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Old 25th June 2008, 12:05 PM   #511
dupyv is offline dupyv  Luxembourg
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Join Date: Jan 2007
Hi Bayermar,

I can confirm Shoogs' statement to drop the global feedback completely. The amp is stable without it, and sound will most probably improve.

Other elements that influenced the sound signature during my tests:

- Value of the cross-connect resistor (R between the two anodes of the
ECC803). I used 16k as per schematic, that's where life comes to the
amp. Higher values up to 47k make the amp sound more
transistorlike. The value also depends on the speakers you have.
16k was great for my fullrange horns.
Gingertube commented this resistor in post #418

- Bypass caps on the EL84 CCS. Swapping the electrolytics against
Blackgates made a big difference in sound to me (although i'm against boutique parts in general)

- The tubes themselves.
Russian military EL84M tubes had a very tight deep bass but were
a bit harsh in the mids & highs (-> read transistorlike)
Finally I ended up with old NOS-tubes (Philips ECC83 and Tungsram EL84).
Tube rolling was very beneficial with the Babyhuey, it's very
easy to spot the tubes you like best here. Unlike my earlier amp
(Eico HF86 topology) where changing a russian EL84M against a
Tungsram EL84 was inaudible. The Babyhuey sounds great
with every tube brand but has the potential to further improve with
a top tube.

Kind regards,

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Old 25th June 2008, 03:13 PM   #512
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Location: Vancouver
Default Fixed bias EL84


My fixed bias has been running for almost 2 weeks and I did some comparison with the original baby huey last night. The solid fast and acurate lower end of the fixed bias is obviously better than the original. It's sound stage is deep and wide with a sparkling highs.

I'll wait a bit longer for the blackgate to break-in. Ah........ it might surprise me with a completly different picture. Will report later.

I'll keep the baby huey for my horns and the fixed bias is sitting on my main listening room with my lately finished speakers "PMS"

I can't wait to see your new design - the 300b. Please do let me know if there is anything coming up.

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Old 25th June 2008, 03:40 PM   #513
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Location: Tuebingen
Hi Shoog and Yves,

Thank you for your fast reply.
The "formal" sound might indeed be due to the global feedback. I will try to leave the global feedback out altogether.
I will further test different values for the cross-connect resistor again, now in stereo this might give different results.
My output transformers are Hammond 1608 (keeping the budget low) and I use a matched quartet of JJ EL84 tubes.
I will see, what else I can find and try to roll them around a bit.
I will report back as soon as I can.
I figured from the descriptions in the previous posts that this amp should sound a bit more like a single-ended amp and the "coolness" of the mids/highs surprised me a bit.

Any comments on the low anode voltages of the ECC803?
I realize I should probably give more data here. I will measure the cathode and grid voltages and report back. This might make it easier to judge the situation.

Thanks again,

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Old 26th June 2008, 12:29 AM   #514
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Location: Adelaide South Oz
Guys - I missed a day, 8 hour trial flight of the new Laser Airborne Depth Sounder for the day job.

110V on the anodes is a little lower than I normally run. Increase the 1K emitter resistor in the CCS to 1K2 to decrease the CCS current a little. If you still have less than 140V then you could try a 1K3 (or 2 off 2k7 in parallel to give 1K35 if you don't have that less usual value).

I think the fixed bias version with the source followers is, as you seem to think, always going to be better. There are a number of reasons for this:
1) elimination of cathode bias and direct coupling of the output tube grids will prevent any blocking distortion.
2) lower value Rg1 on the outout tubes ties them down tight and minimises bias drifts etc. - also better drive of any Miller Capacitance.
3) The one which is often overlooked - diffamps work best into equal and CONSTANT loads on the two sides. If driving the output tubes directly the loads change as the output tubes cut off (the Class A to Class B transistion). The Source Follower buffers provide the diffamp with a constant (Class A) load.

BTW I talked a bit about the "balance" pot in the diffamp cathodes not being shown on the fixed bias schematic. Last night I added these pots in (exactly as per the original scheme). Tweaking them to give exactly equal voltages at the diff amp anodes gave a small but noyiceable improvement to the sound. So if you haven't got justment pots in, then I would suggest adding them.

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Old 26th June 2008, 05:59 AM   #515
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Thanks for the suggestion.
I have a 100R pot there for easy adjust the balance since I found none of my 12ax7 performed as the way I'd like them to.

132v on both the anode of 12ax7
37mA on bias of EL84
22K 2W on anode to anode (NFB)- I'd play with the value a little for better performance. It already shows really good square wave form from 20 Hz to 20K Hz on my scope. Ah.......... I'd be happy if I could tweak the band wave wider.

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Old 9th July 2008, 07:36 PM   #516
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Here is my second and successfull tentative to build the
Baby Huey. In the first I tryed to build a compact layout but I had several problems like as heat dissipation, a bit of hum, difficult to mainteance, etc.
In this version I tryed to use almost all best practice
to build amplifier and I used copper enameled wire and
terminal strips to main wiring - thanks a lot to
rwellerson for tips and tricks as the use of the copper
rivets to fix the terminal strips on the epoxy fiberglass sheet.
I followed the original schematic with 12k global
feedback, 18K shunt feedback, 270R screen resistors (HT is in the upper side 350V) and 1000uF + 1uF in bias
The amplifier is running dead quiet with wonderful sound!
Thanks gingertube!
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Old 9th July 2008, 07:37 PM   #517
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Old 9th July 2008, 07:38 PM   #518
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Old 9th July 2008, 07:41 PM   #519
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Wow - beautiful work, Akirasugai!

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Old 9th July 2008, 07:46 PM   #520
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