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84.28.64.63
hello all
i will try
a C3g (in triode)as as driver for a 300b
i use no preamp only passive attenuation
my dac give 1,6 volt rms out
is it possible driving the 300b with this tube?
and is there on the internet a schematic of a driver stage with a C3g?
or has already someone try it
i hope you can help me with this question
Follow Ups:
Hi,The C3g will work okay as driver for a 300B, it needs to be used wired as triode, this will give, when CCS or Choke loaded a gain of around 40. With 1.6V RMS input this translates into around 90V peak out. If you use the 300B at the standard 350/80mA Operation this leaves a little headroom, not greatly much but enough to get by.
If you use a resistive load you would need to use a vary high value so as not to losse too much swing.
If you look at the triode curves you note that you need to run the C3g with quite high anode voltage and low current (and high anode load) to get good swing and linearity.
With a anodic supply voltage of 225V, a 430 Ohm Cathode resistor (bypassed with 47uF MKT or MKP) and an anode choke of around 100H (or some form of CCS/Mufollower load) you get a gain of around 39 and an output impedance of around 3KOhm with a (peak) swing of 88V from a 1.6V RMS input and around 150V maximum peak swing.
If you where to use a 22K resistive anode load your gain drops to around 34 and the swing from 1.6V RMS to 77V. This might be marginal, as I have noted several 300B's that bias at more than 77V when used in the standard "WE" circuit with 430V +B and 880 Ohm Cathode resistor.
If you want to try anyway, use a 1K/22uF +B decoupling circuit, 22k Anode resistor, 430 Ohm Cathode resistor and 47uF cathode bypass capacitor. BTW, do not forget the gridstopper!!!
For the output stage with 300B you'd have a 1K 12W cathode resistor in parallel with 6K8 2W, a 47uF (160V) cathode bypass capacitor and ideally 12uF (630V) from the 300B Cathode to the +B. The Output Transformer should be 2K3....5K, +B 430V. Coupling Capacitor 0.22uF and 300B Gridleak resistor 240k and 100R Gridstopper. The +B Circuit can be used from any sort of "WE91 Copy" Amplifier, so around 430V under load of 90mA (per channel).
Anyway, that should make a working first cut Amplifier to see how you like the sound in general.
After that you can experiment to your hearts content with Constant Current Source loads (they replace the 22k Resistor), Choke Load (the 22uF +B decoupling capacitor gets shifted to the anode end of the 22k Resistor and the choke is inserted between that and the anode), Interstage Transformer couplung et al.
Finally, you can use the C3g with resistor anode load and your 1.6V RMS input if you can find a good 10K:40K (1:2) line input transformer (something from Stevens & Billington or Lundahl) or autoformer (eg Magnequest EXO-173?) you get an extra 6db gain and this will then make sure that 1.6V RMS can well overdrive the input stage, which goes into overload long after the output stage has given up.
I hope that helps.
Ciao T
hi Thorsten
thanks you very much for the very detailed information
i will try an ccs then
but how much current must the c3g draw?
i have 2 ccs printcarts from Machmat.com
but they must set on the right currentthe powerstage is already finisht it is a fixed bias stage
i wil do 400 volt on the anode and 80ma current
Hi,> but how much current must the c3g draw?
10mA.
> the powerstage is already finisht it is a fixed bias
> stage i wil do 400 volt on the anode and 80ma currentYou may find that in this case the C3g will be VERY magrinal to drive the power stage with 1.6V input, unless you add a stepup transformer either after the C3g stage (1:2 stepup interstage transformer) or before (1:2 lineinput- or auto- transformer).
The C3g can swing enough voltage but is a little short of gain.
For your circuit the D3a might be a better choice, with more gain.
Ciao T
hi Thorsten
okay thanks
can i drive the 300b better with another bias point ?
thats for me also ok
or is the fixed bias wrong?i have no d3a but ec8010 did you try this tube also?
and wat is the sound compered to the c3g?
i find these ec8010 a little "hard" soundingi think i dont need the full 8 watts of the 300b my speakers are 110db/1watt
but if i build a 300b it must be a 300b till the end of his power
Hi,> can i drive the 300b better with another bias point ?
Well, at 350V/80...90mA you get a lower bias voltage and hence need less voltage to drive the output stage to the limit.
> or is the fixed bias wrong?
Nope, fixed bais is neither "wrong" or "right", it's just one of the possible ways of doing things.
> i have no d3a but ec8010 did you try this tube also?
No, never managed to snag any.
> i think i dont need the full 8 watts of the 300b my speakers
> are 110db/1wattIn that case forget to worry about gain, you'll be allright. If you give the Amp a little more input voltage than 1.6V it will be driven to full power and you only loose a db or two at the suggested setup.
Ciao T
you very helpfull
i build the c3g driver with the ccs maby
also grid chokes i have 2 1700H laying around
and let you now the results
the 300b amp that i gone chance runs now with a 6sn7 dc coupled cascade
Hello,I used it as a driver for a 45. Works great.
berlin
hello
do you have a schematic for me?
how do you use the c3g in triode or pentode ?best Robert
Friend of mine very much recomends it as a high transconductance pentode driver. His view is that for pentode drive you get best results from valves of the sort you are talking about.I haven't studied the valve, but for pentode operation you should ideally use a vr tube (paralleled with a series rc network to fix noise) as the stable screen voltage mechanism, plus a small cap screen to cathode (not ground).
For single ended (as opposed to differential) pentode operation you must use resistive load of very low value. Lowest you can get away with sufficient for enough voltage gain. Output impedance is effectively the value of this resistor, gain is increast the greater it's value, linearity particularly at high current low voltage end of the swing is improved the lower the value. You balance the situation for yourself.
Never use an inductive load or a ccs.
Paul,
You know I was using the 12GN7 with a 9.5K load as a driver for the VV32 amp ? So you don't need to use a really low anode load ; and you need something on the anode to 'use up' some of the high B+ voltage with something like the C3g . Ideally a C3g would need about a 350V supply for a driver in pentode mode - then you could use about 8K load and get a reasonable anode value ( say 230 v ) with 15mA and so keep the anode dissipation sensible . This would be a great driver with over 100 gain . The only problem with the C3g in 300B application is the limited dissipation of less than 4 watts . The C3m is a little better in this respect . I'm surprised T has said nothing about using it as a pentode yet .
Mark
Hi,> I'm surprised T has said nothing about using it as a pentode yet .
Look at the curves and try to get reasonable swing (200V PP) into 10K from it (or any sensible load, actually).... Virtually impossible. Compare the C3m.
High Mu Pentodes just cannot swing enough voltage on the anode for driver stages. They are excellent choices however to produce a lot of gain into low impedance loads, making them ideal for low level high gain stages (Phono- or Microphonepreamp comes to mind).
Ciao T
Sorry Mark your splitting hairs.It is well known you get the best from pentodes by looking at the curves on their side, and using low load resistor to keep the loadline away from the knee. That's what I was refering to, I knew this long before you built your first amp. Don't get too big for your boots.
.. to ground or to anode?I like pentode operation driver, based on EF80 from "Revox" or E83F from "Telefunken" tube gear.
Nice to read you here Paul. Be frequently here on asylum.
Thanks.
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