Converted another 50's tube PA yesterday
Posted: Sun Jun 05, 2016 6:09 am
My son brought over a 50's Bogen K130 he picked up at a thrift store last week for $50.
We replaced the old 2-prong cord with a grounded one, pulled the death cap then removed a funky 4-pin mic jack from the back, replaced that with a 1/4" jack and some reducing washers and installed the wire jumper impedence selector that will allow use with 4, 8 & 16 ohm cabs. We soft started it on a current limiter to make sure nothing was going to blow then took it for a test run and.......nothing. I suspected there was something a little odd about this one so we downloaded a schematic and saw this PA is set up with a special circuit to be used with low impedance mics. It was somewhat of a confusing mess. I got up early Saturday and decided to bypass all that shit and use the 6AV6 single triode used for the "mic preamp" as the first gain stage then run that through a .047mf cap that was in the "speech filter" circuit straight to the 12AX7 and she woke up and started singing.
That's what I love about these old point to point amps, they're easy to modify.
The power supply in this thing was really weird and I'm glad I didn't need to mess with that.
Lots of unusual tubes in this one.
It's supposedly a 30 watt amp so the 6AV5GA power tubes must be similar to 6L6's. The amp sounds really nice. Steve gave me the Telefunken ECC83 that was in it for my troubles and beat feet out to lock himself away with it for a little while. I was sad to see it go last night. I didn't even get a chance to get a tone clip of it.
I'll do that next time he comes over.
I think I'm going to drag my CH-18 out and do that one next to scratch my Bogen itch. They make such cool sounding amps.
We replaced the old 2-prong cord with a grounded one, pulled the death cap then removed a funky 4-pin mic jack from the back, replaced that with a 1/4" jack and some reducing washers and installed the wire jumper impedence selector that will allow use with 4, 8 & 16 ohm cabs. We soft started it on a current limiter to make sure nothing was going to blow then took it for a test run and.......nothing. I suspected there was something a little odd about this one so we downloaded a schematic and saw this PA is set up with a special circuit to be used with low impedance mics. It was somewhat of a confusing mess. I got up early Saturday and decided to bypass all that shit and use the 6AV6 single triode used for the "mic preamp" as the first gain stage then run that through a .047mf cap that was in the "speech filter" circuit straight to the 12AX7 and she woke up and started singing.
That's what I love about these old point to point amps, they're easy to modify.The power supply in this thing was really weird and I'm glad I didn't need to mess with that.
Lots of unusual tubes in this one.
It's supposedly a 30 watt amp so the 6AV5GA power tubes must be similar to 6L6's. The amp sounds really nice. Steve gave me the Telefunken ECC83 that was in it for my troubles and beat feet out to lock himself away with it for a little while. I was sad to see it go last night. I didn't even get a chance to get a tone clip of it.
I'll do that next time he comes over. 
I think I'm going to drag my CH-18 out and do that one next to scratch my Bogen itch. They make such cool sounding amps.
