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6AQ5 vs Other 7-pin Tube Dissipation Ratings
6AQ5 vs Other 7-pin Tube Dissipation Ratings
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Old 30th August 2018, 08:03 PM   #1
kylej1050 is offline kylej1050  United States
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Default 6AQ5 vs Other 7-pin Tube Dissipation Ratings

I'm building amplifiers right now using 7-pin miniature power tubes. Can anyone explain to me why tubes of similar sizes have such drastically different dissipation ratings?

6AQ5 = 12W
34GD5 = 5W

Both are 7-pin miniature bottles of the same dimension but the second one has a dissipation rating of less than half of the first. The 34GD5 does have a slightly hotter heater at 3.4W vs 2.8W for the 6AQ5. The 50C5 is another with a plate dissipation rating of 7W though in that one the heater is 7.5W so that can explain away a lot of the dissipation discrepancy.

I hadn't even thought about this until trying to use the 34GD5 in an amp and just switching to a 6AQ5 would increase amplifier output power from 1.4W to 4.5W!

The 34GD5 was a late comer to the tube world. My guess is that the dissipation specification was kept low so they could keep heat down on the newer, smaller radios people were building. Possibly able to handle more but rated lower so people wouldn't burn up radios? I figure the radio engineer would be responsible for that though.
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Old 30th August 2018, 09:47 PM   #2
SemperFi is offline SemperFi  Wake Island
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Don't think heater power is concidered when specifying plate dissipation. Though I see your point that very large heater power must increase temp and thus lower the plate dissipation since the bulb temp sets a limit.
Are the bottles the same height? How do the plate's size compare between the two? Are the plates made of different material? 6AQ5 having a blacker plate?
34GD5 does seem to be punier in most specs, lower max Vplate, and there is probably a limit to how much current the cathode can emit, having little voltage to play with, and limited current makes for low power.
You may be right about the concideration to small and compact builds, but sales wise it could not be a wise move, so I doubt that one.
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Old 31st August 2018, 01:57 AM   #3
Tom Bavis is offline Tom Bavis
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6AQ5 would deliver LESS power in a circuit set up for 34GD5, since it can't supply much current at low plate voltages (at 100V, about 35 mA at zero bias). And idling the 34GD5 higher than the specified 35 mA at 110 V would likely REDUCE the output current swing - there's no curve on the data sheet but it MIGHT do 80 mA peak. So even if it could do more than 5W, it would only reduce efficiency.
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Old 31st August 2018, 02:21 AM   #4
Tubelab_com is offline Tubelab_com  United States
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The 12 watt limit on the 6AQ5 is just shy of the melting point in a typical tube.

A really good tube with perfectly collinear elements will red plate evenly when pushed to the limit. A typical, slightly misaligned tube will red plate on a small area while the majority of the plate remains colorless. The typical 6AQ5's that I have abused will exhibit one hot spot about 1/4 to 1/2 inch in diameter. Pushing these beyond 10 to 12 watts of dissipation in class A will eventually soften the glass over the hot spot resulting in a crater in the glass. Continued operation results in the tube sucking air through the crater and dying. Operation in class A should be limited to 10 watts of dissipation since the dissipation is continuous and no signal or low volume is worse case. A push pull amp operated in AB1 or AB2 could be pushed beyond 12 watts dissipation per tube on signal peaks, provided the idle dissipation is well below 10 watts.

The 6AQ5 is emission limited by its lowish powered heater and tiny cathode. Even when pushed to the limit in A2 or AB2 it will not saturate its plate below about 40 to 50 volts, and that is the limiting factor in power output. Even within these constraints, but otherwise ignoring all other specs, I have been able to squeeze 20 watts from a pair on 340 volts of B+ without blowing them up. This assumes average music, not sine waves or over compressed dance music played well into clipping.

There are a wide variety of the "5 watt" variety of small tubes originally intended for use in table radios, TV sets and other consumer electronics. I noticed that the plates are about the same size as the 6AQ5, and proceeded to "test" some of them. The 4.5 to 6 watt ratings found on these tubes is as I suspected quite conservative, so is the plate voltage spec.

