Super 60 : why would Fender do that ? ???

[ricothetroll]

Hi,
I recently had to repair a Fender Super 60. Here is the schematic :
http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/fenderamps/super_60_rack.pdf
There's a strange design choice in the PSU : instead of using a simple series resistor to drop the voltage for C, there's a voltage divider (R166/2k7, R168/30k). R168k draws a lot of power (14mA / 417V), and so it gets very hot. Is there any advantage of using such a divider instead of the good old / less power wasting series resistor ? Or is it just a bad design ? It seems like a lot of people have problems with this particular resistor on their Super 60.
Best regards.
Eric

[Paul Marossy]

Good question. The only thing I can think of is that they thought that it would make the power supply more stable.

B+ supply "D" is almost half of B+ supply "C", that also may have something to do with it. I would guess that a series resistor would also get quite hot since it's dropping the voltage quite a lot and that energy gets wasted as heat. This voltage divider arrangement may be the lesser of two evils.

[ricothetroll]

Hi Paul,

Thanx for your answer !

Looking again at the schematic, something else appeared to me : C133/C134 acts like a 50u / 700V capacitor so there's a good safety margin on the voltage, while C135 and C136 are only rated 500V. If the amp is run without any tubes (for troubleshooting reasons for ex), the voltage on C could exceed the 500V rating of those caps. Fender might have just prefered the expense of a gluttonous voltage divider with cheap resistors than 600V or 2x 350V capacitors.

Greedy bastards !

[Paul Marossy]

Whatever the case, you can be 95% sure that whatever they do, they do it for cost savings.

[ricothetroll]

I'm currently figuring out how to get rid of the also notorious problem of low volume on the clean channel, and they did some silly economies here too ! Conclusion : don't buy those Super 60 pieces of cr... 

[R.G.]

Fender might have just prefered the expense of a gluttonous voltage divider with cheap resistors than 600V or 2x 350V capacitors.

And/or not wanting to take the risk, however small, of using a MOSFET follower to only let through the few milliamperes at a lower voltage.

It's pretty easy to knock many watts out of a setup like that where the current drain on the output is low. Of course, then the tubes purists would be offended by there being doped silicon junctions inside the cabinet.

[Johan]

Also remember, if the user fail to use the stand by on powering the unit up, those caps see the full voltage until filaments are heated...multiplying sold units with the number of sloppy consumers could result in expensive warranty claims.it makes sense to spend a few cents on that extra resistor

[Paul Marossy]

Also remember, if the user fail to use the stand by on powering the unit up, those caps see the full voltage until filaments are heated...multiplying sold units with the number of sloppy consumers could result in expensive warranty claims.it makes sense to spend a few cents on that extra resistor

That's a good point.

[GFR]

I played a couple of times with a Super 60 combo (about 20 years ago), and I remember liking it a lot. I only used the clean channel, btw.

I have the service manual in paper, somewhere

The combo had a high/low power switch (full power - 1/4 power), like in the red knob "The Twin".  At 1/4 power the power tubes got 1/2 B+ (and compensated bias) while the preamp tubes got full voltage at both power levels. I don't remember if it had such a resistive divider, maybe the hi/low power switch led them to design it like that and they just kept it the same for all versions of the amp, even for those withou the switch?

[ricothetroll]

Hi!

And/or not wanting to take the risk, however small, of using a MOSFET follower to only let through the few milliamperes at a lower voltage.

I agree, that would be a lot better practice !

I played a couple of times with a Super 60 combo (about 20 years ago), and I remember liking it a lot. I only used the clean channel, btw.

I have the service manual in paper, somewhere Smiley

The combo had a high/low power switch (full power - 1/4 power), like in the red knob "The Twin".  At 1/4 power the power tubes got 1/2 B+ (and compensated bias) while the preamp tubes got full voltage at both power levels. I don't remember if it had such a resistive divider, maybe the hi/low power switch led them to design it like that and they just kept it the same for all versions of the amp, even for those withou the switch?

Mine doesn't have a hi/lo power switch. The clean channel is already soooo low volume that it doesn't really need it btw I'm currently experimenting to get that clean channel volume louder, it seems like increasing the clean volume pot to 1M or higher could do the job. The HRD for example is much better designed for example... (especially because of the volume attenuator located between the first two tubes instead of after the second one as in the Super 60)