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 Post subject: I Have Three Telefunken ECC83/12AX7 Testing Very Low
PostPosted: Sep Sun 05, 2021 3:35 pm 
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As the title says, in one of my Scott LK-48 amplifiers, three of the four 12AX7 tubes are Telefunken and the other is an RCA. These tubes are supposed to be ~ 1040 before being rejected (Restored Heathkit TT-1A). They measure between 250 and 450 as does the RCA. An NOS Amperex ECC83 is ~ 1200. Here is the point, even though these tubes measure way low, they sound just fine in the amplifier. No distortion whatsoever. And substituting NOS 12AX7WA tubes, which also test 1200-1300 sound no different.

That kinda supports what is so often said that the device is the ultimate test of the tube. Yes, I am fully aware that they don't have much life in them left. But I intend to sell the amplifier with these "Telefunken Smooth Plate" tubes with excellent markings on them. I guess I will just leave them in there and make sure to "disclaim" that the tubes are weak. Some are more interested in cosmetics of the tube rather than functionality.

Comments welcome!

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 Post subject: Re: I Have Three Telefunken ECC83/12AX7 Testing Very Low
PostPosted: Sep Sun 05, 2021 4:12 pm 
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I once got some supposedly NOS tubes, I do not remember which type, but miniature of some sort, from Playthings of Past, tested them, and found them well below the specified reading on two different calibrated mutual conductance testers I own. Gary (RIP) was upset that I returned them with the test readings written on the boxes. Perhaps they were not really NOS, perhaps the test method is defective, or perhaps they were not made well.


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 Post subject: Re: I Have Three Telefunken ECC83/12AX7 Testing Very Low
PostPosted: Sep Sun 05, 2021 4:22 pm 
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FStephenMasek wrote:
I once got some supposedly NOS tubes, I do not remember which type, but miniature of some sort, from Playthings of Past, tested them, and found them well below the specified reading on two different calibrated mutual conductance testers I own. Gary (RIP) was upset that I returned them with the test readings written on the boxes. Perhaps they were not really NOS, perhaps the test method is defective, or perhaps they were not made well.


You took the liberty of defacing the boxes then sending them back? That's totally uncalled for. A simple note of explanation would have done just fine, rather than ruining property that you had no intention of keeping. I'd be upset too.

Dan


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 Post subject: Re: I Have Three Telefunken ECC83/12AX7 Testing Very Low
PostPosted: Sep Sun 05, 2021 4:38 pm 
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Well...I agree ....marking boxes...just totally not needed.Things seem to be getting worse...people doing things..just because they can.Not only would...and I have indeed in the past done so...just return with a note,It would never even occur to me to do such a thing....The high road is much less traveled in these times.


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 Post subject: Re: I Have Three Telefunken ECC83/12AX7 Testing Very Low
PostPosted: Sep Mon 06, 2021 12:54 am 
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TO be perfectly ruthless you can disclaim any knowledge of how the tubes test, sell on the basis that the amp still sounds great and you can demo that. Test toobs? How do you do that? The drugstore took out the tester decades ago!

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 Post subject: Re: I Have Three Telefunken ECC83/12AX7 Testing Very Low
PostPosted: Sep Mon 06, 2021 5:47 am 
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Defacing?! They did not test anywhere near the specification, despite being advertised as NOS. I was being helpful, giving him the readings. I am sure he had a large supply of empty boxes.


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 Post subject: Re: I Have Three Telefunken ECC83/12AX7 Testing Very Low
PostPosted: Sep Mon 06, 2021 1:50 pm 
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FStephenMasek wrote:
Defacing?! They did not test anywhere near the specification, despite being advertised as NOS. I was being helpful, giving him the readings. I am sure he had a large supply of empty boxes.


If the tubes were packed in generic plain white boxes I would not consider writing the test readings on them as "defacing".
However, writing on genuine Telefunken (or Mullard, etc...) boxes definitely is.


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 Post subject: Re: I Have Three Telefunken ECC83/12AX7 Testing Very Low
PostPosted: Sep Mon 06, 2021 2:24 pm 
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The Scott LK-48(A/B) and the factory wired version, the 222C, put the four 12AX7 heaters in series across the bias voltage supply. The idea was to cut down on heater-cathode hum in the low level stages by running them on DC.

But there are a few issues. First, metallic filaments do not last as long on DC as they do on AC. (Google 'electromigration' to find out why). As the heaters in the 12AX7s narrow, their resistance goes up and less heat is produced so emission falls off after enough time has passed. Series DC circuits are a bit tricky in that the current is the same everywhere in the circuit, but tube heaters don't necessarily have identical resistances. If one tube has a higher resistance, it will "steal" voltage from the others. This could happen naturally since the heaters won't necessarily age the same way, and if somebody mixes different brands of tubes, or new tubes with old ones, it could throw things off. In some versions of the 222, the bias rectifiers were seleniums which would age and develop higher forward voltage drop, and they were also depending on the four 75-uF electrolytics in the circuit to do their job properly. The thing is, there are at least a couple of ways the 12AX7s could receive less than their rated heater voltages and if that was the case they could develop cathode "interface" which would rapidly diminish their readings on a tube tester.

All this was not really a problem back in 1961 when new 12AX7s could be bought for a buck or two at every corner drug store. Obviously Scott designed the amplifier so it would continue working well with weak tubes. I agree that you probably don't want to spring for four new 12AX7s of the same brand and date code (that's the only way to assure the heaters are as closely matched as possible short of measuring the heater currents of a quantity of tubes) if you are getting ready to sell the amp anyway. I would simply sell it as a working amplifier with used tubes, in as-is condition. Don't make any claims that it meets any particular specifications for gain, output power, or distortion--it probably doesn't if the tubes are weak--but there is an interesting trade-off. With any tube it can be shown that noise is related to transconductance, so the higher the gain of a tube, the noisier it is. Thus your low gain 12AX7s are actually a lot quieter than a new set would be. If it was my amp I would not change them until they gave up the ghost.

