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Old 27th January 2013, 06:36 PM   #1
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Default DC7 MK3 up and running.

I am currently listening to the latest iteration of my DC7 amp concept. Its based originally on Gary Pimms Tabor amp.
The latest version has a 12L8GT dual pentode at the front end driven by an input transformer and with a CCS under the tail of a LTP. The load resistor (22K) comes off the plate of the output tube and acts as the plate to plate feedback resistor.
This is direct coupled to a pair of EL86's. These have a grounded cathode for effective fixed bias, however the bias point is set by a combination of the drop over the 12L8GT load resistor and a current mirror feeding current through one side of the output transformer. This current mirror drives a 200uf cap before going into the output transformer. This makes the output self biasing and gives matched current on either side of the transformer for zero DC offset.

After a few trials getting the current mirror to work, it stable and indeed self biases. Remarkably one of the 12L8GT is quite unbalanced and it managed to adjust without issue.

Result - clean and detailed with tight bass. Its a bit zingy at the moment as I need to work a bit on load the input transformer. A subtle but nice step up from the DC7 MK2.

I will trace out the exact circuit when I tidy up the case a bit on the bench, but hopefully it will go into service soon.

Shoog
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Old 28th January 2013, 07:09 PM   #2
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Here it is - any questions ?

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Old 31st January 2013, 05:51 PM   #3
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Some small revisions. Current balance on the current mirror load was rubbish until I moved the 270R resistors.
Output screen dropping resistor increase to reduce current in the ouput.
Correct loading of the input transformer.

Working great and sounding damn fine. Difficult to say exactly if it sounds better than the DC7 MK2 as the input transformer on that was slightly rolled off and so a lot of the changes could be just down to improved bandwidth. Overall gain is down on the MK2 version - which is due to the increased gain of the output stage forcing more feedback signal.

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Old 3rd February 2013, 06:12 PM   #4
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I suppose it was wishful think to expect that the output tubes would bias up to the right 50mA current as if by magic. After a few Red plate moments I realised that it would probably need a little help along the way so I went for the trust TL783 high voltage regulator to make a nice little 120mA CCS.
Everything now settles down nicely all by itself.

Shoog
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Old 21st February 2013, 09:42 AM   #5
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I went back and modified my DC7 Mk 2 6AS7 amp to incorporate this "inverted Bias" arrangement. Once I got it working it sounds cleaner than the version with the cap to cap CCS bias arrangement it replaced. A nice improvement.

However the MK3 definiately sounds better than the modified Mk2. The top end is slightly more extended (probably due to the difference in input transformer), but the main difference is in the bass. The bass is significantly tighter and better defined. Overall this makes the amp sound much more refined. This just goes to show that in a plate to plate feedback arrangement all of the extra gain is been converted into lower output impedence and consequently better damping. Also the EL86 is definitately a very silky smooth sounding output valve which lives up to its sleeper tube reputation.

Its nice to see the concept evolve and get better as it does so, well worth all the effort.

The only issue I have is that everything is in such a tight space in the DC7 MK3 that things run quite hot and this cannot be good for long term reliability. I may have to experiment with some forced air ventilation to keep temps down.

I will take some pictures of the new build as soon as I tidy up and spray the case and add a nice face plate.

Shoog

Last edited by Shoog; 21st February 2013 at 09:47 AM.
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Old 24th February 2013, 12:24 PM   #6
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Amp Porn.

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Old 24th February 2013, 07:00 PM   #7
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Default more CCSs

Hi Shoog,
I have been following your progress with interest as you always have very practical ideas. I don't understand all the points of you amp and biasing arrangement but is the toroid OPT is being used in a sorta parafeed arrangement?
From reading Gary Pimm and Morgan Jones I wondered if you had considered a CCS on the cathodes of the ouput valves as well as the current mirror
on their anodes?
tim
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Old 24th February 2013, 07:22 PM   #8
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Quote:
Originally Posted by dhsettim View Post
Hi Shoog,
I have been following your progress with interest as you always have very practical ideas. I don't understand all the points of you amp and biasing arrangement but is the toroid OPT is being used in a sorta parafeed arrangement?
From reading Gary Pimm and Morgan Jones I wondered if you had considered a CCS on the cathodes of the ouput valves as well as the current mirror
on their anodes?
tim
Not really a parafeed arrangement since there is no cap blocking the DC pathway. The cap has been moved to the least objectionable place, right before the transformer and as part of the power supply. Splitting the primary and fixing the bias allows the arrangemnt to self adjust whilst ensuring perfect current balance in the output transformer. It achieves this by allowing unequal voltages to develop at the input to each side of the primary.
In the DC7 Mk2 (search for that to look at the schematic) the CCS was indeed in the cathode of the output tubes - and it worked extremely well in that position - however it required a large cathode bypass cap to make it work. The objective of this iteration of the design was to see if I could eliminate that cap and whether there would be any benefit in doing so.

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Old 24th February 2013, 11:13 PM   #9
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Default no caps required

Hi Shoog,
I'm only a novice but John Swenson discussed a parafeed scheme that didn't need a blocking cap.

RE: Diff CCS schematic - John Swenson - MagneQuest/Peerless Forum

http://johnswenson1.home.comcast.net...eo/DiffCCS.GIF

I'll look up your DC7 Mk2 posts.

regards
tim
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Old 25th February 2013, 08:09 AM   #10
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Quote:
Originally Posted by dhsettim View Post
Hi Shoog,
I'm only a novice but John Swenson discussed a parafeed scheme that didn't need a blocking cap.

RE: Diff CCS schematic - John Swenson - MagneQuest/Peerless Forum

http://johnswenson1.home.comcast.net...eo/DiffCCS.GIF

I'll look up your DC7 Mk2 posts.

regards
tim
Parafeed very specifically refers to the fact that there is a parallel feed of DC current and AC signal. What that gentleman is describing is a form of series feed where the DC is in series with the inductive element (the OT) which develops the output signal against its reactance. What he is infact describing is the arrangment used in the DC7 Mk2.
He is misusing his terms and causing confusion.

Shoog
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