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AMPEG V4BH 12AU7/12AX7's which???

rodl2005

rodl2005

Jun 1, 2005
19,567
15,063
Tasmania, Australia
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#1
My ampeg V4BH is s'posed to have these tubes in these pos.'s
(sorry for the BAD diagram - but I hope U get the idea



BACK of AMP
-----------------------------------------------------------------------------------------
6L6/5881
O O O O

12AX7 12AU7***
O O

Front
-----------------------------------------------------------------------------------------


------------------------------------------------------------------------------------------
inside pre amp section
--------------------
###(12AX7 in MY amp)
12AX7 12AU7 } inside
O O } inside

-----------------------------------------------------------------------------------
SORRY for bad diagram

NOW I bought MY V4BH NEW & IT came with only ONE 12AU7 in the position marked with the Asterix's ****

I assume this is the Phase splitter tube. According to the service manual-THANKS Jerrold- & the manual my tech got too- it's s'posed to have TWO 12AU7's one where's marked ****(where it was in my amp) & one where it's marked with '####' where there is/ & was when I bought it NEW, a 12AX7 in it!! :ninja:

Anyone know what the REAL story is/ why MY NEW V4BH would've been shipped out with a 12AX7 in the ### marked valve spot??? I'm gonna get a 12AU7 & put it in there asap, but it seems to work OK with a AX7 in there now?????:(
 
#6
I could be wrong, but I don't think there is much difference between the two other than, from what I understand, the AU7 will give you more clean headroom than an AX7. I'll check my V4BH when I get home to see what's sitting in there.
 
#7
Thanx HOWARDMUSIC, appreciate someone doing this for me. I realise the difference IS the more clean gain/headroom outta the 12AU7. Just seems strange my BRAND new amp comin w/ 3x12AX7 & 1x12AU7 when it s'posed 2 have 2 of each-according to manual. The actual manual's seem 2 be right most of the time & mine states 2 of each!!! Appreciate to see what a few other ppl have in their V4BH's! Please do let me know.
 
#8
hey rodl2005, i checked my v4bh, it is an earlier one, not the most current all black version (the v4b lettering is blue and red, rather than white), and it looks like I only have two preamp tubes - can this be right? strange.
 
#9
There is another 2 tubes inside the amp(in my V4BH anyway) There are the 4 6L6's & two SMALL tubes next to 'em in the chassis-a 12AX7 & a 12AU7 then INSIDE behind near the input jacks, are another 2 12**7 tubes. again should be a 12AX7 & a 12AU7 MINE had 2 12AX7's tho..... hmmmmmmmm???? THIS is what I'm querying!

THANX for looking at yrs tho!
 
#10
Mine has 2 12AX7s in the preamp section as well. My understanding of it is that 12AX7s and 12AU7s are pretty much the same thing aside from the gain factor.

If you want to tame a bit of the gain in the preamp section, you could try replacing the V1 12AX7 with a 5751. I recently swapped out the GT (re branded Sovteks) with a pair of old Telefunkens and I have been very pleased with the results. I have read that the 1st gain stage tube in the preamp (V1) has the largest impact on tone of any single tube in the head (assuming all else is working properly). That may be a good place to start if you want to experiment since 12AX7s are fairly inexpensive.

If you end up changing anything, please post your results as I would like to hear what others are doing (tubewise) with their V4BHs.

Cheers
 
#16
12AX7 and 12AU7 are quite different, and yet much the same........

They fit in same places, and look similar, but have very different "plate resistances" and different current capabilities. Differences are almost 10:1 in some characteristics.

I don't seem to have a full parts list for all levels handy, and I don't remember everything, but it looks to me that 2 12AX7 in preamp and one of each in power amp is perfectly correct. Values around the tubes agree with that layout.

Where did it say there should be a 12AU7 in preamp?
 
#17
Is it still super-clean sounding? If you're getting the tones you want from it, does it really matter? Phase inverter valves typicall have a 12AT7 (many suggest getting a "balanced" one for that position). Otherwise, it doesn't really matter what they suggest because your ears should ultimately decide what tubes you want where.

Remember, even among 12ax7s some may be more gainy than others. Same goes for 12at7 and 12au7, though the gain will drop accordingly in relation to the 12ax7s.

The great thing about preamp tubes is that it's pretty much a free for all. Buy some replacement tubes and spend a Sunday afternoon swapping them around in all the positions until you find a mix that suits your tonal needs.
 
#19
Ha ha- maybe.... yeah I see YOUR point re the 3 12AX7's too Jerrold. Dunno where- I think it's in the Service manual MY tech obtained. I sent him the one from you also- so we were both wondering???? It sounds fine w/ 3 12AX7's & a 12AU7 in the PS pos(I think it is). But I'll try a 12AU7 in the other pos anyway & see if results are better or worse, & let U all know MY findings.
 
#20
A 12au7 will result in less gain. If the circuit is already a squeaky clean amp, which it is, a 12ax7 may be the better way to go. Either way, they're all essentially the same kind of tube, just varying degrees of gain available. I've got a bunch of 12at7s in the preamp of my Peavey Butcher head.
 

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