Some time ago, I did some initial experiments with the 4P1L (4П1Л) with the screen performing as anode instead. Some DIYers claim the improved sound of the mesh type anodes. Kees Brakenhoff kindly sent me some PL519 to test in screen mode. He has done multiple builds with this mode of operation with great results. Unfortunately I’ve not had the chance yet to build such an amp.
What I could do instead though, was to mod very quickly my 4П1Л preamp to screen mode. It was a very easy and fast modification. I kept the same heating wiring and just adjusted the screen (anode) current down to 10mA:
The 10mA operating point keeps the screen dissipation within its limit. I’d probably use a BF862 here instead of the BSH111BK, but I reused the existing PCB gyrator board for simplicity.
Some measurements
Before listening to the preamp, I conducted a series of basic tests on the preamp:
Due to its lower quiescent current, the transconductance of the jFET is lower and so it’s it bootstrapping resulting in a slightly worse HF response compared to the 4P1L Siberian preamp. Still is outstanding. You also get a slightly better gain of about 1dB close to 20dB, instead of 19dB.
Distortion profile is very good:
It has the characteristic harmonic footprint of the 4P1L with a predominant H2 and cascaded H3 and H4. A very low distortion indeed.
How does it sound?
I played it for about a couple of hours yesterday. It sounds as good as the 4P1L I think. Unfortunately I modded my 4P1L preamp for this so can’t do a side by side comparison.
A valuable experiment. Thanks. I will try it too later.
A question: as pin 2,4 & 8 are connected together seen in your diagram, how come they exhibit three different voltages, being 126V, 118V & 7.6V?
Paul
Hi Paul
The pin 8 acts as a proper cathode. Anode (2) and suppressor grid (4) are held at same potential. Screen (3) becomes the “anode” in this configuration and it’s held at 126V. Given the filament bias configuration, the cathode is elevated at 7.6V so Vak is 118.4V to be exact. The arrow in the diagram indicating 126V should have been shifted slightly to the right to avoid any confusion.
Cheers
Ale
Nice work again Ale,
How about simulating the screen anode with a couple of volts on the real anode rather than tying it to the cathode?
best
tim
http://www.diyaudio.com/forums/tubes-valves/102352-6v6-line-preamp-4.html#post1263939
Good suggestion Tim. I will try to find the time to test this. Probably will use one of the follower boards to feed the anode from a resistor divider. A pot would be ideal to dial the right voltage at the anode and see changes in gain and sound.
I did some initial tests with FFT. It seems like applying 12-12.5V to the anode can reduce the H2 significantly. The actual response is really good. At 10Vrms output you only get 0.01% with H2 at -63dB and H3 and -65dB against 0.03% (H2 @ -53dB and H3 @ -51dB).
Need to apply the changes to the Mule preamp board and listen to it.
It’s quite sensitive to the voltage I found and I suspect there will be a variance between valves so the average impact may be not that optimistic
Cool
tim
Ale, very interesting, but reading between the lines it sounds like it is not a big improvement over the normal mode, correct ? I can imagine that reducing the bias from 30mA to 10mA alone has an effect…
Hi Frank, I haven’t had the chance to listen to it in detail so far. Too much travel and at the moment I’ve hooked the UV201a stage. Reducing the anode current impacts the output impedance and the LF pole indeed. However, I haven’t noticed a negative impact in the overall response. Again, will have to give it some proper time to it. An interesting experiment as you say, but not quite different from what it is the normal anode operation. I will probably apply some positive voltage to the anode as Tim suggested above. That will be nice to play with.
I think also that reducing the current by 66% could have a less demanding affect on the Power Supply & therefore should keep things on a much more stable platform.
Hi Ale Am in the process of having a crack at the Gen4 Siberian and see in this one as well you have the Coleman reg as providing 550mA, while filaments are running at 1.2V. I know the 1.2V is referred to as running the filaments starved, but from the standard 2.1V at 650mA, should this not be 371mA? Sorry for the Newbie question, but making sure I am not missing anything here.
Regards Grant
Hi Grant,
The filaments (in parallel) are starved to get to 550mA. Without starvation you should expect about 2.1V for 650mA +/- 50mA. There is about an 8-10% variance you should expect between different valves. Those are voltages I measured. As the filament is running colder don’t expect a linear difference in voltages. I wouldn’t be too bothered about it in my view, what is important is to set the current right, which is done by the filament regulator. Any variance will be handled by the Rod Coleman regulator (with more or less power dissipated) providing there is enough voltage headroom at the input.
