Soundcity 120R question

[Clydesdale]

Can someone please tell me what the PI tube in this amp. The amps got all 12ax7 tubes throughout. I hear I should have a 12at7 in the PI socket. Some dude on DFFD mentioned the PI socket was on the right when looking at the amp face on, this doesn't look quite right to me. My amp breaks up a little early for my liking, so I'm thinking this valve may increase my head room a little?

Any help is always appreciated!

Cheers

[Hemisaurus]

Officiallt its an ECC81 you need, I forget what that is in American



Yep 12AT7

[Clydesdale]

Thanks for this.. I slipped the ECC81 back where it belongs, and it cleaned up a bit more. The clean tone really was quite loud before break up.
With my freshly modded Meathead to Meadhead dark, the amp was spitting out quite an evil tone.

[RAGER]

wouldn't a 12AU7 in V1 clean it up quite a bit too?

[dunwichamps]

12au7 would clean it up, but placement is also important also

[Clydesdale]

12au7 would clean it up, but placement is also important also

Placement as in where?

wouldn't a 12AU7 in V1 clean it up quite a bit too?

V1? Im pretty new to all this.

This amp has the active EQ. Off the top of my slightly beer altered state, it has about six pre amp tubes, one for each tone control and one for the reverb.

[dunwichamps]

the 120s are active because the feedback loop encompasses the tone stack, interesting design it even has a DC coupled LTPI where as almost every amp ever has had an AC coupled LTPI. Kinda nice really. Its an interesting design, which is not too far off from a Plexi but it has a more complicated interstage filtering setup.

V1 is the first tube in the signal chain. 12au7 is the tube with the smallest mu, gain factor of 17 compared to a 12ax7 mu of 100. larger mu = more amplification. These tubes have other differences 2 tho (impedance, grid current...)

Phase inverter not only amplifies it provide 2 out of phase signals to drive the push pull amplifier. Higher mu in the PI will result in more focus on preamp clipping while smaller mu will provide more focus on power tube clipping because the PI will push the output tubes harder before it breaks up

[Clydesdale]

Ok, but does the 12au7 result in an overall volume drop in the amp? I dont need to loose any of the volume.

The active EQ is a bit hard to get a grip on at first, but I do really find it rather versatile. 

[dunwichamps]

if you going to front load the amp then no, you wouldnt see much difference in volume. 12au7 is going to extend the clean sweet spot of the amp because it has much less gain. You can get cheap ones if you wanna just experiment, Weber has cheap chinese ones for just a few bucks worth the experimentation

[Clydesdale]

Cool mate, I'll get one and see how it sounds. I've got an old local valve guy, he's got a shed full of valves. I'll hit him up.

Im not going to damage the amp?

[dunwichamps]

Nope

[Mr. Foxen]

If it still has all the original resistors in it, they are kind of age sensitive, so replacing them might give you more volume and less noise. I replaced pretty much all of them in mine, there was one totally screwed one that made the whole thing oscillate.