If its not water based your good. I was saying that more to the guy who started this thread. Its getting harder to find non water based poly, at least here in the USA. It probably because of environmental laws I imagine.
I just wanted let him know not to use the water based stuff.
I personally like shellac. It was used to varnish transformers for many years. I use it as a wood finish too, nice color.
I wasn't questioning your transformer building skills Ajt. I was just trying to let other less knowledgeable people know there's a difference.
Your correct, nothing beats a good baking when it comes to transformers.
Nick
I just wanted let him know not to use the water based stuff.
I personally like shellac. It was used to varnish transformers for many years. I use it as a wood finish too, nice color.
I wasn't questioning your transformer building skills Ajt. I was just trying to let other less knowledgeable people know there's a difference.
Your correct, nothing beats a good baking when it comes to transformers.
Nick
Thanks for the hints about the varnish. I will make sure that it's not water based.
As for the bias, my problem is that the Voltage drop I measure over the resistor is varying rapidly. It jumps up and down by as much as 5 or 6 mV. (Which effectively is 5 to 6 ma, as the resistor is 1Ohm.)
This happens on all 8 valves, so there must be a common source. (Which isn't the supply Voltages. Those are stable.)
Let me try to attach the schematics to this message.
As for the bias, my problem is that the Voltage drop I measure over the resistor is varying rapidly. It jumps up and down by as much as 5 or 6 mV. (Which effectively is 5 to 6 ma, as the resistor is 1Ohm.)
This happens on all 8 valves, so there must be a common source. (Which isn't the supply Voltages. Those are stable.)
Let me try to attach the schematics to this message.
Attachments
Music playing: yup.
All I did in the mean time, was fastening the power tranny bolts a bit more. (In an attempt to silence the darn thing a bit.)
ARC says that bias fluctuation on old amps is normal.
I worked on a D90 of a friend this weekend and that one was also rock solid.
So I am puzzled. Supplies are nicely stable and the only odd thing I see is that the +310V is a bit high at 325V.
All I did in the mean time, was fastening the power tranny bolts a bit more. (In an attempt to silence the darn thing a bit.)
ARC says that bias fluctuation on old amps is normal.
I worked on a D90 of a friend this weekend and that one was also rock solid.
So I am puzzled. Supplies are nicely stable and the only odd thing I see is that the +310V is a bit high at 325V.
Never power up a tube amp without a load of some kind on the output. Doing it can destroy an output transformer. If there is any signal applied or if the amp has a burst of oscillator the voltage on the output transformer secondary can arc to ground without a load because the voltage will rise much higher then it normally would.
Also when setting the bias how long did you let the amp warm up? How old are the output tubes? Bias issues can be a sign that the tubes are reaching the end of there life. Have you check the bias supplies output voltage for issues?
I had a audio research d115 amp. It used 4 6550's per channel plus one for the screen supply. After it ran for an hour the power transformer would also get quite hot. The fact that there black means they will absorb the ir radiation produced by the tubes and cause them to get hot. The cage on mine would block this. But I don't know how yours fits.
try setting the bias on a tube and then let it settle for a few minutes and see if the current stabilizes.
Nick
Also when setting the bias how long did you let the amp warm up? How old are the output tubes? Bias issues can be a sign that the tubes are reaching the end of there life. Have you check the bias supplies output voltage for issues?
I had a audio research d115 amp. It used 4 6550's per channel plus one for the screen supply. After it ran for an hour the power transformer would also get quite hot. The fact that there black means they will absorb the ir radiation produced by the tubes and cause them to get hot. The cage on mine would block this. But I don't know how yours fits.
try setting the bias on a tube and then let it settle for a few minutes and see if the current stabilizes.
Nick
Hi Nick,
In answer to your questions:
-I know the risk of powering the amp without load. I would never do that.
-When I start my adjustments, the amp is fairly warm (about half an hour after power on)
-The outpot tubes are fairly new EH KT90's most tubes in the front amp are new EH as well. But not the 6FQ7's. They are old stock GE.
-slowly biassing one set of tubes isn't helping. No matter how long you wait, the readout is always a bit unstable.
-The bias supply is a very stable -123.8V. (Should be -125V)
-the 310V line is drifting up a bit and measures 328V when warm.
