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Old 1st October 2006, 06:51 AM   #11
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There was a discussion on this idea some time ago. I came up with an idea which I think is superior to the use of a voltage divider for the tube stage and is still cheap.

Firstly - make absolutely certain that the gainclone never clips. This will sound horrible.

Second - use solid state rectifiers and place resistance in series with them, they will behave in prity much the same way as tube rectifiers as the main difference between tube and solid state is their impedence. You will get the rail sag you are looking for.

Thirdly - use a parafeed interstage transformer to drive your gainclone. This will take care of the step down. This has a number of advantages. Namely it will introduce transformer distortion which is the main characturistic which makes a quitar amp sound as it does( along with speaker distortion). A small amount of DC can be introduced into the primary to encourage transformer satuation. This can be controlled by a wire wound pot from the HV rail. The transformer will perform better the cheaper and smaller it is, this is because it will have more High frequency and low frequency roll off. It can be a mains power transformer.

Fourthly - take the trouble to introduce a Marshell style tone stack.

Use back to back transformers for a cheap power supply.

The bottom line is that a simple tube preamp will never distort in the way that you want it to, and quite frankly you are waisting your time. People have done it and have never reported great results.

My idea is in the design and breadboard stage. Will be a few months before I have any results.

Hope that helps.

Shoog
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Old 1st October 2006, 07:58 AM   #12
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Quote:
Originally posted by Sheldon
I don't think that the most commonly used LM series chips are unity gain stable. You'd have to make sure to find one that is.
I'm shocked at how often this is regurgitated on the board; it is simply not true.

http://www.diyaudio.com/forums/showt...061#post112061
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Old 1st October 2006, 02:38 PM   #13
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by leadbelly


I'm shocked at how often this is regurgitated on the board; it is simply not true.

http://www.diyaudio.com/forums/showt...061#post112061
You caught me regurgitating. I'm sorted on that one now.

Sheldon
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Old 1st October 2006, 02:56 PM   #14
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Could someone explain what Parafeed is?


Also, what if I were to take a cheap 1:1 signal transformer (they sell them at RadioShack) and use it in place of the reccomended output transformer on the Moonlight amplifier shown above?
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Old 1st October 2006, 03:11 PM   #15
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by Spasticteapot
Could someone explain what Parafeed is?


Also, what if I were to take a cheap 1:1 signal transformer (they sell them at RadioShack) and use it in place of the reccomended output transformer on the Moonlight amplifier shown above?
There's a good explanation here: http://members.aol.com/sbench/outstru.html This time, I'm regurgitating an expert. Note that you can use either an inductor loaded tube or a resistively loaded tube with the same type of transformer. The key point is that the transformer is coupled through a capacitor, so that it sees no DC. This allows a smaller transformer, and/or, as Shoog suggests, you can add an adjustable external DC source to have controlled saturation.

Sheldon
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Old 1st October 2006, 08:23 PM   #16
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Because your not asking the transformer to deliver any significant power, you can choose your output tube such that you can resistively load it. I did this with a headphone amp (using an ECL82) and used a 3K5 resistor as the plate load. Works well and is simpler than a choke or constant current source load.


Shoog
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Old 1st October 2006, 09:41 PM   #17
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Quote:
Originally posted by Shoog
Because your not asking the transformer to deliver any significant power, you can choose your output tube such that you can resistively load it. I did this with a headphone amp (using an ECL82) and used a 3K5 resistor as the plate load. Works well and is simpler than a choke or constant current source load.


Shoog
Well, I think I understand it. Here goes!

1. A tube puts out DC, but with a sine wave.
2. The inductor or resistor applies a near-constant voltage, without shorting out the tube.
3. AC is the end result.

RadioShack sells cheapo 1:1 signal transformers, and I can get quite a few more off of old modems. Many have high impeadance on at least one of the windings.

By adding some additional voltage to both sides, a DC offset can be created, saturating the transformer and causing distortion.

(My brain hurt now!)

So, how do I apply this to a (simple) guitar amp design like the Skylark, and how do I build a Marshall tone stack (whatever the heck it is?)
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Old 2nd October 2006, 07:13 AM   #18
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Use a step down transformer rather than a 1:1 transformer. Quality isn't really a big issue because you want distortion.
I will scan in my rough plan later and you can use that as a starting point.

Shoog
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Old 5th October 2006, 07:53 AM   #19
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The schematic, untried.

Shoog
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Old 7th October 2006, 10:20 PM   #20
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Quote:
Originally posted by Shoog
The schematic, untried.

Shoog

Neato!

What kind of output voltages can I expect?
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