Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: goldstache on April 11, 2016, 05:59:19 pm

Title: Strange Parisitics in my power section
Post by: goldstache on April 11, 2016, 05:59:19 pm

Ive been combing through an amp I built up. 
Topology:

V1A (preamp) > EQ > Gain control > V1B > PI > PPIMV > PP6L6GC's cathode biased.  Heaters DC floated from Cathode resistor voltage to CT of Filaments.

Here is the rub, when gain control is zero'd out, I still get guitar signal when i turn up the PPIMV.  I hooked up a signal generator running a 1k 500mv signal to the input and traced all the way to the grids of the 6L6's.  There is no AC when the Gain control is at zero.  In fact there is no AC at any point after the gain control all the way to the power tubes.  In this configuration the PPIMV when not on zero I hear the signal generator despite the interstage gain control being at Zero and measuring 0VAC from the gain control to the grids of the power tubes. 

What Gives????????????  Bonafide head scratcher

Title: Re: Strange Parisitics in my power section
Post by: goldstache on April 11, 2016, 06:02:19 pm

Im also using a JJ 40/20/20/20 can for filtering and have a local star ground scheme.  I think this is the reason that the amp has some bad return current noise.  But, this isn't just 60cycle or noise.  It's full on guitar signal. 

Title: Re: Strange Parisitics in my power section
Post by: shooter on April 11, 2016, 06:33:50 pm

Quote

measuring 0VAC

you measuring with a scope or meter?

might try *walking* your meter on every ground.  I clip one lead to the main ground lug from the power cord, then ohms mode probe every ground.  ***make sure you're unplugged and discharged***

Title: Re: Strange Parisitics in my power section
Post by: goldstache on April 11, 2016, 07:42:25 pm

Tried that.   .7 ohms on each ground node.

Thanks for chiming in!

Title: Re: Strange Parisitics in my power section
Post by: jojokeo on April 11, 2016, 11:06:02 pm

Amp in a bathtub? Looking at your avatar... I remember this happening once and I was befuddled to put it mildly. If your Gain pot is making a good ground this shouldn't happen right? But if my memory serves the amp I'm referring to had a pentode in V1 with a bypassed load resistor. I removed the capacitor (either a .001 or 500p) and all went to normal. This was a strange one for me. These are a bit hard without the amp in front to look around at things or schem. Technically this isn't a parasitic rather an oddity issue. Do you get any weird ghost buzz or mosquito sound riding the trail of notes, lack of sustain on certain ones up on the neck, something sounding like an amp cabinet rattle, etc...? These are symptoms of parasitic oscillations.

Title: Re: Strange Parisitics in my power section
Post by: goldstache on April 12, 2016, 07:00:30 am

This is the 4th amp I've built with this schematic.  All DC voltages are on spec, it's the AC characteristics that are strange.  Still probing.

Title: Re: Strange Parisitics in my power section
Post by: sluckey on April 12, 2016, 07:25:58 am

Something must be different with this build?

I would try replacing the cap can with individual caps. Ground the plate and screen caps at the power ground. Ground the preamp caps near the input jack. Does this help?