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Old 9th April 2003, 10:23 PM   #1
Fuling is offline Fuling  Sweden
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Default A very stupid mistake!!!!!

Got a pair of small output trannies and some 6EW7 tubes in the mail today. The plan is to build a small SE amp using one of these tubes per channel, just for fun since I found those cheap surplus transformers that seems to suite the tubes.
The tube in question is a small 9-pin dual dissilimar triode with one low power/medium mu half and one higher power/low mu half. Seems really nice for this kind of project.

Since the transformers are surplus and of some unknown brand I felt that I had to test run one of them to see if theyre any good.
Said and done, I spended a couple of hours making a small breadboarded single channel prototype to see if it worked.
It did, but not that good. Low gain and floppy, distorted bass response. When I checked some voltages and currents I realized that something was very wrong.

In my hurry I hadnt bother to double check the pinout for the tube, so I had mixed up the two (very) different halves!
The power triode is working as an RC-coupled gain stage and the driver triode is acting as a transformer coupled power stage.
Im quite surprised that it did sound at all!!!
Ill rewire the whole thing tomorrow night and then I will report how it works with the right triode at the right place.
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Old 9th April 2003, 10:35 PM   #2
316a is offline 316a  England
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Default 6EW7

I can sympathise , my old Taylor 45D valve tester manual listed the high and low mu triode sections with the grid bias settings the wrong way round . I've tried a simple current source loaded 6EW7 SE and it sounded very nice , but very low power .

'...with the right triode at the right place'

very true ! I've tried to put my view across regarding this fact but some folks aren't listening (for example someone has decided that a 6sl7 section with 100k load , 175v HT 'providing' 70v p-p swing driving a 6080 with 220k gridleak will provide good sound hehehehe) see 6080 parafeed thread )

316a
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Old 10th April 2003, 12:24 AM   #3
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Default Re: 6EW7

Quote:
Originally posted by 316a
very true ! I've tried to put my view across regarding this fact but some folks aren't listening (for example someone has decided that a 6sl7 section with 100k load , 175v HT 'providing' 70v p-p swing driving a 6080 with 220k gridleak will provide good sound hehehehe) see 6080 parafeed thread )
Ya know, you still didn't tell us why it wouldn't.

Tim
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Old 10th April 2003, 12:27 AM   #4
SHiFTY is offline SHiFTY  New Zealand
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Hey nothing went up in smoke, so whats the problem?
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Old 10th April 2003, 09:28 PM   #5
Fuling is offline Fuling  Sweden
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I did some rewiring tonight and, big surprise, achieved some slightly better results

The bass response goes way deeper than expected. The OPT:s are quite small so I didnt expect much on that point. Not the "tightest" bass Ive ever heard maybe, but that must have something to do with the high DCR of the secondary winding.
The measured HF response is not that good, but it did sound OK so why bother? My hearing is far from perfect anyway...

Output power reaches 1,2 W.

Now Ill have to find (or wind) a suitable power trannie, then its time to give it a go!
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Old 10th April 2003, 10:22 PM   #6
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6EW7....COOL...got some of those as well...and am planning one ...one day..

Would love to hear what operating points etc you settle on.

Cheers,
Bas
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Old 10th April 2003, 11:54 PM   #7
7N7 is offline 7N7  United Kingdom
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Default 6EW7 Performance

I have a quantity of this type and indeed am using a pair in a temporary amplifier as drivers cum cathode followers to drive some triode-strapped dual beam tetrodes.

I have tested 6EW7 both individually and as differential pairs and can report that distortion is stunningly low.

I doubt that any significant power could be obtained (unless one paralled four together) but it is an excellent alternative to other medium-mu types, with the added benefit of a low-ra high-current section that could easily drive a big valve directly from its anode.

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Old 11th April 2003, 12:47 AM   #8
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Default DISSIMILAR TRIODES.

Hi,

You can also make nice regulators with this kind of valves.

To bad a lot of them require Novar sockets, other than that these are fine candidates for headphone amp service as well.

And they're available in a wide range of heater versions to boot.

I quite like'm for their compactness and if you haven't noticed yet a lot of them sport these lovely sounding graphite plates too.

Cheers,
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Old 11th April 2003, 02:04 AM   #9
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Graphite plates? Thought they only used those in really big transmitter tubes (e.g. 3-500Z)......

/Insert obligatory

Tim
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Old 11th April 2003, 02:13 AM   #10
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Default DOTTED Is AND CROSSED Ts.

Hi,

O.K.

Want to discuss getter shapes and plate materials?
Guess not...

Nah, sputtered graphite is more like it I reckon...they produce less odd harmonics Timmie boy.

Cheerfully yours,
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