building a Fender "Pro-Amp" from scratch

[Rotorcraft230]

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

 

[Guitar-Rocker]

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

Those are two complete different power transformers / rectifier types. The tweed will lack some of the headroom of the other schematic version, due to the lower plate voltages.

Granted you can go with what your current iron is set up for and change the preamps to suit your needs for which tone is preferred

 

[Rotorcraft230]

The PT I’m using is going to be used with the rectifier from the Knight. I believe it’s a 5AR4,
Not at home to confirm.

 

[Rotorcraft230]

I was wondering what the linear taps would do vs the CT taps used with pin 4

The plate voltages will be in the neighborhood of 420vdc

 

[Rotorcraft230]

Here is a pic of the transformers. I tried looking up the numbers but came up empty,

Anyone recognize them?

 

[neikeel]

Normally the ultra linear taps will be 40% or so.
You have all the gear to test so I would measure and calculate the transformer ratios and taps with variac and DVM. I suspect two 6L6 will be best around 6k which is likely to be the two outer taps and the UL ones can be insulated and hidden inside the bells. Of course it will depend on what secondaries you are going to use and x2 vs 4 outputs or even 5981/6V6 options.
Yes use the 470R screen resistors on each output.
I have posted the primary method before but if you can’t find it I’ll dig it out this evening (work beckons!)

 

[Rotorcraft230]

Thanks Neileel, I can do that and measure. Here is what I have as far as DC resistance, looks like they are not 40%, but I’ll still measure.

 

[Guitar-Rocker]

The top PT looks like it came out of a Baldwin organ.

 

[Rotorcraft230]

The top PT looks like it came out of a Baldwin organ.

A Knight PA with 6L6GCs

Plan on using all the secondary taps on the OT 4,8,16 Ohms.

It also has a 75v, I think that’s a PA thing.

 

[neikeel]

Voltage in (I suggest variac @ 100vac) and an AC voltmeter.
You can use lower voltages but fluctuations will have bigger effect when you multiply up the numbers
Apply 100vac to the primary of the transformer and measure the output voltage on the secondary.
Turns Ratio = V in (to the primary) / V out ( measured in the secondary)
e.g. if we apply 100vac and we measure 3.48 vac on the secondary the Turns Ratio of the transformer is : 100/ 3.48 = 28.7 That means a turns ratio 28.7:1
You can calculate the impedance ratio of your transformer and the impedance in a given load to the secondary.
The impedance Ratio is the square of the turns ratio:
28.7 X 28.7 = 825.7 that means an impedance ratio 825.7:1
So in a given load impedance of 8 ohms the transformer impedance is 8 X 825.7 = 6600 ohm ( it is about 6.6k)
In this case pretty spot on for most 6L6 if you use an 8ohm speaker

Hope that helps.

 

[Rotorcraft230]

Yes it’s surely will help. I plan on doing that when I get home and I will report back. The PA had 2 12AX 7s, a single 6AV6, 2 6L6GCs and a 5AR4. I’ve never done these types of measurements so it should be fun.

 

[Rotorcraft230]

So here is what I came up with. Doesn’t quite look ideal. If it has been used since 1961 and is still working I’m go to assume that this OT and tube combination will continue to work together.

 

[neikeel]

Looks pretty perfect to me for 6L6
I'd probably not use 6V6 at over 460v unless they are JJs but then a tube rectifier will sag a little.
The 75v tap will be perfect for a dedicated bias feed (rather than spurring off the HT)

 

[Rotorcraft230]

Thanks for helping Neikeel, the 75v is actually labeled 70v and it’s on the secondary of the OT and it had 13.38vac on it when I was doing the WR calculations. Here are some pics of what I removed the transformers from.

Oh and also the transformers are SNC, their EIA code is 6.
Eminance speakers EIA code is 67
Jensen speakers is 220
Schumacher transformers 606 (Fender used Schumacher.
That’s just some examples, so 6 is VERY early number.

 

[Rotorcraft230]

I got an undrilled chassis, Marshall JTM45 faceplates. The rest is pretty much gonna be Fender like, oh minus the turret board. The OT extends into the chassis so I had to raise the circuit board off the floor and went with turrets. Still waiting on the filter caps, went with F&T

The preamp tubes are 12AX7, all are from 1961.

I have 2 12AY7s coming.

 

[neikeel]

Going to be good!
Wonder if that is the PA output (75v) in uk often 100v tap

 

[Rotorcraft230]

There is the chassis, that was a little work, very satisfying though.

 

[danfrank]

To OP:
Those maroon EM capacitors are great great great! Much better than the orange drops, so if the EMs are in values you can use, do it! Just do it!

--- Insert Shea LeBeouf clip here ---

DON'T use the selenium rectifier, those are bad bad bad... That black electrolytic is also no good. I'm talking about stuff on the original amp. Those old PA amps make great platforms for guitar amps. I also agree with Neikeel on not using the ultra-linear taps of the output transformer. Ultra-linear has no place in guitar amplification...
Good luck and please report back when it's up and running!

 

[ELS]

Add the 470 ohm resistors, they fix many headaches and don't change the tone much at all.
If you connect the screens to the extra OPT taps, you'll have an ultralinear output which usuallys sounds a bit more plain, And it distorts sharper, sort of like a solid state amp. It stays very clean until snapping into overdrive.
But you can instead just connect the screen supply to the plate connection if you want a more standard config, I'd suggest that.

Also, I'd recommend connecting the OPT center tap to before the power supply filter choke, and the screens after the choke. This puts much less strain on the filter choke, Also it makes the tone creamier because of some intermodulation distortion.
You can make the first filter cap a 47uF. the farther down the line the filter cap is, the lower the value should be because if you put like a 200uF filter cap on the preamp supply, if the power supply sags, it will take a long time to recover and will sound awful. That's why you should do the most filter in the early power supply stages, because there it will recover much faster even with high capacitance values

 

[Rotorcraft230]

Hey danfrank, thanks for the info. Looks like there are 3- .047 caps and 2 .1 caps. Both are rated for 600v. I can use the .1s for the coupling caps for the power tubes. If they check out I’ll use them thanks.

Here are the components bolted on with the tubes in.