Fender "SVT" Diodes?

this Bassman 300 had a bad first filter cap in the bias supply so we had a space heater due to bias voltage not negative enuff,

some similarities between this and the SVT with the direct coupled driver  and minus supply on the cathode,

just wondering what these diodes do in the grid circuit (blue circle)

The diodes stop the grid from going higher than 0.6 volts positive (Hard turn ON).

thanks, that makes sense,

what about C157, 158? FB?

this amp has a unique feature that might be applied to a studio compressor, it lets you pick the bands that you want to compress,
so you could have good punch on the bottom but compress the hi end for sanppin and poppin, or visa versa,  (knobs on the right)

backside has bias and balance with real pots, volt meter lead holes, and a plate that covers the controls,

dual color LED's show red for std by and green if all the tubes are good, otherwise red for a shorted tube/blown screen voltage fuse, screens at about 350 so easier on the fuse holder than 750, smart move,

premp is pretty busy,

control circuits, looks like some logic gates in there,

power supply with bi colored LED circuit,

bias set at 80 ma for three 6550's, = 26.7 ma each,  33 ma would be 70% plate disp, seems low but we have 750 on the plates,

6550 plate disp. of 35 watts is conservative, i bet they do a bit better than that,

Fender likes a cold bias , probably for tube life and insurance against thermal runaway,  how do you get a warm sound out of a fender, reset the bias?  no,  crank it up to 10 ! 

mother board has break apart sections, enough parts for a TV set, or not,

had to do a "solder from the top" replacement of the offending cap  as to avoid an R&R project.

first time we have seen a Faraday shield on an OPT, high AC voltage transient suppression so it does not get into the input circuits,

"what about C157, 158? FB?" They are BOOTSTRAP CAPS and this makes the input Z >> 150k and this unloads the previous stage.
Duke

CJ said:

first time we have seen a Faraday shield on an OPT,

Click to expand...

I believe that is technically not a Faraday shield, but a magnetic shunt, as the copper foil goes around the outside of the core. Is the copper unusually thick, and are the ends of the copper soldered together for a 0 ohm connection?

Basically, it is a shorted turn outside the core, to short out stray magnetic fields that get out from where they should be. Should have no effect on what's going on inside the core, but reduces stray magnetic fields inches away that gets into other sensitive stuff.

Gene

I believe that copper is used for the E field, Mu Metal is used for the Mag field,  if you interleave them, you get protection from both the E field and the Mag field,this is what they do in say, a UTC HA-10-X input coil where you need a lot of protection,

Since the copper 'belt' wrapped around this output transformer is outside the 'line integral"  ie, the closed loop path of the magnetic flux, it does not constitute a shorted turn, thus you do not have to insulate the ends like you might do on an isolation transformer pri/sec shield, or like on a 600:60 line input where the shield is used to reduce capacitance and thus improve freq response,

I guess this shield is more like a Faraday Cage, however it does not completely enclose the transformer as the top and bottom of the transformer does not have the copper folded over like you might see on some transformers.

'Shielding can be anything from using a coaxial or shielded cable, to a sealed conductive chamber for circuit isolation. Shielding serves a reciprocal purpose: it protects the circuit it is shielding from outside noise or unwanted signals; and conversely, it contains its own signals and thus protects the outside world from interference of its own making. Shielding is mostly used to block electrostatic or "E" fields (Faraday shield). However, if ferrous metal (tempered Mu Metal works best for magnetic fields) is used, then both electrostatic and some level of magnetic shielding is accomplished. This is especially useful where open frame transformers or unshielded coils are used and would otherwise exchange signals by mutual inductance. "

hey check this out, a bar owner got fed up with cell phones so he installed a Farady cage! 

https://www.sciencealert.com/a-talented-bar-owner-in-the-uk-has-built-a-faraday-cage-to-stop-customers-using-their-phones

A lot of stuff in that power stage plan looks like someone does not grok tubes.

Yes, 12AT7 has a 1Meg max grid resistor.... in self-bias service. Here it is run in fix-bias cathode follower, grid resistor could be MUCH higher no problem.

But it does not need to be higher, because the 12AX7 stage in front is a 39K source, could easily drive 500K load, so 1Meg (820K+150K) at 12AT7 grids would be fine, no bootstrap.

With 47K(!) series with 6550 grids there is NO need for diode "protection". This is MOSFET thinking. The 6550 grid IS a diode, and was abused much worse than this (much-much worse than 47K can pass) in the Ampeg SVT (and the Fender 300). Lose that diode, put the 47K AT the grid pin, now the 47r is totally silly (but might be needed -with- the Si diode to suppress parasitics when the grid want to go positive).

CR75-CR77 are silly. Protecting resistors that can never get warm? If they protect some frills attached at [6], maybe bias-lights, fix the frills don't protect what could amount to cathode fuses against a shorted tube.

CR98 CR99 limit grid down-swing which should not be needed if the driver is proportioned right.

The long-tail lacks the NFB into its tail, which was probably Leo's goof, but is probably a significant detail in all the large classic Fenders.