Which output tranny should I use?

[ebartlet]

Hi everyone, I have a eico 147 sig tracer that I am corverting to a guitar amp. The amp has a big honk in the midrange. Before condeming the speaker/cabinet, I thought I would check the amp itself. I could not find my 8 ohm dummy load, but stock there is a 47 ohm resistor accross the ouput, so I proceeded to test the frequecy response at the output using a AC voltmeter at sine waves generated by my DAW. Here is what I came up with:

How will the diference in load affect the frequency response?

I suspect the output tranny is the problem. The schematic does not show anything else that I think would affect the bandwidth this much - just a very basic amplifier circuit Any thoughts about this?

What tranny do you recommed I replace it with? (6AQ5 single ended output)

Thanks,

 

[Mark Burnley]

Hi Eric,

Have you tried running a guitar through this amp and having a listen? Is the mid peak particularly bad? I actually like a mid band "honk" in a guitar amp- I'd be tempted not to get it "ruler flat"- you'll end up with a hi fi amp!

You wouldn't have been able to crank the output level that high if you were running into a DAW with a 47R resistor- just remember that the transformer reflects the impedance of the load through to the primary, which is the Plate load, so the output stage will be "sitting funny" with a 47R load. You probably wouldn't notice any severe signal impairment with this load until you reached a higher output level.

The 47R resistor is a DC resistance which is a non-frequency dependent load, whereas a speaker has a "nominal" impedance of 8R (often measured at 1kHz) which will vary with frequency, so running a frequency sweep won't give you an honest picture of what's happening with the entire amp-speaker setup.

Have you got a copy of the schem? Are there any capacitors connected across the primary of the output transformer?

I've used an old signal-tracing amp for a guitar amp, sounded great! Just a single EL84 output.



Mark

 

[Matthis]

hey, first of all check the sound with your guitar.....if you like it to be overdriven the mid´s would be normally very nice to your sound.
after this i would go for different speaker´s......thats, in my opinion the best to try for different soundsssss!!!!
If you need further information on speakers for guit. just mail me!!!
Good Luck,
matthis

 

[ebartlet]

[quote author="Mark Burnley"]Hi Eric,

Have you tried running a guitar through this amp and having a listen? Is the mid peak particularly bad? I actually like a mid band "honk" in a guitar amp- I'd be tempted not to get it "ruler flat"- you'll end up with a hi fi amp![/quote]

I played my guitars through this before I did any testing. My guitars sound great on other amps. The midrange peak sounds very boxy through this amp.

[quote author="Mark Burnley"]
You wouldn't have been able to crank the output level that high if you were running into a DAW with a 47R resistor- just remember that the transformer reflects the impedance of the load through to the primary, which is the Plate load, so the output stage will be "sitting funny" with a 47R load. You probably wouldn't notice any severe signal impairment with this load until you reached a higher output level.[/quote]

I used the DAW to drive the amp. The only thing on the output was the 47 ohm resistor and the ac voltmeter. I didn't drive the unit hard because I did not want tranny saturation to affect my frequency response readings.
[quote author="Mark Burnley"]
The 47R resistor is a DC resistance which is a non-frequency dependent load, whereas a speaker has a "nominal" impedance of 8R (often measured at 1kHz) which will vary with frequency, so running a frequency sweep won't give you an honest picture of what's happening with the entire amp-speaker setup.[/quote]

I understand that the resistor is not frequency dependant. Thats why I used it. I wanted to determine if it was the amp or the speaker/cabinet causing the boxy sound. My first reaction was to condemn the speaker/cabinet. So I thought I would do some testing without the speaker. It would appear that the amp is definately causing some if not all of the honk.

[quote author="Mark Burnley"]
Have you got a copy of the schem? Are there any capacitors connected across the primary of the output transformer?[/quote]

There are no capacitors across the tranny.