Putting a 5751 preamp tube in a AC30 6TB

razorblade_beat

Looking at re-tubing my ac30 from 97, Currently using an OX box but there seems to be very little to no clean and not much top end. I noticed this when I last gigged it jan 2020. Ive never put new tubes in it and I've owned it for about 4 - 5 years now so seems like it needs it. Read some people have put a 5751 preamp tube in for more clean head room. Anyone done this? or shed any light on it?

ecc83

Since the 5751 has almost identical characteristics to a 12AX7/ECC83 I would not get your hopes up for any "night and day" tonal changes!

Dave.

Dodge

"Not much top end" isn't how I'd describe an AC30TB unless you're using the normal channel.

What's it like without the Ox box?

ICBM

I really dislike 5751s. Every one I've tried has had a sort of overly-smooth sound with less gain and top-end than a proper ECC83/12AX7. They're like a jazz valve when you want rock'n'roll .

Personally I would fit standard Shuguang 12AX7s - you only need three, V4, V5 & V6,  unless you use the Vib/Trem channel alone (the Vib/Trem also uses an ECC82 in V3, but you don't need to change that) and JJ EL84s.

razorblade_beat

Yeah was gonna go with the jj’s for the el84’s, so I only use the brilliant channel so which preamps do I need to change @ICBM ;

@dodge exactly that’s why I think a new set of tubes is needed as I only use the brilliant inputs. I can’t turn it up much at all in the house because of how loud it is though it doesn’t sound as clean as it did last time I played it out ( just over a year ago)Was distorting much earlier than normal and not necessarily in a way I like. 

ICBM

razorblade_beat said:

Yeah was gonna go with the jj’s for the el84’s, so I only use the brilliant channel so which preamps do I need to change @ICBM

V4, V5 and V6.

It's worth changing them one at a time to see whether any are more worn-sounding than the others. Also, if you don't use Shuguangs for all three then you still should use one for V5 as it's a cathode-follower and other modern types tend to fail there.

razorblade_beat

Great cheers! Anyone tried cryo'd tubes or is it snake oil?

ICBM

razorblade_beat said:

Great cheers! Anyone tried cryo'd tubes or is it snake oil?

razorblade_beat

@ICBM lol fair dos. How do know which preamp is which v1, v2 etc? 

ICBM

Conventional numbering is that V1 is closest to the input jacks, V6 closest to the power switches.

V1 is the vib/trem channel gain stages
V2 is the vib/trem oscillator
V3 (ECC82) is the vib/trem mixer
V4 is the gain stage for the normal and first gain stage for the brilliant channels
V5 is the second gain stage and tone stack cathode follower for the brilliant channel
V6 is the phase inverter

Some schematics have the numbering reversed so V1 is the rectifier, V2-V5 are the power valves and V11 is the oscillator, or even a more random order!

In any case, if you don’t use the vib/trem channel you only need to worry about the three closest to the power switches.

Bork

I've run 5751 in some of my Burman kit to try and increase headroom (through reduced gain) but I've ended not feeling the results.  Nothing wrong as such but...hmmm, just meh.  Uninspiring.  Lacking in character or interest.  

razorblade_beat

ICBM said:

Conventional numbering is that V1 is closest to the input jacks, V6 closest to the power switches.

V1 is the vib/trem channel gain stages
V2 is the vib/trem oscillator
V3 (ECC82) is the vib/trem mixer
V4 is the gain stage for the normal and first gain stage for the brilliant channels
V5 is the second gain stage and tone stack cathode follower for the brilliant channel
V6 is the phase inverter

Some schematics have the numbering reversed so V1 is the rectifier, V2-V5 are the power valves and V11 is the oscillator, or even a more random order!

In any case, if you don’t use the vib/trem channel you only need to worry about the three closest to the power switches.

Class thanks very much for the info! will try replacing 4,5,6

Bork said:

I've run 5751 in some of my Burman kit to try and increase headroom (through reduced gain) but I've ended not feeling the results.  Nothing wrong as such but...hmmm, just meh.  Uninspiring.  Lacking in character or interest.  

Hmmm, interesting, Ive heard an ac30 6tb with the 5751 in the preamp v1 and it sounded great. Maybe ill just stick with the 12ax7 though. Didn't want to go down the rabbit hole of valves which has opened up lol!

Bork

I'd suggest assembling a small collection of samples for amps and seeing what works.  A bit like strings on a guitar.  And if you find something that works then you can commit to a permanent purchase.