Does my JTM45 sound right?

[EthanB17]

My second amp build is a scratch build off of the Triode USA JTM45 layout.  I previously built the JTM45+ kit from tube depot with great success(wonderful sounding amp).  Back to the triode usa build, On initial startup it was very quiet. I quickly realized i had installed 470k screen grids vice 470's.  After swapping those, out the amp fired up with expected volume and it sounds pretty good too. So whats the issue? I'm not sure if there is one to be honest with you which is where my question is going to come from.  Once i roll either volume past 8 the amp begins to sound very harsh. very fuzzy and muddy(the normal channel is unbearable).  Up until eight the amount of overdrive is pretty linear and good and then, well just not good.  A part of me thinks this may be normal but i have nothing to compare too because i'm pretty new to these old school tube amps and my other JTM45(tube depot) is  quite different than the traditional kt66 layout that this one is based off of.  The tube depot sounds beautiful all the way to the top, this one gets nasty at 8. So is this normal or is something wrong.  I know this is a very newb topic but gotta start somewhere. Thanks in advance.

[Ritchie200]

Sounds like it might be a cold bias.  What do you have these at?

Jim

[EthanB17]

42ma (if i understand correctly that is about as hot as you should safely run them).  Just to verify that i'm doing this correctly,,, they are jj kt66's 27 watts a pop. 70% of that is 18.9.  power tube plate voltage is 442V. 18.9watts/442volts=42ma.  I've adjusted the bias pot to 42mv across the 1R resistor on both power tube sockets. This seems proper to me but i'm pretty new to this game so please correct me if i'm wrong.  One other thing, the data sheet for jj kt66 says 27 watts, however tube depot wrote in 28 as the bias point on the tube label. does bias point refer to the power rating? even if it was 28 watts, that would still call for 44ma vice 42ma(at 70% dissipation) which hardly seems like something that would make the amp sound like crap at high power. especially since it seems like folks typically run their tubes less than 70% anyways.

[sluckey]

Disconnect the NFB wire (purple wire on your layout) at the 27K resistor on the board. Does that help?

I'm not sure what the bias numbers that tubedepot uses means. But I suspect it means milliamps of cathode current.

[EthanB17]

Sluckey, that helped a lot! the overdrive of the amp is much more desirable sounding now(it actually sounded good) it seems to have more gain now though.  while it makes sense that it would have more gain without the feedback it seems to me like the NFB loop is part of the standard design why would mine sound like crap with it? thanks for the help btw!

[sluckey]

while it makes sense that it would have more gain without the feedback it seems to me like the NFB loop is part of the standard design why would mine sound like crap with it?

Because the phase of the feedback loop is wrong. The cure is to swap the OT primary plate leads at the tube socket. Then you'll be able to reconnect the feedback loop for a more refined sound. Give that a try and report back.

[EthanB17]

oh my god i knew it would be something simple, sadly the 6 month old just went to sleep so this will have to wait till morning light!

[EthanB17]

well then, i swapped the wires going from pin 3 to the OT.  fired it up sounded great. then i reconnected the feedback.  after reconnecting the feedback as soon as you throw the standby switch all hell breaks loose. It basically sounds like holding the tip of an instrument cable times a thousand.

[sluckey]

OK, that means the feedback polarity was correct to begin with, so swap the OT plate leads again.

[Tony Bones]

I think the amp is suffering from high frequency oscillations with the volume turned up. These things can be a bear to track down. A scope helps a lot. Usually they're caused by lead dress, but not always. Sometimes a bad component can exacerbate the problem. Even a microphonic capacitor.

The fact that the problem went away when the NFB lead was disconnected seems like a clue of some sort. Does the setting of the presence control affect the problem? Check the routing of that wire.

It's puzzling that the normal channel is worse than the bright channel.

[EthanB17]

Tony,
I'm still hesitant to call it a "problem". I state that because most of my research says that the old school 45's with kt66's were real "flubby" when cranked which seems to be what mine is doing... which of course is the source of my original question does my amp sound right? haha. it sounds awesome with the feedback wire removed however it starts to hiss when cranked. This makes me think that i could increase the size of the feedback resistor and shape that tone a bit. Thoughts? additionally my other 45 has the feedback running from the 4 ohm tap (but it it also has a 3-way selector) before going to the 27k feedback resistor. Does where it taps off the impedance selector affect the amount of feedback?

Thanks for all the help!!!

[Tony Bones]

I've owned several JTM45 style amps , including a 5F6 clone, and none of them had the characteristic that they sounded great up to x on the volume knob and then totally flubbed out above that. Some were tighter than others when cranked, but I don't remember any of them completely giving up at a certain point.

But yeah, try reducing the NFB. A 4 ohm tap will produce 1/2 the voltage of a 16 ohm tap, so 1/2 the NFB.

[EthanB17]

Okay,
so after researching various feedback circuit arrangements i decided to shift to the 8 ohm tap and install a 47k feedback resistor... completely different amp than before! it sounds amazing.  Hopefully i haven't committed a sin here because it's a lot less bluesy than before and way more plexi... if that makes sense.  This appears to be a great balance too because the presence knob seems to do a lot more(even compared to my other jtm45).  Anyone see any issues with this arrangement? thanks for all the tips

Ethan

[Ed_Chambley]

Okay,
so after researching various feedback circuit arrangements i decided to shift to the 8 ohm tap and install a 47k feedback resistor... completely different amp than before! it sounds amazing.  Hopefully i haven't committed a sin here because it's a lot less bluesy than before and way more plexi... if that makes sense.  This appears to be a great balance too because the presence knob seems to do a lot more(even compared to my other jtm45).  Anyone see any issues with this arrangement? thanks for all the tips

Ethan

Certainly fine to "adjust your NFB," I actually have a JTM45 KT66 with a 100K nut locking pot, but even with it turned to zero I still do not get mushy.  More noise, you bet ya, but quite nice.  As a matter of fact, I find tweaking the NFB one of my top priorities on any build.  Sure, there are NFB circuits considered standard, but all amps differ.  It is this characteristic that keeps me building.  It is exciting when I get a new tonal quality.

I have never tried any of Triode's layouts and even different layouts make differences.  Component tolerances come into play and how close the are together.  All amps have some parallel capacitance that will change things some.  If one has Carbon Comp Resistors and another Metal Film the drift in the CC's will give an amp a completely different character, but all these things are nice and inspire in different ways.

So go ahead and try some different things.  If you have 2 JTM 45's what would be the fun in both sounding the same?  For instance, you can increase the first plate resistor a little to add some hair to the notes.  There are a lot of small tweaks I make to my amps.  Sometimes I will simply just try something to see how it sounds.  It is just a good idea to understand and have a general idea of what will happen.  For instance, increasing the plate load resistance will add distortion and lowering bypass caps will reduce bass.