JTM50 Breakup

[Littlewyan]

My JTM50 breaks up with the volume on 2 with my Les Paul. Is this normal? I always thought it should be fairly clean up until 5. This is with the tone controls all set to 5.

Its basically a JTM45 but with 470K mixers and the NFB connected to the 8Ohm Tap with a 27K resistor. Its also got the additional filtering for the preamp that the later amps had. 100pF Bright Cap on the volume control as well which I've tried changing.

[pdf64]

Maybe a Bourns pot would be better? http://vintageamps.com/plexiboard/viewt ... r#p1046035

I keep meaning to buy a few to see for myself http://www.mouser.co.uk/ProductDetail/B ... 8Ep8Vec%3d

[Stevem]

Do you have a O scope to check for what's cipping first, the preamp section or the output stage?
What are your output tubes idling at, and what is the plate and screen voltage on them?
Double check the value of your plate and Cathode resistors in the preamp section, does removing a bypass cap help out any?

[Littlewyan]

I would quite like to upgrade the log pots to CTS or Bourns. Only issue is I need to expand the holes a bit. Shouldn't be too difficult if I get someone to hold a vacuum on the other side so no bits go into the chassis. The Alpha pots I have in there are OK but don't have a smooth taper at all! The volume jumps at various points is very noticeable.

Just checked the amp out with a scope. With a 100mV signal going into the High Treble Input 1 the amp breaks up at around 5.5 on the volume control. Maybe my guitar really does have quite a high output . The output stage was the first to clip, although the PI was clipping at the same time. Then again as this amp has NFB I guess thats expected.

I did notice some crossover distortion on the output stage so I'm going to check the bias to see if thats set ok and I was also planning on trying some different EL34s. The TAD EL34B-STRs are great valves but have a lot of bottom end. Probably not the best combo for a bass heavy amp like this! I have some JJ EL34 IIs to try in it.

[xtian]

I would quite like to upgrade the log pots to CTS or Bourns. Only issue is I need to expand the holes a bit. Shouldn't be too difficult if I get someone to hold a vacuum on the other side so no bits go into the chassis.

I create a dome out of painter's tape (use your thumb as a form!) and apply to the inside of the chassis to catch shavings.

[Stevem]

What I do to keep metal chips out of a wired chassis when I need to drill is to get some play dough , or clay and pack a cone of it on the inside of the chassis.
This way when I drill it catches all the chips and I just throw out the clay !

The JJs have a wider sound stage than the TADs and are smother without the harsh top when drivin hard.

[Roe]

NFB should probably be 27k @ 16 ohms instead, something which will improve clean headroom and tighten bass response

[pdf64]

I create a dome out of painter's tape (use your thumb as a form!) and apply to the inside of the chassis to catch shavings.

That's brilliant!
Note that the standard Alpha audio taper is 15%.
As the signal level increases above max clean output, the phase splitter outputs will be clipped by power tube grid conduction.
If the power tube g-k diodes weren't doing that, the PI is likely capable of much more clean output.
So the phase splitter isn't clipping as such, rather it is being clipped.

[Littlewyan]

All great ideas for catching metal shavings!

I did try the NFB once on that Roe and found it a bit clean. Might give it another go through.

Thanks PDF64, that explains a lot! I always wondered why the PI clipped at the same time as the power valves.

The Alphas are 15% taper which is ok but I think they are actually two linear tracks put together which is where they fall. Either 2 or 3 linear tracks anyway. I read an article on it once. I remember CTS and Bourns being more which is why they have a smoother taper.

[Littlewyan]

I've installed the JJ EL34 IIs and now the amp sounds great. So much nicer. It still breaks up at roughly the same point on the volume but it now cleans up better........Not sure what to make of that. The tone is much nicer though.

Anyway I shall be looking at getting CTS pots as they should make the controls a lot more usable.