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Old 2nd April 2013, 10:50 PM   #1
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Default Biggest noval power pentode?

This has probably been discussed before in some of the A2 design threads, but difficult to search for, so can anybody point me to the biggest (i.e. highest dissipation), commonly available noval power pentode? TIA.
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Old 2nd April 2013, 11:09 PM   #2
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You did say Noval. IF you can find them, 6GM5s are a likely candidate. The innards of a 6GM5 are "identical" to those in a 7591. Anode dissipation is limited to 19 W.
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Old 3rd April 2013, 06:29 AM   #3
gianis is offline gianis  Greece
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EL520 bY Philips ,,27w anode dissipation
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Old 3rd April 2013, 07:48 AM   #4
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EL520 (=EL503) is Magnoval type, not Noval.

EL 520, Tube EL520; Röhre EL 520 ID29781, Beam Power Tube
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Old 3rd April 2013, 09:22 AM   #5
Yvesm is offline Yvesm  France
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Quote:
Originally Posted by Eli Duttman View Post
You did say Noval. IF you can find them, 6GM5s are a likely candidate. The innards of a 6GM5 are "identical" to those in a 7591. Anode dissipation is limited to 19 W.
It's fat ! Look at diameter: 1.188 inch.
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Old 3rd April 2013, 01:11 PM   #6
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The standard size Novals are limited to about 12 or 14 watts of plate dissipation before melting. Physics and the melting point of the glass dictate this. The EL84 / 6BQ5 is the most common of these.

You can squeeze 25 to 30 audio watts out of a pair in push pull AB1, and just over 35 watts in AB2 if you use good tubes (NOS 6BQ5 or JJ EL34) and bend the plate voltage spec a bit, without exceeding the dissipation limit. I use 430 volts to the 6600 ohm OPT CT and 325 screen, fixed bias.

NOS 6HA6 / 6HB6 is a similar tube with more Gm and an odd pinout, but it costs $1. I have couldn't pass up the 500 tubes for 35 cents each sale, so I will be subjecting a few to some extreme power tests to explore the limits of physics.

The previously mentioned 6GM5 is a 7591 with a 9 pin Noval base. It is rare and espensive, and fat. I have two, and will not abuse them.

There are a few other fat bottled Novals with big insides, with a 12 to 16 watt rating. I have not really squeezed any of them for max power yet.

The 7754 is a 16 watt tube, low plate voltage rating....I don't have any.

The 7695 is the same tube with a 50 volt heater intended for cheap line powered HiFi. I have two. Big plates look like they will crank. I will torture them some day.

The 6GC5 is a fat bottled tube with big plates, but only a 12 watt rating. I have found two different constructions, and have several of each. Slightly smaller than the 7695. They look like they could be squeezed pretty hard, but I haven't tried them yet. They are $2 to $4 depending on seller, and quantity.

There are a few more that I can't remember, but they aren't common.
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Old 3rd April 2013, 01:53 PM   #7
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I forgot to mention the Bendix Red Bank 6094, but I wouldn't call it "commonly available." I have a dozen or so but most have no getter left, indicating some possible severe service.

They were designed for severe military service, and defy physics by the use of high temp meterials including Nonex glass good to 300C. All internal spacers are ceramic. They are conservatively rated for 10,000 hours at 14 watts plate dissipation.
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Old 3rd April 2013, 03:38 PM   #8
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Quote:
Originally Posted by artosalo View Post
EL520 (=EL503) is Magnoval type, not Noval.
Ooops, actually Magnoval is OK too, I was thinking 9 pin as the necessary spec.

Thanks for all the info guys. I think there must be some bigger, and more commonly available sweep tubes than those mentioned so far. There is the 13GB5, which is 17W and Magnoval.
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Old 3rd April 2013, 04:09 PM   #9
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Ok, if we include Magnoval types, there are lots of beefy sweep tubes, e.g. the 6KG6A with a maximum plate dissipation of 35 watts.

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Old 3rd April 2013, 05:20 PM   #10
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Ooops, actually Magnoval is OK too, I was thinking 9 pin as the necessary spec.......Ok, if we include Magnoval types, there are lots of beefy sweep tubes
The Magnoval base has 9 pins arranged in a .687 inch circle. Each pin is .050 inch in diameter. These tubes were used primarilly in Europe. The Novar base has the same .687 inch circle, but each pin is .040 inch in diameter. These tubes were used primarilly in the USA.

The cheap Chinese sockets intended for Novars, can be used for Magnoval, but once a Magnoval is inserted a Novar will not work, it will fall out. These sockets are the same diameter as the common octal socket and the 12 pin compactron socket (1 1/16 inch) so that socket swapping is rather easy.

There are a few large sweep tubes in the Magnoval base, but most are Novar. Large sweep tubes come in two varieties. Those that were used in ham and (illegal) CB RF amplifiers, and those that were not. Most of these RF amplifiers were poorly designed so that they ate tubes rather quickly. Some used as many as 10 or 12 tubes. The supplies of any sweep tube that was ever used in an RF amp have been depleted, thus driving the price into the ridiculous region ($50+ per tube). The tubes that were not used in RF amps are still available for $1 to $5 each.

If you are now considering the Magnoval, it makes sense to also consider the compactron, octal, and Novar, so now the choice comes down to price, diameter, height and whether or not you can live with a top cap. So you figure out how much space you have, how much audio power you want, and how much you want to spend, then search for a tube that is in good supply.

The previouly mentioned 13GB5 is hard to beat. It is 1.18 inches in diameter, the same size as the fat Novals, 4.125 inches tall, and can crank out 100+ watts of audio continuously without melting (starts to glow at 150 watts), and costs $1 or less.

Details starting in post #66 of this thread:

Mono push-pull driver PCB

Other tubes I have chosen based on $ per watt:

25DN6 - Octal, still on the $1 list. Makes 100 to 150 watts per pair.

35LR6 - Compactron. I got a bunch cheap at a hamfest, I have seen 250 watts from a pair.

6LW6 - 26LW6 - 36LW6 Octal, these come in several versions. The biggest are huge and the upper power limit is somewhere over 300 watts per pair. I ran out of power supply before the tubes started to glow. Getting a bit hard to find now. The small ones will do 200 to 250 watts per pair.

6CB5 - Octal, I met this tube back in high school (1969). I couldn't blow it up then, and they still crank out 125 watts per pair. About $4 to $5 each.

6BQ6 - Octal, another tube from my distant past. I made guitar amps from them as a teenager. They were free from dead TV's at the county landfill. I recently got a box full for 98 cents each, and another box for 69 cents. I got 125 watts from a pair before they melted. 50 to 75 watts should be OK. 12 volt versions are still $1.

6HJ5 - Compactron, Still $5 each and makes 125 to 150 watts per pair with NO plate cap.

6GV5 - Compactron, Was on the $1 list until Pete stuck them in his engineers amp, and I extracted 100 watts from a pair. Still seen for $4 or $5.

There are dozens more, just see what you can get cheap. Any horizontal sweep (line output) tube should be capable of 50+ watts per pair, usually lots more. All testing was done with one of Petes red boards, or one of my driver boards in conventional G1 drive. Sweep tubes NEED a regulated screen supply of about 175 volts. Plate voltage was 500 to 650 volts in most tests, and optimum plate load impedance was determined by trial and error. 3300 ohms is a good place to start.
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