5E8A based scratch build OT question?

[Dingleberry]

Hi and happy new year.
I'm planning to build a 5E8A-type head using mostly the parts I have on hand. I have Drake 783-257/7 OT which is from Canadian market 2204 Marshall. My question is about the primary Z of the OT. It's kind of in the low side. I calculated it few different times and got same results. ~2k56.
Usually 50W Marshalls have 3k4. Can it really be that low?
5E8A twin has 4k. The drake has only two secondary taps, 8 & 4 ohm.
If I use the existing taps as 8 & 16 ohm taps then I have ~5k primary Z, which will probably be ok for 2x6L6's, little higher than stock, but close enough to boogie.
If that transformer sounds usable to you, I then might just go ahead and try it. Then I could have an option to run it with EL34's also.
But that 2k56 just sounds very low to me. Could it be right?

-T

[Phil_S]

Check the OT for symmetry. Windings are not perfectly symmetrical because the outer winding requires more length than the inner winding for the same number of turns. Nevertheless, it's not a very lopsided thing.

The ohms from each primary to CT should be about the same. The secondary will be harder to measure because it is probably under 1 ohm. If you have 4 and 8 taps. 0-4 is about 2/3 of it and 4-8 is about 1/3.

You can also run low ACV into the secondary and check voltage on the primary, where the CT should be smack in the middle. If it checks out proportionately, the OT is probably good and then, it is what it is. If it turns out lopsided, then maybe part of the primary is shorted?

[Dingleberry]

I measured the Primaries and secondaries with Fluke 87-IV and primary symmetry/balance seems totally acceptable to me.
Resistance of the winding is just that low.

Primary:
CT - a1 ~ 46 ohms
CT - a2 ~ 48 ohms
a1 - a2 ~ 94 ohms

Secondary:
0 - 4 ~ 0,75 ohms
4 - 8 ~ 0,49 ohms
0 - 8 ~ 1,24 ohms

I injected 500mV into secondary and got following results:

500mV into 4 ohm tap : 12,63V from primary a1 - a2.
That gives 638,0676 : 1 impedance ratio = 2,552kohm : 4 ohm

500mV into 8 ohm tap : 9,11V from primary a1 - a2
That gives 331,9687 : 1 impedance ratio = 2,655

Looks like to me that the transformer is ok and just has that low primary impedance.

Any opinions?

-T

[Alexo]

Honestly, that seems like a fine load for EL34's to me, not knowing the plate or screen voltages at least. I always found the mythical 3K3 primary Z on EL34 Marshalls to be almost unacceptably high, at least when the screens are at or near 100% of plate voltages.

[Phil_S]

Looks like to me that the transformer is ok and just has that low primary impedance.

Any opinions?
-T

I don't know that much, but I think the findings suggest the OT has a lower primary z than you expected. The Ohms certainly fall into what one should expect with respect to symmetry and distribution.

[Dingleberry]

I was expecting a little higher primary Z, but it is what it is. No worries;) I'll just go ahead and try. I can use 6L6 and KT66 power tubes just plugging 8ohm cab into 4ohm output. They should be fine with ~5k primary Z. And I've also heard that 5E8A sounds great with EL34's too. It'll probably be a nice platform to experiment. Thank you for confirming my suspicious thoughts.
-T