The Amp Garage

I am pretty far from understanding the schematic myself. I see the four wires that go to the motor, two to energize the field coil (is this with AC or DC current?) and two that carry the variable current (this MUST be DC). I still can't figure out why it is only running on full speed. Can you see w...

Regan wrote:I don't see any need to use the field coil setup for anything then. I would scrap it and just go with a dc power supply of up to 40v and the controller.
Regan

?? The motor won't run without the field coil...

@hossy - why do you want to change the controller you have?

Glad to hear you got it sorted!

What did you done do?

I also ran a new NFB wire with more slack, allowing me to move it far from any other wires, including the MV wires. No change in sine wave on scope. Also, the wire from the PI circuit to the presonance control--pushed it around; no change. Hmm... (I take it the green wire in the last photo is now t...

I tried pushing the NFB wire around, but can't see any change. I'd still put money on it being the problem - its proximity to the wires from the MV wipers to the output tubes. FWIW, I would re-route the wires from the MV wipers so they ran down and back the way the wires from the PI come to the MV,...

Well, I can't find any errors in your wiring. A couple of solder joints that look a bit iffy from the angle that the pictures were taken, but I don't think they're particularly relevant - FYI: The 470R to the 6L6 on the right of the photo, and the junction beteen the 100k grid leak resistor and the ...

Oh no... Could I have my bias voltage and grid voltages on the wrong screens?! I will check when I get home and post pics. Maybe! : I wanted the amp to run 6L6, so I switched the grid resistors to 470r... I hope you meant screen grid resistors (R14 & R15) ? Your signal / bias voltage should end up ...

Cool! I think you're right about the relays: It looks to me like the field coil is permanently energised, with the armature supplied from the rectifier circuit which is supplied with various AC voltages through an autotransformer(!*). I doubt that changing the rectifier will affect anything, but you...

http://ampgarage.com/forum/download/file.php?id=9797 what does the film cap across the grid resistor on pin 2 and 7 do? Thanks Looks like it is an amp equipped with variable HT voltage, so they are probably blocking caps to prevent DC on the volume / guitar pots when operating at low voltage. The r...

...when you increase the bias voltage to say, -40, the plate and screen voltages drop considerably... ...Is this normal? Noooo.... (IMHO) Normally, the plate voltage (and general B+) would rise as you make the bias more negative (-30 to -40, say) due to the lower current draw. Something seems a bit...

How are you determining the bias current?

Did you use the 1 ohm resistors in the schematic? If so, treble check that they are the correct value!

Hopefully not butting in: I wouldn't worry about your V3 (PI) voltages - trying to measure the grids directly with a normal meter doesn't work. (FWIW, the PI grid voltage will be the same as the voltage where the 4 resistors R10..R13 join.) The concern is the non-zero voltages on the EL84 grids - Ch...

Did you follow the ampamker layout?

You could try temporarily disconnecting the negative feedback connection from the 4 ohm tap to R14 to to see if the NFB is causing the problem.

gui_tarzan wrote:...when I turned this on it immediately popped the 470 ohm sandstone resistor. I tried a second one thinking the first one might have been defective and it popped that one too.

Sounds to me like you have a short downstream of that 470 ohm resistor. (Is that middle 22uF cap OK?)

No problem.