RCA 76 Series Preamps Troubleshooting

[Reo]

Hey all, I have four old RCA 76 preamps, all on the same chassis. They each use one 1620 tube in triode mode.

picture of the pres: http://1.bp.blogspot.com/-GTq-wAHxgjk/Uv--rLk3BXI/AAAAAAAAATA/AFv2CkQ7yHg/s1600/RCA.jpg

I don't have grid cap connectors so I changed the socket wiring around on all four so that I can use 6sj7s instead of 1620s/6j7s.

I powered them up for the first time with a heathkit PS-4 for 6.3v and B+.

When I kept the B+ under 53v, all four channels passed audio but at a very low level and hum was pretty minimal.

When I went above 53v on the B+, I started hearing a buzz that got louder as I turned up the B+ (76 consolette manual says plate voltage should be 220).

The buzz sounded the same on all four channels. The cathodes all measured around 10v and the manual says they're supposed to be at 8.3v.

Does anyone know what might be causing the buzz based on those symptoms? I'm not sure if it's power supply or the preamps but I'm not sure how to tell...I have a tektronics  2215 scope if that helps.

I just arrived at the electronics party, thanks for bearing with me 

Reo

Schematic:

 

[Reo]

In the schematic, the bold black line running vertically between the tube and the input TX is the grounding bar. Also, the B+ feed goes into the primaries of all 4 OPTs in parallel.

 

[gridcurrent]

1.  ground one side of the 6.3 VAC supply.  they are floating from the PS-4.
or
2.  save yourself from the hum hassle and power the heaters from a low noise DC supply.  connect the low side of the supply to circuit common/ground.

 

[emrr]

These tend to show input hum and noise if unloaded, so put a 150 ohm resistor across the input. 

You could also just use 6C5's rather than 6SJ7's, if losing the grid caps. 

 

[Reo]

Thanks for the replies - grounding one side of the heater circuit seems to have fixed the problem. I've been reading about loading these kinds of preamps so I'll keep that in mind as well.

Almost done racking these up, then on to the program amp!

 

[gridcurrent]

.

 

[emrr]

I like that chart, hadn't seen it in awhile.  It's interesting to ponder that data against the large proximity effect of a ribbon in usage. 

The program amp is essentially the earlier 40-C or 84-A amp but with an additional gain stage.  If gain seems excessive, consider dropping a front end tube.  I assume the extra stage is there because of the low gain of the preamps  (30 dBish) themselves.