Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Tubes and Hybrids => Topic started by: guitarfreak666 on February 08, 2012, 10:29:40 AM

Title: 18w minimalist
Post by: guitarfreak666 on February 08, 2012, 10:29:40 AM
hi everyone,

i have some questions:
1
id like to build a 18w minimalist, but with a preamp volume. is the schematic alright or do i have to make some changes(i havent changed any values but i just adden a 1m pot in front of the first tube.

2
i live in belgium and i have a tranny with 230 on primary, do i have to get another fuse value?

thanks

gr guitarfreak

Title: Re: 18w minimalist
Post by: mensur on February 08, 2012, 11:18:03 AM
hi everyone,

i have some questions:
1
id like to build a 18w minimalist, but with a preamp volume. is the schematic alright or do i have to make some changes(i havent changed any values but i just adden a 1m pot in front of the first tube.

2
i live in belgium and i have a tranny with 230 on primary, do i have to get another fuse value?

thanks

gr guitarfreak
1.You don't need to change anything, it already has preamp volume,500K after first tube.What you did is just another volume pot of your guitar on amp.
2.1A slo-blow fuse will do the job.
Title: Re: 18w minimalist
Post by: Toner on February 08, 2012, 02:23:14 PM
Yeah, what is your objective to adding the volume on the input? That is the same as your guitars volume control.

  I would put a dual 470k volume on the 8k2/470k junctions for each EL84. That would let you turn up the pre-amp and still have a lower volume out while driving the two 12ax7's into distortion, if that's what your after, often called a Master Volume.
Title: Re: 18w minimalist
Post by: guitarfreak666 on February 18, 2012, 01:18:12 PM
but my guitar doesnt has a volume, and i am kinda looking for a clean tone that can overdrive good but also can stay really clean

btw thanks for the help
Title: Re: 18w minimalist
Post by: phatt on February 21, 2012, 09:09:51 AM
Wow an Electric guitar without a volume control :o

I'd be looking at getting a Volume control.  I'm assuming it is an electric guitar?
Phil.
Title: Re: 18w minimalist
Post by: guitarfreak666 on March 03, 2012, 07:41:51 AM
yes it is but it was a selfbuilt sg and there wasnt enough space to put a pot in
Title: Re: 18w minimalist
Post by: joecool85 on March 06, 2012, 11:15:27 AM
yes it is but it was a selfbuilt sg and there wasnt enough space to put a pot in

Can we see some pictures of this guitar?  Sounds neat.
Title: Re: 18w minimalist
Post by: guitarfreak666 on March 07, 2012, 07:49:52 AM
sorry, i cant right now but maybe later. i have another question. the tube sound is it based on the preamp or poweramp? because if i find out that my minimalist doesnt have enough gain, could i be able to build a ss preamp with massive gain?
Title: Re: 18w minimalist
Post by: J M Fahey on March 07, 2012, 05:11:35 PM
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.
Title: Re: 18w minimalist
Post by: guitarfreak666 on March 10, 2012, 05:22:49 PM
is it posible to add a switch to install a switch for parallel/series for the first 12ax7? i heard some things about coupling capacitors, can anyone explain that to me? my understanding of it is that if u dont have it u will burn ur tube out , is this correct?
Title: Re: 18w minimalist
Post by: guitarfreak666 on March 10, 2012, 05:38:46 PM
also would it be able to do an evh sort of mod to the first tube? can i just change the capacitor to 0.68 µf?
thanks for all the info btw, just by posting on this forum i learnt more from tub amps than i did from reading books about it
Title: Re: 18w minimalist
Post by: J M Fahey on March 10, 2012, 10:30:03 PM
Start by building a classic amp, you'll have a lot of fun doing it, *later* you start experimenting.
Otherwise you do not have a reference sound in your head.
You will never know what any mod is doing.
Title: Re: 18w minimalist
Post by: phatt on March 11, 2012, 08:33:01 AM
I quote Mr Fahey;
"Start by building a classic amp, you'll have a lot of fun doing it, *later* you start experimenting.
Otherwise you do not have a reference sound in your head.
You will never know what any mod is doing
."
-------

What is written above is the BEST Advice you will ever get.
Sadly few will take note of such wisdom. :-X

FWIW, I built my first Valve Amp and quickly built far more complex valve circuits *Assuming* I was improving things,,,,, well 20 Years later all the fancy stuff went and I found that one of the very first simple Valve Amps I built was the best. I still use it today.

A lot of Valve circuits/mods are over rated and often don't reap the rewards that are often claimed by forum blabs.

As to bypassing coupling caps,,, DON"T.

You have DC and AC inside these circuits as the cap *Decouples the AC signal from the DC operating points on the Valve. **a Capacitor in that configuration Blocks DCV but allows the ACV signal to pass onto the next stage.**
Most Valve circuits are AC coupled, you can have DC coupled circuits with Valves but not often used.
Go look at the guts of a simple 741 opamp or similar and you will see that all the transistors Are in fact Directly coupled.

Remember that Amplification is AC voltage (the signal you hear) floating on a DC voltage.

So there are two things happening at the same time when you look at the schematic.
You have to set the DC parameters so that you get the best transfer of the AC signal.

**Think of a schematic as 2 separate circuits drawn as one**

Tiss not very hard to see the AC signal path through a Valve amp but some learning is required to see the more complex DC parameters.

