Power transformer for tweed deluxe

[A.M.]

Hi everybody, lately I've been checking out power transformers' specs in search of the right transformer for the 5E3 I'm building. I've found that Hammond transformers are available here in Europe, and the model I was checking out is the 291BEX. Hammond advertises it as a deluxe reverb replacement, but from the info I've gathered it sounds like it can be used for the tweed deluxe too. I also like the 330-0-330 secondary, it should provide the right voltage for the amp, would you agree?
I noticed though that the 6.3 Volt line does not have a center tap. From what I read I have to use a pair of 100 ohm resistors to create an "artificial center tap". I really don't have the knowledge to do this on my own, so I've searched the web and found with these layouts, will they work?

[A.M.]

I just took a look at the innards of my Victoria 5112 amp, and I think it might have an artificial center tap: here it looks like they did it at the pilot light, by connecting a resistor to each side of the light and grounding the other ends to the on/off pot. It looks very neat, could I replicate the same arrangement on my deluxe?

[pdf64]

Yes.
I suggest not to think of it in terms if an ‘artificial centre tap’. Rather the goal is to make the heater circuit balanced with respect to circuit common, ie 2 x equal but opposing polarity 3.15V feeds, rather than the single ended 6.3V feed of the original 5E3 design.
That can be done by connecting a winding CT to circuit common. Or by using a pair of low-ish value resistors from the heater wires and connecting those to circuit common. Those resistors then evenly reference each side of heater circuit to common.
With 1% resistors, the balance may theoretically be better than with a CT.

The resistors or a CT connect to the balance point. That can be common, or another point with a low impedance to common.
If that point is a few V DC above common, then it can stop any valves with imperfect heater to cathode insulation introducing buzzy hum into the signal path.
Hence one of your layouts shows the balancing resistors connected to the output valve cathodes, which will be around 15 to 25V DC positive to common, but as the cathode resistor is bypassed by a 22uF cap, there’s a low impedance to common.

[martin manning]

I was checking out is the 291BEX. Hammond advertises it as a deluxe reverb replacement, but from the info I've gathered it sounds like it can be used for the tweed deluxe too. I also like the 330-0-330 secondary, it should provide the right voltage for the amp, would you agree?

That would be a good choice to keep the B+ voltage down. What is your typical local line voltage?

I just took a look at the innards of my Victoria 5112 amp, and I think it might have an artificial center tap: here it looks like they did it at the pilot light, by connecting a resistor to each side of the light and grounding the other ends to the on/off pot. It looks very neat, could I replicate the same arrangement on my deluxe?

It does, and you can. Using the pilot lamp terminals and some local ground point is a very common way to execute an artificial CT. Another common solution is to place the 100 ohm resistors on the output tube sockets, one from a pin 2 and one from a pin 7 to a local ground point. In a cathode biased amp like the 5E3, you could place one from a pin 2 and one from a pin 7 to a pin 8 (cathode), for an elevated heater string.

[A.M.]

Yes.
I suggest not to think of it in terms if an ‘artificial centre tap’. Rather the goal is to make the heater circuit balanced with respect to circuit common, ie 2 x equal but opposing polarity 3.15V feeds, rather than the single ended 6.3V feed of the original 5E3 design.
That can be done by connecting a winding CT to circuit common. Or by using a pair of low-ish value resistors from the heater wires and connecting those to circuit common. Those resistors then evenly reference each side of heater circuit to common.
With 1% resistors, the balance may theoretically be better than with a CT.

The resistors or a CT connect to the balance point. That can be common, or another point with a low impedance to common.
If that point is a few V DC above common, then it can stop any valves with imperfect heater to cathode insulation introducing buzzy hum into the signal path.
Hence one of your layouts shows the balancing resistors connected to the output valve cathodes, which will be around 15 to 25V DC positive to common, but as the cathode resistor is bypassed by a 22uF cap, there’s a low impedance to common.

Interesting, thanks for the explanation!

[A.M.]

That would be a good choice to keep the B+ voltage down. What is your typical local line voltage?

Typically around 240volts (235-244). I'm not looking to extract the last watt of power from the amp, quite the contrary. I was considering a transformer that "outputs" even lower B+ like Mercury FTDP-M-E (with a 290-0-290 secondary) but that would be a far more expensive investment. All things considered the Hammond transformer seems to be very good bang for buck!

It does, and you can. Using the pilot lamp terminals and some local ground point is a very common way to execute an artificial CT. Another common solution is to place the 100 ohm resistors on the output tube sockets, one from a pin 2 and one from a pin 7 to a local ground point. In a cathode biased amp like the 5E3, you could place one from a pin 2 and one from a pin 7 to a pin 8 (cathode), for an elevated heater string.

Excellent, thanks for the info!

[ChopSauce]

Yes, great tip.

On a matter more related to 5E3 PT, afterwards I feel like I should have considered the current capacity too, as the original 5E3 is supposed to sag a lot.

[Guy77]

That is a good point Chopsauce. The original spec PT would have more sag.


Guy

[didit]

Hello --

Been pondering whether adding to this thread would be helpful.

Although Hammond markets their BFDR power transformer for a 5E3, in my albeit limited experience the Princeton/Champ power and their 5E3 output transformers are the better option:

• https://www.hammfg.com/files/parts/pdf/291AEX.pdf

• https://www.hammfg.com/files/parts/pdf/1760E.pdf

In a standard 5E3 chassis the power transformer will need adapter to fit the mounting holes. That caveat aside, the voltage and power (VA/Watts) are closer to the 1950s original.

Best .. Ian

[pdf64]

237ohm HT winding; that’s more like it

[didit]

237ohm HT winding; that’s more like it

Exactly so ..

[Guy77]

I used that 291AX on my 5E3 Tweed Deluxe years ago and it worked flawlessly and made for a great sounding amp.
I should have not sold that one lol. It lives on a little island off the coast of Vancouver island where the owner can go outside and crank it every morning!

Guy

[A.M.]

Hello --

Been pondering whether adding to this thread would be helpful.

Although Hammond markets their BFDR power transformer for a 5E3, in my albeit limited experience the Princeton/Champ power and their 5E3 output transformers are the better option:

• https://www.hammfg.com/files/parts/pdf/291AEX.pdf

• https://www.hammfg.com/files/parts/pdf/1760E.pdf

In a standard 5E3 chassis the power transformer will need adapter to fit the mounting holes. That caveat aside, the voltage and power (VA/Watts) are closer to the 1950s original.

Best .. Ian

The 291AEX looks good but... how can you modify a standard 5E3 repro chassis to make it fit?

[A.M.]

237ohm HT winding; that’s more like it

I wish I had the knowledge to undertand these details... does it mean that the 291bex has the wrong specs for a 5E3?

[martin manning]

You do not want to modify the chassis. 291BEX would most likely be fine, but if you want to go with the lower current higher voltage secondary (355-0-355 @ 115 mA, 238.1 ohms DCR), the 290BBEX is the one you want. The primary has options you don't need, but it can be wired for 240V, and the mounting centers are the same as the 291BEX (2.25 x 2.813). The closest one to the original might be 290BBX, with the same HV voltage and current rating, and 204.4 ohms DCR, but it has 120V primary only.