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Post subject: Advice on setting a '65 TRRI up
Posted: Thu Jan 07, 2010 1:02 pm
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Hello, I have a '65 Twin Reverb Reissue that I bought used, I recently bought a bias probe too, and I thought I would check the bias myself. I set it up at approx. 40mA just to find out that the next tube (I biased checking the 1st tube on the right) was reading over 50mA! They are original Groove Tubes 6l6... poor matching? I checked the other tubes too and they read 40 and 50, so I have 2 tubes reading 40 and 2 tubes reading 50mA. Is there a way I can balance those tubes, like pairing them so that the first 2 tubes read 40 and the second pair reads 50? Does hum balance have something to do with that too? Sorry, I'm not an expert on these things, and there seems to be very little info on how to properly bias and generally setting up a Twin Reverb Reissue, for example the schematic does not even tell an approx value to bias it, whereas the Deville schematic, for example, says the right mA to set. Can somebody clear my doubts? Thanks!


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Posted: Thu Jan 07, 2010 5:52 pm
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That is a good question and I have been trying to find that out myself. I have searched all over the net trying to find bias instructions for my 65' reissue twin reverb with no success.
I also purchased a bias probe and biased the amp myself. I was lucky in a sense that all tube measure the same. I adjusted the bias to a little less than 70% of max plate dissipation. Here are a few links that might help. The first one is a video for a fender deluxe reissue but if you read the second paragraph it will give you some good values for twins if using JJ's. The second link is basically a calculator that will calculate what your bias should be given plate voltage readings.
http://www.eurotubes.com/euro-Fender-DeluxeV.htm
http://www.webervst.com/tubes1/calcbias.htm

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Posted: Sat Jan 09, 2010 5:56 pm
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Hi Camazza, the Twin Reverb operates two tubes together to amplify one part of the output signal, and other two tubes to amplify the other part of the output signal. This is a parallel push-pull design. So you can view it as: two tubes"pushing" and two tubes"pulling" the signal. If you view the amp from the rear, the two power tubes on the left are one team,and the two on the right are the other team. If you use a 40ma & a 50ma for each team, then the section would reflect 90ma per side, a balanced output section. If the plate voltage is fairly close for all the tubes, you're in good shape. So two 50's on the outside and two 40's on the inside should do it. Art

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