• TalkBass has been independent since 1998. Add your voice.
    Create a free account to reply to discussions, view embedded media, and browse with fewer display ads.
    Join free Log in
    Want zero display ads or expanded classifieds tools? Compare plans.

1966 Ampeg B-15N issue - capacitors

#41
Other possibilities are leaky coupling caps or bad tubes but not likely since they are both doing it. I have seen brand new tubes red plate(one out of a matched quad) for no apparent reason other than they are bad. We can't really tell you much else until the bias voltage, plate current and voltage is measured.
 
#42
I'm gonna do it on Friday. It may just as well be some broken connection. I travelled 200 km with the amp and as we know - things happen. I'll keep you posted. I translated the bias setting method for my dad and we'll see what's up.

The amp worked well before I brought it back home. I played it for like 3 hours at the shop.
 
#43
I just fired up the amp in order to check it out for a second time - to check whether I was not halucinating.

On Standby the sound comes out quietly into the cab, and the plates become red after you rock the On/off switch. It may be a wiring issue then, may it not?
 
#44
Normally, when the standby switch is on, you can't hear anything being amplified. The wiring needs to be carefully inspected to ensure that nothing is out of place. Look for loose wires or anything that could be causing a short circuit.

Failing that, there are two possibilities that should be investigated: the switch is faulty or a modification has been made to the circuit. The first thing that I would check is the standby switch, it might not be opening fully. Then compare the wiring inside the amp to that of the schematic to see if any modifications have been performed. One modification is to add a resistor across the standby switch for example. This allows sound through the amp when the standby is on.

As for the red plating, the bias circuit needs to be examined. Start by measuring the bias voltage to see what it is. The schematic indicates -50 VDC with a line voltage of 117VAC, see how close it is to that.

 
Last edited:
  • Like
Reactions: Jack-Jack
#45
The bias voltage is -25V so we got the cause. The 470k resistors seem to be OK, the capacitor I marked on the photo may be faulty, may it not?
 

Attachments

  • cap.jpg
    cap.jpg
    116.7 KB · Views: 96
#46
Yes the bias voltage is your problem.

The 100uF 100V capacitor in the bias circuit is often changed when a recap of the power supply caps is done. The one in your image is not original but it should be changed. Also check the other components in the bias circuit, the 100K 2W and 56K resistors. It never hurts to check the diode as well. If you need a replacement, a 1N4007 can be used and they are normally readily available.

It is important to remember that when a new electrolytic capacitor is installed, the (+) end goes to ground. This is the opposite to how the other electrolytic caps are installed.

 
Last edited:
  • Like
Reactions: Jack-Jack
#47
While the cap may need to be replaced, it is not the only potential culprit, and neither is the diode. The diode and cap provide a dc bias voltage. They are simply working with the voltage they receive. If the cap is at fault, it should show signs of leakage or bulging. One or more of the associated resistors are just as likely at fault. They operate at relatively high temperatures which slowly changes their values over time. The reduced bias voltage suggests that voltage may have been dropped prior to rectification (the diode and cap). This could be due either to a series drop or a parallel draw. Either way, excess heat will result (in addition to the red-plated output tubes. A discolored (partially burned) resistor would likely be present.
 
#49
Jack-Jack said:
I think re-doing the whole bias circuit may be a solution.

As I said, we have measured the resistors, and the values seem to be OK.
Click to expand...

When the amp is turned on, the resistors quickly heat up to operational temperature. Old. defective resistors often have a different resistance for room temperature and operational temperature. A resistance measurement at room temperature may not provide the operational resistance. You will need to know both the voltage across and current through the resistor to know its resistance at operating temperature. It is often easiest to simply replace the few parts in question (cap, 3 resistors and diode). If the cap is a section in a 'can' with multiple caps, the can must be replaced, not just installation of a separate cap for the bad section.
 
#52
AstroSonic said:
When the amp is turned on, the resistors quickly heat up to operational temperature. Old. defective resistors often have a different resistance for room temperature and operational temperature. A resistance measurement at room temperature may not provide the operational resistance. You will need to know both the voltage across and current through the resistor to know its resistance at operating temperature. It is often easiest to simply replace the few parts in question (cap, 3 resistors and diode). If the cap is a section in a 'can' with multiple caps, the can must be replaced, not just installation of a separate cap for the bad section.
Click to expand...

There were two caps that orginally were placed 'in a can' and these were replaced (the faulty caps that have fallen off in the beginning). After Bart has fixed them (see the beginning of the thread) up there were no further problems - so I'd like to think that they are not the issue.

DavidEdenAria said:
Those tubes may very well have RUNAWAY CURRENT DRAW at this point.

