JTM50 NFB

[Littlewyan]

When comparing my JTM50 to some of the ones in Johan Segeborn's videos, I'm thinking that mine has a bit too much bass. The amps he has been playing have a tightist lowend even with the bass control up a bit. I have to keep mine on 0. Anyway my amp has a 27K FB Resistor on the 8Ohm Tap and I'm thinking that maybe it should go to the 16Ohm tap.

What do you guys think? Am I right in saying that it will just bring the bass a bit more under control and make the amp brighter?

I don't want to start lowering cathode caps etc because I know these amps can sound great stock.

[Tazin]

From what I've seen the JTM50's with the Drake 784-128 OT have the NFB from the 16 ohm tap on the Selector. The JTM50's with the Drake 784-139 OT have the NFB from the 8 ohm tap on the Selector.

[Littlewyan]

Ok thats worth knowing.

What is the exact effect of increasing the NFB though? I know the amp cleans up a bit but its only in the lower frequencies isn't it?

At the moment I can't have the bass above 0 and if I jumper the channels its just too much.

[Tazin]

Yeah, increasing the NFB voltage cleans up the amp a little. As to whether it's 'frequency related', that I do not know....Someone would have to do the math to determine that. It's easy enough to change the NFB wire at the Selector to a different tap so I suggest doing that and see what results you get. Sometimes, certain output valves (tubes) are more bass heavy than others so have you ruled them out as the possible cause?

[Littlewyan]

Well I believe it would make the high end more prominent as the presence control is dumping the high end nfb to ground.

I was using TAD EL34B-STRs which are very bassy and moved over to JJ EL34 II which was a big improvement.

One night I'm going to put a resistor in parallel with the 27K to bring it down to 18K (same as 27K on the 16Ohm tap) to see if I like it.

[Tazin]

What output transformer are you using?

[Littlewyan]

Classic Tone 40-18025

Their copy of a 784-139