#1
Hi all,
So I have recently acquired a long awaited silverface twin (master vol, push pull) but it appears to be on the quiet side.
Twins as I know and love them are deafeningly loud and rarely make it past 2 or 3 on the volume knob but I had this thing with channel volume on 8 and master on 10 in a small rehearsal room to get the volume I was used to out of my red knob Evil Twin.

The tone is lovely but I am concerned something is amiss.

I don't know if it is simply a case that the biting point for the volume is different on a SF or if something needs correcting.

The only thing I can see out of the ordinary is that one of the power amp tubes is black and singed whereas the others all look fine.

Does anyone have any experience with this issue or do I just need to adjust my expectations for this new amp? I doubt I will ever need the full amount of volume this thing could provide, but I'd like to know it's running happy in the meantime.

All the best!

Sam
#2
Sounds like a tube issue. It could be poweramp or preamp.

My first step would be to take a known good preamp tube (maybe one from your Evil Twin) and try it out in each of the positions. If you put it into a position (say V1) and the volume comes back then you know that tube (the one that was in V1) is no good. Simply replace that tube and you're good to go.

Second, I'd be trying power tubes. Now these you want to change as a set, so you'll need a new set (quad 6L6s, if I'm not mistaken). Swap those into the power section, all at once, and see if that fixes the issue... If so then your power tubes were bad so you can leave the new ones in.

If neither of those fix the issue then it is something more difficult to diagnose, and will probably require a tech's expertise.

Good luck! I'll cross my fingers that it's just a preamp tube.
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Last edited by tubetime86 at Aug 29, 2014,
#3
Had the same issue with my old twin and all it took was a new tube. These amps are pretty solid so I highly doubt it's anything more than that.
#4
Thank you for the replies!
If it's just a tube issue would be a weight lifted! I shall try swapping out the ones from my red knob twin.

As a followup (rookie) question, is it fine to just swap the tubes out myself and power up or should this be done by someone qualified? Does biasing come into this at all or am I just worrying about words I don't understand?

Best,

Sam
#5
It depends on which one it is. Early ones had a normal bias control and had to be set as per normal. Later silverfaces had a bias balance control that allowed you to adjust it for tubes that weren't matched properly but no actual overall bias adjustment. You basically just tweaked them for least hum.
The good ones (AB763 circuit until May ’68) really need biasing, after that they went to the AB568 which just has the balance pot. Which is yours?

The preamp tubes are just plug and play.
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#6
I see, I'm not sure what year exactly but judging by the master volume and push pull pot, definitely a good way post 68. I don't know anything about how to bias an amp myself, control pot or not, so it sounds like I'd do better to take it into a tech rather than straight swap the power tubes myself.