Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => AmpTools/Tech Tips => Topic started by: Bub on September 29, 2011, 09:15:22 pm
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Hi Gent's
I'm just playing around with my scope having fun. I can't find any info on tube scope patterns and don't know what I'm looking at. The amp sounds great with no issues. 5F6A
I have 177 mVpp input at 500 hz. I have my scope set to 5V scale attached to a 4 ohm load on the output. The three pictures are just before clipping occurs. Pic 2319 is normal channel and I know it looks great as output mirrors input.
Pic 2320 is bright channel, just curious about notch top and bottom.
I installed the KOC (best all tube effects loop) with a bypass switch
Pic 2324 is my tube effects loop on, in parallel with nothing plugged in. Send and return pots mid way (it dosen't seem to matter position). It does make the amp sound more sterile. I'm just wondering what I'm looking at.
Thanks for any help
Rob
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Where is your scope connected, at the speaker output?
500Hz is OK. 1000Hz is industry standard. For guitar 400Hz is also a useful standard.
You are looking at distortion, which is defined as any deviation from the input waveform, other than amplitude. Two important types of distortion are clipping, and harmonic distortion. This is the latter. Clipping would show the top, bottom, or both of the waveform shaved off, like a plateau. Often, harmonic distortion occurs in the preamp, clipping in the power tubes.
If you attach your scope's probe to different points along the preamp stages, you can see where distortion occurs. Attach the probe after the coupling cap of a preamp stage, to keep high DC voltage off the scope's input.
E.g., it is possible you are getting preamp distortion which is being cleanly amplified by the power tubes. If you use speakers instead of a dummy load (maybe with an attenuator), you can match-up the sound of the distorted tone with its picture on the scope.
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> Where is your scope connected, at the speaker output?
"I have my scope set to 5V scale attached to a 4 ohm load on the output."
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What does the signal generator look like? Pure Sine, func-gen tit-top "sine", or triangle? Some of that jaggy looks like what you expect from triangles through treble-boost.
Is the 'scope V-gain set in "Cal"? If that is actually 5V/div, it's 12.5V peak which in 4 ohms is only 20 Watts; 5F6A should do 40 Watts.
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This post is very helpful to me. I've read some 50's era "how to interpret o-scope readings" type books as they show waveforms from tube circuits. A year ago I watched a veteran amp tech named Roy diagnose tube amps using an o-scope and I became very, very interested in understanding how to interpret them. Thanks for posting this.
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Signal generator now at 400Hz, pure sign wave, picture 2325 below. Scope channel 1 is set at .1V, 1X scale. Input to amp low channel 1
Cal dials full CW to detent position.
Speaker out is connected to a 4 ohm, 100 watt resistor where I also have channel 2 of my scope connected. 5V/div, 1X scale.
I also have my multimeter attached to the 4 ohm resistor.
Picture 2319 is volume at 5, just before clipping, I'm reading 9.5 VAC on my multimeter at the 4 ohm resistor.
Picture 2326 is volume at 7, clipping, I'm reading 11 VAC at the multimeter
Picture 2327 is volume at 10, I'm reading 14.5 VAC at the multimeter
2 X 6L6GC, plate voltage is 390V, fixed bias, adjustable, each tube set at 50 mA.
Is this formula right? 14.5V divided by 4 ohms = 3.625 amps. 3.625 X 14.5V = 52.56 watts.
Am I only putting out 22.5 watts just before clipping? 9.5V / 4 ohms = 2.375 amps X 9.5V = 22.5 watts.
Thanks for the help
Rob
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I forgot to mention picture 2319 is signal generator at 500 Hz, 2325, 2326, and 2327 are at 400 Hz.
