snap crackle pop JTM45

[chrisrbr]

Hi all,
long time since last post--been enjoying my single ended EL34 reverb I posted here a few years ago.  anyways I just completed a JTM45,sound is wonderful.  but has a crackling like noise that is loud enough to be a concern. I measured all the voltages and they seem ok along with the bias, but have a question about V3
V3:
pin 1 = 259
pin 2 = 27
pin 3 = 42
pin 6 = 241
pin 7 = 28
pin 8 = 42
not sure what pins 2 and 7 should be for VDC.
next I measured the DC voltage coming off of the tonestack caps, and found it to be wandering a bit, (measured between cap and tone control)
250pf = 8.5vdc +/- .1
0.02(bass) = 1.7 +/-0.05
0.02 (mid) = 0.6 +/-0.05

are those voltages typical, and should they me varying?

thanks for any ideas
Chris

[Willabe]

The tone stack caps should block DC. I think 1 or more are leaking. Lift/unsolder the far end of the caps that go to the tone pots and measure for DC. Should be 0v.

Looks like the 250pF treb. cap is bad.

       
          Brad       

[rzenc]

If V3 is a P.I., then it's voltages are ok... which schematic are you using?

Here is Fender 5F6A schem, check it's P.I. voltages against yours. Also, check the NOTICE warning on upper right corner: Voltages read [...] values shown +/- 20%...

http://www.el34world.com/charts/Schematics/files/fender/bassman_5f6a_schem.pdf

Regarding reading DC voltages on caps, unsolder them from tonestack pots and check "flying lead" relative to ground.


Edit: If tonestack caps read ok, check P.I. input cap for DC leakage.

Hope this helps.
Best Regards,

Rzenc

 

[chrisrbr]

Brad - you rock!  thanks so much.  amp is dead quiet now
you know I'm such an idiot, I had unsoldered one end of that 250pf cap and measured its capacitance, which checked out ok, measured DC resistance -over a meg. So I resoldered and continued to troubleshoot.   Should have been more obvious when I saw voltage leaking through to just swap it.  amazing how as $0.35 part can cause you to loose sleep at night.

thanks again for your help---off to a job tonight with my new amp!
Chris

[chrisrbr]

oops forgot to thank rzenc too

[Willabe]

Just glad to help. I've learned --- WAY --- more here than I've been able to give back.

And I think your being a little hard on your self, don't worry , just come on in and grab a hold and learn with the rest of us.        

Hey, that should be the chours in a song.     

BTW, how's she sound and got any pic's of the new build for us???   


            Brad         
         

[chrisrbr]

Hey Brad, and everyone
here's some pics.  she sounded great, it was the first time in 30 years of playing I actually used an amp for the distortion rather than my pedal board.  much more touch sensitive and actually had a use for the guitar volume knob.  the only issue is that with the gain of the amp up fairly high, say around 7 out of 10, I didn't have enough headroom for solos.  I use the old tried and true tube screamer for a boost, but just got a wee bit more volume.
So, I installed a second PPIMV.   
Amp info first would help:  it started as a stock JTM45, then because I just can't help myself I added a few "options" - it's just too easy and cheap when you do it yourself.
1. rear panel switch to change between the standard JTM45 shared cathode to a Plexi split cathode on V1, plus change from a 0.02 to the plexi 0.002uf coupling cap
2. rear panel switch to change the NFB from 27k/16ohm tap to 47k/8ohm tap.
3. front panel bass pull switch to add a 0.68uf cap across the 820ohm on V2
4. front panel vol 1 pull switch for a bright cap
5. front panel vol 2 pull switch to internally jumper channels 1+2.
6. added a PPIMV next to the Presence pot----worked Great!  a must for any venue I play
think that hits all the mods I did before last weekends job---dead quiet amp.
Note on the pics, I don't subscribe to the theory of bundling all wires together.  I prefer to keep a the leads a short as possible and away from each other - especially for V1 and V2.

So back to my need to add a second PPIMV for a volume boost for solos.  to do this I got rid of the presence knob--never used anyways.
Then I added a DPDT relay to switch between the two PPIMV's.
Works great, but when I turn the PPIMV up to say 6-7 out of 10 I get a high pitched squeal, was not there with one PPIMV.  Granted I will never play the amp that loud, but would like to have it behave as before.

Here's where I could use some help.  My guess is the signals are either capacitively coupling in the relay or cross talking.  Perhaps two SPDT would provide better isolation?  or a beefier relay with better isolation?
any ideas???
thanks
Chris

[Willabe]

Looks real nice.

Lizzy? as in Thin Lizzy?

Loved that band, say them twice at the Aragon Ballroom in Chgo. in the mid 70's.


            Brad      

[chrisrbr]

Lizzy is my daughter's nickname.     never realized the band thing until now