The Amp Garage

I put the Kt66's in my bandmaster and they seemed to work however, it's just weird that when I swap them round I get the terrible squeal. Are you saying that swapping positions with the KT66s IN THE BANDMASTER also squeals? Based on the fact that when you put 6L6s in the Marshall you were able to g...

Do I ground them to the same point the bias resistors are grounded to by adding a length of wire? Neither end of the grid stopper resistors will connect to ground. Unsolder the wire connected to pin 5. Connect one end of the resistor directly to pin 5. Connect the other end of the resistor to the d...

I have an original JCM900 100W Output Tranny (Raa=2K).

I really don't need 100W...

You could just use two EL34s. Since the primary impedance will be doubled, you'll also need to double the secondary impedance. IE, 4Ω tap becomes 8Ω. 8Ω tap becomes 16Ω. Etc...

LEO the ac100 is it a 4 el34 cathiode biased amp?

All the schematics I've seen show a fixed bias amp.

Is that a wire going from the fuse holder to the speaker jack. :shock: I'm scared That side of the fuse holder is connected to ground. Not a good idea IMO to fuse the PT HV center tap when you have a negative bias circuit that also relies on that center tap. Funny things happen to the bias supply v...

bt2513 wrote:I'm beginning to doubt the output jack wiring. I followed the diagram exactly.

They are wired wrong. Wire them like this...

I think you can fit it all together with a lot of planning. Is the cab wide enough to fit a 16.75" long reverb tank? If not, you'll need to use a short tank. Fitting the chassis and speaker may need some careful planning and accurate measurements. You may have to experiment with transformer location...

Still nothing using the other side of the HV rectifier... You sure this should work Tubeswell? That will not work with a FWB. You must use the circuit I linked. On the PDF that sluckey referenced it shows a typical bias circuit, but with a .047uF cap off the winding and a 56k to ground, is that all...

-35 is about what you'll get with a bridge on that 24VAC winding. You could use a voltage doubler. Or look at page 5 of this pdf to see how Marshall does bias when the B+ is developed using a bridge.

http://home.comcast.net/~seluckey/amps/ ... apbook.pdf

Could I swap the wires on the Out Jack instead

No. Just swap the red and white primary wires. They are plenty long. Just route them under the board.

Here's the popular one wire mod. Maybe this is what you have...

Set your meter to read AC voltage, not DC.

The green wires from the PT connect to a terminal strip. That's a convenient place to pick up 6.3VAC.

When you remove a pair of parallel tubes on the primary, the impedance increases. Therefore you need to increase the impedance on the secondary as well. Just think of the parallel tubes as two parallel resistors. If you remove one of the parallel resistors the total resistance increases. If the resi...

xtian wrote:Danger! If your amp normally wants to push against 16 ohms (16 ohm tap) but you remove two power tubes, you need to reduce the load by half to 8 ohms. Sounds like you've got a 1:4 mismatch which is a recipe for disaster.

I think you have that turned around?