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Old 15th February 2005, 07:52 AM   #1
argo is offline argo  Estonia
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Default All octal 6AS7 PP amp – is it possible?

Hi everbody,

Having bought a bunch of 6AS7 and 6SN7 double triodes for pennies and gotten my hands on a pair of nice 8kohm PP OPTs, I started to look for suitable schematic.
I found Menno Van Der Veen’s 6AS7 PP amp called Maurits, discussed here on the forum several times as well.
http://www.plitron.com/PDF/PB/Article/Atcl_13.pdf
It’s using 6AS7 for PP output and 6N1Ps for gain and paraphase phase inverter stages.
Although it seems like a well thought out design, Van Der Veen itself admits, that the output power of the amp (7W) could have been raised somewhat, if not the limiting factor of the paraphrase phase inverter’s first tube for providing enough voltage/current to the output (more than 250Vpp).
To make thing even more complicated, I’d like to build an amp using a Maurits output stage providing 10W power and using only octals (6SN7 or 6SL7) for voltage and splitter stages.
What chances I have to succeed here? Or do I have any?
Here’s some options I could thought of:

1. a’la Williamson – grounded cathode + cathodyne splitter + driver, all 6SN7
2. 6SL7 or 6SN7 paraphase + 6SN7 driver
3. 6SN7 SRPP + 6SN7 paraphase
4. ……

Can someone advise the best performing/sounding one? Or am I doomed choosing any of these?

Cheers,

argo
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Old 15th February 2005, 11:33 AM   #2
SY is offline SY  United States
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Why paraphase? What's wrong with good old fashioned diff amps?

Using diff amp drivers, you should have no trouble swinging enough to drive 6AS7-type tubes.
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Old 15th February 2005, 12:07 PM   #3
Previously: Kuei Yang Wang
 
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Default Re: All octal 6AS7 PP amp – is it possible?

Konnichiwa,

Quote:
Originally posted by argo
Can someone advise the best performing/sounding one? Or am I doomed choosing any of these?
I'm not a great fan of the Williamson circuit, but given the swing the 6080/6AS7 demands it's not a bad choice. There is a japanese site that shows a Williamson Frontend driving 6080 and originally used 6SN7 from the RCA Tube Manuals.

Look here for some google translated pages:

http://www5a.biglobe.ne.jp/~jh2clv/6080schematic.htm

http://www5a.biglobe.ne.jp/~jh2clv/6080amp.htm

I personally, for reasons of pure perversity, would probably tend towards a SRPP 6AS7 output stage, with a PFB boosted 6SL7 SRPP as Driver....

Sayonara
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Old 15th February 2005, 01:18 PM   #4
argo is offline argo  Estonia
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He-he - those automatic web translators are amusing

Found this article of 6AS7 SRPP by Flemming Madsen on the other day:
6AS7 "SRPP Power"
He was on the opinion, when designing his amp, that too much good stuff (that is SRPP stages) is not so good idea at the end and went with two grounded cathode stages and cathode follower to the output. He has also shown an option for PP output with diff amp inverter. Unfortunately the quality of those pdf charts are so bad that I couldn’t read out any part values though. Also couldn’t find what impedance OPTs he used for SRPP or PP variant.

Here's another 6080/6AS7 Williamson I found.
6080/6AS7 williamson
Seems like a perfect go. But searching through the forum on Williamson, lead me to the opinion, that this would'nt also be the best solution as Williamson’s circuit tends to be somewhat boring sounding topology and if feedback is used, one can get into the trouble with the stability of the amp?

Kuei -what's a PFB?

Would it be mad to feed the output directly from split load inverter?
I am musing something like: SRPP + common cathode direct coupled to split load inverter?

argo
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Old 15th February 2005, 01:29 PM   #5
SY is offline SY  United States
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Split load inverters are not the first choice if you want to swing lots of volts.
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Old 15th February 2005, 01:31 PM   #6
argo is offline argo  Estonia
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Quote:
Originally posted by SY
Why paraphase? What's wrong with good old fashioned diff amps?

Using diff amp drivers, you should have no trouble swinging enough to drive 6AS7-type tubes.
SY

Forgot to add to same point. Why Van Der Venn used this at first place? If using parafeed, would there be any sonic advantages over other inverters/splitters?
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Old 15th February 2005, 01:37 PM   #7
argo is offline argo  Estonia
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Quote:
Originally posted by SY
Split load inverters are not the first choice if you want to swing lots of volts.
So I have heard. But looking onto this schematic made me wonder still:
http://www.bonavolta.ch/hobby/en/audio/6as7_2.htm
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Old 15th February 2005, 01:38 PM   #8
Previously: Kuei Yang Wang
 
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Konnichiwa,

Quote:
Originally posted by argo
He-he - those automatic web translators are amusing
Yes, very funny.

Quote:
Originally posted by argo
Kuei -what's a PFB?
The opposite of NFB, namely Positive Feedback.

You can use that to increse the swing available from a 6SN7 differential phasesplitter (the sort one might direct coupled drive from a 6SL7 SRPP), simply return the anode resistors of your 6SN7 to the anode of the 6AS7 of the opposite polarity.

Then you can also add some NFB by adding a suitable resistors again crosscoupled to the 6SN7 diff amp grids....

Quote:
Originally posted by argo
Would it be mad to feed the output directly from split load inverter?
You need an awfull lot of swing for the 6AS7. Maybe using an EL34 with 750V +B for the split load phase splitter would get enough swing.

If you want simple, use a 6SL7 coupled to a 6SN7 Mu-Follower/SRPP driving a cheap 115V+115V torroid as autoformer phasesplitter or a better quality splitter autoformer (Magenquest has one, i heard that JB has also designed something that might servethe job....

Sayonara
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Old 15th February 2005, 02:00 PM   #9
Previously: Kuei Yang Wang
 
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Konnichiwa,

Quote:
Originally posted by argo
So I have heard. But looking onto this schematic made me wonder still:
http://www.bonavolta.ch/hobby/en/audio/6as7_2.htm
First note who this this schematic is from (self cencoring removes several potentially inflamatory lines).

Secondly, note that you are dealing with an OTL where the swing needed is much smaller.

Thirdly, note the 550V +B for the Split load inverter, that leaves a lot of swing available, though this in turn has to also be available from the preceeding stage.

Lets calculate a little. If you use 200V anode voltage and -90V grid voltage (which is convenient to get our HT from a 230V Widing, such as a suitably high power mains transformer used to step back up say the 6V or 12V windings you use to heat the 6AS7) we need for each grid a swing of +/-90V.

That suggests at least 150V across the anode/cathode resistor of the Concertina Phasesplitter. The valve needs to handle twice that swing which is 180V, plus some reserve you need a minimum of 250V better 300V across the Valve.

Now if we use a 6SN7 as concertina with 300V across and 150V across each resistor we need 600V +B. Given that 150V anode voltage are a little low for a voltage amplifier stage (except MAYBE srpp optimised for agressive THD cancellation) you may not be able to couple directly to the voltage amplification stage.

My HAm tps copy on using the 6AS7 as Push-Pull Audio Amp suggests a driver stage with 250V Anode voltage and transformer loading, using a 1:1+1 transformer to the 6AS7 Grids.

I'll attatch a link for this little known page for reference....

http://www.clarisonus.com/Archives/TubeTheory/6AS7G.pdf

Sayonara
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Old 16th February 2005, 02:08 AM   #10
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Hi,

Quote:
I'll attatch a link for this little known page for reference....
Good advice from people who seem to know their tubes....

However, in the context of an OTL amp some of that advice is just not practical:

#1.The reason why using parallelled 6080 is not recommended is simply because they're not twin triodes.
If you must paralel them you need to handpick them or at the very least take precautions against current hogging.

From experience, 6AS7G/6080 triodes are far from matched within a single tube so expect to go through lengthy selecting processes.
Preferably using a simplified version of the outputstage they'll be working in.
Give them ample time to stablilize, these are slow buggers.
Putting each half of the bottle in a PPP is not going to help any IMO, unless you really have a truely separate bias supply for each triode...Who does?

#2. As for self-bias as opposed to fixed bias. Unfortunately using cathode bias is out of the question in an SEPP stage as this will up the Zout of that stage considerably.
Having used fixed bias for about 20 years with ten of these triodes in parallel (ahem) I can say that the only time problems occur is when the amps are switched back on prior to cooling down properly.
Until this very day nobody could give me a plausible explanation for the problem but it almost invariably fries a grid on one or more output tubes leaving the other half of the parallelled set intact.
Naturally the B+ fuse blows too making me wonder what it was trying to prevent from blowing in the first place........

Either way, such are the risks involved in using high gm/low mu tubes.

My conclusion for OTL builders would be to keep powertubes to the minimum leaving the 6C33-C as probably the best available candidate nowadays...
The super OTL tubes such as Sylvania's 7246(?) triple triode bottle are long, long gone.

Other than that, the 6AS7G is a great sounding tube for OPT coupled PP stages.
It's one of the cheapest available and is not demanding regarding the OPT given it's low Ri. It does like to be driven with ample reserve which is why I like to drive it from a good CF.

Cheers,
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