Ale Moglia

6e6p-dr HP amp – part 3: measurements

As I promised, I took the HP amp back to the workshop for a bit of abuse on the bench. Here are some interesting measurements which correlate with some of my listening impressions so far.

I added a set of 300Ω dummy loads at the output to simulate the HPs. Firstly I tested the optimal circuit which has the Sowter 8665 output transformer in Parafeed mode (4:1) to drive the load. You can get a very clean and flat response across the audio band from 3Hz up to +90kHz. There is a minor hump to be tuned due to the RLC circuit formed by the parafeed circuit. I used a 4.7μF Mundorf EVO oil cap. It’s pretty flat to me, so will leave it there.

The performance of the circuit is amazing. Distortion is very low, up to 0.07% at 100mW (which is VL=5.47Vrms):

6e6p-dr Headphone Amp performance @ 100mW output level

DHT LT power supply PCB tests

I’ve been working on a few PCBs lately with great results. I started with an HT power supply PCB which allows the use of full silicon rectifier bridge, hybrid rectifier (e.g. for damper valves) and any combination of either CRCRC or CLCLC stages with external chokes. Last capacitor is bulky WIMA DC-Link film instead of electrolytic. That worked really well and I’m using it now in a +50/-300V supply for the fixed bias and source follower drivers in the amps.

Teflon Sockets

I’ve got a set of teflon sockets from Jakeband. These are fantastically made to order. Luciano from Jakeband sent me in addition some samples which I will use in the eTracer which I will bring along to ETF.19.

Honestly, these are fine pieces of craftwork. For example, you can measure and let Luciano know the diameter of your 845 (or any other transmitting valves) when ordering these valves so they fit perfectly.

I’ve used these sockets on my projects for years and am very pleased have to say. Of course you pay a premium price as these are hand-made with fine materials. In my opinion, these are worth every cent.

Also I had a pair of RCA sockets, look at them:

They are made on a single piece of tellurium copper. First coating in pure silver 99.99% thickness 18 microns and the second coating Gold 24 K 3 micron thick. Sterling job!

You can contact Jakeband directly to order your sockets. Just use the form I posted years ago here.

6e6p-dr Headphone Amp – part 3

6e6p-dr HP amp in action – cap coupled output

I’ve been running with this HP amplifier for a few weeks now and I have to say I’m delighted with it. I matched a pair of 6e6p-dr on my eTracer and after fiddling with the operating point a bit, I settled for Ia=25mA and Va=180V. Anode voltage will depend on the valve as said before, the Russian valve parameters tend to be all over the place. Anyhow, expect anode voltage to be around 170 to 185V. The beauty of the gyrator PCB is that you can adjust the anode voltage with the trimpot.

6e6p-dr HP amp – part 2

My adventure with the HT SMPS modules continued. Did further testing with higher power modules which worked ok.

Testing the HP amp

The headphone stage (HP) has a 1MHz bandwidth, impressive. Here is the circuit breadboarded after the modification of the 2P29L HP amp:

HP experiments – part 1

Experimenting on headphone amplifiers, not always turn out the way you expected. Well, that’s the nature of experimentation and some smoke may come along the way.

Here is my new take on the headphone amp. Modified the 2P29L to fit the 6e6p-dr valve. I love this IHT one, so wanted to experiment more.

Instead of the classic HT, I used some Chinese 300-450V SMPS modules. I used the 70W version with success before, not these tiny ones.

Individually tested all parts and worked fine. Fired up the SMPS with the hot filaments and the 6e6p-dr running at 20mA each and the SMPS oscillated. They went to 450V instead of the 250V dialled and one of my cap multiplier PCB boards got damaged. Damn!

Back to the drawing board. I will probably stick to the simple HT supply 🙂

6HS5 triode

More than 8 years ago, I played with this valve after Thomas Mayer wrote about it here. It seemed like a good option but despite a few bench tests, I never got around building an amplifier with it.

I’m preparing for ETF.19 and will be bringing my eTracer to the event so I built a few socket adaptors. It was the time for the 12-pin Compactron to come to life.

Through my few Compactron valves, I bumped into the 6HS5 again, so I decided to put it to the mercy of the tracer. I wanted to see the positive grid current behaviour, as haven’t seen curves around with it:

What can we say of the above curves? Well this triode could do 7-8W easily as Thomas says. It has an anode power dissipation of 30-35W. The anode slope is between 7 to 10K so local feedback will be needed. You will need to aim for 600-700V bias at least, probably higher with slightly negative bias. However, you will need a hefty source follower to drive the grid. Look at the grid current. It can be 50mA at 5V and creeps up the higher the grid voltage it goes.

Interesting valve with a 6.3V/1.5A heater, and the high-mu (80-100) can work out as a SPUD amplifier with local feedback. Driver should be able to handle the grid current and low impedance. Not an easy task though.

Headphone Amp Shootout

A few weeks back we managed to meet at Jon’s with a few of the London Audio Circle members. I haven’t seen the bunch for a long time due to family and work travel commitments, however it was a great opportunity I didn’t want to miss this time round.

One of the intended purpose of the meeting , which for obvious reasons these meetings never goes as planned due to the random nature of DIY, was to shootout two headphone (HP) amplifiers over modded HD800 headphones.

Jon’s HP amplifier was a simple stage formed by a C3g (triode-strapped) valve loaded with a nice amorphous-core Valab choke. Output cap-coupled to a TVC to his main system when he uses it as preamp. Otherwise cap couple to HD800s headphones.

Continue reading “Headphone Amp Shootout”

ETF.19 cartoons

I made the following drawings one afternoon last week in which I took a few days off before getting back to the daily grind in full first week of September.

To SPUD or not to SPUD

Why not? Less is more and if well achieved the task, you will end up with an amazing result. To do this you will need a good quality OPT and the valve which can do this task. It will need high mu where possible and linear. There are just a handful of DHT that could do the job (like EML20/30) or you can look for the high gm pentodes (e.g. E55L, E810F, etc.) which are very expensive generally or Russian valves like my beloved 6e5p which is cheap as chips (still).

Depending on the step down ratio of your OPT you will need likely a step up input transformer to drive the 6e5p to full tilt. Here is what I experimented with many years ago. Unfortunately I didn’t take any distortion measurements but it sounded really nice.