I have one of those Chinese 845 amps with a ridiculous driver circuit and I've been exploring the difficulties of designing a circuit that will swing 300 volts in a linear manner. I'm down a pound of copy paper drawing load lines and I've learned a great deal but I'm still searching.
I've read about screen drive and until this exercise didn't really appreciate the advantages of the concept. I may try that next but for now want to finish with something conventional that I can at least breadboard and measure. I've got my sights set on a big triode, or a pentode operated as a triode. I'm currently enamored of the EL34 because of it's linearity and because I have a few laying around. I'd use a 45 if I had a couple but I don't.
So I wanted to solicit some comments on the EL34. Two questions really.
(1) How close to zero can I swing a triode connected EL34?
(2) Can I get by without a cathode follower with the 845? I'm wondering how to determine if I need the low resistance that a cathode follower offers for driving the 845 with an EL34.
I've read about screen drive and until this exercise didn't really appreciate the advantages of the concept. I may try that next but for now want to finish with something conventional that I can at least breadboard and measure. I've got my sights set on a big triode, or a pentode operated as a triode. I'm currently enamored of the EL34 because of it's linearity and because I have a few laying around. I'd use a 45 if I had a couple but I don't.
So I wanted to solicit some comments on the EL34. Two questions really.
(1) How close to zero can I swing a triode connected EL34?
(2) Can I get by without a cathode follower with the 845? I'm wondering how to determine if I need the low resistance that a cathode follower offers for driving the 845 with an EL34.
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I'm curious, what exactly do you mean by ridiculous? There are several ways of driving an 845. Each has their pros and cons. Here's one not so popular but works well.
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Probably a weak driver like a SRPP 12AX7. It might push enough voltage but runs out of steam quickly.
Did I say ridiculous? Perhaps that's coming it a bit strong. Let's instead say the circuit would be a poor choice for a DIY build. It's very expensive and it doesn't work very well.
The design is a pentode similar to a 6AC7 with a 300B cathode follower. The pentode is not able to swing enough voltage to drive the 845, it is very nonlinear, produces a foul waveform and uses an expensive 300B triode for a cathode follower when any number of inexpensive tubes including a 6SL7 and 12AX7 would do the job nicely.
The design is a pentode similar to a 6AC7 with a 300B cathode follower. The pentode is not able to swing enough voltage to drive the 845, it is very nonlinear, produces a foul waveform and uses an expensive 300B triode for a cathode follower when any number of inexpensive tubes including a 6SL7 and 12AX7 would do the job nicely.
I like your circuit. I just plotted the 6AH4 and can see that it would work very well with the 440v plate voltage available. It looks like the 417A is a remarkably linear tube and I believe I have a couple in my stash. My only concern is that it would take about 50v from the 417A to drive the 6AH4 and that might require about 2 volts with bypassed cathode. Perfect with a preamp.
I was thinking of keeping it as an integrated, but a couple of stages from a medium mu twin triode might work well with the 6ah4. I will give the 417A a look.
Thanks for the suggestion.
In my 845 Class A1 SET design I use a triode connected EL34 with C coupling to the 845 grid. EL34 is operated with grounded cathode, -27 V fixed bias on grid, 20 K Ohm anode load up to 625 V B+ (I know, that's a bit high), drawing 18 mA, +260V on anode. It can easily produce 300 V p-p to drive the 845 biased at -150 V. This is the sweet spot I found after experimentation with measuring lowest distortion. EL34 screen grid is connected to anode via 2.2 K Ohm 1W resistor, this is also a sweet spot. Suppressor grid is connected to anode via 1 K Ohm resistor, though this has little effect, the current is under 1 uA.
In practice 845 requires some 200 Vpp to be fully driven. It has very low amplfication factor and therefore the Miller capacitance is also low.
So 845 do not require particularly low impedance driver.
Any high gm triode with constant current generator (CCS) as anode resistor and some 400 V or more as +Ub works fine.