Thanks again
Rob
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Good, now it's clear there is a sine wave input. 2319 shows the beginning of harmonic distortion. Instead of a rounded curve @ top and bottom, it's more of a point -- like a sawtooth or triangle curve. This particular feature is somewhat symmetrical -- the same at the top and at the bottom portions of the wave. Symmetric distortion tends to produce odd-order harmonics which tend toward clarity, then sterility, then harshness. The bottom of the waveform is asymmetrically bent on its leading edge near the peak. Asymmetric distortion tends to produce even-order harmonics which tend toward warmth, but eventually muddiness. So, there's a complex blend of harmonics beginning to emerge.
2326 shows clipping with slightly imperfect symmetry. Clealry something different is going on from your first pictures.
2327 shows nearly perfect, symmetrical, square waves.
A simple Ohms Law variant: Power in Watts = V2 / R; Watts = 9.52 / 4 Ohms = 23W; and so on. See: http://www.el34world.com/Forum/index.php?topic=3273.0 (http://www.el34world.com/Forum/index.php?topic=3273.0) scroll about 1/3 down the page.
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Yes, 20W-25W where distortion soars.
Can you completely bypass/undo this FX loop mod to be sure that's not limiting?
Or insert signal at the power-amp driver (treb pot wiper) to verify just the power section? (You may need 2V-3V here to get full output.)
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When I built the FX loop I installed a true bypass switch to completly removes the loop from the signal chain.
I will insert a signal at the power amp driver today.
Thanks for the knowledge
Rob
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When I insert the signal generator sine wave into the PI, do I have to physically disconnect the tone stack from the PI, pull V1 and V2, or just turn the tone pots to full to stop the signal from being shunted to ground?
Thanks
Rob
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Just try it?
Unless your sig-gen is incredibly weak, simply setting the TREB pot anywhere except the ends should be all-the-same.
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2.7V inserted into the PI is when clipping just started, 12.5V peak on the scope, same as before. By the time I was up to 3.3V noticable clipping was occuring with no rise in peak voltage. Dropped a pair of 6550s in, plugged the signal generator back into the input jack and I get 18V peak before clipping, 13Vac at the speaker output jack with the 4 ohm load. Pic 2328 below.
The 6550s do sound amazing in there. Using the PT calculator I get 213mA for 2 x 6550s and 4 x 12ax7s and my PT is rated for 287mA.
I think this is correct.
Now I'm going to go through and check the pre-amp stage by stage. Just because I can now.
Is there another way of increasing head room in the power amp other than tubes without major mods. Not that I would do it, this thing sounds to good.
I'm still curious as to what is happening in picture 2324 which is tube effects loop on but still sounds good.
Thanks for the awsome lesson
Rob
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This is kind of interesting: http://home.comcast.net/~machrone/bjr/bjtubes.htm (http://home.comcast.net/~machrone/bjr/bjtubes.htm)
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> Dropped a pair of 6550s
Is this really a healthy "5F6A" original/clone with healthy 6L6, or some "improvisation" or tired 6L6es?
What plate voltage, G2 voltage, plate-plate OT load? Idle and while pushed near clipping.
Also plate and cathode voltages on the driver (PI) tube.
> Pic 2328 below
While not flat-topping, it is clearly distorted.
It almost looks like it's been flat-topped and then run through a hi-boost. Or there's a big choke with not enough filter-cap after it.
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Hi PRR and jjasilli,
This is a KOC 5f6a new build. All tubes are NOS, RCA 6L6GC black plate, Tung-Sol 6550 black plate. Plate voltage is 390V,G2 is 385V.
PI is pin 1, 186V. Pin 2, 19.5V. Pin 3 & 8, 29.6V. Pin 6, 188V. Pin 7, 18.5V. The OT is Hammond 1650K and the PT is a 272JX. I haven't had a chance to check plate voltage sag near clipping. Unfortunatley do to health reasons I can't continue right now. Thanks for helping me out and I hope to be back soon to carry on this lesson. This is one part of amp trouble shooting I really want to learn.
Thanks again
Rob
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Feel better soon!
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Feel better soon!
+ 100 !
I'd make you some home made chicken soup and bring it to you, but I don't think were in the same zip code. Hope you can get some rest and heal up. :sleepy2:
Brad