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: 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

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.

Continue reading “4P1L / 6C6C SE Amplifier Design”

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.

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

The Shunt Cascode Driver

A heavy-weight driver

Rod Coleman came up with a brilliant design recently which baptised as “shunt cascode” driver. For those who cannot stand a pinch of sand in their circuits, I suggest you skip this post now. This hybrid circuit is actually a folded cascode if we consider the book terminology. What makes attractive of this design is its outstanding performance against the classic multistage designs aimed at achieving a large drive signal for output stages such as 300B, 6C4C/2A3, etc. I personally haven’t build it yet but according to Rod the sound is superb.

Before building a stage which will replace my current 45 SE driver, I thought it made sense to analyse the circuit and understand why is claimed to be such a great alternative for today’s designs.

Continue reading “The Shunt Cascode Driver”

6P36S / 6П36С beam tetrode in triode mode

My friend Vyacheslav sent me for testing an 6P36S / 6П36С output beam tetrode which was used for TV horizontal deflection circuits in the old days. Here are the nice curves in triode mode:

Low anode resistance and high current capability with its 12W of anode dissipation make this cheap indirectly heated tetrode an interesting candidate for an amplifier. Let’s have a look at the triode model:

Happy to complete the triode SPICE model if someone can translate the Russian datasheet and provide me with the electrode capacitances.

Well, how will this triode perform in a simple SE configuration? With a low Ra, a 5K anode impedance OT will work well:

With just 55Vpp we can drive this valve to produce 2W @ THD=1.6% (without considering the driver distortion cancellation). The valve can be biased nicely at 55mA and 220V.

I’ve got the (SE) power!

For DHT single-ended (SE) topology, I have to admit that I reached to the conclusion that in my opinion either 6C4C or 4P1L are the way forward in terms of sound and cost after not being happy with the option of running the 45 in A2 mode. Both 4P1L and 6C4C sound lovely in SE despite many will say the 300B is unbeatable. Yes, won’t say a ridiculously thing such as 6C4 or 4P1L are the “best DHTs”. We all know that there are many great DHTs out there, but at a cost. Well, if cost is not a problem for you, you can chose great NOS valves from PX4, 50, 300B to 813 or 845. The latter comes with a hidden price: the power supply. I’ve been there as I’m building the 4-65a SE and most of the budget is used in the power supplies. Sound-wise, we did a side-by-side listening tests on many SE and PSE amplifiers and couldn’t find a significant different between 4P1L PSE and 300B stages. This could easily end up in loosing the tangent and falling into an endless debate about topologies, OT, driver-output stage combinations, bla, bla, bla, but in reality you can’t beat a 4P1L PSE in terms of cost and bias flexibility (i.e. you can easily get 5W from a pair of 4P1L as we will see later). I wish I could achieve the output power I like (i.e. 3W) with a 45. A 45 in push-pull is then very attractive but I haven’t listened (or build) it yet.

I have a very decent stash of both 4P1L and 6C4C, so obviously I will be inclined to get the most out of these ladies rather than continuing burning money on other NOS valves . If you are still reading this is simply because you have (or at least considering buying) 4P1Ls or 6C4Cs and you want to build a good amplifier with them.

So how much power can you get out of the 4P1L? Anatoliy did his own tests and was very pleased with the results in terms of sound. I haven’t run the 4P1L in A2 yet but here it would look like in A2:

You can get clean 5W from a pair of 4P1L running at 50mA (each) and biased at 200V. The driving requirements are only 50Vpp and we can see in the diagram above that the positive grid excursion is to just over 10-12V. Obviously the right driver needs to be used to provide the necessary grid current in A2 and also withstand the changes in grid impedance when transitioning from A1 (high impedance) into A2 (low impedance) with minimum distortion.

I don’t currently have an 2K5Ω OT gapped at 100mA, so won’t be looking at running a 4P1L PSE in A2 like this.

Instead, I have a pair of LL1623/60mA which can provide a varied set of transformation ratios: 5K6Ω, 3KΩ and 1K6Ω.

With this OT I could then easily get 2-3W out from a 6C4C or 4P1L PSE as we shall see looking at the loadlines.

Continue reading “I’ve got the (SE) power!”

4P1L PSE load line

A pair of 4P1L can be easily matched, so 4P1L PSE is a great cost-effective option to deliver +3W single-ended warm sound in A1. Having investigated filament bias, harmonic content, now is time to look at this configuration in a bit more detail.

I have at hand a nice LL1623/60mA which can be configured to 3KΩ:8Ω. After looking at the loadline here is what I think it should play well to deliver 3W:

Va=250V, Ia=60mA, Vgk=-22.9V
The pair of 4P1L will equate to mu=8, gm=12mA/V and Ra=690Ω
Vg= 41.6 Vpp

A driver with some headroom to provide at least 80Vpp should be fine for this SE amplifier. Without looking at harmonic cancellation, this stage should deliver 3W at about THD=1%. Clearly proven that I will not readapt the 45 for A2 🙂

EDIT – 17th March 2013

Just realised after reading Imzen’s comment that the maximum Pa used is incorrect. 4P1L is a 9W device when triode-connected. So here is the correct loadline for a 5K OT:

As we can see, it’s better to run this valve in PSE as you will get just 1W in SE with 1.5% THD when biased at 220V/40mA…

Sweating the 45… (Part4)

Last week I looked at optimising the 45 loadline in A2. Clearly we shouldn’t be attempting to get more than 2W from this valve without a significant level of distortion. However, having about of 3W would be attractive for the transient response of this amp.

So how will this circuit perform in a simulation? Let’s see what the spice results are:

The THD is significantly better due to the harmonic cancellation between the two stages. The driver distortion is 0.3% at full tilt (150vpp) and this could be improved. I guess the 6e5p could do better, but interesting to see how the cancellation of harmonics may play around. The new operating point and the stacked supplies will demand different MOSFET parts of 1kV for sure…