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Old 11th May 2010, 08:47 AM   #21
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Quote:
Originally Posted by eboz View Post
Yes that is very cool from an engineering perspective, but you completely missed my question! How do these things SOUND???

Okay to be fair I must have been thinking (how do they sound) and should have asked more explicitly
Right, the amp was very powerful sounding compared to my benchmark 300B push-pull amps that put out about 12 WPC. I was measuring 20W out and using an output transformer rated at 10W, so power bandwidth did suffer some. Even so, there was plenty of bass down to 40 Hz based on playing through JBL 4320s. Treble extension was great, and overall the amp sounded neutral in terms of not adding any noticeable coloration.

I think with properly sized OPTs, it would have great low end punch with the moderately efficient 92db speakers. Also the larger OPTs would probably have lower DCR and improve the speaker damping some.

The exercise did what I wanted, which was to find out that I like the sound of a powerful SET amp.

Cheers,

Michael

PS measurement wise, the second harmonic dominated until about 4-5 watts, then the 3rd harmonic kept equal with the second until clipping. The driver operates into it's low gm region to swing the power grid positive. I learned that some tuning of the source resistor value can reduce the remnant of the "hockey stick" grid current seen by the driver anode in this region, extending the range of the 2nd harmonic dominance.
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Old 11th May 2010, 01:32 PM   #22
regal is offline regal  United States
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I'm a hybrid enthusiast. To me the advantage is to use a tube gain with mofset follower that way you get rid of the OPT. But I haven't found an elegant method to avoid large coupling caps on the output.

Anyway I do believe that hybrid amplifiers are the future. And am glad to see another method.
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Old 11th May 2010, 02:10 PM   #23
eboz is offline eboz  United States
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Quote:
Originally Posted by Michael Koster View Post
Right, the amp was very powerful sounding compared to my benchmark 300B push-pull amps that put out about 12 WPC. I was measuring 20W out and using an output transformer rated at 10W, so power bandwidth did suffer some. Even so, there was plenty of bass down to 40 Hz based on playing through JBL 4320s. Treble extension was great, and overall the amp sounded neutral in terms of not adding any noticeable coloration.

I think with properly sized OPTs, it would have great low end punch with the moderately efficient 92db speakers. Also the larger OPTs would probably have lower DCR and improve the speaker damping some.

The exercise did what I wanted, which was to find out that I like the sound of a powerful SET amp.

Cheers,

Michael

PS measurement wise, the second harmonic dominated until about 4-5 watts, then the 3rd harmonic kept equal with the second until clipping. The driver operates into it's low gm region to swing the power grid positive. I learned that some tuning of the source resistor value can reduce the remnant of the "hockey stick" grid current seen by the driver anode in this region, extending the range of the 2nd harmonic dominance.
Hey Michael,

Thanks for delving further into the amp's characteristics. I have been using a 211 SE amp for the past two months and love its sound. Unfortunately it's not mine, so I will need to be building myself one. I have been hunting around for the output tube of choice and so far 211 is an easy winner over other thoriated tungsten transmitters. It doesn't take too much heat, it's easy to bias and does not require strong A2 right away, sounds wonderful and passes characteristics of previous stage, and should be in current production for the foreseeable future.

However... It's very tempting to go with something a bit more esoteric such as the 4-65. It seems to be a nice Eimac tube that hasn't been noticed as much as the 75tl through 304tl's. What do you think about this tube's availability?

Thanks!
-Michael Woods
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Old 11th May 2010, 02:12 PM   #24
eboz is offline eboz  United States
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Quote:
Originally Posted by regal View Post
I'm a hybrid enthusiast. To me the advantage is to use a tube gain with mofset follower that way you get rid of the OPT. But I haven't found an elegant method to avoid large coupling caps on the output.

Anyway I do believe that hybrid amplifiers are the future. And am glad to see another method.
regal,

I have not tried this and it might be a completely inappropriate idea in this application, but just for kicks... Do you think it's possible to use a capacitance multiplier here?

-MW
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Old 11th May 2010, 09:21 PM   #25
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Quote:
Originally Posted by eboz View Post
regal,

I have not tried this and it might be a completely inappropriate idea in this application, but just for kicks... Do you think it's possible to use a capacitance multiplier here?

-MW
I guess that's another name for a DC balance servo, which drives one end of the output (transformer) with a slowly changing correction signal to match the average DC voltage of the output node. I would integrate the DC offset across the output or OPT primary to derive the correction signal. This acts in effect like a big capacitor.
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Old 11th May 2010, 09:31 PM   #26
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Quote:
Originally Posted by eboz View Post
Hey Michael,

Thanks for delving further into the amp's characteristics. I have been using a 211 SE amp for the past two months and love its sound. Unfortunately it's not mine, so I will need to be building myself one. I have been hunting around for the output tube of choice and so far 211 is an easy winner over other thoriated tungsten transmitters. It doesn't take too much heat, it's easy to bias and does not require strong A2 right away, sounds wonderful and passes characteristics of previous stage, and should be in current production for the foreseeable future.

