Almost done !!! #12 Rocket of group build

[dansamp]

well the tone section is still doing it's thing
it seems to be a mechanical problem
if I turn the treble pot I get a big jump in volume and gain along
with treble if I turn it to 0 and apply more pressure to the knob
I can hear a pop through the speaker
it also occurs some what with the bass pot
I tightened all the nuts holding the pots just a bit more
not sure what to make of it
I would hate to have to rip out the pots

any more ideas ??

it seems that it is affecting the gain when the problem occurs
and it is sporadic

anyway
this sucks

[dansamp]

another thing to note

this amp really needs to be shielded
it is very noisy with out the bottom plate

has any one else encountered this ??

Dan.....

[Ron Worley]

Yes, the bottom plate makes a huge difference for hum....

Ron

[Fischerman]

dansamp,
Congrats on getting this far...you'll get it right...just stick with it. Whenever I have ANY problems first thing I do is go through the entire amp and measure/record voltages. All B+ 'nodes', all plates, all cathodes. IMO, other than perhaps swapping a tube or two, that should always be the first thing you do.

The thump/click you're hearing might be oscillation. If for example, you start with Treble on zero and the amp makes sound (but is muffled of course) and then you turn up the Treble and then you hear the 'thump' then that is a classic example of what high frequency oscillation can do...the 'thump' is where the oscillation kicks in.

Main tip for preventing/curing oscillation is this: Grid wires are like antennae...they are very sensitive to picking up stray signals. Plate wires are like transmitters...they are giving off a lot of stray signal. So you want to keep the sensitive grid wires away from other stuff...especially those plate wires.

When I built mine there was something going on around the PI. Just putting your hand near the PI input cap and surrounding circuitry caused an increase in hum (unless of course you're touching the chassis...then you become a shield of sorts!). Just laying anything conductive (like a metal amp faceplate) across the chassis (and in contact with it...the shield must be grounded) when upside down will substantially shield it. But somehow that has somehow almost completely went away in my amp. Mark Abbott documented a similar occurence on one of his early attempts. FWIW, I contacted/corresponded with Mark Abbott, Paul Ruby, RG Keen, and Lyle Caldwell on this hum issue and were never able to cure it or even put a dent in it. But it was very low level hum to begin with...just bothered me.

[Fischerman]

dansamp,
It doesn't look like you used shielded cable for the input. Just having that 33k input resistor on the circuit board is a recipe for oscillation...and then not using shielded cable is almost asking for it. I have no idea why he did it that way...I mounted my 33k right on the socket. This is the first grid circuit and is very, very sensitive.

Another thing to consider is using shielded cable from the Volume control to the second stage. It's not really necessary but it's insurance against oscillation and the shielded cable won't affect tone (I know somebody will say something about cable capacitance but please...it's 6" long and you can't hear it and you can't convince me you can hear it ).

Grid wires can be up in the air but try to push plate and cathode wires down to the chassis...and keep those grid wires away from plate wires. Also, the Treble wire can be pretty tricky too. Keep those wires from the Cut control away from the PI input...run those Cut wires along the chassis away from the circuit board.

[dansamp]

OK,
Thanks for the input
I thought about mounting the resistor onto the socket
I will do that and see how it goes

Dan...............

[dansamp]

OK,
here's the latest :
I moved the 33k resistor on to the tube socket and used some shielded wire
seemed to tame the harmonics but did not resolve my tone section problem
shortened the wires to the cut pot and the volume pot
no change
added a shielded wire from the volume pot to V1b
no change
I noticed I kept losing the effectiveness of the bass pot
so again I checked the wires to the tone pots
I move the lead around no change
then I reflowed (again) the jumper wire from the treble to the bass pot
and that seemed to do the trick
no more thump or click sounds and the bass and treble pots are consistent

so it seem to be done for now

Thanks Fischerman,

I know you don't believe that a shielded wire makes a difference
but I do think it reduces some of the upper harmonics and "swirlyness"
going on

I didn't use any shielded wires in my Train wreck style amp

and I don't think this circuit needs it either

but for now it is staying in there

thats my story anyway

thanks all for now



Dan..........

[Fischerman]

I know you don't believe that a shielded wire makes a difference
but I do think it reduces some of the upper harmonics and "swirlyness"
going on

I know...tons of people do. I'm 100% convinced that people think they hear a difference and I'm 100% convinced that there is a difference (otherwise it wouldn't tame oscillations). But when you put them to the double-blind test...fail. People think plain yogurt tastes like strawberry if you blindfold them and tell them it's strawberry. I'm not immune to it either..I just know how easily the human brain can be fooled. I thought I heard cable direction differences but I'm not 'believing' it until I can pass the double-blind test...and I can't. But before I did the double-blind test I would have sworn I could tell the difference.

And besides...we do all sorts of things to reduce high end harmonics in guitar amps so the ole 'it has more upper harmonics' just doesn't fly imo. You want more upper harmonics just dime your treble control and dime the tone control on your guitar and dime the Presence. Do you do that? It's just as vague as 'swirlyness'. It's like we try to come up with these vague terms to describe something that we're not really hearing...we just think we're hearing it. So we have to use some esoteric terms to describe it...because it'e not really there in the first place.

That's just my opinion on it though. There are cases where the person can hear these tiny, tiny differences but 9 times out of 10 when you put them to the real test...fail. I don't like 'fooling' myself.

[RJ Guitars]

Dan,

Long time coming and very sweet sight to see... congratulations! Nice truck too!!

I'm confident that you'll get all together, there are a lot of these units around and lots of good help.

I've got tons of pictures of my build including the tone stack, although as you might recall I did a hybrid build with a power supply board and the ground buss off the preamp board... so it's a bit different layout.

Let me know if you want to see any photos... I posted voltages and I know Fishy has done the same... add that to Ron's work and I think you'll know the ballpark.

Let me know if if you need anything from me... I'll open mine up and measure anything you think you need.

In the meantime, this is the most complete amp I have ever built and my number one favorite to play through of any I've ever plugged in to. It will rock, it will twang, and turn it up to get sweet leads and very cool crunchy chords - lots of bottom, lots of top... very fun amp!

rj

[dansamp]

well I played the amp again tonight at full volume
I don't have a dedicated cabinet yet
but it worked out very well
I had a 1x12 blue bull dog in one marshall style cabinet
and 1x12 no name eminence ceramic in a tweed style cabinet
running both parallel for a total of 8 ohm load

it doesn't sound all that different to my Hoffman type AC30 clone
though

does anyone have any other AC30 type amps that they have compared to side by side with a Rocket ???

Dan........