22+ resistor values at the power section and feedback loop

[gcortright]

My reverb quit working so I pulled the chassis and isolated the reverb tranny as the problem. Ordered one from Mesa.
I changed the 3 filter caps, 470uF16v and 270uF63v caps some years ago. I also added Don's lead drive mod and moved the reverb to the back.
The amp works great but I don't use it much these days. It is noisy.
I was checking resistor values for my amp wondering why it had turned the board black above the EL84 power tubes.
It was like that when I got it off Ebay. I put a fan in the bottom of the cab to cool it some.
People say they run hot but why is that?
.
Here is what I got around the power section. I have the readily available schematics from the internet.
R201 and 202 read 1.5M instead of 2.2M.
R222 and 224 read 295K instead of 330K.
These feed the EL84 pin 2 control grids.

These are in the feedback loop.
R207 reads 1.465K which is close to 1.5K on the schematic.
R252 reads 1.466K instead of 56K on the schematic. This is the big outlier. Could this be a problem?
R207 and 252 are in series on either side and intersect the line from the Presence pot to the V5B pin 7 grid.

Can I change something somewhere to make the amp run a little cooler without messing up the sound?
Also the amp is noisy and they say it is caused by the C6 47pF cap and R104 3.3M in parallel at the input to the grid of V2A which is the first stage of the clean channel.
Can this be modified to quiet the amp down? Is there anything else that would help?

Thanks for any response.

 

[gcortright]

I measured the bias current with a Eurotube bias probe. Tube 6 EL84 reads 30.22 milliamps. Tube 7 EL84 reads 33.82 milliamps at 400 plate volts.
Those are both way too high. That is Class A territory.
70% dissipation for a fixed bias EL84 with a 400v plate voltage is 23.333. I need to knock the bias current down closer to 70%. That might be why it is
so **** noisy. I guess it is time to update lots of components.
If anyone has a suggestion please let me know. I'm thinking about putting all the new parts on the top of the board where I can see them.

 

[pmac11]

Sounds like you may have some drift in the component values, or an improper mod somewhere in the past. If your board is black you may have a burnt resistor. It's my understanding that carbon comps do that. Try replacing the wonky/out of spec resistors with higher wattage metal films.

 

[Nandrewjackson]

Measuring components in the circuit won't jive with schematic values if anything is in parallel with the one being measured

 

[pmac11]

Measuring components in the circuit won't jive with schematic values if anything is in parallel with the one being measured

True. The OP will need to isolate the one(s) he's interested in.

 

[gcortright]

Thanks. I figured that out after awhile. I replaced the carbon comps with larger metal films. Also replaced the orange drop caps with 600 volt panasonics and all the ceramic disk caps. I also changed the input pad R104 with a 1.5M and C6 with a 180pf like later amps. I probably did more than I needed to. Now my A voltage to the output transformer is 450 instead of 390. My B+ is 338 instead of 295 and and C is 275 dc instead of 235 like on the schematic. I also put in a 10K with a 25K bias pot in place 33K R111. I get almost 12v on the control grids of the el84s but only 11.26 ma and 10.5 ma with the eurotube bias probs.
455 plate voltage/12=37.9 X .7=26.5 ma.
Any ideas?