Time ago I asked Rod Coleman about the driving requirements of my 01a preamp whilst investigating the addition of a source follower stage to the preamp. Let’s have a look at a DHT stage loaded with a gyrator (or could well be a choke or whatever you like) driving a power amplifier.
Have you asked yourself whether your pre-amp is capable of driving your amp? How much burden does the cable parasitic capacitance add to the mix?
As an example we will use my current setup. A 45 SE amplifier with a 6j5 driver stage. We can approximate input impedance formed by:
Where Ci is the amplifier’s input capacitance formed by the Miller effect of the valve’s input capacitance (Cag) and the additional parasitic capacitance of socket, wires and so forth. In this practical example where the input valve is an 6J5GT:
If we now add the cable capacitance which could easily be 100pF per metre and at least 2 metres run from my pre-amp to the amp then:
So with an input resistance of 100kΩ and a capacitance of about 330pF let’s have a look at the current requirements to drive this load. The cable current @ 50kHz should be:
Preamps output peak voltage is around 10V, So the current demanded by the cables would be:
So if we want the preamp valve to source 1mA to the load, we need x10 current driving capability to be on the safe side. Clearly with an 26 (or 01a) we are a little on the low side as the bias current won’t be more than 4-6mA.
So there are clearly facts here to support the addition of a source/cathode-follower stage to the amp in addition to the improvement on the bass response due to a lower output impedance. Something we will look at some other time….
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Have you asked yourself whether your pre-amp is capable of driving your amp? How much burden does the cable parasitic capacitance add to the mix?
Hi Bartola !
I am looking at your web site with pleasure. I see that you are using the Rod Coleman filament supply. It’s clearly a major improvement for the DHT world, I think. But, to access at the full life of this supply here are my tips :
1) R1, the current sensor, must be change by a carbon resistor. Cheap one will be far much better than the resistor provided… (if cost no object try Duelund!)
2) C3 (220 nF) : remove this crap, everything like sprague,whatever you want, but not this little plastic crap ! (I use Jupiter…)
Now, the filament supply is flying …
3) Change every resistor. Remove the metal film, go carbon, even cheap ! Next step to nirvana …
Well, have a good flight !
Jérôme Malbrel, Paris, France
Hi Jerome,
I’m glad you like it. It’s always great to have some feedback.
Thanks for the advice, I believe the 220nF you are referring to is C2? I have used russian teflon ones with great results and also oil caps (PIO). I will try Jupiter and report results.
Re metal film, I agree with your recommendation, need to go carbon!
Keep posting your comments please!
Cheers,
Ale
Yes, it’ C2. In fact, I think that anything will be far much better than the plastic one provided ! For the carbon resistor, it’s a little bit painful, because the holes on the circuit are to small… But try it !
By the way, I am using 45 DHT, my driver is a 717 penthode; Next evolution will be: 45 drived by a…45 ! And a 26 at first stage. All with the Rod Coleman filament supply… 🙂
Why not driven by a 46? 46 is a superb valve and not that expensive as the 45!
Haven’t tried 717 pentode, what is your impression?
Why not 46 ? Because I have a little stock of 45 ! (I now, I now…).
The 45 was design at the start of the electrification of USA. In fact, the datasheet are very conservative (tips given by Jack Eliano from Electra Print, Las Vegas) because they are design to sustain huge variations on the electric main. 270v / 45 mA is not crazy at all ! At this moment, I am using a pair of Rogers brand (They were produce in Toronto). There plates are 50% longer than the Cunningham. So, my bias is at 50 mA…
The 717 is a very good tube, but I want to go now to a full DHT system.
Nice! I don’t have many 45 unfortunately. Also got a pair of 245 which sadly found with my tracer that are not in good shape (electrode misalignment and distortion in some regions).
At the moment I’m using the Full Music mesh 45. They sound superb to me.
Re DHT: that’s the way to go! Why not 4P1L, have you tried them? One of the most linear valves out there and a pair of them can do like an 2A3 and cost nothing in comparison!
I have a stock of 45, and 26, and in addition, 6 brand new B406 from the french post…
Later I will try other, but I think I have enough to play !
First time I heard of the B406, what is their reputation? I think 26 and 45 are excellent valves, so agree,with you.