Amp Question

[Structo]

I saw this amp mentioned at the Gear Page and was curious about something.

The company is called Speedster Amps and are located in Gig Harbor, Washington.

Interesting that they would put the bias pot on the front panel.
Also has a bias meter on the front panel.
They encourage you to change the bias for different tones.

But the thing that caught my eye on the schematic which they publish on their site, are the two cap devices in the center of the schematic.

They have one fixed value ceramic disc, 4.7pf
And a variable trimmer cap, 1-10pf, labeled as a variable noise filter.

They have the normal artificial center tap on the heaters with the two 100R resistors to ground.

But what does having the heaters connected through the caps to the return of the reverb do?

[FunkyE9th]

My guess.... Looks to me like they are trying to feedback part of the heater signal to cancel some 60Hz noise from the reverb. By making the one side have a variable cap, you can change the phase and amplitude of the 60Hz signal that's fed back.

[selloutrr]

Didn't these amps start as a spin off company or special order offered from soldano in the 90's?

The bias is a gimmic to some degree it's range is limited within a general safe working level. (not to hot not to cold)... hardly ever right. since vibration will cause the bias to shift while the head is on the cabinet.

[selloutrr]

My guess.... Looks to me like they are trying to feedback part of the heater signal to cancel some 60Hz noise from the reverb. By making the one side have a variable cap, you can change the phase and amplitude of the 60Hz signal that's fed back.

since amps with reverb run out of phase it's probably an attempt at bringing it back in. to make the most out of another stage or minimize additional parts... or a gimmic that lets them apply for a patent. Might be needed to quiet the pan from the faceplate?

[alvarezh]

Additionally, the P.T. center tap is connected to the rectifying diodes. Always seen those diodes going to ground with no C.T. connection.

Am I wrong?

[Structo]

Hmmm, you're right.

Looks like a full wave bridge but then instead of grounding the negative side they use the center tap.
I wonder if that is a booby trap?

[Alexo]

Hmmm, you're right.

Looks like a full wave bridge but then instead of grounding the negative side they use the center tap.
I wonder if that is a booby trap?

...much smoke would be let out!

[alvarezh]

Thanks for that confirmation Tom.

Cheers.

[Structo]

What about the two preamp cathodes marked A and E?

A is usually the first B+.

[alvarezh]

That's it, this schematic is a farce!

Point "A" should be your first B+ node going to the O.T. center tap. They forgot to point that, if you connect the cathode to that point, there is going to be more smoke!

At least point "E" is correct, it's a shared cathode connection, typical of Fender's parts saving scheme.

Tell me about building the whole amp with metal film resistors. Man, that should sound pretty sterile to me.

Reverb driver 12ax7, when it should be a 12at7.

No wonder they published that schematic, like you say Tom, definitely a booby trap!

All the best.

[Firestorm]

That's it, this schematic is a farce!

Not quite a farce; just incredibly sloppy. It's a cut and paste retread of a Fender AB763 schematic. The "A" and "E" cathode connections are straight off the original. They just didn't bother to fill in all the details.

Who in the world publishes that as the schematic of your product???!!! It looks like the prototype schemo, never updated or corrected. Why?

Early on these guys had gotten enough press (thanks to the cabinet design and the gimmicky bias meter) that they needed to hire Soldano to manufacture for them (Soldano had spare bandwidth at the time). Surely they could draw up their own schematic.

As to the noise filter: I've got to think about that. Caps are too small to do anything about 60Hz. Looks like they're deliberately injecting high frequency "noise" from the filament winding onto the third triode grid. Maybe to phase cancel a reverb fault.

[billyz]

Unremarkable . the small adjustable pf cap does affect the hum level dramatically. Not sure it would not have been quieter without it, very finicky to adjust. Sounded pretty good, smallish PT and OT by Mercury.