The Edcor meets the 6AV5

[tubelab.com]

That´s bad news, I recently bought a few NOS Sylvania as future replacements for my 6B4G amp.

Well, I thought about this for about 10 seconds, and then I opened up some 6AV5 boxes. One of the tubes in the original test is a Dumont brand. Upon further inspection these are made by Sylvania. I have 6 distinct types of 6AV5 tubes. Two types of RCA's (with and without the slots in the radiating fins), two GE's, a Raytheon, and the Sylvania. I have just finished torturing (I mean testing) each type. The dissipation figures that I use are total (plate and screen) tube current times actual plate to cathode voltage. All tubes were tested at 300 volts triode strapped unless otherwise noted. All of this testing was done at very low volume (milliwats), this is worst case for dissipation. Sherri has freinds over, so I can't crank it up loud anyway.

The slotted RCA's start glowing at about 15 watts, this is only slightly above the 13.5 watt rating. These all have the newer style lettering on the boxes and two letter date codes.

The non slotted RCA exhibited no visible redness at 30 watts. At this plate dissipation, I could see the screen wires begin to glow if operated at 325 volts. No glow was seen at 275 volts and 30 watts. These tubes all have taller glass and 1963 date codes. 4 tubes tested.

The Raytheon (I only have one here) started to glow at 23 watts.

There are two types of GE's. The newer ones have plates with rounded corners that look identical to those in the 6FW5 which carries a 21.6 watt (total) rating. I tested 2 of these. One showed a slight redness, only visible with the room lights off at 27 watts. The other showed no color at 30 watts.

The older GE's have the smallest plates and smallest envelope of any of the tubes tested, but they seem happy at 27 watts, and just show slight redness at 30 watts. 4 tubes tested, all the same.

The Sylvanias are clearly the best of the bunch in the overload department. When you push them hard enough to get them glowing the entire radiating fin glows evenly, no hot spots. I got a few minutes to listen to the Dumonts at 315 volts, 80mA, which is the sweet spot for the Edcor transformers. They do sound nice, but I am not quite raedy to trade in my (Chinese) 300B's. There is no visible at this level, and it takes 30 watts to see any. 2 Sylvanias, and 2 Dumonts tested. Both Dumonts have the Sylvania EIA code 312 on them. The included photo was taken at 315 volts and 130mA, this is 41 watts.

Considering that a 6B4 is only good for 15 watts, I would use any of these tubes except the slotted RCA. Guess which one I have the most of?

[isaacc7]

Hey, great stuff! I have changed my mind and have decided to try a parallel single ended 12av5 amp, maybe three tubes per channel. I've got mostly Sylvania tubes, so it looks like I got lucky:-) Any recommendations on good transformers for this sort of load? How about some cheap ones to experiment with? Thanks and keep up the torture on those sweep tubes!

Isaac

[isaacc7]

Oh one more thing, that sure is one purty picture of a glowing tube:-) Which Edcor transformer are you using?


Duh, never mind, it's right in your first post... Sounds like a great transformer to experiment with and not sweat it too much if you screw up and fry them... I'll probably order some later on. Do you see any that they make that would be OK for paralleled 6av5 tubes?


Isaac

[tubelab.com]

3 tubes in parallel, that would want a load of 1.5 to 2 k ohms. The only Edcors that come close are the CXSE25 series. They are $75 each. Another possible choice is the One Electron UBT1 for $85. I have not tried either transformer. I have used One Electron UBT-3's in 300B amps and they work very well. The Edcors are the only ones with UL taps.

If two tubes would provide enough power and you wanted to save money, you could get the 16 ohm version of the same transformer and run it with an 8 ohm load.

SY planted an idea in my head that I can't shake, so I am going to try 2 6AV5's in P-P with screen drive. I have all of the parts, so I am going to make some sound, or some smoke. I don't know which yet!

[isaacc7]

Thanks for the suggestions. I'll have to see how it goes. My first project is making a bench supply or two based on what's on your web site:-) Let us know how your experiment with screen drive goes!

Isaac

[isaacc7]

Does the trioded 6av5 actually respond like a 6b4g? I ask because when I look at the data sheets for the 6b4g it specifies an output impedance of around 2.5k instead of the 5k that you're using. Are you using this transformer just because you have them there and you wanted to see what they'd do despite being the wrong spec, did you do something else to make it a good match, or am I making a basic newbie error?:-) I can't find any info on 6av5 in triode mode other than it is supposed to pass for a 6b4g, so is that a good place to start? Thanks!

Isaac

[tubelab.com]

The trioded 6AV5 = 6b4 story has been around for a long time. It has surfaced many times over the years, and I always dismissed it as an urban legend. The story is that Phillips or Sylvania built some triode wired 6AV5's with 6B4 pinouts to satisfy a military contract. Well if you look at the "tube gallery" on the Angela instruments web site (www.angela.com) you can see a picture of one, so the legend is true. This means that the tubes were similar enough to be interchangeable IN THE ORIGINAL APPLICATION. There is no way that the two can ever be identical.

