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Dear sir,
I am going to build a power amplifier with a differential phase splitter. I found one in the sound pratice , it is using the 6SN7 + 6SN7 + 2A3.
Could you please share your experience or any link I can refere to similar design??
Wilson
Follow Ups:
With that tube complement, I'd for an all-differential design.
Stage 1 would be an LTP splitter, using a 6SN7 with a CCS in the tail (requiring a negative rail for the CCS). Stage 1 would be DC-coupled to Stage 2, a 6SN7 differential driver. Stage 2 would be cap-coupled to Stage 3, the 2A3 PP output stage. Such all-differential designs have extremely low hum and noise.
You could apply balanced cross-coupled NFB between the plates of the 2A3 and the plates of the first 6SN7 if you needed to bring down the amp's OP impedance (better damping).
You can look at my PP-1C circuit, near the bottom of the page from this link.
Best phase splitter I know. We use it with a 6H30 in both of our production poweramps, an 18 watt 300B amp and a 150 watt KT88 amp.
Regards, Allen (Vacuum State)
HI Allen,
I am your fans. I need to spend more than ten years to understand your pre-amp cook book & your article on Sound Practice.And I am still keep reading to learn more.
I saw the circuit. Is it OK to use E288CC instead of 6H30? It should be better than ECC88. BTW, why the circuit need to have current source loading instead of a resistor ( just like the text book differential amplififer). Is it enough gain of a signle stage drvier? In fact, I agree that only one stage should be better than two stage 6SN7 differential driver design.
look forward your sharing
> > I am your fans. I need to spend more than ten years to understand your pre-amp cook book & your article on Sound Practice.And I am still keep reading to learn more < <
Great, keep on it!
> > I saw the circuit. Is it OK to use E288CC instead of 6H30? < <
THe 6H30 is better, because you can use a lot more current, and to directly drive output tubes, you need LOTS of current.
> It should be better than ECC88 < <
Maybe, but not much. 6H30 is a far better choice.
> BTW, why the circuit need to have current source loading instead of a resistor < <
It doesn't. All my circuits use resistive loading.
Are you confusing this with the CCS in the cathodes? That's NOT the loading, and is used because it sounds far better than using a resitor to a high negative voltage.
> > ( just like the text book differential amplififer). Is it enough gain of a signle stage drvier? In fact, I agree that only one stage should be better than two stage 6SN7 differential driver design < <
It is far better, by actual test. And it has enough gain.
Regards, Allen (Vacuum State)
Hi Allen
> > I saw the circuit. Is it OK to use E288CC instead of 6H30? < <
THe 6H30 is better, because you can use a lot more current, and to directly drive output tubes, you need LOTS of current.
How about 5687 or E182CC? they has more power dissiplation. I don't have 6H30 on hand.
> BTW, why the circuit need to have current source loading instead of a resistor < <
It doesn't. All my circuits use resistive loading.> > >
I saw the lower ECC88 has connected with upper ECC88 with Fixed biase on the gride. SO, I guess the 2nd ECC88 is a current source. What is this circuit? I don't see on the text book.
Wilson
If it's not in your text book, you need a better text book!
Try Valley & Wallmann "Vacuum Tube Circuits". That's where the cascode was first revealed. (Page 440)
The cascode is a series connection of a common cathode stage under a common grid stage.
Very wide bandwidth, high gain, low noise and very linear. Similar to a pentode but quieter,and no screen current.
Regards, Allen (Vacuum State)
Allen,
What B+ are ideal in your circuit?
Also - how much curent is each EL34 pair set to draw?
Thanks,
Adam
If you find yourself frequently repeating the phrase, "garbage in, garbage out," you may need to rethink your system.
> > > What B+ are ideal in your circuit? < < <
With EL34, I'd like a max of 425 B+. Subtracting the cathode bias voltage gives a cathode to anode V of circa 400.
