Hey all,
I have acquired an ARC Dual 75 from a local guy here and it sounds OK. Upon taking it home, I checked the bias for the hell of it and noticed it is only 1.5 to 3 ma on each tube. I have swapped other tubes and get only a few ma current...schematic says to bias it up to 50ma each tube.
The guy I bought it from had his tech look it over and said it was ok, but wonder if he really did anything if bias is so low.
All tubes (five 6550, two 12AX7, and four 6FQ7) test out good (two 6FQ7 were 65%) on my Knight 600 tester.
I put the settings in for the 6550 and it pegged out the 100% very quick, so I think the settings on the Knight 600 may not be quite right.
I have also tried other 6550 that I had, KT88 too, and cannot get more than a couple ma each tube.
6550 Plates show the correct 585v, the bias voltage is at -21v...and ideas?
I have acquired an ARC Dual 75 from a local guy here and it sounds OK. Upon taking it home, I checked the bias for the hell of it and noticed it is only 1.5 to 3 ma on each tube. I have swapped other tubes and get only a few ma current...schematic says to bias it up to 50ma each tube.
The guy I bought it from had his tech look it over and said it was ok, but wonder if he really did anything if bias is so low.
All tubes (five 6550, two 12AX7, and four 6FQ7) test out good (two 6FQ7 were 65%) on my Knight 600 tester.
I put the settings in for the 6550 and it pegged out the 100% very quick, so I think the settings on the Knight 600 may not be quite right.
I have also tried other 6550 that I had, KT88 too, and cannot get more than a couple ma each tube.
6550 Plates show the correct 585v, the bias voltage is at -21v...and ideas?
Not sure what 'across the cathode' means. Is it 'across the cathode resistor(s)'? And are there four jacks or one?
A schematic would be helpful... Make sure the test jack does not measure imbalance instead of total current.
Greetings,
Andreas
With RV7, I can move the current a tiny bit, but still within a couple ma, not the suggested 50ma
I have tried with and without load resistor across 0 and 4 ohm tap, does not make a difference...
EDIT: Just noticed the 4 ohm tap is grounded (why?)...maybe put the load across the 4 ohm and 8 ohm taps?
I have tried with and without load resistor across 0 and 4 ohm tap, does not make a difference...
EDIT: Just noticed the 4 ohm tap is grounded (why?)...maybe put the load across the 4 ohm and 8 ohm taps?
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John
You're supposed to measure current not voltage . Set the multimeter to mA range .(300mA is OK) plug the jack , turn bias to 55ma check the other tube and turn DC balance so it's approximately the same on both tubes than turn the bias again to get close to 55mA and so forth with DC balance to the point you have 55mA on both tubes . Than go to other pair.
You're supposed to measure current not voltage . Set the multimeter to mA range .(300mA is OK) plug the jack , turn bias to 55ma check the other tube and turn DC balance so it's approximately the same on both tubes than turn the bias again to get close to 55mA and so forth with DC balance to the point you have 55mA on both tubes . Than go to other pair.
The 585V are the unregulated B+, as I read the schematic. The series regulator supplying the screen grids should put 310-320V to the screens, as shown on the schematic. Are those as they should be?
Greetings,
Andreas
PS. As pointed out by limono, make sure that your way of measuring current is correct.
I checked the resistance from pin 8 cathodes on V16 to 16 ohm tap and its 0 ohms, and V14 to 0 ohm tap and it is also 0 ohms. I plugged in the amp meter into both jacks and got the same continuity )0 ohms), so I think the jacks are OK.
Limono, I ma in Plainfield...I may take you up on your offer...
Limono, I ma in Plainfield...I may take you up on your offer...
John
I doubt it. I had exactly the same thing happening and needed to replace jacks. there are two pairs of pins make and brake.. I cleaned them and it helped for a few plugs but that was it. Once the spring is distorted it's hard to make it work. There is no way the amp can sound Ok with 1ma on output tubes, so actually the tubes operate more or less Ok .
I doubt it. I had exactly the same thing happening and needed to replace jacks. there are two pairs of pins make and brake.. I cleaned them and it helped for a few plugs but that was it. Once the spring is distorted it's hard to make it work. There is no way the amp can sound Ok with 1ma on output tubes, so actually the tubes operate more or less Ok .
OK, I pulled one of he signal tubes (V6 - same as pin 4 of 6550 screen ) and its plate is only at 140v and not 310 to 320 as stated on the schematic...I have tried other regulator tubes and all the same, so it looks like those three ZD5, ZD6 ZD7 diodes?
Limono, all four jacks went bad at the same time?
Limono, all four jacks went bad at the same time?
No, When I got the amp two were already replaced and I could't get any reading on the other two, but the amp was playing more or less OK so I replaced all 4 and all was fine.(amp was serviced by ARC in 94') Low reading on signal tube may indicate that they are alredy weak. If amps is operating more or less OK and you can't measure any current it means that jacks are bad, there is simply no other explanation.(unless you're not measuring it right) Check signal tubes on your tube tester.
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I checked all signal tubes and all look good except one pair 6FQ7 that is only 60%...
You know, I really don't evn know if this signal tubes are in the right sockets...they all have same pinout...I have the 12AX7 at opposite ends of the bunch of 6FQ7...does that sound right? there are no markings on the pcb or schematic.
You know, I really don't evn know if this signal tubes are in the right sockets...they all have same pinout...I have the 12AX7 at opposite ends of the bunch of 6FQ7...does that sound right? there are no markings on the pcb or schematic.
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To check it out just break connection between jack and transformer or cathode and jack , plug your amp meter in and measure the current. There may be other issues with your amp like low screen voltage or so but if amp operate's more or less OK like you stated there must be more than 1mA running trough 6550. It's 12AX7 current , you wouldn't even move your tweeter
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