Tube stash strategy and testing (long)

[mashaffer]

I picked up a few tubes that are somewhat valuable but don't have enough of each to put them to use (onesies and twosies) so rather than try to scrape up the money to get more I thought it might be a better idea to try to trade some of the more valuable tubes for an equal valued lot of other less expensive tubes or even sockets and iron.

Here is the trick though. I don't have any idea whether the tubes I have are good or not (except that the getters are not white). I also don't have the sockets for many of them (compactrons of various descriptions) or top caps for testing.

I do have some un-needed computer cables however so I thought that maybe I could pull the pin clips out of some connectors and carefully use them and alligator clips (for top cap) to wire each tube into a test circuit and just see that I get reasonable DC bias. Stupid idea?

I have attached a list of my tube stash. As you can see I have some tubes like 6LQ6, 36KD6, 6SJ6 and 6JB6 that seem to be selling for pretty good prices. The only one that I have more than one of is the 6LQ6 of which I only have three. So enough for a mono block but then I don't have anything to build another comparable one.

So one approach is to trade off the 36KD6, 6SJ6 and 6JB6 for another 6LQ6 and some sockets. Or I could trade off the 6LQ6s and some of the other higher priced tubes for some lower priced power tubes and some iron or for more of the less expensive sweep tubes like the 6GE5 or 6JN6 or even some more 6T9s or 6EM7s.

Since I have enough 6EM7 and 6GF7As to make a couple of small SETs my biggest need is the tubes and iron to make at least one stereo PP amp of about 35 wpc or more. A higher powered (100W or more) mono block might be in the cards later (a reason to keep the 6LQ6s) but is not really critical.

So what approach would you take if you were me? What would you try to trade for what? How would you check the tubes that you intended to trade off to make sure you weren't ripping someone off?

[dsavitsk]

It depends on what your time is worth to you, and what you think about economics. You can sell them as untested on ebay, and you'll get a little less than you would for a known good tube. Or, you could put the time in to test them. If they are indeed, good, then you might get more. Of course, you are shifting the risk that they are in fact good onto yourself, and the economists would probably say that you should see the same amount in the end.

That is, say there is a 50/50 chance that some tube is good. If you could sell it for $10 good and $0 bad, then a buyer should be willing to pay $5 for untested, which is the tube's expected value. If you put an hour into testing the tube, and your time it worth $10 per hour, then the tube has in fact cost you $5. If it tests good, you are at $0, and if it tests bad, you lose the $10.

So, I'd let the buyer bear the risk, and sell them clearly labeled as untested.

[mashaffer]

Thanks for the input. Hadn't thought of Ebay as I have never sold anything there. I kinda of prefer doing a trade if I can but would consider the other approach if it isn't too difficult/expensive. I have a paypal account but only set it up so I could buy stuff online. No idea what I have to do to go the other way.

Based on the needs I expressed do you have any input on which tubes to dispose of and what to try to get in return? Would 6GE5 or 6JN6 make nice medium power PP pentode amps (or screen drive)? Or maybe 6T9 (already have one) or even 6BM8 (actually have the right sockets for these)? I probably need to get some 12pin Compactron, Novar and 9pin compactron chassis mount sockets out of the deal somehow to.

The idea of using the horizontal sweep tubes is intriguing but I must admit that their special needs are a little intimidating for a novice.

And speaking of sweep tubes, in pentode mode would there be any advantage to powering the screens from a separate transformer? I have a lot of SS transformers (i.e. low voltage) lying around which could probably supply the low screen voltage preferred by sweep tubes. I suppose the normal approach is voltage divider off of B+ and then SS regulation.

[Miles Prower]

Originally posted by mashaffer
And speaking of sweep tubes, in pentode mode would there be any advantage to powering the screens from a separate transformer? I have a lot of SS transformers (i.e. low voltage) lying around which could probably supply the low screen voltage preferred by sweep tubes. I suppose the normal approach is voltage divider off of B+ and then SS regulation.

For any pentode operated as a pentode, good screen voltage regulation from a Lo-Z source is always a good idea. You could use a separate, dedicated PS for the screens. However, an active voltage regulator operating off the positive rail will also work quite well. You can implement the active regulator with either hollow state (which I did for two projects) or solid state. Either way works just fine.

If you use a separate screen supply, make sure its voltage drops faster than the main positive rail so's you don't overvolt the screens during power down.

[mashaffer]

As I have been reading about screen drive more it seems that it is really the easier way to get clean power from a sweep tube as long as you use a MOSFET to drive the screens (for good current drive) and have a PI stage that can handle the gate capacitance of your FETs. No regulated PS needed.

Would a cathodyne be OK for a PI as long as the idle current was reasonably high or do I need to go LTP? I am thinking that the sand shoud buffer the PI sufficiently from the screens-acting-as-grids (scrids?) so that imbalance issues should not arrise.

I guess I can get into design details on this project after I get the 6EM7 project done (I am always getting ahead of myself).

Here is a thought though. Based on the list of tubes I have I wonder if any of them would be close enough (if I use fixed bias) to use as a push pull pair even though not the same tubes... For example... can 6GE5 and 6JN6 be used as PP pair with appropriate socket wiring for each if bias for each tube is set to the same current? 17JM6 and 17JN6? I don't intend to do a finished amp that way but thought I might be able to do some prototyping that way and then buy the full set of tubes for the final amp.

No takers on which tubes to trade off and which to stock up on?

[tubelab.com]

For example... can 6GE5 and 6JN6 be used as PP pair with appropriate socket wiring for each if bias for each tube is set to the same current? 17JM6 and 17JN6?

