6L6 replacement for HD-150 bass amp

Hi,I juz got an HD150 the bass version with RH115 cab.It came with 4 EH EL34's in it.Asking about tube power section,if I want to change it,what types of tubes can I use?as far that I know,it's the SS pre and tube power section,so I dont have to worry about the bias thing.I did alot of searching and found that this head was originally wired for 6L6GC tubes.there are many threads talking about them but I juz havent see any thread in here talking about this amp (bass version)
So,should I change them to 6L6GC or the 7581a or 6CA7s?How about the JJ E34L ? is the EH EL34 safe for this amp?I want more bass response as I use it for bass guitar only.
thank you sir.
Saras

Is the tube layout label still stuck inside the back of the amp? What does it say about the tubes? If nothing has been modified inside the amp, I'd think that staying with the factory specified tubes is the best policy. I'd certainly be getting that bias checked if the tubes are non factory spec.
I'm not sure that your having the SS preamp section is going to relieve you of having to bias the amp for the tubes. Most of the experts here suggest keeping the bias on the low side of these high-powered amps, something along the lines of 25ma max. Take this with a grain of salt, as my only experience is with HD130's with the 12AX7 phase inverter tube.
Caution: As everyone around here will tell you, don't go inside the amp and start probing around unless you know what you're doing. These big MM powerhouses have 700 volts running through them. This juice can lay in wait for you, in the capacitors, even after the unit is unplugged. There is information all over the web that will help you be safer, should you choose to work on your amp. Believe the warnings and stay alive.

If the amp is designed for the EL-34/6CA7 tube, you need to stay with that. 6L6 and EL-34 are not interchangeable. The very first HD-150 and 75 amps (I learned recently) were made with EL-34/6CA7. Just switching to another brand of output tube will NOT give you more bass response if the existing tubes are working within specifications. Low bass output can happen for a number of reasons. Electrolytic capacitors (power supply, the 20uF coupling caps) in my opinion, is a a more likely culprit. Have you tried playing with the different filters? The deep/bright switches in combination with the two different set of tone controls for channel 1 and channel 2 allow for a lots of possibilities.
Personally I like Winged-C tubes. Whether you buy the 'cryogenically burned-in' version is a matter of how much you love your pet Music Man amp.
Cheers
Lars Verholt
(who keeps all his tone controls on 5)

Hi Brian and Lars,firstly I have to thank you u guys so much for the answer.my HD-150 is the bass version that has no reverb or trem.it only has Normal and Bass Channel.I've already asked the amp tech and he said its ok if I wanna use 6L6GC.I dont know why but he guarantees me that.also,the in "Looking for tubes" thread,the MM tech here recommended me to get a quad of 6L6GC or KT family.it sounds pretty OK to me rite now but I juz wanna try a new or new old things that might give me a better result.anyway,thank you u guys again and I might try both new and NOS.maybe Winged C and RCA.