I’m sure others have said, it’s not enough to just say “set it between 25-48 mA.” You really need to measure the plate voltage to be able to calculate where the bias should actually land. If you have a plate voltage of 480 V and you’re running EL34’s, they should be set around 36 mA if you are going for 70% dissipation at idle. However, if you had an amp that ran the plates higher, say at 513 V, it will be more like 34 mA for that same 70%.
but you can't adjust the plate voltage? so lets say i don't know the plate voltage on my specific amp, but i measure my 4 power-tubes, and i see we are drifting around the 30 mA - 40 mA area, then i can safely bias them all to exactly 35 mA without doing something stupid, am i right? I can safely assume that the four KT88's most likely were biased at around 35 mA by the manufacturer, is the idea.
@@Lasse3 You need the plate voltage to know at what mA to set the tubes depending on how hot or cold you want your tubes, 70% of the max voltage is pretty much the sweet spot, so for example lets say that the plate voltage is 450 VDC and you're using an EL34 tube (25W tube) you divide 25/450 and get 0.0555, that multiplied by 0.7 (your 70% sweet spot) gives you 0.388 which is 39mA. This kind of probes are useful, but you still need to know the plate voltage of the amp in use in order to get an accurate reading.
nice video. @7:29 for potentiometers' maintenance, that procedure may be good enough.. if they are dirty (as in noisy, etc) the procedure is a two-step if you want the cleaning to last for longer than a few weeks. spray the pot cleaner of your choice, work the potentiometers REAL good and let them sit for a few hours. spray the pots again to flush out the dissolved dirt and depending on the cleaner you use, it may be a good idea to spray a pot lubricant for both the electric surfaces and the shaft. (the presented brand has the F5 for extra lubrication for faders and such) (some pots have grease on the shafts, especially the double pots, and that grease can and will get on the tracks and blades with time. it collects dirt too)
Very good promo but a terrible idea to use a metal screwdriver in a hot amp even if you have one hand in your pocket. Use a plastic tool or other non-conductive tool. If you're buying matched tubes, is it really necessary to measure all 4 tubes in order to establish correct bias since there's only one adjustment for fixed bias amps that adjust every power tube's grid voltage in the same direction? Also, how far off does current need to be in a tube to be considered not matched?
Personally I check them all anyway just to make sure that each side of the power amp is balanced as best as can be. For instance, let’s say I have a set of tubes that are measuring 40 mA, 37 mA, 43 mA, and 40 mA respectively. I would likely move the 37 mA one to the outside as it would be a closer match to the 40 mA one as opposed to the 43 mA one. Sorry if that’s a bit convoluted. Does it actually matter that much? I can’t say for sure, but my mind feels better about it lol
Take a 50-watt EL34 amp. If the recommended range is 20-40 mA, why not start at twenty and raise it while playing until you get the sound you want, without going over the max, of course? Seems like sound is what we are after, within the safe range. Numbers being just numbers. Color of the plates under power is also valuable info.
I have a question is you can answer it. When people are working on the amp, how do you replace and solder components with one hand or do they discharge the capacitors?
Bias is one thing, but what about plate dissipation? You need to know the cathode to plate voltage to calculate that. 40 mils at 300 volts will give you 12 watts of plate dissipation. This could be the high end of some tube ratings.
I know this is off topic, but I need two, ceramic octal tube sockets to fit a Marshall JCM900 chassis, the holes to mount the sockets for a Marshall JCM900 are larger than most American chassis holes. The JCM900 tube sockets are bottom mount, and the hole diameter is approximately 1-1/4". I hope Amplified parts is listening!
Mashall 100 watt, 2 new power tube sockets,crimpon connecters on the power switch, one new speaker jack,looks like an orange drop for the presence control cap and by the looks of the added resistor next to the bias pot, I'd say that this was converted from 6550 power tubes to EL34s, 180 k bias feed resistors as well,maybe a new V1 tube socket,can't see it that well. How'd I do ? Thanks,I love your videos.
Did you mean voltage, because last time I checked, "mA" is a measurement of current? The eurotubes measures both voltage and current (amperage), this one only the current.
