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Old 17th August 2012, 04:52 AM   #11
cancon is offline cancon  Canada
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After cutting the plate wires I still measure the same resistance across the primary taps.
I measured zero continuity between the plate sockets and chassis.

Does the circuit give you any indication of the required OT for replacement? I know the secondary is 8 ohm, so i'm thinking 15W Tweed deluxe, 8000ohm primary, or 20W Deluxe with 6600 ohm primary. go 20W to be safe?
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Old 17th August 2012, 05:39 AM   #12
Enzo is offline Enzo  United States
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Take your pick. ANy OT made for a single pair of 6V6, and an output winding to suit your speaker should work.
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Old 17th August 2012, 12:51 PM   #13
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If you liked the tone of the amp (or what you got to hear of it) try to find an OPT that is about the same physical size and weight as the original. Much of the amps "tone" is determined by how hard the original OPT was pushed toward (or into) saturation.

Fender tended toward undersized OPT's that were near saturation. This often leaded to disastrous results is someone played bass through say a Bandmaster at full crank for a while.

Marshall tended toward oversised OPT's and excess preamp gain for the classic screaming sound.
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Old 19th August 2012, 11:21 PM   #14
cancon is offline cancon  Canada
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Thanks for the excellent points.

What do you guys think about the excessive hiss that is produced even though nothing is plugged in? The hiss goes away if the volumes are off, or it is greatly reduced if the tone knob is completely rolled off. I noticed one of the plate resistors is way off (stock is 250k, one is reading 500k+), in general should I be replacing the caps and resistors around the preamp tube to help eliminate the noise?
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Old 20th August 2012, 01:18 PM   #15
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The two main causes of hiss are resistors and tubes, in that order. It is also possible that his can be caused by an oscillation, which could be due to a bad capacitor or sloppy wiring.

A resistor measuring twice its marked value should be replaced. I would replace the plate load resistors first, and any others that carry current.
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Old 21st August 2012, 08:00 AM   #16
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Quote:
Originally Posted by cancon View Post
I noticed one of the plate resistors is way off (stock is 250k, one is reading 500k+), in general should I be replacing the caps and resistors around the preamp tube to help eliminate the noise?
By FAR the most common fault in valve amps are the anode loads of the triodes - they go high and eventually O/C. One reading twice it's value is 100% faulty and should be replaced ASAP - I'd also advise replacing the others while you're doing, as it's likely they are faulty (or on their way to faulty) as well. Such faulty resistors are a prime source of noise!!.

Use decent quality resistors, 1W metal film would be a good idea, and should prevent them ever failing again.

Historically capacitor problems were mostly leaky coupling capacitors, making the grid of the next valve positive - but anything remotely 'modern' shouldn't suffer from that.
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