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Thanks for the update.

I am progressing on the build. I have more or less completed the power supply and am about to start wiring the circuit. Late February is the time of year here where the heat (90+) and humidity make conditions tough for enjoyable soldering so I can only work in the early mornings. Still, I progress slowly and once March arrives, things should be much more pleasant.

ray

Some random thoughts after 5 days with these amps....

Most of you probably want to know how they sound.... Kinda hard to describe that, but let me tell you this: I plan on spending quite a bit of time going forward optimizing this design.

This design, as it sits now, is merely a proof of concept. No effort has been put into choosing the best tubes for the job (yet), so it's quite possible that there are better choices. The EL84 for a driver comes to mind, and there's a whole host of sweep tubes that might be better choices for output tubes. I haven't tried it yet, but the math suggests an EL34 will bias up correctly as an output tube if the cathode bias resistor is changed to 250ohms. Time will tell.

Being direct coupled, the amp can be twitchy to get the values right since any change to the driver stage affects the output stage. Those of you who primarily build existing designs, rather than doing your own design work, would probably be better off building known working variants of this design rather than wandering off on your own.

Roscoe

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I can explain it to you, but I can’t understand it for you.

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Walt

Made some minor updates to the amps yesterday. Not really changes looking for improvement, but changes to make them serve better as development platforms. Replaced the 220ohm cathode bias resistor for the output tube with a three resistor network consisting of three 345ohm resistors and a switch. Now, I can change the cathode bias resistor from 230ohms to 345ohms at the flick of a switch (two 345ohm resistors in series which can be optionally switched in parallel with the third 345ohm resistor). The 230ohm value is good for 6550s at near max dissipation (should also be good for KT88s, KT90s etc), the 345ohm value is good for 6550s at reduced current, or EL34s, 6l6s, 5881s etc at near max dissipation (voltage is a little higher than the 400v spec for 5881s, make your own decision...).

Ray, you making any progress?

Roscoe

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I can explain it to you, but I can’t understand it for you.

We went away for a trip and will get back to it shortly. Thanks for the info.

ray

It's finally in operation after many missteps. The final one was the placement pin of one of the 6550 tubes had broken off and I had inserted the pins into the wrong holes.

So far I have only had a brief listen and I will be off to get my second Astra Zenica jab in a few minutes. I won't be able to tell if the bass is of the 'deep-diving' kind because I bi-amp, but I do notice the treble is not yet as 'high flying' as I expected. The midrange sounds sufficiently 'navel-gazing' so I will keep listening when I return from the jab session.

I've had to increase the gain on the crossover as expected.



The build was complicated by the two huge old Audio Note transformers which I wanted to incorporate. They weigh 22 lb each and I have to lift them separately. I therefore just lay them on top of the rest of the amplifier and connect them via plugs to the rest of the amp. I placed the amp on a trolley so that I can move it around.

In the background, you can see the step-down transformer 240 to 115 for the subwoofer amp and crossover. The new amp, hereby dubbed, 'the Roscoe', uses 240.

Next to the transformer is one of the two bucket subwoofers I built here.

Next, I will finish off the Nelson Pass F6 and then move on to a headphone amp.

By the way, our area in Oz is not in lock-down and we do not have to wear masks. However, we are required to use an app on our phones to check in to any shops, bars or restaurants that we enter for contact tracing.

ray

Which tubes are you using for outputs? I've tried 6550s, two pairs of 5881s, and a pair of EL34s, the 6550s were definitely best in my system....

Roscoe

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I can explain it to you, but I can’t understand it for you.

6550. I don't have 5881 or EL34 tubes.

It's been an interesting process building in a different country. One of the best things is that I can still order from Mouser and Digikey. Provided the total cost of the order is above $50, shipping is free. Delivery takes about 7 days and is often faster than delivery from Sydney or Melbourne. I have no idea why.

So thanks for the schematic. Now there are two of the amps on either side of the world.

ray

At least the world is balanced now

Roscoe

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I can explain it to you, but I can’t understand it for you.

Even if us folks are not.