Working on an Ampeg V3

[TubeGeek]

Someone brought me an Ampeg V3 to sort out.  I have a couple questions to ask.


I found r41 (1k 2W) burned up. The dropping resistor, r41, is connected between the first couple of filter caps.  It made it to the point of burning off the colour bands before the mains fuse (3A) blew.  Even though the resistor is close to dead, it actually measured 1k. Still replaced it of course.



This amp is using a solid state rectifier plugin for the rectifier, instead of a 5AR4.  Clearly too much current was flowing through r41. Now why would that happen?


I notice that Ampeg did not include screen grid resistors.  I am thinking that adding 1k 5W would be fitting for extra protection.  This amp has Winged C 6550's installed.  Would you guys recommend screen grid resistors?


I have replaced the electrolytic's in the circuit.  At the first filter stage I connected two 40µF caps in parallel for 80µF at 525VDC. The schematic calls for two 30µF caps in parallel or 60µF at 600VDC total. Will someone confirm that I am ok in doing this.  Since the amp has a SS rectifier installed, the limit of filtering that a recto tube has is not really a concern.

[John]

I *think* that the 525V rating on your filter caps is barely enough. The schem shows 500v at node A, and at startup that will probably go much higher. I got in the habit of letting a 500 volt analog meter hooked up to my B+ on a little 5 watter I just got done with, and was really surprised that it goes to 380 on startup, before settling in at around 285 after everything gets done warming up.


Just my 2 cents; I know you've fixed more amps than I've ever seen yet! :)

[sluckey]

This amp is using a solid state rectifier plugin for the rectifier, instead of a 5AR4.  Clearly too much current was flowing through r41. Now why would that happen?

The only thing drawing current thru R41 is the screen grids. C26 could have been leaky, but I see you've replaced that. If R41 still gets too hot, look to the 7027 tubes. Screens may have been pulling too much current. Installing screen resistors on each tube is a good idea. Also look at bias.

I'm also concerned that the replacement filter caps are not rated for a higher voltage. Just how much B+ do you have on C25/C28 immediately after you turn the amp on with the standby switch closed? I would feel better with two 100µF/350vdc in series. The extra B+ due to the SS rectifier could very well be at the root of your problems. That 5AR4 has a long warm-up time that will let the power tubes get hot before the B+ rises, and this will put a load on the B+ that may keep the voltage from exceeding the rating of the caps.

Is the amp working now?

[2deaf]

Screens may have been pulling too much current.

Does a 6550 draw more screen current than a 7027?

[TubeGeek]

This amplifier has been serviced by a tech previous to me.  I have corrected at least 4 or 5 things that were done poorly.


I first saw the amp back in May for a cap job.


The amp then came back to me in Sept for installation of 6550's to replace the 6L6's.


The owner has been experimenting with an amp selector switch at home and thinks he may have had some technical issues with his connections.  I am not exactly sure.  He was experimenting with a weber cap WZ34 and then a generic ce dist ss plug.  The weber copper cap fell apart when he pulled it out so it's gone.


I was thinking the same thing about the high voltage on the caps.  The replaced cap was also not original (assuming) but it was rated for 525VDC.  I didn't take the time to analyze the caps before ordering and replacing.  I can install series caps to resolve this concern.


The amp will not remain powered up long enough to measure the voltage.


The 6550's test good on my tube tester, although I don't rely on this 100%, it at least tells me the tubes aren't dead.


I have installed 1k5w screen grids from pin 6 to pin 4 on both power tube sockets.


When I power up I hear a few clicks, sizzles and then immediately the mains fuse blows.


The noises I am hearing are coming from the OT I think. 


The owner of the amp did mention that he was hearing a noise coming from the OT when things went bad.


Now I am suspecting a blown OT.


I removed the power tubes and ss rectifier plug and powered up.  Fuse does not blow.  As soon as I install the ss plug, the fuse blows.  I don't want to blow up one of my 5ar4 tubes until I think this through a bit more.

[sluckey]

Disconnect the OT primary centertap. Does the fuse still blow? Check for low resistance on the B+ rail

And check the ss rectifier. It may be bad. You can temporarily replace it with a couple diodes soldered to the rectifier tube socket.

[TubeGeek]

I am in the middle of replacing the cap can with series caps.  I'll finish this up and continue troubleshooting tomorrow. Thanks for the help here.


I tried another ss plug rectifier and the fuse blew, so both plugs I have are behaving similarly.

[PRR]

It is what it is, and resents tomfoolery.

I assume you have fresh high-voltage first filter cap. (Your symtoms suggest it is shorted.)

Get a GOOD *vacuum* rectifier. Not these silly-state things which are not made for this much current, and also deliver more voltage than this over-volted amp can stand.

6550 is correct. But expensive. Factory installed 7027 as "starter tubes". These served some players for many years. But factory said to use 6550 in heavy use. Since 7027 has become scarce, and decent 6550 are available and robust in this amp, that's what it should be.

[TubeGeek]

Good news!


The amp is working again.


I installed discreet 100µF 450V caps in series in place of the cap can at part A and B on the schematic.


Double, triple checked my work, installed a JJ GZ34….powered on….success.


It's idling nicely now. 


Biased to 43mA per tube which is about 23W, close to 70% max plate dissipation.


Happy so far.  I'm going to monitor and play the amp for a few days now.