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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Feb Sat 02, 2019 7:17 am 
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Location: Mission Viejo, southern California
I have an output transformer good for 60Watts, so some negative feedback would be good. The transformer is a Hammond 1650PA: http://hammondmfg.com/pdf/EDB1650PA.pdf

I have a 6SL7, a dual triode with separate cathodes and a gain of 70. I also have two NOS 6N7GTBs, which are dual triodes with separate cathodes and a gain of 20. What if I use both and build something like this from the 1950's:
http://www.r-type.org/articles/art-154.htm

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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Feb Tue 05, 2019 7:36 pm 
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Far as phase splitters go here's one I got from a GEC 912 Plus amplifier schematic and it works quite well with lower mu triodes such as a 6SN7 and 12AU7 and others.

Attachment:
Phase inverter 1.PNG
Phase inverter 1.PNG [ 3.27 KiB | Viewed 1010 times ]


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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Feb Tue 05, 2019 8:12 pm 
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My 6SL7 was bad, but I have two NOS 6SN7 tubes. I've been studying various other designs, and considering the voltages provided by the power transformer. The radio used about 167.1ma of plate current and 3.3A of filament current, while this design will have about 140ma of plate current and 3A of filament current.

Image

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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Feb Tue 05, 2019 10:50 pm 
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Joined: Sep Thu 23, 2010 6:37 am
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Location: Powell River BC Canada
The circuit is coming along, and now some hum abatement should appear
in a diagrammed approach to ground management.

Another thing about the Hammond transformer, while the use of the 4 ohm tap
makes sense for the speaker at hand, given the cone compliance, the feedback
might become 'motional' to the extent that cone excursions may intermodulate
higher frequency components.

Here just for reference is how a Western Electric 6L6 (350B) used the output
transformer to maximize copper usage. Whatever impedance was desired,
it took the output from the ends of the secondary winding. Also it uses two
feedback loops. It can run at 20 watts, just by moving the HV secondary
taps. Note resistor in series with 6L6 plates and screens connected to
voltage divider.

This amplifier was not intended to be solid gold. Just a bread and butter unit
that would fit into a multiple of applications a telephone company would need
to amplify speech under the license terms of the tubes. Typically, It would
just sit in a rack working until the hours were up on the tubes, and they would
be changed out.
Attachment:
W N E  124 A shown as 12 watt 7 and a half ohm output  VE7ZSO.jpg
W N E 124 A shown as 12 watt 7 and a half ohm output VE7ZSO.jpg [ 230.55 KiB | Viewed 1002 times ]
Attachment:
N W.jpg
N W.jpg [ 162.44 KiB | Viewed 1002 times ]
Attachment:
W N E   f   VE7ZSO.jpg
W N E f VE7ZSO.jpg [ 254.37 KiB | Viewed 1000 times ]

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Amateur Radio Literacy Club. May we help you read better.
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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Feb Wed 06, 2019 2:51 am 
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Yes, I see that "star" grounding is often recommended, that is, bringing all grounds to one point.

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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Feb Wed 06, 2019 3:15 am 
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Location: Powell River BC Canada
With the good Hammond transformer, you can check the effect of the feedback
by running the amplifier into variable load.

With a 1000 Hz tone producing a sin wave across the 4 ohm terminals. into a
100 ohm variable, measure the voltage.

Then reduce the load to 4 ohms. Measure the voltage again.

Then reduce the load, until it is exactly half the voltage that it was at 4 ohms.

The ratio of 4 ohms to the half voltage load resistance indicates the damping factor.

The value may increase with the feedback. connected.

Once the damping factor is known, you should have enough ammunition to start
up a debate that probably has been on going for seventy years.

Also I doubt the Mr. Hammond would offer the warranty if a damping factor test
was known to have been done . :shock:

http://www.cieri.net/Documenti/JBL/Docu ... te%20(1967).pdf

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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Feb Wed 06, 2019 3:19 am 
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Location: Texas. USA
FStephenMasek wrote:
I have an output transformer good for 60Watts, so some negative feedback would be good. The transformer is a Hammond 1650PA: http://hammondmfg.com/pdf/EDB1650PA.pdf

I have a 6SL7, a dual triode with separate cathodes and a gain of 70. I also have two NOS 6N7GTBs, which are dual triodes with separate cathodes and a gain of 20. What if I use both and build something like this from the 1950's:
http://www.r-type.org/articles/art-154.htm
You might want to consider this recent build right here in Homebrew.
viewtopic.php?f=12&t=338702&start=980

That is a Williamson derivative I designed for John, from a design I was doing for myself. He built it and is quit satisfied.

While the design shows 807s for the output that is for 'historical nostalgia' (the tube that "won the war"). The 807, however, is electrically equivalent to the 6L6 and either can be used with no changes other than the socket (which was my intent).

My mods essentially make the amp compatible with modern sources. Basically, there's more gain from the 6SL7 and the preamp plate bootstrap, which both maximizes gain and lowers distortion.