Consider the intended application. The tube gets stuck in a radio in the late 1950's through the 1960's. The average family had one, maybe two radios. My mother's table radio was on for at least 8 hours a day whenever she was home. I can assume that other users may have done the same. The output stage in these radios ran class A. Assuming the typical 50C5 using the "typical operating" conditions given in the tube manual the 50C5 burned 110 volts * 50 mA which is 5.5 watts ALL the time. That's right, the book states to run the tube at 100% of maximum dissipation ALL THE TIME! Yes, its a conservative spec. I dissected and fixed these radios and even made guitar amps out of them as a kid. The real B+ voltage even in the 60's was about 125 to 130 volts. When the radio finally died from a weak tube, it was the 35W4 rectifier, not the 50C5.

My testing shows that 10 watts is no problem and red plate doesn't start to show until 15 to 18 watts on most tubes. That's the first bit of good news. I stated that there are a wide variety of these little 5 watt tubes, and they are all different. The plates on any that I have abused are all good for 10+ watts, but they are not all created equal. I briefly mentioned the cathode capabilities when discussing the 6AQ5 and its 2.835 watt heater. As you mentioned the 50C5 has a 7.5 watt heater in a nice fat cathode. That turns out to be a good thing. That hot fat cathode puts out zillions of electrons. When driven hard in A2 or AB2 the 50 C5 can be saturated down to 15 volts or so putting them in TV sweep tube territory. What does this do for us, hoe about 25 to 30 watts from a pair of 50C5's while cranking out continuous sine waves at 2 to 3% THD , or anything else for hours without melting or even looking upset about it. Here the SCREEN GRID is the limiting factor the screen grid voltage spec should be respected. My testing was done with 120 volts on the screen, but 340 volts on the plates. The plate dissipation is under 10 watts per tube, but the screen dissipation touches the 1.25 watt limit somewhere between 28 and 35 watts depending on the tubes. This was done with continuous sine waves. Higher power could be acceptable when used with average music.

Note the published curves for the 50C5. The GE spec sheet that I have gives curves with zero G1 volts at various values of G2 voltage. Note that the G1 = 0 and G2 = 120 volt curve goes well over 150 mA, and the "knee" is around 25 volts at 120 mA. This is the sign of a tube that will exhibit good efficiency, especially when G1 is pushed positive (A2 or AB2).

I have not tested the 34GD5 since I don't have any. The suggested operating conditions for a class A radio is 35 mA on 110 volts, considerably less than the 50C5. The curves top out at 100 mA with zero G1 volts and 110 G2 volts, with a rather rounded knee. My guess is that this tube is also emission limited with a 3.4 watt heater, and may not be capable of the extreme power levels that I have seen from a 50C5.

On paper the 35GL6 with it's hotter 5.25 watt heater should do better, but I couldn't get more than 20 watts from a pair. The 60FX5 was also a disappointment. I suspect that some of the tubes that were optimized for "low drive requirements" give up power output capability to get high Gm.

The odd surprise in my testing was the 32ET5. These have a wimpt 3.2 watt heater, but a newer cathode coating formula. These babys can crank out 25 watts per pair on 340 volts into a 6600 ohm OPT.......8ut wait there's more.....I decided to sacrifice a pair in the name of since, so I set the power supply on 400 volts of B+, screen voltage 150 volts, load 6600 ohms, and hit the go button. What happened? 40 watts per pair happened......no big bang, no glowing plates, just lots of power. After about 10 minutes a pale red golo on both the plates and screen grids could be seen in a darkened room. This power level is probably safe on music, but who want's to push the tiny tubes that far. Measured plate dissipation is 13.6 watts per tube, but the screen grid is 1.8 watts per tube.......So, turn the knob to the right.......turning up the drive yielded 45 watts per pair, but a visible glow was on the plate and a rather obvious glow came from the screen grids. Measured plate dissipation was 17 watts and the screen grids were eating over 4 watts each....don't go there!

As with any operation beyond the specs YMMV. I have been way beyond the specs without blowing a single tube. You might not be so lucky.
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Old 31st August 2018, 04:41 PM   #5
kylej1050 is offline kylej1050  United States
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Wow, that's a lot of info! Thanks!

The 100mA tubes appear to be a bit anemic (32ET5 and 45B5 notwithstanding). My current project is running a 60FX5 pair in SE but my attempt at running AC heaters while sharing a secondary with the DC supply has been a failure due to hum. Pretty sure it's the result of my build as the heater wires needed routed funny to allow me to build the unit the way I did.