Cheers
Ale
Sorry to ask this but as there are no schematics of Ros’s reg in public: IS there an easy way to measure the actual current though the regs at the regs ?? I am not using filament bias, but a CCS…
Currently I am running mine on 2.05V as this what I can measure easily, but they are VERY microfonic…and Iam already usinf Kevin C.’s decoupled sockets. It is so bad that I am thinking about changing to 10y.
Running them at 1.2V…that is quiet low, I would never considered to go this low as I would have thought that going that low would have given me a 550mA…
Hi Frank
If you’re not using filament bias, then you can measure the current by either placing a current meter in series temporarily ( you only need to do this once) or measure the voltage across R1 (I think is the big wirewound) in the regulator. Alternatively, you can add a 10R 1% series resistor before the regulator and measure the voltage drop. The regulator will consume some mAs, but that’s ok to get you an idea of what the current is.
Whether it’s 1.2 or 1.5V is not the point. The point is the current running through them, not the voltage across them when you’re starving the filaments.
Try other 4P1Ls if yours are very microphonic. If you’re running them at 650mA, they will sing like a lady!!
Try them at 550mA, I’ve been there…many years ago.
Just turn them down to 550mA…measuring across R1 which is 1.5ohm, so easy to read. I think I need to give them a more heavy base than the littlepcb they are on right now…
As I am using four (in differential mode right now): Interesting result: two are at 1.6V at 550mA, two are at 1.8v at 550mA. So one needs to decide wether you want to regulate for voltage or current…
Filament variance is not small. Regulate current. If you have a differential stage without gyrators then is more difficult to balance.
I’d suggest a thick top plate. 4mm aluminium or 1cm hardwood is ideal
Hi, You guys, I found that any noise issues microphonic or otherwise did subside considerably after 200 hours, there is still a little, but most has gone now & inaudible, I am using Ale’s latest mods etc too & starving filaments to 550mA. My PS is also in the same chassis albeit separated by an aluminium partition!
Hi Barry
Glad to hear that someone else is reporting the same. I highlighted this point time ago in other posts and comments but since I’ve been using the same pair of valves for over +300 hours I couldn’t not confirm if there was an expected change or not.
I suspect that the expansion and contraction of warming up and cooling the valve which each cycle may contribute to the mechanical behaviour
Well…even with many hundred hours on them and starved filaments I have quiet a bit of microphony on them. I used a tube socket especially designed by a well known and respected manufacturer for the 4p1l with subchassis etc …still really so bad, that I am currently plan to exvhange this tube with a 10y.
I am leaving this comment as warning for others…
What obviously helps is a step-down opt as this reduces as well the ringing echo.
Last try I will do now on this tube is to build a really heavy socket construction with some silicon…just as Ale did. Plus maybe a little enclosure around the tubes…if that wont help, there will be a large batch of utracer tested and matched 4p1 from 1959 on ebay…
Frank, try a thick top plate like 4 or 5mm aluminium. Also a 5mm-1cm dense hardwood top plate will work better.
One more consideration. Before ditching the 4P1L, try the 2P29L instead. It will be a nearly drop in replacement. It has zero microphonic noise, same loctal socket, similar gain, smaller filament current which can be easily accommodated with your LT supply and the Rod Coleman boards. A great valve, which sounds superb. As good as the 4P1L in my view.
Changing to 2P29L would be far easier than moving to the VT-25/10Y.
Ok, it seems that one trick works: I have got some pearl tube dampers, which are metal and add mass to the thin envelop of the 4p1l. The reduces the microphony to nearly zero…my first impression at least…running them at 1.95V, so much higher than you do, Ale, but still lower than specs….I hope others can benefit from this finding as well…
hello finally after the first construction that did not give me satisfaction I built a dual mono version and I found myself with all the measures in the standard the absorption of the filaments remains constant at 640 millampers and 2.13 volts the anode voltage is rectified at 278 volts and on the R5 at 147 volts and 26 milliamperes the micronocity solved it by not fixing the circuit to the frame leaning on 2 5x5cm sponges. fluffy 1 cm thick. placed under the valve base
I want to ask a few questions:
1. there is a difference in listening if you use 4p1l valves from the 60s with valves from the 70s and 80s, someone tested which ones could be better
2. What happens if I change the 220nf output capacitor to 1microfarad?
3. how can I bring the absorption from 26 to 30 milliamperes
4. why after a few hours of operation a channel begins to produce crypts of intermittent noises and only by switching off and moving the valve does it disappear
Ciao. Apologies, I’m travelling and short of time. You can ask for help in DIYaudio as there is a thread about this. I’d use a 2P29L valve instead. Or buy a few different sets of 4P1L to find the least microphonic ones. If you have intermittent noises is likely due to poor socket connection. Cheers