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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: 24 Nov 2017, 16:31 
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Location: Arizona, USA
Hi, In response to an earlier post that indicated the CCS will distort when over driven. For sure. The design is a forced class A one. If you over drive it will distort. That is the reason for different sizes of amps. When operated such that the negative swing (it is more sensitive than the positive one) is no closer to ground than about 2 -3 volts there is no distortion. Cross that level and it will distort instantly at any frequency. You can fudge the 2-3 volts a bit by applying a small positive voltage equally to each grid. About 4 volts will do. It should come from a supply through high impedance resistors and be well filtered. Since it is common mode to the tubes much of the noise and crud is ignored. I do this in the newer small (EL84) Poddwatts. It allows for about 1 watt of extra output as the 2-3 volt penalty on the cathodes is a relatively large fraction of the idle operating voltage. The limiting factor in larger amps is the actual voltage on the cathodes will be bigger by the amount applied to the grids. It is relatively easy to exceed the ratings for dissipation and voltage in the LM317 if you are not careful. Using different types of CCS I have found increases the voltage penalty as all I know of have minimum operating voltages. I have yet to find anything that is as cost effective and works as well as the ancient and lowly LM317. Use the HV version though in the bigger amps.

Good listening
Bruce

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PostPosted: 30 Nov 2017, 14:34 
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Joined: 18 Jun 2016, 06:52
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Hi Bruce,

I would like to build dual mono Oddblocks in one case, with one power tx to feed the whole thing. I'm thinking of using the EdcorXPWR267, the one in the Oddwatt monoblocks. I would use mainly EL34 tubes, but maybe I could even get away with KT88s? Would the 10.9V at 4A heater winding be enough for four filaments in series at 12V and two 6SL7 filaments at 6.3V? I don't know if 6sl7 is meant to be connected in series at 12V. What do you think?
Greetings,

Jan


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PostPosted: 30 Nov 2017, 14:52 
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Joined: 16 Feb 2016, 19:45
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Location: Boston, USA
I built a dual Oddwatt Octal and run with KT88s. However, I used an XPWR-117. It runs hot to the touch with KT88s, so the 267 which is rated for 10% less power (450mA vs 500mA at 360V) might be marginal on the HV winding for KT88s. For the lower bias of the EL34s it would be okay.

I think a 10.9V heater winding would not be sufficient to drive two 6.3V heaters in AC. Both transformer output voltage rating and heater rating are RMS for AC operation.

In any case, you would have 1.5A x 2 = 3A for the EL34s and 0.3A for the 6SL7s
for a total of 3.3A on a 4A rated winding. Probably marginal. With KT88s I think you would exceed the rating.

If you don't mind paying a bit more the XPWR-117 works dandy! I started with EL34s and upgraded to KT88s, both sound wonderful but the KT88s have a bit more punch.

Here are some details on my project (scroll down to Oddblock Octal):
http://ohm.bu.edu/~hazen/TubeAmp/

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PostPosted: 30 Nov 2017, 15:14 
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Thanks Eric for the quick reply. I'm going to run the heaters in DC as in the Oddwatt monoblocks I built last year. There I use Bruce's 12sl7s. I'm going to have a good hard look at the XPWR-117's specs and of course I'm going to study your project.
Thanks again

Jan


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PostPosted: 30 Nov 2017, 15:24 
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Location: Arizona, USA
Hi, The only odd reason for the 10.9 heater windings is that when rectified and filtered you get right around 12 VDC. It will vary a bit depending on your AC mains voltage, but will be within the range necessary. I would not use either transformer for a dual set up. 360V at 600ma would be better or a 720 (up to about 750 CT) at 300 ma would be fine too. Watch for the heater load as well. BTW the weight of the trannies will be large and is one reason why I didn't post a dual mono or stereo version of the amps. To maximize performance I would actually build two amps and two supplies on the same chassis. The costs of the second power supply would not be all that great and you could use less costly transformers. The Poddwatt DMB (Dual Mono Block) does just this with great results.

Good listening
Bruce

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PostPosted: 30 Nov 2017, 16:00 
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Thanks Bruce, I'll consider the pros and cons carefully. The size of the case I ordered is a determining factor too. http://www.vt4c.com/shop/program/main.p ... group_id=2 AV200-S(ilver)

Jan


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PostPosted: 30 Nov 2017, 18:16 
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Joined: 18 Jun 2016, 06:52
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Bruce, one more question if you don't mind: in your description of the revised Oddblock circuit you recommend the Edcor xpwr002 as an alternative for your oem tx. The weight is 7.33 lbs x 2 = 14.66 lbs. Can you recommend a smaller and lighter Edcor tx for the dual mono version? As I live in Belgium it must be a 240V type.


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PostPosted: 30 Nov 2017, 18:21 
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Joined: 19 Jul 2012, 10:45
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I'm finally pulling the trigger on building an Oddwatt. One thing that I'm hung up on is going with either an octal or noval for the phase splitter.
I'm looking at either a 5751 tungsol/sovtek new production noval or a 6SL7 tungsol new production octal. Any advantage going one way or another?


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PostPosted: 01 Dec 2017, 14:10 
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5751 and 6SL7 are electrically identical, so choose the one that sounds better to your ears, I'd say.

Cheers!

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PostPosted: 01 Dec 2017, 14:42 
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Joined: 19 Jul 2012, 10:45
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Geek wrote:
5751 and 6SL7 are electrically identical, so choose the one that sounds better to your ears, I'd say.

Cheers!



Thanks, my only experience with valves are 12au7 tubes in my hybrid SOHA II amp. I presume choice between 5751 and 6SL7 boils down to ascetics and heater voltage options, but was interested if anyone can steer me one way or another.


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