Peavey 4x6L6 question

[jutsin]

I've recently acquired a 5150 head (again), and measured the OT's primary. It came out to be about 4235 (average between 4,8,16 ohm). From what I've read and researched here, that seems a bit high for a 6L6 PP quad. I inquired Mercury Magnetics and they said their 5150 OT is just over 3K primary. If I were to order a new OT would it be beneficial to go with a lower primary- which seems to be normal compared to the stock 4K+ OT? If I did go with a lower one, say Mercury's 3K OT what would the effect on the tubes be?

[Wilder Amplification]

I've recently acquired a 5150 head (again), and measured the OT's primary. It came out to be about 4235 (average between 4,8,16 ohm). From what I've read and researched here, that seems a bit high for a 6L6 PP quad. I inquired Mercury Magnetics and they said their 5150 OT is just over 3K primary. If I were to order a new OT would it be beneficial to go with a lower primary- which seems to be normal compared to the stock 4K+ OT? If I did go with a lower one, say Mercury's 3K OT what would the effect on the tubes be?

What's the plate voltage in that amp?

[jutsin]

I didn't measure the plate voltage before I started tearing into it, but from what I've found it looks like it averages about 475

[DIY_Guy]

I didn't measure the plate voltage before I started tearing into it, but from what I've found it looks like it averages about 475

Mines at 476, so I'd say you are correct.

[Wilder Amplification]

At that plate voltage 4200 is a bit high. Running a pair at 450V the data sheet states a 5.6K Zp-p so 1/2 of that for 4 x 6L6 running push-pull would be 2.8K Z-pp, which is very close to the 3K that MM specs theirs at.

This may explain why 5150s are known for taking out screen resistors. I would definitely consider the 3K OT. With your current PT you actually have some room to mismatch down one if you wanted, like run an 8 ohm load on the 16 Ohm tap, which would reflect a 2.1K Zp-p, which is exactly what Mojo's replacement OT for 100 watt Fenders is spec'ed at.

[jutsin]

At that plate voltage 4200 is a bit high. Running a pair at 450V the data sheet states a 5.6K Zp-p so 1/2 of that for 4 x 6L6 running push-pull would be 2.8K Z-pp, which is very close to the 3K that MM specs theirs at.

This may explain why 5150s are known for taking out screen resistors. I would definitely consider the 3K OT. With your current PT you actually have some room to mismatch down one if you wanted, like run an 8 ohm load on the 16 Ohm tap, which would reflect a 2.1K Zp-p, which is exactly what Mojo's replacement OT for 100 watt Fenders is spec'ed at.

Thanks for the reply! So in a push pull class AB 6L6GC quad, as the plate voltage increases, do we want the primary impedance to raise with it, or lower?

[Wilder Amplification]

Thanks for the reply! So in a push pull class AB 6L6GC quad, as the plate voltage increases, do we want the primary impedance to raise with it, or lower?

In ANY push-pull circuit you'll wanna increase the primary impedance. It's just a question of how much it should be increased to keep the tubes from exceeding the max dissipation curve. However in your case you already have too high of a primary Z for 4 x 6L6s so you've got some room to drop it.