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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 18 Mar 2009, 21:51 
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Joined: 27 Oct 2008, 18:40
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I have a number 807 tubes and a couple of 30W Transcendar UL output transformers. I was thinking about "going oddwatt" with the 807s. 807s are 25W max plate dissipation tubes, just like KT77s, 6L6GCs, El-34s, etc.

I was wondering about the B+ voltage. You are running 350V x .065A for a 22.75W plate dissipation on the KT77s. The 807s have only a 300V max screen rating, yet they can take ~450V in UL mode, without issue, and triode-strapped at 400V 807s are quite common. 400V @ .055A seems nicer in theory (at least to me) than 350V x .065A. Does 400V sound better than 350V?


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PostPosted: 23 Mar 2009, 22:11 
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Location: Arizona, USA
Hi, 807s can be used in this circuit. The circuit values should be rather close as I seem to recall that the 807 is quite similar to 6L6GCs and they were the tubes for the original design. I would personally go for UL mode at about 350 B+. You could go a bit higher as you noted, but there is no particular advantage other than slightly greater output. I have tinkered with the circuit with various tubes and in particular KT77s and 6L6GCs. Nearly any B+ from as low as 300 to 475 (for tubes that can handle that) works well. You can adjust the idle current to insure you stay inside the tube ratings for dissipation. I like to keep mine around 75% of max. It is good for tube life. The most recent oddwatt style amps in the Odd Block project are running at 475 B+ and 125 ma per pair of KT77s. This provides about the maximum output from that tube type and retains the sound quality. If more output is wanted KT88s or 6550s can be used and the idle current upped to 180 ma per pair. The only problem I have had doing this is not all KT88s work well in the circuit. I am presently working on a "test bed" amp to try to find out why. The new Gold Lion KT88s work perfectly. EH KT88s will not stay biased and are consequently likely to fail (most likely not the fault of the tube, just they don't work in this application).

No back to the original question. I have not been able to measure nor hear any significant difference with the B+ settings. So I would initially use what ever power transformer you had in the right power range. I like to have some margin of reserve in the power supply and select transformers that can deliver about twice the current that I expect to need at the voltage wanted.

Good listening
Bruce

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PostPosted: 24 Mar 2009, 11:04 
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Joined: 29 Jul 2008, 08:12
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Location: Wiltshire, UK
This is looking very promising. I have yet to have the guts to build anything with valves, I've got my hands on my first valve for me to build something. My first ever solo build of a valve circuit. :-) I have been given an old Mullard book which I'm very intreagued by.

It's the second edition of "Mullard Circuits for Audio Amplifiers". From what I understand, it's hard to get hold of but not to fear for those who want access to it, I'll be scanning it to PDF when I get a chance, drop me a PM with your e-mail address if you'll be wanting it e-mailed to you.

Kind regards, John.


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PostPosted: 24 Mar 2009, 17:59 
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Hi John,
I think a lot of the book is already online: http://www.r-type.org/static/mull-cir.htm
Cheers

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PostPosted: 30 Mar 2009, 22:54 
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Hi Everyone, The Mullard book is available from AES (tubesandmore.com). I forget the price, but I have a copy. Not bad on the theory and overviews. A bit dated on the actual designs. I would read it for general knowledge and then build any one of the many more current designs on the web.

Good listening
Bruce

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PostPosted: 31 Mar 2009, 03:50 
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It seems it's not that hard to get at all. :-D Perhaps a good thing really.

Kind regards, John.


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PostPosted: 06 Apr 2009, 22:19 
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Hi Everyone, Update two thousand, seven hundred and fifty six (or so it seems). Nothing is ever as simple as it would seem. Delays in parts suppliers and testing are nearly over. The last parts are on the way (at least I hope they are). The testing of the preproduction kit amps has finally been completed and they are better than the prototypes. Frequency response is a bit wider at 8HZ to 24KHZ and output remains at 15 w rms (an arbitrary level set by me such that distortion never goes above 1%). Actual distortion is in the .15% range at low powers and except at close to 20KHZ under .6% at full power. At 20K it goes to about .9% The final kits and a supplemental project update (for folks that what to build from scratch) are coming soon. Starting a company can be more of a challenge than designing equipment, but if it results in the availability of something that can put quality sound reproduction equipment into the hands of enthusiasts it will be gratifying (we are not going to get rich doing this, actually I hope to come out even). The Web site (http://oddwattaudio.com) is in place, but is still under construction so bear with us until it is fully populated. Some of the material and photos are just place holders for the final items.

Good listening
Bruce

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PostPosted: 07 Apr 2009, 01:58 
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Joined: 27 Dec 2008, 04:48
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Location: QLD, Australia
gofar99 wrote:
Another possibility is to use a power FET as a CCS in the cathode circuit. I have a tentative design for that as well using IRF730s. It appears to work, but will need good heat sinking as there seems to be a thermal drift issue. If the FET works, it promises to have even better results than the 317s. It can go down to nearly zero volts on the cathodes under high levels of drive and get around the 4-5 minimum that exists with a 317.

Hi Bruce, I was wondering if anything came of this? It sounds like the LM317 is doing a top job in this application in this amp anyway, but just curious. Thanks for the updates. I have not yet done the SRPP preamp stage on my amp yet, because it sounds so good already, but after I reread this from the 1st page of this thread, I feel much more motivated :

gofar99 wrote:
It would appear that neither by itself is all that wonderful. The pair though work very well together, far better than I would expect. Both parts have things in their favor. The SRPP is highly immune to power supply noise and can drive difficult loads easily. The output stage is simple in the extreme and also is rather immune to power supply hum and noise. Together they make a quiet amp.

I hope the kits are a success, I'm sure every builder will be very impressed with the results.

Ian.


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PostPosted: 10 Apr 2009, 20:10 
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Location: St Paul, MN
Looking good. Can't wait to see the kits when ready.

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PostPosted: 13 Apr 2009, 05:29 
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Joined: 13 Apr 2009, 05:14
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Hi Bruce,
Your ODDWATT looks very interesting , similar to ALLEN WRIGHT VACUUM STATE differential amplifier
CCS coupled on cathode.Any similarities in sound reproduction despite ODDWATT is much simplier.
I am very interested can QUAD II alike cathode feedback be implemented in your design .
Did anyone try or think about it?
All the best
Zoran


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