this is a FEEDBACKLESS amplifier, the only(very little) feedback is the one
provided by the emitter resistors, and of course the local feedback of
the valve vas stage...
there s no global feedback that can be used to servobias the functionning
point since the final stage has a gain less than 1 and the valve stage is
connected through a capacitance, making it irrelevant to be used as a
servo bias....
about the bootstrap, it allows a higher output impedance to load the high output impedance of the valve stage...
yes, current source are high impedance, although they can t match
a bootstrap when it comes to providing a high voltage to the
bases of the followers...the mid point of a bootstrap reach
voltages in excess of the supply voltage, in this case with a
+- 50V supply , the bootstrap reach +- 75V.....
it s true that the current source can be kept as they are
in the original..
no,because of limited space,they are not mounted on main heatsink,they mounted on individual heatsink for each transistor,but actually the transistors whose get warm when this amp operate are q8 and q9 (q44 and q56 on my original schematic),i'm using 2N6488 and 2N6491
thank you
thank you very much wahab for the help
,is it an opamp wahab?(at lower left of your schema),the resistors which connect to v+ and v- of opamps from rail supply are about 8k to 10k for dropping voltage,so it will give about 12 to 15volt to this opamp,is it right?once again thanks
Hi bobodioulasso
yes
,two pairs only when i give bias a bit high,two days ago after i replace burnt transistor,i set the bias much lower,below 10ma,with this amp connected to speaker as the load,i playing them for about 2hours,everything is fine,but the sound is not make me happy,maybe i set the bias to lowthank you
Hi, I don't know much about this but the bias is done with q50/51 and 12k resistors. I have seen this type of circuit before somewhere, but if one of them transistors are faulty then you will lose your drive symetry, which may swing the output to 55V. You could check that capacitor accross the collectors of q50/51, is it 47000pf i can't quite tell , but make sure it isn't short. I hope this helps, unless you have already fixed it.
Tony
thank you Tony
the 47000pf capacitor now change to 47uf,the schematic change to this one below at this moment
Tomat,
Larger heatsinks make a better temperature regulation too. (acting onto the bias)
It looks like you must use low or complex impedance loads as you need four output pairs / 55v rails. (2 pairs are enough for a 8 ohm load and 150 watts power)
Your amp, about 300 watts / 4ohm capable needs big heatsinks to work properly.
So, what is your load and how big are your heatsinks?
Larger heatsinks make a better temperature regulation too. (acting onto the bias)
It looks like you must use low or complex impedance loads as you need four output pairs / 55v rails. (2 pairs are enough for a 8 ohm load and 150 watts power)
Your amp, about 300 watts / 4ohm capable needs big heatsinks to work properly.
So, what is your load and how big are your heatsinks?
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Hi bobodioulasso
you are right,when this amp in normal condition,it can produce 300watt at 4ohm,the heatsink are using from old adcom gfa 565 monoblock,it is large enough.
since i'm lowering the bias,the amp is fine,today i'm still running in the amp,i let the amp play music from this morning and will see is it still fine this afternoon? and hope
there is no 
on my amp
thank you very much
you are right,when this amp in normal condition,it can produce 300watt at 4ohm,the heatsink are using from old adcom gfa 565 monoblock,it is large enough.
since i'm lowering the bias,the amp is fine,today i'm still running in the amp,i let the amp play music from this morning and will see is it still fine this afternoon?
and hope there is no on my ampthank you very much
Hi Tomat,seems your amp sounds nice, but its reliability problem prevent the owner away from worrying something.
lets try to solve it.
-Thermal runaway: The easiest way to do with some bigger heatsink and fan. Your amp has three stage that each transistor participate on this problem. Don't let them hot (all 3 stage, not only the final).
-Overflow/storage problem: Toshiba's has deep implant and small chip, that resulting bad base storage time characteristics. This is a relaibility problem when operating on high freq. Smaller Rbe to speed it up, or place 33ohm resistor (or smaller) between driver emitors (need reset the bias).
-Crossover distortion: more bias but higher risk in thermal runaway.
-High voltage loss: helpless, actually not a reliability problem since its heating maintained well.
-Some Offset voltage : no problem.
-Low SRR: no problem.
-No problem
nice amp.
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seems your amp sounds nice, but its reliability problem prevent the owner away from worrying something.
lets try to solve it.
-Thermal runaway: The easiest way to do with some bigger heatsink and fan. Your amp has three stage that each transistor participate on this problem. Don't let them hot (all 3 stage, not only the final).
-Overflow/storage problem: Toshiba's has deep implant and small chip, that resulting bad base storage time characteristics. This is a relaibility problem when operating on high freq. Smaller Rbe to speed it up, or place 33ohm resistor (or smaller) between driver emitors (need reset the bias).
-Crossover distortion: more bias but higher risk in thermal runaway.
-High voltage loss: helpless, actually not a reliability problem since its heating maintained well.
-Some Offset voltage : no problem.
-Low SRR: no problem.
-No problem
nice amp.
Hi too
thanks very much for the advice
,onto aban.. may i know where you live?
Hi too
thanks very much for the advice
Kudos , Valley of the Mount of Muria, Java. I like to live in peace and fresh-air quiet-night place.
To deal with your room problem, you could mount the Vbe mult transistors on top of some already heatsinked device.(that one heating the most, for instance)
I am actually starting to build this buffer with four Sanken outputs 3264/1295, 3263/1294 as drivers and A1837/D4793 as pre-drivers.
Drilling an tapping of one heatsink have been done this morning...
Hi bobodioulasso,
If you can't find any tubes for the input stage, Linear lateral mosfets will work (or use the firstwatt f1 topology that working in small current). The output always directly affecting this stage (through its emitor follower topology), and so this input stage is the quality.
If you can't find any tubes for the input stage, Linear lateral mosfets will work (or use the firstwatt f1 topology that working in small current). The output always directly affecting this stage (through its emitor follower topology), and so this input stage is the quality.
thank you
,i will try to do it, what kind of input stage that you will be use bobodioulasso?
yesterday after i'm run in the amp for almost one day,everything is fine,there is a little problem after i turn off the amp,when i turn the amp on again and let it idle,the bias is fall to only a few ma,i check the vr (i'm using vr for replace R19-12K resistor),the value have change,i think i should replace with better vr or fix resistor.
bobodioulasso..,does your buffer finished yet? ,how is the sound?
bobodioulasso..,does your buffer finished yet?
,how is the sound?- Status
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