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Old 27th February 2004, 05:34 AM   #1
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Default Newbie First Amp

Hello All,

I am going to start building my first amp. I have started to research and am currently reading Valve Amplifiers by Morgan Jones and Vacuum Tube Valley magazine. Most of the concepts are slowly sinking in.

I have searched different threads, and it seems like everyone wants something just a little bit different. This will be no exception. Here are some of my preliminary requirements:

1. Push-Pull KT88 design. Yes, there are many wonderfull designs out there, put for the sake of time, I'm going to stick with something tried and true, like the Williamson design. You have to start somewhere, and a SET probably wouldn't drive low frequencies through my speakers very well. (They will be upgraded in the future.)

My current speakers:

http://www.bayareaaudio.com/products...gm/7semk3.html

2. Seperate 17" wide chassis for the pre-amp and amp. This is to facilitate future upgrades such as remote control motorized volume and source selection, tone controls, and tube phono preamp stage. Plus it just looks better, IMO, to seperate the two. The amp can be 1 chassis (preffered) or mono blocks.

3. Budget < $ 2000.00 US for everything.

4. Have a working amp in 180 days. I do want to get this done and working quickly (but safely), as my receiver is giving up the ghost.



My first question: Has anyone tackled the John Eckland design in issue 19 of Vacuum Tube Valley?

http://www.plitron.com/pages/Products/Audio/vtvkt88.htm



I am curious to know the thoughts of you learned scholars. I could use a kit, if there is one that can be hard-wired (e.g. no PCBs) and puts out > 30W and can be separated into pre-amp and amp.

Thanks in advance. I will post all progress and the finished amp in the picture section.

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Old 5th March 2004, 05:28 AM   #2
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Hello all,

OK, no responses. This leads me to believe that no one out there has tried to build this iteration of the Williamson amp. I would still like to try building this amp, or an amp of similar design. Are there any good sounding kits of a similar nature available? I would like to start building soon.

Any thoughts or suggestions you could give to a first time builder would be appreciated.
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Old 5th March 2004, 05:54 AM   #3
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Default Hale Fellow, Well Met!

Hey Tone Loc,

It seems Newbie minds think alike, for I am pondering
the same type of project, reading the same book
(Jones), and surfing the same website (Plitron).

To the extent that we *do* think alike, I pity you.

I have perused the same article, to these Newbie
eyes, it looks like an excellent and proven project.

Once my posts emerge from moderation, you
will see that I have posted a similar question,
but regarding a more radical Menlo van der Veen
Super-Triode design, shown on the same site!

For your project, have you seen the

circuit board that Vanderveen / Plitron sells?
I have not scrutinized the design, but believe it
is similar to the one you mentioned. The board would
certainly simplify design and assembly--if you
want that.

The only thing I don't like about it is the photo
of the completed board shows the surface mount
components (resistors, caps) on the same side
as the tubes. To my eyes that messes up the aesthetics,
but maybe one could devise a workaround?

Anyway, I hope this helps, and would appreciate
your own comments.

Best,

George Ferguson
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Old 5th March 2004, 09:12 AM   #4
SHiFTY is offline SHiFTY  New Zealand
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Looks like a nice amp. However, for simplicity I would choose cathode bias with individual resistors & caps instead of fixed bias. Less hassle and better reliability, but very slightly less power. No biggie.

Also use a 6.3V DC supply for the input & driver tubes to lower noise and hum.

I like the driver stage, will be ample to drive KT88s and the ultralinear output stage will be great sounding.

With the right output transformers, (think high power handling!) this would be a great sounding amp.

You should buy the bits and build the thing. Don't get sucked into audiophile nonsense though, spend the money where it matters like output and power trannys. Vintage stuff can be very good indeed. The heavier the amp the better!
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Old 7th March 2004, 07:34 AM   #5
kmj is offline kmj  Sweden
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DrDeville and Tone Loc.

Like you DrD, i too would like to thank the members of this forum for their help and well based comments.

And also for their patience with newbies who are trying to learn how to swim in the deep end of the pool (such as me).


So, here's another newbie.....

I to have surfed the piltron/amplino sites....and done a couple of googles on "menno van der veen"..

And since probably ANYONE here knows more about valves than me and everyone has different opinions I went to the source and bought Mr Van der Veen's book "modern high-end valve amplifiers" along with "Valve amplifiers" by Morgan Jones for some more basic knowlage.

Since I have a tendancy to missquote and putting my foot in my mouth i'm only saying this.....if you are planning to build a vdV-amp it could be good to take a look at the book. In it, theres extencive info on every detail of the 5 or 6 types along with comments, pros and cons.

basicly everything you need to know about building the amp and designing it. what i fell for was his comments on that anyone with basic knowlage of electonics should be able to build a high-end amp and his designs are at least to some part based on this idéa.