So 845 do not require particularly low impedance driver.
Any high gm triode with constant current generator (CCS) as anode resistor and some 400 V or more as +Ub works fine.
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I will have to did out my notes on tests that I did nearly 2 years ago. Basically its a compromise between the fairly low mu of the EL34 triode configuration and having a high enough grid bias (-27V) so that no grid current is drawn (ie. EL34 in class A1) and achieving 300 Vp-p output.
And now for something completely different
The EL34 is only OK in triode. You can get better performance with the TV line tube EL36 (6CM5) or PL36 (25E5), if it's triode-connected and operated in Shunt Cascode.
Tom Schlangen's characterisation of the EL36 shows why this works so well. With a cascode voltage of 300V, and about 47mA of idle current, the 845 can be clipped with less than 2V (zero-to-peak) - an output to the 845 grid of 150V +/- 150V.
So you can go from CDP to 845 grid with one triode.
The shunt cascode can give an output of 0V, too, so there are ways of dc-coupling it, for the cost of some stabilisation of the currents.
The EL34 could probably be used in this circuit, too, but would need to be run much hotter than the EL36.
I use a slightly scaled down version (200V EF184) for 300B driver. This works and sounds far better than the usual 6SN7 circuits that are often found to be ruining the potential of the DHT.
Edit: The EL36 should be run from fixed bias, not the autobias shown.
The EL34 is only OK in triode. You can get better performance with the TV line tube EL36 (6CM5) or PL36 (25E5), if it's triode-connected and operated in Shunt Cascode.
Tom Schlangen's characterisation of the EL36 shows why this works so well. With a cascode voltage of 300V, and about 47mA of idle current, the 845 can be clipped with less than 2V (zero-to-peak) - an output to the 845 grid of 150V +/- 150V.
So you can go from CDP to 845 grid with one triode.
The shunt cascode can give an output of 0V, too, so there are ways of dc-coupling it, for the cost of some stabilisation of the currents.
The EL34 could probably be used in this circuit, too, but would need to be run much hotter than the EL36.
I use a slightly scaled down version (200V EF184) for 300B driver. This works and sounds far better than the usual 6SN7 circuits that are often found to be ruining the potential of the DHT.
Edit: The EL36 should be run from fixed bias, not the autobias shown.
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A2 drive?
Hi
Do you intend to drive the 845 in A2 or A1?
You can have a look how I managed this on my TB3/1000 150W SE amp. You could swap the SRPP stage with e.g. D3A in pentode mode to obtain sufficient gain & low output impedance. You can get around 40W in A2 with a large portion in A1.
ward's 150W SE amp blog part 3
By the way, we have a very nice 845 output transformer in our range.
http://www.monolithmagnetics.com/images/stories/datasheet_S-5.pdf
I wish you succes with your project.
grz, //WDC
www.monolithmagnetics.com
Hi
Do you intend to drive the 845 in A2 or A1?
You can have a look how I managed this on my TB3/1000 150W SE amp. You could swap the SRPP stage with e.g. D3A in pentode mode to obtain sufficient gain & low output impedance. You can get around 40W in A2 with a large portion in A1.
ward's 150W SE amp blog part 3
By the way, we have a very nice 845 output transformer in our range.
http://www.monolithmagnetics.com/images/stories/datasheet_S-5.pdf
I wish you succes with your project.
grz, //WDC
www.monolithmagnetics.com
Hi Rod. With the design constraint of a simple RC coupled and common cathode amplifier, without CCS, cascode or any other solid state "helpers", can the EL36 still do better than EL34 for extremely large and clean output voltage swing?
I built an EL36 pentode single ended amp many moons ago but always thought it was wimpy compared to EL34. I haven't seen triode curves for EL36. Having a Top Cap is slightly annoying.
