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Old 3rd March 2004, 12:07 AM   #11
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Hi,

Why dont keep the split load phase inverter with the 6SL7 but change the resistors at anode and cahode to more conventional values, say 100k?

A long time ago, (~35 years ago) when I was very young I used a 6SL7 with 47k + 47k to drive 6146's with 150k grid leaks, it is not ideal but it worked surprisingly well, I changed the 6SL7 to a 12AX7 later but the difference was mimimal.

Check old conventional circuits for normal component values, it is quite common to have grid leaks ~2-4 times the anode resistors, in your case you have a factor of 12.

Regards Hans
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Old 3rd March 2004, 12:15 AM   #12
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Hi,

Quote:
Why dont keep the split load phase inverter with the 6SL7 but change the resistors at anode and cahode to more conventional values, say 100k?
Funny, I was thinking the exact same thing. It's simple, has "Maintenance Free" written all over it so it will just work and work and work...

Cheers,
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Old 3rd March 2004, 12:16 AM   #13
SY is offline SY  United States
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Quote:
i thought the -1.5V tail would be nice for a battery source?
That's where you least need a voltage source like that and a small value resistor. High impedance at that point is a GOOD thing and will let you get the plate resistors more equal, dropping distortion. And that's the point at which you've got a high supply rejection, too. If you're determined not to use a silicon current source (bipolar or fet), at least make that tail resistor a proper tail and take it down to a nice negative rail, like -50 or -100 V. Or you can capacitively couple the input to the amp, raise the B+ on that stage, and perch the diff amp on top of a nice big tail resistor to ground.
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Old 3rd March 2004, 02:27 AM   #14
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hans and frank,

yes, why not. i'd give it a shot.

sy,

with 47K tail i will need -44V and Ra2 - Ra is close with only 2.32K between them (Ra = 75K, Ra2 = 76.8K). gain dropped from 34 to 26 though. my Vout (max) is quite balanced at -71/+71.2 V. i could probably play with B- to find a better operating point of the 6SL7 probably up to 1mA @ tube so my B- will be around -92V.

thanks a lot gentlemen
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Old 3rd March 2004, 11:43 PM   #15
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Quote:
my goals are:
get 30W something from it


You mention you needed something like 30watts. It would be hard to do so using this design. UL should give you around 18watts only plus or minus(I recalled with in class A, not sure for Class AB). How about you try working around direct coupling the drivers with the el34. THis would allow you to go into AB2. Might atleast push the power up more.

I find when using ltp, it is worthwhile to use high B+ to allow a better swing and linearity. You might want to use a pot between the anode resistors to balance for AC. I would go for a high B+ as well as a high B-. I used something like 300 volts for my diff stage 6922. The plate was 50 volts above the cathode, which is the right voltage to run the 6922.

For some view on how i did my direct coupling check out my website. I direct coupled a 6922 to a el34. Sounded fine and great. However i used triode mode and biased them into A. was getting around 12 watts with a 6k trans
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Old 4th March 2004, 12:14 AM   #16
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hi nick,

i'm going AB and i think is the only way to get those 30W. textbook figures show at AB1 with ~420V 130mA Rk=470 Zout=6K6 i should get 37w at 1.3% THD

ok, i can push the B+/- of the LTP up to max where i won't need to lift heater voltages.

i visited your website, i'm sure i'll learn more with the information contained therein.

cheers!
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Old 4th March 2004, 02:42 AM   #17
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That sounds right, you ought to be good for at least 25-30 watts with that load and B+. If you want to squeeze a few more watts out, just use so-called fixed bias. The cost and complication of a negative rail and a couple of pots is probably less than putting in a direct-coupled cathode follower- and it's fewer tubes. But really, it may not be worth it- we're quibbling over a couple of dB in max SPL.
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Old 5th March 2004, 02:19 PM   #18
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i'd like to thank everybody who shared their thoughts and expertise with me.

i just completed a mono version using the 6SL7 split load and it is currently driving my test speaker, a Bose 301. i will try to play with this first until i find the recommended tubes, or go with the long tail pair version, or try both.

cheers!
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Old 6th March 2004, 12:39 AM   #19
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Hi,

Quote:
and it is currently driving my test speaker, a Bose 301.
Talk about contradictios in terminis:

Bose and loudspeaker...???
Bose and test loudspeaker...????

Just pulling your leg, Arnold...

Keep is Boseted,
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Old 6th March 2004, 01:32 AM   #20
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he, he, frank, everytime i power up a new amplifier i use that Bose. it is 16 years old, so if the drivers blow up, i won't feel bad.

it is now playing with my resident Infinity Kappa 200 (made in Denmark ) listening to some Carmen Cuesta, Jacintha, and Metallica.

mids are ok, a bit warm. bass is firm and tight, i think there's a top end roll off. i'm using really cheap parts - carbon resistors, white coffin cathode resistor, taiwan electrolytics, and the only branded one is the pair of coupling caps made by Electrocube. output transformers are custom wound.
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