Sound City 50 PA Plus missing bias presets!

[Stratfordade]

I'm taking a look at a Sound City 50 PA Plus. Differs from the 50 Plus guitar amps in having 6 inputs with tone stack for each channel, but the same EL34 pair output. Even managed to find the attached schematic which is for the 100W but states on schematic same aside from having 4xEL34s. The schematic I've found doesn't show any of the rectifying components (cap and diode) around the bias winding because the scan has missed top of circuit.

The task was just to locate source of hum on one channel which I've traced to a missing ground connection and all otherwise sounds ok but I haven't measured output power yet.

What is baffling me is that the schematic shows presets to adjust bias for each EL34, and a photo I found from someone restoring same amp (first photo before schematic) clearly shows the presets. But the example in front of me has no presets (photo after schematic). The bias voltage measures pretty much OK though. -41V at the points on circuit where -35V indicated so a little cool.

Almost all the (limited) discussion I've found on these amps is of the guitar version, and usually it's about the bias being too hot and using a doubler to tame it! Does anyone know if the design was changed at any point to remove the bias presets, or do you think someone has modified the amp I'm working on to delete the bias controls? This version is ‘Custom Built Mark 4’.

[drewl]

I can't see it big enough to tell if it was modified, but I wouldn't be surprised if it came this way from the factory.

I needed to add a voltage doubler for the bias circuit of my L120

[TomCarlos]

I did some digging.... apparently, finding a schematic for the 50 Plus PA version is almost impossible. Funny thing about this head - in my first band, we borrowed a PA head for our first gig. Sure enough, it was this same head.

Did you look at the schematic I am posting here? It is the schematic for the guitar amp version of the 50 Plus. BUT.... except for the inputs, the Power Supply and Power Amp section might be the same.

Sound_City_LB_50_Plus_IssueB_Schematic.pdf

[Stratfordade]

I did some digging.... apparently, finding a schematic for the 50 Plus PA version is almost impossible. Funny thing about this head - in my first band, we borrowed a PA head for our first gig. Sure enough, it was this same head.

Did you look at the schematic I am posting here? It is the schematic for the guitar amp version of the 50 Plus. BUT.... except for the inputs, the Power Supply and Power Amp section might be the same.

[ATTACH]n1009773[/ATTACH]

Thanks Tom I did look at that schematic first and it does have the same PS and supply arrangements, save for the removal of bias presets and a very different preamp.

Anyway the owner has let me know the amp now sounds “fantastic” so a happy ending.

His amp is slightly different from the version you posted as it has the ‘smartie’ knobs and lacks extra logo so there may have been variations.

[Stratfordade]

I can't see it big enough to tell if it was modified, but I wouldn't be surprised if it came this way from the factory.

I needed to add a voltage doubler for the bias circuit of my L120

Thanks Drewl yes that would make sense as a factory change given the problems I’ve read with the amp not having a high enough bias supply. I believe some owners resorted to a doubler as you did, or roped in the heater winding for an extra 6V. Maybe the amp I worked on had the heater winding mod? Back with owner now so can’t check. I will post larger picture of the inside though.