Hey Anyone,
Has anyone ever heard of an amplifier using 6AS7GA output tubes? I first saw them in Bruce Rozenblit's OTL power amp. They were intended for use as DC amplifiers not as audio tubes. But in looking at their specs they are really interesting. Its a low mu twin triode, mu=2, with and extremely low plate resistance of 280 ohms. I'm assuming that if you parallel a single tube the plate resistance would be 140 ohms and then if used in push pull the plate to plate resistance would drop to 70 ohms?
If the 70ohm plate to plate is correct, (My math is always suspect.) in push The output transformer would need a turns ratio of 2.95 to 1 to drive an 8 ohm load. With such a low turns ratio the drawbacks of an output transformer would be minimized. It might even be possible to use a toroid?
But the real question? Has anyone ever heard this tube? Would its sound be worth building an amp with it?
Has anyone ever heard of an amplifier using 6AS7GA output tubes? I first saw them in Bruce Rozenblit's OTL power amp. They were intended for use as DC amplifiers not as audio tubes. But in looking at their specs they are really interesting. Its a low mu twin triode, mu=2, with and extremely low plate resistance of 280 ohms. I'm assuming that if you parallel a single tube the plate resistance would be 140 ohms and then if used in push pull the plate to plate resistance would drop to 70 ohms?
If the 70ohm plate to plate is correct, (My math is always suspect.) in push The output transformer would need a turns ratio of 2.95 to 1 to drive an 8 ohm load. With such a low turns ratio the drawbacks of an output transformer would be minimized. It might even be possible to use a toroid?
But the real question? Has anyone ever heard this tube? Would its sound be worth building an amp with it?
The tube is 250V 100mA. A good-power load would be 2.5K per plate, not the matched load you have figured. You can go a long way either way from 2.5K/plate, but not matched if you want serious output.
You will basically be looking at 6L6/EL34 iron.
> mu=2
Which means HUGE drive voltage. For audio, it tends to need a driver supply B+ higher than the power stage B+, an awkward combination. Clever design and large cathode resistor drop can lead to a fine audio amp. (Note that in DC regulator service we can accept less gain than in audio, because distortion is less of a problem and we can load-off the long tail of the cut-off.)
You will basically be looking at 6L6/EL34 iron.
> mu=2
Which means HUGE drive voltage. For audio, it tends to need a driver supply B+ higher than the power stage B+, an awkward combination. Clever design and large cathode resistor drop can lead to a fine audio amp. (Note that in DC regulator service we can accept less gain than in audio, because distortion is less of a problem and we can load-off the long tail of the cut-off.)
6S19P push-pull.
Do you see an additional transformer?
It is to add voltage for the driver, QQE03/12 tetrodes.
Testing Wavebourn Constellation - 2 amplifier prototype - YouTube
Wavebourn Canstellation - 2 playing in the darkness - YouTube
Do you see an additional transformer?
It is to add voltage for the driver, QQE03/12 tetrodes.
Testing Wavebourn Constellation - 2 amplifier prototype - YouTube
Wavebourn Canstellation - 2 playing in the darkness - YouTube
http://www.triodeguy.com/Triodeguy PDF files/6AS7 Current Balanced PP Amp.pdf
Never heard it but it seems like a good design.
Never heard it but it seems like a good design.
I breadboarded a hybrid PPP 6AS7 amp using a 1K output transformer. It was a successful design that delivered 25~30W, but I never moved forward to chassis construction after I got it working to my satisfaction. Here's the schematic: http://www.diyaudio.com/forums/atta...36a-6as7-output-tubes-perveant-breadboard-pdf
This is my version. About a dozen of these have been built by Finnish hobbyists.
This produce some 10...12 W. If you want a fair priced class A triode PP-amplifier, this is one candidate.
http://kuva.termiitti.com/image/43328.GIF
This produce some 10...12 W. If you want a fair priced class A triode PP-amplifier, this is one candidate.
http://kuva.termiitti.com/image/43328.GIF
Attachments
Has any of them run them on very sensitive speakers or as headphone amps ?
Unless they have their experience isn't comparable.
I have used just about every type of 6080 variant and they all show similar behaviour.
If you do extensive searching on 6AS7 based headphone amps you will come across people reporting the same thing.
Even though I loved my 6080 PP amps, I still think that there are better candidates for the job. I personally think that the PL84/EL86 is a better bet and not excessively expensive. Dealing with the massive drive requirements of a 6080 amp makes for a more complex build in the end. The MU of 2 will really equate to a gain of 1 or less in a real world circuit.
Shoog
I don't know about their speakers. Mine are 91 dB Radiotehnika Giant FS-100.
Giant FS-100N | VEF Radiotehnika RRR
But I have measured the amplifier's S/N, which is > 80 dB (unweigted).
True. The driver stage needs to be carefully designed. The MU in actual circuit is (in my case) 1.44 as can be seen from the schematic.
EL86 as a PP/UL is very good choice, better that typical EL84. With 300 V suppy voltage, more than 20 W is available.
