Fender bassman 100 blowing fuses

[Enzo]

In the first photo, right in front is a silver cap and you can read "60 v" on it. THAT is your primary bias supply filter. Then from the right hand end of that cap, there is a 22k resistor running back to the bias control. where it joins another cap that is white. That white cap is the second bias filter.

The caps in the pan are all your B+ filter caps. You can call all of them filter caps, but for some of them, their most important function is decoupling.

[g1]

Here's the schem, thanks for the link Justin.

[Chuck H]

Jam, if this is a customers amp you could (and probably should) goad them into replacing ALL the electrolytic caps. The three Atom caps in the power supply probably aren't that old and are clearly replacements, but since they've looked like that forever it's hard to tell. The only reason I can come up with for a tech to replace ONLY the preamp filters, and not the main filters, bias supply and preamp bypass caps, is that those were the only ones that were leaking. It's a strange decision otherwise. So, with that being the most likely case I'd like to say that if that amp were on my bench and three of the original caps were leaking I would have replaced ALL the electro's in the amp at that time. The caps are old and untrustworthy. Well past their intended service life. It's extra work for you and your customer would be better off for it. Especially if they're investing in new power tubes.

[tubeswell]

Leave the tubes out of the amp for the testing.

What Chuck said.

The mains fuse will blow if too much current is being pulled through it. So you need to think about what would pull lots of current through the mains fuse.

The mains fuse is connected to the PT primary, which is 'connected-at-AC' to the PT core and to the PT secondaries (including the HT winding, which is connected to the rectifier and filter caps and supply resistors). If there's little or no load on the secondary windings, then there is no extra work being done across the PT core and the primary side is just experiencing 'normal' AC current between the active/phase mains wire and the neutral/return mains wire (after a initial surge of startup current has charged the filter caps). In that situation the mains fuse shouldn't blow - assuming startup surge current within the PT on an unloaded Pt doesn't cause the fuse to blow)

Now if one of the secondaries is shorted to ground (on both ends of the winding), then there will be a high load across the PT as the secondary works overtime to supply power to the ground. This situation would draw heaps of current across the PT, and cause the mains fuse to blow.

Now if there's no tubes (and nothing else like bleeder resistors or other DC voltage divider etc to put a load on the B+) and the B+ supply voltages are sitting at maximum after the amp is powered up, then there will be very little current being pulled across the PT core, and you can fairly safely assume that if the fuse was blowing with tubes installed, then the fuse blowing was caused by a bad tube.

In terms of whether the filter caps might be bad or not, with (little or) no load (i.e. without tubes installed), the filter caps will simply sit at the maximum B+ voltage that the PT (and rectifier) can put out. And if they hold up at that voltage without a load, then they should be fine with a load.

However if one of the filter cap nodes is pulling way down without a load, then you more than likely have a bad cap on that node (shorted or 'partially-shorted' cap). If this is happening, you may also notice the supply resistor that is connected to the bad filter cap is getting really hot or smoking, because it could be passing much more DC current than it is designed to cope with, because the shorted filter cap is just dumping that DC to ground (effectively just like having that supply resistor connected straight to ground).

So measuring DC voltages on the power rail is a good way to verify the situation.

[Jam]

Thanks guys their is a lot of great info to take in. So I tested the sliver cap and I get -74.9VDC and the white cap I get -63VDC

[g1]

So I tested the sliver cap and I get -74.9VDC

The picture shows that cap to be rated at only 60 volts, so it must be replaced, best to use a 100V. The white one looks to be original, so might as well replace it too.
Remember that the bias caps are installed with + end to ground.

[Jam]

Thanks again everyone. I was going to order some parts where is a good place to get caps from? When I get the bias caps should I match the MFD and they both can be 100V ? If I replace any other caps do I just match the MFD and Volts off the old one?

[The Dude]

There are plenty of suppliers on the web. I usually get them from mojotone.com, but as I said there are others. It's difficult to get exact values for the older amps. They are not standard values. You can use for instance 22UF for 20UF, 47UF for 40UF, etc. Value isn't that critical- VOLTAGE IS! Don't use caps rated for lower voltage than what is in the amp currently. The filter caps do what their name implies- filter AC voltage. Go over rather than under if the exact part is not available. More filtering is better than less filtering in most instances.

[Jam]

I replaced all the electrolytic caps what would be the step to make sure amp is stable?

[Chuck H]

Start it up with no tubes in and take voltage readings before inserting them. If you have a current limiter, use it as a matter of course. If you don't, just be sure all voltages look correct before inserting known good tubes. If you don't have known good tubes, make a current limiter and use it as you insert the tubes one at a time.

[Jam]

Thanks again when you say take voltage reading do you mean the bias cap voltage? What are the best test points to check? What I did was hooked it up to my light bulb limiter with 100 watt bulb and (output tube out) on the two bias caps the primary one I got -69.9 VDC and the other one was -59.64 DCV

[Chuck H]

Take HV rail reading just to see that it's accurate to the rectified voltage off load. And yes, take a bias voltage reading to be certain there IS bias voltage. It's good you have built a limiter. The limiter should glow dim when no current is being pulled by the amp. If it glows brightly with no tubes there is a problem. If you don't see a problem then start plugging in tubes. If they aren't of known goodness you should plug them in one at a time so that you can be prepared to quickly shut down if a shorted tube is drawing excess current through the amp. The limiter bulb will glow with more brightness, but less than full brightness as tubes are installed. If any tube instigates a very bright bulb there may be a problem.

[Jam]

With out the output tubes the bulb was very dim but as I put new output tubes it would glow little brighter ? The main bias cap I had -50.71 and on the plate pin I had 307.3 VDC is that what you mean for HV rail? Thanks

[Chuck H]

Yes. The HV rail is the high voltage supply. They are usually set up in stages along a shared path. Thus the term "rail" and often just called the HV rail.

Your description of the limiters response and verification of bias voltage all seem right. Remove the limiter from the equation now and check/adjust the bias current.

[Jam]

Thanks again I removed the limiter and now on the main bias cap I have -73.6VDC and my plate pin is 462.0VDC . So this does not have a bias adjustment but does have a balance pot how do you check that? Thanks