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Old 10th July 2007, 07:19 PM   #1
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Question Unknown Tube Amp

Recently A friend of mine was cleaning his house and came upon an old tube amp in his attic. Now, according to him, this thing was originally a power supply and amp for an electromagnetic speaker (I am unsure as to whether or not this is true). He gave it to me because he has two businesses to manage and can't "mess around anymore".

Now, onto the amp! <-- I have all the needed pictures on my website so you can see it without the pictures bogging down diyaudio's server

This thing uses two 6L6GC tubes as it's power tubes and a 12AU7 as it's onboard pre-amp tube, it also has one other tube that I am unsure of as it's writing has completely worn off. The tubes and amp itself are all over 40 years old (I think). If someone could identify this unknown tube I would really appreciate it. The amp only has taps for a single 16 ohm speaker, so I hooked up two of my 6 ohm full range monitors in series for 12 ohms on the 16 ohm tap and it sounds BEAUTIFUL! I have never heard a tube amp before this and was completely stunned by how defined the sound was, and how clean the tone of this ancient amp was. Frankly I was stunned it even worked, let alone work as well as it did. I say did in past tense as I (being a complete klutz and utter bafoon) knocked one of my monitors over on the one of the power tubes. I felt so horrible that I immediately turned around and order some "winged C" SED 6L6GC's from tubedepot.com

Sorry for the story but it may help some of you understand where I'm coming from, basically my questions are as follow's


1. What is the unknown tube?
2. Is it bad for me to use my 12ohm speaker load on the 16 ohm tap?
3. I was using it hooked up to my computer and somtimes I am not playing constant music through it, is the absence of sound on the input a problem for a tube amp? I read somewhere that it was, is this true?
4. I sometimes listen to music for long periods of time and the transformers get quite warm, so much so, that I don't like leaving my hand on them for long. Is this ok? they're not burning hot, just uncomfortable to touch for a long time.
5. Are the paper-oil caps in it safe to continue using or would a change be good? I know for sure that it will probably sound even better using new caps.
6. Were the "Winged C" tubes a good choice? I read some people's opinions on them from here and they look pretty good.



Pictures of it can be found Here

Sorry for the long post but I'm very excited about this amp!
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Old 10th July 2007, 07:36 PM   #2
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Well your first clue is that it says pilot on it. Pilot made some good tube amps. The output transformer has obviously been replaced , as it says Sansui on it. Sansui made good speakers and pretty good tube amps. Judging by the size of the power supply caps (100uf, 250v) it uses a voltage doubler for the power supply to obtain high B+ from low windings. The chassis looks pretty cobbled together, so it is very likley that this is a home made amp. That unknown tube looks like some sort of small signal pentode, maybe an EF86? try and trace the circuit to see.
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Old 10th July 2007, 07:54 PM   #3
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The tube looks like a 5879.
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Old 10th July 2007, 08:03 PM   #4
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If "Pilot 240" is marked on the bottom, perhaps the bottom doesn't really belong to this amp... the 240 is a stereo amp with 6BQ5 (smaller) output tubes. If it's on the top of the chassis... it ain't one any more!

The output transformer is marked "Sansui", a well known Japanese maker of amps and transformers. But this could be a home built project (unlikely, with silkscreen marking of chassis), or the transformer may not be original. So the speaker marking MIGHT not be correct, either. But the wrong speaker impedance will only affect maximum power - it won't hurt the amp, nor will running without signal.

The big tubular electrolytic capacitor appears to have a date code of 1960 from Micamold in Brooklyn (EIA code 240). Those should be replaced. Are there any numbers on the bigger transformer? The brown film capacitors will still be good, in my experience.

The mystery tube loks like a 9-pin pentode - an EF86/6267 seems most likely. If it's a Mullard, Valvo, or Amperex, there will be small etched numbers near the base that will ID it. The circuit will probably be the "Mullard" circuit, similar to the EICO HF-22: http://www.triodeel.com/eicohf22.gif
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Old 10th July 2007, 08:38 PM   #5
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Well I thought it was a home brew amp until you pointed out the silk screening Tom, I don't think the bottom of the chassis is from the same device. I know for a fact that it is a push-pull amp capable from anywhere between 17 and 47 watts according to an old tube book my friend gave me. The tube book is even older than the amp and only has tubes in the 6L6 series upto the gb. It has very loud output with the little 30 watt aiwa monitors that are connected (which by the way I know little about as well, other than their wattage, and that they have flat cones). So I suspect that it is in the higher output of that range.

