Albert sent me recently some great pictures and feedback of his 26 pre-amplifier using gyrator as the anode load similar to my design on the 26/01a.
Here are Albert’s notes on his 26 preamp:
“I have my 326 with separated power supply, 5U4G – 8uf PIO – 20H – 50 ASC – 30H – 50 ASC + 18uf pp feeding the Salas’ HV Shunt V2 Regulator, then to 2 ( left and right ) cascoded DN2540 gyrators. Rod’s filament supply coupled with filament bias @ 850mA with anode @135v. I don’t have anything to check the distortion, but this is the most good sound preamp that I have finished to date. It pleases my ears so much that I have the intention to turn it on once I’m in the house. It sounds gorgerous with very good image seperation. Sound stages are wide, deep and absolute quiet. The highs are sparkling smooth with lots of details. Mids are lush and warm with admirable vocals. I’ve found it even better at the lows if compare with 2x 156c. It goes deeper with clear notes.”
It took me probably half an hour to build this brilliant shunt regulator. I waited for a long time until picked up this board and stuffed all components. I will use it in my next generation of DHT preamps.
Output ripple is below 5mV, can’t even measure it given the noise in my workbench. The test gig included my variable power supply (600V) feeding the shunt regulator which was set for 40mA. output load is three 3K3 power resistor clads (50W each).
I adjusted the regulator to provide about 160V, so current is about 16mA. Interesting to seer the harmonic profile to have a higher peak at 200Hz compared to the 100Hz harmonic….
When moved to the UK 6 years ago faced the challenge that I couldn’t move my personal CD collection. So ripped as many as I could on mp3 / m4a / AIFF with the best quality I could. Didn’t take a hi-fi approach on those days as didn’t have the time nor the knowledge. Just wanted to get my huge CD collection with me and had just a few days to address it. Currently I have a collection of about 100GB of digital music. Can’t fit in an iPod or not very interested in using an iPod for playing a part of it. Problem is that my computer is not in the same room where my 45 SE is 🙁
Looking at how to address this problem, Rob (DHTRob) suggested using voyage linux + mpd as a music server. I was considering a Raspberry Pi, but long lead times at the moment pushed me to look at alternatives.
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So here is the first quick build for this digital music station. A tiny Linux server with WLAN and mpd running on it. My collection is all stored now on an SSD 128GB usb disk. The voyage Linux server is powered by a shunt 18V regulator.
What comes next is the USB DAC, which is on its way 🙂
Initially did some tests with fixed bias, normal DC heater supply from my workbench and HT from a passive capacitor multiplier also available in my workbench. Circuit breadboarded has long cables and we shouldn’t expect good 50/100Hz noise levels as have many transformers and things around 🙂
Test 1: fixed bias
Vgk=-6,8V
Ia=6mA, Va= 119V
THD=0.033% @ Vo=1.4Vrms
;
Test 2: fixed bias with Rod Coleman regulator
Vgk=-6,8V
Ia=6.5mA, Va= 119V
THD=0.031% @ Vo=1.4Vrms
Vf=1.4V (minor starvation)
Test 3: Filament bias with Rod Coleman regulator
Vgk=-6,3V
Ia=6.5mA, Va= 119V
THD=0.031% @ Vo=1.4Vrms
Rfilament=5Ω, Vf=1.4V (minor starvation)
As we shown earlier in other tests, filament starvation reduces THD slightly as expected. The OPT performs really well. Probably will look at starving a bit more filaments whilst doing a listening test
Will now proceed to rebuild the 26 DHT preamp with LL1660 with this circuit 🙂
After some tests found that initial design of the power supply had some nasty transient response when HT switch was used. Rod Coleman suggested the addition of snubbers and also end up re-arranging the location of the HT SW at the output of the supply. An UF4007 was also added as a protection for any negative spikes at the output due to leak inductance:
Andy Evans built a pre-amp with the 4P1L and was delighted with the sound of it albeit the 4P1L was running below its optimal operating point: 15mA given the limitations of the 126C interstage transformer.
I went to my workshop to test this configuration and looked at biasing 4P1L with fixed bias and driving it with 1Vrms or more to see what the results were:
So here is the first test at Vg=-4V, Va=74V and Ia=15mA
4P1L test circuit
(all tests were done with the 100k input impedance of the Pete Millett Sound Card interface as the secondary load of the OT. 4P1L had both filaments in parallel and If=600mA)
4P1L test 1
You can see a richer harmonic profile with the OT and distortion is around 0.13% when driven with a 1Vrms providing an expected Vo close to mu (Vo=8Vrms)
The distortion gets very high when output voltage is higher than 9Vrms:
4P1L test 2
Now if we bias the valve at a more convenient operating point:
We get a slight improvement in THD down to 0.11%. However the distortion above 9Vrms is still high:
4P1L test 3
So what if we compare the performance of the OT against the CCS?
4P1L test 5 (CCS)
As we can see from above the distortion is halved. Now if we look at how well this valve could perform if biased in a better operating point, we can see that distortion can be reduce down to 0.03%
4P1L THD 6 (CCS) minimum distortion figure
Minimum distortion from a CCS (or gyrator) doesn’t mean that it will sound better. Clearly the OT doubles the THD of the CCS equivalent circuit. Gain here is nearly same on both as OT is in 1:1. Only way of judging both is to do a listening test….
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:
Recommended anode voltage: 200V with 150V on the screen
Maximum operating voltage 250V anode or screen.
Maximum cathode current: 50mA
Anode dissipation: 7.5W
Screen dissipation: 1.5W
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.
4P1L Siberian DHT Preamp
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…
4P1L Siberian DHT pre-amp
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:
G2 and G3 tied to anode
Filaments are in parallel, so If=650mA powered by DC supply.
4P1L-triode curves
You can create your model or use the curves to produce your load lines, etc.
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 🙂
Here are Albert’s notes on his 26 preamp:
