Fender Super Twin.

[Simon Barden]

While waiting for Windows to finish updating, I thought I would see what valves were installed in the chassis, here's what I found:


Three 12AX7 valves, one 7025 (basically a lower-noise version of the 12AX7), one 12AU7A, and six 6L6 power valves!!

OK, bad news. Looking on the web for a schematic of the amp, one thing is certain - it shouldn't have three 12AX7s in it! http://acruhl.freeshell.org/mga/sche.../supertwin.jpg

The valves should be:

V1 = 7052
V2 = 6CX8
V3 = 6C10
V4 = 12AX7A
V5 = 12AT7

The 6C10 is a triple triode valve, so certainly can't be replaced with a 12AX7 unless the amp has been seriously modified. Maybe by omitting the reverb unit they've been able to do this, but the amp certainly isn't standard, and you don't know how well any modifications have been made.

The 6CX8 and the 6C10 aren't made any more, so you have to rely on a dwindling supply of NOS valves (which aren't cheap if you can get them), so I can see why someone made some changes. However, it isn't a proper Super Twin amp any more, so is worth what someone is willing to pay for what it is now. The big question is, were the changes made by someone who knew what they were doing and all the supporting resistors for the valves were changed to the right values so they work properly? You can't simply change valve types and expect them to do the same job without changing the supporting circuitry. Even the 12A_7 range (12AX7, 12AT7 etc.) aren't 100% compatible and the bias resistors for them need to be tweaked to get the best from them.

In the light of this, I wouldn't advise even powering it up, but it needs to go straight to an amp specialist who should be able to tell if it's safe to use and whether the modifications have been done correctly. If not, it could be lethal.

Also note that it needs a 4 ohm load (no alternate tappings are available), so a standard 4x12" cab will need to be modified so that all four 16 ohm speakers are wired in parallel.

[DrNomis_44]

OK, bad news. Looking on the web for a schematic of the amp, one thing is certain - it shouldn't have three 12AX7s in it! http://acruhl.freeshell.org/mga/sche.../supertwin.jpg

The valves should be:

V1 = 7052
V2 = 6CX8
V3 = 6C10
V4 = 12AX7A
V5 = 12AT7

The 6C10 is a triple triode valve, so certainly can't be replaced with a 12AX7 unless the amp has been seriously modified. Maybe by omitting the reverb unit they've been able to do this, but the amp certainly isn't standard, and you don't know how well any modifications have been made.

The 6CX8 and the 6C10 aren't made any more, so you have to rely on a dwindling supply of NOS valves (which aren't cheap if you can get them), so I can see why someone made some changes. However, it isn't a proper Super Twin amp any more, so is worth what someone is willing to pay for what it is now. The big question is, were the changes made by someone who knew what they were doing and all the supporting resistors for the valves were changed to the right values so they work properly? You can't simply change valve types and expect them to do the same job without changing the supporting circuitry. Even the 12A_7 range (12AX7, 12AT7 etc.) aren't 100% compatible and the bias resistors for them need to be tweaked to get the best from them.

In the light of this, I wouldn't advise even powering it up, but it needs to go straight to an amp specialist who should be able to tell if it's safe to use and whether the modifications have been done correctly. If not, it could be lethal.

Also note that it needs a 4 ohm load (no alternate tappings are available), so a standard 4x12" cab will need to be modified so that all four 16 ohm speakers are wired in parallel.

The wood cabinet that the amp chassis is in needs replacing with a new one because I noticed that the corner-joints are starting to come apart, good call about the 6CX8 and 6C10 valves, I didn't notice that, I'm going to wait till I've paid the guy the money I owe him for the amp before I go and do anything with it, I'm familiar enough with the safety procedures for working on valve amplifiers so I'll see if I can do the work myself, I'll certainly give it a good check out electrically to make sure it's electrically safe beforehand, that's for sure, I don't think I will sell the amp once it's all sorted out, I think I will keep it.

One thing I have noticed is that the standby switch definitely needs replacing because it appears to be stuck in the on position, the power switch seems good though, but I might just replace both as a matter of course, I think I can get a new knob for it from Evatco.


On the back panel of the amp chassis, near the speaker output socket, it says that the minimum load impedance is 4 Ohms.