However... It's very tempting to go with something a bit more esoteric such as the 4-65. It seems to be a nice Eimac tube that hasn't been noticed as much as the 75tl through 304tl's. What do you think about this tube's availability?

Thanks!
-Michael Woods
I like the 4-65A a lot; in triode mode it looks a lot like the 845 wrt. plate resistance, gm, even the plate curves... with a little lower dissipation rating. In practice, the 4-65A *should* be run at about 55 watts to keep the getter healthy, so it's a lot like a lot of 845 amps. Filament is manageable also.

The 4-65A seems to be in fairly good supply but I will stick to NOS from now on. Used ones are 80%+ junk, and even new ones can be about 50/50.
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Old 11th May 2010, 09:31 PM   #27
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Quote:
Originally Posted by eboz View Post
regal,

I have not tried this and it might be a completely inappropriate idea in this application, but just for kicks... Do you think it's possible to use a capacitance multiplier here?

-MW
I guess that's another name for a DC balance servo, which drives one end of the output (transformer) with a slowly changing correction signal to match the average DC voltage of the output node. I would integrate the DC offset across the output or OPT primary to derive the correction signal. This acts in effect like a big capacitor.
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Old 12th May 2010, 03:12 AM   #28
Ejam is offline Ejam  Australia
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Default Using an 811A

Michael

Was impressed by your circuit in Post 19 and thought that it could be easily adapted to an 811A. Have tried several three tube circuits with 811A in Class A2 and have found the sound to be particularly good. There is a nice operating point of Vpk = 430V, Ip = 95mA and Vpk = +22V into a 5k load. The grid current draw is around 22mA, This is similar to your 35TG valve. The thing I like about the Post 19 circuit is no need for a negative supply. A little unsure about adapting especially the source resistor for the above operating conditions. Would appreciate any comments or help. Thanks in advance.

Ejam
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Old 12th May 2010, 05:56 AM   #29
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Why MOSFETs? A 6DJ8 or 5687 will drive 50mA peak grid current no problem.

Heck, and if it's still too squishy for 'ya, you could use a pentode-driven "mu" stage with mondo gain, and wrap NFB around the thing to reduce the drive impedance to a couple of ohms. Going from ~2k gain down to 20 or so, you'll drop Zo from maybe 100 ohms to...just a couple?

Speaking of ridiculously low impedances, 2N7000, LEDs, resistors: jokes, all of them. TL431 is adjustable and comes in at an ohm or two, and is priced similar to a 2N7000 or red LED. Rated up to 100mA and 30V, it will handle anything you put it under; output tubes will need an additional pass transistor or voltage divider to accommodate the higher power dissipation, of course. And the voltage is a hell of a lot more stable than some crapMOS (check the tempco of Vgs(th)!).

Of course, a regulated voltage at the cathode of a class A stage is momentously pointless, a resistor is the best solution. Don't bother with anything else, and don't let liars mislead you otherwise!

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Old 12th May 2010, 07:34 AM   #30
eboz is offline eboz  United States
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Quote:
Originally Posted by Sch3mat1c View Post
Why MOSFETs? A 6DJ8 or 5687 will drive 50mA peak grid current no problem.

Heck, and if it's still too squishy for 'ya, you could use a pentode-driven "mu" stage with mondo gain, and wrap NFB around the thing to reduce the drive impedance to a couple of ohms. Going from ~2k gain down to 20 or so, you'll drop Zo from maybe 100 ohms to...just a couple?

Speaking of ridiculously low impedances, 2N7000, LEDs, resistors: jokes, all of them. TL431 is adjustable and comes in at an ohm or two, and is priced similar to a 2N7000 or red LED. Rated up to 100mA and 30V, it will handle anything you put it under; output tubes will need an additional pass transistor or voltage divider to accommodate the higher power dissipation, of course. And the voltage is a hell of a lot more stable than some crapMOS (check the tempco of Vgs(th)!).

Of course, a regulated voltage at the cathode of a class A stage is momentously pointless, a resistor is the best solution. Don't bother with anything else, and don't let liars mislead you otherwise!

Tim
Yes of course we know mu can be used here and it has been tried. FET source followers actually sound better!

I am still going to try a JFET current-biased in an input tube's cathode. Hadn't thought of the TL431 though! Sometimes cathode bias with a resistor sounds nice, sometimes it doesn't. It's just another flavor.
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