JCM900 4100 Correctly Biasing

[gibson17]

Thank you for your reply. I reconnected R30. That resistor is a 22.5k (red, red, orange, gold) in my amp. I pulled the tubes and turned the amp on. The measurements on pin 5 are -46.01vdc and -58.25vdc. I am using a dual bias probe from SRS into my DMM which is a Fluke 115.

 

[anitoli]

Next question what type of power tubes is this amp set up for?

 

[Spanngitter]

Thank you for your reply. I reconnected R30. That resistor is a 22.5k (red, red, orange, gold) in my amp. I pulled the tubes and turned the amp on. The measurements on pin 5 are -46.01vdc and -58.25vdc. I am using a dual bias probe from SRS into my DMM which is a Fluke 115.

The Resistor you targeted but missed is R28 on your amp and should be 47k (some have also 56k)
Pls check R31/32/33/34 (Screen Grid Resistors) on what value they have, if they have 470R then your amp should be equipped with 5881, if they are 1.5k then it is an EL34 Version.
Due to your bias Voltage not being equal pls check also the 1.5k Grid Stopper feeding to Pin 5 on each socket, they might be mangled by a failing power tube and therefore give you different readings.

 

[gibson17]

Next question what type of power tubes is this amp set up for?

It had EL34's in it, so that's what I put back in it. After some research I see that 5881's might have been a thing when this amp was produced.

 

[gibson17]

The Resistor you targeted but missed is R28 on your amp and should be 47k (some have also 56k)
Pls check R31/32/33/34 (Screen Grid Resistors) on what value they have, if they have 470R then your amp should be equipped with 5881, if they are 1.5k then it is an EL34 Version.
Due to your bias Voltage not being equal pls check also the 1.5k Grid Stopper feeding to Pin 5 on each socket, they might be mangled by a failing power tube and therefore give you different readings.

R28 in this amp is a 56k. R29 is 15k. These values differ from the schematic that was linked by anitoli. R31/32/33/34 are 2.2k 10% 4w. I checked the resistors on the tube sockets pin 5. They are using one resistor on each of the outside tube sockets with a jumper to pin 5 on the inside sockets. The measurements of the two resistors are 1.482k and 1.548k. Interestingly, one resistor is metal film whereas the other is carbon comp.

 

[Spanngitter]

R28 in this amp is a 56k. R29 is 15k. These values differ from the schematic that was linked by anitoli. R31/32/33/34 are 2.2k 10% 4w.

This is an EL34 Amp with the 2.2k (which are in my eyes and ears not required and choke the amp, better to have 1.5k instead) and the schematic for it should be this one shown below. They did an awkward series connection for the Grid Stoppers where one set has 1.5k (on PCB) and the other set of tubes has 1.5k +1.5k in Series. I usually convert those by adding bridge on PCB and equipping each tube with a separate MOX/MF 1.5k.
CC was for sure not used in 900s and is (even on very old models) a constant source of failure due to the heat it is exposed (especially in Combos). Better use MOX, MF or (if you want to stick with Carbon) CF at this point...

 

[gibson17]

This is an EL34 Amp with the 2.2k (which are in my eyes and ears not required and choke the amp, better to have 1.5k instead) and the schematic for it should be this one shown below. They did an awkward series connection for the Grid Stoppers where one set has 1.5k (on PCB) and the other set of tubes has 1.5k +1.5k in Series. I usually convert those by adding bridge on PCB and equipping each tube with a separate MOX/MF 1.5k.
CC was for sure not used in 900s and is (even on very old models) a constant source of failure due to the heat it is exposed (especially in Combos). Better use MOX, MF or (if you want to stick with Carbon) CF at this point...

A few questions:

If I change the Screen Grid Resistors should I go with 1.5k 10% 4w?

Is the value of the two grid stoppers that I measured cause for the imbalance you were referring to?

Did I miss two grid stoppers on the PCB? The only resistors I saw that looked connected to pin 5 were the two I mentioned and measured.

Do you have a recommendation what value I should use in R28 to correct the bias issue to get it into range or should I just experiment with the decade resistor to find an appropriate value?

 

[Spanngitter]

I usually do not like and use these cement block resistors, I normally use enamel coated Welwyn or Flameproof Vishay_Dale or Ohmite 5W Wirewound Resistors 1.5k 5% (1.8k if the Amp is beaten hard) which are available from RS Components, Mouser, etc.
Regarding your grid stoppers you say there is none on the PCB? Normally they should be located on the same PCB where the Screen Grid Rs are, check where the orange and green wire feeding to Pin 5 of the inner set of power tubes do branch off, they should be clearly visible there

For your Bias Circuit there had been normally (at least per my information) 2 Versions for R28/R29:
EL34 => 47k / 15k
5881 => 56K / 8.2k
Since your Amp has 15k I would first check if you are able to get into range of -40Vdc at Pin5 for any socket without PTs installed before you swap R28 as the PCBs are delicate and any fail in modding Bias Circuitry might end in core melt down of the power tubes if not done properly.
If this is possible and readings are fairly equal (< 1Vdc delta) then proceed with inserting power tubes and recheck Bias.

 

[gibson17]

Just reporting back for some closure to the additional content I posted to this thread so that it might be helpful to someone later on down the road. I dislike it when I am researching an issue and find a thread that just goes cold. Like did you solve the issue?

I wound up recapping the amp along with power filters. Some of the caps were bulging and getting ready to leak. I understand that some boards are very fragile. I have a pretty decent metcal workstation that has soldering and desoldering. I got it pretty cheap when everything was switching over to lead free... so disassembling the through hole is a pretty straightforward process. I replaced R28 with a 1/4w 47k. I replaced the 1.5K resistors on the tube sockets. Everything biases up right and the amp sounds great. The reverb is having an issue but I've narrowed it down to the tank, but that's another problem for another day. Thank you very much for the help. It is greatly appreciated.