Re-tubing a MusicMan HD130 - bias or no bias?

[shizoz]

Hey guys,
I'm about to re-tube this head, which I bought from a friend, so I haven't had it long. Wondering if it has to be biased if I slam a matched quad in it. If you don't bias an amp that needs to be biased, what are the implications?
I would like to have the tubes in it now as backups, but if it is biased for a new set, then is that even possible?
Thanks all!

FYI - Old Tubes are Valve Arts, new ones are Mullards....

[sunbeast]

I used to have one of those- they can be pretty hard on tubes, and really work ideally with only certain tubes (I can't remember the options that were recommended, but I'm sure you could find out more with a Google search)... you can use others, but they may not last too long (especially if you don't bias the amp). Definitely have it biased, the schematics are out there somewhere and it will guarantee you the longest life from your tubes.

Great amp- one of the sweetest clean + bright tones I ever had!

[johnk_10]

they used four 6CA7's in most of them that i've seen, and they DO need to be biased when replacing them. the only ones that i know of that are still being made are:

the EH 'fat bottle' 6CA7 (which i've had excellent results with):
http://thetubestore.com/ehx6ca7.html

and the Shuguang (which are great but cost a fortune @ $100 each!):

http://thetubestore.com/sh-6ca7-z.html

[shizoz]

EL34's are the same as the 6CA7's, right? At least that is what is in there now.
And yes, I do love this amp, it has a great sound paired w/ 8-10's!
Thanks for the feedback!

[BassmanPaul]

Originally Posted by shizoz

EL34's are the same as the 6CA7's, right? At least that is what is in there now.
And yes, I do love this amp, it has a great sound paired w/ 8-10's!
Thanks for the feedback!

No they are not the same. They have different pinouts. 6CA7 has no pin 1. EL34 is a true pentode and has its suppressor grid, G3, brought out to pin 1. If the octal base has pin 1 connected to pin 8, the cathode, then using EL34s is fine.

The Musicman amps that I have seen were basically a SS amp using a grounded grid tube output stage. These have to be biased correctly or the power transistors driving the cathodes can be damaged.

Paul

[blockinlay]

I would always check the bias. I've gotten used amps that had the bias set way too high. Probably because the owner wanted a more distorted sound and didn't care about tube life. Check the old tubes bias, because as tubes wear, the bias can be adjusted again to get the optimum dynamic response. New tubes always make an amp sound great, but it would be a waste of money to change them if the old ones are working fine. Buy new ones, and keep them as your spares till you hear or see problems. The implications are that improperly biased tubes will have a shorter lifespan. If you think something is wrong with the amp, have a tech give it a go over.