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I don't know if my amp is too hissy but if i turn everything up on tmb side
it's very hissy, not so on normal channel.
What is the norm,where are what dials and amount of hiss.
With guitar and without etc.
Also if i turn the gain to 10 and volume i get squeal or is that just feedback because everything is maxed out.it goes away at about 8 on the gain.
If i change to metal resisters does it affect the tone.
Thanks.

The TMB channel has quite a lot more gain than the Normal channel. You could replace carbon composition resistors with carbon film resistors, which were what Marshall mostly used in the original 18W. Carbon films are less noisy than carbon comps. It also makes sense to use metal film resistors for values above 100k, which will also produce more noise. The tone of the amp will still be really nice, as well as clearer. There is an argument for using carbon comp resistors just for the PI's 100k load resistors - http://www.geofex.com/Article_Folders/c ... oncomp.htm There is very little gain at this point so noise won't be significant. Possibly, the effect will be more pronounced if you only use one CC resistor there, so that the nonlinearity doesn't get cancelled out in the push-pull power amp.

Sometimes hiss in the TMB channel can also be parasitic oscillation at the upper edge of the hearing range. It may be a good idea to try moving various wires around with chopsticks while the amp is running to see if the hiss or squeal can be made to go away or even made worse.

Measures to look at include, running shielded wire to/from the volume and gain pots, twisting any long lengths of output transformer wires together (primary and secondary separate), and also twisting together the pair of wires that go from the PI stage to the EL84s. You could also try a 47pF 450V cap across the anodes of the PI valve.

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