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Author Topic: 18w minimalist  (Read 13443 times)

Roly

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Re: 18w minimalist
« Reply #30 on: June 12, 2012, 12:51:13 PM »
The normal value for a fixed bleeder is to allow 1k ohm per volt, so for around 300 volts use 280k or 330k.  The power rating;

P = E * I;  300V * 1mA = 300mW or a bit over 1/4 watt, so use half or one watt resistor.

Quote from: guitarfreak666
how i could put a 3band tonestack in this amp?

Perhaps the most important aspect is to drill enough holes for all the controls you are going to need before you start any actual assembly or wiring.  If you intend to tweek a lot then add a few extra tagstrips scattered around, but always try to do all your metalwork before you start mounting any components.

First build the basic amp and get it going, then you can have endless fun trying different tonestacks, etc.  It's a good rule only to make one change at a time and see what effect that has, any more at once and it quickly gets confusing.

Quote from: guitarfreak666
i cant find a transformator with 215 on primary, if i use a 230, what value of resistor should i use to drop the voltage?

I think you mean secondary, the rectifier/amp side.  The primary is the mains side.

The DC voltage is found by multiplying the secondary voltage by the rectification factor, root(2) or 1.414.

Originally;
215 * 1.414 = 304.01 (which has been rounded on the circuit to 305V)

If you use a 230V secondary the new HT voltage will be;

230 * 1.414 = 325.22, or about 20 volts more.

The valve databook gives typical conditions for push-pull EL84's on 300V as an idle of 2x 36mA, or 72mA, which we can round up to 80mA for the entire amp.  We therefore want to lose 20 volts at a current of 80ma.  Ohms Law;

R = E / I;  20V  / 0.08A = 250 ohms (available values either 220 or 270 ohms)

Power dissipation;

P = E * I; 20 * 0.08 = 1.6 watts.  I'd use a 5 watt ceramic wirewound so it has ample reserve for full drive.

The overvoltage is, however, only (230/215) * 100 = 107 or 7 percent which I personally wouldn't worry about if it were my build.

I agree that running first double triode in parallel is a waste, so see the attached circuit for a minimalist two stage preamp.

Preamp notes: the first 1Meg resistor could be changed for a pot, but there is no need to use both a pot and a fixed resistor; this only lowers the input resistance and loads the guitar pickup making it less lively.

The stage coupling caps are shown as 5nF but these can be tweeked up for a fatter sound or down for a thinner more trebly sound.  These need to be rated for the HT voltage (300V) or greater.

The 25uF cathode bypass caps are fairly nominal in value, and can be rated anywhere between about 5 volts and 25 volts.

The 1Meg on the grid of the second stage can also be changed for a pot if a more conventional "gain" control is desired, the "volume" then becoming the "master volume".

Any tonestack can be inserted after either of the stage coupling caps (5nF or whatever).

In any case I would increase the value of the main volume control from 500k to 1Meg, and include a 100pF "top coupling" cap as shown to maintain brightness at lower volume settings.

Extra HT decoupling is required in the form of a 1k 1 watt resistor and another high voltage electro (33uF 450V).  Neither the value nor the voltage is critical and anything more than say 10uF and 320 volts will do.

This amp generally provides almost endless scope for tweeking and mods, but get a basic version going first, then start playing.

HTH
If you say theory and practice don't agree you haven't applied enough theory.

Jack1962

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Re: 18w minimalist
« Reply #31 on: August 12, 2013, 07:20:30 AM »
You are "wasting" a triode by using them in parallel.
Build a conventional, 2 triode preamp and you will have *a lot* of gain.
Check the Marshall 18W, or similar amps.

A common mod to Marshall JMP's is to parallel V1
 on this circuit I see a 1meg pot on the grid of V1 that I would guess is the pre and the other pot is your post . just to answer  the question that was ask to begin with