Capacitor Testing

[roadkingbluzs]

I am working on an VT22 Ampeg. Do I need to disconnect from circuit cap cans to read a good / bad cap. I only have multimeter, no cap funtion. My understanding is 15K ohms minimum to be good. I was going to replace caps but now have gotten word to do minimum and just get up and running. I do not like changes to just part of power supply, well I think most would agree. Bob P.S. time to invest in another meter

[Structo]

How old are the caps in the amp?

Sometimes you can reform old caps in an amp, but if it was powered up after sitting a long time, I believe you missed the window to reform them.

If the caps are older than 15 years, most likely all the E caps need replacing, including the bypass and bias caps as well.

[roadkingbluzs]

I totally agree. Belongs to a freind. He borrowed the amp and fired it up and played for a couple months, in studio made intermittent sounds a little smoke and then DOA. Owner and borrower do not what to flip for complete overhaul. Very early ampeg and looks totally stock. It was in storage up to a few months ago. I told these young guys I may get this up and running but no guarantees. May work for five minutes or five years(unlikely), but there will be many weak leaks in the chain. Most here on forum are familiar with vintage gear but your regular musician /player just wants to plug and play. Any recommendations on multimeter with cap funtion would be appreciated. Bob

[Structo]

Most guys here will probably recommend a Fluke meter.
It is pretty much the top brand name in meters.

I bought a Meterman that I have been happy with.
It has a cap tester.

If you don't want to spend an arm and a leg, there was one I bought at Radio Shack a couple years ago that wasn't too bad.
It was a True RMS meter. Most of the good ones will have that feature.

Check ebay for deals on used Flukes. I think one model I recall is the 87.


You can pick the one you want here then try to find it cheaper elsewhere.


Since you don't have a meter, are you aware of the dangers that lurk inside a tube amp?
Filter capacitors can hold a charge depending on the amp, so if you touch the wrong thing you can get zapped and if you get it across your chest (by having it go in each hand), it can kill you.
So first thing to learn is how to discharge the filter caps.

READ THIS:
http://www.aikenamps.com/SafetyTips.html

[Phil_S]

I have a Triplett 9045 with a cap meter in it. It is primative and that's what you get for low cost. It will tell you the value of the cap, i.e., in farads (or mf, nf, pf, etc.). You can't control the scale. It usually shows guitar amp caps in nf.

You need to test the caps for leakage. I think that calls for an ESR meter and it will run into a few bucks.

What the other guys said...an amp this age needs all the electrolytic caps replaced, especially in light of what occurred and the missed window for the remote possibility that the old caps might have successfully been reformed.

[roadkingbluzs]

I am trying to convince owners to do at least cap repair as this amp is a '70's model. They'r trying to go cheapo but do not realize they will pay in the end. Thanks for any and all info. Bob

[Rick]

Yep, they should spring for all new power filter elytics at the very least. Not doing them will end up costing them more, maybe much more in the long run not to mention almost no reliability. With all new filter caps, it at least has a chance of running somewhat trouble free for a while and maybe protect power section and sound much, much better. Probably do for retube if you really want any kind of reliability also. BTW, The first symptom I usually notice with leaky filter caps is very noticeable loss of power and maybe some strange noises.

[MBD115]

My last meter was a Craftsman $99.00 meter that also tests caps. I've tried in many caps and it has always gave me the proper value for the cap, so it must work ok. lol

It is cheap but its working and I've used it a lot.

Good luck

[roadkingbluzs]

Update, last night as I was looking to see if could check cap values on tone controls I noticed star lock washer stuck under some wires laying flat against chassis. Wow lucky that didn't ground anything out. Well after closer inspection washer had two arc marks and seems to have arced on two terminals of power supply cap can, they match up exactly. So I will inform owner cap job or I will not do it. Pretty simple. Lock washer from pre/power out jack. Bob

[paulster]

I think it was Brandon who had exactly the same loose star washer fault on an SVT. Must be an Ampeg favourite!

Bear in mind that some of the can caps they used are hybrid values so can be difficult to find, although I can't remember whether this was the case on the VT-22/V4. Fliptops.net are a great resource for spares for these if you need obscure values.

[roadkingbluzs]

Looks like Fliptops has a capacitor package deal, seems reasonable. Trying to find a quad of 6550 or 6l6 to replace 7027's. I will be calling owner today.

[paulster]

7027s are still available as new production from JJ.

You can change to 6L6es or 6550s but expect a change in tone and the sockets will need a wiring tweak because of a slight pinout change.

If they're really cheaping out then it might be more cost-effective than springing for the 7027s, which I'd imagine (without checking) will be more than new 6L6es.

[selloutrr]

becareful of the traces on the PCB they peal up very easily when over heated. The V4 is a very popular amp here in the desert for stoner rock, QOTSA, Unida, HOBP... Every one i've had on the bench needed the ceramic power resistors replaced and at least 1 of the power caps. I recommend adding an adjustable bias, the removable plate is a very cool place to install the pot so you don't have to drill the chassis.

I can not stress enough!!! do to the voltage this amplifier series pushes well over 500V it's not uncommon for one fix to cause another problem to show it's head. Give it plenty of time to break in before you give it back.

[selloutrr]

capwizard is a sweet incircuit tester

[roadkingbluzs]

I will be doing a complete cap job on power supply. including coupling caps. What is recommended on bias. Replace 75K with a VR or? Remember owner or kid paying is going cheapo. I will be investing in another meter, so thank for your input. Bob