With EL84 and +Ub = 300 V, some 12 W is max.
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Hi mr2racer. you probably know this OTL circuit:
6AS7GA OTL
Did you ever think of trying 6AS7 as a driver?
6AS7GA OTL
Did you ever think of trying 6AS7 as a driver?
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You can also use this tube in a SE configuration: my CELLINI triode amp
I still use it occasionally.
I still use it occasionally.
Thanks everyone for the help. Especially the data sheet with audio information. I was confusing internal plate resistance and plate to plate load resistance. That data sheet was a bit of a disappointment though as it warns to never use the tubes in parallel. Another disappointment was the expected life of the tube of only 400 hours.
As for the noise/whine Shoog mentioned I've found posts dealing with it. Most are for OTL headphone amps. They seem to find it is a product of individual tubes rather than the design of the tube itself. Also, rolling 6AS7, 6080 and 5998 tubes to find examples that are quiet. So it doesn't seem to be limited to 6AS7. Winged C built 6AS7G which would be relatively late model tubes. And yes they are pretty!
On the up side the tubes are inexpensive when compared with other power triodes like 2A3 and 300B. They are NOT DHT tubes. Built a couple of those amps and don't want to again. Also, both power transformers and output transformers would be fairly straightforward. As for the drive requirements from the interelectrode capacitance's, (I'm on thin ice here.) they don't seem more difficult to drive than 2A3?
The audio datasheet mentions the low mu/voltage swing problem and recommends using an interstage transformer to swing the grids. So I would probably start with a known quantity differential amp like Pete Millet's uniamp. Or possibly design from scratch using EDCOR line matchers as interstages and 6N6P as drivers, I have a few.
I've wanted to build a ten watt pure triode amp for a while. Which would be relatively high power I might add. One without feedback. And the differential push pull layout seems the most attractive option. Plus the amp could be done with only four tubes. I'm going to get a quote on the transformers and see where it takes me?
I really didn't need another project but this one feels good!
As for the noise/whine Shoog mentioned I've found posts dealing with it. Most are for OTL headphone amps. They seem to find it is a product of individual tubes rather than the design of the tube itself. Also, rolling 6AS7, 6080 and 5998 tubes to find examples that are quiet. So it doesn't seem to be limited to 6AS7. Winged C built 6AS7G which would be relatively late model tubes. And yes they are pretty!
On the up side the tubes are inexpensive when compared with other power triodes like 2A3 and 300B. They are NOT DHT tubes. Built a couple of those amps and don't want to again. Also, both power transformers and output transformers would be fairly straightforward. As for the drive requirements from the interelectrode capacitance's, (I'm on thin ice here.) they don't seem more difficult to drive than 2A3?
The audio datasheet mentions the low mu/voltage swing problem and recommends using an interstage transformer to swing the grids. So I would probably start with a known quantity differential amp like Pete Millet's uniamp. Or possibly design from scratch using EDCOR line matchers as interstages and 6N6P as drivers, I have a few.
I've wanted to build a ten watt pure triode amp for a while. Which would be relatively high power I might add. One without feedback. And the differential push pull layout seems the most attractive option. Plus the amp could be done with only four tubes. I'm going to get a quote on the transformers and see where it takes me?
I really didn't need another project but this one feels good!
No big deal to use four tube per channel for parallel push-pull, just use individual cathode resistors and bypass caps.
I would take that with a grain of salt. Expect 5,000+ hours.
I'm not sure how much better/worse this would be than using something like a pair of EL34's. Consider that your push-pull output transformer needs to handle a lot of current but only be sized for modest output.
A 2A3 sits at 50-60V of bias usually depending on the design, while you may have 100V of bias on the 6AS7. The 6AS7 sure will need more drive voltage, and if you use some global feedback around the circuit, then you'll need even more!
I think there may be an assumption in that datasheet that one would want the simplest possible amp, and they may be thinking that you could run something like a 6J5 off the same B+ supply as the output stage, then just step that first stage up a ton to get the required drive voltage for the output stage.
You may be a little disappointed when you put your efforts up against a half way decent 300B amp (which can also easily be made to make 10W).
The datasheet and recommendation is from 1947 when it was common to use interstage transformers.
As you can see from my schematic, there is 370 V supply voltage available for the driver and required drive signal for the output tubes is 245 Vpp, so no problem at all.
The driver stage has also a huge effect to the overall THD. Now it is biased to perform most of the distortion cancelling. The output stage alone has some 3.5...4 % THD at 10 W. With properly biased 6SN7 driver the overall THD is below 1 %.
With an interstage transformer as a driver you won't get such improvement to THD.
Attached the test results of my amplifier.
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What is even more dissapointing in the datasheet is the large power loss in the cathode resistors. But this is common to all self-biased power triode applications. Thus, I'd prefer smaller cathode resistors, primarily for balancing purposes, and fixed bias, assisted by a servo circuitry.
Best regards!
Best regards!
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