What ever happened to looking at the amp on a scope and adjusting bias right to the point cross notch distortion disappears? Of corse, one needs to look at idle current also, but when you adjust for the point of cross notch elimination, the amplifier will sound its best????
Do companies really test every tube, record its characteristics, and say "hey this one matches just right with the one I tested 30 minutes ago! now... which one is it?" I really would like to see how far off 2 individual tubes bought from the same reseller/company, at the same time, etc. are. Aren't you curious? One more question Can one just use parasitics to make them match?(push-pull) (assuming one has put in the circuitry to do so)
"Do companies really test every tube, record its characteristics, and say "hey this one matches just right with the one I tested 30 minutes ago! now... which one is it?"" They do! A cataloging process has to be used to keep everything in order but that's precisely how it happens. I'm not sure what you mean by balancing the tubes using parasitics but amp builders have definitely used separate bias supplies, one per tube, to balance the bias on unmatched tubes.
Hi I have JMP-50 2204 amp with EL34’s! Well.... it sounds horrible! I have replaced everything I could think of and I could! Tubes, filter caps, trim pot,bias caps, pots, jacks tube sockets, a lot of other caps! .....still sound awful:-( On low volumes it is not that bad, but the more I push it, the worse it sounds! Well....with Marshall tube amps is supposed to be the other way around isn’t it :-) I was checking the bias from time to time and the last time I did I left the probe and meter on V5 , and found something that definitely didn’t look right to me!? Before that :My amp plate voltage is 400V! Most JMP’s are on 600V, but few run at lower : at 400V! My JMP is one of them! So I have set it around 37mA idle! There was around 0.4~0.5 difference between V4 and V5 at idle! I have one of these socket probes that goes between tube and amp socket with probes to be plugged in meter! I am biasing the amp, on mV setting on the digital multimeter! After about 15 min waiting for the bias settling in ,I left the probe and the meter on, and start playing! The readings on the meter went up, which it should! When I was swapping the probe back and forth between V4 and V5 , I found V5 was jumping a lot higher then V4! And as I said above,the more I was pushing the amp, the bigger that difference became! For example: The highest readings on louder lower notes on V4 was reaching 45mV, while V5 was going way higher to 130mV! If I stop playing, both will go back to 37.0 37.4! The lauded the amp pushed, the bigger the reading differences between V4 and V5 ! If I swap the tubes, the higher readings are still on V5! I have well matched pair of svetlanas, and I would guess, there is crossover distortion caused by the big difference in the current draw between V4 and V5! What possibly can cause such a dissbalance Between the two tubes when amp is pushed!?? All transformers are room temperature! No hum or any other signs of something being wrong?! I have Marshall JMP, and now i’am recording with amp simulators .......yuck ! Please Help! Any suggestions would be highly appreciated! Thanks for Your Help, and Kind Regards! Steve! :-)
amplifiedparts Well there was 100K resistor on the phase inverter measuring way above 200K ! V4 was barely moving while V5 was popping off the lid!i put some metal oxide and both tubes start working about the same! I have ordered from guy from Italy 1/2W pihers and he promised to me to match the cathode couple 82K/100K, and the following 220K couple so i have all resistors as close to the spec as possible! It works pretty good now, but I wanna make it all with carbon film pihers as it comes originally! Thanks again for the Help, and Best Regards! Steve:-)
I’m sure others have said, it’s not enough to just say “set it between 25-48 mA.” You really need to measure the plate voltage to be able to calculate where the bias should actually land.
If you have a plate voltage of 480 V and you’re running EL34’s, they should be set around 36 mA if you are going for 70% dissipation at idle. However, if you had an amp that ran the plates higher, say at 513 V, it will be more like 34 mA for that same 70%.
Best video on how bias your amp❤
You need to know the plate voltage to set the plate current. 25-40 mA is a huge range.
but you can't adjust the plate voltage? so lets say i don't know the plate voltage on my specific amp, but i measure my 4 power-tubes, and i see we are drifting around the 30 mA - 40 mA area, then i can safely bias them all to exactly 35 mA without doing something stupid, am i right? I can safely assume that the four KT88's most likely were biased at around 35 mA by the manufacturer, is the idea.
