Fender Twin Reverb UL 135watts

Carlosraj

Member
2010-01-17 11:12 am
Hi


I have an old Fender Twin Reverb Ultralinear 135watts(1981) amp with a blown output transformer. I have ordered a replacement from Hammond, model No: 1750WR. It is stated that it is a replacement for Fender's 013691 output transformer. This 1750WR transformer has a primary impedance of 2000 CT Ohms. Will this impedance match 4 of 6L6GC tubes. I do not understand how the output impedance of a output transformer is used to match the load impedance of the tubes. I have attached the datasheet for this transformer and 6L6GC tube. Please assist.


Thanks


Carlos
 

Attachments

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I have an old Fender Twin Reverb Ultralinear 135watts(1981) amp with a blown output transformer. I have ordered a replacement from Hammond, model No: 1750WR. It is stated that it is a replacement for Fender's 013691 output transformer. This 1750WR transformer has a primary impedance of 2000 CT Ohms. Will this impedance match 4 of 6L6GC tubes. I do not understand how the output impedance of a output transformer is used to match the load impedance of the tubes.
Twin Reverb Ultralinear 135W usas 4 6L6 tubes in push pull parallel (2 tubes per side) and expects tp "see" a 2000 ohm plate to plate lload.

The Output transformer does not have impedance by itself but converts speaker impedance to that needed for the tubes.

So IF you connect an 8 ohm speaker to 8 ohm secondary tap (from black to green) or a 4 ohm speaker (from black to grn/yel), you will have 2000 ohms plate to plate (brown toblue).

No need to dig deeper, that´s enough data to repair that amplifier.

Hammond is excellent quality iron so it will work fine.
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
...two pairs of those 6L6-GC's I'd expect half the datasheet's plate-to-plate load value...

Many combinations of V and Z will make the same power.

Fender could estimate or experiment his own answers.

The 4k/pair loading seems to come from 6L6 AB2 condition (3,800) where Vg2 is held down to suit original 6L6 spec and Z is dropped to suck the current. The 5881 had higher Vg2 rating and could pull 4k/pair in AB1 for about 40 Watts (Bassman 5F6a). The 6L6GC has even higher rating for P and G2, and you can do 65 Watts per pair in about 4k, and even in UltraLinear.

You can build a marketable radio or amplifier on the datashet suggestions. Building a market-beating amplifier may need a little thought and much smoke-testing.