6as7/6080 Se Otl

[poynton]

To make this cicuit work you have to take out max currents through the 6080s which means their workingpoint should be Ia 125mA, Ua 100V.

You should also use a choke or a CCS from the cathodes to ground. A B+ of 130-170V will do.

Every triode has an theoretical (simplified 1/Gm)Rout of ca 140ohms, so adding a separate 10 ohm to each one will not make you loose so much. Without NFB Zout will be high anyway.

If going for a 8ohm load I would suggest using at least 5 but preferably 10 6080s/channel.

I suggested using a CCS some time ago.

http://www.diyaudio.com/forums/showthread.php?s=&threadid=91200

However, there was concern that oscillation would (had) occured.
Maybe a CCS per cathode?


Andy

 

[revintage]

Have anyone really tried a CCS as I know you suggested? Must be the most costeffective way to go as chokes cost a lot. Just using a resistor will make you lose to much power and worsen THD.

Think a CCS robbed from Pass would be fine......

 

[poynton]

To be honest, I could not see why a CCS would cause oscillation and instability as it is well used, as you say in Pass designs.

And as you say, cost is minimal !!!

So far, I have not had chance to try anything due to work. ( I am now 200km off the east coast of India ! )


Andy

 

[Fuling]

If you can find transformer cores, bobbins and magnet wire there´s no need to pay 60 bucks each for cathode chokes. I made my own for my SE OTL with 4x PL504 (per channel):

600 turns 0,45mm wire on EI78 cores with ~0,5mm airgap.

 

[Klimon]

60 bucks each

30$ each actually; appealing idea to diy however I'ld have to find a parts supplier here (maybe in Germany?)

Regards - Simon

 

[revintage]

Hey Fuling,
As I know you are keen to try new solutions, have you tried CCSs?

My (theoretical) idea is that one could lower B+ to consist of 100V over the 6080s, the negative swing at decided load and the voltage drop over the CCS.

So a std 115V secondary transformer should be sufficient.

 

[revintage]

This is a theoretical model of what I mean. Simmed it works well. Stole the CCS from Pass Zen and turned it upside down and changed P to N !

Almost like it is too simple as the grids are at ground level. 12,5W through each 6080. At 5% THD output is 12W/16ohm and 6W/8ohm. At 1% THD output is 2,5W/16ohm and 0,5W/8ohm

 

[poynton]

That's pretty much what I had in my previous postigs although I cannot remember where my CCS came from!

What current have you through the CCS ?

How about -ve bias for the tubes -?

Andy

 

[Fuling]

Klimon: I bought my stuff from www.svebry.se
Look under "produkter", then "transformatorer" and then "transformatorbyggsatser".
I guess you can write to them in english and ask if they ship abroad.

 

[revintage]

As I said earlier 100V/125mA through the tubes. No need to bias the tubes otherwise than with the CCS. If you check UaIa-curves, Ug is in the ballpark of -30V. So there is enough room for negative swing even if we should loose 10V over the CCS.

 

[Fuling]

Lars: Good idea but you´re not the first one to think about using an Aleph-type CCS in an SE OTL amp, I wrote something about that on this forum at least two years ago. Never got any replies though...

The whole thing would be sort of a White follower.

 

[revintage]

Fuling, I asked you a few posts back, remember? Just because I was sure you had already been there !

 

[Fuling]

Must have missed that, I´ve been busy lately and haven´t followed this thread as closely as I should.

 

[revintage]

Fuling,
Anyway, did you ever try it? Maybe we should give it a try? I have a ICA-plasticbag with at least 50pcs 6080s somewhere in my warehouse.

Gain is -7dB with 16ohm and -10dB with 8ohm so the driver must have good voltageswing capability.

 

[desperateaudio]

Lars

Do you feel the design I originally posted has good driver voltage swing as is? I have already boarded up the amp I posted just need to do some tweaks and I will try the CCS and report back over weekend

 

[Fuling]

Lars: No, I never tested any kind of CCS load. I remember being concerned about the startup process but I don´t remember exactly why

 

[revintage]

It wont work, in theory anyway! Not enough swing. Will come back with an adjustment if your circuit.

 

[revintage]

I was wrong! It will work, but the sensitivity is very high at 100mV rms. But keep it that way, you might want to add a tad of NFB to it later.

 

[poynton]

NFB would be good anyway to reduce the output impedance etc.

Andy

 

[revintage]

Fuling is the guy who have tried this IRL.