SF 135Watt: Dropping the B+ voltage, and bias mod

[FFM]

HI,

What is the safe and simple mod for dropping the B+ of a 135 watt UL SF twin?

I am getting over 500 Volts on both the plate and screen grids.

Also, to add a bias pot of the output matching pot: can one just replace the resistorcoming off the pot with a 100K pot?

Thanks,

Sekt88

[pdf64]

The best (but expensive) option is to replace the mains transformer with a BF spec type. The power levels in the B+ are such that trying to reduce the B+ with the existing transformer aren't realistic, or at least aren't simple.
To give your tubes a better chance of longevity, increase the screen grid resistors to 2k2 5 watt.

[Enzo]

Are you taking these readings with the tubes removed? The only readings that have any meaning are taken when the tubes are in the amp and running.

[FFM]

tubes in.

[FFM]

what about using a weber v-dump/25 watt zener diode to drop 50 v from the b+?

[pdf64]

When it's cranked, there will be well over 0.5A drawn from the B+, so that zener's going to get overheated and pop.
For good reliability, don't run resistors and semiconductors at more than half their rated dissipation.
Your chassis would run hotter, thereby decreasing the life of other components, especially the electrolytics.
I guess you could fit a string of 4 x 12V zeners, but by the time you've got them all mounted properly, might as well have replaced the power transformer.
Re the bias mod, a 100k pot would work, wouldn't be the best option. Poor resolution, and the the bias could inadvertently be turned damagingly low, or even off.
Better to use a 22k resistor in series with a 47k pot.

[ThermionicScott]

How much over 500 volts are we talking? 6L6GCs are rated for 500V plate, 450V screen, and this wouldn't be the first Fender amp to exceed a couple voltage ratings.

My preference for the bias supply would be to replace it with the blackface circuit, and add a 25-ohm (or so), 3-watt trimpot to the power tube cathodes, as seen in this Bogen: http://www.schematicheaven.com/hifiamps/bogen_m120.pdf

It allows you to balance the output pairs via the cathodes (which has a nicer failure mode than the SF circuit), and has a side-benefit of reducing the plate-to-cathode voltage slightly.

- Scott

[FFM]

520 volts on both the plates and screens with the 220v ac tap. 480 with the 240v ac tap

[ThermionicScott]

520 volts on both the plates and screens with the 220v ac tap. 480 with the 240v ac tap

Aha, didn't take your location into account. What are your filament voltages using the 240VAC tap? If they're not too low, I'd stick with it!

- Scott

[FFM]

240AC Tap: 480 on the plates and screen grids, 31 mA, 6.0-5.8V AC heaters
220AC Tap: 521 on the plates and screen grids, 36,3 mA, 6.57 -6.0V AC heaters

[clydey]

remember that in the uk when we dropped the voltage from 240v ac to 230v ac mains in ,that this was just a paper exercise playing on the 240v tolerance
and no reduction was done from the substations.
what this effectively means is that we are still on 240 volts ac (we just record it as 230)
i test premises in britain every day as an electrician
cheers

[ThermionicScott]

240AC Tap: 480 on the plates and screen grids, 31 mA, 6.0-5.8V AC heaters
220AC Tap: 521 on the plates and screen grids, 36,3 mA, 6.57 -6.0V AC heaters

That 240VAC tap sounds like a winner! Love it when solutions are this simple.

- Scott

[FFM]

yes, I believe you are right. I put a new set of JJ 6l6GC and the voltages worked out nicely. I also replaced the B+ filter caps . Do you have a schematic for the exact place to put the bias pot? Whomever drew up the schematic for the 135 W UL fenders must have been on acid .

[FFM]

anybody know how the tone and response of this amp would be running 4 x KT66?

[ThermionicScott]

yes, I believe you are right. I put a new set of JJ 6l6GC and the voltages worked out nicely. I also replaced the B+ filter caps . Do you have a schematic for the exact place to put the bias pot?

Are you going to rebuild it as the BF circuit, or add adjustable bias in addition to the balance circuit?

Whomever drew up the schematic for the 135 W UL fenders must have been on acid .

Or worse -- they were engineers that had worked on much more complex designs before.

- Scott