Twilight Zone experience.... can someone explain??

[rascalseven]

A friend brought by his Ampeg SVT-VR head that he smoked to have burned resistors replaced.  Upon checking out the resistors on the power amp pcb I found that many of the resistors (5 ohm 3W and 47.5k 1/2W) are open -- no reading whatsoever.  No resistance, no apparent connection from lead to lead.  Many, if not most of the resistors on this board are this way.

Even those that have no scorching at all.  They all appear to be of the same make, carbon comp.  Other resistors of clearly different manufacture all read fine, but the green ones (all look the same, even though two different values) have no measurable resistance that my meter can read.  It's as if the meter leads aren't touching anything.

I removed a couple of them completely and measured, and can't get a reading, so it's not adjacent components affecting my meter.

How can several resistors (about 8 of them, IIRC) all go 'open' at once?  I'm guessing this had something to do with the thing burning up?

Has anyone had a similar experience?  I'd LOVE to understand what happened.

Thanks for any light y'all can shed on this!

JC

 

[mad.ax]

In this type of PP amp the current flows through several power tubes (6 in the SVT). There are resistors in series with the plates and grids, so if the current is too high, all those resistors may be affected at once...

Have you got a schematic for the SVT VR? If not you should trace out the difference with the original SVT. The 47K are probably grid resistors, but I see no 5 Ohms. Either 10 Ohms/5W in the plates, or 22 Ohms/ 1W in the screens.
If the plates resistors have been replaced by 5 Ohms/3W, that's twice the current and half the dissipation of the original... May explain...

Axel

 

[hobiesound]

common problem with svt amps. I just repaired 5 classic's and they all had this problem. But not with all of their resistors. If the bias circuit does not work most of the time it's either those resistors or bad tubes. I've got a SVT VR at my shop that "doesn't sound right" but also the bias circuit doesn't work 100% (we bought it as an ex demo model) Haven't opened it up yet but  i have a feeling it's the same problem as you are having. I just replace the resistors with higher wattage higher quality resistors. Doesn't need to be exactly the same ohmage just close enough also make sure to match them for each tube.

in old schematics these resistors are 5 watt in the slm and loud tech these are just 2 watt.

greetings,

Thomas

 

[hobiesound]

oh yeah forgot to say. in the slm amps the resistance was either off of open. in the loud tech versions they were mostly open or still working.

 

[rascalseven]

Interesting and helpful information, gentlemen, thank you!

I'll replace these 2 watters for 5 watts.  It also looks like some of these that are supposed to be 1/2 watt are really 1/4 watt, so I'll probably replace all of these as well just to be thorough.

Thanks again!

JC

 

[kato]

We've all had to recap an amp, but this is the first I've heard of reresistoring an amp.