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Old 15th September 2009, 04:20 PM   #41
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Default F5 reversed Transistors

Hi

Well, I am pleased that I finally got the variac to 118 volts. And nothing smoked this time. I have done in the source resistors before. The DC offset on the output never got higher than 16 millivolts. The voltage across R11 stayed at 0. I didn't have time to do any adjustment. In another post, I saw that someone suggested turning the potentiometers 5-7 full turns and then turning on the amp to start adjusting. Is this a good idea? Do I need to put across the outputs a resistor of 10 ohms? I saw on the gainclone page advice to do that.

I am pleased to say the least. Again thanks for you posts.
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Old 15th September 2009, 04:36 PM   #42
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Quote:
Originally Posted by scranton View Post
Hi

Well, I am pleased that I finally got the variac to 118 volts. And nothing smoked this time. I have done in the source resistors before. The DC offset on the output never got higher than 16 millivolts. The voltage across R11 stayed at 0. I didn't have time to do any adjustment. In another post, I saw that someone suggested turning the potentiometers 5-7 full turns and then turning on the amp to start adjusting. Is this a good idea? Do I need to put across the outputs a resistor of 10 ohms? I saw on the gainclone page advice to do that.

I am pleased to say the least. Again thanks for you posts.
The 5-7 turns is a good suggestion. Remember to do it on both sides. Over R11 if it climbs above .400mv off the bat, I would turn down the variac quickly (assuming a short). It should drift up at that point somewhere between .050v and .1v. Then as manual says set it at 400mv for a few hours, then adjust to to .590 and wait a few hours and back it off, etc as it warms up. Trying to set it somewhere around there, or where it is most stable in bias.

I just run one DVM across R11 and one between the outputs. I would not use a shunt resistor across it. Good luck.
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Old 15th September 2009, 04:37 PM   #43
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Quote:
someone suggested turning the potentiometers 5-7 full turns and then turning on the amp to start adjusting
I think that's just to get you there a bit quicker. I started from zero and it was fine. It's kind of nice to see those number climb!
My output actually started in negative volts.

It will warm up over an hour or so. When cold, adjust it close to optimum and then again after it's hot. You should go back after a few days and check it again, just to be sure.

Only had mine running a few days. Needs more chassis ventilation and bias re-check.

Throw some pics up! Let's see what you have!

V~
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Old 15th September 2009, 04:44 PM   #44
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Quote:
Over R11 if it climbs above .400mv off the bat
I've seen mine drift above .7 and gradually drop back down to normal.
I have 2 CL60 thermistors per channel. Not sure what the deal is there.
Mine is dual mono down to the power cord. Maybe way too much in-rush of current.
Lights dim for a milisecond...on same circuit.
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Old 15th September 2009, 04:47 PM   #45
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Originally Posted by vdi_nenna View Post
I've seen mine drift above .7 and gradually drop back down to normal.
I have 2 CL60 thermistors per channel. Not sure what the deal is there.
Mine is dual mono down to the power cord. Maybe way too much in-rush of current.
Lights dim for a milisecond...on same circuit.
Vince makes a good point. It goes up quickly when set normally, then settiles. I think this is the 4.7k Thermistor dropping value as the FET heats up.
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Old 15th September 2009, 05:15 PM   #46
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Mine also start above .7 cold but come down pretty quickly to .600. Offset stays super low and both channels behave the same way - I am not worried about it.

What I am seeing is that when I blast it for a while, after it starts clunking into AB mode, the bias will settle at around .640v. I have also seen that the transistor temp drops when output exceeds class A level (but dissapation over the .47 ohm resistor goes up(gets hotter?)).

I am not worried about this as my heatsink seems to be sufficient. with 1.26 amps of bias my sinks stay in the high 40C range and the transistor backplates never reach 60C

The only thing I am concerned about is the temperature the .47ohm resistors run at. I am wondering if they are getting to the point of the resistance changing - but I have not measured their temp. Nevermind - I measured them - barely hitting 60c.

If I should worry about any of these things, let me know.

My speakers that I have been testing it with for the last 3 days are Infinity QLS5's which are 4 ohms except for where they are less (gotta love the watkins woogers), and Infinity RSIIIa's which are also 4 ohms except for when they are less.

I have also played with it through some Beta 20's and some ZaphAudio SR-71's that I built. I am prefering the sound through the lower impedance speakers which is strange since they are also inefficient.

Last edited by CBRworm; 15th September 2009 at 05:22 PM.
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Old 15th September 2009, 06:18 PM   #47
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I am prefering the sound through the lower impedance speakers which is strange since they are also inefficient.
I noticed this also. Tested with a set of TEAC 6 Ohm speakers from a mini-system.
Might be just that the amp is providing more power. More of a sense of force.
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Old 16th September 2009, 08:26 PM   #48
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Default F5 Help

Hi

Thanks for following up. These comments are helpful. Going through this process I am suddently understanding...on some minor level...what I am doing here. I am finding another problem. I would describe it as if my output readings and R11 readings are reversed. I get 15mv on the output after the 5-7 turns and nothing across R11. The more I turn both pots very little voltage appears on R11 or R12. However, the output gets as high as 1+ volt.
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Old 16th September 2009, 08:30 PM   #49
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Default COnt.

If the adjustments are small, perhaps I have been too gross in the adjustment so that when I am adjusting R12, I have aleady gone too far. I wasn't thinking to turn the pot the other way. This is obvioiusly the talk of a newbie. I was never any good at adjusting the timing on my old Karmann Ghia either. Thanks

Jan
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Old 16th September 2009, 08:46 PM   #50
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Yeah, it's a real balancing act. You can't get carried away, take it slow.
I had to go beyond the recommended bias setting a few times to help zero in the DC output and sometimes go negative on the DC output to get the bias set. It doesn't help that just by breathing on the amp board from a foot away can make the bias jump up a few hundredths of a volt.

Quote:
the timing on my old Karmann Ghia either
Yeah, what was that? Just a single allen wrench bolt?
You see the new Blue Sport VW diesel roadster convertable coming out in 2013?
I also hear Porsche will have an offering below the Boxster.

http://germanera.com/Articles/27/VW-Roadster.aspx
http://www.leftlanenews.com/porsche-...vel-model.html
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Last edited by vdi_nenna; 16th September 2009 at 08:54 PM.
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