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Old 7th April 2011, 04:54 PM   #1
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Default Guitar Valve Amp Output Transfomers

Hey,

I was thinking about building a valve guitar amp following this Schematic:
Marshall JMP 2204

I was going to put a 100w Power transformer from an old 'Sound City' amp (Making the 50w version of the Marshall amp as I don't really have that much money for valves! But can always add in the extra part of the circuit )

Basically, Its the Choke and output transformer that is worrying me.
I have a Utc a-20 Output transformer from a previous Transistor amp build and was wondering if that will do the trick?
And does anyone know of any cheap but good Chokes that I could buy?


Many Thanks!
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Old 7th April 2011, 05:47 PM   #2
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Quote:
Originally Posted by shunt94 View Post
I have a Utc a-20 Output transformer from a previous Transistor amp build and was wondering if that will do the trick?
And does anyone know of any cheap but good Chokes that I could buy?
Surely you have the number wrong. A UTC A-20 is a 30 milliwatt line output transformer. For the choke, try Edcor or Hammond.
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Old 7th April 2011, 06:12 PM   #3
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Mercury Magnetics -- The Guitar Amp Transformer Company

These are among the best.

Regards, Peter
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Old 7th April 2011, 06:15 PM   #4
ChrisA is offline ChrisA  United States
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[QUOTE=shunt94;2530811]

Quote:
Basically, Its the Choke and output transformer that is worrying me.
I have a Utc a-20 Output transformer from a previous Transistor amp build and was wondering if that will do the trick?
Almost certainly "No". You will need an Output transformer with an impedance ratio of roughly about 6,000:8

Quote:
And does anyone know of any cheap but good Chokes that I could buy?
Buy the transformer and choke at the same time and save on shipping. stf-electronics.com has good prices on Hammond parts. Choke for maybe under $20 and a output transformer suitable for your amp for about $75

They have metal chassis too, good prices, may as well get it all at once.
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Old 7th April 2011, 06:53 PM   #5
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I bought transformers from this site when I built my Champ 5F1 clone and I'm very happy with the quality.

They have Marshall style transformers also. Great prices.

Transformers for Tube Amps
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Old 7th April 2011, 06:57 PM   #6
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by shunt94 View Post
Hey,

I was thinking about building a valve guitar amp following this Schematic:
Marshall JMP 2204
looks complicated
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Old 7th April 2011, 07:03 PM   #7
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Pick a transformer recommended for the application (50W Marshall) as it will have a critical impact on the sound you get, you can use an oversized one safely, but it will tend not to produce the distortion (typical sound) Marshall users expect.

w

Oh, this applies to the power transformer AND the output transformer.

Last edited by wakibaki; 7th April 2011 at 07:11 PM.
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Old 7th April 2011, 07:05 PM   #8
Gilgy is offline Gilgy  United Kingdom
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Quote:
Originally Posted by shunt94 View Post
I was going to put a 100w Power transformer from an old 'Sound City' amp (Making the 50w version of the Marshall amp as I don't really have that much money for valves! But can always add in the extra part of the circuit )
I'm guessing the Sound City had four EL34's and you're talking about making a 50 Watt version with two EL34?

You can't simply omit two valves and use the same transformer, the impedance ratio will be wrong.

EDIT: ignore me, I didn't read the bit that said "power transformer". You'll be fine using the power transformer, just the output transformer will have a different impedance ratio from the design value on your schematic.

Last edited by Gilgy; 7th April 2011 at 07:09 PM. Reason: I must read posts before replying
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Old 7th April 2011, 07:52 PM   #9
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It's a strict rule to know a certain degree of amp-design. I have never, ever gone to more extend than switching an output-tranni in a Super-reverb to one from a Showman just to be able to use 4 Ohm instead of 2 Ohm, loading the secondary near perfect. And the heavier iron was just disappointing soundwise.
Keep values as they are - unless tweaking with eq-components.
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Old 7th April 2011, 08:29 PM   #10
tinitus is offline tinitus  Europe
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from the nature of your questions I conclude this is your first tube amp, since you ask about chokes

this one looks a lot easier to build
I have seen it in several places, so I'm not sure who designed it
but I guess its a proven design
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