Did my oscilloscope find a parasitic?

[David Root]

My 100W #124 has had an annoying parasitic on the OD mode since I built it. I tried all the usual band aids, it's still there.

Just scoped it on OD mode with a 400Hz sinewave. V1A and B clean. V2A slightly flattened, V2B squaring but no ringing. V2A cathode, thickening of sinewave, V2B cathode, full wave sawtooth, thickened. V2A and B ground fuzzy baseline.

Input to PI entrance cap square wave with downward spike entry at bottom left and upward spike at top right of square. Now the interesting part....moving the scope probe off the cap on to the board, the amplitude of the spikes increases and the upward spike moves left to the center of the square wave top. Maximum of both is with the probe tip on the board up against the left side of V2B coupling cap. Moving up into the air or to the left or right of that cap diminishes the effect very quickly.

Could someone explain this, did I find a parasitic, should I try higher frequency?

[67plexi]

David the fuzz is what I call it on the scope not good. Check for a marginal ground will cause that symptom, use jumper leads to get a solid ground, see if that does it. Start at the 250k OD level Pot. I don’t recall if you used PEC. If you did the ground washers are to small use a larger star washer like CTS.

[David Root]

Thanx plexi, I wondered about that. It is a PEC pot, I'll check it out. I also noticed that the wire from the return jack to the PI input cap if moved towards V2 the apparent volume increased, so I moved it left towards V3 (PI). The output of the OD level pot goes to that jack.

[Bob-I]

I wouldn't call myself an expert, but the PO I've found was without any input at all. It's seen as a sine or square wave with no input at all.

[David Root]

Thanx Bob, I'll check that too.

[David Root]

Checked out the OD level pot washer, it's tight.

Checked out the ground from the OD level pot ground lug thru the ground scheme with the DMM ground probe in the ground meter jack in the cathode measuring system. 0.20 ohms, probes crossed tightly 0.17 ohms and this is clear across the chassis from one end to the other.

The baseline fuzz disappears when I ground the probe. Duh!

Nothing shows on the scope with the gain up in OD mode, nothing plugged in. Waved the scope probe around the chassis, got most action over the PT area.

If I plug in the 400Hz sinewave again, in OD mode, and turn up the scope sensitivity the squared sine wave goes up in amplitude off the scale then a new double peak comes up from the bottom of the screen (not the baseline) at the same interval. maybe something similar at 4KHz, hard to tell. What is this?

[67plexi]

David Try the test in this order: signal generator. Amp. Dummy load. Oscilloscope.
Use a 300 watt dummy load resistor it will get hot.
Signal generator input of amp
Dummy load output of amp, speaker out
Oscilloscope. to dummy load.
On Dumble amps the PI v-3 is critical (any other amp v-1 is the one for the best ecc-83)
Sorry for my first post I have had poor earth strange spikes on the wave form
Also it is possible on your v-1 or v-2 the bias it too hot.

Steve.

[David Root]

Thanx Steve. I have a 225W dummy load made of two 225W 2 ohm Ohmite wirewounds in series.

I have not heard that V3 in a Dumble is critical, I had recently seen a post that said Dumble delivered one amp with three GE longplates in it, I can also try that.

My bias on V1 and V2 is currently between 1.45 and 1.50V.

Will try your recommendation this morning as I have a day off.

Update. Used a 200W 16 ohm dummy load instead. No spikes, just a squared sine wave. Ran it flat out in OD mode, nothing unusual on the scope.

[67plexi]

David. Low plate. I matched the 5uf and 1k5 resistors 18AWG solid for the grounds.
V-1A pin. 8. 1.5v bias. V-1A plate pin. 6. 190v Clean
V-1B pin.3. 1.5v bias V-1B plate pin. 1. 190v Clean
V-2A pin.8. 1.74 bias V-2A plate pin.6. 205v Overdrive
V-2B pin.3. 1.74v bias V-2B plate pin.1. 206v Overdrive
The voltages listed V-1 & V-2 gave me the best even, first, second, and third order harmonics
Used GE long plate 7025 matched V-1 & V-2
PI .V-3. I used super strong ECC-803s Seimens made in Germany date code 1958 seam top rare gold pin
Gm. 3.8 ma/Gm.3.6 ma.
David if you need I can send you some mil-spec Dale 2 watt 1% 1k5 resistors I have a full box.
I’m not sure if this helps but it worked for me on the preamp.
Next base bias was set at 70% power section set the 5k trimmer for best odd order harmonics readjusted bias cooler to around 60% by ear with guitar signal.
I think that’s is where the bloom is made not sure I only have one year experience with the
Low plate Skyliner amp.

Steve.

[Structo]

Man the cathodes and plates of those preamp tubes are very well matched!

Of course I am using the high plates so they won't be the same anyway.

[David Root]

Steve, I am nowhere near 70% dissipation on the power tubes. 34mA cathode at 437 Vp= 14.5W or 48% dissipation.

I think that explains why when I put three GE longplates in I got all six plate voltages way high compared to yours. Tone was not good in OD. I thought that these amps sound best at about 50% or so dissipation, but I will try moving 'em up to 60%, that's about 41-42 mA cathode.

I'm using RN65 Dales on all 1K5 cathode resisors.

For V3, I don't have an ECC803S but I do have a '60s RadioTechnique 12AX7S and a couple of '60s Telefunken long smooth plate 12AX7. Maybe I'll try those in the PI. Also a '62 Tungsol 12AX7 which is not a bad tube either. I think I'll try these and the GEs and see what dissipation gets the plate voltages in line.

[67plexi]

David, I hope that works. I started out with 3 matched GE-7025
It sounded ok with my Telecaster but my 335 sounded like crap.
After the PI V-3 change any guitar sounds great.
Bias 41.5ma. 465v plates.


Steve.

[67plexi]

The most important tube in your amp The phase inverter.

http://www.guitaramplifierblueprinting. ... verter.pdf

[David Root]

Thanx again Steve. I am trying to set this amp up for a Epi Elitist 57 Goldtop with pickups from a 65 SG Standard, great sounding guitar.

I had not seen that piece on the PI, very interesting. Dumbles are supposed to not overload the PI but in playing around with this amp on the scope I am not certain of that. If it does, it's not a huge effect I think.