|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
69.125.143.12
I am refurbing a 1976 Sound City 50R guitar amp for my son. The original B+ caps were two 300uF/350V in series to effectively get 150uf/700v for the first capacitance after the bridge rectifier. B+ according to the schematic should be 420V at 115V input but with my local line voltage at 123V I'm getting 460V and 490volts in standby. My desire is to use a twin 100uf/500v F&T in parallel to get 200uF. I'm worried that I might not have enough margin with a 500V cap. I already put a CL-80 on the primary side of the PT which got me down to the numbers I listed. Am I worrying unnecessarily?
The PP outputs are EL-34's. One has 22ohms cathode to ground. The other has 63ohms cathode to ground. At first I assumed some yahoo stuck whatever he had in there to measure bias but I looked up the schematic on the web and those values are on it! There are individual fixed bias pots with a recommended bias voltage of -36V. Early versions (1972/73) grounded the cathodes. Why would a PP deliberately have different cathode resistors?
Thanks for any opinions.
Gary
Follow Ups:
Standby B+ voltage is with the output stage tubes heated and loading down the amplifier. Given the amp uses ss diodes, there is 10-12 seconds when a higher DCV is applied to all caps in the amp.At power turn on, a modern e-cap meeting typical performance standards can withstand +10% of rating for a short time and infrequent events (such as during power on of a guitar amp).
Unless you are prepared to confirm by measurement what the peak turn-on B+ voltage can reach, and are happy with the margin for the caps you want to use, and the local line voltage variations, then imho don't go there.
I'd suggest the different cathode resistor values in the output stage are to counter-balance the difference in output transformer primary half-winding dc resistances, given there is already individual EL34 idle bias level adjustment pots. However that reasoning makes no sense for normal operation as the EL34 plate resistance in pentode mode would dominate nominal frequency response. It may make some sense during gross over-drive operation, where each EL34 saturates in turn, and an imbalance in saturation current levels could noticeably offset the output transformer flux swing symmetry.
Edits: 10/01/20 10/01/20
Make it reliable. It gets hot in there.
(By the way, adding more power supply capacitance in a guitar amp can make it sound worse. This ain't hi-fi)
Follow the schematic!!
The "unbalanced" cathode resistors are not there to provide much of the bias. They are probably there to provide a little difference in the operating point of the output tubes. Remember that guitar amps are distortion generators. The quickest way to ruin one is to "clean things up".
nice to have confirmation.
Don't mess - restoration first, then you can learn 1st hand what messing with the designers intention means.
I had a guy bring in a Marshall amp with a crapload of capacitance he put in the power supply and it sounded like doggy doo and wanted to know why.
"Bass is the place..the rest is filigree and lace" Doug Sax
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: