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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 13 May 2016, 02:39 
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Joined: 13 May 2016, 02:02
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Location: Javea, Spain
Hi all,
Brand new on the forum today. 45years ago I wired Hiwatt guitar amps for a living. I could wire one beautifully but had no idea how it worked. I'm in the process of learning about valve amps now (50% light bulb moments when something sinks in. 50% head in hands with frustration when it doesn't). This weekend I'm building the NP-100v12: 12AU7 (ECC82) / IRF510 Headphone Amp so that I can play around with a multimeter without risk of killing myself. My main aim is to build and hopefully understand the Lacewood Amp in the next couple of months. I'm sourcing the parts at the moment and have found a European supplier for Edcor transformers but they don't stock the XPWR163. Can anyone tell me if I could use this one..... https://www.don-audio.com/Edcor-XPWR139-120-240 and what modifications I would need to make to the power supply circuit. Also can anyone give me a part number for the Edcore main choke.
Thanks in advance John.


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PostPosted: 14 May 2016, 05:06 
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Location: Australia
Should you build the amp you will be pleased. I did and I'm happy. Still have it with Australian made black glass 6V6.

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Projects:"retro2308" - chip based headphone amp | ”Calibre 834” - tube phono MM preamp | ”najah” - Raw 180W Tripath Class D power amp | "Icon" - Shuguang CV-181Z preamp


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PostPosted: 21 Sep 2016, 13:42 
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Hi everyone, name's Ron, and my first time posting here. I have been having a discussion with Matt on his website regarding the 6V6 Lacewood amp, and he suggested moving the topic here so here goes.
I completed the amp several days ago, and it seems to have very low volume. Also no bass response, though I suspect those two are related. Matt asked me to first check all B+, bias and plate voltages and they seem to be somewhat low but within spec.
Today I did a signal tracing test from input to output, and got the following results:
Input from function gen of 1.5V P, 1Kz sinewave, volume at max, output connection to 8 ohm dummy load
At grid (pin 5) of 6V6 tubes = 11V P
At plate of (pin 3) of 6V6 = 125V P
At output transformer, across secondary with scope gnd lead to white wire, I get 4.2V P, however, when I gnd the scope lead to to the gnd bus, and measure at the yellow wire, I get 60V P.
While the amp signal measurements look good, I don't know what to make of these last two at the output transformer. I may not be thinking right on this, but if the 4.2V P signal is accurate, that would generate barely over 1W.
I should also say that in normal operation I'm using an iPad at max volume for the input, and thats only about 400mv. I use a pair of small Polk Audio Monitor 30 speakers, and at full volume on the amp, and it's less than a comfortable listening level.
Matt asked to see a picture of the wiring, so I am including that here. Would appreciate any suggestions. I have ample test equipment to work with, and am willing to try about anything. Thanks so much, and apologize for the lengthly post!


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PostPosted: 22 Sep 2016, 19:08 
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I have the same design except I have split HT rails from the first cap. Mine also suffers from no low bass. It rolls off at about 63hz. I took the amp to a friend's place who has 6' tall, 400lb, 94db efficient, double 12" woofer speakers and the bass sound good. My friend thought there was good bass extension where I had said to him that the amp doesn't do bass well.

Lately I beefed up the split PS by adding a 450uf cap on both legs. I did this because my 98db efficient exposed a very slight hum which is now gone. I haven't yet re-tested the bass response but felt the bottom end now better.

Firstly test your bass response with a CRO.

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Projects:"retro2308" - chip based headphone amp | ”Calibre 834” - tube phono MM preamp | ”najah” - Raw 180W Tripath Class D power amp | "Icon" - Shuguang CV-181Z preamp


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PostPosted: 24 Sep 2016, 20:01 
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Location: US Pacific Northwest
Ron;

I can find no error in your wiring and it looks nice and clean. Your DC voltages are in the right ballpark. With a 1.1v RMS input it should be producing about 5 volts RMS across an 8Ω load. This is in the neighborhood of 3.2W. You said that with a 1.5Vp (1.1 v rms input) you are getting 4.2v peak (3v rms) then you are correct this is about 1.1W and your output is about 5dB low.

You need to isolate the response to see where the falloff is occurring. The 6SN7 stage is designed to give a gain of about 8 v/v with an touch of 2nd harmonic. So a 1.5v peak sine wave input should be giving a signal at the volume control of ≈12v peak. First step is to investigate that voltage and see if is correct. If not, you may have a low gain driver tube.

Check the voltage and let us know what you find.

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PostPosted: 24 Sep 2016, 22:49 
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The output is floating, ground the white speaker wires from the output transformer.
.


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PostPosted: 26 Sep 2016, 13:03 
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Matt,
Based on your post, I changed the driver tube with a different 6SN7 and took all my signal measurements again.
With an input of 1.5V p, 1Khz, I now have 13V P at both plates on the driver tube, but still just under 5V P on one channel, and 4.5V+ P on the other. I also checked the signal at the output tubes, and have 140V on one, 130+ on the other. Does that seem like reasonable gain for the 6V6?
Before I shut down the amp, I made one other test - I connected the output to a pair of JBL 8ohm speakers and checked the output voltage. Surprisingly it measured 7V P on both channels. I had been feeding the output to a pair of 50W 8ohm finned resistors. Does this make any sense? It did seem quite loud.
Also, I see one post here suggesting I ground the white output lead. Is that appropriate?
Thanks for all your help, Ron


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PostPosted: 02 Oct 2016, 10:02 
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Ron;

The fact that your output voltage changed when using the resistors vs the speakers is intriguing. I would like to ask a question. Can you tell me exactly how you are measuring the ac voltages? I would like to know exactly what equipment you are using, how it is hooked up, how it is set, and what frequency you are using for your test signals.

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PostPosted: 02 Oct 2016, 11:02 
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RDrew wrote:
With an input of 1.5V p, 1Khz, I now have 13V P at both plates on the driver tube, but still just under 5V P on one channel, and 4.5V+ P on the other. I also checked the signal at the output tubes, and have 140V on one, 130+ on the other. Does that seem like reasonable gain for the 6V6?


Gain looks fine.


RDrew wrote:
Before I shut down the amp, I made one other test - I connected the output to a pair of JBL 8ohm speakers and checked the output voltage. Surprisingly it measured 7V P on both channels. I had been feeding the output to a pair of 50W 8ohm finned resistors. Does this make any sense? It did seem quite loud.


Were you measuring the AC voltage at the speaker with a hand held multimeter or a scope?

Most scopes I believe reference the input to ground (neutral in panel) and if your output of your amp isn't referenced to ground then it might mess up the readings. I don't recommend floating the scope but try lifting the chassis (safety) ground of your amp when taking reading with the oscilloscope. I just had a bass amp on my bench that floats the output and my scope would take a reading until I lifted the safety ground of the DUT.

Or reference the output to ground as I suggested by ground one of the output leads, it doesn't matter which one because you aren't using any feedback. I think it's a safety thing too because of leakage, given a certain scenario the secondary could reach a relatively high potential. In fact most MI power amps that put out over a certain amount of power require a speakon connector for safety.


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PostPosted: 05 Oct 2016, 12:26 
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Joined: 05 Oct 2016, 12:10
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Hi I'm new to building tube amps and I'm looking forward to starting with this amp build. I am a little confused with the schematics however. My question is, where is the Input suposed to hook up? And what do the little points mean that don't seem to go anywhere? They come off the bottom of the audio transformers and are labeled B+272v. Any advice would be greatly appreciated.


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