Here are lines on one of the designs from the book, slightly modified by Rudy Godmaire. (my project to be...after a lot of reading.) :
http://www3.sympatico.ca/r.godmaire/VDV3070.htm


disclaimer:
I am learning, with meens that i don't yet know much. Since i'm known to make mistakes, from time to time. it's wise to read, check the facts (often links) and judge for your self. and comments from me that seems to be rude are just misstakes in translation.
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Old 7th March 2004, 12:47 PM   #6
EC8010 is offline EC8010  United Kingdom
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Hello all. The proposed design should make a good first project. There aren't any fiddly fragile bits, and it stands a good chance of working first time. Things you might want to consider:

Toroidal transformers are very sensitive to out-of-balance anode currents between the output valves. This causes saturation of the transformer core, and distortion. Cathode bias prevents severe arrors, but is unlikely to be as good as carefully adjusted fixed bias. However, be prepared to check regularly, and adjust if necessary. Rather than using a moving coil meter (inaccurate), you might prefer to change those 10R resistors to 1R and make each end available as a test point outside the chassis for connection to a DVM. They're at a low voltage, so safety isn't a problem. Changing them to 1R means that the mV range of your DVM reads directly in mA of anode current.

Make the chassis bigger than you need, then you can add/modify bits later when you have learnt more.

Your might prefer to use loctal 7N7 instead of (octal) 6SN7GT. It's electrically identical, but much cheaper. Make sure you get the sockets first, though.
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Old 7th March 2004, 01:51 PM   #7
SY is offline SY  United States
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To follow up on EC's point, for best balance, once you've gotten the idle currents set fairly close, trim it by connecting the meter between the test points of the two output tubes. Then you can trim for a null.
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Old 7th March 2004, 06:50 PM   #8
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KMJ wrote:
Quote:
And also for their patience with newbies who are trying to learn how to swim in the deep end of the pool (such as me).
Welcome, and thanks for joining! It is always good to have another companion on a challenging journey. The Fellowship of the Tubes?

And thanks for your comments and information on MvdV's designs. I have his book on order, and it is great to know that it is useful to newbies like me. I have heard that it also has some very complex design calculations and analysis.

And good tip on Rudy Godmaire. In fact, we are in contact--he is helping a friend build the Super-Triode amp that intrigues me. Please contact me if I can be of any assistance--I'm glad to help.

Quote:
comments from me that seems to be rude are just misstakes in translation
Thanks for the warning, but your English and your manners are both excellent.

EC8010 wrote:
Quote:
Things you might want to consider:
Thanks for the tips and encouragement. Guidance like yours means a lot when venturing into new waters.

SY wrote:
Quote:
trim it by connecting the meter between the test points of the two output tubes. Then you can trim for a null
Thanks for the helpful suggestion. MvdV recommends that, after getting the currents matched, the final adjustment be done by ear--adjusting one of the pots with an ear against the speaker, and listening for lowest hum. He says that even though this adjustment will not match the currents, it will minimize their effect on the particular transformer's magnetic signature.

Of course I am just a newbie, and merely parroting what I have read. From what I understand, torroidal transformers offer superb performance, at the expense of making this DC adjustment critical.

Thanks again to all for your insights and encouragment.

Best,

George Ferguson
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Old 9th March 2004, 04:43 AM   #9
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Hi All,

Thanks for the feedback and/or advice! I am going to start sourcing parts for this project this week. The transformers will come from Plitron, but as for the other parts, can anyone recommend suppliers? Antique Electronic Supply seems to have most components that would be needed.

DrDeville wrote:

Quote:
For your project, have you seen the circuit board that Vanderveen / Plitron sells?
I have seen them, and they probably would save me time and money, however I would like to do all of the wiring myself. I think you would get a better understanding of how the circuit works. Plus, for me at least, the fun is in the building.

ShiFtY wrote:

Quote:
However, for simplicity I would choose cathode bias with individual resistors & caps instead of fixed bias.
I thought that the meter and pots in this design were meant to adjust the cathode bias. Am I mistaken?

Quote:
You should buy the bits and build the thing.
Amen!

kmj wrote:

Quote:
I went to the source and bought Mr Van der Veen's book "modern high-end valve amplifiers"
I think I may pick up a copy of that book, also.

EC8010 wrote:

Quote:
Cathode bias prevents severe arrors, but is unlikely to be as good as carefully adjusted fixed bias.
Fixed bias of the cathodes? I am a bit unclear as to what you mean. However, I am still learning exactly how this amp works.

Thanks again for your help!
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Old 9th March 2004, 07:55 AM   #10
EC8010 is offline EC8010  United Kingdom
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The amount of current each output valve passes is in inverse proportion to the voltage between its grid and cathode. One way that you could set the current would simply be to apply a fixed negative voltage to the grid, and adjust it until you get the current you require. This is known as fixed or grid bias because valve current is controlled by setting a fixed grid voltage.

Alternatively, you can put a resistor in the cathode circuit. As current flows through the valve and resistor, a voltage appears across the resistor. The cathode becomes positive. In this instance, the grid is connected to 0V, so it is effectively negative with respect to the cathode. Thus, more current through the valve and resistor tends to produce a voltage that turns the valve off. The two effects stabilise one another, and this is known as cathode or automatic bias.

If all valves were perfect and identical, there would be little to choose between the two forms of bias. But they're not, and because toroids are so sensitive to out-of-balance currents, it is better to have a scheme where differences can be adjusted out, hence this circuit's variable resistor in the fixed bias scheme.
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