Tubes for Sound City 120

[telenorman]

Hi, not too long ago I had some work done to my SC120. (recap, couple of sockets few resistors etc). Last week I bought the 5 preamp tubes. 4 ECC83's and 1 ECC81. Birthday coming up soon so will be looking for 6 EL34's.
Should I buy 3 matched pairs, matched quartet and matched pair or can I just go mad and put different types where I want?  The SC120 comes with the infamous MK4 preamp so are some tubes more suited to it than others. I'm not a pro and I doubt if I'll ever turn this thing up to '11' more like '5 or 6'. Any suggestions anyone?       Merci

[bobmegantz]

you either need to use 6 matched tubes (you can order them as a set from some vendors) or you should install cathode current sensing resistors on each tube and measure the bias current to make sure they're all drawing similar currents.  Actually, you should have the resistors anyway...

[RicharD]

Having been inside of a SC120, the last thang you wanna do is add parts.  Them thangs is a bit crazy on the inside.  Other than a bias mod, I would try to keep it as original as possible (excluding the usual cap job etc).  The one I worked on had been modded to hell and back and was a ground loop nightmare.  There's not a whole lot of them thangs floating around that still work "properly".

Order a matched set of 6.  Bias it a little cold and rock on.  It's a geetar amp, not a hifi.  Don't worry about a little bias imbalance. 

[DummyLoad]

those things are a nightmare + they hummmm worse than a room full of buddhists. the one buttery and i worked on had a bad socket and a couple of screen resistors that seemed to drift wildly with temperature causing a pair of the tubes to red plate. in an effort to "fix" the hum problem a tech prior installed a piece of paper/copper laminate shield over the tone stack area. we removed the home-brew shield and the hum problem actually got better. we ended up installing a sextet of svetlana EL34s that were mix-n-mach from octets that survived a pair of conrad-johnson premier 5 hi-fi amps i used to own. we tweaked the divider ckt on the bias ckt so we could bias colder and matched the "survivors" best as we could and sent it on it's way. it's been hammered on for over 3 years now by a punk rocker. it had 4 well matched 6CA7 sylvania fat-bottle that survived the carnage. the owner flipped the 6CA7s on fleabay and made enough to recover our repair bill with enough left over for beer money. in retrospect, perhaps we should have charged him more...  ;)

[TubeGeek]

I'd go with a sextet of winged c el34's

[terminalgs]

Tube vendors sell sextets (not on the online catalogs, so just call em). I had
A sc120L, it came with a mix of EL34s, I put JJs in it.   Get a nice tube for V1,
Good advice for any amp, but especially for the SC120, otherwise there
Isn't much character (just volume!).

What ever tubes you put in it will last forever, the SC120 only runs 345v
At the plates.

[telenorman]

well, finally, ordered a sextet of RT166 STR TAD DR fromwww.tubeampdoctor.com Would have loved to have bought some NOS Winged C's but don't deserve them yet!  just one question; I can just plug these things in and go? No need to bias with the SC 120?     

[telenorman]

2nd thing.not sure if my transformers are Partridge, how can I tell?      Merci               thanks

[rafe]

2nd thing.not sure if my transformers are Partridge, how can I tell?      Merci               thanks

Check for a pear tree?   sorry I tried to resist......

[overtone]

You can usually tell if they are partridges when you need two people two move the amp!

The transformers seldom have the Partridge label on them. Typical Partridge transformers of that time, fitted in guitar amps, had a distinctive frame mounting allowing a choice of mounting positions. But that was used by many UK transformer builders then anyway! Below are a few I have to give you an idea.

If the amp has Partridges fitted, take a real good look at the state of the fixing bolts and chassis. Due to the weight, the fixings usually need attention, tightening down or proper replacement.