Metropoulos Forum

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.

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.

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.

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?

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.

What output transformer are you using?

Classic Tone 40-18025

Their copy of a 784-139