The two tubes meed to have the same characteristics in order to have a reasonably low distortion, so ordinarilly, no. The 17JM6 and the 17JN6 are very similar, it might work with these.

I guess I can get into design details on this project after I get the 6EM7 project done

Yes, the 6EM7 makes a nice 1 to 2 watt triode amp. So do the 6GF7's. Make that one first. It will cost far less for all the other parts.

As I have been reading about screen drive more it seems that it is really the easier way to get clean power from a sweep tube

There is a thread here called Tube Sale at AES. A few of us bought up some of the cheap tubes and are in the process of finding their limits. The last few posts (starting from #94) are concerned with screen driven sweep tubes. I am getting 80 to 150 watts from a pair of $2 tubes. Your 6LQ6's (and the other sweep tubes) can be used to make a big P-P amp. However the 4th one will cost you more than 4 lesser known sweep tube.

I have some 24LQ6 somewhere, and I will get around to testing them sooner or later, but I tend to avoid super expensive tubes.

Screen drive is still far from mainstream. Unless you copy a working design, plan to do some experimentation.

[mashaffer]

Thanks George! I will start wiring the 6EM7 project as soon as I scrape up some cash for some of the passive components. I have the EM7s, the chassis, Sockets, Trannies, Pots (I think), crossover tubes, jacks, and most filter caps that I need so all I need to get is some resistors (wire wound power for PS and EM7 cathodes), signal caps, PS choke and some misc. trinkets.

Maybe I should figure out how to use paypal to accept payment and post the 6LQ6s for sale or trade on the Trading post forum. I still feel a little uneasy offering them with no idea of their condition. I could use proceeds to finish the EM7 project and maybe even have a little bit left over.

Of the other horizontal sweep tubes that I have...

$30 6JB6
$10 6GE5
$10 6JN6
$50 6JS6
$ 8 6T9?
$12 17JM6
$ 7 17JN6
$12 22JF6
$30 36KD6

Which would you recommend for moderately powerful PP screen drive? I am sort of leaning toward the 6JN6 as it is among the least expensive, has no anode cap to complicate things and seems to be pretty well regarded. I think I even remember seeing a screen drive schematic using that tube somewhere. It seems like one ought to be able to get well over 50Watts with a pair of them without stressing things too much.

Is the 36KD6 identical to the 6KD6 other than heater voltage? TDSL indicates that the two are acceptable substitutes but I could not find an actual datasheet for the 36 volt version. TDSL also lists 6JN6 and 17JN6 as substitutes as well.

Really appreciate your input as I know of no one else who has used, tested and abused as many different tubes as you.

[dsavitsk]

Originally posted by mashaffer
$50 6JS6

Not sure where you are getting your prices, but these can be purchased new old stock, tested, and guaranteed to work for $35.

As for where to set prices, only you know what condition your tubes are in. If they are in original boxes, and appear unused, there is a good chance that they are just that. But, if the boxes are missing, or there are scorch marks, then you will have trouble convincing people they aren't used.

Also, keep in mind some of the value in tubes from dealers, besides the guarantee, is that there are generally multiples and thus pairs/quads are available. Singlets, especially when they are not dual tubes, can be less valuable.

[mashaffer]

Yes the prices were not meant to indicate an asking price but rather were the prices that I had gotten by a quick internet search just to give me an idea of the relative cost of getting more of the same.

I posted the prices in part so folks could do just as you did and correct any misinformation that I had amassed and also to give us a framework for deciding the best bang for the buck.

I assume that I would need to ask somewhat under the real market price since I have not tested. I will take a closer look this week and see what I can determine from looking at the tubes and boxes I do have branded boxes for a least some if not most of them but whether they are original boxes is a trickier thing to determine.

I will try to borrow digicam and post some pics.

[mashaffer]

As I lay trying to get to sleep my twisted mind came to rest on an off the wall thought so I figured I might as well come down and give it some air so that I might get some sleep.

I was thinking about our horizontal sweep tube amplifier ideas. No doubt the sensible approach is to use a fairly conventional PI with an intervening high voltage MOSFET for a screen drive. However, a random thought struck me. Why not an all TV tube amplifier.

The natural question then was why could not a vertical sweep tube (VST) provide the high current high voltage drive required by the screen grid of the horizontal sweep tube. Something on the order of 6GF7A or 6EM7 springs immediately to mind. They can run quite comfortably at 50mA or so of idle current. If additional gain is necessary I am sure a suitable medium to high mu triode of TV lineage could be found for use as an initial VAS.

I could envision three possible approaches.

1. Use low current section of the VST for voltage amplification and the high current section as a cathodyne which then drives the output tube.

2. Use the low current sections of two VST as a LTP PI and use the high current sections as CFs to drive the output tubes.

3. Same as (2) except use a choke tail ala Boozhound to allow for more voltage swing and possibly to ease the heater to cathode DC voltage spread.

My immediate concern with (1) is the possibility that non-symmetrical load on the PI might cause a problem given the current delivered to the screen grids. I am alert enough to recognize the possible problem but not informed enough to evaluate the extent to which it might actually be a problem.

The obvious concern with (2) and (3) is that the two triodes forming the LTP or CTP are not in the same bottle and therefor might not be as closely matched as one would desire. Again I am not smart enough to know if this would really be a problem. I suspect that a CCS in the tail or large inductance should reduce the effects but a 100mA CCS might not be exactly trivial.

So am I barking in the wrong forest here? Go ahead, be brutal, I can take it.