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Old 16th August 2013, 07:47 AM   #11
cnpope is offline cnpope  United States
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Originally Posted by homeskillet View Post
As a point of possible interest, though, there appears to be some sort of feedback taking place as is. The effect of the 8 ohm tap to the cathode of the 12AT7, in this configuration, is not subtle. The bass is boosted quite a bit and the high frequencies are lowered quite a bit.
Assuming the schematic you posted is correct, any feedback effect you are getting at the moment must be depending on the capacitive coupling between the primary and secondary windings of the output transformer, I suppose. It seems hard to imagine that that could have any significant effect, especially on the bass, since it is feeding back into 100 ohms at the cathode.

If you have a meter, it would be interesting to know what resistance you measure between one of the loudspeaker terminals and ground. (Measured while amp is switched off, of course.) This could serve to confirm whether the OPT secondary really is otherwise floating, or not.

Chris
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Old 17th August 2013, 03:05 AM   #12
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you can triode connect the 6550's,
do a Broskie Aikido configuration
Hi. I love reading Tubecad. That guy is a great writer.

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If the secondary winding is floating, the feedback won't work.
I think I confused myself by trying to imagine what balancing action is taking taking place there. To look at the output, I have the scope on one lead to the speaker and the other on the common ground. The oscilloscope is closed when hooked up this way, so definitely not floating. Thank you.

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I would strongly encourage the person who drew the schematic to use solder dots rather than hoopti-hoops to indicate whether two wires are connected.
I'll use dots and hoopties from now on.

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any feedback effect you are getting at the moment must be depending on the capacitive coupling between the primary and secondary windings of the output transformer
The idea that this could be what is taking place is really exciting to me. I realize my designs need a feedback loop that includes the output transformer.

I hope to have the skills in the near future to compensate the phase of the feedback. I'm still pretty early on in my studies in this regard though.

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it would be interesting to know what resistance you measure between one of the loudspeaker terminals and ground
I measured about 100 ohms between each speaker lead and ground on an old kludgy analog meter I got at goodwill.

I've got an old giant 50 watt 500 ohm rheostat as the 12AT7 cathode resistor. If I sweep it it's kind of like a mild tone/presence control. I have it set about 1/5th of the way so the meter is in the ballpark.

Thank you guys for talking with me about this thing. -Fred
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Old 17th August 2013, 10:51 AM   #13
seta1 is offline seta1  United States
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20k square wave:
Direct Connected 12AT7/6550 to Single Driver/Speaker - 2k square wave - YouTube
I increased the volume to full output but started it out lower just to be more gentle on things.

This is at 20Khz or 2Khz? Man, that lead edge of the square wave is brutal. I have found that any spike in the lead edge of a square wave, when I test my amps, means a harshness in the sound quality and for me, a lot of ear fatigue when listening to the amp. What does the square wave look like through a simple resistor load? I am assuming, from what you have written here, that the load is a speaker. Did you have any negative feedback enabled when you produced this wave from?
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Old 17th August 2013, 06:47 PM   #14
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This is at 20Khz or 2Khz?
Hell, I need an editor. I see my error. It's a 2kHz square wave.

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I am assuming, from what you have written here, that the load is a speaker.
Yes, the load was the speaker on the video and everything was connected as it is on the schematic.

Without the feedback loop, the rise is higher on the square wave. Also with the speaker as the load.

BTW, this is the best square wave performance I've had out of any amp I've made so far. Earlier ones were totally crazy. I mean totally as in not safe.

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What does the square wave look like through a simple resistor load?
The results are attached below. I hooked up an 8 ohm 25 watt rheostat as the load. The shape didn't change much from low volume to max on my sound card. The first pic is without the bypass resistor and the 8 ohm tap run to ground. The second pic is what's on the schematic with the bypass resistor and 8 ohm tap running to the cathode of the 12AT7.

I remember seeing a square wave distortion guide with pictures and the likely distortion culprits and their deviations on the shape of the wave. I hope I can find it again.

I ran it at full volume for quite a bit and the rheostat was warm but not hot like I was expecting.

Thanks for taking a look and giving me a use for my morning coffee.

-Fred
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File Type: jpg 2 kHz no FB.jpg (392.1 KB, 158 views)
File Type: jpg 2 kHz w - FB.jpg (379.8 KB, 148 views)
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Old 17th August 2013, 06:55 PM   #15
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Originally Posted by homeskillet View Post
I measured about 100 ohms between each speaker lead and ground on an old kludgy analog meter I got at goodwill.
-Fred
So, your output is not grounded except for the feedback circuit. I think this is not what you want. Not an expert on these things, but I had odd warts and farts on my mark III build until I grounded the secondary of the opt. Ground the ground speaker tap. 100 ohms does not mean grounded. 0 ohms means grounded.
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Old 17th August 2013, 08:35 PM   #16
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100 ohms does not mean grounded. 0 ohms means grounded.
I hadn't thought of that. I was merely concerned whether it was floating or not.

