4P1L – LL2746 driver test

After a recent discussion in the DYI Audio forum about the 4P1L drivers, I decided to do some quick tests on an idea I had around to use a step up transformer (1:4) – 4P1L and step up interstage transformer (1:2) to drive a 300B or similar using the 4P1L in filament bias.

First suspicion is on whether the 4P1L has the grunt to drive a capacitive load which would be a real challenge in a 1:2 step up as load capacitance is multiplied by 4 when impedance is reduced by a factor of N^2=4.

I built a test rig with the 4P1L in filament bias using a 15Ω wire-wound filament resistor and connected the filaments in parallel to obtain easily a nice bias voltage with 650mA of filament current. Also lower Rf will improve the low frequency response as helps keeping low the output impedance:

The valve was biased at Ia=30mA / Va=160V and grid bias is about -10.2V. A 10KΩ resistor was added as a primary Zobel as per recommendation of the datasheet. Then it was replaced by a 25kΩ potentiometer (P1) and the right value was found by looking at the frequency response.

Initial tests showed a very good response at 1kHz with only 0.24% THD @200Vpp output. The gain is approximately 16. The mu of the 4P1L with paralleled filaments is around 8 and lower than when used in series which is approximately 9-10. Albeit the results were promising initially, the real test of this stage is by looking at high frequency response where the capacitance will makes it real pain.

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HT Power Supply Noise Measurement

I posted recently a great idea to measure noise levels from our power supplies. Yesterday I managed to put together this small interface circuit. I used a remainder piece of double layer PCB big enough to fit the bulky capacitor, the transformer and the output BNC connector. The input is a just a simple set of copper turrets. Special care is taken in laying out the ground planes to avoid ground loops. Also the transformer is grounded at one side only of the case. A finished interface looks like this:

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HT ripple measurement

Measuring HT supply ripple with your Sound Card

Despite having used the Pete Millett’s SC interface to measure HT supply ripple, it does have a limitation as the input capacitor is rated 400V. Morgan Jones recommends on his latest great “Building Valve Amplifiers” (second edition) three methods for doing this. I opted the one shown above as it does help rejecting common mode noise and also protecting the SC interface. I’m careful enough to not revert the input by accident. If that should happen, the transformer used in this case provides a 500VAC (700VDC) isolation. Normally I play around 400-600V, so should survive in case of misuse. I played with LTSpice to tweak the secondary resistor down to 200Ω when input capacitance is raised to 10μF. Other transformers may allow reducing size of capacitor and increasing resistor up to 10KΩ. I have an OEP E187F at hand so this is what I will use. f-3dB is about 30Hz and provides flat response at 50Hz and 100Hz to measure accurately input ripple.

Will build and report 🙂

814 SE A2 Amplifier

Goodbye 4-65a SE, at least for now

After enjoying the 4-65a SE amplifier for many months, I couldn’t resist myself from upgrading the output stage to the 814s. I just needed changing sockets and filament raw supply transformers to fit the requirements of this lovely transmitting valve. Needless to say, my recent tests on 814s were very encouraging. The 814 seemed to perform much better than the 4-65a in delivering 10W of class A2 sound at half the distortion levels. This to me, was only worth trying.

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814 SE A2 Amplifier (Part 1)

It’s time for the leap of faith. Having tested the 814 in triode mode, I will proceed now to upgrade my 4-65a SE amplifier and replace output valve for the 814. To ensure it can withstand the 540V in the anode, the remaining grids are all tied together through a resistor to the anode. All grids and anode are fitted with ferrite beads as well. A pair of UF4007 in series are placed to protect the Output Transformer in case load is accidentally disconnected.

I added to the UX-5 socket a small bar to place two turrets to provide the anode (top connector), the strapped grid connections through the wire-wound resistor and the pair of UF4007 diodes.

Given that the 814 will run @ 540V / 100mA, I will only need to adjust the Rod Coleman regulators to set current down to 3.25A after replacing the raw filament transformers, as the 814 are 10V instead of 6V filaments of the 4-65a.

Minor DC adjustment will be required on the driver circuit via the gyrator load, so can easily implement this new amplifier.

Stay tuned…

4-65a Triode Curves

Tracing the transmitter valve curves

I posted several times about my 4-65a SE Amplifier and also traced in the past the curves using my analogue curve tracer to get a view of the loadlines of this fantastic DHT in triode mode.

