How do you determine the circuit impedance that feeds the OT?

[5thumbs]

I am building a 5e3 head out of some leftover parts I have laying around. I found 2 output transformers that supposedly will both work.

One is a PTF22913: Replacement for Princeton Reverb, Tweed Deluxe, 8 ohms. Equivalent to #125A10B & #022913. Specifications Audio 12 W (RMS) Primary Impedance 8,000 Ω Secondary Impedance 8 Ω.

The second is a PTF41318: Replacement for Deluxe and Deluxe Reverb, 8 ohms. Equivalent to #125A1A & #041318. Specifications Audio 25 W (RMS) Primary Impedance 6,600 Ω Secondary Impedance 8 Ω .

It seems that the 2-6v6's in a Princeton Reverb circuit have an 8-8.5k ohm output for the OT into 8 ohms, and a Deluxe or Deluxe Reverb circuit's 2-6v6's have an output of around 6-6.5k ohm feeding the OT into 8 ohm. What is the reason for this difference in these amps? Is it caused by the cathodyne verses long tailed pair phase inverter? How would I calculate which transformer I should use for optimal performance in a build?

Thanks for your help!
Tim

[potatofarmer]

First off, either of those OTs will work fine with a pair of 6V6s. Check out the 6V6 data sheet; there is a plot of power and distortion versus plate-to-plate impedance.

You pick the primary impedance based on the tube's characteristics and the supply voltage you're using.

Merlin explains it well here: The Valve Wizard -Push-Pull

[eschertron]

Somewhere on this forum I was pointed to this : Interactive Valve Data Sheets, which is a tool that can help you pick the best B+ for a OT impedance (or vice versa!), and helps to visualize how the factors interact.

[Alan0354]

I am building a 5e3 head out of some leftover parts I have laying around. I found 2 output transformers that supposedly will both work.

One is a PTF22913: Replacement for Princeton Reverb, Tweed Deluxe, 8 ohms. Equivalent to #125A10B & #022913. Specifications Audio 12 W (RMS) Primary Impedance 8,000 Ω Secondary Impedance 8 Ω.

The second is a PTF41318: Replacement for Deluxe and Deluxe Reverb, 8 ohms. Equivalent to #125A1A & #041318. Specifications Audio 25 W (RMS) Primary Impedance 6,600 Ω Secondary Impedance 8 Ω .

It seems that the 2-6v6's in a Princeton Reverb circuit have an 8-8.5k ohm output for the OT into 8 ohms, and a Deluxe or Deluxe Reverb circuit's 2-6v6's have an output of around 6-6.5k ohm feeding the OT into 8 ohm. What is the reason for this difference in these amps? Is it caused by the cathodyne verses long tailed pair phase inverter? How would I calculate which transformer I should use for optimal performance in a build?

Thanks for your help!
Tim

You ask a question I meant to ask for a while, I compare the two schematic, they both use +B=410 volt ( +/- a few volts if you really insist). Both have grid bias in about -35V. I don't even see why the two have different output power. Could it be Fender intentionally mismatch Princeton to lower the power for marketing reason?

[g1]

I don't even see why the two have different output power. Could it be Fender intentionally mismatch Princeton to lower the power for marketing reason?

The output power is usually a function of (and limited by) the output capability of the power transformer. To make an amp put out more power usually requires a larger power transformer.
As ChuckH mentioned in another thread, an amp can be thought of as a valve that modulates the power supply in accordance with the input signal. Even if the valve is wide open, it can only deliver as much power as the supply is capable of.

[Alan0354]

The output power is usually a function of (and limited by) the output capability of the power transformer. To make an amp put out more power usually requires a larger power transformer.
As ChuckH mentioned in another thread, an amp can be thought of as a valve that modulates the power supply in accordance with the input signal. Even if the valve is wide open, it can only deliver as much power as the supply is capable of.

Yes, but I still think it's a marketing thing. Making a slightly bigger PT is not going to cost that much more......$5? Fender can easily make the Princeton 20W single channel amp. But they choose to make it 12W to make more of a distinction from the DR.

[Dave H]

You ask a question I meant to ask for a while, I compare the two schematic, they both use +B=410 volt ( +/- a few volts if you really insist). Both have grid bias in about -35V. I don't even see why the two have different output power. Could it be Fender intentionally mismatch Princeton to lower the power for marketing reason?

They both have 410V B+ but the Princton Reverb has a cathodyne PI with a supply voltage of only 240V. I don’t think it has the output swing to drive the 6V6s to full power. I'm not sure if this was done deliberately to make it have less output than the Deluxe Reverb.

[Alan0354]

They both have 410V B+ but the Princton Reverb has a cathodyne PI with a supply voltage of only 240V. I don’t think it has the output swing to drive the 6V6s to full power. I'm not sure if this was done deliberately to make it have less output than the Deluxe Reverb.

I don't doubt Princeton is lower power, I more think it's a marketing thing. I don't think Leo Fender dreams the small throw away Champ and Princeton will one day be the more sorted after and high resale value. I think they just want to make a lower model and sell it cheap those days. As I said, they could have make it to 20W for about $5 extra!!!.....Well.....ok!!! Say $10......I doubted!!!!

I think it's more like they want to sell the Deluxe higher.

[JoeM]

My Princeton has a B+ somewhere around 440V, the PT does not have the current capability the DR PT has, so as soon as the amp is pushed the B+ drops a lot. Max power on mine is at most 15W. My DRRI B+ around 410-420 (lower value if the tubes are biased hotter). Gets the 22W rating pretty easily maybe slightly more.