Curious_George

In my experience, changing the output transformers will contribute the most difference when comparing two identical amps. Other components or toplogy changes (such as stereo vs mono) will have much less effect on the sound difference.

Plate voltage is 430 - 33 bias voltage, so 397V plate voltage 397*70mA = 27.79 watts Typical EL-34 are rated at 25 watts dissipation. I am using what Eico used based on the schematic and voltage reference measurements listed in the build manual. My goal was to emulate the HF-87, not redesign it (yet...). I have a quad of Svetlana EL-34's I bought probably 20 years ago I am using. They should last for the next 20 years in circuit.

@rmlowzhas a Dennis Had amp. Post at least 5 times and you can private message him.

Cappey, I see your are having fun with that idiot on the cable thread. Ugh, a waste of brain cells replying…

70mA per tube and plate voltage is about 430V.

I do. They are one of the few film & foil caps that are nominally priced and have a long history of use. I still love metallized film, but for the price, it is hard to not use an Orange Drop for signal coupling. "Drops of Jupiter" are priced out of this world.

I may put a drop of super glue on each of the leads where they meet the main body of the cap to seal it.

Here are a few close-ups of the Orange Drops. I was going to use Orange Drops too for the grounded grid half of the phase inverter, but the radial leads did not lend themselves to a nice clean layout, so I used Illinois Capacitor (IC) metallized film (polyester) caps in those locations.

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High voltage, filament, input, driver, output and feedback.

The new Orange Drops I used in the HF-87C are different than ones I’ve seen in the past. The orange epoxy coating seems to be a different composition too. The epoxy coating around the leads is now “machined” clean around the entire lead out area. Previous Orange Drops had the epoxy coating free flowing around the leads. This made bending the leads a challenge as you mentioned because small pieces of the epoxy would break off and not in a clean manner. I’ll get a close up pic of the new Orange Drops with the leads. The new Orange Drops are much easier to work with and they look clean.

I don’t have any reactance loads per say, just resistors. But I have been wanting to build a reactive load for testing. It’s on my project list.

I’ll do a more thorough test regimen. I was so excited to get this amp built and up and running. I’ve been waiting a year to build it.

15.5dB. I'm sure I have headroom to use more, but I am happy with the sound and the distortion figures with 15.5dB.

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I had the wrong cap selected for use in the formula...

f = 1 / (2 pi sqrt[LC] )

I calculated 14Hz. What formula are you using? I have the formula in an Excel spreadsheet. Maybe a factor is off.

Cappey - I ran a 20kHz square wave through the HF-87C (c = clone) and it looks fairly clean, minimal ringing. Looks like the same response as pictured in a lot of the advertisements when they were selling these kits back in the day. I'm still going to dive deeper, but just giving you an update. More listening today. It's hard to tell but, I think this amp definitely sounds better than the Williamson I built. The Williamson clone was the first tube amp I built... nothing can sound better than it!

I can see how UL mode would cause compromised operation near max power. However, there were millions of units made (kits and DIY) with UL as the chosen topology. Even the Marantz 8B was UL. The book "An Approach To Audio Amplifier Design" by G.E.C (England) has a bunch of amps, some fairly high powered. Good reading.

Although (it seems) a lot of people think the "old iron" was better such as Acrosound, Stancor, Partridge, UTC, etc than Edcor or Hammond, I have had good luck with new modern equivalents of those transformers. The main problem with buying those older transformers, is you do not know the history of the device. Certainly the old cloth covering is brittle and will crack as soon as you move it and/or you don't know if the core is OK, meaning if it ever had any heat problems to due a tube failure or oscillations, etc. Going with a new OPT makes life easier, but the compensation network will be different. I'd like to hear an amp with 20% UL taps, I think it might sound a bit better than 40% taps. The 20% tap will move operation a bit more toward "pentode" mode and give you slightly more power too. According to the Philips tube book, 20% UL taps is optimum. Going to 40% makes less distortion but not by much. I doubt it would be audible. I think the 40% taps was made popular by Hafler after he published his work on UL back in the day. With 20 ~ 40% UL taps, the output tube is also more forgiving of its primary impedance and it can vary from about 6k ~ 9k and performance does not change hardly any. The HF-87 circuit can also use 6L6's with a cathode resistor change. I might try it soon, just to hear the difference. Right now, input sensitivity is more than I like, but it is quiet, so I can't complain. It is right on the verge of being at the threshold of too sensitive because even though it is quiet, it is not as quiet as my Williamson clone which requires 1.65VRMS to reach full power. 0.550mVRMS will drive the HF-87 clone to full power and that is using a 6SL7 instead of the original tube, a 12AX7!

I have not looked at the square wave response yet either. I wanted to get it up and running and burn it in to ensure it was working ok. It seems to be fine and now comes the fine tuning. Thanks for all your suggestions, I’ll definitely post results of the network results and further testing. Maybe later this week I can do the Bode plot.

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JJ - How are you doing after your fall? Hope you have recovered.