STRANGE PROBLEM WITH MY '79 JMP 2204

[demonufo]

They all vary a little.

What condition are the filter caps in (when were they last changed)?

They are a pretty bright sounding amp by their very nature though.

[hotguit]

original filter caps by Daly.
thunk they are still good anyway; no hum or noise
yep i know that some of these little bastards are brighter than others

maybe a tube replacemnt could help
Any suggestions for preamp tubes' brand?
hot

[demonufo]

For current production pre's I only really like the Tung-Sols, although JJ's might help darken it a little.

[hotguit]

agree.
thank you

hot

[neikeel]

i know, i know
i found that the sweet spot for me is gain about at 8 so the bright cap should not work to much.
The sound is rich in mids and high mid but very poor in mid bass and bass and that is strange (comparing to other 2204 - same era - i've heard live and on you tube)
The tone has a heavy mid - high character and sounds almost metallic and not rounded (considering the eq: bass 3, mid 7 high 4, pres 4)
The previous owner added a res in series to the bias res (think a 56K); without that res it is not possible biasing the tubes (the trim - 22K - can not set up the tubes' bias)
Why?
Can it be part of the problem?
I can not solve this issue
hot

Sounds like you need a set of NOS mullard EL34s

[demonufo]

Hmm, what are your power tubes currently biased to, and at what plate voltage?

And what are they?

[hotguit]

power tubes are SVETLANA C LOGO
BIAS is about 40 or 41 mA (if i remember right)
Plate Voltage (on pin 3) is about 422V
hot

[hotguit]

i'd like my amp had this tone:
<iframe width="420" height="315" src="http://www.youtube.com/embed/cayc_UdFklc" frameborder="0" allowfullscreen></iframe>
bass and mid bass are here present and powerful; mine has not that tone...
hot

[hotguit]

ok, shielded wire istalled and oscillatione problem fixed.
swapped V1 tube with another one (E.I. 12ax7) and V2 with a Delta lab "chinese" 12ax7 and now the tone in as it should be.
Rich and full of bass and mid bass.
Great AcDc tone from this amp now
hot

[HTH]

You're welcome

[hotguit]

I also put a 33K res in parallel with 10K V1B catothode; the gain is a very littlle higher than stock without altering tone at all
beautifull
hot

[HTH]

I also put a 33K res in parallel with 10K V1B catothode; the gain is a very littlle higher than stock without altering tone at all
beautifull
hot

Thats around 7.5k btw for anyone wondering.

I like 4k7 rather than the 10k and also a 0.68uF cap bypassing the cathode resistor.

[hotguit]

yes. around 7,5 K
Don't you think that with 4,7 K there are some harsh overtones added?
4,7 K + 0,68 u is not too much gain? (for acdc or classic rock tones i mean)
hot

[demonufo]

Personally I think 4.7K is waaaaay too far. I like 8.2K myself. But have been known to use a 6.8K and just a smidge less on the preamp gain pot.

[hotguit]

Personally I think 4.7K is waaaaay too far. I like 8.2K myself. But have been known to use a 6.8K and just a smidge less on the preamp gain pot.

doesn't it affect the tone a bit? (because of the bright cap on gain pot which should work a bit more at gain pot's lower settings)
hot