building a Fender "Pro-Amp" from scratch

[Rotorcraft230]

Not really happy with the grounding of the pot components, but it’s done now, don’t want to re do it now. Anyway it’s the weekend and it’s like 110 degrees outside so I’ll be here with the amp moving forward .

 

[tonyigaar]

I have a 1961 Knight 3036 PA amp that has 2 preamp tubes a PI, tube rectifier and 2 6L6gc tubes in it (tubes are vintage as well dated 1961 RCAs) I'm gonna pull the transformers and tubes and build a Tweed Pro with them. When I was pulling the OT I saw there were 2 extra wires on the primary side stowed ( not used in the PA), they are taps between the CT and the plate wires. I was looking at the schematic and the Tweed pro taps the CT and puts them on pin 4 (screen grids) on the 6L6 tubes. Can I put the taps there instead of the CT taps? Fender did make an ultralinear Pro and that is what they had done.

some questions

Do I need to put the 470 Ohm resistors on as well? The Tweed pro didn't use the 470 Ohm resistors but the ultralinear did.

Also what will my outcome be if I wire it this way? Cleaner and more power?

https://schematicheaven.net/fenderamps/fender_ultralinear_pro_rev.pdf

https://schematicheaven.net/fenderamps/pro_5e5a_schem.pdf

you may want to change the screen resistors to 1K at 5watts for each 6L6, this is what the RCA tube manual suggests for all power tubes, leo fender used the 470 at 2 watt because it made the amp louder quicker. back then tubes cost 2 bucks for a 6L6 not 19 bucks and up in todays economy. i build all my amps that way, they are loud and have controllable bass and i change all amps coming into my shop to that config. for el84 i use a 2 watt 1k resistor, also 7189 but 6v6 a
5 watt 1k.

 

[Rotorcraft230]

The screen resistors take a beating in old Fender amps. I like using the Vishay 3watters.

you may want to change the screen resistors to 1K at 5watts for each 6L6, this is what the RCA tube manual suggests for all power tubes, leo fender used the 470 at 2 watt because it made the amp louder quicker. back then tubes cost 2 bucks for a 6L6 not 19 bucks and up in todays economy. i build all my amps that way, they are loud and have controllable bass and i change all amps coming into my shop to that config. for el84 i use a 2 watt 1k resistor, also 7189 but 6v6 a
5 watt 1k.

 

[Rotorcraft230]

getting the power tubes wired up.

 

[BlueX]

It's a real pleasure to look at your builds

 

[Rotorcraft230]

Inputs and the pots side is done

 

[Rotorcraft230]

Just heaters and the power tubes are done.

Added jacks for biasing

 

[Rotorcraft230]

Little more work

 

[TXOldRedRocker]

Purdy.

 

[BlueX]

Just heaters and the power tubes are done.

Added jacks for biasing
View attachment 135042View attachment 135043View attachment 135044

What's between the trafos? 1/4" jack? What's it for?

 

[Rotorcraft230]

What's between the trafos? 1/4" jack? What's it for?

It’s for biasing the power tubes. The wires from the jacks between the transformers go to pin 8 before the 1ohm resistor so you can measure the ma in mv.

With a DVM, you put one lead to the jack and the other to ground, the result is the current flowing through the tubes at idle in mv.

The hole through the chassis is a potentiometer to adjust the bias. This method allows you to adjust the bias without pulling the chassis out. Here is a pic if the inside, that’s the pot by the indicator light.

 

[Ttepper]

I’ve always been confused by this. How can you measure current in volts? I think the idea is to set DVM to millivolts and take measurements, but don’t we want to know current draw in milliamperes?

 

[Rotorcraft230]

Ohms law says if you measure voltage across a 1ohm resistor it’s the same as ma.

DVM is set to mv. And across the 1ohm resistor toe two will be the same.
ma and mv.

I’ve always been confused by this. How can you measure current in volts? I think the idea is to set DVM to millivolts and take measurements, but don’t we want to know current draw in milliamperes?

 

[Rotorcraft230]

The only hitch with this method is you’re measuring the screens as well. The screens draw approximately 3-5ma. That’s no big deal though.

 

[Ttepper]

Thanks for the explanation. Makes sense and greatly simplifies taking measurements. Nice looking build by the way!

 

[Rotorcraft230]

Glad you asked, yes it simplifies the biasing.

Thanks, should have it fire it soon. Maybe this weekend.

 

[Rotorcraft230]

This morning’s progress

 

[Marcomel79]

This morning’s progress
View attachment 135137

Beautiful build as always

 

[Resident 217]

What's between the trafos? 1/4" jack? What's it for?

Its a great mod that Ive seen used by many. Ive added the resistor to my amp as well but I still have to pull the chassis.
I was wondering about your thoughts on safety concerns of a 1/4 inch jack used in tis manner.?
for example in the future, If the amp was to fall into the hands of someone unfamiliar with its function.
I suppose it could be labelled?

 

[Rotorcraft230]

Its a great mod that Ive seen used by many. Ive added the resistor to my amp as well but I still have to pull the chassis.
I was wondering about your thoughts on safety concerns of a 1/4 inch jack used in tis manner.?
for example in the future, If the amp was to fall into the hands of someone unfamiliar with its function.
I suppose it could be labelled?

Thank you, it’s been a very fun build. Cutting and drilling the chassis in particular.

It’s a hollow body shaft potentiometer, you use a small screwdriver to adjust it. I will label it with:

“Bias adjustment “

We have plastic screwdrivers we use.