Old Mallory Caps

[TerryD]

Will old cardboard type Mallory caps bulge on the end before they hit the crapper or are their other signs?  I'm torn between replacing them all and being done with it but then on the other hand the amp sounds pretty good.  Thanks, Terry

[tubenit]

EL34  Library of Information has instructions on how to check for "leaky caps".   I think a cap can be bad without a bulge.

With respect, Tubenit

[jjasilli]

Some thoughts:


*  If it ain't broke, don't fix it.
*  To the contrary: an ounce of prevention is worth a pound of cure.  A wise saying which may not apply to a valuable vintage item.
*  If it's under repair anyway:  especially for someone else's amp, for a host of reasons, you don't want the amp coming back after you worked on it.  [size=78%]For your own amp, it's a buckdancer's choice.[/size]

[TerryD]

I'm looking in the library for "check for leaky caps" and not finding anything.  Thanks, Terry

[Ed_Chambley]

If you have one on hand, why not.  If you have never had one explode you may consider keeping it.  Even in the amps I collect, I still replace them.  I keep them in-case the next buyer is insane and wants old caps in their amp.


However, I do not substitute values if I can help it.


Sorry, I forgot.  I do keep very old ones in old radios.  I often have to clean then out and it smells pretty bad and makes a huge mess.

[Tom_Hull]

CAPACITOR Installation Tips

• When restoring an antique radio it standard practice to replace certain of the radios capacitors. This is known as "recapping" a radio. An old radio may work with it's original caps….but for how long ?? ….and how safely ?? If the radio is going to be sold with a guarantee or is being given to someone as a gift, you should "recap" the radio.
• You will want to replace all the paper and electrolytic capacitors. However, "do not replace the mica capacitors". Mica capacitors very rarely go bad and if you replace them it will throw off the radios tuning. Replacing the mica capacitors will do more harm than good. Only replace a mica if you are sure it is bad (which is rare).
• Like mica capacitors, ceramic capacitors also very rarely go bad. Do not replace ceramic disc capacitors unless you are sure one has gone bad.
• Some radios use what are known as "line-filter" capacitors. These capacitors connect across your radios power line and/or go from your power line to ground. When replacing these capacitors, you should use special AC Rated Safety Capacitors. These special capacitors with improve the safety, performance and reliability of your radio. If you would like to learn more about these "safety capacitors", there is a link to the ABC's of Safety Capacitors near the bottom of this page.
  
• Get a schematic (and parts list) before you start your recap job. It is often impossible to read the values that are on the original capacitors. Also, if the radio was repaired at some time in the past, there is a good chance someone threw in the wrong size capacitors, just to get the radio working. Without a schematic you'll be guessing.
• Before replacing the capacitors, check all the radios' resistors. Since you will be replacing the capacitors, you should snip one lead of each paper and electrolytic capacitor. This will help prevent false resistance readings. All resistors that are off-spec should be replaced. When it come to tube electronics..."all resistors are not created equal", so when you replace a resistor be sure to use a resistor that has both a "voltage rating & wattage" that is equal to or greater than the original resistor. More information on resistors for tube electronics.
• Put heat shrink (spaghetti) tubing on the leads of the capacitors and resistors before you solder them into the circuit. This will help prevent dangerous shorts. If you need some heat shrink tubing, just "let us know" and we will be glad to add some to your capacitor order at no charge.
  
• Always check a capacitor before installing it. Although it is very rare, every once in a blue moon, a new capacitor will be defective or off spec. Taking ten seconds to check a capacitor can save you hours of troubleshooting…..only to find out you accidentally installed a brand new "bad" capacitor

[jjasilli]

I'm looking in the library for "check for leaky caps" and not finding anything.  Thanks, Terry

It's in th bias section

[stratele52]

Will old cardboard type Mallory caps bulge on the end before they hit the crapper or are their other signs?  I'm torn between replacing them all and being done with it but then on the other hand the amp sounds pretty good.  Thanks, Terry

If you want to gig with this amp , you are better to replace all filter  caps . If only for home use , you can leave like it


Bypass electrolytics caps need to be replace too . For best Fender tone.


 New buy  ? Check amp fuse .
 

