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Old 20th June 2011, 09:21 PM   #1
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Default Fender Twin low power and distortion

Hi all,
I,ve checked everything but the OT. Everything else seems normal exept the power and the sound (no pounch and farty distortion) Power supply is fine so is the bias all voltages within range even under full signal with 1khz sine wave. Traced the PI 120pk to pk on each side....the pre amp seems fine.(both channels) Power tested it....barely 50v pk to pk into a 12 ohm load! this is a 100w amp! Even less with 2 tubes pulled. Am I missing something or is my OT toast. What the best way to test the OT?
Thanx.
Mario.
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Old 20th June 2011, 09:27 PM   #2
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Depending on the age of your Twin, the old black face units had a set 4 ohm output impedance. Your 50v pk to pk would translate into 78 watts if a proper 4 ohm load were being used. Those units were rated at 85 watts originally, so given any one of a number of variables, 78 watts would be well in the ballpark. Either that, or I'm simply missing something here.....

Dave
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Old 20th June 2011, 09:39 PM   #3
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This is a 1976 100 watt model. The 50v pk to pk was into a 12 ohm load so isnt that 26.5 watts . I can play the thing on ten sitting next to it. It sounds bad too. My Bandmaster 45watt sounds a lot louder. The bandmaster put out about the same into the same load(12 ohm about 26w fine forr a bandmaster)
Thanx for your reply
Mario.
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Old 23rd June 2011, 04:05 AM   #4
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output transformer is probably bad. coupling caps is the second suspect.

if you need a full schematics look at this:

Amp Guide Fender*Twin Reverb (Silverface)
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Old 23rd June 2011, 04:15 PM   #5
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by mario13 View Post
Hi all,
I,ve checked everything but the OT. Everything else seems normal exept the power and the sound (no pounch and farty distortion) Power supply is fine so is the bias all voltages within range even under full signal with 1khz sine wave. Traced the PI 120pk to pk on each side....the pre amp seems fine.(both channels) Power tested it....barely 50v pk to pk into a 12 ohm load! this is a 100w amp! Even less with 2 tubes pulled. Am I missing something or is my OT toast. What the best way to test the OT?
Thanx.
Mario.
How did you get a 12 ohm load? Don't say you placed a 4 and 8 ohm speaker in series. Also how is this 12 ohm load connected to the output transformer? Don't say you are using the 4 ohm secondary. If you've done both of those things I'm not surprised at the result.
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Old 23rd June 2011, 05:00 PM   #6
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this may seem ludicrous, but just in case - check all the pots and switches if you haven't. I had a bum treble pot on an old Pro Reverb that totally sounded like something major had gone wrong and had led me on a wild goose chase to see if an output tube had fallen out and checking cables - turns out it was just a dirty pot, and wiggling it brought it back to life! Also had a bright switch somehow causing a similar problem.

probably not your issue, but pots are so much cheaper than transformers that it would be worth the doublechecking, in my opinion.
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Old 24th June 2011, 05:53 PM   #7
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Mario -- Your output transformer may in deed be bad, but regardless, using a 12 ohm load will never let you test the unit for proper performance. Using a 12 ohm load on this unit will only let it develop about 33 watts before clipping sets in, based on a 100 watt rating.

Also, do not let the "100 watt" rating fool you. The original black face Dual Showman uses the exact same power and output transformers by part number, and is only rated at 85 watts. In fact, that is a very accurate rating regarding what these units will actually produce. CBS in all of its infinite wisdom decided to stretch things a bit and called it a 100 watt unit. Call it what you will, the original transformer set in your unit with RCA 6L6GCs will only produce in fact about 85 watts RMS. Based on that rating then, driven into a 12 ohm load, a properly operating "Fender" blackface Twin will produce less than 30 watts RMS, or less power than a Bandmaster will operating into a proper 4 ohm load.

Dave
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Old 26th June 2011, 09:07 AM   #8
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Thanx for the input guys. I,m new to using a scope and making measurements so I didnt know what to expect. I knew the amp would put out less with the 12ohm but didnnt know what. I only used this load because it was the highest powered resistor i had and man did it get hot. I think the speakers are at fault here there old and farty. After giving it a full service (caps, screen resistors, moding the bias, etc) i pluged it into my fender 2x15 and it sounded amazing. now it sounds like a proper twin.
Regards
Mario.
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