Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: SoundCity85 on July 03, 2020, 06:28:45 am

Title: Sound City Mark IV PA 50
Post by: SoundCity85 on July 03, 2020, 06:28:45 am

Hi guys,

New to the forum, and relatively new to amp building. I do have an account on the marshallforum, but I can't keep annoying these guys with the Sound City and Carlsbro amps I'm messing around with :-) My first build was a JMP 2204 preamp into a power section of a Sound City Mark IV PA 120R, and that worked out pretty good. I'm still learning though.

This new amp I got is a 6 channel all tube 50W 2xEL34 amp. There is no schematic around as far as I know. Does anyone know where to find it perhaps?

My thoughts are to mod it instead of gut it, because basically the preamp tubes are there, the good Philips mustard caps, and I think it hasn't been used that much so it should be in good shape. So I was thinking to modify it this way to at least have one high gain channel. The basic concept of the amp is that each input goes straight into the grid of a ECC83 triode (not the typical 68k resistor), 220k plate resistor, and then a 47 µF coupling cap before the signal runs back towards the tone stack (volume, bass & treble). There all 6 channels are mixed on one position, running towards V4 and V5. There is a master volume control somewhere between V4  and V5. So it's not notorious active preamp of the Mark IV L120 or 50 plus. Also, all 6 cathodes are interconnected.

So my main question, how should I approach a high gain channel? I would assume cascading one channel into another is the way to go here, but what to do with the tone stack, I don't need two tone controls for 1 channel. And possibly I would need something to control the gain. Do I need to modify the inputs?

Cheers

Title: Re: Sound City Mark IV PA 50
Post by: sluckey on July 03, 2020, 08:31:05 am

What is the model number of the amp in question? There are several Carlsbro schematics in the library on this site...

     https://el34world.com/charts/Schematics/files/Carlsbro/Carlsbro_Schematics.htm

Title: Re: Sound City Mark IV PA 50
Post by: SoundCity85 on July 03, 2020, 10:00:56 am

Sound City Mark iv PA 50

Title: Re: Sound City Mark IV PA 50
Post by: SoundCity85 on July 04, 2020, 09:26:57 am

Nobody?

I don't think you even need a schematic to help me, I explained the concept of the amp above, and it's probably the same or comparable as Mark III PA

Title: Re: Sound City Mark IV PA 50
Post by: SoundCity85 on July 06, 2020, 07:41:27 am

Better head my ass back to the marshallforum I guess :-)