I've horded tubes for 30+ years and have bags of faulty power tubes with runaway current draw.

The whole bias section needs to be rebuilt first though.
Click to expand...

That's what I'm gonna do - the bias section and then, if that does not help, I'll get myself a pair of black plate TAD 6L6s to replace the old tubes. Anyway, the power tubeys look as if they were replaced not so long ago, so they are the last element on our list of suspects. Anyways, I'm gonna get new ones in two months I hope. Rebuilding the bias circuit is cheap. I'm just happy it's not the transformer.

I just hope it's for sure the bias circuit and not anything else.

beans-on-toast said:
Yes, that would be the best thing to do. As was mentioned, the resistance could change when the resistors are hot. Mostly the 2W one, the other ½W ones don't run hot.
Click to expand...

I measured them 'on line', with on and on on standby switch.

Your insight guys is invaluable. I think I am gonna get you all a beer after the struggle is finished.
 
#54
beans-on-toast said:
Don't worry, you'll get it fixed.

Forget about the beer, I'd want some polish food!
Click to expand...

pierogi_ruskie_4.jpg


Here you go, sir! :)

I'll get you an update when I get all the parts and put them into the amp.

EDIT: Correct me if I'm wrong - if I change the tubes, do I need to adjust the bias with additional resistors? Or is that not necessary for the B-15, as it has only 1 pair of tubes...?
 
Last edited:
  • Like
Reactions: BurningSkies
#55
Jack-Jack said:
Here you go, sir! :)

EDIT: Correct me if I'm wrong - if I change the tubes, do I need to adjust the bias with additional resistors? Or is that not necessary for the B-15, as it has only 1 pair of tubes...?
Click to expand...

Those pierogi look very good! Just like my baba used to make.

If you change your power tubes you will need to check the bias and adjust it if necessary by changing the resistor.

The B-15 power amp comes in two versions. The earlier 25W one is cathode biased. You plug in a pair of matched tubes and the amp is self biasing. No bias adjustment is necessary. The later 30W revision, the one that you have, is called fixed bias. With this version, you need to check the bias and adjust it, if necessary, each time you change the power tubes. Sometimes, to set the bias to the correct voltage, more than one resistor in a series/parallel combination is needed.

In the Heritage B-15 they added a potentiometer that allows you to adjust the bias without the need to change a resistor. This is a common modification. An example of a fixed bias circuit with a potentiometer for adjusting the bias is shown below in the lower left.

 
Last edited:
#58
beans-on-toast said:
$1 fix. Experience that you gained in learning how to repair the amp, priceless.

What was the problem in the end?
Click to expand...

The unoriginal, gold 100uF/100V cap.

You will laugh If I tell you. It was a cap made by Ducati company, and the date of manufacture on the capacitor says: 12. Apr. 1933. Made in Italy. I have no idea how it ended up in my amplifier, I'm telling you. But on the flipside, no wonder it ceased to work properly.

I used a bipolar cap [without polarity marked], and it works well.

PS - The amp still gets quite hot, but the logo is intact, it is not turning into liquid, and the cables in the back sit well in their places. And redplating is gone.

Is 56V ok Bias voltage?
 
Last edited:
#59
Jack-Jack said:
Is 56V ok Bias voltage?
Click to expand...

Yes, if the amp sounds good and is not red plating. When it comes to bias there is no right answer. You are close enough. The bias voltage can fall within a range where it is acceptable. To know what the optimal bias should be you have to take some readings in the amp. Measure the plate voltage and cathode current. Do the calculations outlined in the wiki procedure. The plate voltage will depend on the wall line voltage and how healthy your capacitors are. The cathode current will depend on your tubes to some extent. As you can see there are some variables.

Some people stop there. If you have some test equipment, there are additional tests that can be performed. Once the bias is set, you can look at the output signal with a scope to look for distortion. A spectrum analyzer can used to see the harmonic distortion. A piece of test equipment called a distortion analyzer can also be used. Then you can iterate and change the bias setting and see how this affects the performance of the amp. From this, an optimal bias can be determined.

Remember, keep the amp away from the wall and give it some air. If the plexi logo is getting too hot, remove it and put it in a safe place. That's what I do.
 
  • Like
Reactions: Jack-Jack
#60
As a side note to this bias problem, one of my cathode-biased Ampegs had a similar issue, after I got it up and working. Turns out, the wiper on the Hum Balance Pot was grounded. On a fixed biased amp, this would not be an issue. But, as soon as I replaced it with one that had an isolated wiper, the bias fell right into place. As the Plexiglass badge had warped on it, long before I got it, I'm sure that it had this same problem for no telling how long before it died. Glad you found your problem and now have a fine amp.

Jack
 

Latest posts

Top Bottom
Back