2.5k vs 3.5k primary on output transformer

[hembrook]

The tube sheets for the 2A3 and 6B4G and 6A5G all list a load of 2.5k. I notice that many of the folks here use the James transformer with 3.5k primaries. Is this because of availability, or is there a benefit to the slightly higher impedance? You loses a bit of power, bvut I am not worried there.

I am ordering some custom wound EI output trannies, so I could spec out either 2.5k or 3.5k (or even 5k) plate loads, so I was wondering.

Thanks

Robert

[Shannon Parks]

The James outputs I used were due to what was available at the time - 3.5k/5k primaries. Also, the original Simple 2A3/JE Labs design used 3.5k. So for those two reasons I went with 3.5k. I would like to experiment eventually with a 2.5k primary.

[hembrook]

I am ordering 4 sets of the 2.5k ones (to match my 6A5G tubes, as well as a some 45s). I will let you borrow a set for a time if you like.

So I will spec the 2.5k.

Thanks

[EWBrown]

2.5K will give more power (around 3.7W) at a slight increase in THD, 3.5K slitghtly less power (around 3W) and THD, etc, and even 5K works well, with around 2.5 watts out at around 3% THD.

The James trannies come in 2.5 / 3.5K and 3.5 / 5K versions.

I've heard good reports of using the Magnequest RH-60 OPTs (5K, 60 mA) with 2A3s and 6B4Gs (and 6A3s and 6A5Gs should be the same, as well).


I'll run some 6B4G or 2A3 numbers with SET CAD and the G*S*G design, and post the results later.

/ed B in NH

[hembrook]

I think I should be pretty safe with 2.5k, but it is always interesting to see what is out there. I have not run sims on this stuff at all, maybe I should learn how. So many projects, so little time.

[EWBrown]

I did a quick SET CAD run on the GSG, using 6B4G, B+ = 320V, RK = 900 ohms, IP = ~ 50 mA (varies slightly with different OPTs)


PO = Power Output (into 8 ohms); 2nd and 3rd refer to harmonic content.


James 6112HS 2.5K: PO = 3.48W; 2nd = 6.8%; 3rd = 0.8%

James 6112HS 3.5K: PO = 3.05W; 2nd = 4.7%; 3rd = 0.2%

James 6113HS 3.5K: PO = 3.01W; 2nd = 4.7%; 3rd = 0.2%

James 6113HS 5.0K: PO = 2.34W; 2nd = 3.3%; 3rd = 0.1%

James 6115HS 3.5K: PO = 3.01W; 2nd = 4.7%; 3rd = 0.2%

James 6115HS 5.0K: PO = 2.34W; 2nd = 3.3%; 3rd = 0.1%



MagneQuest RH-60: 5.0K PO = 2.53W; 2nd = 3.4%; 3rd = 0.1%

Hammond 125ESE 2.5K: PO = 3.80W; 2nd = 7.1%; 3rd = 0.8%

Hammond 125ESE 5.0K: PO = 2.73W; 2nd = 3.4%; 3rd = 0.1%

Hammond 125ESE 10 K: PO = 1.64W; 2nd = 2.2%; 3rd = 0.3%

DARED (Chinese) 2.5K: PO = 3.49W; 2nd = 6.7%; 3rd = 0.7%

Transcendar 10 3K: PO = 3.01W; 2nd = 5.1%; 3rd = 0.3%

Transcendar 15 3K: PO = 3.04W; 2nd = 5.2%; 3rd = 0.4%

HTH

/ed B in NH

[hembrook]

Seems to be an OK trade to me. I guess listening would be the ultimate objective. 2.5k it is.

[EWBrown]

The nice thing with the James 6112HS is that you can choose either 2.5 or 3.5K. I suppose a SPDT switch could be used, but don't hot-switch it during operation, as this can kill the tube and the inductive "recoil" can fry the OPT primary.

/ed B in NH