Fender Princeton ----> Twin

[Chuck H]

For a time Kendrick (R.I.P. Gerald Weber) used to offer an OT for tweed Deluxe amps that was carefully interleaved and idealized in impedance to increase fidelity and volume. The trend for Deluxe amps now seems to be contrary to the effort but some players wanted the stage volume and dynamics. I'm sure the difference was small but probably real. It was a boutique unit being made by a local (to him) winder and, so, expensive as I recall. I don't know if Kendrick still offers it.

[glebert]

When he has the bass tone cranked the amp starts to flub out.

Oddly enough I was going to suggest a bass CUT in order get more clean (perceived perhaps) volume, but I went and reread the OP and this part of it makes me think that there is no way to get this amp to do what this owner wants it to.

[pdf64]

Was it ever confirmed that the OT made the difference?
If so, the "upgraded" OT might have lower impedance.

There's not been any update as yet.
I think the Allen OT has a 6k6 primary, so yes, that'll be contributing to the characteristics under discussion.

[Helmholtz]

I think the Allen OT has a 6k6 primary, so yes, that'll be contributing to the characteristics under discussion.

Using a loadline calculator I found that optimum plate load is ~8k.
6k6 passes above the knee, so means strongly increased Vsat and actually lower output.
I measured a Fender Princeton OT at 8k.

[pdf64]

Using a loadline calculator I found that optimum plate load is ~8k.
6k6 passes above the knee, so means strongly increased Vsat and actually lower output.
I measured a Fender Princeton OT at 8k.

Yes, it's just those Princeton OTs are such ratty little things, they seem much more insubstantial than eg the OT Marshall or Watkins used for equivalent EL84 amps; might that lead to lower efficiency?

[Chuck H]

Hammond 1608

It's a big thing compared to the Princeton OT so it would be a retrofit. But it's 8k, interleaved and VERY effecient. Having been designed for mid level hi fi. It's what I have in my own amp and I just couldn't ask for better fidelity or volume from a pair of el84's.

That said there's still the low capacity of the PT to contend with. Like I outlined earlier, I really don't see the point in trying to rebuild the amp as a performance machine. More trouble than it's worth and would destroy any vintage value.

[35L6]

If the power supply sagged would that 6k6 load line shift to the left and run through the knee ?

[pdf64]

If the power supply sagged would that 6k6 load line shift to the left and run through the knee ?

I think it works opposite to that.
As g2 voltage sags, all the g1 plots move down on the chart.
A loadline that's through the knee at idle voltages will be to the right of the knee when the g2 voltage sags.

So if it starts off right of the knee, sag makes it worse, audio power reduces, anode dissipation with signal increases.

[Helmholtz]

If the power supply sagged would that 6k6 load line shift to the left and run through the knee ?

All those 2x6V6 Fender amps sag a lot.
Using schematic voltages, output would be around 35W.
A sim with B+ = 350V and Vscreen = 315V still gives around 2W more output with 8k compared to 6.6k. Loadline way above the knee with 6.6k.

I like this calculator: https://www.vtadiy.com/loadline-calc...ge-calculator/

[Helmholtz]

Hammond 1608 ..

Acc.to spec it's a 10W OT. Not clear if this a performance limit only.
Interleaving does not increase efficiency.
From the resistances given in the datasheet I calculate copper losses to around 8%.
The Fender Princeton OT (022913 = 125A10B) has 12% copper losses. Not a huge difference.
There will also be core losses which are usually smaller than the copper losses with an OT.
From the looks I seems the Fender OT uses extra thin M6 grain-oriented laminations allowing to use a smaller core size compared to standard non-oriented M4.

My Super Champ uses the same OT as a Princeton and puts out 22W with a B+ of 413V (at idle, SS rectifier)..
I like the amp.

[35L6]

pdf64 , that makes sense , thanks for clearing that up . Helmholtz , I bookmarked that calculator .

[Chuck H]

Acc.to spec it's a 10W OT. Not clear if this a performance limit only.
Interleaving does not increase efficiency.
From the resistances given in the datasheet I calculate copper losses to around 8%.
The Fender Princeton OT (022913 = 125A10B) has 12% copper losses. Not a huge difference.
There will also be core losses which are usually smaller than the copper losses with an OT.
From the looks I seems the Fender OT uses extra thin M6 grain-oriented laminations allowing to use a smaller core size compared to standard non-oriented M4.

My Super Champ uses the same OT as a Princeton and puts out 22W with a B+ of 413V (at idle, SS rectifier)..
I like the amp.

FWIW I spoke to a tech at Hammond when I was reproducing my proto amp for Dean Markley. He said (paraphrasing) that you could use 16xx series OT's at half their rating for any guitar amp and they would be bullet proof. And this has indeed been my own experience.

Also FWIW, There was a member here named Shea that used the 1608 for a regional "18 watt" amp shootout in his area and won. This was the endorsement that inspired me to use it and I've never been sorry for the choice and I've recommended this OT at every applicable cirumstance ever since.

EDIT: Just to add a non technical observation about the 1608... I did build the same amp using a similar spec Heyboer OT. When I say "same amp" I mean the same PT, components and even tube brands. Full disclosure, the layout was a bit different. All I know about the Heyboer is that it was very slightly smaller and was also 8k primary. The Heyboer was a little "smoother" sounding. Not in a bad way but different from the Hammond 1608 which sounds/feels more dynamic which suits this design a little better (subjectively speaking).

[Chuck H]

All those 2x6V6 Fender amps sag a lot.
Using schematic voltages, output would be around 35W.
A sim with B+ = 350V and Vscreen = 315V still gives around 2W more output with 8k compared to 6.6k. Loadline way above the knee with 6.6k.

I like this calculator: https://www.vtadiy.com/loadline-calc...ge-calculator/

Fun calculator. Thanks for the link.

What I notice running this calculator with the parameters you did was that while output is greater with 8k vs 6.6k at nominal g1 output is actually greater with 6.6k at max g1. Of course this is not a proper quantification because of the sag you mention. Real voltages analagous to g1 drive would be significantly lower.

EDIT: And I also bookmarked the calculator

[pdf64]

Fun calculator. Thanks for the link.

What I notice running this calculator with the parameters you did was that while output is greater with 8k vs 6.6k at nominal g1 output is actually greater with 6.6k at max g1. Of course this is not a proper quantification because of the sag you mention. Real voltages analagous to g1 drive would be significantly lower.

EDIT: And I also bookmarked the calculator

Max g1 here requires linear operation in the positive grid region, ie class AB2.
AB1 power is the g1 = 0 figure.

[Helmholtz]

What I notice running this calculator with the parameters you did was that while output is greater with 8k vs 6.6k at nominal g1 output is actually greater with 6.6k at max g1.

That "Output Power at max g1" makes no sense with guitar amps, where no significant positive grid drive is possible.

Even more confusing is that the uppermost plate curve often belongs to a positive grid bias and one needs to identify the relevant Vgk = 0 curve by using the pointer.
I coudn't figure out what makes the calculator choose the max. positive grid bias.

For "Load" one needs to enter the Raa with PP amps. The slope of the loadline then correctly corresponds to Raa/4 in class B and Raa/2 in class A.

Unfortunately all former settings are lost when changing the tube type.

That "Output Power at class A" is a useful feature when analyzing a cathode biased amp for operating class.