AN Wonderland/JM 100w layout

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jelle
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Re: AN Wonderland/JM 100w layout

Post by jelle »

Bombacaototal wrote:
jelle wrote:Ok, what is the idea with NFB? What does it do? :D
I've seen some old posts with a lot of confusion about the GNFB which also added a question mark to myself

Here is one:
http://ampgarage.com/forum/viewtopic.php?t=606

There were other posts but cannot find them again
Great, read the posts by Gil, and there is your answer. :D
Bombacaototal
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Re: AN Wonderland/JM 100w layout

Post by Bombacaototal »

jelle wrote:
Bombacaototal wrote:
jelle wrote:Ok, what is the idea with NFB? What does it do? :D
I've seen some old posts with a lot of confusion about the GNFB which also added a question mark to myself

Here is one:
http://ampgarage.com/forum/viewtopic.php?t=606

There were other posts but cannot find them again
Great, read the posts by Gil, and there is your answer. :D
Thanks Jelle. For me the thread was not conclusive so I was not sure if I should follow the rationale

Basically the ratio will be between 1.4 and 1.47. So for the JM amp from this thread I would need to divide by ratio once for a 50watt version. The 12k NFB (8 ohm tap) would then be between 8.2k and 8.6k (ie 12k/1.4=8.6k or 12k/1.47=8.2k). So a 25W version would be between 5.6k and 6.1k (8.6k/1.4=6.1k or 8.2k/1.47=5.6k)

Found this post which seems to have landed at the same range although the starting value was the 4.7k and not the 12k
http://ampgarage.com/forum/viewtopic.ph ... ght=reverb

My view of the 4.7k is a bit different. 4.7k from dumbles has a reference of 4ohms. For an 8ohm it should be 5.6k (100watts)*. So to use Gil's ratio it should get between 2.6k and 2.8k for 25watts

*reference: http://pdfelectronics.com/dumble-ods-124/
Finally since I was using a 8Ω speaker while ODS #124 used a 4Ω speaker I had to change the NFB resistor to ensure I was getting the correct amount of NFB when using a 8Ω speaker as I would if I used a 4Ω speaker. To do this I simple increased the 4.7KΩ NFB resistor to 5.6KΩ.
Last edited by Bombacaototal on Thu Feb 23, 2017 5:06 pm, edited 1 time in total.
amplifiednation
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Re: AN Wonderland/JM 100w layout

Post by amplifiednation »

I agree with sticking close to a layout as you start out, but just build it...then see how you like it.

And if there's something you aren't sure about (like an NFB value) then put it on a switch. I find it much more rewarding to execute a clean, well thought through build with solder joints that will last forever then agonizing over a few resistor values.

Like try building one without using zip ties, only shrink wrap... :D :shock:
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Bombacaototal
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Re: AN Wonderland/JM 100w layout

Post by Bombacaototal »

amplifiednation wrote:I agree with sticking close to a layout as you start out, but just build it...then see how you like it.

And if there's something you aren't sure about (like an NFB value) then put it on a switch. I find it much more rewarding to execute a clean, well thought through build with solder joints that will last forever then agonizing over a few resistor values.

Like try building one without using zip ties, only shrink wrap... :D :shock:
Thanks Taylor. As far as NFB my intention was to have a 3-way switch: no NFB, Mayer NFB (12k ->8ohm tap), #124 NFB (5.6k -> 8ohm tap equivalent to 4.7k -> 4ohm tap).

Now just confused if I should do this scaling thing. I might just try both and see
bluesfendermanblues
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Re: AN Wonderland/JM 100w layout

Post by bluesfendermanblues »

A small pot (eg. 10k) in series with the feedback resistor, gives a nice "loudness control" for low level playing.
Diva or not? - Respect for Mr. D's work....)
Bombacaototal
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Re: AN Wonderland/JM 100w layout

Post by Bombacaototal »

I was doing some more detailed calculations on the GNFB and also added other reference amps

I am assuming Taylor is connecting to the 8ohm Tap for the 12k/390

Conclusions are:
To have my 25W at same range as the 100W Dumbles (ie #124) with 4.7k/390 (4ohm) I need 3.2k/390 (8ohm)

To have my 25W at same range as Taylor's Mayer Two rock with 12k/390 (8ohm) I need 5.8k/390 (8ohm)

Does this look about right?

