50 watt problems

OnTheFritz · Post by **OnTheFritz** » Mon Mar 05, 2007 4:47 pm

I just put a board in a '76 1987, and new filter caps, and while I was at it I redid the heater wires cause they were green and orange from the factory!

The horror! So at that point I decided to replace the tube sockets as well, and everything went fine, put in the new metro board, and tried to fire it up, and Im getting no sound at all.
Have 396 on the plates biased at 65% max diss, and I got some real screwy voltages on v-3. I used the 100 watt voltage chart as a general reference, and v-1 and v-2 both looked good on all pins.
here's v-3:

pin1 = 284 vdc
2 = 126
3 = 250
4 = 3.0 ac
5 = 3.0 ac
6 = 280
7 = 125
8 = 250
9 = 3.0 ac

Some crazy high numbers there, and when measuring the pins one and six, I was getting a noise out of the speaker, and a buzz when measuring the cathode(s).
Here's some pics, any help would be much appreciated.
Thanks!

SDM · Post by **SDM** » Mon Mar 05, 2007 6:46 pm

Your presence pot has no connection to ground (through the cap alone no good). The pot, or resistor sometimes slapped across it serves as the end of the PI tail there, so the PI needs that to work right. Typically in later amps like yours it'd be as you have it, but a 4.7K resistor is needed across the pot (from purple wire lug to ground, pot being a 22K usually).

OnTheFritz · Post by **OnTheFritz** » Mon Mar 05, 2007 7:31 pm

Thanks for the tip steve, I'll get that straightened out, and go from there.
one thing I forgot to mention, is you can't see the yellow wire from v4 pin 6 attached to the board, cause I soldered it from the bottom into the lug, just thought it looked a bit cleaner.

OnTheFritz · Post by **OnTheFritz** » Mon Mar 05, 2007 10:53 pm

That was the ticket Steve!!
I never would've thought of that, but I know now.

Thanks for the input!

neikeel · Post by **neikeel** » Tue Mar 06, 2007 9:17 am

He's sharp isn't he

Either that or we are dumb

Flames1950 · Post by **Flames1950** » Tue Mar 06, 2007 4:56 pm

Steve's eyes are sharp as a tack......I'd never have seen that from the posted pics!!!!

OnTheFritz · Post by **OnTheFritz** » Tue Mar 06, 2007 5:25 pm

Flames1950 wrote:Steve's eyes are sharp as a tack......I'd never have seen that from the posted pics!!!!

Wow, coming from you, Steve should really take that as a compliment!!

This amp screams, wow. Very "rip your face off" aggressive! I'm thinking about removing the .68 cap over the 820 ohm cathode resistor, just to hear the difference.
What year did Marshall add that cap? Just curious.

SDM · Post by **SDM** » Tue Mar 06, 2007 9:08 pm

OnTheFritz wrote:
Flames1950 wrote:Steve's eyes are sharp as a tack......I'd never have seen that from the posted pics!!!!
Wow, coming from you, Steve should really take that as a compliment!!

I do, thanks guys, glad the amp is breathing fire now.

neikeel · Post by **neikeel** » Wed Mar 07, 2007 4:03 am

OnTheFritz wrote:
Flames1950 wrote:Steve's eyes are sharp as a tack......I'd never have seen that from the posted pics!!!!
Wow, coming from you, Steve should really take that as a compliment!!

This amp screams, wow. Very "rip your face off" aggressive! I'm thinking about removing the .68 cap over the 820 ohm cathode resistor, just to hear the difference.
What year did Marshall add that cap? Just curious.

Seems to disappear on the 50 watters about the switch to PCB (late '73-'74), along with 100k onto 4ohm NFB change and the 0.68uF/25k/27k on the presence pot, not sure about 100watters, probably a bit earlier as I have seen quite a few 71-73 superleads without it.