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Old 11th June 2008, 09:33 PM   #501
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Default EL84 fixed bias

Ian,

Thanks so much for guilding me along and the amp is running happily now. It sings within an hour and my impression is this amp sounds more solid and punchy with lots of weight. I thought I better let it run for another weeks then to compare with the baby huey. Will report later.

You're exactly right that I put the leds the opposite side, and everything seems working ok after I switch them back.

Anode of 12AX7 127v, 126v
the top BC546 at point B -25.8v
the bottom BC546 point E -29.3v
between 1k8 & 2k2 -31.72
point after ZVN0545 -10.25

I've set the EL84 running at 37mA, and feed back at 22k

Thanks again

Albert
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Old 11th June 2008, 11:47 PM   #502
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Albert,
Glad to help and are happy to hear that its all running.

All the voltages you quote look good. Well - the 12AX7 anode voltages are a little lower than I normally would aim for.
If you want to try finessing this, you will need to drop the CCS current a little. That means adjusting that 1K8 resistor up in value a bit.
You could try 2 x 3K9 in parallel (1K95) or even a 2K2.

This MIGHT make a slight improvement in sound.

Cheers,
Ian
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Old 12th June 2008, 02:24 AM   #503
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Ian,

Will try that for sure.

Thanks

Albert

BTW I only run that for five hour and I surely can tell the difference.
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Old 23rd June 2008, 02:04 AM   #504
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I'm hip deep in my first amp build, and I have a few ultra-newbie questions...

I'm somewhat confused as to the wiring for my O/P transformers.

Specifically, how do I figure out which terminal is the center tap for the primary side?

I would also like to confirm that the terminals marked S and G are the ultralinear taps that I should be using for the baby huey build. (I do not have any docs for the O/P transformers).
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Old 23rd June 2008, 02:08 AM   #505
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"SG" stands for screen grid and are the ultralinear taps.
The primary CT is connection 3 AND 4 ("B" for B+). Connect to both.
Cheers,
Ian
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Old 23rd June 2008, 02:17 AM   #506
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Thanks Ian for the speedy reply... Does "connect to both" mean that I should tie terminals 3 and 4 together for the CT?
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Old 23rd June 2008, 02:57 AM   #507
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Yes - tie terminals 3 and 4 together.
Cheers,
Ian
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Old 23rd June 2008, 03:22 PM   #508
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Hi all,

I just finished to build the EL84 version of Baby Huey.
One channel is working, the other one needs some debugging.
The sound is very impressive as far as I can tell at this stage. So far I only built guitar amps and a K-502 kit, which I modified heavily, so not so much to compare to.
The Baby Huey sounds very detailed with an amazing bass.
I can not wait to have the other channel working (the CCS does not work correctly, I guess I did something nasty to one of the transistors).

I had to include some modifications to the power supply since my power transformer gave too high HT voltage. The power supply looks like this now: UF4007 diodes -> 4.7uF PP film cap -> 1.5H choke (with 0.22uF snubbers) -> 22uF electrolytic -> 100R ->330uF. From there I split up for the two channels as in the original schematic (47R -> 100uF electrolytic paralleled with 4.7uF film caps).

I was curious about the feedback and did some testing there.
The shunt feedback seemed a bit tricky because there is definitely some range of values where the effect of change is not so obvious. I ended up with 18k shunt feedback (I could not decide if it sounded better than 20k) and a variable global feedback. For global feedback I used the 12k (suggested in the original schematic) in series with a 50k stereo pot.
It is very nice to listen to the effect of feedback.
My wife and I both (independently) decided that it sounds better with less feedback. The amp sounds more lively. If you dial in the feedback it is as if someone would put a cloth in front of the speakers. Somehow muted, less "emotional". I have to think of increasing the pot value or omitting it (although I like the idea of variable feedback).
BTW the amp is amazingly quiet, no hum at all (even with my ear to the speaker). There is some very very weak noise (ssssh) when cranked up all the way and no input signal (the amp would blow out the windows with input signal). At normal listening levels there is nothing coming out of the speakers with no input signal.

Once the second channel is working I will post again -maybe with some pictures. This is my first built from scratch and if I compare to the quality of craftsmanship usually seen here on the forum I wonder if this is a good idea. Especially the interior is quite crowded and messy (after a few modifications here and there it looks quite similar to a bowl of pasta).

Thanks to gingertube for this incredible amp and his support on this forum.


Cheerio,
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Old 24th June 2008, 03:39 PM   #509
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Hi everyone,

I could not wait and had to finish the second channel last night.
Now it is much easier to tell the overall sound.
I really like what I hear, but somehow I have the feeling there is still something missing. The amp has a very good bass response, actually there is really a LOT of bass.
But in the high frequencies the amp sounds very correct, a bit emotionless.
There is definitely room to tweak.

My version of Baby Huey runs B+ at ~300V. The idle current of the EL84s is set to ~38mA. I realized that the CCSs for the diff. amp might need to be adjusted because I read ~110V on the anodes of the ECC803.
Seems like the idle current is a bit high there. What I learned from the previous posts this should be between 140-190 Volts.
This is my first "from scratch build" and I am quite a nooby, so please be patient with my questions.
As far as I understand this CCS arrangement, it is the LED forward voltage that is important, not so much the LED current. I am asking this because I picked just two standard 3mm red LEDs from the parts bin. Now I am wondering if this might have been low current versions which would end up with ~2mA instead of the typical ~20mA LED current. The voltage drop should be the same AFAIK.
I measured all resistors in the CCS which are fine (+/- 5% tolerance). Should I just increase the 1k resistor?

This might be just a technical improvement, since the amp is running quite fine at the moment. But I really want to get the most out of the circuit and I have the feeling that my version is not running at the optimum.
Thanks for any input/help.

Martin
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Old 25th June 2008, 09:50 AM   #510
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If the sound is a "bit formal" sounding it could be due to the overall global feedback. The plate to plate feedback doesn't really have this failing.
Try substituting a suitable pot for the global feedback resistor and then varying it and seeing what you like.
The circuit shouldn't really need any global feedback, if the output transformers aren't totally crappy.

Shoog
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