformer (except for one that a guy wants $4000 for ) and was thinking parafeed may make it workable with cheaper iron. I decided I need to learn a little more before I go throwing around big bucks on the parts.
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Old 14th November 2005, 09:35 PM   #6
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After spending considerable money to have the prototype transformer made that prompted that web page, and finding out that it wasn't suitable for HiFi, I realize that there are only two reasonable alternatives. Hammond 1642SE for about $250 USD each, or Electra-Print CU5KB for about $350 each. I haven't decided which to use yet. I would like to hear from someone who actually tried the Hammonds in a high power amp. They are the largest (28lbs) of the big SE transformers, so they should have some impressive bass. The prototype transformer that I had made was 20 lbs and it was capable of serious bass, it just couldn't go past 15KHz.

I have the power transformer for the HiFi amp, got it on Ebay from a fellow member of this forum, but no OPT's. I have the prototype OPT which is good enough for a guitar amp, but no power transformer yet. Whenever I find all of the parts for either amp, I will build it. The results will be posted on the web site.
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Old 14th November 2005, 09:51 PM   #7
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Perhaps plate chokes, high voltage PiO-caps and non-gapped OPTs would be easier to find?
Im thinking parafeed.
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Old 15th November 2005, 12:55 AM   #8
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I have only limited experience with parafeed amps, but those experiences convinced me that getting a flat frequency response with generic chokes, caps, and transformers is almost impossible. I have had good luck on low power designs using a CCS instead of a choke. This takes one reactive element out of the circuit. Unfortunately the efficiency and voltage requirements preclude its use for a mega power SE amp.
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Old 15th November 2005, 03:46 AM   #9
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***** WARNING *** AMATURE HACK IDEA *****

How about plate choke with a servo to control bias to avoid the cap. Calculate the voltage drop due to resistance at the bias point, put a 15 V or so (whatever it takes) above the B+ to feed the hot end of the choke, and run the OPT from plate/choke node to B+ and use a servo to regulate the bias and keep the DC off the OPT. Now the only problem is finding a big **** choke that won't arc over at 1500V or saturate on 400 ma.



I got the pattern size way off on my tube sockets, Now i'll have to cheeze one on with pigtails. I knew I should have waited for the sockets before I did the board. I considered soldering the tubes straight to the board, but that would really be bad form.
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Old 16th November 2005, 01:55 AM   #10
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Pinche Mierda

I was about to put power to the thing, and I was looking it all over and noticed that my paralleled resistors that should add up to 500 ohms were 7 @ 75 ohms each. I devided 500 by 7 instead of x 7. Now Ace electronics is closed and I'm stuck reading instead of adjusting and listening. Don't you hate that.
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