duffrouillard
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Can anyone direct me to a good a-z thread on doing this. thanks in advance
duff
duff
Changing out my tubes need some help
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Julia
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First question is are you sure you need new tubes? Are they redplating? Is your volume fluctuating? Or do you just want to try a slightly different sound?
Get a multimeter that reads mV DC.
Before you do anything make sure you know the information about the amp and tubes. Go to the valve manufacturer's website and look up the spec sheet on the tubes. There is a number called Maximum Plate Dissipation (PD). This is given in watts. You do not want your bias setting (B in mA) to exceed 70% of this.
Also bear in mind that your line voltage can fluctuate +/- 8 VAC, so I'd strongly recommend not exceeding 60% of this value. When the line voltage is high your bias will be hotter. When the line voltage is low your bias will be cooler. Make sure you have a good power conditioner.
Then you need to know your amp's transformer plate voltage (PV) rating. I contacted Randall and they gave me "approx" 530V for the RM100. If you do not know this number contact Randall.
The equation follows Ohm's Law.
PD = PV * B
where PD is the max plate dissipation; PV is the plate voltage rating of the transformer measured at a couple of pins in the tube socket but I don't know which ones so I let Randall tell me the number; and B is the bias setting in mA
So your bias setting B = PD/PV, but you're measuring in mV because there's a 1 ohm resistor across the test point, and because it's 1 ohm mA = mV.
EL34/E34L/KT77/6CA7 = 25W Max Plate Dissipation
6L6GC/5881WXT = 30W
6550 = 35W
Since they recommend tube bias between 50% and 70% we get
B = (0.50 or 0.60 or 0.70) * (PD/PV)
So your bias settings for the tubes for an RM100 are
EL34 family = 23.6 mv (50%); 28.3 mV (60%); 33.0 mV (70%)
6L6 family = 28.3; 34.0; 39.6
6550 = 33.0; 39.6; 46.2
1) Make sure your amp is turned off and cold.
2) Do not eat anything greasy while you're changing tubes.
3) Pull the old tubes. You'll need to use a GENTLE rocking motion. Be careful you do not break off the center post of the tubes. Mark the bad ones. Keep the old ones that still work for backups.
4) Install the new tubes.
5) Turn the bias screws counter clockwise.
6) unplug your guitar and turn all the volume knobs to 0. Make sure your speakers are connected to the amp!!!!
7) turn the amp on standby for a few minutes.
8 ) turn the amp on play and leave it on for about 20 minutes to let the tubes warm up. Watch the tubes for any red spots on the plates. If you see them at this point, you've probably got a bad tube.
9) Set your Multimeter to read mV DC and insert the black meter lead (ground) into the Common hole.
10) insert the red lead into Test Point 1.
11) use a small flathead screwdriver to gently turn the screw until the bias reading is where you want it in the range of the tube type you're using.
be real careful when doing this since the trim pots (screws) are very sensitive. a little movement can cause a large shift in bias.
12) if you've got an RM100 move the red lead to test point 4 and match the setting for test point 1.
13) now do test point 2 the same way you did test point 1
14) now do test point 3 the same way you did test point 4
15)Leave the amp on like this for another 30 minutes and watch it in a darkened room. If any of the tubes start to show any red spots on the plates you may either have biased the tube too hot, or have a weak tube. Check the bias settings and even them out.
It will take about a month to burn in the tubes. Remember the hotter you bias them the shorter the life. The harder you play them, the shorter the life. They should last about 1000 hrs of normal use.
There are some techies in this forum who know a lot more about this than I do. This post is more than you probably wanted to know.
Get a multimeter that reads mV DC.
Before you do anything make sure you know the information about the amp and tubes. Go to the valve manufacturer's website and look up the spec sheet on the tubes. There is a number called Maximum Plate Dissipation (PD). This is given in watts. You do not want your bias setting (B in mA) to exceed 70% of this.
Also bear in mind that your line voltage can fluctuate +/- 8 VAC, so I'd strongly recommend not exceeding 60% of this value. When the line voltage is high your bias will be hotter. When the line voltage is low your bias will be cooler. Make sure you have a good power conditioner.
Then you need to know your amp's transformer plate voltage (PV) rating. I contacted Randall and they gave me "approx" 530V for the RM100. If you do not know this number contact Randall.
The equation follows Ohm's Law.
PD = PV * B
where PD is the max plate dissipation; PV is the plate voltage rating of the transformer measured at a couple of pins in the tube socket but I don't know which ones so I let Randall tell me the number; and B is the bias setting in mA
So your bias setting B = PD/PV, but you're measuring in mV because there's a 1 ohm resistor across the test point, and because it's 1 ohm mA = mV.
EL34/E34L/KT77/6CA7 = 25W Max Plate Dissipation
6L6GC/5881WXT = 30W
6550 = 35W
Since they recommend tube bias between 50% and 70% we get
B = (0.50 or 0.60 or 0.70) * (PD/PV)
So your bias settings for the tubes for an RM100 are
EL34 family = 23.6 mv (50%); 28.3 mV (60%); 33.0 mV (70%)
6L6 family = 28.3; 34.0; 39.6
6550 = 33.0; 39.6; 46.2
1) Make sure your amp is turned off and cold.
2) Do not eat anything greasy while you're changing tubes.
3) Pull the old tubes. You'll need to use a GENTLE rocking motion. Be careful you do not break off the center post of the tubes. Mark the bad ones. Keep the old ones that still work for backups.
4) Install the new tubes.
5) Turn the bias screws counter clockwise.
6) unplug your guitar and turn all the volume knobs to 0. Make sure your speakers are connected to the amp!!!!
7) turn the amp on standby for a few minutes.
8 ) turn the amp on play and leave it on for about 20 minutes to let the tubes warm up. Watch the tubes for any red spots on the plates. If you see them at this point, you've probably got a bad tube.
9) Set your Multimeter to read mV DC and insert the black meter lead (ground) into the Common hole.
10) insert the red lead into Test Point 1.
11) use a small flathead screwdriver to gently turn the screw until the bias reading is where you want it in the range of the tube type you're using.
be real careful when doing this since the trim pots (screws) are very sensitive. a little movement can cause a large shift in bias.
12) if you've got an RM100 move the red lead to test point 4 and match the setting for test point 1.
13) now do test point 2 the same way you did test point 1
14) now do test point 3 the same way you did test point 4
15)Leave the amp on like this for another 30 minutes and watch it in a darkened room. If any of the tubes start to show any red spots on the plates you may either have biased the tube too hot, or have a weak tube. Check the bias settings and even them out.
It will take about a month to burn in the tubes. Remember the hotter you bias them the shorter the life. The harder you play them, the shorter the life. They should last about 1000 hrs of normal use.
There are some techies in this forum who know a lot more about this than I do. This post is more than you probably wanted to know.
duffrouillard
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A bit more info than i needed but thank you You rock!!!!
Julia that information is amazing, thanks a heap for posting that 
If I want to replace two of the 6L6's with EL34's (leaving 2 6L6's still in the amp), will it just be a matter of setting the bias on the EL34's to the appropriate amount? Or will the remaining 6L6's require some adjustment as well?
Anything else required to be done in this situation?
If I want to replace two of the 6L6's with EL34's (leaving 2 6L6's still in the amp), will it just be a matter of setting the bias on the EL34's to the appropriate amount? Or will the remaining 6L6's require some adjustment as well?
Anything else required to be done in this situation?
djkal
Well-known member
anyone would know the pv for a rm 50?
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