no presence on Sound City 120

[telenorman]

Hi, last year I had my 120 re-capped and changed all the tubes just after. the tech did an excellent job and the amp is working well except for the loss of presence.  could this be because the ecc81 is in the wrong socket? ( there are 4 ecc83 and 1 ecc81)   if not........what could be the problem???

[PRR]

> could this be because the ecc81 is in the wrong socket?

No, but why don't you put the tubes right?

[telenorman]

normally they are in the right place
originally the ecc 81 wasn't in this socket, it was the tech that told me it should be here. as I said I have every confidence in him........... but was just checking, I've only ever had the best advice from here.
I'm trying to put some more photo's on...........need to change the format

[telenorman]

2

[telenorman]

well.......that ecc81 was originally in the v1 position. I'll swap them around and see sometime this weekend. would it be a good idea to just use ecc83's?
thanks

[telenorman]

Haven't flicked the switch yet..... 81 is in v1 position and will be trying tomorrow. The six el34's are matched, I have four ecc83's the same, one electro-harmonix 12AX7, one marshall  ecc83 out of a marhall 8080 and two ecc81's. Can I just keep swapping these aound until I find 'my sound' without damaging the amp or indeed damaging myself?   

[telenorman]

actually.....;swapping an el34 for an ecc.....didn' think of that!    won't be fiddlin about with them big boys just the preamp tubes.  I believe the SC120 has a bias capacitor. Does this mean that them el34's just get on with that biasing stuff all on their own?

[gldtp99]

I believe the SC120 has a bias capacitor. Does this mean that them el34's just get on with that biasing stuff all on their own?

You're making a joke, right ? If not --- in the pics below, the bias cap is the black new looking capacitor between the two power amp turret boards---- at the neg end of the bias cap is one bias trim pot (this sets the bias voltage for one bank of three EL34's)---- look to the right of the first bias pot, next to the yellow twin shielded wires leading to back panel jacks{Reverb In/Out}-- and you will find the other bias trim pot that controls the bias voltage level for the other bank of three EL34's:



Sorry, i don't know why the Presence on your SC 120 is inop---- but i'm sure it has nothing to do with whether an ECC83 or ECC81 tube is in whatever slot---- the ECC81 should be in the PI position but having an ECC83 there shouldn't be a critical issue.
The pic is a SC120 that i used to own--- before it was converted to Hiwatt specs--- here's the "After" pic:

[telenorman]

no wasn't joking but I do like making people smile (I'm a dental tech) . ok found my pots. thanks for that!  
seems I need a special tool if I want to bias this thing myself right?  is there a way of checking that it needs to be biased?

[gldtp99]

seems I need a special tool if I want to bias this thing myself right?  is there a way of checking that it needs to be biased?

Here's some good info about biasing amps: http://aga.rru.com/TechTips/disp.cgi?file=LV/BiasAdj.tt

Any time a set or any individual output tube is changed in a Class AB push/pull amp (such as your SC 120)--- the output tube bias needs to be checked and adjusted, if necessary.
It's also good practice to check output tubes bias every so often because it's not unusual for output tubes to "drift" as they wear---- sometimes re-adjustment is needed................gldtp99

[telenorman]

merci beaucoup!   Will check it out; Thanks again
by the way.............excellent job with that conversion