-The Voltage coming out of V9 and V10 isn't all that stable. Finally, the supply to the tubes (-fourtysomething Volts) is drifting as well.
Hmmm. How could this be caused? Would it be the too high 310V line?
The bias pots are good, though. Stable V coming out of them.
Cheers,
Joost
In answer to your questions:
-I know the risk of powering the amp without load. I would never do that.
-When I start my adjustments, the amp is fairly warm (about half an hour after power on)
-The outpot tubes are fairly new EH KT90's most tubes in the front amp are new EH as well. But not the 6FQ7's. They are old stock GE.
-slowly biassing one set of tubes isn't helping. No matter how long you wait, the readout is always a bit unstable.
-The bias supply is a very stable -123.8V. (Should be -125V)
-the 310V line is drifting up a bit and measures 328V when warm.
-The Voltage coming out of V9 and V10 isn't all that stable. Finally, the supply to the tubes (-fourtysomething Volts) is drifting as well.
Hmmm. How could this be caused? Would it be the too high 310V line?
The bias pots are good, though. Stable V coming out of them.
Cheers,
Joost
Then check your screen voltage at the pens or as close as you can to it. But don't run the amp without the power tubes as it could cause a failure in the screen voltage regulator. Try and tap the screen pins on the bottom of the board or if you can on one of the screen resistors. It might help if you could look at it with a scope to see what's going on if you don't find other issues. I don't know what v9 and v10 are. But if there part of the screen regulator that might be your problem. The screen directly effects plate current as much as grid bias.
I'd give all the power supply a good checking out to make sure your regulators are functioning properly.
If there is something funky going on in the regulator it could be loading the transformer more then it should.
Nick
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I'd give all the power supply a good checking out to make sure your regulators are functioning properly.
If there is something funky going on in the regulator it could be loading the transformer more then it should.
Nick
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Hi Nick,
First of all, thanks for your help
The screen supply is absolutely stable at 409V. Doesn't jump up and down even a 10th of a Volt.
But that's not the case with the negative grid bias. This one is fluctuating a bit.
If youd liek to know more about V9 and V10, check the diagram I've uploaded a few messages ago. It looks like they emit a slightly unstable DC. The negative grid bias is derived from the output of these tubes.
Cheers,
Joost
First of all, thanks for your help
The screen supply is absolutely stable at 409V. Doesn't jump up and down even a 10th of a Volt.
But that's not the case with the negative grid bias. This one is fluctuating a bit.
If youd liek to know more about V9 and V10, check the diagram I've uploaded a few messages ago. It looks like they emit a slightly unstable DC. The negative grid bias is derived from the output of these tubes.
Cheers,
Joost
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the cathode followers are directly feeding the output tube grids...
so that what you do with biasing is adjust the grid bias on the 12at7 drive followers,
what i will do in that case is to replace the bias pots with better quality ones
like the bourns multi turn types and see if that improves the situation...
so that what you do with biasing is adjust the grid bias on the 12at7 drive followers,
what i will do in that case is to replace the bias pots with better quality ones
like the bourns multi turn types and see if that improves the situation...
Just a word to the wise. Changing anything that solder into a pcb on a audio research amp is a royal pain in the butt. I was originally going to put pio caps in mine until I tried de soldering the caps that was in the amp. The copper traces are extremely heavy plus everything is soldered from both sides. I just thought I'd let you know that little detail too. I sold my arc amp because there so hard to work on. And I wasn't using a cheap soldering iron either I use a weller wes50.
Nick
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Nick
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My friend and forum moderator Kevinkr use to service arc amps. He told me about the issue. To be honest I thought how hard could it really be. Boy was I wrong.
I honestly thought about stripping the amp for the iron but I decided against it. In retrospect I should have stripped the iron, lol.
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I honestly thought about stripping the amp for the iron but I decided against it. In retrospect I should have stripped the iron, lol.
Sent from my iPhone using Tapatalk
AR likes complicated circuits in my opinion...
my experience is that even simpler topologies like the mullard 5-20 or a concertina amp can really sound nice....
if that was my amp i will rebuild it to a much simpler amp...