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 Post subject: Re: I Have Three Telefunken ECC83/12AX7 Testing Very Low
PostPosted: Sep Mon 06, 2021 3:14 pm 
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Maybe they're like a candle burn efficiently until they don't or a car with a tank full of gas Which runs just fine till its empty and then starts sputtering. Just because it tests low is like shopping for a car based on how much gas is in the tank.

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 Post subject: Re: I Have Three Telefunken ECC83/12AX7 Testing Very Low
PostPosted: Sep Mon 06, 2021 3:21 pm 
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I have been playing this amplifier all day for about four days in the garage/workshop whilst I am trying to straighten up. (Found stuff that I packed 10 years ago, "Oh, that is where it went!!!) So far, so good. Chris has given me quite an education, explaining the "Why" with this. I have seen Telefunken tubes test weak many times, not knowing why. I always thought that they were just loafing along. Guess not.

My problem is solved, thanks to all who took the time to read and respond.

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 Post subject: Re: I Have Three Telefunken ECC83/12AX7 Testing Very Low
PostPosted: Sep Mon 06, 2021 7:25 pm 
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A properly calibrated Heathkit TT-1 will test a new or near new 12AX7 tubes at around 1600. The meter circuit calibration Step 3 is often incorrect due to the resistance when the Meter control is set at max clockwise. It should be a low resistance the closer to zero the better, but in several of these I have found over a 100 ohms. The calibration can still be done by shorting out the meter pot. during calibration, but remember to remove it when your finished. It would be nice if the pot could be replaced, but these are difficult to find. Unless the resistance is very high the test results will still be good.
Bill


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 Post subject: Re: I Have Three Telefunken ECC83/12AX7 Testing Very Low
PostPosted: Sep Mon 06, 2021 9:06 pm 
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FStephenMasek wrote:
Defacing?! They did not test anywhere near the specification, despite being advertised as NOS. I was being helpful, giving him the readings. I am sure he had a large supply of empty boxes.

If you were returning the tubes, you should have returned them in the condition in which they were received. The boxes, being returned, were not yours to write on. I'd have called them defaced as well. It's not your decision whether the owner you are returning the tubes to wants the boxes all marked up or not. That's thoughtless at best. A simple note would have sufficed.

How would you return something to any store.... mark all over the box it came in and then expect a full refund ?

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 Post subject: Re: I Have Three Telefunken ECC83/12AX7 Testing Very Low
PostPosted: Sep Mon 06, 2021 9:23 pm 
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Barry H Bennett wrote:
FStephenMasek wrote:
Defacing?! They did not test anywhere near the specification, despite being advertised as NOS. I was being helpful, giving him the readings. I am sure he had a large supply of empty boxes.

If you were returning the tubes, you should have returned them in the condition in which they were received. The boxes, being returned, were not yours to write on. I'd have called them defaced as well. It's not your decision whether the owner you are returning the tubes to wants the boxes all marked up or not. That's thoughtless at best. A simple note would have sufficed.

How would you return something to any store.... mark all over the box it came in and then expect a full refund ?


+100

Perhaps they tested fine according to the NOS manufacturer's specs. In the 1950's and early 1960's, 12AX7 tubes were used in many hydronic/steam heating systems in differential controller amps, which would balance indoor heating requirements based on outdoor temperature. (Barber Coleman made many of these; I've worked on 'em.) They were never meant to be Audiophile tubes in the first place. They were routinely changed yearly, since they were hot 24/7 in most systems. They were sold in egg-crate boxes of 48 tubes. Just sayin' . . . . . . .

Dan


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 Post subject: Re: I Have Three Telefunken ECC83/12AX7 Testing Very Low
PostPosted: Sep Tue 07, 2021 3:47 am 
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Barry H Bennett wrote:
FStephenMasek wrote:
Defacing?! They did not test anywhere near the specification, despite being advertised as NOS. I was being helpful, giving him the readings. I am sure he had a large supply of empty boxes.

If you were returning the tubes, you should have returned them in the condition in which they were received. The boxes, being returned, were not yours to write on. I'd have called them defaced as well. It's not your decision whether the owner you are returning the tubes to wants the boxes all marked up or not. That's thoughtless at best. A simple note would have sufficed.

How would you return something to any store.... mark all over the box it came in and then expect a full refund ?
They were defective and I was the victim. I fully expected him to put them in the trash, as they tested nowhere near the specified level. They were not some special brand.


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 Post subject: Re: I Have Three Telefunken ECC83/12AX7 Testing Very Low
PostPosted: Sep Tue 07, 2021 10:13 am 
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Victim? Since you were returning them for a full refund or exchange, they were not yours to do with as you saw fit. If you get a defective washing machine and the seller tells you they will exchange it for a new one, is it your right to smash it up with a hammer before sending it back?

What you expected the seller to do is irrelevant, and absolutely none of your concern. All you were "entitled" to is a full refund, or an exchange for tubes that did what you expected. You don't have the right to deface the sellers' merchandise, even if you decide to somehow feel victimized by an error. There was no intent to defraud, just some unknown mistake lost in the distant past at this point. MISTAKE ... which the seller agreed to correct. And somehow, you feel like a victim.

Geezuz.

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