Developing the DIY "Champ" for first time builders

[Firestorm]

RJ, pin nine on the 12ax7 is a heater pin, I believe, and you have it going to a ground lug. You also have both heaters going to 4/5 on the 12ax7 and one should go to 9, the other to both 4and5 (again I think).

That's a good catch (also good catch on the EL34 pin 5 wiring). You can either wire the heaters directly to the pins, OR ground one heater lead to the chassis. But not both ways. (I don't know if the PT has a CT for the heaters, but you absolutely can't use it if you elect to ground one side of the heaters). This raises an interesting question. Fender connected one side of the heater supply to the chassis in all of the Champs and wound up with amps that were surprisingly hum-free for a single-ended design. The assumption is that this induced two out-of-phase hums that cancelled. If you wire the heaters conventionally (wired to the pins, not to ground), you may wind up with hum you'll need to eliminate in some other way. The usual fix is to bias the heater supply up to 50VDC or so with a voltage divider off the screen node.

[RJ Guitars]

Good Stuff guys, I appreciate your help. The heater wiring was the residual from the Russian preamp tubes we used on the Eagle 100. I'll fix that and repost the corrected values.

Firestorm,

I've also had another request to have the tonestack bypass option so that seems like a good plan.

I'll follow on these ideas,

thanks again,

rj

[surfsup]

This raises an interesting question. Fender connected one side of the heater supply to the chassis in all of the Champs and wound up with amps that were surprisingly hum-free for a single-ended design. The assumption is that this induced two out-of-phase hums that cancelled. If you wire the heaters conventionally (wired to the pins, not to ground), you may wind up with hum you'll need to eliminate in some other way. The usual fix is to bias the heater supply up to 50VDC or so with a voltage divider off the screen node.

I'd like to know more about this...hint hint... I have a SE amp that was a humming machine but I split the preamp smoothing cap off from the ground bus and put it to the input groudn that knocked 95% of the hum out of the amp. As this is planned to be an experimental amp, are you saying I could wire one of the leads to both octal pins and all three 459 pins on the noval and the other lead goes to ground on the chassis? Or should one lead go to one side of the octal, then to ground, and the other goes to the other octal pin, the 45and9 and then to ground?

RJ, if you're taking requests for options, how about a switch to swap secondary PT voltage supply? I didn't look more at the layout/schemo...

[Zippy]

If you wire the heaters conventionally (wired to the pins, not to ground), you may wind up with hum you'll need to eliminate in some other way. The usual fix is to bias the heater supply up to 50VDC or so with a voltage divider off the screen node.

How do you feel about tying the heater supply to the output tube cathode bias resistor to raise its reference voltage?

[Firestorm]

How do you feel about tying the heater supply to the output tube cathode bias resistor to raise its reference voltage?

It works up to a point, but the voltage derived that way isn't always high enough to kill off all the hum.

[surfsup]

How do you feel about tying the heater supply to the output tube cathode bias resistor to raise its reference voltage?

I tried this on mine, didn't work. I also elevated the heaters to 46v and that didn't work either. The thing that worked was splitting the ground bus at the preamp smoothing cap and grounding the cap and V1 at the input ground. Hence my interest.

[Firestorm]

I'd like to know more about this...hint hint... I have a SE amp that was a humming machine but I split the preamp smoothing cap off from the ground bus and put it to the input groudn that knocked 95% of the hum out of the amp. As this is planned to be an experimental amp, are you saying I could wire one of the leads to both octal pins and all three 459 pins on the noval and the other lead goes to ground on the chassis? Or should one lead go to one side of the octal, then to ground, and the other goes to the other octal pin, the 45and9 and then to ground?

RJ, if you're taking requests for options, how about a switch to swap secondary PT voltage supply? I didn't look more at the layout/schemo...

The preamp filter ground really ought to go to the input ground anyway. What you probably solved was first B+ node and output tube currents getting into the preamp. It's really best to power SE amps with SS rectifiers and use a lot of filtering or add an extra filtering stage upfront (before the plate connection) to get rid of most ripple currents.

As to wiring heaters, running one side of the heater circuit through the chassis provides a ground reference that keeps SE amps quieter. In that case, usually, octal pin 7 is connected to the chassis as is noval pin 9. If there's a 6.3VAC center tap, you don't use it.

Alternatively, you bias the heater supply up by 50 or 75VDC to "shield" the 6.3V hum. Apply the offset to the center tap or to the junction of two 100 ohm resistors.

Or there's DC heaters.

[RJ Guitars]

I updated the schematic and layout drawings (page 19). I'll avoid the temptation to draw in extra switches on the layout for now, but will consider listing them as optional on the schematic. I am still clinging to the simplicity of the Champ to some degree.

I'm not entirely clear on what all would need to change if I had a switched power primary so I'll leave that alone for now.

I am not sure where I borrowed the tone stack I had in Rev 0.0 but it wasn't complete so I've replaced that whole thing with my iteration of the Fender version. It's pretty simple although it may need some adjustment on the tone caps to get us where we need to go.

For those that have the inclination, see if the suppressor grid, screen resistor, and tonestack are sensible. Hopefully each iteration is an improvement and we'll have good documentation on what we want to do before the parts arrive.

thanks,

rj

[surfsup]

RJ I still think the blue lead off pin 5 on the octal needs to be connected upstream to the 02uF cap instead of the resistor at the turret at the middle of the board just to the right of where the purple/red dashed leads cross.

[RJ Guitars]

RJ I still think the blue lead off pin 5 on the octal needs to be connected upstream to the 02uF cap instead of the resistor at the turret at the middle of the board just to the right of where the purple/red dashed leads cross.

Yes thanks - I kept missing that. Drawings updated. rj

[telentubes]

I built my premium champ kit this weekend. It fired up without a hitch. I stuck it in a cab with a Celestion Blue. Nice little amp, as is. Thanks RJ and the rest for this inclusive project.

[RJ Guitars]

Telentubes that's a great report. Any photos or audio clips for us?

In the meantime I got the report that we are at the top of the queue for a CNC machine at the shop. If nothing bumps us we should have some Supre chassis out soon.

The first of the Supre transformer options showed up yesterday. These massive 25 watt SE trannies from Heyboer. They should handle everything all the way up to the KT-120... I want to see it built but don't know if I'll go that extreme or not.

rj

[M Fowler]

Now that would make for a great DLM which is an amp I have been itching to build for a long time.

Mark

[RJ Guitars]

Now that would make for a great DLM which is an amp I have been itching to build for a long time.

Mark

Ah yes grasshopper you read the signs very well.... or your equally demented as I am. This tranny just happens to fit exactly on a Wreck Chassis. I am thinking Express chassis with a plug in one of the output tube holes etc..

rj

[M Fowler]

Yes master that is exactly what I was thinking about.

I don't care if I have more room then needed for this SE amp the idea is to eventually have the DLM and Songwriter to go with my three other TW amps all in cherry cabs.

Mark