Caps Questions?

[IIIBOOMERIII]

Caps Questions?

*Does the preamp V1 pin 1 method of draining caps always work?

*Can you replace caps with a higher voltage rating without any
problems? Ex. Replace 350V with a 400V cap with the same uF
rating.

*How far off can you be with the uF rating without causing problems?

*Is a cap a cap, are there caps that sound better than other caps?
What caps should we stay away from when working on guitar amps?

I do apologize if these are elementary questions. I just recently
started getting heavily into working on amps. I still have a lot to
learn, as you can tell.

Thanks in advance

Eddie

[John Phillips]

Does the preamp V1 pin 1 method of draining caps always work?Yes, but it can take a long time.

Can you replace caps with a higher voltage rating without any problems? Ex. Replace 350V with a 400V cap with the same uF rating.Yes.

How far off can you be with the uF rating without causing problems?It's not usually a good idea to go smaller - amp designers mostly used the smallest they could anyway, for cost reasons, but the worst that will happen is excessive hum. It's not a good idea to increase them too far, especially in a tube-rectified amp, since it can overload the tube. Generally a small increase is safe (eg the common old 32uF value to 50uF), but it may also change the tone slightly too.

Is a cap a cap, are there caps that sound better than other caps?For electrolytic filter caps (what I assume you're asking about here) - IMO no, or at least probably not. I have heard some anecdotal evidence that the physical size of the caps could make a difference (and a reason why it could be so), but I'm not completely sure.

Modern caps are usually so much smaller than old ones that getting a larger value into the same physical space is not a problem - in fact, it's more often the opposite, that getting a modern cap to physically fit an original chassis clamp can sometimes be tricky, because the cap is too small.

What caps should we stay away from when working on guitar amps?'NOS' ones more than a few years old. You'll often see these being sold as 'genuine vintage parts for vintage amps', with the implication that you'll preserve your original tone by using them. In fact, if they've been unused for more than ten years they're probably scrap before you even install them, and will certainly have a short working life even if they don't blow straight away.


This only applies to electrolytic caps BTW - main filters, bias filters and cathode-bypass caps. These are the only ones that need changing routinely. Non-electrolytic signal caps do not need to be (and IMO should not normally be) changed unless they actually fail, usually by leaking DC. They most certainly do affect the tone IMO. When restoring vintage amps I deliberately seek out and use old ones of these (tested, of course) in order to keep them as close to original as possible, if the amp warrants it.

You also need to check a few other components which may need replacing due to age - high-power carbon-comp resistors (eg screen grids) and old diodes, especially Selenium ones (usually found in bias circuits in older amps).

[IIIBOOMERIII]

John, thank you so much for taking the time to answer my post with such great detail and insite. I must say I find all of your post very informative. Again thank you for taking the time.

Eddie

[jjasilli]

NO!!! draining the caps at a tube pin does not ALWAYS work (at least not well enough). Sometimes filter caps upstream from the cap feeding that tube remain charged. Even 30 or 40 volts might kill or permanently injure you (depending on the particular circumstances). So after draining the caps at one location, the safest thing to do is to clip your voltmeter to ground (black lead), and with ONE HAND ONLY in the amp, use the red lead to test the voltage on ea cap. Any cap that shows 10 volts or more should be drained individually. Then test all the caps again. (Sometimes they manage to recharge themselves during the process).

It is always safe to go to higher voltage ratings. As to tone, for caps in the signal path it is better to stay near the lowest safe voltage rating. Like using 400 volt caps instead of 600 volt caps. This is because caps have a number of effects on current - which are undesirable side effects on audio signal. The larger the cap, in terms of voltage capacity, the more pronounced these undesirable side effects will be. (This may be more theoretical than noticeable.)

Changing uF values may or may not make a difference. Unless you already understand design parameters, or are following the recipe for a tried-and-true-mod, then you should probably stick with stock values. If you want to experiment, then change only one component (or one set of related components) at a time. If you do more than one such mod at a time, it cold be difficult to locate a problem.

[John Phillips]

NO!!! draining the caps at a tube pin does not ALWAYS work (at least not well enough). Sometimes filter caps upstream from the cap feeding that tube remain charged.Not true. If you connect pin 1 (of a 12A*7 preamp tube socket, which is what this method is intended for) to ground, there is a permanent resistance path from all the caps via the B+ chain resistors and the tube plate resistor to ground, which will drain the caps right down to zero eventually. It will also provide enough of a drain to keep 'memory charging' down. Measure the voltage on the caps while doing this if you think it isn't so.

The only thing to remember is to keep the standby switch closed or the first stage caps won't discharge if they are upstream of the switch.

The reason I don't like to do it this way is because it's too slow. There are other far faster ways which are just as effective (which one is best depends on the exact power supply configuration).

Even 30 or 40 volts might kill or permanently injure you (depending on the particular circumstances).Extremely unlikely unless you're soaking wet. Yes, I once had a painful shock from a 12V car battery when I was soaked, but I doubt it was life-threatening, and if you work on a guitar amp while you're wet, you're an idiot anyway. 50V is not a serious hazard in normal dry conditions, that's why it's the rating on low-voltage connectors.

It is always safe to go to higher voltage ratings. As to tone, for caps in the signal path it is better to stay near the lowest safe voltage rating. Like using 400 volt caps instead of 600 volt caps. This is because caps have a number of effects on current - which are undesirable side effects on audio signal. The larger the cap, in terms of voltage capacity, the more pronounced these undesirable side effects will be. (This may be more theoretical than noticeable.)I agree, but for the exact opposite reason. A cap which is operated close to its voltage rating will give more 'side effects' (ie harmonic distortion) on the signal, because voltage-dependent capacitance variations will be more pronounced. This is a bad thing for 'pure' signal path applications (eg hi-fi) but good for guitar amps where you want harmonic distortion (usually!).

Changing uF values may or may not make a difference. Unless you already understand design parameters, or are following the recipe for a tried-and-true-mod, then you should probably stick with stock values. If you want to experiment, then change only one component (or one set of related components) at a time. If you do more than one such mod at a time, it cold be difficult to locate a problem.Agreed, but I've never heard one case where increasing the value slightly - usually just going up to the next standard value when replacing old caps where the original value isn't available nowadays - made an amp sound worse. OTOH, I'm not a fan of underfiltering and ghosting anyway. (Or limited bass response.)