71 Ampeg V4B Questions.

[shiner555]

Hi All!

I'm finally going to try and tackle my 71 V4B that has never worked quite right. It seems to have some kinds of thermal issue where once it heats up, it can make cracking/popping sounds, and sometimes slight motor boating. It also seems to push the speakers much harder when hitting a single note when compared to another amp of its size (DC offset?, not sure if that makes sense, or not) Anyways, I re-capped the PS a few years ago when I first started working on amps and now I'm going back to do a total re-cap. In looking at the schematic and actual part values in the amp itself, it has 6550's in it, I had a couple of questions I hoped some of you may know.

http://www.ampegv4.com/images/schematics/V4B.jpg

1) Does anyone know the proper values of R39, 40, 45, 46?? The schem just says PR1 5W...but the amp has 3.3k(or 3.3Ohm not 100% sure, hard to read on the parts) 3W??
2) The amp has 6550's but R41,42,47,48 have not been changed out, so I plan on putting in 1k 5W's. Maybe this could be the cause of the speaker movement/popping sound? Possible high freq oscillation?
3) Should R37,38,43,44 be changed from the schematic for the 6550's?
4) In going over the pre-amp caps, there is this weird 4 pack cap PEC#6470000 on the bass/vol pots, any ideas on a replacement?
5) Any other ideas on things to check out in the chassis for possible popping/clicking? I'm going to clean all of the pots/tube sockets as well as re-tension them.

Many thanks to taking to time to read this!!

[Diablo]

http://www.ampegv4.com/images/schematics/V4B.jpg

1) Does anyone know the proper values of R39, 40, 45, 46?? The schem just says PR1 5W...but the amp has 3.3k(or 3.3Ohm not 100% sure, hard to read on the parts) 3W??
2) The amp has 6550's but R41,42,47,48 have not been changed out, so I plan on putting in 1k 5W's. Maybe this could be the cause of the speaker movement/popping sound? Possible high freq oscillation?
3) Should R37,38,43,44 be changed from the schematic for the 6550's?
4) In going over the pre-amp caps, there is this weird 4 pack cap PEC#6470000 on the bass/vol pots, any ideas on a replacement?
5) Any other ideas on things to check out in the chassis for possible popping/clicking? I'm going to clean all of the pots/tube sockets as well as re-tension them.

Many thanks to taking to time to read this!!

1) The plate resistors R39, R40, R45, R46 are 3.6 ohm - 5 Watt cement fire proof. Some amps had 10 ohm - 5 Watt according to the Joe Piazza schematic.

2) The screen grid resistors are 470 ohm - 2 watt stock, but you can sub in 1K - 3 to 5 watt.

3) I don't know if you need larger grid resistors for the 6550. I would guess the stock 47K would be adequate.

4) The PEC#6470000 is an encapsulated group of resistors and caps that make up the Baxandall tone circuit. It consists of two resistors 220K and 22K, and four caps .00047, .0047, .001, and .01. I replaced my PEC with the individual components on a little board. The schematic for the Bax circuit is shown in the Ampeg B12XT schematic.

http://www.drtube.com/schematics/ampeg/b12xt.gif

5) When I fixed up my V4, I just took the shotgun approach and replaced nearly every cap and resistor. That got rid of my gremlins.

[Diablo]

Here's a link to the PEC replacement with all the details in the AmpegV4 forum. You may have to register there to see all the pictures.

http://www.ampegv4.com/forum/viewtopic.php?id=301

[Enzo]

And unless it has failed, there is little reason to replace that PEC.


There is no DC offset in a tube amp. DC won;t travel through a transformer.

[Gtr_tech]

Maybe someone had been in there and fooled with the feedback loop? That would make it "floppy" sounding if it was eliminated or decreased a bunch.

[shiner555]

Hi Guys,

Many thanks for the replies!! A couple more questions.

1) Any suggestions for a good source for caps in the volume PCB? I'd like to replace the disk caps, but what I normally use (orange drops) won't fit in the space.
2) Enzo, your totally right....you'd think I'd remember that back from school. DOH!
3) I'll also check the feedback loop. From the looks of it a bunch of stuff has been messed with in this amp from stock....various traces are cut, then jumpered back, parts are tacked in all over the place.

Thanks again!!

[melvin]

I have found popping and such to be caused by broken/cracked leads on the PEC part. It's just a combination of caps encapsulated.

[Diablo]

Hi Guys,

Many thanks for the replies!! A couple more questions.

1) Any suggestions for a good source for caps in the volume PCB? I'd like to replace the disk caps, but what I normally use (orange drops) won't fit in the space.
2) Enzo, your totally right....you'd think I'd remember that back from school. DOH!
3) I'll also check the feedback loop. From the looks of it a bunch of stuff has been messed with in this amp from stock....various traces are cut, then jumpered back, parts are tacked in all over the place.

Thanks again!!

For the caps, just use some tubular metallized film polypropylene. Did you check out the AmpegV4 forum link?

[shiner555]

Diablo,

Just checking it out now, thanks for the link! Looks like the right site for the ampeg stuff.