Leaky capacitors in Guitar amps

[jsel84]

I have some questions on leaky caps in the preamp stages in guitar amplifiers.

I've noticed that the amp sounds OK yet the caps in the preamp stages are leaky. The ones I am asking about are the ones that go from the preamp tubes to ground all the supply caps and the coupling caps to the Phase inverter I changed the amp other wise would not preform.
What is going on here when they are leaky going to ground.

I haven't done a playing test yet but would like to know what happens to the amp with these leaky caps to the preamp tubes etc... I've read so many different articals on this, I'm a bit nitty at this point.

Thanks for any tips and info on this. You guys have always helped in the past with my radios.

Regards,
jim

[Ed_Dinning]

Hi Jim, from your description it sounds like you are refering to decoupling caps. These normally go from a valve electrode to earth where that electrode is fed by a resistor from the HT line, effectively giving that electrode a lower voltage. They are there to reduce the signal impedance at that point. A leaky cap at this point will reduce the voltage on the electrode. BUT if the leak of the cap is, say, 1 meg and the feed resistor is 33K, it will have little effect
(generally if the cap leak is 10x greater than the feed resistor it will have no effect). They will however ultimately fail and should eventually be replaced.

HTH Ed

[jsel84]

Hi Ed
Not sure if I understand you. Yes they are the coupling caps at .047uf.
There are two resistors from the B+ that are 1 meg, each going to 2 of the preamp tubes each having a .047uf cap on the same lug, caps going to ground. The tubes are 6SJ7.
Regards
jim

[AndiiT]

Hi,
Are the capacitors in question electrolytic ones by any chance?
I believe that electrolytics are inherently leaky due to their design. That said looking at the values given I think it is unlikely that the caps will be electrolytics and as mentioned above should be changed if they are leaky to prevent future problems.

Andrew

[Ed_Dinning]

Hi Jim, these caps, if leaky will then form a potential divider and "rob" the electrode of some of its voltage. As the feed resistors are 1M, then the cap "resistance" needs to be greater than 10M or it will have an effect on the electrode voltage. It could well be that with these valves and at the signal levels used it has little effect on the signal, but they need to be replaced.

Ed

[jsel84]

Hi Folks
There are 5 caps in question here the .047 cap are at 400 volts and are not
electrolytic type
There are two that are at 20uf at 25 volt these have a 1K resisitor accross them. I do know when I changed the 1K resisitors the amp got much quiter. The orginal resisitors were very noisy. Is there any way to meausure at the caps to see if they are doing harm to the amp, or robbing any sound of the amps preformance
thanks for the help so far.
jim

[Ed_Dinning]

Hi Jim, these electrolytic caps have probably dried out and reduced in value. they will be worth changing, especially if you have changed the associated resistors. As they are in the feedback path you should notice a change in "sound" of the amp.

Ed

[jsel84]

Hi Ed
If I understand you correct you are saying I should change all five capacitors
the two 20uf @25 volts and the 3 .047 caps at 400volts.
Are these all known as electrolytic caps?
Thanks for the guide here.
When playing the amp tonight I did notice a strange hum when I rolled of all the tone It sounds like the .01 cap is leaky. I can here the tone control scrachy as though voltage is leaking back into the control.
I had the same sound on the volume control I changed that cap (.022) and now its clean when you turn ther volume up or down.
I will say amp needed lots of work, resistors caps, and the proper tubes.
The other owner had a GZ34 in place of the 5V4 and a 6V6 in the place of the 6SN7 tube, he did say the didn't work.
Thanks for advise and any comments you may have.
jim

[Sean Williams]

A gZ34 will be fine as a repacement for the 5V4, the 6v6 however will never work....

Probably would pay to re cap the whole amplifier if you plan to use it in anger....

Cheers
Sean

[Ed_Dinning]

Hi Jim, scratchy tone control is usually a sign of a leaky cap as it no longer blocks all the DC from the pot (I've even seen one go on fire when turned to the low resistance end with a faulty (very) cap.
The 20uF's are electrolytic caps, make sure you fit the correct way round.
The other caps are non-polarised and fitting direction does not usually matter.
Resistors will probably be oK unless looking obviously damaged. As long a sthey are within 10% they will be fine. High value parts (over 100K) are the most likely suspects for value change, as are high power resistors.

Ed

[jsel84]

Hi Guys
Yes I have had to change a few resisitors the two at the phase inverter
were way out as were the caps. I really noticed a big difference in noise when I replaced the 1K res. at the preamp stage with the 20uf caps.
I also had a problem with the tremolo NOT going off a 51K resistor went to 500K.WOW! After that the trem worked correct.

I'll take your advice on the other caps to be replaced. There is the question of the type of cap to use. I'm going to try and get the paper oil filed ones not NOS but new ones I've read so many articals on this subject. I would like to say with the same type that were used orginally.

Regards
jim