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Old 31st December 2007, 02:27 AM   #11
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Gotta agree with some previous posters. An OTL is not something for a beginner. I tried building the Rozenblit design a number of years ago and had all kinds of stability issues. Not saying its the design but merely my fault for sloppy layout and not enough planning.
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Old 31st December 2007, 04:53 AM   #12
alexg is offline alexg  Philippines
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Quote:
Originally posted by Steve Cresswell
Hi Alex

A 6AS7 or 6080 pp is a superb sounding amp. I've attached a schematic of my own take on the 6AS7 pp idea.

I use low current drive in my amp and love the sound it makes. Others might prefer higher current drivers but either way the 6AS7 can be made into a stunning sounding push-pull amp.

Mine uses differential amps with constant current sinks in the tails of both the driver and power stages. The driver uses a simple LM317 as a sink with an STC 12E14 regulator tetrode as the sink for the output stage.

It is a work-in-progress and I'm thinking about reducing the number of stages to 2 by putting a 6SL7 or similar as a driver and removing the first stage. By doing this I'm trying to take full advantage of the low Miller capacitance of the 6AS7 by using a high gain low current driver. That's for the new year.

As it is though the amp sounds simply beautiful.

On another thread Shoog has done the opposite to me and used high currents and interstage transformer coupling. Also, his constant current sinking arrangements are a bit more sophisticated than mine.

Both approaches are equally valid and will produce an amp that sounds superb. I can guarantee you won't be disappointed whichever option you choose.

Steve.

Thanks Steve.

I have been reading the progress of your build on another forum last night.

I will source an appropriate input transformer to do the phase inversion, maybe have a winder custom wind a pair for me.

BTW, I forgot the bookmark the other forum that you narrated your progress, can you please email it to me: alexg_ht (at) yahoo.com

Thanks.
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Old 31st December 2007, 11:00 AM   #13
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Hi Jaime

Thanks for the suggestion regarding the input transformer.
I never thought of changing that.

Alex
you have an email.

Steve.
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Old 31st December 2007, 07:30 PM   #14
Jeb-D. is offline Jeb-D.  United States
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Hey Steve,
I saw your amp in another thread and it has to be said that chassis looks sweet
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Old 1st January 2008, 03:21 AM   #15
alexg is offline alexg  Philippines
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Hi Steve,

Forgot to mention, am driving my 6080 parafeed with a 12at7, the 12at7 cannot drive the 6080 into full power. I have a 5670 preamp with a lot of gain, paired with the 12at7/6080 parafeed, then I am getting decent power.

Am thinking of doing the 6080 PP using a 6n1p or 5670 driver into a center tapped choke phase inverter (just like the Seth Amplifier of Jean-Francois Lessard, which I have tried on a PP 832A with excellent result) then into 6n1p voltage amp, then into the 6080. B+ for the 6n1p at 400VDC and B+ for the 6080 180 to 250VDC.

This might give me enough voltage swing on the first two stages to drive the 6080 to produce adequate power.
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Old 1st January 2008, 07:29 PM   #16
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That should sound nice Alex.

832As make good pp amps also. A friend of mine uses one in his system

Steve
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Old 2nd January 2008, 10:47 AM   #17
alexg is offline alexg  Philippines
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Hi Steve,

How much current is going through each triode of the 6as7?

Thanks,

Alex
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Old 2nd January 2008, 11:28 AM   #18
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There is around 50mA going through each triode section plus around 35mA set aside for the sink.

The 12E14 sink is pulling approx 135mA to ground.
50mA per triode might not seem much for a 6AS7 to be passing, but as I said, this is a deliberately low current design, which I like the sound of.

Halving the cathode resistor on the sink will of course double the current passing through the 6AS7s and gives a more forward sound. This setting for the output tubes is more suited to commercial speakers. The low curent setting suits my Fostex driven Metronomes better.

Steve
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Old 2nd January 2008, 03:27 PM   #19
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Hi all,
I've just started another thread looking for anyone interested in group buying some Lundhal LL1517 output transformers to use as input/interstage transformers. These would appear to be ideal for my 6080 amp design (see attached schematic), and a bargin at about 20.00 per pair.

Shoog
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File Type: jpg 6080 amplifier with interstage and 5687.jpg (32.9 KB, 1462 views)
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Old 24th August 2010, 01:39 PM   #20
Brit01 is offline Brit01  United Kingdom
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Bringing up an old thread.

I'd like to start an OTL project (as I have no access to any transformers here, not even toroidal!!!).

Was looking at the 6C19P tube. they have a 120 ohm plate resistance and are fairly cheap. Better option than 6AS7 no?

I guess I could use these in an OTL design. Not sure which one yet. Just doing some research.

Also to detect DC on the outputs, could I use a simple LED connected to ground (along with fuse protection and possible relays of course for large failures)?

If the LED is connected to the output it will illuminate if there is any DC present no?

I realise a small amount of DC can be present in SS amps, I had 300mV on one channel I had to fix. But anything up to 15mV can do no damage to speakers.
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