Someone posed the question in the original thread:

Anyone know anything about a 6AV5GA?

So I decided to try this out. During those tests I used a One Electron UBT-3 transformer which presents a 3K ohm load. I believe that this is about optimum, but I never did load VS power VS distortion testing. I did swap back and forth between the 6AV5 and the 6B4, and they are a lot more similar than I would have thought.

Then there was another thread:

Cheap SE output transformers

This thread was about an $18 SE output transformer. The lowest impedance that this transformer comes in is 5K ohm so I bought two of those. I tested them with 45's and 300B's.

Then I was at the Orlando hamfest where I got a box full of 6AV5's for $3 each. Well, I just connected all of this stuff together because it was sitting on my bench asking to be tested. I have been listening to this amp for over a week now, and I like the sound of it, so I have been listening instead of testing. I still don't know how much power or distortion I am getting. I will get around to testing sooner or later.

I think I am going to get another pair of the SE transformers with a 16 ohm secondary. This will give the tube a 2.5K load with an 8 ohm speaker.

SY's comments have got me building a P-P 6AV5 amp with screen drive. I'll get a pair of the cheap Edcor P-P transformers for this one too, but the order will have to wait until I find out what the optimum load for this amp is.

[isaacc7]

Thanks for the info! I'm back to trying a 12av5 SE amp as my first scratch build since I have them and they're really cheap. Your dissipation tests really piqued my interest, but I want to make sure that I get the right transformers, even if they are cheap:-) If 2.5k is indeed the ideal load for a single tube, I might pony up the money for the 25watt speced transformers from them. Keep us in the loop with your experiments, good data on audio usage of sweep tubes can be difficult to get and I appreciate anything I can get:-)

Isaac

[tubelab.com]

As I said earlier I have about 75 pounds of sweep tubes, and more in boxes that I haven't gone through yet. I will try more experiments as time permits, but I don't have much time right now.

I may have not explained it fully in my earlier posts, but If you intend to build an amplifier and keep it around for a while, don't push the tubes too hard unless you plan to change them often. Transistors just blow up when you push them too hard. Tubes can be operated above the maximum dissipation rating at the expense of shorter life. It is not uncommon for a guitar player who plays 4 or 5 two hour shows a week to need new tubes in six months or less. NOS tubes seem to live longer than many current production tubes due to better vacuum, and most sweep tubes were over designed due to the severe service they were in.

6AV5's, 12AV5's and 25AV5's are cheap (for now) so if you plan to push them hard get some spares. My rule of thumb is to turn off the room lights and look for red spots on the plate, and glowing grid wires. Back off the bias until this goes away.

Some of these tubes seem more sensitive to voltage than others. I am using a variable power supply so I can watch the current as I turn up the voltage. All triode connected sweep tubes will have a point where the current starts to rise quickly. Operation at or above this point could cause an uncontrolled runaway condition. I beleive this is because we are way above the screen voltage rating. This happens at about 295 volts on some 6AV5's while others are happy with 350 volts.

If you are going anywhere near this you should put a 100 (or 125) milliamp fuse in the cathode leg. It may save your transformers if a tube fails.

[isaacc7]

Originally posted by tubelab.com
As I said earlier I have about 75 pounds of sweep tubes, and more in boxes that I haven't gone through yet. I will try more experiments as time permits, but I don't have much time right now.

No problem, I think I speak for everyone on this board when I say that we appreciate you sharing whatever time you can spare. With any luck, I'll be able to add to the knowledge pool before too long...

I may have not explained it fully in my earlier posts, but If you intend to build an amplifier and keep it around for a while, don't push the tubes too hard unless you plan to change them often. 6AV5's, 12AV5's and 25AV5's are cheap (for now) so if you plan to push them hard get some spares. My rule of thumb is to turn off the room lights and look for red spots on the plate, and glowing grid wires. Back off the bias until this goes away.

Some of these tubes seem more sensitive to voltage than others. I am using a variable power supply so I can watch the current as I turn up the voltage. All triode connected sweep tubes will have a point where the current starts to rise quickly. Operation at or above this point could cause an uncontrolled runaway condition. I beleive this is because we are way above the screen voltage rating. This happens at about 295 volts on some 6AV5's while others are happy with 350 volts.

If you are going anywhere near this you should put a 100 (or 125) milliamp fuse in the cathode leg. It may save your transformers if a tube fails.

Good advice, I'll probably do the fuse thing even if I'm not going close to those numbers. Hopefully it'll protect them in case a tube fails either due to it being driven too hard or my stupidity... In any case, I was figuring I'd back off a bit, I prefer designs run well within the limits of the tube. I have half a mind to make this a present to someone and i want to make sure that they could put in any 12av5 and have it work well. Plus, it's nice to be able to set up an amp and forget it for years... With the dissipation numbers you were posting, it sounds as though I could back off from 10-12 watts of output power to 6-7 and keep the tube around for a long time to come. I don't think the miniscule difference between 7 and 10 watts is a good enough reason to shorten tube life:-)

Isaac