I'll play with the 60FX5s and see where I can crank them to. I have a few pairs so experimenting is no problem! That's part of the fun! With a solid state B+ regulator I have an easy time changing B+ levels as well.
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Old 1st September 2018, 01:25 AM   #6
Tubelab_com is offline Tubelab_com  United States
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The 45B5 is a 45 volt 6CW5 and can be made to rock even at data sheet values for a 6CW5 amp.

I built a high gain guitar amp using series string 100 mA tubes. I tried several things before finally chasing the hum out by running the heaters on pulsating DC created by rectifying but not filtering the AC. This way the energy content is the same as hanging the heaters directly across the secondary of the isolation transformer, but the heater of the first tube (most sensitive to hum) is grounded at one end. I added a 47 uF cap to smooth the edges of the pulsating DC for the preamp tubes only to kill the last bit of hum without really changing the heater power. This is a high gain guitar amp, so this cap is probably unnecessary on a HiFi amp with normal gain.

The schematic of the amp is included here. The unusual mosfet load on the input stage allows for a high and variable load impedance on the pentode offering a voltage gain of nearly 1000 when maxed out. The ultra high gain brings up the issue of microphonics in most tubes.

This amp runs from a B+ of about 165 volts depending on line voltage. This allows about 4 watts of power from the $5 70 volt line transformer used as an OPT.

I also included a schematic for a 5 tube guitar amp that uses a unique power supply that runs a 100 mA heater string on DC, makes 165 volts for the output tube screen grids, and 330 volts for everything else. The transformer is a 100 Va isolation transformer. The output tubes are UL84 / 45B5's and the amp makes about 25 watts of power. The preamp circuitry has been hacked up considerably since the schematic was drawn, but ir used UCC85 / 25AQ8 tubes.
Attached Files
File Type: pdf GTA_4T_11-10.pdf (22.6 KB, 51 views)
File Type: pdf HBAC_5T.pdf (29.1 KB, 39 views)
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Old 1st September 2018, 03:22 AM   #7
PRR is offline PRR  United States
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Originally Posted by kylej1050 View Post
...explain to me why tubes of similar sizes have such drastically different dissipation ratings?...
You start from a Need.

A low-price product needs a small power output. Kitchen radio.

A high-price product may need a higher power output. Deluxe livingroom radio, small PA system.

You size the guts appropriately.

Then you reach for a bottle to put them in. Since these are not-large guts, you consider the 7-pin base: it works. It may be a bit roomy for small guts but so what?

The 34GD5 is the spiritual descendant of the 25L6. It makes about a Watt from 110V DC. The 25L6 would do more, but the bulk of the market was down around a Watt, so the 34GD5 has smaller guts. Costs got to be very critical (everybody selling 5-toob radios for $9.98) so everything was shaved.

6AQ5 is a small 6V6, itself a small 6L6. It *depends* on having much more than 110V, but then can do much more. While the 6V6 sold well {g}, a smaller cheaper 6V6 would sell well also. In the 7-pin bottle it is a little tight and shouldn't be pushed as hard as a 6V6 will stand.
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Old 1st September 2018, 02:54 PM   #8
Tubelab_com is offline Tubelab_com  United States
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The 34GD5 is the spiritual descendant of the 25L6.
Radios were expensive things in the early days. As they grew more popular and the demand increased, cost reduction was needed. Ditching the power transformer was the obvious and largest piece of "low hanging fruit."

This meant wiring all the tube heaters in series from line power and figuring out how to get good power output from a 110 to 130 volt B+ derived from tube rectified line voltage. The radios of the day still had wood cabinets and decent sound from a 5 or 6 inch speaker and 2 or 3 watts of power.

Early attempts at getting 2+ watts from a low B+ were not so successful. I have a couple of weird old tubes around here somewhere that actually have two cathodes inside mounted next to each other. I don't remember the type numbers. There were attempts to stick 50 volt heaters inside the cathode of some of the biggest tubes of the day......the 50C6 was a 6Y6 with a 50 volt heater. It was modestly successful, but was instantly replaced with the home run hit, the 50L6.

No, the 50L6, 25L6, and 12L6's are NOT 6L6 derivatives, they are 6W6's with series string heaters. The 35L6 is an entirely different tube created for FM radios so that an extra tuner tube can be added.

Compare the data sheets for the 6W6 and the 50L6, you will see that they are identical with respect to low voltage operation. The 6W6 does have a higher plate voltage spec, and some TV vertical sweep specs. I have some 25L6's that are marked 25L6 / 25W6.

