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 NEW  Bruce Heran outlines the details and construction of his simple DIY 6DJ8 (ECC88) Tube Hi-Fi Headphone Amplifier Project.

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PostPosted: 28 May 2011, 08:42 
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Location: Lisbon, Portugal
Hi!

The other day, while talking on HF with another Ham, he suggested me to give a try on these tubes. I found them extremely pricey and wonder why? Does anyone have a listening impression or any comments regarding them?

6094 : http://tubedata.tubes.se/sheets/131/6/6094.pdf
6973 : http://www.jogis-roehrenbude.de/Roehren-Geschichtliches/Roe-Neuvorstellungen/BTB/6973/6973.pdf

The 6973 is a big tentation to me! Extremely low distortion! :P

Cheers in advance,
Miguel


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PostPosted: 28 May 2011, 09:49 
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The 6094 is just a replacement for the 6AQ5/6005 (as even stated in their data sheet) and I'm not sure why it would be expensive, except perhaps because someone thinks they can get it. This entire family is just a small envelope 6V6 design. I picked up 10 6005s (military versions of the 6AQ5) a while back for about $3.50 each and they sound great. The thing to remember about this family is that the sound can vary greatly dependent on the design of the amplifier.

I'm not familiar with the 6973 but from the data I found it does appear to be one of the later pentode designs made specifically for UL PP operation. However, the RCA data sheet for this tube does recommend transformer input coupling when using the tube in AB1 UL PP mode. I find this a curious requirement. It may indicate a propensity for this tube to draw excessive G1 current in over drive conditions leading to excessive bias excursions if capacitive coupling is used.

I am always a little suspect when someone recommends a "magic tube" that is either rare or expensive. In my experience, few of these ever pan out as truly exceptional devices. My advice would be chose a less expensive (but well made) tube and put the money saved into quality amplifier design and construction. This will almost always yield superior results. :soapbox:

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It's all about the Glass!
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PostPosted: 28 May 2011, 10:19 
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Hi Matt! Thanks for your input! :)

Meanwhile I found a nice schematic, which I believe also designed by RCA, but the filename is ampex, maybe Amperex?
Attachment:
ampex6973sch.jpg

Despite of which company designed it, it looks OK to me.

The 6973 output tubes are driven with some 7199 (don´t know them either).
7199 Datasheet: http://www.drtube.com/datasheets/7199-rca1963.pdf

Miguel


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PostPosted: 29 May 2011, 12:10 
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Hi Miguel,

It is Ampex, not Amperex. It is Ampex amp. stereo version. I have a pair mono block 6973 Ampex. Recently rebuilt, upgrade the power supply section and they sound beautiful. I have listenning some amp with 6L6, 6V6, EL84, 7591, 7868 and KT88 tubes, but I have to say I like the sound of 6973 tube better. It is even better than my MC-30. The bass is sonic and very deep, the high is clear and the mid range is fabulous. You are right, it is pricey and scarce.


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PostPosted: 29 May 2011, 13:23 
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Binh wrote:
It is Ampex, not Amperex. It is Ampex amp. stereo version. I have a pair mono block 6973 Ampex. Recently rebuilt, upgrade the power supply section and they sound beautiful. I have listenning some amp with 6L6, 6V6, EL84, 7591, 7868 and KT88 tubes, but I have to say I like the sound of 6973 tube better. It is even better than my MC-30. The bass is sonic and very deep, the high is clear and the mid range is fabulous.

WoW! :P That´s quite an experience! I wish I could hear them playing, but for that... I need to build my own! :idea: I´m so tempted!

Thank you very much for your opinion!
Miguel


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PostPosted: 31 May 2011, 08:14 
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You have to look for the output transfomers. If you can find then you can build. It is Triad 658-0024, UL special wound for Ampex. But I'd build the mono block instead.

Binh


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PostPosted: 31 May 2011, 15:46 
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Interesting stuff Binh!Do you know any technical details of those transformers?

Just a guess, Triad 658-0024... Maybe 6.5K for primary and 8 ohm for secondary, rated at 24W?

But according to RCA´s datasheet, the typical operation at 400v is loaded with 8K transformers with U/L taps for 24W (2%THD), which is quite common to find in the market. RCA also talks about 50% UL taps, but I guess 43% taps will be fine.

These tubes need about 50vpp voltage swing (PP) on the grid to drive them to full power! :o

Impressive little tubes! :P
Am I missing something?

Cheers,
Miguel


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PostPosted: 01 Jun 2011, 07:55 
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This transformers operate ate 385V (based on the schematic for mono block that I have). So your guess maybe right.
If you would like to make these transformer you might try this man http://heyboertransformers.com/tubeamps.shtml . He is good.

Binh


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PostPosted: 01 Jun 2011, 10:15 
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Miguel,

Read this http://www.high-endaudio.com/RC-Vintage.html. The Ampex section.

Binh


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PostPosted: 02 Jun 2011, 15:49 
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There are some very nice reviews on that webpage Binh!

I´m pretty sure that 6973 tubes have a big smile to offer us!

But i´m forced to put this project on standby, because the tubes are very expensive and quite rare.

There are a couple of them listed on eBay right now, including a pristine NOS NIB Westinghouse quartet! And that´s a big headache to me, because Westinghouse is one of my very few prefered tube brands!

Cheers,
Miguel


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