I’ve got asked the question on how an EML20 (or EML30) can drive an EML45B. Answer is straight forward and hopefully this brief blog entry can shed some light to this request.
I recently looked back again on the EML20A. With a gain of 20, it’s a good candidate but it hasn’t got just enough gain to drive the output stage to full power:
As you can see on the curves above, the 45B needs 176Vpp to produce about 4.6W. You can push it up to 6W in class A2 with 200Vpp input signal. I’m not sure how the Emission Labs grid will work with positive grid current so I wouldn’t try this without checking with the manufacturer.
With a 2Vrms input signal you will get about 113Vpp at the output of the EML20A. Not enough for full power and will give you 2W. You will need to add an extra input stage or a simple 1:2 SUT at the input.
The EML20A has to be biased at higher voltage. Here is my suggested operating point:
The EML20A has to be run at 300V at least. The EML30 is an alternative. I don’t know how linear and good this one is as I haven’t tried it myself. However looking at the curves you will need to run it at 400V at least. With the voltages involved and swing required, probably better to run it with choke load.
As I’ve done it in the EML20A preamp circuit, the valve is biased with filament bias. The SiC diodes get hot, but can handle it. They drop close to 1.5V when running through large currents like in this circuit. So we will need 3 diodes to get about 4.5V. In this way we can allow 180Vpp swing. We may want to increase this to 5V bias and give some further headroom for 200Vpp or more. The HT is 410V and you notice as well that the negative raw bias supply has to be around -200V as the 45B needs -88V at its grid and you need at least 100V swing voltage (in both directions).
I’m sure this would make a great sounding DHT amp with these 2 nice valves!
I thought you are done with the EML20A ? In comparison to a normal 6sn7 it sounds actually quiet closed, very rigid, not to say not very musical…IT coupled at 400V and 25mA…now its back ?
It would be interesting to understand if this character is similar with the EML45 or if the EMl45 is a different animal…
All the Best
No, it’s not back for me Frank. I’d try it as driver as clearly wasn’t the best sounding DHT as Preamp. I don’t want to rule it out myself without testing it as driver first.
I’d likely use a D3a in this position instead though.
The EML45B is very promising providing it still sounds like a 45
Cheers
Ale
Hi Frank and Ale,
this discussion is more than informational for me. I just completed my 7N7(6SN7) and 300B-Mesh amp. But I am flexible for a later upgrade to a 20A / 300B-Mesh 2-stage (with IT) solution. According to you I should not take that “upgrade” into consideration.
Or do you have new findings?
Best regards,
Bernhard
Hi Bernhard,
There are far more superior drivers than the popular 6SN7. The use of the 6SN7 to drive a 300B is far from good. I won’t dive into this point at has been discussed at length in many forums.
I’d suggest to try a D3a, E810F, E180F or E280F triode connected which will give you what you need in this position:
1. high gain to drive the 300B
2. Very low distortion at full swing (i.e. about 200Vpp). Primarily H2, with very low odd harmonic contribution
3. Sufficient current drive to drive the 300B grid
4. great sound, detail and transparency.
Try SiC cathode bias or LED bias. Both sound great. I personally prefer SiC.
With little modification you can improve your driver. Make sure your IT matches the driver anode resistance and has good specs for this job, which means you will pay a lot of money for it 🙂
Cheers
Ale
As always: nice work Ale!
I have attached snap shots of the EML 30a loaded by the iso nc20f2
Secondaries open. 415V ak@20ma
Vg1-k : -4,25V
First pic : output is 77,3 Vrms 1 kHz
Second output 77,3Vrms 30 kHz
Third: 100 V rms 1 kHz
For sure one hell of a driver tube
I use it in my GM-70 se.
I have a single spare (4 years old, but little use, If you find the time, and want to check it, I will cover post both ways
Cheers Vegard
Hi Vegard. Great to see other measurements on the EML30a. Surely is a great valve. These EML mid-mu DHTs are intended to swing large volts. Clearly the EML20a for preamp wasn’t at its best and am confident it will perform better like the EML30a in your circuit.
Thanks for sharing your experience and I love your builds!
I wish I had the time for more testing these days. I will take your offer in future when I get some time.
Thank you!
Hi Ale,
I asked the A2 question some years ago and they said it is possible without issues. However the impression I got from their response (and also from what people write after some experience) was that they tend to live less than expected if one demands continuously high peak currents. This is also true for standard class AB1. NOS types suffer a lot less this working conditions. NOS 45’s are some little tanks. Unfortunately the 45B has the same filament as the standard EML type. With the 300B they have solved this issue increasing the filament power and plate dissipation (i.e. 300B XLS and even more with the 320B XLS).
I have found that the new Gold Lion 300B’s are excellent if properly selected (I can tell you where to get proper ferocious selection with an Amplitrex if you want). I rate these one step above the JJ’s and 2 steps above the standard EH. A super-selected pair of these cost just $330 + taxes…..a lot cheaper than 45B’s with similar drive requirements and OPT and more efficient! Some elite current production has gone a bit crazy for my taste….
P
Ciao Paolo. Great insight. I suspected it could be the case. I may adventure in the 45B in the future, but not for now. Too many projects on the list.
I have a pair of EML300B and will be testing it against my other 300B collection when I get around it.
Interesting feedback on the Gold Lion.
We should do the belated curve tracing of the 45s when you want.
Cheers
Ale
Yes. We need to trace them. Unfortunately bringing 12 tubes (8 of those in their original boxes) has proven impossible when travelling with family. There always is something extra to put in the luggage….. but this year with both kids going to school in October I will have “my” holiday… :-))
I am actually trying to find more but it has become very difficult to get 4 identical ones and new. In the meantime KR also have released their copy of the 45 and this looks the closest in appearance to the original. Especially the plate design. Again it’s expensive at 500 euros a pair. Specs are all identical except power rating which is 15W. I don’t know if this is a mistake though. Need to ask.
Cheers,
P
P.S.
The 4 used ones should measure like the one you posted a while ago at about 85%. These can give the same 5W output class A1 power just increasing a bit the anode voltage (330V/30mA with 10K). No problem if those 330V are regulated and one uses self-bias with common plate-to-cathode film cap. Transformer coupling. A little Karna basically….except DC regulated filaments and only mild positive drive.
Hey Ale,
look in your e-mails….
Cheers,
P