Does the fact that my OT primary is 8k influence any calculation I might have ignored?
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Bombacaototal
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Re: AN Wonderland/JM 100w layout

Post by Bombacaototal »

Doing some calculations and simulations I realised that the same NFB and shunt resistors (ie 4.7k/390) would work the same way (have the same effect) on the following configurations: 100W with 4ohm, 50W with 8ohm, 25W with 16ohm

Is this a correct assumption? If so I can just plug the GNFB to the 16ohm and use the values of the 100 W reference amps
Bombacaototal
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Re: AN Wonderland/JM 100w layout

Post by Bombacaototal »

Reviewing the layout there is one more thing I cannot understand. Is the 10KL pot (trimmer) close to the 6L6s the Phase Inverter Trimmer which will be used to unbalance the PI Loads (6.5 to 7 volts apart) in order to maximize harmonic feedback? Or is it used for Biasing purposes?
telentubes
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Re: AN Wonderland/JM 100w layout

Post by telentubes »

10K Biasing Pot
Bombacaototal
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Re: AN Wonderland/JM 100w layout

Post by Bombacaototal »

telentubes wrote: Fri Mar 03, 2017 3:00 am 10K Biasing Pot
Thanks a lot :)

I heard that fixed bias with a balance pot adjustment is not a good idea:
1. If valves are swapped for a pair with different characteristics then it could potentially ruin the valves and/or the power transformer
2. With use, valve characteristics vary with time so bias should be adjustable to compensate
3. Power valves are generally matched so a single variable bias voltage control is perfectly adequate for the job
4. An additional balance control should only be installed if valves are likely to be unmatched (e.g. NOS)

Just wondering what is everyone else's thought on this
Bombacaototal
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Re: AN Wonderland/JM 100w layout

Post by Bombacaototal »

EDIT: I spent some time checking the gut shot of the Two Rock mayer Prototype (attached) and comparing to Taylor's (the limited edition signature) and they do look fairly identical
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Last edited by Bombacaototal on Thu Mar 30, 2017 10:00 am, edited 2 times in total.
Bombacaototal
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Re: AN Wonderland/JM 100w layout

Post by Bombacaototal »

Last edited by Bombacaototal on Mon Mar 13, 2017 11:09 pm, edited 3 times in total.
sturgeo
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Re: AN Wonderland/JM 100w layout

Post by sturgeo »

What chassis are people using for these?
I want to build a 50W version and have just missed the boat as AN has stopped selling them :(

I'm in the UK but use mouser on a weekly basis, they have a decent selection of Hammond models.
Bombacaototal
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Re: AN Wonderland/JM 100w layout

Post by Bombacaototal »

Bombacaototal wrote: Tue Feb 28, 2017 11:10 pm Doing some calculations and simulations I realised that the same NFB and shunt resistors (ie 4.7k/390) would work the same way (have the same effect) on the following configurations: 100W with 4ohm, 50W with 8ohm, 25W with 16ohm

Is this a correct assumption? If so I can just plug the GNFB to the 16ohm and use the values of the 100 W reference amps
Anyone?
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rogb
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Re: AN Wonderland/JM 100w layout

Post by rogb »

sturgeo wrote: Fri Mar 10, 2017 12:26 pm What chassis are people using for these?
I want to build a 50W version and have just missed the boat as AN has stopped selling them :(

I'm in the UK but use mouser on a weekly basis, they have a decent selection of Hammond models.
Could the normal CE chassis be used?
http://cepedals.com/Dumble-Style-Chassis.html

Otherwise give RJ a shout on here

http://ampgarage.com/forum/memberlist.p ... ile&u=1709
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