@@Lasse3 You need the plate voltage to know at what mA to set the tubes depending on how hot or cold you want your tubes, 70% of the max voltage is pretty much the sweet spot, so for example lets say that the plate voltage is 450 VDC and you're using an EL34 tube (25W tube) you divide 25/450 and get 0.0555, that multiplied by 0.7 (your 70% sweet spot) gives you 0.388 which is 39mA. This kind of probes are useful, but you still need to know the plate voltage of the amp in use in order to get an accurate reading.
nice video.
@7:29 for potentiometers' maintenance, that procedure may be good enough.. if they are dirty (as in noisy, etc) the procedure is a two-step if you want the cleaning to last for longer than a few weeks.
spray the pot cleaner of your choice, work the potentiometers REAL good and let them sit for a few hours. spray the pots again to flush out the dissolved dirt and depending on the cleaner you use, it may be a good idea to spray a pot lubricant for both the electric surfaces and the shaft. (the presented brand has the F5 for extra lubrication for faders and such)
(some pots have grease on the shafts, especially the double pots, and that grease can and will get on the tracks and blades with time. it collects dirt too)
Very good promo but a terrible idea to use a metal screwdriver in a hot amp even if you have one hand in your pocket. Use a plastic tool or other non-conductive tool. If you're buying matched tubes, is it really necessary to measure all 4 tubes in order to establish correct bias since there's only one adjustment for fixed bias amps that adjust every power tube's grid voltage in the same direction? Also, how far off does current need to be in a tube to be considered not matched?
Personally I check them all anyway just to make sure that each side of the power amp is balanced as best as can be.
For instance, let’s say I have a set of tubes that are measuring 40 mA, 37 mA, 43 mA, and 40 mA respectively. I would likely move the 37 mA one to the outside as it would be a closer match to the 40 mA one as opposed to the 43 mA one. Sorry if that’s a bit convoluted.
Does it actually matter that much? I can’t say for sure, but my mind feels better about it lol
Always have the amp connected to a speaker cab or dummy load when setting the bias. And I prefer Caig Faderlube F5 for cleaning pots.
Great info. Thanks man 🤟🏼🤟🏼🤟🏼
Hi Justin Thanks for the great video, helped me a lot.
Very useful explanations TKS
Take a 50-watt EL34 amp. If the recommended range is 20-40 mA, why not start at twenty and raise it while playing until you get the sound you want, without going over the max, of course?
Seems like sound is what we are after, within the safe range. Numbers being just numbers. Color of the plates under power is also valuable info.
Why 😎 would you not have special non conductive TOOL for this procedure... AIN'T IT JUST AIN'T IT 😜.... Awesome video content... thanks
I have a question is you can answer it. When people are working on the amp, how do you replace and solder components with one hand or do they discharge the capacitors?
Bias is one thing, but what about plate dissipation? You need to know the cathode to plate voltage to calculate that. 40 mils at 300 volts will give you 12 watts of plate dissipation. This could be the high end of some tube ratings.
Also discharge the capacitors.
I know this is off topic, but I need two, ceramic octal tube sockets to fit a Marshall JCM900 chassis, the holes to mount the sockets for a Marshall JCM900 are larger than most American chassis holes. The JCM900 tube sockets are bottom mount, and the hole diameter is approximately 1-1/4". I hope Amplified parts is listening!
try going on their website and... i dunno... looking!
Mashall 100 watt, 2 new power tube sockets,crimpon connecters on the power switch, one new speaker jack,looks like an orange drop for the presence control cap and by the looks of the added resistor next to the bias pot, I'd say that this was converted from 6550 power tubes to EL34s, 180 k bias feed resistors as well,maybe a new V1 tube socket,can't see it that well. How'd I do ? Thanks,I love your videos.
This product would be perfect if it measured the plate current as well.i know the Euro Tubes one does,but it only has 1 probe :-(
Did you mean voltage, because last time I checked, "mA" is a measurement of current?
The eurotubes measures both voltage and current (amperage), this one only the current.
