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Old 21st January 2012, 09:00 PM   #11
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Double up on the output tubes that givs you more power.
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Old 21st January 2012, 10:20 PM   #12
djn is online now djn  United States
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I'll be watching this build. I want to build one but have another project ahead of it.
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Old 22nd January 2012, 10:53 AM   #13
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To me, and no offence to Tim or anyone else--The design is fundimentally flawed in its feedback loop.

The FB gain changes depending on input vol-pot position, this is sure to change the character of the sound depending on the volume control position!

Ive tried this type of FB, and was never happy with this 'colouring' effect.....
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Old 22nd January 2012, 08:05 PM   #14
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Quote:
Originally Posted by Alastair E View Post
To me, and no offence to Tim or anyone else--The design is fundimentally flawed in its feedback loop.

The FB gain changes depending on input vol-pot position, this is sure to change the character of the sound depending on the volume control position!

Ive tried this type of FB, and was never happy with this 'colouring' effect.....
I tried measuring the input impedance at the grid of the input tube V1a, and it seems to be about 1.5K. This is very small compared with R1, which is 34K. I think this means that the effect you are speaking of, although real, is rather small. If my estimate is correct, I think it means the feedback, which is nominally 26dB, changes by less than about 0.5dB as the volume pot is varied. Certainly, in practice, I find no discernible change in the feedback as a function of the volume pot setting.

As Tim Mellow remarks in his article, an alternative is to hook the input to the grid of V1b instead.

Chris

Last edited by cnpope; 22nd January 2012 at 08:20 PM.
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Old 22nd January 2012, 11:24 PM   #15
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Quote:
Originally Posted by Alastair E View Post
To me, and no offence to Tim or anyone else--The design is fundimentally flawed in its feedback loop.

The FB gain changes depending on input vol-pot position, this is sure to change the character of the sound depending on the volume control position!

Ive tried this type of FB, and was never happy with this 'colouring' effect.....
I don't prefer this type of NFB too , but if you put a 100Kohm resistor and connect the volume in the input of the preamp then it will have no problem . And something else , if a small amount of DC voltage occurred in the output it will lead to a change in the bias point of the input tube ..
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Old 23rd January 2012, 06:11 PM   #16
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Interesting comments RE the Tim Mellows design..

I chose it for two main reasons,
1) the total cost of the build will be about the same as the tubes and transformers for something like a 300b build.
2) In my experience... the more simple a design, the better results.

I am still working on perfecting the anodising.. once the top plate is anodised the build can start hopefully this should not take me too long as I have booked a week of work
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Old 23rd January 2012, 06:23 PM   #17
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These pics are the mock up of top plate now stripped ready for anodising
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File Type: jpg 6c33 top.JPG (333.6 KB, 817 views)
File Type: jpg 6c33.JPG (330.3 KB, 778 views)
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Old 24th January 2012, 10:24 PM   #18
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"Double up on the output tubes that givs you more power. "

It can't be as easy as JUST doubling up on the OP tubes. You would need to increase the filament transformer capacity and the split supply power transformer capacity if you want to do this. And can the driver stage deal with two more O.P. tubes? Perhaps the best choice for me is to make two monoblocks using the existing transformers specified. Now that might increase the power out to about 50 watts @ 8 ohms IF the driver stage is capable of handling the two extra tubes. WOuld anyone care to comment on this?

Thanks,
Mark
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Old 24th January 2012, 10:42 PM   #19
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Quote:
Originally Posted by Mark Allen View Post
"Double up on the output tubes that givs you more power. "

It can't be as easy as JUST doubling up on the OP tubes. You would need to increase the filament transformer capacity and the split supply power transformer capacity if you want to do this. And can the driver stage deal with two more O.P. tubes? Perhaps the best choice for me is to make two monoblocks using the existing transformers specified. Now that might increase the power out to about 50 watts @ 8 ohms IF the driver stage is capable of handling the two extra tubes. WOuld anyone care to comment on this?

Thanks,
Mark
Of course its not so easy to double the output power , and the anodes of the two EF86 can't drive 4 6C33C , with large input capacitances plus the Miller capacitances , in this case you want cathode follower to drive the output tubes .
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Old 24th January 2012, 10:53 PM   #20
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Of course you need enough power and drive e.t.c

Doubling the tubes is the basic aproach.
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