misbias issue??

[jmaf]

[ATTACH=CONFIG]16753[/ATTACH]
i tried bypassing the 2nd stage, volume pot and tone pot and everything is perfectly clean (no motorboating). as seen in the picture i jumpered from after the first stage coupling cap to the grid of the power tube.
i hope this means i've narrowed the problem to in between the gator clips. does this reinforce the additional decoupling of the first two stages theory? if so, why do tons of amplifiers not have this problem cause it seems pretty common for the first two stages to share a node? plus the two stages are out of phase.
if you listen to the clip i posted, this is not your typical hum.

Motorboating requires big capacitance, it's a low freq. oscillation that requires a big "pump", so normally you'd suspect the big capacitances and the power section in the amp: the reservoir caps and accidental feedback from the output stage modulating the power supply or even mechanical feedback from the speaker into a microphonic tube. Aluminum chassis don't block magnetic flux so you can even suspect that, but since this is a tiny amp I'd go for the low hanging fruit first(for example are you 100% certain those caps are good?).

On my first suggestion I mentioned the cap, but as Enzo pointed out I hadn't seen C6 on the upper left corner. So that stage is supposedly decoupled, assuming those caps are good(like I said(half jokingly) and JMFahey too, I'd add another resistor there anyway just before the first plate resistor). That 12ax7 will not suffer at all from some more added resistance.

The 12at7 is in phase with your first gain stage when you have 2 gain stages(full circuit), so when you jumped over the second gain stage you set the preamp and power amp to be out of phase. If that 12at7 is modulating the power supply, that'd be a plausible explanation for your motorboating. Again, two small tubes should not be able to mess with your power supply. I made a tiny 12ax7 + 12au7 amp with just 20uF reservoir and 10uF for second filter....no oscillations of any kind....so like others said, there is enough capacitance in there to keep it stable.

[yunger]

....are you 100% certain those caps are good?).

i didn't measure any voltage on the negative side of the caps, i tried paralleling equal value caps next to each, and they are new so i assumed they were ok.

On my first suggestion I mentioned the cap, but as Enzo pointed out I hadn't seen C6 on the upper left corner. So that stage is supposedly decoupled, assuming those caps are good(like I said(half jokingly) and JMFahey too, I'd add another resistor there anyway just before the first plate resistor). That 12ax7 will not suffer at all from some more added resistance.

this will be my next expirament. how did i get so lucky?

The 12at7 is in phase with your first gain stage when you have 2 gain stages(full circuit), so when you jumped over the second gain stage you set the preamp and power amp to be out of phase. If that 12at7 is modulating the power supply, that'd be a plausible explanation for your motorboating. Again, two small tubes should not be able to mess with your power supply. I made a tiny 12ax7 + 12au7 amp with just 20uF reservoir and 10uF for second filter....no oscillations of any kind....so like others said, there is enough capacitance in there to keep it stable.

your right...i didn't think of that. my next step was going to be jumpering from input to the second stage input. sounds like that wouldn't reveal much.

[jmaf]

i didn't measure any voltage on the negative side of the caps, i tried paralleling equal value caps next to each, and they are new so i assumed they were ok.

You won't find any voltage on the negative side of the caps, if you do then something's really really wrong with your grounding scheme.

I Googled this up just now: Testing capacitors

I like the quote at the end: "A heart in love with the beauty of glowing tubes never grows old. "

[MWJB]

I'm not mad keen on your heater wiring, your 12AX7 plate wires seem to run very close & parallel with the heaters wires? Does moving the plate wires away from the heaters make any difference (being sure to keep them away from grid wires too)?

Double check that pots, jacks & any grounds are tight to the chassis.

