Marshall Amp Forum - View Single Post - Preamp Tubes?

MartyStrat54 · 04-25-2009, 03:04 PM

Joe, I did a lot of online researching and I couldn't find a thing about the bias question. Now I do know with VOX they sometimes market an amp as Class A when in fact it is a high AB. The Night Train can go from pentode to triode power on the EL84's. This usually means Class A and Class AB operation which means that the power tubes need to be biased. I looked at the owner's manual and it doesn't say a thing. I know nothing about VOX, other than I have played their top of the line stuff like an AC-30. It is possible that the amp is cathode biased and that means you can plug and play the power tubes. You would think the owner's manual would cover a critical topic like this. Did you buy the amp new? If so, maybe the music store knows the question. I have a Traynor all tube Custom 100 that plays in both pentode and triode modes and it must be biased. Better to know before changing out power tubes. I did see where V1 is the gain tube and V2 is the tone stack. It doesn't use a phase inverter, just two 12AX7s and two EL84's. Now that is simple. I looked at it for a while and I would say it is a nice little amp, bigger than a TT. And yes it is nice to be able to carry a 15 watt head up a flight of stairs versus the DSL50. I have an old 1972 Randall 150 watt amp that will break your balls just walking on a level floor. Man that thing is heavy. It makes my DSL401 seem like cotton candy.

Yes, EL84's are sort of like preamp power tubes. They are just a little taller and they have low output. However, depending on who made the tubes can mean a big difference in tone. They all will put out the same power, but the tone can vary wildly. I have three premium sets for my 401. RCA Black Plates,
RCA Gray Plates and Telefunken Gray Plates. In addition, I bought tubes direct from Russia to include premium 1967 Military EL84's. I think I have eight of those left. Of course you know I sold my old 1999 401 and have the cold biased 2006 model. I put a set of these Soviet missiles in my amp and biased them as hot as I could go and they sounded great, almost as much top end as the Telefunkens, but the Soviet's were meatier in the low end.

Well, I hope all is settled here on our tube rolling forum. I'm a hippie from the flower power days. Peace and love.

04-25-2009, 03:04 PM	#154 (permalink)
MartyStrat54 Senior Member Join Date: Feb 2009 Location: Formerly Phoenix, now north of KC Posts: 2,583	Re: Preamp Tubes? - Who, What, When, Where, Why, and How? Joe, I did a lot of online researching and I couldn't find a thing about the bias question. Now I do know with VOX they sometimes market an amp as Class A when in fact it is a high AB. The Night Train can go from pentode to triode power on the EL84's. This usually means Class A and Class AB operation which means that the power tubes need to be biased. I looked at the owner's manual and it doesn't say a thing. I know nothing about VOX, other than I have played their top of the line stuff like an AC-30. It is possible that the amp is cathode biased and that means you can plug and play the power tubes. You would think the owner's manual would cover a critical topic like this. Did you buy the amp new? If so, maybe the music store knows the question. I have a Traynor all tube Custom 100 that plays in both pentode and triode modes and it must be biased. Better to know before changing out power tubes. I did see where V1 is the gain tube and V2 is the tone stack. It doesn't use a phase inverter, just two 12AX7s and two EL84's. Now that is simple. I looked at it for a while and I would say it is a nice little amp, bigger than a TT. And yes it is nice to be able to carry a 15 watt head up a flight of stairs versus the DSL50. I have an old 1972 Randall 150 watt amp that will break your balls just walking on a level floor. Man that thing is heavy. It makes my DSL401 seem like cotton candy. Yes, EL84's are sort of like preamp power tubes. They are just a little taller and they have low output. However, depending on who made the tubes can mean a big difference in tone. They all will put out the same power, but the tone can vary wildly. I have three premium sets for my 401. RCA Black Plates, RCA Gray Plates and Telefunken Gray Plates. In addition, I bought tubes direct from Russia to include premium 1967 Military EL84's. I think I have eight of those left. Of course you know I sold my old 1999 401 and have the cold biased 2006 model. I put a set of these Soviet missiles in my amp and biased them as hot as I could go and they sounded great, almost as much top end as the Telefunkens, but the Soviet's were meatier in the low end. Well, I hope all is settled here on our tube rolling forum. I'm a hippie from the flower power days. Peace and love.