All about electronic valves and hi-fi
Playing around today with the 4P1L chap found a very good operating point where distortion is minimised at maximum anode power disipation:
Interesting to see this valve swinging beautifully at just 0.027% THD….
4П1Л (or 4P1L) is probably one of my favourite valves. It was an unknown device to me until was suggested by some friends in the forum. Many discarded it as being a howling beast in pre-amp stages :). I found that albeit it can be microphonic, this can be controlled to a certain extent, but in my opinion this is a great valve in most of the roles: pre-amp, driver or output stage. Preferably is such a linear valve that can easily match 2A3 and 300B characteristics (when arranged in parallel) at a fraction of their cost. You can get a view of this beauty in the datasheet here.
Looking at the specifications, the key points to highlight are:
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I became aware of this valve when Anatoly (a.k.a. Wavebourn) recommended the 4П1Л directly heated pentode which was used in military transmitters. It is very popular now among Russian audiophiles. Apparently is the Russian equivalent of the WWII era German Wehrmacht RL2 / 4P6 RF oscillator / transmitting amplifier tube. It’s a brilliant valve when triode strapped, better than 2A3 / 300B in terms of linearity. See my post around THD here and will see why 4P1L is at the top of the chart with less than 0.03% THD @ +22.22dBu!
As Anatoly suggested, they are very nice for class A in triode, and give up to 2.5W per valve when driven with up to +12V on control grid. It is easy to parallel them, since they are consistent and very linear: paralleling linear valves you are loosing power on mismatch, i.e. the valve with higher transconductance will draw more, no distortion raise caused by mismatch. 2A3, for example, has a pair of paralleled triodes inside. You can parallel ten of 4P1L matching them (it’s easy), to get 100W dissipation and 25W output.
Many found a sweet spot around Va=235V, Ia=40 mA, Vg=-18V providing Pout= 2.5W on a 5K OT, triode connected. Capacitance between anode and first grid for 4P1L is 0.1 pF. Capacitance between screen grid and control grid is about 1 pF. It has a 10 pF Miller capacitance which is not high value, and for 20 KHz it is slight less than 1MΩ impedance. Any driver with 10 mA idle current will make it happy.
I tested this valve a lot as a DHT preamplifier with great results. Starving filaments and suspending the socket with cord can reduce significantly its microphony to very low levels. I could listen to it perfectly fine whilst my friend Tony still have some issues with a 30sp DHT stage bolt to the aluminium top cover 🙂
I loved the sound of the 4P1L pre-amp. I will build a 4P1L SE in the future, is on my list…
Recently, a friend from the diyaudio forum asked me for the 4P1L curves which I posted previously. Here is a new trace of the curves under the following testing conditions:
You can create your model or use the curves to produce your load lines, etc.
Hope this helps
I was warned, I can’t say I wasn’t. But I think I learned my lesson. Morgan Jones clearly said it on his book: don’t buy on eBay from sellers who claim they don’t have the means of testing the valves. So if they don’t, why are in the business of selling valves?
Mr. William Donzelli (aka toober in eBay), sold me a pair of used 4-65a for the amount of $43.55. I took the risk, I’m not denying that, but after receiving the tubes on the post I did test them accordingly. One was completely flat despite filament being alive, and the other was completely gassy and filaments burned after 20 seconds of use. I do own many 4-65a and can test them accurately on my curve tracer and valve testers. If you don’t have one, you can always do some pretty basic tests anyway with basic lab equipment.
I contacted Mr Donzelli to try solving this out amicably. But his response was harsh and clear: no returns and items sold as-is. What kind of seller are you if you are not able to share the risk and compensate your buyer if you literally sold him a useless product and you got away with the money. Is that fair? Not even accepting the return of the useless product? Offering a similar one? a discount on the next purchase?
Not, that wasn’t offered. claim was raised on eBay and responses from seller were only made to protect himself:
“[…]I have certainly learned my lesson winning auctions the resulted in disappointment – I saw that those wins were my doing, not forced by anyone, and I had to adhere to the rules. So do you. Good buyers do this”. (Mr Donzelli responding to my claim on eBay case)
So Donzelli is telling me to learn the lesson: there are sterling buyers out there that the find a way or covering their back in the best way to screw their customers legally over eBay. What we had in the real world, clearly exists now in the virtual one 🙂
If you don’t want to make this man happy by selling you everything at your own risk, DON’T BUY FROM HIM! – you were also warned, so don’t be as stupid as I was next time 🙂
A simple PMOS gyrator for driver stage anode load
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This is a very simple PMOS based gyrator with only one transistor. You can use a Zetex ZVP0545a which is easy to obtain around here and is not a surface mount type (in case you don’t feel confident in working with SMD components).
Steps to complete the design on this gyrator are as follows:
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Quick test on a EIMAC JAN-8165/4-65a NOS. Here is the THD response at output level of +22.22dBu:
Artefacts at 50Hz and above are all a result of a poor filament power supply 🙂
Nice to see this valve providing 0.14% THD at a low operating point: Ia=85mA, Va=246V. At least this is a nice test to show how linear this valve it is.
As I’m proud of my 26 DHT pre-amp and also looking to use this valve as the first stage of my 4-65a SE amplifier shortly, I looked at how linear this valve is.
Unfortunately I don’t have an extensive set of valves of same brands, so albeit I have about 40 valves if this type there is a big mix of different brands and many of them are used ones. Having said that, I think this may be the case of many of you out there, so I think that probably the results of this test may be relevant to you (if you are still reading this post).
So in summary, this is the sample set I tested:
All DUT were tested with the same test set and operating point:
So in summary the results showed that you should expect a 26 to have around 0.08% (Std Dev = 0.00047). Here is an histogram showing a summary of the tests done:
Looking at brands, the following average THD ranking was produced:
Interesting to find National Union leading the chart. I found RCA and Sylvania to be my preferred ones in terms of sound. THD shown above are average of sample sets of 4 valves or more. Although I tested 12 brands in total, some of them were just a pair so they are not a representative subset.
Finally, here is a sample THD of a very nice 26 valve:
I wish I could have a bigger collection of 26 to improve the accuracy of this statistical analysis. Either way you can get a view of what you should expect from this great valve…
A warm and subtle tone, CX301a…
Superb sound!
When your mind is on some other place (not at work really)
I’m still in the process of testing valves, here is how the ranking is coming up so far. This is a mix of driver and output valves. All tested at Vo=+22.22dBu:
Looking at the chart above a couple of interesting points to highlight:
Have so many other ones to test, but limited time….
Expect this chart to be updated in the future, so stay tuned 🙂
6CB5a, as many TV power pentodes, is a great valve for a single ended amplifier. Specially now that popular valves of similar anode dissipation are getting really expensive.
Thomas Mayer came up with a great design for this superb TV pentode that wired in triode mode is very linear and can sustain at least 25W of anode dissipation. If you want to check Thomas’ design, have a look at his website which is very interesting.
If you want to look at the specifications, you can download the datasheet here.
When testing this valve on the curve tracer it was evident the fantastic features and linearity at every point.
As a power valve, testing a 6CB5a for harmonic distortion showed THD=0.07% @ Vo=+22.22dBu (10Vrms) at the following operating point:
Transconductance in triode is high: 6.4 mA/V @ Ia=40mA, Vg=14.6V, Va=100V. In summary:
Here is a set of triode-strapped curves that you may find very useful if you’re planning to use this valve:
Also you may want to try the following spice model. It can be improved, but I think is an accurate representation of the curves shown above. Please try it and let me know what you think about it…
