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Old 12th August 2007, 02:48 AM   #11
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Default Motorboating and NFB

Oh yes....

Motorboating as a problem with NFB is meant as a stability problem. Yes, it is eventually positive fb, but then because of poorly proportioned l.f. time constants all phasing together to create positive fb from the mid-band nfb in the end (below the audio band).

That statement is also conditionally true. Insufficient decoupling can also cause motorboating without any signal nfb as we know it. It is then as above; the wrong time constants (as in all co-inciding in frequency) again being the problem. But the possibility is less with a low-gain circuit such as this.
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Old 12th August 2007, 04:06 AM   #12
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Hi Soren,

Nice amp. I've seen your others on facebook and they always look good. I had a few things to add/ask.

Check your cathode voltage on your second 6SN7 triode section. They are only rated for 100V htr-cthd, and since your plate resistor value isn't shown, I'm not sure what your plate voltage on the first section would be. Sounds like when you tap it, something mechanical inside one of the preamp tubes might be shorting (mainly the grid of the second triode stage contacting the heater, and when it cools it retracts back into place.) Are they both russian 6H8C?

Also, how do you like the Edcor output transformers? I was thinking about getting some for another project.

good luck,
-micah
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Old 12th August 2007, 07:19 AM   #13
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Quote:
Originally posted by anatech
If you drew it and replaced the positive supply wires with small value resistors and the same for your ground traces, I think you may see the answer.

Does that help?

Also, I should have said this earlier. Nice Job!

-Chris

Edit: I should have stated that you created a feedbak loop via the supply, which includes your ground.
thanks! i'll still try to puzzle out where the grounds go but the replies below might just have taken the burden off. it makes sense to put the ten ohm 6SN7 cathode caps to the PSU ground since the bulk of current passes through that point. in fact, the large amounts of current run past all the small-signal ground stuff, where i would really want it at one end or another. in this case, re-orienting the ten ohm resistors is kind of a hassle, whereas all I have to do to equalize the ground is make it an I-shape, with PSU caps bottom left and bottom middle, HV and filament CT on bottom middle, chassis ground bottom right, and signal ground top left and power/OPT common on top right. basically move the vertical ground connection to the right between signal ground and the 6L6's. sound good?


Quote:
Originally posted by Johan Potgieter
Sorenj07,

You do not have a high gain circuit, so motorboating is also rather funny to me. The usual weird first question, with respect, in detective "where were you at the time of the murder" style. You are sure of all the connections, etc.?

After that I would support Chris. From what I see (I did not enlarge) your earth (common) goes from your loudspeaker terminals picking up everything on the way to the input socket and then with a length of wire to the power supply caps. The first thing I would try is common of output and 6L6s directly to the reservoir caps' negative. All the other commons (drivers last) with a separate wire to the same reservoir caps' negative point. You could also earth the chassis there.

Forget about mechanically fixing caps etc. at this stage. There is neither your problem nor microphony there (urban legends exist thataway). Then also as others suggested a bad tube, though I doubt that; still. (Hope we are not all overlooking something silly.)

Good luck!

Quote:
Originally posted by Boris_The_Blade
Hi Soren,

Nice amp. I've seen your others on facebook and they always look good. I had a few things to add/ask.

Check your cathode voltage on your second 6SN7 triode section. They are only rated for 100V htr-cthd, and since your plate resistor value isn't shown, I'm not sure what your plate voltage on the first section would be. Sounds like when you tap it, something mechanical inside one of the preamp tubes might be shorting (mainly the grid of the second triode stage contacting the heater, and when it cools it retracts back into place.) Are they both russian 6H8C?

Also, how do you like the Edcor output transformers? I was thinking about getting some for another project.

good luck,
-micah
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Old 12th August 2007, 07:26 AM   #14
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Quote:
Originally posted by anatech
If you drew it and replaced the positive supply wires with small value resistors and the same for your ground traces, I think you may see the answer.

Does that help?

Also, I should have said this earlier. Nice Job!