With the huge success of the 50L6, the 6W6 guts found their way into lots of different tubes, and were the basis for the 50B5. The specs for the smaller 50B5 and 50C5 are slightly different due to the closer plate spacing required to stuff the guts into the 7 pin envelope. The 9 pin versions share most of the same specs.

The 50B5 was the table radio tube of the day until impending UL safety regulations forced the re-pinout creating the 50C5.

The later vintage table radio tubes were largely cost reductions of the basic 50B5 / 50C5 design. The 32ET5 output, 36AM3 rectifier, and its 18 volt cousins for tuner duty were the last generation of tubes created before the line voltage operated transistors chased them into obsolescence.
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Old 1st September 2018, 06:58 PM   #9
smoking-amp is offline smoking-amp  United States
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The little 7 pin tubes may look cute and all, but they only produce 2 or 3 Watts in SE mode of table radio Lo-Fi. The OT needed to do P-P with them typically is above 8K Ohm primary (for Hi-Fi). Putting them on the edge of (or into) OT issues.

If you want some Hi-Fi quality with more than 3 Watts, I would suggest using 9 pin size tubes at a minimum, with at least a 65 mA max DC spec. (or typically 3X to 3.5X that for the peak mA spec) to get the OT primary Z down low enough so a cheap OT will give quality sound.

And if that needs to be done dirt cheap, then the following tubes are on the $1 list:

38/12HE7 230 mA DC, 10/15 Watt Pdiss
(suggest 38HE7 as best, since -usually- one can run just the pentode in them, use 21V heater on pins 10 and 12, good for 15 Watt Pdiss then)
6GF5 160 mA DC, 9 Watt
17/12BQ6 110 mA DC, 11 Watt
15/6KY8 70 mA DC, 12 Watt
15/6MF8 75 mA DC, 12 Watt
33GY7 155 mA DC, 9 Watt
33GT7 140 mA DC, 9 Watt
6JQ6 70 mA DC, 10 Watt
24/17JZ8 70 mA DC, 7 Watt
15HB6 65 mA DC, 10 Watt
16AK9 65 mA DC, 10 Watt

Splurge to $3 or $5 a tube:
26DQ5 315 mA DC, 24 Watt Pdiss ($3)
6CB5A 240 mA DC, 26 Watt ($5)
6EX6 220 mA DC, 22 Watt (21EX6 $3 )
16GY5 230 mA DC, 18 Watt ( $3) (6GY5 $5)
12JN6 175 mA DC, 17.5 Watt ($3) (6JN6, 6GE5 $6)
6LU8 75 mA DC, 14 Watt ($5)
12AV5GA 110 mA DC, 11 Watt

Last edited by smoking-amp; 1st September 2018 at 07:19 PM.
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Old 1st September 2018, 09:40 PM   #10
Tubelab_com is offline Tubelab_com  United States
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The OT needed to do P-P with them typically is above 8K Ohm primary
Many of the wimpier tubes may run into the higher impedance range for OPT's, but there are several small tubes that can be made to rock and do it with considerable power output into a reasonable OPT.

Of course I played with these tubes as a kid in the 60's, they were free by the hundreds complete with OPT's and all the other necessary parts. It was a rare trash day walking home from school when I didn't come home with a discarded radio, sometimes I was known to tie a TV set on to my bicycle. Those days faded as I got into TV tubes and tubes from old HiFi sets, mostly mono consoles.

I rediscovered the 7 pin minis when The gauntlet was thrown down for the Hundred Buck (guitar) Amp Challenge in 2011. The little series string tubes proved to be the absolute lowest cost path to a decent sounding low powered guitar amp. I designed and built the little 4 tube amp whose schematic is in post #6. I modified it several times since the original challenge, but it still works good today and it's the only working guitar amp I currently own. Built for under $50, it makes 4 watts with a $5 OPT (5,000 ohms). I did use some 9 pin tubes to see just how much amp I could make for the $100 limit posed in the challenge. The 5 tube amp makes over 20 watts with an Antek power toroid for the OPT (about 3000 ohms).

My testing squeezed 20 to 30 reliable watts of power from some 7 pin tubes and the OPT used for all tests was 6600 ohms, a reasonably common impedance. The key here is to use tubes that were derived from the 6W6, 6Y6 or other high perveyance tube which was originally conceived for TV vertical sweep.
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