"Matching" Power Transformer to Output Transformer to Circuit

[jonyoungyi]

I'm in the process of sourcing parts to build a Tweed Bluezmeister. A friend of mine is giving me a Power Transformer for next to nothing, but the secondary voltage is 15v to 20v higher than the "Mojo762". (See compared PT specs, below).

I know I have to match my Output Transformer impedance to the tubes I'm using (4.2k for the Bluezmeister's tubes), but do I simply just find an OT that has the required impedance and has the same Wattage as my PT? Is it that simple or are there other criteria used to "match" a PT, OT, and Circuit? (Excluding Resonant Frequencies, I guess, since I just want a functional amp?).

Along with "matching" the PT and OT, I'm not sure what changes need to be made to the circuit to accommodate the extra voltage. The Bluezmeister is solid state rectified and from what I remember, SS rectification has less "sag" than a Tube rectifier, so that means I would be running closer to higher than normal voltages than I would with a Tube rectifier due to the voltage sag, right?

Would I just need to change the voltage drop resistors to lower the B+ to the "recommended" B+ the circuit calls for?

Mojo762
PT: http://www.mojotone.com/transformers/Fender/Power/MT-FENDER-MOJO762.pdf
Sec: 354vac @ 250ma, 50vac, 6vac @ 4A, 5vac @ 4A

Used PT
PT: http://www.hammondmfg.com/pdf/EDB290YX.pdf (013895 Marking)
Sec: 372vct @ 400ma, 56v @ .10A, 6.3vct @ 4A

I've built several amps from hoffman and they all worked great, but I always stayed within the BOM and parts/order list, so this one is new to me =X

Any and all help is much appreciated, thank you!

[PRR]

> find an OT that has ....the same Wattage as my PT?

No. The PT must be a LOT more power than the amplifier audio output; losses everywhere.

IMHO, the Mojo762 at 250mA  is over-specced for any two-6L6 amp. The Bluezmeister idles at 23V/360r or 64mA, maybe 70mA with all the little tubes. It could use less PT.

The EDB290YX is _400_mA!! Over-over-size. But hey, it's free.

I would not over-think a 18V difference.

However the Bluezmeister uses a 5V bottle rectifier. The EDB290YX has no 5V winding. (And you have no need for the 50V winding.)

372V AC will rectify out to 520V DC peak, an awkward value since electrolytic caps only got to 450V maybe 500V. With a tube rectifier this will sag once the 6L6es warm up, maybe to 400V-450V.

Just for jollies, let us back-calculate what is a "good fit" for the EDB290YX. Expect say 500V 400mA of DC. This suggests a 2.5Kpp load. No single pair of any common tube will drive this, we need four power tubes. We can't hang four EL34 or four 6550/KT88 on the 4A heater winding, so gotta be 6L6es. Data. Bottom of page 2, we are near the 5.6K 55 Watt condition, except double. So four 6L6GC with 2.5K load will make well over 100 Watts. However the most available 100W OTs are 2K and 1.8K, while on the other hand our B+ promises to be higher than the suggested 450V. We can't just resistor-drop because the total DC current nearly *doubles* from idle to full roar. If the idle is well below 520V, the full-roar B+ will be way low, saggy. May be a fine amp. But a LOT of tonal risk (buying a 100W OT just to use-up a "free" PT, and maybe it all sucks).

Further the four 6L6 eat 3.6A of your 4A heater winding, so you can feed _one_ small bottle. The PI/driver, but no preamp.

I'm wondering what the EDB290YX may be good for, and not finding anything. It is a LOT of PT, with odd outputs.

[DummyLoad]

290yx is replacement for  75 watt fender   


2 x 6L6GC in UL.



--pete

[jonyoungyi]

Oh, man... I completely forgot about the voltage rating for the filter caps... Way to much voltage and I doubt I'll find 600v caps...

Looks like this PT is "stuck" between some common amp designs...

In theory, if I were to buy a 5v transformer and tube rectify, that would lower voltages to acceptable ranges, right?

Oh, and I assumed there was no need for a 5v winding since I see the BOM had a "solid state rectifier plug" line item, but the schematic does show a 5v winding now that I look at it..

Hmm... That over current you mentioned too...

[jonyoungyi]

Oh, and thank you for clarifying the PT and OT "matching"!

[tubenit]

In theory, if I were to buy a 5v transformer and tube rectify, that would lower voltages to acceptable ranges, right?

372v x 1.2  (5V4) = 446v         which are fine for 6L6/5881  .............. and JJ 6V6's would probably work fine also.

You probably would need to increase the dropping resistor on the B+ rail between nodes B and C.

With respect, Tubenit

[Tony Bones]

In theory, if I were to buy a 5v transformer and tube rectify, that would lower voltages to acceptable ranges, right?

According to the Hammond datasheet, the 290YX is 372V CT but but we're getting nomenclature mixed up. It's not 372-0-372, which is what you would use with a tube rectifier, but is actually 186-0-186. You can still get 372VAC out of it, but you need to use a bridge rectifier.

You could use a tube rectifier plus two silicon diodes to make a bridge. It's not a big deal, but just one more thing to consider.

[kagliostro]

.... is actually 186-0-186. You can still get 372VAC out of it, but you need to use a bridge rectifier

As told, one of the possible solutions (note that this way you don't lost the SAG effect due to the tube rectifier)





this alternative permit to use a Tube rectifier that is rated for enough current but that is underrated in voltage


the max admitted voltage of the tube is summed to the max admitted voltage of the diodes





Franco