Mark iii blue stripe 60w redplating

[user 41014]

But now I have a red stripe in front of me, I wonder if the blue stripe has other issues as well.

This ^

If changing "the" bias R doesn't choke off the idle current, the amp likely needs service but I really can't see what could act as a constant V source. That leads me to think in a pretty basic circuit you aren't effecting the bias circuit as you think you are for whatever reason.

Here is a 60W blue bare bones S. It looks like there used to be a R102 but it's gone and from the flux mess to the left of the radial bias caps the bias caps may have been checked or replaced but it may be the other connection there. So the bias circuit is prob not original but here is the way mine is configured on the board.

Tracing really should be done to know for sure the signal path. If R102 is a parallel resistor it's possible someone pulled it in my blue and adjusted up the remaining single resistor (Is R117 the 22k ohm?). A known stock blue might be helpful.

On mine if I can trust my eyes it looks like it goes like this after the diode allows the negative through:
10.5k ohm --> 1st bias cap --> 3.32k ohm --> 2nd bias cap --> 22k ohm

The other neg allowing diode that feeds the 332R must be for EQ this amp does not have.

The last pic with '57.3' marker on the output driver may be the tested neg bias volts for the grid.

And come to think of it, there is a way to put a ceiling on your bias resistance, and hence a floor on your idle current- kinda hits me like duh. This is wordy so I'll put the answer on a clean post

 

[user 41014]

Since you called out R117 as a possible bias R I'm thinking it must be the 22k ohm right next to R102.

Based on what you are getting all around here is my rubix cube or forrest fenn solve:

The answer is you are making big changes in a single R and getting minimal results because your effective R is capped by the || R you didn't change- 22k ohm.

22k || 68k = 16.62k
22k || 50k = 15.28k and you increased the idle current
22k || 100k = 18.03k and you added 1.4k, not the 32k you thought

changing the 22k:

68k || 50k pot = 28.81k max - this puts you in the ball game
68k || 100k pot = 40.48k max

I would bet on this solve but still, make sure R102 and (R117?) are parallel. If that is the hammer on the nail head my effective must be ~22k since there is no ||R.

All things being the same which you can never trust, you may have just defined for this amp that going below 16k ohms for effective resistance is too much bombardment for the plates and dropping below 15k would likely have turned your hot plates quickly into momentary heaters doing more damage than just tubes.

 

[BoogieJim]

Since you called out R117 as a possible bias R I'm thinking it must be the 22k ohm right next to R102.

Based on what you are getting all around here is my rubix cube or forrest fenn solve:

The answer is you are making big changes in a single R and getting minimal results because your effective R is capped by the || R you didn't change- 22k ohm.

22k || 68k = 16.62k
22k || 50k = 15.28k and you increased the idle current
22k || 100k = 18.03k and you added 1.4k, not the 32k you thought

changing the 22k:

68k || 50k pot = 28.81k max - this puts you in the ball game
68k || 100k pot = 40.48k max

I would bet on this solve but still, make sure R102 and (R117?) are parallel. If that is the hammer on the nail head my effective must be ~22k since there is no ||R.

All things being the same which you can never trust, you may have just defined for this amp that going below 16k ohms for effective resistance is too much bombardment for the plates and dropping below 15k would likely have turned your hot plates quickly into momentary heaters doing more damage than just tubes.

You’re probably right on the money.
When I started the process there was no clear information telling me whether I should remove both resistors or only 1. Since US amps, as I understand, only had the 1 resistor, whereas export models had both.

I still feel like there maybe another issue with the amp because I don’t believe the bias current should have such a massive effect on the sound.
Now that I have purchased a red stripe I have returned the blue stripe to stock with the stock R102. No red plating.
I will give it a final listen tonight before it is sold.

Red Stripe = OMFG!!!!

 

[user 41014]

You’re probably right on the money.
When I started the process there was no clear information telling me whether I should remove both resistors or only 1. Since US amps, as I understand, only had the 1 resistor, whereas export models had both.

I still feel like there maybe another issue with the amp because I don’t believe the bias current should have such a massive effect on the sound.
Now that I have purchased a red stripe I have returned the blue stripe to stock with the stock R102. No red plating.
I will give it a final listen tonight before it is sold.

Red Stripe = OMFG!!!!

Out of curiosity, I would be interested in what you think of the sound lifting a leg of the 68k, just leaving the 22k.

Stock or not you might want to mention to the buyer that the bias needs adjusting. It sounds like you are thinking it was meant to be that way but no Mark III 6L6 should be dissipating at the edge of "slowly redplating". The factory would change that if it was on their bench.

Edit: As a side note the way I understand it, pushing a tube to the max gives you a more clean tone whereas the lower the idle current, the more distortion. Most people would probably not think a Mark III at the verge of tube kill is the "Mark III" sound.

 

[BoogieJim]

I have put the stock resistor back in and it’s running at about 46mA.
But there is some other issue going on. The lead channel is down on volume and has nowhere near the drive it should have.
I wonder what could be going on

 

[mark1406]

I have put the stock resistor back in and it’s running at about 46mA.
But there is some other issue going on. The lead channel is down on volume and has nowhere near the drive it should have.
I wonder what could be going on

I wouldn’t get rid of the pot. A bias pot is a good thing as it’s adjustable. Fixed resistors have only one advantage over pots and that is resistors can have a higher wattage than the pot. Other than that the pot is the way to fly.

Regards

Mark

 

[BoogieJim]

I wouldn’t get rid of the pot. A bias pot is a good thing as it’s adjustable. Fixed resistors have only one advantage over pots and that is resistors can have a higher wattage than the pot. Other than that the pot is the way to fly.

Regards

Mark

I have removed the pot so I can sell the amp in as close to factory condition as possible.

I’m more concerned with the lack of gain and volume on the lead channel at the moment