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Old 14th March 2013, 08:41 AM   #21
RuiR is offline RuiR  Portugal
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Originally Posted by Osvaldo de Banfield View Post
This wiring is true for constant current sources (like LM336) but isn't true for MOSFET's. You will need to rewire your circuit to use P channel MOSFET's as CCS or BJT's. You need to generate a voltage of about 3-4V between gate and source (depending of the polarity of the MOS transistor) to turn it on (Class A biased), and then adjust source resistor to get drain current you need. Power in it will be directly I * E across the mos, then multiply it by the thermal resistance from die to air, and you will get temperature rise in the MOS without the heatsink. If this rise is too high, use a proper heatsink needed to reduce temperature to normal (< 70 deg Celsius) value.

But, why not to use the other half of the 'AT7 as a SRPP?
I don't have enough knowledge to understand what you are say I just learned enough to understand and build this amp.... at least i thought

Each half of the AT7 amplifies a channel... at least i think it does.

My problem is with hardware... when it played it showed ~320V at mosfet imput and right now i have the same voltage as B+ The 12AT7 plate voltage was ~200V and now it's residual

Once again i will do a thorough look .... i hope i willl find something wrong

Thank you Osvaldo
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Old 14th March 2013, 12:15 PM   #22
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Try this MOSFET CCS: as I say, the diode must be tried at between 2 to 4V depending of the MOSFET's VGSth, and then the source resistor selected to give the current you need from it. R2 must be gives say, 1mA to zener diode, and the voltage of VDD-Vz/Iz. Discard gate current. Power in the MOSFET is VDD times Io.
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Old 14th March 2013, 02:13 PM   #23
RuiR is offline RuiR  Portugal
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Originally Posted by Osvaldo de Banfield View Post
Try this MOSFET CCS: as I say, the diode must be tried at between 2 to 4V depending of the MOSFET's VGSth, and then the source resistor selected to give the current you need from it. R2 must be gives say, 1mA to zener diode, and the voltage of VDD-Vz/Iz. Discard gate current. Power in the MOSFET is VDD times Io.
Thank's Osvaldo for the time and help given
I will study it and find diodes.
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Old 14th March 2013, 02:17 PM   #24
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I forgot to say that also you can use a TL431 in place of the zener, and trim the gate voltage for the FET. Observe the P channel MOSFET, it may be IRF9530 or 9560 easy to find.
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Old 14th March 2013, 03:48 PM   #25
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I think it would be much easier just to install the 10M45S. It will "just work".

IXCP10M45S IXYS | IXCP10M45S-ND | DigiKey

If getting them from Digikey is cost prohibitive, perhaps you can find a proxy? Peter Millett was going to sell them for his popular DCPP amp, which also uses them.
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Old 14th March 2013, 04:06 PM   #26
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I think it would be much easier just to install the 10M45S. It will "just work".

IXCP10M45S IXYS | IXCP10M45S-ND | DigiKey

If getting them from Digikey is cost prohibitive, perhaps you can find a proxy? Peter Millett was going to sell them for his popular DCPP amp, which also uses them.
Surely, if the circuit has been designed for it, but the guy has the liberty of try other solution(s) as all of us.
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Old 14th March 2013, 08:03 PM   #27
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I think it would be much easier just to install the 10M45S. It will "just work".
...
I already buy them on the auction site and have them installed.

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Are you using FETs in place of the CCS? If the CCS is doing it's job, the voltage will vary as the CCS maintains a constant current. What is the voltage *across* R13/23 and R14/24?
I don't get readings across these resistors... 0 VDC

B+ Rises until ~470/480 VDC and decrease and stables around 430/440 VDC

Thanks
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Old 14th March 2013, 08:06 PM   #28
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Originally Posted by Osvaldo de Banfield View Post
Surely, if the circuit has been designed for it, but the guy has the liberty of try other solution(s) as all of us.

Your help is apreciated I think the problem lies in a hardware problem and i just don't have the knowledge to diagnosis it

Thank's
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Old 14th March 2013, 08:48 PM   #29
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Quote:
Originally Posted by RuiR View Post
I already buy them on the auction site and have them installed.
OK, so you are no longer trying to use a FET here? Good.

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I don't get readings across these resistors... 0 VDC
Zero volts across all four resistors? That would mean that the CCS and/or tube are not passing any current at all. So let's review:

You originally had DN2540s here, but were experiencing part failures so you switched back to the 10M45S. You have a 330ohm resistor in there now, right? I assume the resistors still measure the correct resistance.

Looking at your other voltage measurements and assuming the resistors and PCB are good, it seems that the 10M45S is an open circuit right now. You have B+ on R14/24 and a few volts on R13/23.

So the question is, what is taking out your FETs/CCSs? Perhaps you have a shorted input tube? I assume that the heat sinks/hardware are not touching your chassis anywhere, right? The tab on the CCS is connected to the "anode" of the CCS, so it will have B+ on it.

What are you using for an input tube? Do you have a spare to try?

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B+ Rises until ~470/480 VDC and decrease and stables around 430/440 VDC
That sounds normal...just the output tubes warming up and conducting.
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Old 14th March 2013, 08:50 PM   #30
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Quote:
Originally Posted by Osvaldo de Banfield View Post
Surely, if the circuit has been designed for it, but the guy has the liberty of try other solution(s) as all of us.
Of course, but we are still trying to diagnose an unknown problem with his amplifier. It is a reliable design, so some other factor is at play. Introducing more complexity at this stage is probably not going to help.
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