Some reading and experimenting with simple BJT transistor circuits will soon reveal biasing procedures and help you get a handle on DC biasing. (setting the Q or quiescent point)

This will also reveal that transistor and valve circuitry is not so different.
The beauty of transistor circuits is that you can mess with them LIVE and not kill yourself.
(at low voltage they are a very safe learning tool)

It takes forever to rewire valve circuits and AB test stuff. :grr

Learning on transistors is good as It will speed up the Valve understanding.
Blowing up small transistors is way way cheaper than blowing valves or worse melting transformer winding which is an extremely expensive way to learn. :(
Phil.
Title: Re: 18w minimalist
Post by: guitarfreak666 on March 11, 2012, 05:12:41 PM
wow, nice advice. i now decided to built it according to the schematic. if it works and i dont like it i can still make some mods, maybe a tone stack or something.
thanks for all the good advice. when my amp is ready i will post some pics and samples.
Title: Re: 18w minimalist
Post by: phatt on March 12, 2012, 09:28:07 AM
If you are hell bent on finding how Valve stuff works this will help. :tu:

http://www.freewebs.com/valvewizard/index.html

If you are new to electronics and struggle with it then a book might be of more use but with a reasonable grasp of how amplification works then web pages like V wizard can fill in a lot of gaps. :tu:

And don't forget google,,, 
Let's say you see an odd word in text.

Example; *Cathodyne Phase Splitter*
OK it's obviously some kind of PI but if you search for *phase splitter* (a common term) you will get a million meaning less hits.

Type *Cathodyne* (Not so common)  leave off  *phase splitter* may give even better results.

I've found some really good stuff by typing in words that are only used by scientific geeks.
Heck some words the scientific chaps use I can't even pronounce.  :lmao:

Have fun and don't forget to turn off and pull plugs Before you work on Amplifiers.

And Fur Gads sakes don't work on live Amps when you are alone :trouble 8|
 
Title: Re: 18w minimalist
Post by: guitarfreak666 on March 12, 2012, 12:52:49 PM
awesome advice. i have the power suply almost ready, but how do i discharge the caps????
Title: Re: 18w minimalist
Post by: phatt on March 13, 2012, 05:26:13 AM
If your power supply is setup correctly with bleed down resistors then you won't need to worry.

If not sure then bridge them with a low value resistor,, say 500 ohms.
Use DMM to check how much Voltage is present on the terminals.

Once the DCV is below about 70 Volts it's fairly safe and unlikely to kill you but ideally you want to see close to Zero.
Phil.
Title: Re: 18w minimalist
Post by: guitarfreak666 on March 13, 2012, 06:35:39 PM
so across the caps i put 500k resistor? how much watts should the resistors be?
Title: Re: 18w minimalist
Post by: phatt on March 14, 2012, 04:10:43 AM
100 Ohms~ 500 Ohm. Not 500,000 Ohms ,,, as you will be waiting a fair while to bleed the voltage. Winky.

Use 1 watt or bigger,, the lower the R value the hotter it gets while holding it,,, but then you should not be holding it with wet fingers in bare feet?????? winky.

If inserting these into a build THEN you use 200k or bigger.

Bigger just means it bleeds down slower and does not load the supply while running.
Normally a PSU in a valve Amp will bleed down in about one minute But wise to check with meter if you are not sure.

With no bleeder setup they can stay charged for quite a while.

You can of course just short the terminals with a screwdriver but it will likely shorten the life of the Cap.  Not recommended but I admit to doing such things when in a hurry. :-[
Phil.
Title: Re: 18w minimalist
Post by: guitarfreak666 on March 14, 2012, 02:22:10 PM
id like to built them in, since i probably will do a couple  of mods.
Title: Re: 18w minimalist
Post by: guitarfreak666 on March 28, 2012, 09:14:57 AM
i have most of the parts, just waiting for the pt, but could any of you tell me how i could put a 3band tonestack in this amp?

grtz
Title: Re: 18w minimalist
Post by: J M Fahey on March 29, 2012, 04:14:51 AM
Copy some similar Fender amp, which has bass/mid/treble controls and a similar tube setup.
Title: Re: 18w minimalist
Post by: erokit on April 01, 2012, 01:10:22 AM
There used to be forum 18-watt.com. The builder of Gabriel amps, Gabi Bucatereu was an active contributer. He is extremely helpful and enthusiastic. He will love to help you out.

guitargabi@gmail.com

Tell him erok sent you!
Title: Re: 18w minimalist
Post by: guitarfreak666 on April 01, 2012, 09:14:34 PM
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?
Title: Re: 18w minimalist
Post by: J M Fahey on April 01, 2012, 09:35:20 PM
Apply Ohm's Law.
Title: Re: 18w minimalist
Post by: guitarfreak666 on April 01, 2012, 09:41:36 PM
i know how it works but i cant seem to make the formule work to calculate the value of the resistor
Title: Re: 18w minimalist
Post by: J M Fahey on April 01, 2012, 09:48:51 PM
You need to provide the current value first.
Title: Re: 18w minimalist
Post by: guitarfreak666 on April 01, 2012, 10:18:02 PM
the problem is that i dont know how much the current will be
Title: Re: 18w minimalist
Post by: phatt on April 02, 2012, 09:22:12 AM
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?

Hum?
Do you intend to drop the *Primary voltage* via a resistor?

Phil.
Title: Re: 18w minimalist
Post by: guitarfreak666 on April 02, 2012, 02:50:38 PM
no, i will use a transfo with a bridge rectifier and then let the voltage drop to 305v(the first voltage on the schematic) but i dont know how much current it will draw. could i get that info by the voltage drop across the second resistor, then i could calculate it.
Title: Re: 18w minimalist
Post by: Roly 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
Title: Re: 18w minimalist
Post by: Jack1962 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