> > > Also - how much curent is each EL34 pair set to draw? < < <
A real EL34 is rated at 25 watts anode dissapation. So at 400v this means a MAXIMUM of 62.5 mA/tube. But I like to run things a little under maximum, so lets say 50 Ma/tube should be safe.
Regards, Allen (Vacuum State)
I'm just wondering if DHTs could be used in such a cascode design. 6N30 has a Rp of about 1k and mu of about 15. I don't know any DHTs that are similar, so I don't know what to prioritise - the mu or the Rp?
there are some bigger output DHTs like 2a3, 71a etc but that would be very wasteful.
I'm wondering if something could be done with the 31. cheap and easy to heat - Rp of about 3.6k and mu of about 3.8. We've strayed quite a way from the 6N30, but the 31 sure sounds nice to my ears. Any use?
andy
The cascode gain is set by the lower tube gm and plate load. The swing available( w/o getting into the curvy part at left hand side of plate curves ) depends on the upper grid voltage and how much more or less voltage is required to flow current across it v. the bottom tube.
Take a look at GE's 6BQ7 cascode curves. The cascode plate curves are constructable with some single triode curves; take the single triode curves, and superimpose a second set shifted to the right by the upper grid voltage. The horizontal part starts with the intersection of shifted g=0v with the original grid line. using the 6BQ7 curves, it takes 125V to flow 20 mA on the single triode curves; the cascode turns horizontal at 250. 10 mA is reached at around 75V, so add 75 to the 125V upper grid voltage and that is where you'll see the -1.2V grid line turn horizontal.
The cascode creates a pentode-ish set of plate curves, and they can be extended closer to the upper grid/gate voltage with a device that requires less voltage across it than the lower tube. Lower mu tubes will require more lower plate voltage swing though...and that will effect the composite curves too. Lower voltage drop and high gm is nearly the exclusive domain of the SS device. Triode/MOSFET cascode works very well here.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Andy,
A tube for good cascode usage needs HIGH(!!!) transconductance, which eliminates ALL DHT tubes. The perfect tube would be the Russki 6S45, second the 6H30, 3rd the ECC88.
Nothing else needs to be bothered with if you want cascode. IMO and IME.
If you want to play with 60 year old DHTs, no problem, but don't force them to work in a circuit topolgy they are not intended for - horses for courses!
Regards, Allen (Vacuum State)
So with DHTs it's a case of some combination with a Mosfet? I'm really unfamiliar with this topology. Is it worth looking into? I really try and work only with DHTs these days since I so prefer their sound! I have no production constraints like Allen, so I can use anything.
andy
MOSFETs have no place in an audio signal path, IMO and IME. None! Been there, done that, and left!
They simply sound ugly, due most likely to their variable input gate capacitance with signal level.
If you want to use DHTs, then use them in the circuits they work best in, there are hundereds of possibilities out there, mostly proven by tests of time. I'm not against such circuits and tubes, they are just not my choice for production.
If you want to use "modern" (post WW2) circuits like cascodes, use suitable tubes.
If you want your audio to sound good, forgetabout MOSFETs!
Regards, Allen (Vacuum State)
hey-Hey!!!,
The cascode is driven by gain requirements( delivered gm and plate load ), and voltage availability to deliver a space where they're supposed to work in( the horizontal part of the load line ).
As far as DHT in the bottom go, the same gm requirements are in force. There are single element DHT with gm greater than 6 mA/V to choose from, though implementing them would pose some interesting challenges...:)
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
The highest gm in DHTs is about 5.6, with the 1624 in triode, which has a mu of 10 and Rp of about 2.
Not sure if that's any use. There's really not much higher, and most are very much lower!
andy
hey-Hey!!!,
Type HY51B has gm of 6.5 mA/V...and a plate resistance of ~4k once current is flowing. That sort of characteristic makes it a bit of a challenge. I'd consider type 2A3 if I wanted to build a DH triode cascode. The IDH valves are indeed easier to use, with gm in the 8-20 mA/V range and plate Z low enough to allow low Ec2 voltages( the upper grid/gate voltage ).
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
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