Standby switch squeal round 2

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Mpollard
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Standby switch squeal round 2

Post by Mpollard » Wed Aug 29, 2018 12:11 pm

I’ve read through the previous thread about the standby switch squeal. The last post was back in 2014. I’m just wondering if the new kits have implemented the 5.6k grid resistor fix. And just to be clear on how it’s implemented, does it replace the 470 Ohm resistor? My kit is in the mail and it should be here by the weekend.

danman
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Re: Standby switch squeal round 2

Post by danman » Wed Aug 29, 2018 7:15 pm

470 ohm resistors are the screen grid resistors attached to pin 4 (or bridged from 4 to 6) that are used for over current protection on the screens. It's smart to use anything from 470 to 1k here to protect the power tubes. 5.6k resistors are control grid resistors (sometimes called swamp resistors) mounted directly up against pin 5. They are used to squelch oscillation in the circuit and with high enough values, they can roll off a bit of high end. I usually go 10k in everything I build. They could be considered a cure for a squealing amp depending on where the oscillation begins. I'm afraid that I don't have any info on squealing caused by the standby though. Is this a new build your working on? If so, and the amp squeals when taken out of standby, have you tried reversing the OT primaries first?

Mpollard
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Re: Standby switch squeal round 2

Post by Mpollard » Thu Aug 30, 2018 12:12 am

I haven’t started building yet. Just preemptively reading about and preparing for things to go wrong.

Thanks for the advice!

danman
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Re: Standby switch squeal round 2

Post by danman » Thu Aug 30, 2018 6:49 pm

Gotcha. Most of the time when squealing occurs as the amp is flipped out of standby, its the negative feedback loop causing positive feedback in the circuit. The quick fix is to swap the OT wires that attach to pin 3 of each power tube. Always leave these a little long in a new build until you have confirmed that the amp works properly. Then you can shorten them up and attach permanently.

Mpollard
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Re: Standby switch squeal round 2

Post by Mpollard » Thu Aug 30, 2018 9:54 pm

Again, thank you. Very helpful info.

Mike

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neikeel
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Re: Standby switch squeal round 2

Post by neikeel » Fri Aug 31, 2018 1:52 pm

When you build I suggest leaving the plate wires (pin 3) a bit longer than needed.
- it is really annoying to have to splice in a new bit of wire when you have to reverse the primaries.

We are talking about two different things here.

However a quick way to see if the banshee howl (or sometimes just motorboating) is reversed primaries you unsolder one leg of the NFB resistor (it breaks the unwanted positive feedback loop and squeal goes away) the you know what the problem is and you can wire the primaries neatly and all is good.

BTW if you wire it neatly with good lead dress you do not need the 5k6 resistors on a stock JTM45.
Neil

Mpollard
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Re: Standby switch squeal round 2

Post by Mpollard » Fri Aug 31, 2018 2:40 pm

Thanks.

You guys are a wealth of information!

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