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Old 12th March 2003, 02:09 AM   #41
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Default NOT TO MY WORST ENEMY.

Hi,

Quote:
But please!!!...Not to a friend!!
Morals aside, I would either have it corrected it or not sell it at all, it is going to turn up like the proverbial bad penny anyway.

Somehow I knew that pot should left alone...

Tshhhh,
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Old 12th March 2003, 02:31 AM   #42
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Default Frank...the only interlocutor in town....

Quote:
Morals aside
Moral must have a company that sell this piece of crap...as a High End amplifier...for all over the world...imagine the number of
people that spend their money in a thing like that...

Quote:
I would either have it corrected it or not sell it at all, it is going to turn up like the proverbial bad penny anyway
To spend more time and money in a amp like that seems to me dobius...as if the output transformers are made by the same people....they dont give me any trust....

And with the money of the sell he can chose another project ,even a kit and learn something in the processe...
Quote:
Somehow I knew that pot should left alone...
The more you talk...the more i learn about your expertise in amplifier design....

But now talking to Foe...do what you think is best for you....

Arrivederci!!
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Old 12th March 2003, 03:39 AM   #43
DUG is offline DUG  Canada
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it would seem to me that the pot within the feedback loop would work (although I would not do that) as a gain control.

yes linearity would suffer at low levels as it does not have the feedback to clean it up

with the gain set high enough the maximum gain would be determined by the external loop

review the formula for gain of a feedback amplifier

Vo / Vi = a / ( 1 + a*b)

where Vo is output, Vi input, a is open loop gain and b is feedback (as a fraction)

as a becomes large (high open loop gain) the overall gain aproaches 1/b

as a is lowered the gain aproaches a/1 and the exernal feedback is not significant to aid in linearity and the linearity of the amp is dependant on each stage
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Old 12th March 2003, 04:32 AM   #44
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Default The lost post....

Sorry Foe i miss your post #35...only now i see it...
Quote:
Oh Sorry, i forget de most important R10,R11,R12 are connected to the ground
Yes...if not the amp can't work at all....

Quote:
Why R8 and R9 have different values.
Because R9 is drived by a comum cathode stage and R8 by a ground grid stage...the diference in resistors values is for to compensate this diferences in gain...and reduce distortion.


Quote:
Why R14 is just 1K in my amplifier, due to higher gain of the EL90 ./.EL41 ?
If the R14 that i "think" is the feedback resistor is 1K the overal voltage gain will be 6...only....can't be...
Quote:
Doyou know Synthesis amplifier ??
Yes!
Quote:
What do you have @ home ?
I have Jadis Orchestra heavily transformed even with remote control...and a Luxman SQ38 also heavily transformed...im a danger for the amps...heehehhehe...

For ending...are you sure that the pot at the input as been properly conected?'

Cheers

PS . if your final decision is to try to fix your amp...teel me...we will try!
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Old 12th March 2003, 06:18 AM   #45
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Quote:
Originally posted by bournville
Yes, it may act as a volume control, but it may not have been designed for that function, but to control the level of feedback.

We've agreed that putting a vol control in a feedback loop is bad practice, so looking at the circuit it appears wrong. However, the designer may have wanted to give the listener some degree of control over the level of feedback (for whatever reason) so the pot was placed there for that purpose.

It's not what we would do, but who knows what was in the head of the designer? Incidentaly, it's not the first time I've seen a vol control within a feedback loop!
I haven't actively followed this thread, but pls note that in this amp the feedback is very weak, probably less than 10dB. So, the fb is not really "fighting back", and the volume control will have much more effect than you might think. I think it really is a volume control, but I agree it is quite inelegant. This is Not Done.


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Old 12th March 2003, 06:25 PM   #46
Foes is offline Foes  Belgium
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Smile I will survive !

Hi all,

Even if my amplifier is pure scrap, i have the secret hope to transform it info a true (not hi end but not so bad) amplifier.
Then, I will not sell it to my best frend.
I am now bak at the original shematics, and i can hear 50Hz when the volume is at the maximum.
OK, but how to solve it ? More feedback or less gain in SRPP ? I have no idea !
Anyway, does someone know ao "Tubes for Dumies" book other thas "Audio classroom". The question is how tocalculate the different usual kind of stage ?

Thanks to all and especialy to Jorge.

Gauthier
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Old 13th March 2003, 12:10 AM   #47
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Default The amp that becomes famous...

Quote:
i have the secret hope to transform it info a true (not hi end but not so bad) amplifier.
OK,let's work in it....

First of all....measure the values of the resistors R14 and R3 ...for that you need to take one end of the resistor out of the circuit...

After measure the voltage at the point where R4 conect to C2 and the pin 3 of the tube...

Check this two points ...and let me know!

Cheers!
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Old 13th March 2003, 01:23 PM   #48
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Foes,

No reason why your amp should not give good service - the circuitry from V2 onwards is fairly straightforward and non-controversial.

How loud is the 50hz hum? You say you can hear it at the full setting of the volume control - is it noticable at other levels?

(If you have no use for the EM80 magic eye in the amp, why not remove it and the associated circuitry? It will mean that your power supply will have less demand on it)
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Old 13th March 2003, 05:21 PM   #49
Foes is offline Foes  Belgium
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Hi Jorge,
Why do you want to mesure the resistor, i know the values :
R14=1K 1W
R3=200Ohm 1/8W
I say that because it's difficult to desolder double side PCB's...
Anyway, i will mesure the voltage...

Hi Bournville,
The 50Hz is only noticable at high level, i mean from 3/4 of the max position of the pot @ the original position...
Indeed, i can remove EM80 but if the PS is so underdimentioned... no hope!
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Old 13th March 2003, 06:08 PM   #50
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Default The resistor

Quote:
Why do you want to mesure the resistor, i know the values :
Quote:
R14=1K 1W
Quote:
R3=200Ohm 1/8W
Hi Foe if is this the values...they are wrong ...see the gain of the amp in this case will be 6X .. .

I'm wating for the voltage measurement...

Regards

PS : In the schematic R14 is 5,6k a value that will produce a closed loop voltage gain of 29X...(a more creadible value).
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