How much bacground hum or noise is acceptable in a JCM 800 2205

[jalexquijano]

Does anyone know how much hum or hiss is aceptable on a JCM 800 2205? Will upgrading to Mercury Magnetics transformers reduce the hum and hiss or interference?

[Mick Bailey]

It depends what you're using it for. Someone playing in a live situation may tolerate more noise than a bedroom player. Swapping out components without analysing the noise source is a bad idea and you can spend money just to arrive back where you are right now.

If you think the amp is too noisy, where is the main problem; is it hiss or hum? Also, what speakers are you using? It seems an odd question, but speaker sensitivity can play a big part of perceived noise issues. Sensitive speakers can easily double your noise level.

When you turn all the controls to zero, does the noise disappear?

[jalexquijano]

i have it plugged to a Marshall 1960AX 16 ohms cab which carries 4 X 12 Greenback Celestion rated 25 Watts each. when I turn all the controls off the noise dissapears.

[Tom Phillips]

See http://music-electronics-forum.com/t42448/#post427669

[Tone Meister]

Read Mick's post #2 and ponder his questions.

The 2205 is a two-channel amp and its Boost channel has a cascading gain which will almost always have some background hiss at moderate gain levels and above, even if you install the finest preamp tubes money can buy. A certain amount of background hiss is to be expected with that type of high gain circuit. Poorly functioning preamp tubes (especially V1, V2, V3) are a likely culprit for other types of excessive noise and/or picking up environmental RF signals from who knows where. As Mick asked, is it hiss or hum you're hearing, or both?

[Steve A.]

Posting a sound sample here would help. A spiral filament like with the Sovtek 12AX7LPS should reduce AC noise in the first preamp tube as I recall, not as well as a DC filament supply but then again it requires no modification of your amp.

Its been many years since I've used the 12AX7LPS for the initial preamp tube (I prefer old stock 5751's) so you might want to wait for someone to second my nomination. Yes, I am getting ready for the Presidential elections!

Steve Ahola

P.S. I'm very familiar with the 2203/2204 "MVP" schematic, a real classic copied by many, but never looked into the 2205/2210 preamp before, What a strange beast... definitely not your grandfather's Oldsmobile!

[pdf64]

...I'm very familiar with the 2203/2204 "MVP" schematic, a real classic copied by many, but never looked into the 2205/2210 preamp before, What a strange beast... definitely not your grandfather's Oldsmobile!

Yes, it's full of 'I wouldn't do it like that' circuit features, eg the fx send with rather high impedance, the W005 bridge rectifier (which presumably stops overdrive getting too assymmetrical).
Can't even blame hiss on the grid stopper or bright cap.
Whatever, it clearly delivers the goods, which is the criteria that matters.