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Old 24th August 2008, 03:56 PM   #11
nhuwar is offline nhuwar  United States
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Quote:
Originally posted by astouffer
Build something from a stock schematic at first to get experience and then modify if necessary. You're right about never finding a schematic that everyone would agree on. The two you just posted are fine performers. Whats important is how the amp sounds to you.

I'll buy that for a dollar.

You payed for it, you built it, and if you don't like the way it sounds when it's done mod it.



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Old 24th August 2008, 04:24 PM   #12
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What I do is build amplifiers with modular top plates. If you want to do this, do as follows.

Think in terms of a standard rackmount size chassis - 30mm deep, 420mm wide and 2U high (=88mm). My top plates are 120mm, 100mm, 70mm and 50mm all out of 4mm aluminium. I can do anything with this method and I recycle constantly. All top plates are 275 deep.

So, think PP amp. Make a power supply of say 450v. Put that on a 120mm plate. Put two PP OPTs of about 6.6K on a 100mm plate next to it. These will be fixed. You can then experiment with the rest of the amp. If you want something big, use PP 845 or 813 low voltage. For those you'll need a pair on two plates of 100mm and the input stages will have to go on a seperate chassis. Or you can use four 300b on one plate of 100mm and you have 100mm left for the input stage. You can experiment knowing that you will always have a working amp in minutes by putting back whatever you started with.

I'm just starting to build exactly this. I will use the same power supply and OPTs and try 845, 813 and 300b and also 2E22 in triode. I will probably put the input stages on a different chassis with a LL1671 PP output.

I went modular about 3 years ago and it was the best thing I ever did. It's kind of like a breadboard but permanent if you want it to be.
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Old 24th August 2008, 05:27 PM   #13
Yvesm is offline Yvesm  France
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Quote:
Originally posted by andyjevans
What I do is build amplifiers with modular top plates. If you want to do this, do as follows.

Think in terms of a standard rackmount size chassis - 30mm deep, 420mm wide and 2U high (=88mm). My top plates are 120mm, 100mm, 70mm and 50mm all out of 4mm aluminium. I can do anything with this method and I recycle constantly. All top plates are 275 deep.
I suppose you mean 300mm deep ?
That will match with 275mm top plates

Great idea !
Front and back rack chassis may even receive "permanent" connectors, switches, pots, and so on !

That makes me dreaming ...

Yves.
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Old 24th August 2008, 06:18 PM   #14
athos56 is offline athos56  United States
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That modular idea is indeed a good one. My other amp projects were all from complete schematics. Its easier to find generally well accepted SE amps, it seems harder to find the same consensus on PSE or PP amps. I guess I should clarify, mainly I'm looking at amps without any sand or outrageous part counts I've also looked at Mikael's KT-88 which fits the bill, I wish it had a few more tubes

Yves, I also looked at your 845 amp, was the bias -145? Plus you had two, one with some iron, and one without, which was better?
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Old 24th August 2008, 07:26 PM   #15
Yvesm is offline Yvesm  France
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Quote:
Originally posted by athos56
. . .

Yves, I also looked at your 845 amp, was the bias -145? Plus you had two, one with some iron, and one without, which was better?
Yes, more or less -150V IIRC and 1000V on the anode for some 80 mA current.

The better is ... can't tell !

The simpler (with a single tube and no iron) is really stunning looking to its vy lo complexity.
This was the final choice of the builder.

I must say the the one with two tubes and the iron sounded much more "dynamic" to my ears and had my preference ...

I beleive the builder rejected it when, while explaining him how it works, I pronounced the word "feed back" !
Some audiophiles have real avertion against that.

The only way to know is to check yourself

Yves.
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Old 24th August 2008, 08:19 PM   #16
athos56 is offline athos56  United States
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Default hmmm

I think I'll try and learn how to make a negative bias supply today and put your 845 on the list of possible builds.

On a side note why not Self-Bias?
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Old 24th August 2008, 08:58 PM   #17
Yvesm is offline Yvesm  France
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Default Re: hmmm

Quote:
Originally posted by athos56
I think I'll try and learn how to make a negative bias supply today and put your 845 on the list of possible builds.

On a side note why not Self-Bias?
Oh, two at least !

- I dislike to vaste precious and costly volts from the B+ (a bias supply is cheap).

- I dislike to add any impedance in the cathode return, very good bypass caps here is a must, and good caps are not cheap.

My two pence !

Yves.
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Old 24th August 2008, 10:11 PM   #18
athos56 is offline athos56  United States
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Default Ok, let me se if I got this straight.

How about something like this. For example one 6D22S damper diode reversed, one leg of the transformer to ground.

Question 1, I assume I can work out the Transformer/resistor/cap values as if it was a positive half-wave rectifier using PSUD II.

Question 2, if the winding had a center tap I would just leave it unconnected.

The other implementations had both legs of the transformer going to the anodes of a full-wave rectifier, the 5v heater going "nowhere" ie not the usual b+ rail. And the Negative supply comming off the center tap. The assumption from question one would still apply. What low current rectifier would you use? I assume that the 5u4 would be an overkill.
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Old 24th August 2008, 10:30 PM   #19
kevinkr is offline kevinkr  United States
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My thoughts would match Yves pretty much exactly, but I would put even more emphasis on the cathode bypass cap - there is no cap like well, no cap. It is impossible to build completely transparent sounding caps in the value required to do this chore.

I also strongly recommend against using tube rectification for bias supplies - if the rectifier fails for any reason you will loose the output tube and possibly the output transformer and your HV supply as well.

Tube rectifiers are great for their slow warm up (depending on type chosen) and their soft rectification characteristics, however mundane a ss rectifier may appear to be in this application its simplicity and ruggedness make it the best choice. I use tube rectifiers in all of my designs except in the bias supplies..

Not a bad idea to interlock the HV supply to the bias supply - no bias means no HV applied to the output tube. I use a simple circuit to sense bias is present and use this to energize a relay which either switches the primary of the HV transformer or the CT if the transformer is used for other purposes.
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Old 24th August 2008, 11:00 PM   #20
athos56 is offline athos56  United States
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Default Ouch

Ok then something like this? A diode bridge with the polarity reversed?

I've seen a few with a relay I'll look at them some more
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