[HotBluePlates]

Bypass electrolytics caps need to be replace too . For best Fender tone.

Years ago, I would have agreed; now I might not.

If you like the sound of the amp as-is, leave the bypass caps alone. If you want the amp to sound as-new, replace the bypass caps.

A lot of builders find their Fender amps have too much bass (or a muddy sound) for distorted playing. They typically reduce the bypass caps to 1-5uF, which doesn't seem to reduce bass so much as get rid of the impression of muddy bass. Marshall reduced bypass caps in some amps/channels to 0.68uF for the same reason.

I once had an incredible-sounding '67 Princeton Reverb. The stock bypass caps had dried from 25uF to about 1-2uF per cap. I never had to turn the bass above 4-5 to have plenty, even with single-coil guitars.

So you may want to decide if you like the amp's sound as-is before swapping bypass caps. Or at least keep the old ones in case you want to go back.

[EL34]

Will old cardboard type Mallory caps bulge on the end before they hit the crapper or are their other signs?  I'm torn between replacing them all and being done with it but then on the other hand the amp sounds pretty good.  Thanks, Terry

Get rid of that old crap and service the amp properly
If you ned help, I have a full fender service section on the library page
Look for the link at the top of this page

Servicing fender amps

http://el34world.com/schematics.htm

[Willabe]

I agree with Doug but,

If you like the sound of the amp with weak/drifted K bypass caps then just measure their value and replace them with new caps of the same uF value.

That way you don't have to worry about a cap dying.


               Brad     

[EL34]

It's not a value drift issue with old caps


It's that the guts are 50+ years old and not working up to stuff if they work at all


Would you leave a 50 year old paper bias cap in a Vintage fender?
Think about the consequences of that action and what can happen if the amp losses bias.


On a vintage car, you don't keep the original oil and air filters just because they are original


Just my .02

[Willabe]

I agree change all the e-caps.

I was just talking about the point the HBP brought up about smaller value cathode bypass caps.

If they have dried out to a smaller value and you like the sound of the amp now then I'd measure their value and replace with new caps.


                 Brad     

[tony_hunt]

Yes, HPB was talking about bypass caps, not bias caps.
At least in the pre-amp section the bypass seem to be unstressed, or do they see a higher voltage on start up? I have never checked!
I always replace filter and bias caps with the best I can get. I have left bypass caps in on my own amp when there many, like nine or so.

[Willabe]

Welcome Tony!

They are made the same as any e-cap and they dry out with age.

Most guys change them along with any e-caps when doing a cap job but HBP did bring up a good point. That some guys like a smaller uF value for the K bypass caps.

   
                  Brad     

[tony_hunt]

Thanks for the welcome :-)
I think I will measure the bypass caps before I pull them next time, out of interest. I like to gather useless information like that.

But on these British amps, where I have nine in the pre-amp, all are at 100uF so fully bypassed, so I doubt they will ever drift down to single numbers to make an audible difference. You never know.

[HotBluePlates]

At least in the pre-amp section the bypass seem to be unstressed, or do they see a higher voltage on start up? ...

A filter cap in the high voltage B+ supply will see a higher-than-normal voltage at startup, because zero tube current draw means zero voltage dropped anywhere in the power supply.

But preamp cathode bypass caps are a different story: The voltage source is the high-voltage supply. If the tube hasn't warmed up and is passing no current, then its plate rises to the value of B+, but the cathode is sitting at 0v, because there is no current drawn through the cathode resistor to set up a voltage across the cathode resistor. Therefore, the cathode bypass caps will have no voltage & no stress at startup.

You should evaluate your individual amp to determine if this is always the case, as some circuit configurations may have an additional voltage source that is across a cathode bypass cap (it's just very uncommon in guitar amps).

[Jack1962]

A cap that is bulging may not be blown or it may be blown , however if it is replace the cap. If the amp is to be played thru replace the caps , if the owner is concerned about the "vintage thing" put them in a bag for him .

[EL34]

I'm looking in the library for "check for leaky caps" and not finding anything.  Thanks, Terry

The search feature on EL34 world works


At the bottom of this page or any page on EL34 world, use the search link or search buttons


Typing "leaking capacitor" into the search provides you with 3 pages
Give it a try