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Not satisfied to leave well enough alone, I opened up my V4 again to check the bias voltage. I was happy to measure it at -62.5 volts. Close to the -62V listed on the schematic. The problem I found is that Pin 3 and Pin 4 of the 7027A power tubes only measure 492V. I traced back to point A on the circuit board and it also measures 492V. I checked the voltage coming into the amp at 124V. The odd thing is that before I recapped my amp, I checked the voltage at Pin 3 and Pin4 on the power tubes and measured 537V - close to the 540V on the schematic. Anyone have a clue where the voltage went or how to trace down the likely problem? The amp was working just fine after the recap job for about a month. Played fine at loud volumes this morning. As soon as I turned it off standby to do the voltage measurements tonight, it started humming. I have a loud hum with no input and the hum doesn't change with the volume control. The amp works now, but the hum is driving me nuts. It was dead quiet this morning.
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I figured out the problem by chopsticks. One of my solder joints on the power supply filter caps in series was poor. When the connecting wire moved slightly the hum went away and the voltage on the power tubes came back up to 545V. -
The bias voltage is unimportant. What you need to measure is the current the power tubes are drawing. The voltage on the schematic is kind of "in the ballpark" as a service guide line.Comment
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What's the easiest way to measure the current that the power tubes draw? I've read about soldering in a 1 ohm precision resistor between the cathode and ground and then measuring the voltage drop across the resistor. I would have to do that for four tubes. Can I leave the resistors in place forever so I can use them in the future to check the bias, or is it a bad idea to leave them in and run them amp?Comment
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Yes, you can use the one ohm resistors and leave them in place. If you feel ambitious you can install tip jacks through the chassis from the junction of the resistor and tube socket so you can easily connect a meter from the outside. If you also install a bias pot, you can do the whole bias operation without having to remove the chassis.Comment
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Thanks J Martin for the good info. Since you've seen a few V4 amps, what is the typical bias % for the stock bias voltage circuit combined with 7027A tubes? Do they come biased from the factory close to 70% or a lot colder for tube life and drop in replacement? I'm curious about what you've seen and measured.Comment
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I have never measured the original bias setting. By the time they get to me there is usually nothing to measure. People think the original tubes will last forever. So many people are averse to preventative maintanance they just wait for a meltdown. My regular customers learn to be more observant.
Just use the standard calculation to determine bias percentage. In that respect the Ampegs are no different from any other amp. Most of the time the bias current ends up in the low 30's (Ma.)Comment
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Just another bias method,
If you have installed a matched set of power tubes, I feel removing the center-tap of the output xformer & inserting your ammeter is pretty convenient. Less possiblity of breaking the tube pins & such. glenComment
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Folks,
I've got an original 1970 Ampeg V-4 service manual. On page 153 it lists the OT primary DCR as 110 ohms, ACI 3K (while on page 215 of the next section it lists the V-4B as 300 ohms, DCR 6K ACI). So I've got 545 VDC at the OT Pri. CT and 540 VDC at each 7027A anode - there is a 10 ohm, 5W 10% resistor in series with each 7027A socket pin at the OT pri lead. Anyhoo, looks like each side at idle with two tubes in parallel is 5 volts dropped across 5 ohms (2*10 in parallel) in series with 55 ohms (60) yeilding a current between tube tubes of 0.0833 amps of 41.7 ma/tube. Hmmm, not counting screen current, we get a dissipation of 540*0.042 = 22.68 W and 35/22.7 = purdy damned close to 65% (considering it's 5:55 am and I've been up since 2:30). So I ubermajestyrannically declare that the Linden NJ Ampeg biased their V-4s at 65% dissipation.
Which may or may not please your drummer, tube salesman, significant other/companion animal or and other molassas covered marsupial ("goo Roo") in your life.
RobComment
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And for something completely different here is a DIY Bias Probe from http://www.geofex.com/, under the link Schematics and more. I am building a version of this without the DMM since I can use the DMM I have. I am just using a copper tube, banana jacks, one octal plug connected to one octal socket the way the DIY schematic shows. It will look something like Weber's Bias Rite BA-AH. Or you could just buy the BA-AH for $35.00. https://taweber.powweb.com/biasrite/br_page.htmOriginally posted by Mars Amp Repair View PostComment
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IMHO, the OPT DCR method is more convenient than to R&R the CT connection. What's more convenient (for me) is the OPT shunt method, but it's not for Nu-Bs. The safest way is to install current-sensing resistors on the cathodes, then the only HV to deal with is measuring the plate V.Originally posted by Mars Amp Repair View PostComment
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With all of you V4 experts here, let me ask another couple of questions. My amp has a soft hum/buzz through the speakers when the standby switch is open. Am I correct in thinking that there should be silence through the speakers with the amp on standby? How should I trace this problem down? Likely sources?
2nd question - My diodes are all original. I find no values listed for these on the schematics. What should I use to replace them?
Thanks in advance!Comment
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It's your decision, but as far as I know solid state devices don't degrade over time. They are sensitive to voltage transients mostly come from the wall outlet. They can be caused by power appliances switching on or off in the house or the nasty ones come from the power system during thunder storms, etc. These voltage transients can then be "transformed" by the power transformer to even higher voltages on the secondary. That's why they put that line capacitor across the primary (see your schematic) as it sends this relatively high frequency energy to ground. Hopefully!Originally posted by Diablo View Post
So rather than replace the diodes you could simply get a good surge suppressor, plug it into the wall outlet and plug your amps into the surge suppressor.
Electrolytic capacitors on the other hand start to degrade after ten years to my knowledge. The current hum/hiss aside, many people in TGP feel it is good insurance to replace them all even if they sometimes last much longer than 10 years. I just replaced all mine on my 1977 VT40. I figured they are right I was living on borrowed time and it was beginning to hum. I was already in there adding a variable resistor to the bias circuit and changing the screen resistors and I don't really want to pull that chassis again, though it was straight forward to remove, if a little hefty.Comment
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The diodes I'm talking about are the four that make up the rectifier power supply. They convert the AC from the transformer to rectified DC. All of the electrolytic capacitors were recently replaced.Comment
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If the diodes work, why replace them? They don't wear out, go stale, or whatever.
Hum on standby is probably just magnetic coupling between the power and output transformers, it's nothing to be worried about."Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"Comment
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