Capacitor purpose question

[JonO]

A friends amp blew a cap after new power tubes were installed.
Its a Crate VC5212, cap C57
what is the purpose of the cap?
The pdf is from a Crate VC3112, but the layout is the same
Thanks
Jon

[HotBluePlates]

The heater winding (H1, H2) is first balanced with respect to ground by an artificial center-tap (R60, R61). Then the heater voltage is applied to a bridge rectifier (D15, D16, D17, D18), which then has a d.c. output voltage filtered by C57. Oddly, the output of the bridge is again balanced with respect to ground by R79 and R80.

You might want to figure out if the cap "blew" due to old age, or if there is some kind of short from the heater circuit to something else.

[JonO]

Thanks!
I noticed D15 has a black spot where its soldered to the board.
Will test that as well.
The new tubes got destroyed. Never seen any like that

[HotBluePlates]

There is probably something else going on. New tubes would not cause the heater rectifier circuit to blow up. And a simple failure of the heater circuit shouldn't damage the tubes.

There is something else to be found here that really caused the destruction.

[Tiny_Daddy]

I would replace that bridge rectifier with something larger. 1 amp diodes are not sufficient for charging 10,000uF and powering the heaters. Most amps use a 3 amp bridge which still gets hot.

[JonO]

dusting this topic off lol
i should be able to see the heater voltage on the tube socket pins when the amp is on and no tubes correct?
think I need to retouch all the solder joints
the standby light stays lit until some cap drains
strange things going on

[Willabe]

Would the lamp limiter be useful in troubleshooting?

Look in here (thanks to Sluckey) for instructions on how to build and use a light bulb limiter;

http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf

It is a must have tool for amp repair and building. Easy and inexpensive to build/use.


                          Brad     

[JonO]

edited my last post so it made sense
the power tubes are still el84, the ones that fried were JJ's originals were Sovtek
will build the limiter
still wonder why I dont see heater voltage on all the sockets, only v1 and v2
thanks

[PRR]

> wonder why I dont see heater voltage on all the sockets, only v1 and v2

H1 and H2 are AC.

+H and -H are DC.

You have to change your meter mode.

[JonO]

ahhh, thanks did not notice that
surprising on a buget amp, they put the dc circuit and then use it sparingly

[thelonious]

surprising on a buget amp, they put the dc circuit and then use it sparingly

Kind of surprising that they put in a dc heater circuit at all in a budget amp... but expected that they used it sparingly. It allows them to use lower wattage (i.e., smaller and cheaper) components for rectifying (changing AC to DC), regulating, and smoothing the DC heater voltage. The more tubes they run on the DC heater supply, the bigger and more expensive the DC heater supply components get. Also, from what I've read, it's a good cost-saving and space-saving decision to use DC heaters only for the first couple tubes in the preamp, since you get by far the most benefit from reducing noise in the first couple tubes compared to tubes further down the line, where the signal voltages are usually much higher (and therefore the signal-to-noise ratio is better). And there's comparatively little benefit to running the power tubes on DC heaters, since they draw so much heater current and aren't as sensitive to heater noise as the preamp tubes are.

[PRR]

Careful twisted point-to-point heater wiring can be AC, no big hum.

Un-twisted flat meandering intertwined mixed heater and audio traces on a PCB are sure to hum on AC.

Only a few stages: first stage, volume and tone recovery, reverb recovery. Anything higher-level, the induced hum is small compared to the stronger signal.

The 1A (marginal) diodes and cap are cheaper than hand-wiring proper twisted heater lines.