What ever happened to looking at the amp on a scope and adjusting bias right to the point cross notch distortion disappears? Of corse, one needs to look at idle current also, but when you adjust for the point of cross notch elimination, the amplifier will sound its best????
Do companies really test every tube, record its characteristics, and say "hey this one matches just right with the one I tested 30 minutes ago! now... which one is it?"
I really would like to see how far off 2 individual tubes bought from the same reseller/company, at the same time, etc.
are. Aren't you curious?
One more question
Can one just use parasitics to make them match?(push-pull) (assuming one has put in the circuitry to do so)
"Do companies really test every tube, record its characteristics, and say "hey this one matches just right with the one I tested 30 minutes ago! now... which one is it?""
They do! A cataloging process has to be used to keep everything in order but that's precisely how it happens.
I'm not sure what you mean by balancing the tubes using parasitics but amp builders have definitely used separate bias supplies, one per tube, to balance the bias on unmatched tubes.
And if u check the bias and its way higher than before whats tht a bigger sign of ?
Any idea what voltage should be on a Krank Krankenstein 100watt amp. Just bought Sovtek 5881/6L6 wgc quad matched at 19.9 mAh
Hi
I have JMP-50 2204 amp with EL34’s!
Well.... it sounds horrible!
I have replaced everything I could think of and I could!
Tubes, filter caps, trim pot,bias caps, pots, jacks tube sockets, a lot of other caps!
.....still sound awful:-(
On low volumes it is not that bad, but the more I push it, the worse it sounds!
Well....with Marshall tube amps is supposed to be the other way around isn’t it :-)
I was checking the bias from time to time and the last time I did I left the probe and meter on V5 , and found something that definitely didn’t look right to me!?
Before that :My amp plate voltage is 400V!
Most JMP’s are on 600V, but few run at lower : at 400V!
My JMP is one of them!
So I have set it around 37mA idle!
There was around 0.4~0.5 difference between V4 and V5 at idle!
I have one of these socket probes that goes between tube and amp socket with probes to be plugged in meter! I am biasing the amp, on mV setting on the digital multimeter!
After about 15 min waiting for the bias settling in ,I left the probe and the meter on, and start playing!
The readings on the meter went up, which it should!
When I was swapping the probe back and forth between V4 and V5
, I found V5 was jumping a lot higher then V4!
And as I said above,the more I was pushing the amp, the bigger that difference became!
For example:
The highest readings on louder lower notes on V4 was reaching 45mV, while V5 was going way higher to 130mV!
If I stop playing, both will go back to 37.0 37.4!
The lauded the amp pushed, the bigger the reading differences between V4 and V5 !
If I swap the tubes, the higher readings are still on V5!
I have well matched pair of svetlanas, and I would guess, there is crossover distortion caused by the big difference in the current draw between V4 and V5!
What possibly can cause such a dissbalance Between the two tubes when amp is pushed!??
All transformers are room temperature!
No hum or any other signs of something being wrong?!
I have Marshall JMP, and now i’am recording with amp simulators .......yuck !
Please Help!
Any suggestions would be highly appreciated!
Thanks for Your Help, and Kind Regards!
Steve!
:-)
You may want to check that both your phase inverter, and output transformer are balanced.
amplifiedparts Thanks amplifiedparts!
I have figured it out! My phase inverter circuit was all messed up!
Kind Regards:-)
Awesome to hear you got it figured out! How does it sound now that everything is working right?
amplifiedparts
Well there was 100K resistor on the phase inverter measuring way above 200K ! V4 was barely moving while V5 was popping off the lid!i put some metal oxide and both tubes start working about the same! I have ordered from guy from Italy 1/2W pihers and he promised to me to match the cathode couple 82K/100K, and the following 220K couple so i have all resistors as close to the spec as possible! It works pretty good now, but I wanna make it all with carbon film pihers as it comes originally! Thanks again for the Help, and Best Regards!
Steve:-)
JJ EL34 jaki Bias na parę w mV ? / pacient Marshall JCM2000 DSL100
There isn't a set mV rating, it is up to the player's preference.
@@amplifiedparts I set a pair of new JJ EL34 tubes at 80mV, the other pair at the same 80mV
Too hot sounds muddy and indistinct.