[yunger]

here's an update...although mostly for me to keep track.

what i tried:
-reconfigured my grounding scheme; grounded the filament false c.t. directly to the chassis. placed diode bridge + & - directly to the first filter cap..as well as the power tube's cathode, grid leak and the tone circuit that feeds it. the second filter cap connects the first two stages. both caps ran to a single point on the chassis. instability still there
-tried a third decoupling filter node(10k resistor and 22uF cap) to supply the first stage with it's grounds to that node. no change
-temporarily removed the first stage rk bypass cap. no change.
-moved all the wires going to the tube sockets a million different ways. no change
-ran o.t. plate wire directly to the tube socket
-unbolted the o.t. and speaker jack and moved it every possible way and orientation in a 12" diameter. no change
-tried different good tubes, different speaker, swapped speaker wires, swapped heater wires, resoldered everything, tested continuity and resistances where i could

despite all these changes there is still instability when i turn the volume knob way up. jumpering over the second gain stage kills the problem. touching components with my finger in the first and third(power) stage quietens the motorboating. seems like they are still talking to each other.

on deck:
-seperating the two filter cap to chassis ground points to opposite ends of chassis.
-changing existing filter caps for other new ones.
-putting copper sheet on the back of the chassis for additional shielding.
-any other suggestions here?

is it possible that my chassis is too small and everything is just too close together? i'm starting to lose patience with this project.

[Steve Conner]

You found that touching some nodes of the circuit made it stop. So put a little capacitor (47pF) from that node to ground. If it was a grid resistor you were touching, then the cap goes between grid and cathode of the tube whose grid resistor it is.

I had a build where this was needed on the first stage to stop it oscillating.

If touching the circuit affects it, the actual oscillation must be ultrasonic and the motorboating is caused by the oscillation pulsing on and off (aka squegging) You'll need to check that you are really stopping the oscillation as well as the squegging.

If adding caps makes no difference, an alternative explanation is that you have classic motorboating from low-frequency feedback, and touching the circuit injects enough buzz to break it up.

[MWJB]

"is it possible that my chassis is too small and everything is just too close together? i'm starting to lose patience with this project." Yes, it's possible, it is never a good idea to try and make a circuit fit a given space, better to go a little large on the chassis. Where you do go small on the chassis I wouldn't complicate/compromise the layout with an eyelet board, it could probably be wired point to point.

It's never a good idea to daisy chain grounds (your 1st & 2nd stage cathode resistors "ground" eyelets are out of phase, but they tie together a long way from chassis ground), yes, some folks get away with it *with proven working layouts*...but that's not what you have. Have you lost that insulated input jack yet? There are people who can undoubtedly make a single star ground "work"...but I have to admit that I haven't seen this yet!

Any reason why you don't want the vol & tone controls both placed between the 1st & 2nd triodes?

[yunger]

If adding caps makes no difference, an alternative explanation is that you have classic motorboating from low-frequency feedback, and touching the circuit injects enough buzz to break it up.

must be this. i tried the pF capacitor and it didn't fix.
i did find out that the motorboating happens even with the guitar cable out and the input jack grounded. dead silence though when i lift the input and first stage cathode from ground.

Yes, it's possible, it is never a good idea to try and make a circuit fit a given space, better to go a little large on the chassis. Where you do go small on the chassis I wouldn't complicate/compromise the layout with an eyelet board, it could probably be wired point to point.

it might take a little more time for this one to sink in. i'd like to exhaust all options before reaching this conclusion.

It's never a good idea to daisy chain grounds (your 1st & 2nd stage cathode resistors "ground" eyelets are out of phase, but they tie together a long way from chassis ground), yes, some folks get away with it *with proven working layouts*...but that's not what you have. Have you lost that insulated input jack yet? There are people who can undoubtedly make a single star ground "work"...but I have to admit that I haven't seen this yet!

i seperated the first stage and grounded at the input jack. maybe i can explore this further

Any reason why you don't want the vol & tone controls both placed between the 1st & 2nd triodes?

actually, if all else fails i might just parallel the first two triodes and call it a day.