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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Feb Fri 08, 2019 5:06 am 
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FStephenMasek wrote:
Yes, I see that "star" grounding is often recommended, that is, bringing all grounds to one point.
A true 'star' ground isn't practical in a tube amp because of the long wires that would be needed to take the various stages to the common point. "Local staring" is preferred, as in
Attachment:
Grounding.jpg
Grounding.jpg [ 71.82 KiB | Viewed 960 times ]


Each stage is starred to it's respective filter capacitor. The current that circulates through each stage is returned to the capacitor from whence it came leaving only (quiet) DC (and whatever ripple that propagates) between filter caps. Then the most sensitive stage is taken to chassis ground.


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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Feb Fri 08, 2019 12:26 pm 
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Location: Warner Robins, GA
Grounding and how it is done is very important as it can be the difference between a ground loop and no ground loop.

I find the best option for grounds that are not soldered to the chassis is to use the ground wire from a piece of romex as it provides a good solid low resistance path.


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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Feb Fri 08, 2019 4:56 pm 
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Location: Mission Viejo, southern California
I've got an aluminum chassis, so soldering to it is not a realistic option. I did read about the localized star approach. Yes, I did think about using a piece of number 12 copper as a ground bus to connect the localized grounds, so it is nice to read that it works.

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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Feb Fri 08, 2019 5:19 pm 
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Joined: Sep Thu 23, 2010 6:37 am
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Location: Powell River BC Canada
There are two ripple components in amplifiers. They are countered somewhat in
the output stage. The main component is the cathode current which flows up
the cathode resistor because that furnishes the plate and screen current.

The lessor component is both hum and ripple fed to the grid that is picked up
along with the signal. To the extent that modulates the (output) cathode current depends
on the gain and phase that was added along the stages. Feedback is a way of
countering hum. There are other sly ways.

Changing capacitors because 'bigger must be better' is a sure path to making
sure it can't ever sound the same.

Some of these things raise spectres of finding out the hard way.

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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Feb Sat 09, 2019 10:10 pm 
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Steven your new schematic looks better to me, in that it could have been be copied from a tried and true design already established by engineers in the 40s and 50s.
So then I would just go with the resistor values specified for that design and those tubes. 8)

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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Mar Mon 04, 2019 12:39 am 
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It is playing nicely and it is fun to listen to my 1951 Browning RJ-12B tuner with it. I'd have to buy a different power transformer to get significantly more out of these tubes. With the voltages shown, they are being driven gently, so should last a long time.

Image

With the Browning tuner and Renegade speaker I intended to use:
Image

The schematic
Image

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Last edited by FStephenMasek on Mar Tue 05, 2019 5:13 am, edited 1 time in total.

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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Mar Mon 04, 2019 2:02 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 20393
Location: Warner Robins, GA
For the cathodes of the 6L6 tubes try shorting them together and see if it does any better.


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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Mar Mon 04, 2019 3:32 am 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 11807
Location: Powell River BC Canada
Nice EMT installation. A question. How many feet does the circuit
take to run back to he breaker panel, and there is a separate green grounding
used used in the conduit ?

Attachment:
Nice EMT installation.JPG
Nice EMT installation.JPG [ 14.63 KiB | Viewed 762 times ]

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VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Mar Mon 04, 2019 5:45 am 
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Location: Mission Viejo, southern California
It is quite loud with the tuner volume control full on, even though all I've done with that 68 year old tuner is replace the power supply filter capacitors. I suspect it will be even louder once I go through the tuner chassis. That speaker cone really moves on the bass notes.

Regarding the EMT- Thanks! Yes, there is a separate ground conductor, the conductors are number 12, and the distance is less than 60 feet.

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Last edited by FStephenMasek on Mar Tue 05, 2019 6:15 am, edited 1 time in total.

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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Mar Mon 04, 2019 7:05 am 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 11807
Location: Powell River BC Canada
Something to consider is instantaneous fault current you have at your
receptacles compared to what was available in the era the radios you restore
had. Ground wire is very good, however EMT rattles and buzzes if a phase
to pipe happens. Especially the fix bolts at coupligs and offsets.

Today, this factor has been recognized by agencies certifying devices providing
power through other than the cord set provided.

Years ago, an instructor said a common 15 Amp fuse will blow before any
breaker will trip.

I have a 15 Amp glass fuse in series with stuff here most times.

Attachment:
Usage warning  for product rated at 19500 spike amps and 1875 watts.jpg
Usage warning for product rated at 19500 spike amps and 1875 watts.jpg [ 82.88 KiB | Viewed 756 times ]

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Steve Dow
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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Mar Mon 04, 2019 12:49 pm 
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What exactly is EMT?


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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Mar Mon 04, 2019 3:57 pm 
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Location: Mission Viejo, southern California
The thin steel conduit is, in electrician's lingo, Electrical Metallic Tubing.

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 Post subject: Re: How about this 6L6 amplifier
PostPosted: Mar Mon 04, 2019 4:23 pm 
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Oh ok.


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