Fender Blues Junior Harp mods

[Watto]

Does anyone know of some harp mods to lower feedback on these amps.
If you change the input load resistor to 5M like in say the bassman ,is this going to work.I've got the schematic and I'm guessing the resistor in question is R2.

I'm new at this and was wondering if I'm on the right track

Cheers
Mark

[MWJB]

Increasing the input load won't really reduce feedback, it will improve sensitivity.

There are 2 Blues Jr schematics, pre '01 and Mar '01 onwards. Many resistor cap numbers are completely different from one to the other. So if your amp is pre '01, then yes R2 is the input load.

However, these amps have a TON of gain so I'm not sure that it's the best basis for a harp amp. That said, you could try snipping the bright cap off the volume control (C2), if you can live without the fat switch try snipping C4, reduce the size of the feedback resistor (R27-say add 47K in parallel?), go a little larger at R13 (add 22k in series)?

Of course there are also preamp tube subs to try & rebiasing?

All these are easily reversible if things don't work out.

[Watto]

Thanks for that MWJB

I'll give them a go and let you know the results.

Cheers
Watto;)

[dsouder]

Simple mod is replace 12ax7 with 12at7 or 12ay7. This will reduce the gain and help with feedback a bit. Some like to drop the preamp B+ to around 90-100v by increasing the resister between the last 2 filter caps. Larger bypass and coupling caps for more bass also helps for harp.

[arnenym]

I cut off the c1-cap and change r1 to 100k.
That leaks out the signal and increased gain a bit.
Change speaker to a harpfriendly speaker. A Jensen or Weber Alnico.
Then you do some tubechanging experiments.