Here is an interesting and very informative thread related to grounding the output:
Tubelab SE Output Transformer Grounding?

My plan isn't to build a stand alone amplifier but a powered speaker that is all contained but I'm still very glad to know this. Currently it's on the bench with a powder fire extinguisher nearby.

You've given me much food for thought. Thank you. -Fred
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Old 17th August 2013, 08:53 PM   #17
freddi is offline freddi  United States
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is the main purpose of the CCS in this case to allow getting things biased so it can be direct coupled? - eat up half the B+ ? what is the dynamic Z of that CCS? capacitors have "seasoning flavors" plus comfortable operating points aren't a bad idea
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Old 18th August 2013, 05:43 AM   #18
cnpope is offline cnpope  United States
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Originally Posted by homeskillet View Post
Hell, I need an editor. I see my error. It's a 2kHz square wave.



Yes, the load was the speaker on the video and everything was connected as it is on the schematic.

Without the feedback loop, the rise is higher on the square wave. Also with the speaker as the load.

BTW, this is the best square wave performance I've had out of any amp I've made so far. Earlier ones were totally crazy. I mean totally as in not safe.



The results are attached below. I hooked up an 8 ohm 25 watt rheostat as the load. The shape didn't change much from low volume to max on my sound card. The first pic is without the bypass resistor and the 8 ohm tap run to ground. The second pic is what's on the schematic with the bypass resistor and 8 ohm tap running to the cathode of the 12AT7.

I remember seeing a square wave distortion guide with pictures and the likely distortion culprits and their deviations on the shape of the wave. I hope I can find it again.

I ran it at full volume for quite a bit and the rheostat was warm but not hot like I was expecting.

Thanks for taking a look and giving me a use for my morning coffee.

-Fred
It's not entirely clear to me what your test setup is. If I am understanding you correctly, that second scope trace is obtained with the amplifier configured as in your schematic, with the OPT secondary supposedly "floating" aside from the connection to the lower cathode of the input stage? However, you then connect the output to an oscilloscope (or computer with sound card?) Does that mean you are now grounding one of the OPT secondary terminals through the grounding of the oscilloscope? In which case, you now have the OPT centre-tap connected to the lower cathode of the input stage and, and one or other of the endpoints of the OPT secondary connected to amplifier ground via the two mains cords of the amplifier and the oscilloscope.

So apparently the 100 ohm cathode resistor in your schematic is being bypassed by one half of the OPT secondary, when your oscilloscope is hooked up for the measurement. This would completely change the operating point of the tube. And it's not clear which end of the OPT secondary is the one you are grounding, so the feedback, such as it is, could be either positive or negative depending on which OPT lead you chose to connect to oscilloscope ground. Anyway, maybe this might go some way to explaining the rather bad traces you are getting.

(Actually, it's not clear from you schematic what measures, if any, you have taken to connect amplifier signal ground to mains ground. The above was written under the assumption that your amplifier signal ground is connected directly to mains ground. If not, then depending on how you have it configured, some of the above statements might need modification.)

Chris
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Old 18th August 2013, 11:08 AM   #19
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Originally Posted by freddi View Post
is the main purpose of the CCS in this case to allow getting things biased so it can be direct coupled?
Yes, you hit the nail on the head. Also I wanted to see if there was any balancing action that could counteract drift in the circuit. Unfortunately it eats a bunch of current but then it sounds nice and that's why I'm building it. So, yes, I'm conflicted.

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what is the dynamic Z of that CCS?
If that tube stays there much longer, I'll try to find out since I'm now curious.

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capacitors have "seasoning flavors" plus comfortable operating points aren't a bad idea
I'm planning on trying a bypassed resistor in place of that tube. I've got to buy one though unfortunately. Hopefully tomorrow.

BTW, my transmission line has a Karlson cut on the end and my name's Fred too! -Fred
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Old 18th August 2013, 11:36 AM   #20
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Originally Posted by cnpope View Post
It's not entirely clear to me what your test setup is.
This is where things could get confusing. I'll do my best.

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Does that mean you are now grounding one of the OPT secondary terminals through the grounding of the oscilloscope?
Nope. The ground of the scope is on the common ground and the test lead is on either of the speaker lead.

The only difference between the tests is that one was taken as the circuit is shown in the schematic.

The other test was without the feedback loop. Meaning the 8 ohm tap was connected to common ground along with the oscilloscope ground and then the test lead of the scope was on either speaker lead (as well as the "bypass" capacitor removed).

To be sure, at no time was the oscilloscope ground connected to either of the speaker leads. This would cause some some unwanted behavior for sure as you mention.

I hope I cleared things up O.K. I've never written a description of a circuit or anything electronic before posting here. It's quite challenging. -Fred

BTW, I don't believe the secondary of the output transformer is floating since there is a ground reference through the 8 ohm tap and 100 ohm resistor and I'm to the point of loop equations with two batteries.
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