Ampeg SVP Pro 12au7 switched to 12ax7

anotherlin · #1 09-28-2011, 03:37 AM

Just purchased an Ampeg SVP PRO preamp on ebay recently. It is probably one of the early models.

I opened it and saw that the 12au7 tube (v4) has been switched for a 12ax7.
The unit works fine, except the sound is not loud at all.
I play it (at home) through an Apogee ONE soundcard. I have to put master volume to the max to have a correct level.

So, I've a few questions:

1) 12ax7 and 12au7 are supposed to be compatible, except 12au7 has 20% of gain compared to 12ax7. Does it change the sound of the preamp? Will it damage it?

2) The preamp is not "loud". I think the tubes are old (they are all Groove Tubes brand). If I put new tubes, will it be better? Note that the volume is not loud, but the clipping light just lights up when I play hard.

BillyIVbass · #2 09-28-2011, 03:47 AM

Old or tired tubes will put out less signal. A 12AU7 will take more gain to distort.

lem8r · #3 09-28-2011, 04:35 AM

You can't put ax7 in place of au7 because au7 is conducting more current and has lower output impedance (drives tone stack and other stuff properly) ax7 just can't do that properly, especially conducting too much current can wear out tube very quickly.

staindbass · #4 09-28-2011, 06:07 AM

people swap 12au7 and 12ax7 all the time, never heard of a problem, done it myself. whats the output impedance difference between those two tubes?

father of fires · #5 09-28-2011, 06:36 AM

Quote:

Originally Posted by staindbass

people swap 12au7 and 12ax7 all the time, never heard of a problem, done it myself. whats the output impedance difference between those two tubes?

12AX7 is about 80k where as a 12AU7 is at about 10k. There will be some signal loss but it's always worth experimenting. Where you run into trouble is how the 12AU7 is set up. They can handle about 3 times the current of a 12AX7. Depending on how the stage is configured the 12AU7 could have been operating outside of the 12AX7's max current range. This'll cause the 12AX7 to run hot and burn out early.

Jerrold Tiers · #6 09-28-2011, 06:46 AM

The difference is the specs known as "plate resistance" and "amplification factor". Works out to the fact that a 12AU7 in place of a 12AX7 will generally have lower gain .

And a 12AX7 in place of a 12AU7 will probably have somewhat less gain, and likely less signal swing.

staindbass · #7 09-28-2011, 07:21 AM

i see what you mean now. a 12ax7 in a 12au7 socket will not last as long as a 12au7 in a 12ax7 socket because of the current handling capability. makes sense. i hope thats the only problem as its an easy fix.

father of fires · #8 09-28-2011, 07:26 AM

Quote:

Originally Posted by staindbass

i see what you mean now. a 12ax7 in a 12au7 socket will not last as long as a 12au7 in a 12ax7 socket because of the current handling capability. makes sense. i hope thats the only problem as its an easy fix.

A square is a rectangle but a rectangle is not a square.

anotherlin · #9 09-29-2011, 03:27 PM

Ok, it seems likes I'll have to buy a new set of tubes. This time with an 12au7.

Jerrold Tiers · #10 09-29-2011, 08:27 PM

Quote:

Originally Posted by staindbass

i see what you mean now. a 12ax7 in a 12au7 socket will not last as long as a 12au7 in a 12ax7 socket because of the current handling capability. makes sense. i hope thats the only problem as its an easy fix.

Not necessarily so..........

Only if the current capability is NEEDED will there be any problem. Most of the time the problem is not really current, but plate resistance issues, and that has little influence on tube life.