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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Mar Mon 04, 2019 6:52 pm 
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If you are happy with the speaker, I imagine so, you could downgrade to 6V6 or 6F6 power tubes, no?

It just looks like you are powering a sports car with a honking big V8, altho there is some appeal to the Cobra concept. Looks good. 8)

If the EMT was causing any kind of phase issues with the grounding conductor, a length of PVC tubing could be inserted, I think.
Re; the glass fuse vs breaker, agreed. An aging breaker might need to be replaced in case it is starting to seize up, a mechanical device, while an antique fuse will always be good.
But either way, 15 amps will smoke the amp, so it needs about a 1 amp fuse.

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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Mar Mon 04, 2019 8:50 pm 
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I have a pile of 6V6 tubes, and started with them, but since I have the 6L6 tubes, I may as well use them.

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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Mar Tue 05, 2019 5:21 am 
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One of the things I found while working on this is that it worked much better with a large capacity after the choke in the power supply. I had a 1,000 ohm wire-wound resistor in the B+ to the 6SN7 tubes, with a 30MFD on either side of it. While it was playing, I (carefully!) put a jumper across that resistor. I learned that the resistor was not needed, and that making the capacitor after the choke 60 MFD caused it to sound better. I then temporarily connected an additional 68 MFD radial capacitor, and that was better yet. I removed the 1,000 ohm resistor, and used a 50 MFD I had with the two 30MFD units already in the chassis to come-up with 110 MFD.

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 Post subject: Re: How about this 6L6 amplifier
PostPosted: May Thu 09, 2019 6:07 pm 
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Here is the bottom (the brown wires are chassis ground)
Image

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 Post subject: Re: How about this 6L6 amplifier
PostPosted: May Sun 19, 2019 11:23 pm 
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It has a strange problem, but not all of the time. Even with no input, the speaker cone will be moving at a low frequency, perhaps 1 to 2 cycles per second. It will do it sometimes, and not other times. It played for a few hours yesterday with no problem, then I changed the station and came back to see the speaker doing the low frequency movement, even with the tuner off. Any suggestions?

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Last edited by FStephenMasek on May Mon 20, 2019 7:14 pm, edited 1 time in total.

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 Post subject: Re: How about this 6L6 amplifier
PostPosted: May Mon 20, 2019 1:21 am 
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Thinking the input filter capacitor might be too small, I changed it to 40MF. That did not fix the problem.

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 Post subject: Re: How about this 6L6 amplifier
PostPosted: May Tue 21, 2019 2:46 am 
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Did you unplug the tuner from the amp to narrow it down?

If the fault is in your amp, does it seem like a capacitor is discharging slowly, applying a voltage to the speaker, then recharging and discharging again at 1 or 2 cycles/sec?

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 Post subject: Re: How about this 6L6 amplifier
PostPosted: May Tue 21, 2019 3:14 am 
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Yes, the first thing I did was try it without the audio input. It is definitely in the amplifier.

I tried a few things:

1) Changing the 6SN7 tubes. The Philco one I had in it gave different reading for each triode section on my Hickock tube tester, so I replaced it. No difference.

2) Disconnecting the negative feedback was the same or worse - I think worse.

3) I tried it with just one cathode resistor for both audio output tubes, with a 220 MFD capacitor. No difference, so I put it back to two resistors, but with 220MFD capacitors.

4) I added a 10,000 ohm stopper resistor before the input grid, and a 400MMFD anti-ringing capacitor from there to ground. Seeing that quite a few other amplifiers use a 1 Meg. resistor from the input connector to ground, I replaced the 470,000 unit I had there with a 1 Meg. unit. No change.

5) I increased to 10 MFD capacitors to 47 MFD each. No change.

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Last edited by FStephenMasek on May Tue 21, 2019 5:55 pm, edited 1 time in total.

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 Post subject: Re: How about this 6L6 amplifier
PostPosted: May Tue 21, 2019 1:05 pm 
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On your schematic back on page 2 I noticed the coupling capacitors at the 6SN7 grids as well as the 6L6 grids are all .25µF. This will cause instability like you are experiencing because the phase shifts introduced by the capacitors will add up to 180° at some low frequency. Try reducing the value of the capacitors at the 6SN7s to .022µF and see how that works. This is known as "pole splitting"; it places the low frequency cutoffs at two different frequencies so the phase shifts don't add together at the same frequency, and one will dominate over the other and cause the gain to fall below unity before 180° of phase shift can occur and cause oscillation.


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 Post subject: Re: How about this 6L6 amplifier
PostPosted: May Tue 21, 2019 3:17 pm 
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Another solution is to make the capacitor values large enough to where the cutoff frequency is well below 20Hz.

For determining what coupling cap values to use in an amp I use this site http://sim.okawa-denshi.jp/en/CRhikeisan.htm

That site will show you the response and phase shift on separate graphs.

Here's what I calculated.

