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Old 1st February 2013, 06:43 AM   #1331
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I've been checking my voltages and the -48v supply is actually at -38v supply. I will need to make a few tweaks tomorrow to increase the negative supply....
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Old 3rd March 2013, 03:02 PM   #1332
Hylle is offline Hylle  Denmark
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Hi, I have alle the parts for building the 6V6/ECC83 with mosfet driving the 6V6. I just need to know, does it have to be a blue LED for the voltage referance in the input diff. circuit?

Where does the name Baby Huey come from?

Also there are many small parts in this amp, have you build PCB for it or how do you handle alle these small parts, glued them to a piece of wood or how do you handle such designs?
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Old 3rd March 2013, 03:16 PM   #1333
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For the small parts I used perforated pcb. Not sure what you call it. Looks like pcb with lots of holes. Most electronics shops have them.

Verstuurd van mijn GT-N7000 met Tapatalk
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Old 3rd March 2013, 10:21 PM   #1334
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Hylle,
Baby Huey was a comic book character,very large and immensely strong (for a baby).
Hence the name. I have run this little amp up against BIG Amps like Aranov LS9100 120W Monoblocks, Jadis JA80 80W Class A Monoblocks etc. and while it won't go as loud as some of those it kicks their butt sound quality wise. As far as sound quantity goes it has the bass authority of an amp with X2 the power.


The Blue LED is the Voltage Reference for the current source in the tail of the diffamp.

The "quality" of the current source (it's output impedance) is a function of the beta of the bottom transistor and the value of the emitter resistor. For a given current that resistor increases in value as you increase the reference voltage at the transistor base., so I maximised that voltage by using a Blue LED. It doesn't have to be Blue, It will work well with Green or Red LEDs but since the reference voltage is then lower the emitter resistor will need to be lower and the current source will not be quite as good. Would you be able to notice the difference? I don't really know but I suspect you may not. An alternative is to not use a LED at all, but use a reverse connected NPN transistor with open base as a "low noise " approximately 6.8 volt zener. (Connect the NPN reference transistor emitter to the current source transistor base, connect the reference transistor collector to the -ve supply where the current source transistor's emitter resistor goes, leave reference transistors base unconnected AND limit the current flowing to say 1mA max so that the reference transistor acts as a zener rather than just blowing up.) That arrangement is shown on one of the posted schematics and it is one I've used several times and found to work well.

Drewaudio,
The "Rule of Thumb" for the negative supply on the source followers is that it needs to be a minimum of about 3 x the bias voltage. This allows the signal to swing up to 0V and down to 3 x the idle bias which will certainly be enough for the tube to go into cut off for normal Class AB operation. For an EL84 idle bias is about -12 V and a -38V rail would be fine. For 6V6 you will probably want a bit more. Measure the bias voltage at idle and then do the X3 calc, or just check with an oscilloscope that you have enough voltage there to run the tube into cut off (zero current). Note that you do not want to be running the source follower into cut off. Mosfets can do nasty things on the edges as they go into and out off cut off.

Cheers,
Ian

Last edited by gingertube; 3rd March 2013 at 10:32 PM.
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Old 3rd March 2013, 11:00 PM   #1335
AJT is offline AJT  Philippines
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Quote:
Originally Posted by Drewaudio View Post

The only problems so far is that the 6v6 bias pots (50k) are fully wound and I can't drop the current below 42ma (they started off at about 100ma at very first power on!!!). The balance pots (100R) are also fully wound to one side and only just come close to balancing....
you can make changes in the bias pot circuit/resistors so that those pots are not fully wound...
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Old 4th March 2013, 06:13 AM   #1336
Hylle is offline Hylle  Denmark
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Thanks Ian. I will try and modify this current source to be a double transistor type instead. Does it matter how much voltage there is across the current source?
As I see it it is most important that the current is correct?
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Old 4th March 2013, 06:21 AM   #1337
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Thanks for the information. I've ditched the 6F6's for the time being as I couldn't adjust for a low enough current through the tubes in order to stay within the dissipation limits. I know I could change some components to achieve a lower range for the bias pots but I just love 6V6's in the amp.

I've also tweaked the bias PSU and have -48v. I'm struggling to understand why the amp cannot go into ab2. Is it because the fets are fed by a CCS that means the 6V6's cannot draw very much current from the grids? Or am I just missing something??

Cheers
Andrew
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Old 4th March 2013, 08:29 PM   #1338
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Hylle,
If you have followed any of the schematics posted then the current source is a "double transistor" type, we engineer types call it a cascode bipolar current source. There is a minimum voltage requirement but the maximum voltage across the current source is limited only by the voltage rating of the transistors. I like to use 2N5551 which have a 160V limit.

Andrew,
I have not actually checked but I would expect that with the source follower drive it is going into Class AB2 operation. Ditto on the 6V6. I'm running some 1950 6V6G from the Sydney AWV factory( the ST "Coke Bottle" shape) in mine with Philips ECG 6SL7.

Away for work at the moment (in Cairns) so I'm missing the sound system.

Cheers,
Ian
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Old 23rd March 2013, 11:16 PM   #1339
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Thanks for the comments. I'm still trying to get my head around why I cannot get into AB2 territory (not that I want to, I just want to understand...). The conclusion I have reached is that the CCS for the fet is limiting grid current in AB2 therefore the tube won't go very far into AB2. Have I got that right??

Please correct me if I'm way off track!!
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Old 1st April 2013, 02:53 PM   #1340
shanti is offline shanti  France
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Hi,

I am another diyer wanting to build this Baby Huey amp. A bit lost among the various updates brought along this thread, could someone direct me towards the final version aka the best sounding version?
- Does the wiki page written by Bas Horneman represents the final version?
- The speakers I want this amp to drive are Fostex Fe207, any experience with those?
- I'd be interested in a mono bloc schematic if it exists.
- Is there an existing list of components? I am staying in France and would like to order the lot to get started soon. (my SS Onkyo amp is dying)

Thank you very much for help and congratulations to those who built very nice unique amplifiers!
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