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Old 25th May 2008, 06:25 PM   #481
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Th eoriginal idea was to tie the cathodes together for AC with big back to back caps, forcing a quasi differential operation, butwith each tube having it's own cathode R to (sort of) enforce some sort of DC balance. Never tried CCs's but they could properly enforce DC current balance, which I consider far more vital than do many other designers.
Thats the arrangement I am currently using in all my amps. I use CCS' which seem to work well. I have a 1meg resistor from the caps tied node to keep them polarized. What value did you find worked best for you Allen ?
I like this arrangement because it is self adjusting. I appreciate that its going to be slightly sound compromised but the thought of fiddling with bias adjustment every month or so leaves me cold. I understand that the 300dpa need manual adjustment.

The problem I am currently having is at power up the output tube is zero or positively biased and asking to pass huge currents, but the dinky little MJE350's can't cope with the current slam - hence pop. Its a particular facet of the DC coupling I am using. I need to find a good transistor with 600V capability and about 4amps current capability. The nasty thing is because the CCS are tied together through the big differential caps, if one CCS goes down then the other instantly follows.

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Old 25th May 2008, 07:43 PM   #482
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Shoog,
I have never had any luck with using bipolar trs under hi voltage condidtions. I always now use MOSFETs as , in my experience, they are VASTLY more reliable. I've had them so hot even with 350v across them the black burnt off the heatsinks (!!!) but they keep on running just fine - can't do that with bipolars.

I find the optimum value for the R to gnd from the center tap of the cathode C's by distortion measurements, start around 100ohms and go up or down from there.

Regards, Allen
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Old 25th May 2008, 08:10 PM   #483
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Thanks for that.
I'm going to do one more CCS lash up with what I have left and then will do a whole new design so I might try my hand at a MOSFET.

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Old 25th May 2008, 08:28 PM   #484
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Originally posted by jkeny
Thanks Miles,
So is it called partial cathode degeneration? Have you tried it? Any findings?
Only with BJTs.
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Old 25th May 2008, 08:46 PM   #485
jkeny is offline jkeny  Ireland
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Hey Shoog,
Why don't you try Gary Pimms CCS - reportedly excellent sounding. It can take the voltage but I'm not so sure about the 6A current? He has Eagle files on his site for PCB ordering.

I'm going to order some DN2540 depletion mode Mosfets from Mouser in the next couple of days - let me know if you want in! Could also pick up some good Fets too.

I've just built his CCS using LND150s & John Swenson's 250 & 130V voltage regulators based on this CCS for the BDT preamp. They're a pain to do P2P so pcbs would be of interest to me too!
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Old 26th May 2008, 07:06 AM   #486
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Albert,
ZVN0545 in the design dissipate only about 50mW or less so heatsink is not required. I had a huge vinyl session with these amps over the weekend. Showed that these are the best amps I've had on the system so far BUT can't rest on my laurels - there is still plenty of room for improvement.

Interestingly enough - when listening to CDs (that digital stuff) I prefer the "Jessica" (6V6G with 6SL7) version of the amp. Its not technically as good but it does a nice job of covering up the digital artifacts and so sounds better with CDs.

Cheers,
Ian
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Old 26th May 2008, 08:25 AM   #487
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I rebuilt my CCS last night and plugged her in. Two power ups so far and she's OK. Not really as robust as I would like but baring a fault situation elsewhere in the amp they should last.
Current balance isn't quite as good as I am used to, and this may be effecting the bass which sounded a bit ropey last night.

The only problem with the Gary Pimm CCS is that they are very complex compared to all the others, and the level of performance they offer just isn't warrented in a cathode duty with bypass caps.

Shoog
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Old 26th May 2008, 11:29 AM   #488
jkeny is offline jkeny  Ireland
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Shoog,
Glad to hear it's working - so you did something different? Are your power up issues as a result of the quasi differential O/P stage or elsewhere in the topology? I want to try this O/P configuration on my Baby Huey & just need to be aware of potential probs

I agree about GP's CCS being complex but if you consider the problems you've had with this CCS ........ maybe not such a big build?

Maybe even consider cascoded DN2540 if this CCS fails!
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Old 26th May 2008, 02:54 PM   #489
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Ian,

Thanks for the infos.

I've ordered the ZVN0545A and they're on the way to me. I'll go for the fixed bias in this monment so to compare the baby huey.

I also have some 6V6 RCA and some 6SL7 RCA on my collection.
I certainly will give a it a try in the later date.

About the sound of the fixed bias EL84, Can we play a bit on the feed back resisitor? I mean to lower the value to have smoother / warmer sound.

Albert
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Old 26th May 2008, 05:26 PM   #490
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I spoke to soon. On a bass heavy test record I realised something had gone wrong. I'm thinking that the IXY chip might be a simple robust solution. Don't think farnell do them though.

Shoog
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