A Rockster Variant Mutant Donster Monster

[Littlewyan]

Is the input jack shorting to ground when you unplug the guitar?

[donzoid]

Its got a 2watt metal film 1meg there, so it "shouldn't be" but I will check.

EDIT: Just checked, with the cable unplugged its right at 1.02meg tip to grd, but plug in, at about 3.x ohms. That seems correct, but I "do" have the preamp up almost all the way to hit the sweet spot on this amp.

[Littlewyan]

There is your problem. When the guitar is unplugged the signal (tip) tab on the jack socket should be connected to ground, otherwise it will act as an antenna picking up whatever it can, hence the oscillating. Have you used a switched Jack Socket?

[donzoid]

LOL, thanks Wyan, I do have a switched jack, but noticed I had not patched one of the switched side to ground when the plug is removed...duh! I just soldered in a cable to connect those, and will re-test.

[donzoid]

Yep...that's it. Sometimes the simplest little things...

[donzoid]

I thought I'd post updates, and some ponderance(s).

This amp is quite different in that it:
...has this "meaty" drive in almost all the stages of the gain knob, underneath the tone of the whole thing - hard to describe;
...has amazing note clarity (especially low notes on wound strings) even with the gain knob dimed. Reminds me of my 100watt JMP's that way, but thank God at much less raw dB level;
...Is really easy to dial "too trebly"; but I keep finding myself with the cut control all the way up (basically off), the treble on 8 of ten, and the bass on 6 or 7 of ten
...gets quite fizzy when my PPIMV is anything less than halfway (though that spot AT or until halfway is very effective at cutting just a bit/to some 25% of the overall volume, and I almost never cut an amp back farther than that anyway...)
...Is very pedal-friendly (more on that later)
...Seems to like the low-secondary switch setting (at least I like the tone of it better).
...runs 6550's, kt66's and even the JJ 6v6's without any changes to the cathode resistor on the power tubes...pretty handy, the kt's are sounding best so far though. I wouldn't want to try the 6v6 on the high sec. setting without adding more cathode resistance though.
...the PI divider resistor lift/V1b cap "add" relay is perfect for a lead boost - I ended up with a 250K resistor, and only 2uF there but good boost and not too wooly (though if used strictly for leads, the 5uF and even the 10uF were OK).

...for some odd reason I just keep wanting "a little more gain" out of the beast, even when she ain't got any more to give - but that's EASY to fix using a pedal (in my case a voodoo sparkle drive with more clean boost than TS type gain, but a bit of that too). And that... sounds very angry!

I am getting some odd DC scratchy-ness from my guitar volume pot that I don't get on my other amps. Still re-checking grounds etc. but so far it all looks solid.

I'll attach the low-secondary voltage chart if anyone wants to take a look.

In all a really good sounding amp with lotsa chimey vox-ness, but I am surprised I can't get more flat-out grind out of it. I knew it wouldn't be like an Express (what is?) but for some reason I thought I'd get a bit more over-the-top at the highest end of the gain knob. It's there when I really get aggressive with pick attack though. Perhaps its the volts differences from a "stock rocket" at the anodes/or cathodes of V1 or V2? I've done zero tube rolling yet so that may change my mind also.

Heater volts are a bit high too, curious if you folks think I should drop them a hair. 6.7vac at power tubes, 6.65vac at the 9pins.

[Littlewyan]

The scratchyness on the guitar knob is normal for these amps, with certain 12AX7s in V1 you can make it go away but it wont cause you any issues if you leave it, just a nasty noise when you turn the knob.

The heater voltage is ok, its not so high that it'll cause issues. What power transformer are you using? I saw in your voltage chart its a P-Train Heyboer but can't find it.

[matt h]

gee, lots of note definition? Now why might that be... *ahem*

[donzoid]

Hey Wyan, the PT is one I got from a TAG member here, as I was collecting parts for the project last year...I had to go to Heyboer to get my info (Arlyn was very helpful there) but its label says what is on the chart - "Fat P-Train" and it has a 5v tap that's powering the relay, heater tap, and two sets of secondaries (those are what I wired to the switch) 260-0-260, and 300-0-300. The most unique part of the PT according to heybeor is that it purposely used M26 layer materials, thus the "fat" part of the label.

One thing I didn't put on a switch was the OT primary taps, I used the 6.6k, there is a set of 5.2k-ish leads also. I figured that would give me a few more options for output tubes.

Matt, yes therein lies the engineering rub. It has a PI filter up front quite similar to the usually-used designs of the home stereo crowd.

[Littlewyan]

What are the primary windings? As if you're using a 115VAC Primary then it would explain why your heater voltage is high. Is your wall voltage at 120V

[donzoid]

yep its pretty steady at 120.5 here.

Arlyn at Heyboer told me which pairs to combine as it has taps for your side of the pond.

black+
blk/blue
and
blk/white+
blk/yellow

was what they told me would be correct for 120vac.

[Littlewyan]

Hmm odd, well its not a massive issue like I said. With heaters in valves you can go up to 10% above or below 6.3V so you could safely go up to 6.9V. I'm just concerned that the wall voltage might be high somewhere and raise the heater voltage just a bit too much but you'll probably be fine.

Is it a non centre tapped heater winding?

[matt h]

(deleted)

[donzoid]

The heaters are center tapped... I asked because according to Merlin, 6.9 is right on the edge 'o 'too much, and I don't want to unnecessarily tax the tubes, but yeah Wyan you are right if one of the towns outside here is running a bit higher it could get right into tube eater territory.

I'll keep an eye on it.

Made some clips will post in a few hrs.

[donzoid]

Not too great of playing, but here's some tones:

Starting with vol. pots on 3 or 4 both pickups, then playing around with vol's upward, then switching to bridge pup only, at a couple vol. settings, then the neck pup at the last few seconds...

I like the way it's mic-ing up...that is 2" condenser about two feet off of 2-12's this round was the 65watt celestions in half of my jcm800 cab.

This clip isn't using the relay boost, just the "standard" setting with the pre gain "all the way up". At this point I think I might leave the mid control in there as it makes a pretty huge difference between guitars for finding "just the right spot".