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.
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.
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~
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.
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.
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:
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.
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.
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
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
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.
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-strongly-considering-new-vw-based-entry-level-model.html
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.
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-strongly-considering-new-vw-based-entry-level-model.html
Last edited:
Karmann
As I remember it, and it was many moons ago, it was loosening a clamp nut and trying to find the high point on a cam. That engine did drive a '63 VW for a few years and then a '65 VW Van across country. Nothing like these beauties tho. I liked the guy who said it was hard to put VW and roadster in the same sentence.
Jan
As I remember it, and it was many moons ago, it was loosening a clamp nut and trying to find the high point on a cam. That engine did drive a '63 VW for a few years and then a '65 VW Van across country. Nothing like these beauties tho. I liked the guy who said it was hard to put VW and roadster in the same sentence.
Jan
are you using PD boards or Cviller boards?
PD boards are pots are reversed, counter clockwise to get increase voltage, crank them all the way to right to start, if I remember right.
F5
Playing music through the one channel...building monoblocks...Sounds very good. How stable is stable on the dc offset? It ranges between 0 and 10 mv. I was concerned about my heatsinks 6" X 9" X 2+" but they are never too hot to keep my hand on. Had it on for many hours today. The voltage across r11 is .655v. I brought it a bit higher to see if it would be more stable. The B1 and the one channel F5 was lite so I brought out my adcom preamp. That gave it a little more, so that a mono sinatra recording was very enjoyable.
Did you see the new 6 moons article about Nelson? He's going to write about those baffles of his. I have a set up like that with a fostex 166ers.
Playing music through the one channel...building monoblocks...Sounds very good. How stable is stable on the dc offset? It ranges between 0 and 10 mv. I was concerned about my heatsinks 6" X 9" X 2+" but they are never too hot to keep my hand on. Had it on for many hours today. The voltage across r11 is .655v. I brought it a bit higher to see if it would be more stable. The B1 and the one channel F5 was lite so I brought out my adcom preamp. That gave it a little more, so that a mono sinatra recording was very enjoyable.
Did you see the new 6 moons article about Nelson? He's going to write about those baffles of his. I have a set up like that with a fostex 166ers.
scranton,
0mV to 10mV is good. Can you hear any hum at the speaker? How close do you have to get to hear it?
Are you using only 1 qty of 6" X 9" X 2+ or 2 per channel? If using 2, that's plenty as you are using slightly larger HS than are recommended.
The .655 is a bit high. Recommended is .59v or .60v. This figure will drift, especially if you start playing music thru it and monitoring the bias. Take measurements with nothing at the input and output of the amp. Remember all the parts will need break in time for a while, and the quality of the pots matters too.
Also, if you left out the protection circuit, adjusting the amp will take more time to balancing the DC and bias
0mV to 10mV is good. Can you hear any hum at the speaker? How close do you have to get to hear it?
Are you using only 1 qty of 6" X 9" X 2+ or 2 per channel? If using 2, that's plenty as you are using slightly larger HS than are recommended.
The .655 is a bit high. Recommended is .59v or .60v. This figure will drift, especially if you start playing music thru it and monitoring the bias. Take measurements with nothing at the input and output of the amp. Remember all the parts will need break in time for a while, and the quality of the pots matters too.
Also, if you left out the protection circuit, adjusting the amp will take more time to balancing the DC and bias
where does this come from? It's not a ClassAB until one asks for more than 2.6A of output and that will only happen on transient peaks, if at all.
A ClassA output disiipates most heat at quiescent condition. As output power increases the junction dissipation drops.
PSU power = Pq + Pout.
If the output goes into ClassAB then the dissipation increases again back up to the level of the quiescent dissipation.
Only at very high ClassAB output powers will the device dissipation exceed Pq.
If the temperature compensation is set close to optimum then the bias should not alter significantly as device dissipation drops.
If you have a bit of patience, you don't even need the thermistor -- and the difference between 5K and 4k7 is immaterial.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.