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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4-65a SE Amp: Building process (Part 2)

Earth and grounding and input Kiwame resistors

Close look at the 46 driver sockets

front view

filament heat sink and SSHV2 anmeter

filament resistor array and supply connectors

Top view of the breadboard

Further progress today: more cabling done for the SSHV2 boards and A2 current raw supplies for the 46 driver gyrators….

4P1L / 6C6C SE Amplifier Design

Pushing to the limits

We’re constantly obsessed to get the most out of our lives. Not a product of the capitalist world we live in, but a fact of our human nature. Its evolution.

When it comes to sonic power, unfortunately we are not too distance from this thought. We want more Watts. Yes, pure power. My generation back in the 80s got misled by the audio product marketing and their unrealistic metrics (e.g. PMPO) to fudge the real power of a solid state amplifier.

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4-65a SE Amp: 46 filament supply tested

Finished the second channel and tested the filament supply. The filament array which is formed by two paralleled pairs of 20W 10Ω wirewound resistors gets hot as expected. The array temperature is about 110-126°C at an ambient temperature of 24°C. The anode of the 46 gets to 49°C after 20 min of use and the heatsink stays at 42°C whereas the regulator TO-220 transistors are about 45°C. There is about 30W dissipated on each array. Yes I know, a lot of power but the filament bias is hard to beat in terms of sound in my view.

Push-Pull fascination (Part 4)

Continuing with this series of blog posts around the 6C4C push-pull design. As suggested my 45 in my previous blog entry, here is the 4P1L-4P1L version:

The first 4P1L driver stage remains unchanged, as does the output stage. The addition of the 4P1L differential pair with CCS tail and LL1660/pp IT is the main change of this design. The LL1660 is configured in ALT M or 2.25+2.25:2+2. I guess that a different IT could be used instead to get a lower output impedance on the diff pair and improve the performance in A2. The amp has more gain that I need in this configuration as it delivers its maximum power (circa 8W) when input is 1.2V peak.

So how does it performs?

Very low distortion indeed. About 0.06% up to 8W. Mainly odd harmonics dominating the sound of this amp.

Interesting findings…

4-65a SE Amp building process

Some drilling and mounting work done this morning on the 4-65a SE amp breadboard:

Filament bias resistor array

Closer view of the filament bias resistor array

Mounting pilars for the SSHV2 regulators

46 valve sockets

Closer view of the 46 valve sockets

46 sockets, filament bias array and filament and HV supply connectors

During the afternoon, I managed to wire a filament supply for one channel 46 driver. Tested and working ok, now can move to the next one:

Filament regulator with its heat-sink at the back, anode loads and the 46 DHT drivers at the centre.

46 DHT driver under test. Gyrator anode load can be seen at the front

46 DHT driver

Push-Pull fascination (Part 3)

In the quest of refining the 6C4C push-pull design I started not long ago, it was suggested by 45 to try a different driver. The famous E180F russian equivalent with gold grid: 6J9P-E. So I went back at my design and replaced the 4P1L driver with the 6J9P-E. Instead of filament bias, the bias was provided by a pair of red LEDs. The low dynamic impedance of these avoid the use of a bypass capacitor. Setting the operating point with the existing supply is a bit tricky but I found that Va=210V / Ia=14mA provides good swing and minimum distortion. Biasing the valve at around -3.7V keeps the valve away from grid current which will increase the distortion unnecessarily. So here is the circuit:

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4-65a SE Amp layout

Back to work and starting to laying out all part of the 4-65A SE amp.

Push-Pull fascination (Part 2)

Thanks to the great feedback from 45, we found out that I made a mistake in modelling the LL1682 OT in my previous post. In a nutshell, I was getting half of the power, doh!

I should have started from scratch, looking at the push-pull curves and estimating at least the A1 power from a pair of 6C4C in push-pull. So following the B.J. Thomson method plotted the curves in push-pull:

Push-Pull fascination

I love the sweetness of my 45 SE amplifier, but you know what? A great push-pull (PP) amp has a fantastic presence, bass and dynamic response. Whenever I listen to a good PP amp, I get to the conclusion that I need to have different amps ready to be played depending to the music I want to listen to! My last two years have been devoted to what Morgan Jones calls in his book “single-ended madness”. And yes, my 4-65a SE in class A2 is slowly coming to life and when ready so then I will be properly mad.

6C4C amps I listened so far made a great impression both in SE and in PP. Owning all components required, I embarked on refining a full DHT push-pull design and again, cap-less (excluding the power supplies of course).

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