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…

Trane’s Psalm hipnotism

I normally will write about the sound of Coltrane’s music in a sweet class A amplifier, but not this time. Of course I was listening to trane’s music in my 4-65a SE amplifier when I got tele-transported back in time. The 4-65a SE output valves are really setled in and the amp is playing at its glorious sonority capability. I’m afraid of touching it as it sounds good.

Anyway, if you haven’t listened “A love Supreme” by Coltrane, then you’re definitely missing a master piece of the jazz music. In fact, I recall playing this record back in the early 90s to a friend who was very fond of 60s rock and some of the 90s electronic music. The monocorde progression of trane’s scales with Elvin Jones and the rest of the Quartet creating the most incredible mood is magnetic, or hypnotic. I’m not looking to review one of the greatest records of all. Simply, Psalm (Part 4) is one of those tunes that gives me goose bumps. Every time I play it, is unique and that is the lovely thing about music. When I played music before, I was really into bebop but then discovered how beautiful was to express with fewer notes and slowly. Is like when I draw cartoons. You can express a lot with fewer traces and just plain indian ink. But a very small number of people have mastered the art of reflecting the most incredible things and experiences through fewer notes.

That Quartet was unique and I’m sure they were in trance when they played Psalm. They were taken to another dimension. We can enjoy their music still, but only God knows what they felt as part of that collective experience.

Anyhow, I’m still sitting here with my Single Malt in one hand and just decided that these lines were enough (hopefully) to reflect what I experience every time I listen to this record.

Cheers

Ale

4-65a SE Amp: testing it finally!

The much-awaited moment finally arrived. After yesterday’s driver tests, I did a lot of work this morning to assemble cables and test the output stage. What I clearly know now is that I won’t be needing any heating this winter! What on earth was on my mind when I decided to build this amp? God only knows…

Here are some pictures of the first tests in the workshop and then when I hooked it up to my system downstairs in the sitting room:

26 preamp and 4-65a SE Amp just before testing

Burning in the 4-65a!

Setting output bias

Measuring output

I did a quick measurement of the output THD without burning in the 4-65a or the amp. The operating point is not optimised but clearly shows a nice picture. First of all, the amp is absolutely quiet. The Rod Coleman regulators plus the extensive filtering on all supplies (LCLC and CLCLC) make this the quietest amp I’ve ever made! The distortion is higher than predicted. With the valves at 100mA/540V and with a non-inductive resistor load of 10Ω, the THD is about 2.7% for nearly 6W of pure class A power. Only 4% of the harmonic content is H3 and with a nice H2 component. The footprint of an SE amp is clearly on this amp.

Hooked it downstairs and after a lot of wiring I finally got to play some good records on this amp. I used my 26 DHT preamp. First record to be played was “a love supreme” (John Coltrane). Here are my impressions so far:

I’m surprised with the bass. It is powerful and not something I was used to in a single ended amp
Definitely needs some burn-in time. The amp improved after 1 hour of use
It’s loud! You can get 10W easily in class A2. Very loud for my room!
The tone is warm and very sweet. you get the sound of the DHT clearly
Dynamics are its forte. This amp responds very well to them

Some more pictures:

4-65a Amp finished

Cherry red anode!

First time up

A power supply madness!

Input transformers and 46 drivers

Now is time for proper listening after so much work. A real accomplishment and I’m feeling very proud. The amp fits within my cabinet so wife is happy 🙂

4-65a SE Amp: first driver test

After completing the last power supply, I finally did some real tests on the 4-65A SE amplifier. Given the DC coupled design it is a bit tricky to do the initial calibration. I had to set the operating point of both 46 DHTs by adjusting the anode voltage through the individual gyrator load presets. Also had to balance at the same time the Salas Shunt current and output voltage to the desired levels. After playing a while with it I managed to stabilise the Salas shunt regulator.

Set the 46 to drive the output stage to 200Vpp with a 3.7Vpp (1.33Vrms) input. That is a gain of approximately 54. Here is the distortion profile:

Breadboard is really quiet with the 50 and 100Hz noise below -95dB. It’s great to see the nice 46 Super Silvertone performing only 0.09% at 200V peak to peak!

Results are promising, just need final tweaks to 600V supply and then hook the 4-65a!

Ale

4-65A SE Amp: A2 grid current supply

Finished today the penultimate power supply of the 4-65a SE amplifier. This one is the A2 grid current one and doesn’t need a lot of filtering as the 46 driver gyrator will do most of that job. The DC stacked design of this amplifier will allow an independent loop of the A2 current between this supply and 4-65a grid-cathode.

Just only one more supply to go and I’m done!!!

40uF and 8uF oil caps

Mains transformer and bridge recitfier

Main switch and bridge rectifier

Choke and output connector

4-65a SE Amp: Fitting the HV regulators

A bit of further progress today as managed to build two Salas Shunt HV regulators (SSSHV2) with the slight tweaks I tried recently. Both regulators will provide a very stable voltage reference (+280V) for stacked supplies. Now time for completing the wiring of the 46 drivers and do some further testing…

Salas SSHV2 Regulators

4-65a SE Amp: Building the OTs

More work done this afternoon. Built the LL9202/100mA OTs in their supporting structure and fitted the crowbar protection boards for the primary windings. Also added the speaker posts. A heavy part of the amp!

LL9202/100mA with crowbars

Crowbar protection for primary winding

Crowbar fitted on the back of the OTs

Module completed

Speaker binding posts

Crowbars and LL9202/100mA

Crowbar protection

Protecting your valuable output transformer

When using fixed-bias you don’t want to take any risks and damage your output transformer. What happens if your output valve (e.g. a transmitting power valve) makes false contact at the grid or the bias supply fails to start for whatever reason. Well, your valve will conduct fully and probably melting the anode and if not damaging the primary winding of your OT. Both things are catastrophic and you don’t want to try it…too expensive!

You may think there is a simple solution: a fuse. We all know fuses are inaccurate and can be dangerous if they don’t blow properly. Also HT fuses are expensive and they don’t come up with a wide range of values.

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