FWIW, i used to repair pc motherboards with bloated caps in the past, what i did was yank out the bloated caps,
i do not recommend this however....
my experience is that even simpler topologies like the mullard 5-20 or a concertina amp can really sound nice....
if that was my amp i will rebuild it to a much simpler amp...
FWIW, i used to repair pc motherboards with bloated caps in the past, what i did was yank out the bloated caps,
i do not recommend this however....
Hey Guys,
I've removed quite a few parts from my amp and it's not a big hassle, provided that you have a good soldering iron and patience. I have to admit that I did work on old IBM mainframe boards for a few years and those can really be considered a very big pain in the ***.
Anyway: I have narrowed down the issue to 2 prossible causes:
1) The 6FQ7's seem to be pretty worn on my unit. I am ordering new ones to see what that will do.
2) The 310V line is bad. That is to say: the Voltage creeps up with rising temperature. It looks like the zeners that are used as a reference, became pretty temperature sensitive after 35 years. I will replace them as well. Hopefully this will solve all issues.
The 325V has a direct relation with the bias, although the 'real' bias supply is -125V.
On a side note: The amp sounds wonderful and (apart form the bias) it measures great as well. I've never had a tubed amp that measured this low in distortion. Most amazing.
Cheers,
Joost
I've removed quite a few parts from my amp and it's not a big hassle, provided that you have a good soldering iron and patience. I have to admit that I did work on old IBM mainframe boards for a few years and those can really be considered a very big pain in the ***.
Anyway: I have narrowed down the issue to 2 prossible causes:
1) The 6FQ7's seem to be pretty worn on my unit. I am ordering new ones to see what that will do.
2) The 310V line is bad. That is to say: the Voltage creeps up with rising temperature. It looks like the zeners that are used as a reference, became pretty temperature sensitive after 35 years. I will replace them as well. Hopefully this will solve all issues.
The 325V has a direct relation with the bias, although the 'real' bias supply is -125V.
On a side note: The amp sounds wonderful and (apart form the bias) it measures great as well. I've never had a tubed amp that measured this low in distortion. Most amazing.
Cheers,
Joost
Installed the new 6FQ7's with no result at all.
I checked the power lines with my scope and... Bingo.
There's a big saw tooth on the 410V line.
The ideas about bad rectifiers might be correct after all.
I just wonder why this riple isn't suppressed by the fairly large bank of (new) capacitors on this line.
Can you guys explain this?
I checked the power lines with my scope and... Bingo.
There's a big saw tooth on the 410V line.
The ideas about bad rectifiers might be correct after all.
I just wonder why this riple isn't suppressed by the fairly large bank of (new) capacitors on this line.
Can you guys explain this?
About two years after, it is time for an update.
I still run the D90 in my main system, with great satisfaction. (Although I wasn't too happy when I found that my power consumption went up with as much as 40% after I started using this power amp.)
After helping out another friend with the same amp, I found that stable bias is not possible on these. So be it.
I did change the rectifier diodes and that improved the sound quite a bit (one of the old ones appeared shot) but the transformer buzz stayed.
Well, that is... Until I got my hands on a PurePower 1050 regenerator. I connected it in the power line and bingo: dead-silent transformer!
I did measure my wall outlet and found 1,5V DC on it. That is not good for any tranny. (core saturation.)
A borrowed Classé Ten dit exactly the opposite: It buzzes on the PP and not on the normal line. Weird...
Cheers to you all,
Jacques
I still run the D90 in my main system, with great satisfaction. (Although I wasn't too happy when I found that my power consumption went up with as much as 40% after I started using this power amp.)
After helping out another friend with the same amp, I found that stable bias is not possible on these. So be it.
I did change the rectifier diodes and that improved the sound quite a bit (one of the old ones appeared shot) but the transformer buzz stayed.
Well, that is... Until I got my hands on a PurePower 1050 regenerator. I connected it in the power line and bingo: dead-silent transformer!
I did measure my wall outlet and found 1,5V DC on it. That is not good for any tranny. (core saturation.)
A borrowed Classé Ten dit exactly the opposite: It buzzes on the PP and not on the normal line. Weird...
Cheers to you all,
Jacques
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