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Old 27th January 2010, 01:46 PM   #1
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Default DC7 MkII :New Schade 6080 amp.

Hi all,
A few years ago i built a PP 6080 amp with toroidal interstage phase splitting which I called the DC7. I liked it a lot and others did to, but despite pushing 30mA of driver current through the interstage it still had slight high frequency roll off issues. Eventually I went on to build an adapted Tabor clone with 807's and a front end input transformer phase splitter. This took some getting right, but once done it sounded powerful and detailed. Still it has a very slight amount of pentode edge.
I knew that the 6080 could give magical sounds so I thought about using the principles learned in building the Tabor clone to building a 6080 amp. The eventual design consists of a 12L8GT dual pentode on the front with the grids driven with a little input transformer. i used a negative rail for this which allowed me to tie the screens (common) to the earth rail so simplifying the design. This is direct coupled to the 6080's and the plate's of the 12L8GT are supplied from the output transformer via a 20K resistor.
The output is run at 100V and 100mA per triode and this is tightly regulated with CCS in the cathode. I have bypasssed the cathodes to each other with 1000uf caps in series. This makes the design differential input to output.
The outputs are configured with 100% plate to grid feedback so function purely as current pumps. The output voltage is developed across the 12L8GT 20K plate load which allows for 160V Peak to Peak from just 8mA of plate swing. This gives an input sensitivity of about 2V for full output.

Sound wise it is a bit different to the Tabor clone. It has a very smooth sound with just a hint of Triode bloom. It is detailed with great sound stage and a very powerful bottom end. My sensitive vintage Isophon speakers are easily driven to heavy bass distortion if pushed to hard and this amp is more than capable of pushing them beyond their limits.

I will throw together a schematic later if anyone is interested.
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Old 27th January 2010, 02:08 PM   #2
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So Schade might better the unlinearity of the 6080s.
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Old 27th January 2010, 02:15 PM   #3
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I suspect there may be a reasonably high level of distortion - but the sound suggests mostly second harmonic. Unfortunately I haven't got the equipment to test it.

Shoog
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Old 27th January 2010, 05:54 PM   #4
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i'd love a schem. i have some 6as7g's lying around..
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Old 27th January 2010, 06:10 PM   #5
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Here it is.
The 12L8GT might be a tad tricky to get hold of as they are not commonly known. Still there are many out there in dusty corners. I got mine for small money from a great Ebay seller in America, I can probably drag out his details if anyone is interested. Otherwise a pair of small signal pentodes could be installed.
The LTP on the front end wants a bit of adjustement built in so that you can trim the voltage drop across the 20K resistor to 130V. Since the 12L8GT is not particularly linear it is essential to use a high quality cascode design in order to enforce best symmetry and achieve maximum distortion cancellation. I found them easy to drive and had the input transformer configured as a step up initially but found it had far to much gain. If the 12L8GT is unobtainable then any pentode which passes 6.5mA at the plate for 190V on the screen would do fine. I just wanted a four tube PP amp (better to confuse the snobs into thinking it was a super SE amp)

Unfortunately there won't be any amp porn as its still in a semi-finished case.

Shoog
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File Type: png DC7 Mk II.png (36.9 KB, 954 views)
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Old 27th May 2010, 04:29 PM   #6
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Someone asked me to sketch up the power supply so I thought I would post it here.

I thought I would add a bit of amp porn for good measure.

Shoog
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File Type: png DC7 Mk II PSU.png (30.6 KB, 799 views)
File Type: jpg dscf1406 (DC7).jpg (901.6 KB, 802 views)
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Old 27th May 2010, 06:20 PM   #7
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Are those pots on the transformers? Potted?
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Old 27th May 2010, 06:41 PM   #8
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No just covers.
The case is a bit peeling, so one day I may just do a spray job on the whole thing.

Shoog
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Old 28th May 2010, 12:00 AM   #9
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6080 is low mu Triode already, what purpose Schading it to an even lower Mu?
I see the 20K, where is the other half of the Schade voltage divider? Surely not
the plate resistance of the twin Pentode?

Normally, we might take a high impedance pentode and Schade it to behave like
a triode. What you've done, defies my ability to Jethro down and cipher...

Does 100% PG feedback make for a Mu of exactly 1?

Last edited by kenpeter; 28th May 2010 at 12:21 AM.
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Old 28th May 2010, 07:38 AM   #10
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Hi,
Its 100% P to P feedback.
I was making an adjustment yesterday and ran it for a little bit without the heaters on the 6080's. It played at slightly lower SPL but without any authority. So the outputs are indeed working at about a mu of 1. When I built the original DC7 it ran the 6080's without any form of feedback and the gain was still less than 1, so in this situation the gain is no less than running them straight with all the output voltage been developed across the 20K resistor. The 6080's are been asked to do what they do best - which is deliver current.
I estimate the output 6080's impedance at about 100Ohms, and the output triode is working purely as a current pump for the input pair. Sort of like tube current dumping. Anyway it works extremely well with the triode character of the 6080 coming through but with amazing bass punch.

Shoog
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