Stock value for 6SN7

Attachment:
6SN7 coupling.png
6SN7 coupling.png [ 25.68 KiB | Viewed 823 times ]


Different value for 6SN7

Attachment:
6SN7 coupling 2.png
6SN7 coupling 2.png [ 25.25 KiB | Viewed 823 times ]


Stock value for 6L6

Attachment:
6L6 coupling.png
6L6 coupling.png [ 25.25 KiB | Viewed 823 times ]


Different value for 6L6

Attachment:
6L6 coupling 2.png
6L6 coupling 2.png [ 25.36 KiB | Viewed 823 times ]


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 Post subject: Re: How about this 6L6 amplifier
PostPosted: May Tue 21, 2019 11:26 pm 
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Quite interesting!

Have you seen the article in the April 1957 issue of Wireless World on building a 50-watt amplifier? The have resistors and capacitors between the first duo-triode and the second to prevent both high and low frequency problems, and a discussion of them in the article (it starts on page 8 of 52):

https://www.americanradiohistory.com/Ar ... 957-04.pdf

That article, and this one were the two main ones I studied when designing the first version of this amplifier:
https://www.bonavolta.ch/hobby/images/audio/6l6.gif

This one on grid stopper resistors was also very interesting, especially since I found quite a few other designs without them: http://www.valvewizard.co.uk/gridstopper.html

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 Post subject: Re: How about this 6L6 amplifier
PostPosted: May Thu 23, 2019 3:29 am 
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Here is the power supply and output section of the Stromberg-Carlson 1201-H I started with. It had a resistor in the B+ between the feed to the output tubes and the feed to the other tubes. I'd put that in, then remeoved it, thinking it did noting. I will put it back into Version II of this amplifier.

Image

Several people here and on diyaudio.com have recommended smaller coupling capacitors. I respect and appreciate that, but wonder why Messrs. Heath and Woodville who wrote the April 1957 article in Wireless World (starting on page 158, see American Radio History) "Design for a 50-Watt Amplifier" 0.25MFD coupling capacitors between the first and second tube, and 0.5MFD units between the second tube and the output tubes? Here is their schematic:

Image

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Last edited by FStephenMasek on May Thu 23, 2019 9:39 pm, edited 1 time in total.

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 Post subject: Re: How about this 6L6 amplifier
PostPosted: May Thu 23, 2019 12:25 pm 
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A larger coupling cap would be to reduce phase shift as seen in my earlier post.


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 Post subject: Re: How about this 6L6 amplifier
PostPosted: May Thu 23, 2019 1:24 pm 
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The KT88 amplifier in the schematic above gets away with the larger coupling capacitors because it has dominant pole low frequency compensation in the form of the 4.7Mohm resistor/.005µF capacitor parallel combination; this rolls off the low frequency gain below unity before 180° of phase shift can accumulate from the multiple RC time constants involving the larger value coupling capacitors and turn the amplifier into a low frequency oscillator.

Simply increasing the values of the coupling capacitors might work, but this is merely hoping that the natural low frequency rolloff of the output transformer becomes the dominant pole instead. IMHO this is not good design practice.

Paradoxically it is possible to use a better output transformer in a given tube amplifier design and get worse performance due to the wider bandwidth of the transformer allowing oscillation to occur where it did not before with a lesser transformer. That is the reason for all the frequency compensation networks in that KT88 schematic above.

So, the solution for the amplifier design being discussed here is dominant pole compensation, whether in the form of a reduced coupling capacitor value between the first and second voltage amplifier stages or adding a parallel RC network between them like the KT88 example above.


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 Post subject: Re: How about this 6L6 amplifier
PostPosted: May Thu 23, 2019 1:45 pm 
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Reducing coupling cap value will increase the phase shift plus it may roll off the bass some depending on what value the capacitor is reduced to.

Phase shift I don't think is beneficial and rolling off of the bass can be beneficial if the speakers cannot reproduce below a certain frequency.


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 Post subject: Re: How about this 6L6 amplifier
PostPosted: May Thu 23, 2019 9:40 pm 
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Interesting - "dominant pole" is a new phrase for me.

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 Post subject: Re: How about this 6L6 amplifier
PostPosted: May Fri 24, 2019 9:59 pm 
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I found this web page in a search:

http://www.angelfire.com/electronic/funwithtubes/Amp-Compensation.html

It mostly covers high frequency compensation, but has a paragraph about the low end and says about the same thing I did; keep the low frequency cutoffs very low except for one place.

The other way to ensure stability is to keep the RC coupling to only one place; but that requires direct coupling in the voltage amplifier stages like the pentode-split load phase inverter designs use for example. The OP circuit can be made to work, it just needs a few minor changes.


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 Post subject: Re: How about this 6L6 amplifier
PostPosted: May Fri 24, 2019 10:09 pm 
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First an oscope should be used to find where the oscillation is in the amp as that will give more of a clue as to what is causing it.


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 Post subject: Re: How about this 6L6 amplifier
PostPosted: May Sun 26, 2019 5:13 pm 
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A slightly larger chassis came, so I can get going with building Version II of this amplifier. The old power transformer was running hot, significantly more so than what seems normal for radios, so I finally decided to buy an Edcor XPWR011.

I found another of the schematics which I'd found and studied before building Version I of this amplifier:
Image

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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Jun Tue 04, 2019 1:11 am 
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What I wrote on the drawing is incorrect. I measured the second 6SN7 plate voltages (to ground) as 181 and 222. Perhaps I need a balancing rheostat?

It is working better than ever with the changes shown on the schematic below. The bass seems better with these changes.

Image

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