Old Silvertone PA

[jcollins]

I've got an old Silvertone PA on my bench. It looks like this one:

http://guitardiner.com/cpg/displayimage.php?pos=-38

The customer complains that it's not as loud as he thinks
it used to be. He brought it over. I reseated the pre-amp
tubes and he plugged his harmonica into it. At that point
he commented that it didn't sound as weak as he remembered
it and that maybe it was ok afterall.

I also thought it sounded plenty loud for an old amp with two
6L6's at 3 on the volume knob. At 5 it was very loud. He still
wanted me to check it out.

I told him I would clean the tube connections/speaker/input connections
and check the caps. He left one of the field coil speakers with me.

I opened it up. Looks like all the caps, including bypass, have been
recently changed. Also, swapping the pre-tubes around didn't make
any difference in the volume.

What I'd like to do is test output power, but I'm not sure how to do that
with field coil speakers. Usually I just hook up a dummy load. I don'
want to blast this thing through that old field coil speaker he left me. I
can handle the noise but I'm afraid I'll blow it.

Any tips on how to safely test this for output power?

[Enzo]

Field coil speakers are just speakers, except they have a large coil in place of the magnet. The field coil is usually either serving as a choke in the power supply or is wired to ground as a bleeder or as a voltage divider in the B+. I didn't look up your schemo.

So leave the field coil connected and disconnect the voice coil. Now connect the load of your choice to the amp output in place of that voice coil.

Keep in mind that if the power cut in half, it would only be a 3db drop. Or if the power somehow were to double, it would only be a 3db increase in loudness. I think most complaints of not as loud as it once was are sensitivity complaints about the preamp. The difference in loudness between a 20 watt amp and a 50 watt amp is not really a lot. The difference between the high and low sensitivity input jacks on all the Fenders in the world is only 6db.

You do all this, and find out the thing makes 25 watts. OK, now what? How much was it designed to produce? What can we goose it up to? Who knows?

In this day and age, we routinely think a pair of 6L6s means 50-55 watts or so. I seriously doubt that SIlvertone intended that much to come out of this little amp. REmember a PA amp was intended to be clean. A 50 watt MArshall is anything but, and those 50 watts were never measured clean.

[petemoore]

output tubes

Output Tubes this may see some serious duty, especially if lack of loudness or diminished performance is an issue.
New output tubes can provide an excellent comparator to 'barometer' an amps existing output tubes quality, good to have if you like the amp.
I have some in the shelf cupboard [Bill called and needs them], which I'd replaced because I thought the amp would perform better, I couldn't tell a dimes worth of difference.