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Post subject: Pulling output tubes on silverface Twin Reverb?
Posted: Sat Mar 21, 2009 11:22 pm
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Hi all:

Is it possible to pull one or more output tubes on a '70s silverface Twin Reverb, to decrease output power? Reckon I should ask before experimenting. Thanks!


Last edited by gwailoh on Sun Mar 22, 2009 6:37 am, edited 1 time in total.

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Posted: Sun Mar 22, 2009 12:40 am
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If you pull power tubes in a Twin you have to pull in pairs. Pull either the outside 2 or the inside 2, xOOx or OxxO. I pulled the outside pair once. There is no big audible difference pulling 6L6s, the volume decrease is only -3 dB. If your amp started to get too loud with 4 tubes and the volume on 6, with 2 tubes it will start to get too loud with the volume on 5 1/2. If you want a quieter amp you can pull all 4 6L6s and put in 2 TAD ToneBones. I have only 2 pentode ToneBones in my '79 UL Twin. The output dropped from 135 Watts to about 22 Watts. That is a very noticable difference. The ToneBones with EL84s really chime in my amp. With 4 power tubes in the amp the output transformer in the '79 Twin looks for a 4 Ω load. The speakers are 8 Ω each, wired parallel to create a 4 Ω load. With only 2 tubes, the transformer is looking for an 8 Ω load. You can disconnect one of your 8 Ω speakers to match the 8 Ω that the transformer will seek. Normally, Fender transformers are built to withstand that load difference. But, just to play it safe, I bought a Z-Matcher to match the impedence differences.

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Posted: Tue Mar 24, 2009 2:04 pm
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Last edited by tinker tech on Wed Jul 29, 2009 10:39 am, edited 1 time in total.

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Posted: Tue Mar 24, 2009 11:46 pm
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Tinker - I can't give you a technical answer there. I've learned from several amp forums that when 2 of 4 power tubes are pulled, the output tranny looks for twice the resistance as with 4 tubes. Maybe a Techy could chime in here and explain.

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