sub bass odd harmonics?

[lowell]

this amp is sounding pretty good thus far, I've been tweaking things for 6 months now. however, there is this weird sub bass harmonic thing going on that makes certain intervals sound out of tune. 5ths and octaves sound good but anything else and it sounds awful. This is on both channels but only noticeable on the clean channel when it's almost cranked-cranked. I've thought about the power supply and wonder if the source is my decoupling b/t stage supplies.

below are the schem and power supply.

[loudthud]

You might try doubling up on the 47uF and 32uF series caps. What does the B+ run in the various modes?

[lowell]

not sure what you mean by modes LT...? so you think it's a PA decoupling/filter issue and not the preamp? Just curious what you're thinking behind this is.

[Tom Phillips]

Interesting amp.
Besides the heater CT, where is the 6V6 cathode circuit completed? (Point K)
If it's there in plain sight I missed it.
Tom

[lowell]

Hi yeah sorry I left that out of the schem here on ampage. it's to a Rk not shown, no bypass cap. forget what value Rk is at this moment as I don't have schem in front of me.

[loudthud]

not sure what you mean by modes LT...? so you think it's a PA decoupling/filter issue and not the preamp? Just curious what you're thinking behind this is.

Well, there is what looks like a switch in the power supply schematic that bypasses the 5Y3. Another switch looks like it silences the 6L6s but that shouldn't affect the B+.

From your description I think you have a classic case of "ghost noting", usually caused by power supply ripple. In the 6V6 only mode, the 6L6s are still drawing current and causing more supply ripple than if they were turned off somehow. But this may be necessary to keep the B+ from going too high for the 6V6s.

[RudeBoy]

Maybe an intermod product. I wonder if you could be getting some blocking on the cathode biased output tubes? Those 470k are pretty large. You might try reducing C23 and C24 to say - .01u?

Is it actually wired like the schematic? G1 to shield, input sig to screen, supressor grid to what is normally the screen supply?

[guitician]

By "Cranked Cranked" do you mean distorted? Once your output tubes are clipping the harmonics generated are hard to control. Different tubes may have better transfer characteristics, and get less of the objectionable "sub odd harmonics".

[lowell]

interesting. Loudthud is there a way to see this on a scope or w/ a meter?

Sorry if the schem power tubes aren't wired right, they ARE wired correctly in the amp.

I can't image it's tube type that causes this. I've used many 6L6 6V6 amps that do not do this.

[guitician]

Exactly my point. With your particular amp layout/schem you've hit on just the right conditions to produce the "Odd sub bass". It may be that less filter on the PSU will fix it.

[loudthud]

Ghost noting is usually caused by ripple on the B+ that is rejected by a push-pull output stage until the amp starts to clip. There is a certain amount of this in most guitar amps but it's not to the point that it produces undesireable tones. The way to look at this with a scope is to simply look at the speaker output but sync on the power line frequency. The guitar signal will just look like a jumble of random lines but at the top and bottom of the waves you'll see these sawtooth divits. Use a sweep speed of 10 millseconds per division. If the peak to peak voltage of the sawtooth wave gets to be more than (I'm guessing) 20% of the peak to peak voltage of the guitar signal, it should be audible. So you are injecting a 2x line frequency buzz into the guitar signal and that can produce distortion products at frequencies that were not in the original guitar signal.

The first filter in your amp is two 47uF caps in series. Effectively 23.5uF. (You do have equalizing resistors don't you?) That's enough filtering to an amp like a 5E3 but maybe not enough for a 6L6 push pull amp. There are a lot of unknowns in your amp like what the static B+ current and voltage is so I just suggest an experiment to increase the filtering to see if that is what is causing the problem.

[lowell]

hmmm cool. ok so how do I "sync to line" w/ my scope?

Finally, this sawtooth is 120hz because it's been rectified at this point right? I want to understand all aspects of this as best I can.

thanks for your help w/ this!

[loudthud]

Just about any scope will have some way of triggering (starting the horizontal sweep) from an external source or possibly a second channel input.

If your scope has a "Trigger Select" switch with a position for "Line", use that. If there is no position for "Line", select "External" and connect a probe to one side of the heater winding from the "External Input" jack. Use the "Normal" Horizontal mode where the trace won't sweep unless it is triggered. Adjust the trigger level until you get a sweep. Connect the available Vertical input to the speaker output.

Yes, the sawtooth wave should be 2X the line frequency. There could be a bad diode or wiring error in the power supply that would cause the sawtooth to be the same as the line frequency. That may indeed be the problem with your amp. It would cause the ripple to be twice as big as normal. I'm just mentioning this because your amp isn't 100% yet.

[lowell]

ok i'll try that tomorrow.

I was thinking of using the screen supply for my PI plates. I'm wondering if this will cause any crosstalk issue in the filtering, or maybe it's fine? thought I'd pick your brain on that too.

[loudthud]

With a multi-Henry choke between the plate B+ and the screen B+, there should be enough filtering that the PI would not operate abnormally as long as the voltage is not too high.