-Chris

Edit: I should have stated that you created a feedbak loop via the supply, which includes your ground.
thanks! i'll still try to puzzle out where the grounds go but the replies below might just have taken the burden off. it makes sense to put the ten ohm 6SN7 cathode caps to the PSU ground since the bulk of current passes through that point. in fact, the large amounts of current run past all the small-signal ground stuff, where i would really want it at one end or another. in this case, re-orienting the ten ohm resistors is kind of a hassle, whereas all I have to do to equalize the ground is make it an I-shape, with PSU caps bottom left and bottom middle, HV and filament CT on bottom middle, chassis ground bottom right, and signal ground top left and power/OPT common on top right. basically move the vertical ground connection to the right between signal ground and the 6L6's. sound good?


Quote:
Originally posted by Johan Potgieter
You are sure of all the connections, etc.?

After that I would support Chris. From what I see (I did not enlarge) your earth (common) goes from your loudspeaker terminals picking up everything on the way to the input socket and then with a length of wire to the power supply caps. The first thing I would try is common of output and 6L6s directly to the reservoir caps' negative. All the other commons (drivers last) with a separate wire to the same reservoir caps' negative point. You could also earth the chassis there.
Good luck!
thanks! I'm pretty confident that between me and all of you out there, this problem can get sorted out. as for connections, I can always reflow a few for good measure. I hear what you say about where signal ground is on the buss; the I-shape ground that I mention above is probably how I'm going to try to deal with it, or at least the thing I'm going to try first. It's a quick thing to do.

Quote:
Originally posted by Boris_The_Blade
Nice amp. I've seen your others on facebook and they always look good. I had a few things to add/ask.

Check your cathode voltage on your second 6SN7 triode section. They are only rated for 100V htr-cthd, and since your plate resistor value isn't shown, I'm not sure what your plate voltage on the first section would be. Sounds like when you tap it, something mechanical inside one of the preamp tubes might be shorting (mainly the grid of the second triode stage contacting the heater, and when it cools it retracts back into place.) Are they both russian 6H8C?

Also, how do you like the Edcor output transformers? I was thinking about getting some for another project.

good luck,
-micah
Love the Edcor transformers, first off. I've used two 6.6K 60W 4-8-16 OPT's in a stereo push-pull EL34 amp and I really like how it sounds. I've also used 7.6K 25W OPT's in a PP 6BQ5 amp that I have since sold; both I and the client like the sound. Of course it's hard to separate transformer coloration from the rest of a completely scratch build, but at least you know they function to magnetically couple audio My next project, after trying to sell these monoblocks (I'm tired of them already ) will probably be some type of PP 811A monoblocks. A nightmare to bias them, and a 10K a-a 50W OPT is no joke. Should be fun!

I just forgot to write in that value, it's 47K 1W. I took it straight from the good Mr. Williamson so I assume that the heater to cathode ratio is all right. If not, should I ditch the CT on the heater line and tie the 6.3V to B+ with a resistor?
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Old 12th August 2007, 10:47 AM   #15
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I'm not going to dive into the problem portion of the thread because the experts have already done so.

What I will do is give a hearty two thumbs up for your workmanship and design on this project. Very nice work!!!
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Old 12th August 2007, 03:22 PM   #16
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Quote:
I just forgot to write in that value, it's 47K 1W. I took it straight from the good Mr. Williamson so I assume that the heater to cathode ratio is all right. If not, should I ditch the CT on the heater line and tie the 6.3V to B+ with a resistor?
It's probably ok with the CT at ground. I plotted the operating point of the first section of the first 6SN7, assuming an approx. B+ there of 310V (using psud). It's about right at 100V, as are other williamson designs (I own a pair of heathkit W4's), but they had slightly lower B+ on the first section, so they may be more around 90V (if i remember test resutls correctly) . The cathode of the next section will be a few volts higher than the plate of the first. You could change your 33K series drop resistor to 47k or 68k and see if that helps. Maybe an older American tube might tolerate it better, that is of course if this is the actual problem... Also note that your first 47k plate resistor is dissipating about 0.9W+, so you may want to change that to a 2W for long term reliability.

Can you put the meter on the plates and cathodes mentioned to see? (do you have a working meter?)
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Old 12th August 2007, 06:48 PM   #17
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Hi sorenj07,
Great looking chassis, do you remember where in Dorchester you went to get this work done, I live next town over and could use someone that good..