Now that I have the uTracer I traced again the curves including grid current and A2 anode curves which are very handy for this type of transmitter valve.

My tests were conducted with the following heating and bias setup:

DC heater using Rod Coleman regulator @ 6V and 3.5A
Cathode connection to the negative filament terminal of the regulator and valve.

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A collection of latest images

After upgrading my 26 preamp, turntable and adjusted the system to a great sounding state. Looking forward to play many records now…

HT power supplies

Latest System

26 preamp and 4-65a SE Amplifier

DHT filament supplies (and many of them)

26 and bigger brother 46

Reddish 4-65a

26 preamp Gen3

Lenco turntable

Setting up to play in the evening

4-65A SE Amp: first tweaks

As discussed previously, I replaced the gyrator’s polypropylene caps with better ones I had at hand which are 1uF 450Vdc Mundorf Mcap EVO Silver Gold Oil Cap (EVOSGO-080). I should look at lower value. A 220nF or even 100nF one should provide a 1Hz -3dB point with the 4M7 resistor. Anyway, the sound is a tad better in the bass I would say. Nice upgrade but should listen to it more to find what other changes has this capacitor made to the amp:

Replacing the capacitors

Mu-follower gyrator boards finished

4-65a SE Amp: output valve

One of the output 4-65a was actually a military JAN-8165. Wanted to test a pair of them but in the rush of building the amp I ended up with a mixed of the two. Not an orthodox approach buy who cares! After playing the amp extensively for a couple of weeks I noticed that this valve droped anode current after 1hour or more of playing. Anode current could go down by 10-15mA. Perhaps it has to do with the pin oxidation, but I also suspected on the filament regulator due to the heavy current required on this. The regulator heatsink gets really hot as you can imagine so I was already blaming on the regulator FET before I even suspected on this output valve.

A simple test was to replace this valve by a new NOS 4-65a. I did that and surprised to find that the filament regulator wasn’t to blame. The 4-65a SE filament stayed rock solid at 100mA after 1h30m of fantastic music. Yes, the bass of this SE is unique. It exceeded everything I previously listened to. Is the 46 driver in filament bias? Not sure yet, but hey ho. What an amp!

Adjusting the shunt regulator

One of the problems I found when using the 4-65a SE amp was that my version of the Salas SSHV2 drifted significantly with temperature. This was due to the 01N100D characteristics (see extract from datasheet below). We can see that @ -1.5V VGS the temperature dependency is as much as 50mA/100°C. With the smaller heatsink I had before and the significant voltage drop given raw supply voltage levels this caused a problem:

01N100D temperature dependency — IXYS IXTP01N100D datasheet extract

I then reduced the input raw voltage level to 300V to ease the power dissipation across this FET. This was not sufficient so I proceeded to look at various things on the regulator as described below:

Change log

Replaced M1 for DN2540. The DN2540 has a better temperature response. The voltage levels now allowed me safely use the DN2540 and no need for a 01N100D.
Added bigger heatsinks for M1 and M3. Even 2.5W across M1 would need a bigger heatsink if we are looking to run the CCS as high as 90mA.
Removed TP resistor to allow higher CCS current
Set CCS current to 85mA
Load will be now 30mA per channel (30mA each 46 driver valve)
Set output votage to 270V

All tested well in the workbench. I used an external CCS set at 60mA and the output voltage stabilisation was rock solid at 270V. Temperature of the heat-sink was around 40-45°C after 30min of continuous use.

Back in the 4-65a SE Amp, the raw supply level at full load was 297V. The regulator played nicelly at 270V and the CCS current drifted only 5mA from 90mA down to 85mA after 2 hours of continuous play. The temperature of the CCS increased only to 47°C whereas the shunt FET got up to 54°C given the continuous 7W dissipated on it. I may try to reduce the CCS current a tad to ease here.

The regulator is fitted back into the 4-65a SE amp

Played the amp for about 2 hours and slowly started to appreciate more and more the sweet sound of this beast. Bass has increased significantly in my systems and previously haven’t been able to extract the deep bass out of the SE amp through my Fostex FE167E full range drivers.

I’m really going to enjoy this amp for a while until I start looking at changes. Oh yes, many come to mind, but easy for now. Other projects are awaiting for a long time so far.

Ale