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Old 7th December 2011, 08:03 PM   #541
M Gregg is offline M Gregg  United Kingdom
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Components arrived today..


I have fitted the 270 ohm gate stoppers on first stage ccs..

47K now on each side of the PI...

I have used a 10K as grid stopper on PI..

Took about half an hour..

Sound has changed....more detail in the highs...

just breaking in at the moment ..then some scope readings..

Next step to look at the 6c33c cathode resistors...

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M. Gregg
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Old 7th December 2011, 09:01 PM   #542
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Ive made some Mosfet CVS for the cathodes of O/P bottles.

Still running the PL509 and nothing else changed, the 11V P-P Maximum before clip has increased to 13V P-P. I carefully set this up for a Perfectly clean and nice sine-wave, before measuring.

--The MOSFETS got Nice and Hot, during the 2-3 mins I had it at full-chunter!

Not a Vast improvement in power.

Checked the PI, and looks like I'm running out of steam in that stage, Currently it manages about 65V P-P on each of its outputs, further increasing drive exhibits that flat-top and bottom nipple-distortion as illustrated on 'The Valve Wizard' website the Grid-Resistor is now 100K instead of the 470K I originally had there to prevent this.

For the 6SN7, we need more +B to get much more outta it I think, My +B to the PI is only 320V.
Any suggestions welcome for this issue, but we would prefer to keep the 6SN7....

So, Partial success. For further power increase with my current PL509 pair I need to get more drive from the PI as well as probably more +-B rails to O/P stage.

The drive and cathode-resistor/mosfet mods would carry over to any O/P triode used, and particularly the Cathode resistor/mosfet mod should show benefits for higher current triodes like the 6C33C
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Old 7th December 2011, 09:54 PM   #543
tubemax is offline tubemax  Netherlands
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Hi Alastair E, some time ago I read on Tube CAD website about EL509 OTL version but with different driver tubes...I saved a picture of tubecad design of EL509 driven by 6N1P - maybe helps to try out...

Max
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Old 7th December 2011, 11:22 PM   #544
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Hi Tubemax,
Interesting.....

LTP Phase-splitter with CF drive, although I wonder why its designer didnt direct-couple the driver CF's for better drive capabilities to the O/P pair instead of the phase-splitter LTP to the CF's...

I think I need a higher Transconductance tube for the PI set up accordingly if I am to use the 320V supply I have......

Thinking maybe a 7044 or 5687 as I have some of these, The 12BH7 is another that is known to work here but I aint got any! Unfortunately, these are 9-pin all glass valves....
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Old 8th December 2011, 09:37 AM   #545
tubemax is offline tubemax  Netherlands
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Hi Alastair E, maybe as a option buying a book ( Audio Reality ) from transcendentsound.com where is in great detail described T8 OTL amplifier based on PL519 triode connected as output tube.
I have this book but due to copyright I can't share anything from it...
In any case designer is using ECC8x driver tubes and not octal ones...but with 8 tubes per channel is claiming 80 WATT's of power or with 4 output tubes 25 WATT on 8 Ohm load.

Max
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Old 9th December 2011, 07:42 AM   #546
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by kevinkr View Post
Is that V peak (Vpk) or V peak to peak (Vpp), you have not specified which.. You would normally use Vpk, and you can calculate rms power directly as Vpk^2/2RL or Vrms^2/RL if you have a good audio voltmeter. (Note the round off error if you convert Vpk to Vrms - it is significant) You can also use vpp just divide by half and then use the first equation..

More than likely the driver stage cannot quite make up for the voltage losses in the output stage which are quite significant recollecting from my 6C33 OTL debacle..

IIRC the source impedance of a 6C33 pair in a futterman configuration without feedback is in the vicinity of 12 ohms.. Not sure if the reverse futterman is better or worse than this.

I'd see how much more swing you can extract out of the first stage, and how much additional swing you can get in the second without compromising the sonic qualities you like. I'd run the 6SL7 at half the available supply and play some games with the concertina to maximize voltage swing while trying to keep the current above 6mA. (I like 9 - 10mA for the 6SN7)

FWIW: Mine with all of its design issues still managed to sound really good, noting that my six 6C33 "200W" design only achieved 25W as my driver stages could not deliver the required voltages and current.. It was my epic fail, and given this one works reliably and sounds good you are most of the way there.

One other comment I would make is that in characterizing and matching 6C33 for my amps I saw some very strange behavior when I only ran half the tube. (So long ago I do not remember what, so take it as anecdotal.) Obviously for matching I ran one section at a time and calculated the transconductance based on several different grid voltages and measured plate current at the operating plate voltage. Unloaded there was a measurable difference in linearity and source impedance obviously doubles. I always ran all filaments and just removed pairs when less power was needed.
Kevinkr,

Can you post the circuit of the driver stage you used in the 200W 6c33c..Just interested..Thank's

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M. Gregg
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Old 9th December 2011, 01:52 PM   #547
M Gregg is offline M Gregg  United Kingdom
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Alastair,

Try putting the 6SL7 in the first stage and turn it up watch the speaker cones....

I will drop the scope on the amp when I get chance curious...

Regards
M. Gregg
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Old 9th December 2011, 11:28 PM   #548
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Hi M Gregg , my opinion is to try the fixed bias for the output tubes ( as I told Alastair ) , because the self-bias increase the output impedance of the amp , and this may be the mainly reason that the amp can not deliver more than 4W into 8ohm load .
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Old 10th December 2011, 12:32 AM   #549
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by M Gregg View Post
Kevinkr,

Can you post the circuit of the driver stage you used in the 200W 6c33c..Just interested..Thank's

Regards
M. Gregg
I'd rather not, there is a reason why over all these years I have never shared it - it is very badly flawed and I'd rather not take the risk that someone would build it not understanding that the design was just bad. At the end I understood why it did not work properly, but got distracted by DHT SE amps. This saga ended about 14yrs ago now, but here is a link to what I wrote about that experience: 6c33
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Old 10th December 2011, 09:20 AM   #550
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by kevinkr View Post
I'd rather not, there is a reason why over all these years I have never shared it - it is very badly flawed and I'd rather not take the risk that someone would build it not understanding that the design was just bad. At the end I understood why it did not work properly, but got distracted by DHT SE amps. This saga ended about 14yrs ago now, but here is a link to what I wrote about that experience: 6c33
Thank's Kevinkr,

Thanks so much for the link..
You know you can learn a lot from other people’s experiences.
I found that building OTL even "feels strange" I guess it’s the current as opposed to the HV that causes this! Even when I have worked on higher current amps the feeling is not the same as OTL... Perhaps it’s because you have to keep reminding yourself think current. Think heat "cooker element”. Or perhaps working on cookers is a greasy pain in the a.. and effects the feeling of OTL build..LOL

Perhaps its the thought that normaly you have a Tx between your mistakes and the speaker..

I must admit they sound good...after going back to my other amps..there now seems to be something missing...a sort of veiling that "was not there before”. Of course it was, just that I had nothing to compare it against. You can hear the OP Tx after listening to OTL..or is that the lack of one. Depends on your point of view I guess..

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M. Gregg
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