#1
Hey GG&A,

About a year ago I made a really good deal on a 10/10 mint condition upgraded Fender Twin Reverb with a friend of my dad's. I've only had solid state amps before that so I'm truly illiterate when it comes to tube amps. So far, I haven't experienced any problems with it, but as I'm going to be gigging soon I thought I'd but some spare parts. So I started to google up some information (before this I knew nothing about tube amps whatsoever).

Holy crap. Some of the user tips and reviews describe tube amps as babies who haven't skulled over their soft spot yet. I started to worry even though I've always treated it with extreme care.

Afterwards, I looked into buying a set of new tubes (see tube layout below, I don't know if it's any difference but my power tubes have "6L6GC-STR SELECTED" written on them. I have no idea what this means, I just read it off the back) but holy crap they are expensive to replace. A new set of tubes would set me back a good €200? **** me, I can't really afford that right now as I'm a college student who's in between jobs.

So I started wondering (and you're probably going to start cringing right at this point): "Hey, I'm not even using the vibrato/reverb channel at the moment. Can't I just use those tubes as a spare? Can't I just disconnect them for later use? Or switch them if the other ones break down?

It's probably the same with the power tubes, I guess each channel is powered by 2 tubes, right? ... right?

Maybe I'm overlooking a fundamental electronics law here but I'm an absolute nobody when it comes to things like this.

I've googled the subject, but I'm not really sure what to look for. So if any of you could help me on this, that would be awesome. It's such a great amp it would be a shame for me to fiddle with it and hurt it.

More pictures will follow if requested.



Tube layout

AB763 Tube layout (Seen from behind, V1 is to the right side)

V1 12ax7 = Preamp normal channel
V2 12ax7 = Preamp vibrato channel
V3 12at7 = Reverb send
V4 12ax7 = 1/2 Reverb recovery and 1/2 gain stage for vibrato channel
V5 12ax7 = Vibrato
V6 12at7 = Phase inverter
V7 6L6 = Power tube #1
V8 6L6 = Power tube #2
V9 6L6 = Power tube #3
V10 6L6 = Power tube #4

(from guitarguru/twinreverb, more info and pictures there.)


Thanks in advance for any response. Yes, I'll even tolerate flaming (because I guess it's fully justified given my knowledge on the subject) but keep it decent
Wouter
Last edited by Lord Waltaa at Nov 6, 2014,
#2
Quote by Lord Waltaa
Afterwards, I looked into buying a set of new tubes (see tube layout below, I don't know if it's any difference but my power tubes have "6L6GC-STR SELECTED" written on them. I have no idea what this means, I just read it off the back) but holy crap they are expensive to replace. A new set of tubes would set me back a good €200? **** me, I can't really afford that right now as I'm a college student who's in between jobs.


it sounds like you're in Europe?

you can go to thomann and they have sets of 6L6GC's for about 100 euros. i generally prefer JJ's though, and i am unsure of where to order those in Europe. but 120 euros is about the most i'd expect to pay for a quad of 6L6GC's.

still pretty expensive.

tubes generally last a few years at least, it's nice to have an a few extra tubes (or an extra set) floating around just in case though.

there are also two basic types of tubes:

power tubes: the big ones, about 4 to 6 inches long with the big plastic octal bases. these are the 6L6GC's

preamp tubes: the small ones, about 2 inches long with the smaller pins and no plastic base. there are like 12ax7's or 12at7's.

preamp tubes aren't as expensive as power amp tubes. i have no idea if you knew any of that already.

Quote by Lord Waltaa
So I started wondering (and you're probably going to start cringing right at this point): "Hey, I'm not even using the vibrato/reverb channel at the moment. Can't I just use those tubes as a spare? Can't I just disconnect them for later use? Or switch them if the other ones break down?


some circuits you can pop out a tube, but i wouldn't recommend it.

basically all those tubes do different things in the circuit, sometime a single tube will be relegated to one function or channel and it can be removed without a fatal effect but i wouldn't do it unless i know exactly how it worked in the circuit.

in general it is not worth it.

Quote by Lord Waltaa
It's probably the same with the power tubes, I guess each channel is powered by 2 tubes, right? ... right?


no, the power amp uses all the tubes regardless of channel.

there is ways to run off two power amp tubes though, you'd have to figure out which tubes are pushing and pulling and then remove the proper tubes. you'd also have to change the amp's output impedance. people do this to make an amp 'quieter' because this cuts the amp's power in half.

imo this is also not really worth it. but if that interests you then make sure you are doing it right before you attempt it.
#3
A Twin is built like a Sherman tank and should be good for 5-10 years on a set of tubes. It's loud as hell so unlikely you will be pushing them very hard unless you are gigging Glastonbury every week. It's a lotta amp so enjoy.

As spares go you could get by with 2- matched, medium bias 6L6 and 2- 12AX7 just in case but I wouldn't worry about it. I have been gigging with Fender tube amps since 1969 and only had a tube fail once during a gig. If you play a lot, have your amp and tubes tested once a year by a qualified tech and only change tubes when needed. Brit amps eat tubes 10x faster than Fender amps. Always have a plan"B" in case your gear fails. This applies equally to tubes and SS.
Last edited by Cajundaddy at Nov 6, 2014,
#4
I play my 65 Twin almost everyday and haven't had to switch out the tubes for the last couple years. Got them checked a year ago, no need to replace.
#5
An easy to put this: if ain't broke, don't fix it.

Unless it starts making weird noises, loses output or feedbacks a lot, don't worry about replacing the tubes as they will be fine for many years.

If you're curious about the tubes, you can tap the tubes with a pencil to see if any of them are ringing through the amp. A tube that rings through the amp is bad, so you can only have to swap out a pre-amp tube rather than the entire section. Power amp tubes need to be replaced together, so if one rings, they all gotta go. V1 preamp tubes seem to always ring through the amp, so swap that one out into a different location to test it.
#6
The power tubes are V7 and V8 for one half and V9 and V10 for the other side. So if you are going to pull two tubes you have to take one from each of those two groups. So for the sake of simplicity, pull the inner pair or the outer pair.

However, and it's a big however, they don't have an impedance switch do they? They are designed to run into a 4 ohm load. If you pulled the tubes you'd have to switch the amp down to 2 ohms - which you can't do.
On the originals you could still do it anyway without switching anything because they were built like a tank. The reissues, I'm not sure about that. I'd guess that it would work fine but I can't offer any assurances that you won't fry the tubes if you try it.

If I were you, I'd buy a set of spare tubes as you can afford them. You have to buy the power tubes as a matched set but the preamp and phase inverter tubes can be bought separately.
Hunt around for good preamp tubes. V1 and V2 particularly, have a big impact on sound. Try to find some good NOS American, British or Western European tubes for those positions as you find them and can afford them. They'll cost more but they'll last longer and sound better.
This site is good for NOS tubes: http://tctubes.com/
Ebay can also be a good source of old tubes but be very careful. Look closely at the photos and make sure they are actually NEW Old Stock. If it looks like they have been used, leave them alone and keep looking. Ignore bs statements like "tested NOS." That means they are still working like a new one but it doesn't really give you much of an idea how much life is left in them.
In the other position, just buy JJ's.