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Old 17th January 2009, 06:02 PM   #1
nhuwar is offline nhuwar  United States
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Default Guitar amp question

Hi all I have a question for ya.

If I build a guitar amp is it ok to use ultra linear configuration?

And also If I build the output stage with a higher then needed output transformer will this have any effect on removing distortion created by the preamp?


I know this is probably a stupid question just not sure.


Thanks for the help.


Nick
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Old 17th January 2009, 08:15 PM   #2
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Nick-
If you want to get distortion (ie blues-y or more) from your guitar amp- and most of us do- it's probably better to stay away from 'hifi'-type circuits, like UL. However, it could be an interesting experiment.


When you say 'higher than needed' output transformer, are you referring to the primary impedance or the power rating?
Too-high impedance will reduce output and lose some of the 'tubey' sound .
A higher power-rated transformer won't give the same distortion characteristics as a smaller unit but may be smoother- which could be a good thing.

It's only a guitar amp- just use what you have and do some experimenting.

Cheers
John
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Old 17th January 2009, 08:22 PM   #3
nhuwar is offline nhuwar  United States
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Sorry I didn't mention it I meant the primary impedance of the output transformer.

I was planning on using an edcor push pull transformer for a circuit with 4 el34's and the impedance in 5k which is about 2k high. I know it will drop the output power just wondering about anything else.

I wanted to try it in ultra linear just for the sake of simplicity, you know one less voltage to come up with.

Thanks for the help,
Nick
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Old 17th January 2009, 10:50 PM   #4
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Nick-

You are correct that UL avoids the screen resistors.

You should be blowing out the windows with that amp!

Cheers
John
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Old 18th January 2009, 01:17 AM   #5
kevinkr is offline kevinkr  United States
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Hey Nick,
I would just use a large dropping resistor (1K -4.7K 5 to 10W rating) to get the screen voltage you need for the EL34, that is pretty much what everyone does anyway. 22uF cap will probably provide enough filtering to keep hum reasonably in check, and should allow for some nice quick sag as the screens start to draw some current.

UL has been used in some obscure accordian type amps (Gretsch??) and I have heard some diy amplifiers using it, and you won't be surprised to hear that IMO poorly regulated pentode sounded much better, nice sustain and crunch, UL just seemed to clip a lot harder and sounded bad.

Avoid the temptation to do your usual overkill psu, you want poor regulation in this supply. Check out some of the old Marshall EL34 designs for some inspiration.

If that edcor has an 8 ohm tap just connect a 4 ohm speaker load to it and you will reflect about 2.5K to the primary which is in the right range.

I haven't been posting much as my current consulting job keeps me really busy.. Nice to have work in this economy.
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Old 18th January 2009, 02:26 AM   #6
nhuwar is offline nhuwar  United States
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I had thought about putting half the load on the secondary of the out put. That good to know about using it in ultra linear, I won't do it.

I'm basically going to clone a rather obscure guitar amp,one I can't afford LOL . It's a Orange GRO 100.
Here's a link to a schematic
http://www.schematicheaven.com/newam...ge_gro_100.pdf

I like the amp and I also like the simplicity of the design. Just trying to find the secondary specs of the power transformer though to see if one I have on hand will work.

As you can see the power supply has decent filtering for a guitar amp, I have found most of the expensive one at least have clc filter network.


Nick
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Old 18th January 2009, 03:12 AM   #7
Tweeker is offline Tweeker  United States
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Guitars also often use deliberately poor OPTs as part of their tone. Edcor offers these now (high leakage inductance, strong insulation to deal with its consequences).
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Old 18th January 2009, 03:29 AM   #8
nhuwar is offline nhuwar  United States
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What do you do just tell them it will be used for a guitar amp when you order from edcor?


Nick
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Old 18th January 2009, 10:47 AM   #9
Tweeker is offline Tweeker  United States
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Ask for the GXSE or GXPP series if you are looking for a higher leakage OPT. These are for lower power levels though, Champs and the like.

Not all guitar iron is designed this way, my Hiwatt has a large hunk of Partridge iron.
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Old 18th January 2009, 03:38 PM   #10
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by nhuwar


<snip>
I like the amp and I also like the simplicity of the design. Just trying to find the secondary specs of the power transformer though to see if one I have on hand will work.

As you can see the power supply has decent filtering for a guitar amp, I have found most of the expensive one at least have clc filter network.


Nick

Despite the stack electrolytics in the power supply you can see that there is a 450V cap at point B and probably less than 4mA of plate current total to the rest of the pre-amp and driver circuitry - figure not much more than 400V at point B and probably a bit less, as a SWAG I would figure on B+ at point A in the 425V - 440V range or so..

You don't need to use a center tapped power transformer for this application, but you do need one with a seperate bias winding if you go this route. Alternatively you could possibly go with something commonly available like a 600 - 650VCT transformer (Hammond?) using full wave center tap rectification (Use UF4007 in series for reliability) with a 50V bias tap. I only suggest this if you cannot find better information than this. That VCT number also depends a lot on the dcr of the choke you will be using, and might need to be adjusted upwards to compensate for DCR. Umm, current rating should be something like 300mA.

The fact that the amplifier has a UL connected output and the nature of the tone control design suggests to me that this amplifier is oriented more towards a clean sound than anything else. Perhaps a hifi type output transformer would not too far off the mark given the nature of the rest of the design and comments about its sound online. Seems like this might be a good amplifier for use with acoustic guitars as well.
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