Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dude on April 07, 2020, 12:52:26 pm
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I know this issue has been discussed numerous times here, I've searched and good info but didn't find my answer. In a Marshall 1987 the feedback resistor is 47K with (100K) as an option I assume , nothing else on the schematic here about the 100K.
I was using the Hammond AO-43 OT in one of my 6V6 Plexi's with 47K FB resistor, but found a bigger 18 watt OT (8K) that I had marked "bad" years ago, it wasn't bad. Anyway, I installed the bigger 18 watt OT as another 6V6 Plexi I build has an 18 watt OT and sounds great. For some reason this newly installed OT doesn't sound as good as the other 18 watt OT. They look the same but are obviously different manufactures, both have 4, 8, 16 taps. The better sounding 18 watt OT has the 47K FB on the 8 ohm tap, same with the not as good sounding OT but it doesn't sound as good, seems turning Presence down past half starts to produce a muffled tone, like a high ohm p/u in the neck position of a start. If put the 47K on the 16 ohm tap wouldn't I get more negative feedback and the Presence wouldn't cut the highs as much. I believe from what I read this would be the case...?
EDIT: It's the strat's pick up causing the bassy tone on the neck, was using a different guitar that I though had a low ohm neck p/u, it's close to 7.6 ohms. I'm losing it in this lock down.
Thanks,
al
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The first thing you MUST do when installing an unknown OT in an amp with NFB is determine if the feedback phase is correct. The easiest way to do this is to swap the OT plate leads. If things improve, then you probably had the OT plate leads connected backwards originally. If things get worse, then the phase was probably correct originally. If so, put them back like they were.
Ever hear this before?
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Sluckey thanks for the reply. This is a known OT, l had asked a while back about a breadboard to check an OT out of an amp, this is that OT. You gave me instructions and the OT that l thought was bad was not, back ten years ago l had the squeal with this OT but didn't know about phase, switching plates. When I did hook up this OT outside the amp, big squeal, changed the plate wires, bingo, played. So, l install this known 8K impedance 18 watt OT in one of my 6v6 Plexi's. That's when I notice the issue of the Neg FB (above). But l was using a different strat with a overwound neck P/U, with other guitar's Neg FB didn't act this way, this lockdown is getting to me :sad2: . Guess l should delete this tread, no issues.
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Sluckey thanks for the reply. This is a known OT...
Don't nitpick my words! Maybe you know this OT but your amp doesn't. YOU MUST VERIFY THAT THE NFB PHASE IS CORRECT!
It's a 50/50 chance. No way can you know without testing it.
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Steve, not nicpicking your advice. This is a misunderstanding. I known the plates of an OT installed need to be checked in every amp, 50/50 shot, you told me that years ago. I may have made it seem that this OT was unknown. It was an OT for an 18 watt but like you taught me, every amp is different even the same amps and it's a 50/50 shot each time you hook up the plates.
I'm just saying l didn't know this ten years ago, now I do and learned from it. If it wasn't for your help here l probably would never have been as far as l am, l appreciate ever reply.