Originally posted by vintagekiki
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70s Ampeg V4B Blows Fuses
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Originally posted by ca7922303 View Post( I had a V4 stack in 1875)
I have a bias adapter from eurotubes if that would work just to see what it's doing. 1)
1)
Since you have a bias adapter (for octal tubes) skip Red 1 on schematic.
Bias adapter mount in tube socket V5
Setting bias is continued, according to the procedure described under Red 2 on schematic
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Originally posted by ca7922303 View PostAny information on setting bias on this amp?
Red 1 on schematic
In the cathode tube V5 (pin 8) make a test circuit for testing and matched tubes 6L6GT.
Red 2 on schematic
Replace bias resistor R49 (75k) with resistor 68k 1/2 W
Connect 68k resistor in series with trim pot 50k, so you can regulate bias voltage.
With trim pot (50k) set bias voltage (on schematic point -64V) to max.
Insert one tube 6L6GT into the tube socket V5, and leave for 5 min to tube warm up.
Turn on StandBy, and with 50k trim pot set the bias to value 25mV (200mV DVM range)
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Originally posted by g1 View Post
There is no bias adjustment on this amp. They usually do fine without it.
If you want to, you can check the idle current of the tubes by a few different methods.
One is to cut the ground connection from pin8 on each power tube and install 1 ohm cathode resistors. Then you measure the DC voltage at idle across those resistors.
There is another way that does not require any circuit modifications. You measure the resistance of the output transformer primary sections and record them. Then you measure the idle DC voltage across those sections and calculate the idle current, using the resistance numbers that you recorded..
Checking the idle current of the tubes will only tell you how well matched they are.
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Originally posted by ca7922303 View Post
Any information on setting bias on this amp? Thanks.
If you want to, you can check the idle current of the tubes by a few different methods.
One is to cut the ground connection from pin8 on each power tube and install 1 ohm cathode resistors. Then you measure the DC voltage at idle across those resistors.
There is another way that does not require any circuit modifications. You measure the resistance of the output transformer primary sections and record them. Then you measure the idle DC voltage across those sections and calculate the idle current, using the resistance numbers that you recorded..
Checking the idle current of the tubes will only tell you how well matched they are.
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Originally posted by ca7922303 View PostHow about a 30 uf / 20v, I found a couple laying around?
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Originally posted by g1 View PostSo not just errors on the schematic but on the layout drawings as well. Sure makes troubleshooting a lot more difficult.
Once you get new caps in there (for C106 & C112) as well as the 12DW7, it will be worth it to do one more run through of heater, plate, and cathode voltages for the tubes.
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Originally posted by ca7922303 View PostI'll post those voltages after I get hum balance pot installed, also do you think either one of these film capacitors would work in C106/112 while I wait for them to be deluvered?
C106 / 112 is 6.8uF / 20V electrolytic capacitors
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Originally posted by g1 View PostSo not just errors on the schematic but on the layout drawings as well. Sure makes troubleshooting a lot more difficult.
Once you get new caps in there (for C106 & C112) as well as the 12DW7, it will be worth it to do one more run through of heater, plate, and cathode voltages for the tubes.4 Photos
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So not just errors on the schematic but on the layout drawings as well. Sure makes troubleshooting a lot more difficult.
Once you get new caps in there (for C106 & C112) as well as the 12DW7, it will be worth it to do one more run through of heater, plate, and cathode voltages for the tubes.
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Originally posted by g1 View Post
Back at post #270 I advised you to disconnect C106, which you said you did and it did not make a difference. When you cut the leg in post #271, was it some other cap than C106? Did you reconnect it?
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Originally posted by ca7922303 View Post
R14/15 with C106/112 one leg lifted now read 3.3k in both positions of ultra lo switch.
Originally posted by g1 View Post
Remove C106 and then re-check V1 pin 3 voltage.
Originally posted by ca7922303 View Post
Cut one(1) leg of C106, V1 pin#3(11mv).
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Originally posted by ca7922303 View PostIs it possible that these two(2) caps could be left out of circuit and amp operate correctly?
The amplifier will work quite correctly without caps C106, C112, it will only have a shade less ultra lo.
If you do not have caps 6.8u, you are free to install caps 10u. Pay attention to caps polarity.
Just go ahead boldly. Soon V4B will be playing somewhere on stage.
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