[yunger]

curse this demon to hades!! nothing seems to be working.
-tried a fresh set of filter caps and tried different values of dropping resistors....all to no relief.
-unbolted transformers and moved them as far as they could get away from each other and inputs.
funny how touching certain parts/wires of the circuit make the noise go crazy(o.t. blue wire) and some quiet things down(power tube grid). i still suspect some phase problem.
it's also weird to me that i can hear through the speaker when i touch grounds with a multi-meter. noticably at the preamp filter cap ground and the tone pot itself.
i'm determined to post a definative solution at some point.

[MWJB]

"it's also weird to me that i can hear through the speaker when i touch grounds with a multi-meter. noticably at the preamp filter cap ground and the tone pot itself."

This suggests that these aren't at ground potential, can you measure any resistance, or do a continuity test between these components & ground. Run a wire from the tone & vol pot bodies to the input jack ground. I would also move the preamp filter to the circuit board side and mount from the junction of the plate resistors to the input jack ground.

[jmaf]

funny how touching certain parts/wires of the circuit make the noise go crazy(o.t. blue wire) and some quiet things down(power tube grid). i still suspect some phase problem.

Same issue MWJB said. If it's getting quieter when you touch the power tube grid with a multimeter with the other probe on ground, that means that stage is missing a proper ground reference. Your multimeter's probably 1M to 10M internal resistance, when you touch the grid to ground, you're setting that ground reference for the grid so it quiets things down.

[yunger]

not having too much luck with moving grounds around. i'll keep trying.
here are some other clues that are probably just nonsense...i'm bottom feeding at this point:
-my o.t. is a champ output transformer, 7000 primary 3.2 secondary. i assumed reflected load of 17,500 primary into an 8 ohm speaker.
-wrapping the tip of the 12at7 with aluminum foil stops the motorboating but the sound from the guitar is horrible. like a ring modulator.
-grounding shorting everything but the treble portion of the tone circuit passes a high whistle/squeal. seems like the noise is coming from before that point but nothing seems to be adding up right now. what a pain.

[J M Fahey]

Sorry friend, but your amp is oscillating like crazy.
Even the "motorboating" may be a symptom of *high* frequency oscillation, which periodically gets rectified by a grid, charges the coupling capacitor, blocks the tube (killing its gain, so the HF oscillation stops for a brief time), said capacitor discharges with a LF time constant, and the cycle repeats itself.
You do not hear the HF oscillation but the LF blocking cycle, which sound similar to motorboating.
That "ring mod" effect, is also *part* of a more complex problem you donīt hear.
Sorry , would like to be of greater help, but these pesky problems are hard to solve by "remote control", are difficult even if I had the beast in my own bench.
Only path I can suggest, is to strip everything, rebuild only the *last* stage, test it, check that itīs stable, only then add 1 extra gain stage, do the same, and so on.
That way you fight 1 problem at a time.
In this moment you can have 4 or 5 problems (grounding/layout/decoupling) at the same time; even if you solve one of them you wonīt know, because others will hide it.
Donīt know if what I'm saying is clear, but I think *in the long run* rebuilding step by step may be the "shortest" path.
Good luck.

EDIT: needless to say, *this* is one of those cases where a Scope is mandatory. Oh well.

[MWJB]

The OT is fine for the purpose, you could also try 12AY7 (without a rebias) or 12AU7/12BH7 (rebias cooler) for the power tube.

Have you tried placing vol & tone controls both between V1a & V1b yet?

[Steve Conner]

Have you tried placing vol & tone controls both between V1a & V1b yet?

Funnily enough... The build where I had trouble with the oscillations had no controls between V1a and V1b at all. The two stages were cascaded, and all that broadband gain made it want to oscillate with even the slightest bit of capacitive coupling in the layout.

Edit: Did you try adding a grid stopper resistor to the "A" section of the 12AT7? As in, each grid has its own stopper, and both of them mounted right at the socket pins. The 12AT7 has gain right up to VHF and could be oscillating.