Glenn,
I've been following the thread as I have a blown up Chinese Music Angel 845 and new (decent) power and output trannies to re-build it into something worthwhile. I had'nt considered the 6CM5 as a driver but should have because as you would appreciate there are a s..t load of NOS 6CM5 floating around Oz from TV duty. I also have 3 old STC/AWV/Philips PA Amps, one with a pair of 6CA7 (EL34), one with a pair of 6CM5 and one with six (6) 6CM5. The 6CM5/EL36 has anode dissipation rating of 12 watts about half that of of the EL34 and about the same as the EL84 or 6V6. I have not tried it myself but the "Popular Wisdom" (and therefore probably wrong) is that the 6CM5/EL36 is one of the best tubes around for screen drive, and is reputed to give amazing linearity in that mode. I would be looking at either triode strapped 6V6 or screen driven 6CM5.
Note that the 150W SE 833 project on this site uses a triode strapped EL34 to drive the 833 tube but it is very different from an 845. The 845 does not need to be driven into significant grid current so a really beefy driver is not required. Also check out Mr Turner in "Cantberra", he uses 3 parallel triode strapped EL84 to drive parrallel 845 to 55W.http://www.turneraudio.com.au/monobloc845se55.html
Cheers,
Ian
I've been following the thread as I have a blown up Chinese Music Angel 845 and new (decent) power and output trannies to re-build it into something worthwhile. I had'nt considered the 6CM5 as a driver but should have because as you would appreciate there are a s..t load of NOS 6CM5 floating around Oz from TV duty. I also have 3 old STC/AWV/Philips PA Amps, one with a pair of 6CA7 (EL34), one with a pair of 6CM5 and one with six (6) 6CM5. The 6CM5/EL36 has anode dissipation rating of 12 watts about half that of of the EL34 and about the same as the EL84 or 6V6. I have not tried it myself but the "Popular Wisdom" (and therefore probably wrong) is that the 6CM5/EL36 is one of the best tubes around for screen drive, and is reputed to give amazing linearity in that mode. I would be looking at either triode strapped 6V6 or screen driven 6CM5.
Note that the 150W SE 833 project on this site uses a triode strapped EL34 to drive the 833 tube but it is very different from an 845. The 845 does not need to be driven into significant grid current so a really beefy driver is not required. Also check out Mr Turner in "Cantberra", he uses 3 parallel triode strapped EL84 to drive parrallel 845 to 55W.http://www.turneraudio.com.au/monobloc845se55.html
Cheers,
Ian
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why don't you open up a thread in the vendor's forum, the tenor of your post is like trying to advertise....😉
Hi Glenn,
The answer is No, the EL36/6CM5 cannot give large linear swings without assistance. Like the majority of triode-connected pentodes, they work fine until faced with high voltage. Just take a look at the EL36 triode curves I posted, above. The triode-mode serves well until somewhat over 300V, where the mu starts to shift.
With the larger EL34, the curves are markedly compressed at high voltage/low current too.
But as Ian says, there should be a reasonably good supply of well-made EL36/6CM5 and the (higher heater voltage equivalent) PL36/25E5. There's even a super-EL36: the Mullard EL360. And if your preferred source of valves is in Russia, the 6П31С is worth investigating - an EL36 analogue.
With shunt cascode, you make this cheap beam-tube into a real contender, with much better linearity than a simple triode stage, and far more gain too (90 in my example).
If you don't like the top-cap, you can just use a EL84 in the same position, with the same current.
Actually, with a Transmitter in the output, I think the Frankenstein look of the topcap-octal driver is the presentation to aim for. The amps with a huge End-tube and tiny driver valves look out-of-balance to me.
Hi Ian,
I think that popular wisdom is on to something this time.
The Mullard PL36 curves show 300mA of anode current for Vg1= -1 and Vg2 = 120V, with curves that look as regular as any.
this sounds like a solution for an end stage to me though, or you will need an input stage that delivers +/- 50-V peak, or more.
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