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Old 28th July 2012, 11:56 PM   #1
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Default adding a preamp to tube power amp

I have a "no name" no schem. ppp mono power amp that may have come out of a tv/phono console. It is up and running and stable but has very little volume. I am guessing that I need a preamp stage to boost input signal(?). I have tried using an outboard preamp but to no avail. It changes nothing.

tubes: 4xEL84, GZ34 rectifier, 1x12AX7.

Can I build a preamp stage into the amp itself? There is plenty of room under the chassis. Would any of you know where I can find a tutorial or something that I can use to accomplish this?

Thanks
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Old 29th July 2012, 04:03 AM   #2
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The problem is that there is usually not enough filament current to run more tubes.
Lets say for example 2X 12AX7...now you need another 1/2 amp of filament current.
And since it's hi fi, maybe DC heaters for the preamp, even more current, to keep the voltage up.
The power transformer would need to be enlarged, most likely.
When a transformer is selected by a manufacturer, it's usually designed to run "just" the tubes in the existing amp. It is not over-designed, for more tubes to run...
OR adding a small filament tranny to the chassis would give you more current to heat more tubes.
As far as high voltage, you would probably have enough, especially if you beefed the capacitors. So the problem is mainly filament current.

Last edited by soundguruman; 29th July 2012 at 04:09 AM.
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Old 29th July 2012, 04:08 AM   #3
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If you have already tried using an outboard preamp (I'm guessing it has some gain) then maybe there must still be something wrong with the amp.

Have you checked the tubes? Bad tubes can also be up and running and stable but there is little sound.
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Old 29th July 2012, 04:54 AM   #4
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Use the phono input. Change the rca phone jack to a 1/4 inch. Change the caps & resistors around the 12x7 as per any common guitar amp using a 12X7 in its first stage. U see the phono input has an eq that was used for phonograph players, sounds like crap with a guitar. The Aux input needs a signal at a level as you would get from a tape player.
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Old 29th July 2012, 07:10 AM   #5
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Tubes are all brand new.

There were 2 inputs. One for phono and one for tv. They were hooked together. So I installed a 1/4' and a new rca. Replaced the output rca jack with a 1/4" jack.

I have changed all of the caps including filters.

When I hooked the the outboard preamp (actually a sansamp DI) there was a bit of improvement but still weak and distorted.

I will try the preamp in my bass rig and see if that does anything.

As it stands, this seems really difficult at my level of experience. I will be more discerning next time and purchase a vintage "integrated" mono amp for this type of use.

Thanks for your help.
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Old 29th July 2012, 07:11 AM   #6
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What's the intended use of this amp, and what country are you in as regards getting parts?

Andy
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Old 29th July 2012, 09:45 AM   #7
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I would like to turn it into a guitar amp.

I live in the USA

Thanks
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Old 29th July 2012, 02:30 PM   #8
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Quote:
Originally Posted by drgroove View Post
I would like to turn it into a guitar amp.

I live in the USA

Thanks
It depends on what preamp you have.
Some have about 1/2 volt output. This won't be enough to drive a tube power amp. Really intended as a line out into a mixer.
Some have a much higher output.
If you want it for guitar, you may be able to tweak the last gain stage of the preamp.
You may be able to tweak the input stage of the power amp.
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Old 29th July 2012, 02:37 PM   #9
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The output shouldn't be distorted at low volume. I suggest the amplifier has a defect.
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Old 29th July 2012, 03:16 PM   #10
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Quote:
Originally Posted by drgroove View Post

When I hooked the the outboard preamp (actually a sansamp DI) there was a bit of improvement but still weak and distorted.
DI box means direct, so I suspect it has no gain, or very little
its basicly just a cable driver
but having tone controls there is some gain
says +/-12db on tone controls, thats all
but I doubt its enough

try adding a 'booster'(gain)
if you have way too little gain it could be lack of 'headroom', and what you experience is dynamic distortion

I expect you have a new battery in your Di box
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