As for the electrolytic cap, whereabouts would a good one be bought from, what cap would you recommend? Online shopping for components like that is about the only place I can get things like that around here. I plan on building a new chassis for the amp out of some birch plywood that I just bought. I would eventually like to copy the design of this amp and make another so I can have stereo again, although I'm quite impressed by mono at this point

If I take a DMM to the ouput terminals while it is off will that give me the correct impedance of the output terminals?

How are the tubes I selected for it?

Also, thanks for all the help so far, keep it coming, I really appreciate it!
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Old 10th July 2007, 08:57 PM   #6
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Well, it is possible that it's a home-built amp on another chassis, but it could be a commercial amp from Sansui or someone else who used Sansui transformers... or maybe it's a replacement. Are there any extra holes in the chassis? There may be a date code on the can cap any markings on that or the larger transformer?

It might be possible to find information on the Sansui transformer - impedance ratio can't be measured with an ohmmeter.

Small quantities of caps can be bought from justradios.com
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Old 10th July 2007, 09:11 PM   #7
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Hi everybody,
The output transformer came from a Sansui 1000A tube stereo receiver. In its original application, it was good for 45 watts from a pair of 7591a tubes. The primary impedence is 6000 ohms and the secondaries are 8 and 16 ohms for this transformer. This is an extremely good output transformer made with extensive interleaving, more so than a lot of old Peerless units that I have opened up in the past.
The output transformer is definately a keeper!
Daniel
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Old 10th July 2007, 10:47 PM   #8
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There are alot of extra holes all over the chassis. So it has a very good OPT huh? No wonder it sounds so good, thanks for the info on that transformer. The larger Transformer may have had markings on it but they have since left it as it has some rust spots and looks as if someone had repainted it once already. I have never heard of 7591a tubes, are they similar to the 6L6's that were in there? As for the can cap, it has the following on it

Sprague
20MFD - 400VDC
10MFD - 350VDC
50MFD - 50VDC
Can. Com. Neg.
D31994 6017


I will try to replace those big internal caps as soon as I can, aren't electrolytics the ones that explode? As for the Can Cap, should it be replaced or is it ok? I tried cleaning the top of it with alcohol since I took the photos and it looks allot nicer.

Anymore info? and thanks again
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Old 11th July 2007, 06:56 AM   #9
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check www.tubesandmore.com or www.tubedepot.com for can caps, although you can just replace the all in one with a bunch of single caps. The ones in the can are the electrolytics, and the smaller ones will be either wax/paper caps or some sort of polypropolene. Wax and paper caps should always be replaced, poly's are ok if there is no DC leaking between ends.

You should probably replace the can with seperate caps under the chassis, you could use larger values for the 20 uf and 10 uf (ie, 47uf and 22uf etc. )You don't have a tube rectifier so you don't have to worry about the extra strain larger capacity caps have. Ground can common just means that all the caps are in one can and that the metal outer case of it is the common ground for them.

The 7591 is a very good tube , and is somewhere between a 6v6 and 6L6 as far as output power goes. It was used extensivley in the 1960's in a lot of hifi amps and receivers, namley hh scotts and fishers.
Edit: by home made I meant put together using parts from a few amps, or at least an amp that has been extensivley worked on It has at least a replacement OPT, and I saw a few areas under the chassis where it has been worked on.
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Old 11th July 2007, 10:26 AM   #10
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Thanks for the info on the capacitors, with so many needing to be replaced do you think there is a danger to my speakers being hooked up to this amp? Or can I use it regularly until I get more new caps?


I have 2 old books that specs for tubes but unfortunately they don't tell how they sound or how well they work. The info on the 7591 is interesting, thanks again.
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