Do you have a scope? One technique to apply is to try and isolate the problem to the culprit stage. I'd remove the output tubes use a variac and check to make sure none of the tubes in this thing are a subsonic noise generator. (Do you have extra 6SN7 to try?)

This should not be making noise even without negative feedback so if you are thinking about adding it later fix this problem first..

Definitely look at your grounding scheme as well.

Good luck..
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Old 12th August 2007, 11:05 PM   #18
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Regarding the h-c voltage of the 6SN7, the peak spec. is 200V, so that there should not be a problem with the design. Williamson indicated 105V on the phase splitter cathode, with 450V h.t.

But the solution in case of doubt is easy. Elevate the heater voltage to about (say) +60 - +70V, by using a resistive divider from h.t. The standing current needs not be more than 1mA. Bypass that point to common with an electrolytic. This will also assist in eliminating any hum resulting from a small h-c leakage, where the positive heater cannot radiate to the cathode. This is an often used measure, especially for pre-amps.

Soren07, I still feel uncomfortable - we are overlooking something. This behaviour of yours is uncharacteristic, especially in absence of NFB. Of course, one way to eliminate possible microphony is to simply unplug the loudspeaker. Hope you can find a scope, although a meter (especially analogue) would also show whether the l.f. instability is still there. I would also try different tubes as suggested by Kevinkr, but that may not be practical. Also, while "fishing", does shorting the input make a difference? (Further, while you are testing and this occurs, do not let the condition go on for too long. You could be generating spikes in the output transformer with detrimental effect.)

I am really interested in the solution, and wish you luck.
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Old 13th August 2007, 01:00 AM   #19
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Quote:
Originally posted by burnedfingers
hearty two thumbs up for your workmanship and design on this project. Very nice work!!!
Thanks! I'm shipping off to college in less than a week and hope to get these suckers done before then. People might remember that about this time last year I was making noises about building a 6L6 amplifier in the style of the first Hafler/Keroes modification of the traditional Williamson. It's been a long-*** time and I've definitely learned a bit since then, no small part because of people on forums like these. Thanks guys!

Quote:
Originally posted by Boris_The_Blade
You could change your 33K series drop resistor to 47k or 68k and see if that helps. Maybe an older American tube might tolerate it better, that is of course if this is the actual problem... Also note that your first 47k plate resistor is dissipating about 0.9W+, so you may want to change that to a 2W for long term reliability.

Can you put the meter on the plates and cathodes mentioned to see? (do you have a working meter?)
This makes sense. I have half a mind to give the 6SN7's a break and use 47K 2W and 33K 2W instead of the 33K and 22K resistors. Not to mention that bigger resistors will be easier to mold in 3D into pleasing rectilinear shapes 0_o

Quote:
Originally posted by kevinkr
Hi sorenj07,
Great looking chassis, do you remember where in Dorchester you went to get this work done, I live next town over and could use someone that good..

Do you have a scope? One technique to apply is to try and isolate the problem to the culprit stage. I'd remove the output tubes use a variac and check to make sure none of the tubes in this thing are a subsonic noise generator. (Do you have extra 6SN7 to try?)

This should not be making noise even without negative feedback so if you are thinking about adding it later fix this problem first..

Definitely look at your grounding scheme as well.

Good luck..
Will do. The place is called "Metalsmiths" and they're located at:

15 Banton St.
Dorchester Ctr, MA
(617) 265-4040
http://www.bostonmetalsmiths.com/index.html

The people who work there are really chill, especially the water cutting guy. A bit of advice if you plan on working with the guy. He programs CAD into a terminal to operate the water cutter, so if you can come up with the CAD files yourself, you'll save yourself a lot of money that you'd have to pay him to draft it himself. If you have any doubts about the precision of the machine, ask him about the clock he made out of thick stone cut to the exact outline of Germany that he made for a friend

I do have a smallish scope kicking around but not a Variac. I also have an ancient Eico signal generator that desperately needs some calibration/cap replacement, but seems to be working decently.
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Old 13th August 2007, 01:55 AM   #20
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Use those. The scope does not need to be calibrated, must just be able to show something for this exercise.
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