Using Small Power Amps as Line Amps

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tardishead

Well-known member
Joined
Aug 11, 2004
Messages
623
Yes I know very decadent and inefficient but what are the dos and don'ts in this topic.

My friend has some awesome old SE tube amps 10 watts. I was wondering if I could push the output tubes and use the units for processing line level signals.
So it needs some kind of power soak. How about simulating a speaker with a 10 watt 4 ohm shunt resistor and then parallel another pad for line level tap????

I had posted about building a 2a3 mic preamp but the overall response was not worth it. I was thinking padding down the output of tube amps might be an easier way of getting the colour from these old tubes at different levels of drive.
 
111 smackers says that PRR has done something similar, for reasons unknown.

After this thread, please start a Line Amp Used As Power Amp and see what happens.

112 smackers says that...



 
Common on old broadcast mixers to have the monitor amp (usually PP 8-10 watts) set up with switching to be the 'emergency' program amp.  Usually 20 dB padding on the output side for program use.  The tube monitor amps I've measured are generally more linear on the bottom end than program or preamp. 

I understand RCA/NBC used the RCA SS monitor amps at Rockefeller Center for program rather than the RCA program amps.  I think because they had central equipment rooms and lots of real estate, so they wanted to have a higher bus level to help overcome potential interference. 
 
> 111 smackers says that PRR has done something similar, for reasons unknown.
> 112 smackers says that...


That's 223 smackers. Who is paying?

10 watt SE tube amp?? Ya sure?

8 watts at 8 ohm tap is 8 volts. Which is just past +20dBm in 600 ohms. You DO need a dummy-load; Radio Shack has 8 ohm 20 Watt $2. You don't need a pad, except to protect the amp from a shorted line; a couple 10R series resistors would work. Actually most SE tube amps don't mind a dead-short.

Another issue: if it takes NFB from the speaker winding, it is an unbalanced output. People like to think their main out is balanced.

OTOH if it lacks NFB the THD may be awesome.

But to really grind the vacuum, I know you wanna aim for 15 or 20 watts from a 10W amp. Now you may need a pad, or a smoke-sucking fan above the input chip of the next device in the chain. Try two 1-ohm 10W resistors in series across a 4 ohm tap, take output from one of the resistors. That's unlikely to deliver more than about a Volt, meanwhile the tube will be straining (bit not strained) to drive the under-impedance.

The old consoles often used the same module for Line and Monitor, although this would be incidental monitoring with multiple speakers and 25V transformers. One I recall was two 6V6, rated +30dBm (1 Watt), easily did 8 Watts, and delivered 5 measured Watts via a nasty lash-up of mis-applied transformers to get from 150/600 down to 16 ohms. As our other boards were 2W and 4W monitor channels, this was ample.

You know, small tubes distort too. 6SN7 with 600 ohm load will not be too very different in sound than a hard-flogged 2A3, with much less heroics. Save the big tubes for sandwich warmers. My Fisher hi-fi makes a passable stereo line-amp but makes a GREAT burnt-cheese sandwich.

 
What would be the bias current for a BLT?  :D

I'll stick to the intake manifold on the Saturn Twin Cam, which handles much like the first Honda Civics, you know, the beer can frame that folded up like a pup tent if you did not pre load the suspension?
 

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