Pumpkin Preamp - Perfect for F4

[Zen Mod]

ground heatsinks , just for test

 

[ZUM911]

I've tried grounding the heatsinks with the same result. Probing around I see the distortion is on the the output side of the 5pF caps but not on the input side.

 

[Zen Mod]

increase those 5p to , say, 33p , just for test

you can use whatever you have in drawer - cheap ceramic ones

just solder 33p across existing ones

what's scope with non-powered ?

 

[ZUM911]

Thanks ZM. I had 33pF mica caps and put them on one channel. It seems to have increased the frequency of the oscillations I see by about 5 times. The amplitude stays the same. The scope shows a straight line with the Pumpy off.

 

[Bonsai]

Gate stoppers in place?

 

[ZUM911]

Thanks Bonsai,

Everything as per cookbook, gate stoppers are all 220 Ohm

 

[Zen Mod]

to determine which side ( upper or lower) is responsible for oscillation , make this simple test -

take 220R with plastic tweezers and put it in parallel to one gate resistor

you can start from lower side

if osc. increases , that's most probably problematic side

btw. it can help to gnd heatsinks

 

[grimberg]

Despite my best intentions to finish the Pumpkin preamplifier last year, things got in the way of completing that project. Zen Mod's documentation is good so purchasing the parts and assembling the PCBs was a breeze. The problem was finding a suitable case that would fit the boards and allow for short signal wires. I purchased a 2U Pesante from the DIYAudio store, but it was a tight fit.

2U Pesante (Maximally Vented) - Full Width with No Heatsinks - Chassis

So I decided to build one tailored to the project.

 

[grimberg]

I started with a 1/8" aluminum plate purchased as remnant from a local supplier. Using the table saw with a Freud blade for non-ferrous metals I cut the bottom, sides and back panel. From a 3/16" aluminum plate, also purchased as remnant, I cut the front panel. The top uses perforated aluminum from the same supplier.
The pieces are joined with aluminum angle and #6 socket head screws. The bottom uses stainless steel #6 screws and the holes are countersunk.

 

[Zen Mod]

something is wrong with images

 

[grimberg]

The transformers are Antek and I decided to build an EMI shield, just in case. The wires pass through it at the edge near the front panel, away from the input signal.

 

[Zen Mod]

congrats !

bloody neat work

 

[grimberg]

Thank you! Are you still having problems seeing any of the pictures?

 

[Zen Mod]

just in #829

those in #831 are enough visible to me to make a comment

 

[grimberg]

I am able to see them using a different machine and browser. Could you be more specific? Maybe a moderator can help resolve the issue.

 

[Zen Mod]

dunno

I can't see those in #829 neither in FF nor in Chrome nor in IE , logged in .


report that post to Mod Team , if pics are different to ones in #831

 

[grimberg]

I'll contact the moderators. In the meantime, maybe you can give me an idea.
I am having a problem adjusting the Pumpkin and may be something simple, which I am missing.
The Shunty is delivering +/-36V absolutely stable. When I connect the probes to signal ground and TP1 or TP2 I get 0.114 mVDC. Yes, that is the scale, mVDC! Great, isn’t it? Well, I am not sure. Turning WR1, which according to your schematic should correct any imbalance between the J309s, moves that voltage very little. Turning WR2 I only managed to make the Pumpkin run hotter, to the point of only being able to keep my fingers on the heatsink for 3 seconds.
What am I doing wrong?

 

[Zen Mod]

help me - with TP1 and TP2 , you're meaning on output nodes , before output caps ?

compare with values in attached schm , let me know do you need some other value

 

[grimberg]

Yes. Here are the two places where I soldered handmade eyelets to hookup the probes.

 

[grimberg]

Well, those voltages will certainly help, but first I need to resolve another issue. This schematic shows R22 as 180R as opposed to 47R in your guide. Has any other value changed?