Ampeg B-15-NF hum problem

vinylivo · #1 03-22-2011, 03:33 AM

Hello bass amp folks!

I hope it's fine to start a new thread instead of posting to the portaflex club thread.

I got an Ampeg Portaflex B-15-NF for repair/fix. Because we have 230V here it's powered via a 230 to 115V step down transformer.

The main problem is that there is a strong hum/buzz if you turn up the volume on either channel.
The power supply filter caps were already replaced by somebody else. I already swapped all the 6SL7 tubes in various possibilities but it didn't change anything. I checked AC ripple on the preamp voltage and it's below 10mV rms. If I switch off power the buzz disappears immediately even while sound is still continuing for another 20 seconds. It seems like the buzz is injected via the AC heaters of the preamp tubes. I already tried different connections of the center tap from the hum control pot. originally this is connected to the negative bias and reconnect to ground makes things slightly worse. did any of you have similar problems with a B-15-NF?

now i'm thinking about modifying the preamp tube heating to DC, what do you think about this?

BTW the rectifier tube is a 5V4 instead of the 5AR4/GZ34 in the schematic. can this be a source of problem? when checking the rectifier tube datasheets it looks like the 5V4 is somewhat 'weaker' (has more voltage drop) compared to the GZ34. and the guy who replaced the power supply caps did so by a 80uF type instead of the original 30uF. also the GZ34 datasheet says the rectifier should be used with a maximum of 60uF.

beans-on-toast · #2 03-22-2011, 05:40 AM

I would repair the amp to use the proper capacitors and 5AR4 tube. The voltage drop / lack of headroom / early distortion of a 5V4 might be good for guitar but not bass. There are some things that you can try first though.

Re-solder all the connections that were made when the capacitors were installed. There should only be one ground point, at the end of the ground bus at the input jack. As this is the only ground point, make sure that the jack contact to the chassis is free of corrosion and that the jack nut is tight. The caps should not be grounded to the chassis. If there are any grounds in the power supply there will be a ground loop and hum. If there is a cap can, it needs to be isolated from the chassis with a cap can washer and not in contact with the metal. This is a common mistake people make when re-capping.

Pull the rectifier tube and clip in a set of diodes. Use two series 1N4007 or equivalent on each leg of the high voltage tap. Like this this. This will help you determine if the larger capacitor/5V4 is causing a problem.

Converting the heaters to DC is not necessary. This is a very quiet amp as designed. You could unhook the heater circuit and inject DC as a temporary test to see if this is the source of your hum.

If the speaker cable was removed and replaced with a jack, the jack needs to be isolated from the chassis to prevent a ground loop. The speaker return should not be grounded to the chassis. There also needs to be an isolating washer on the ext speaker jack. Not having one on the ext amp jack is normal.

vinylivo · #3 03-22-2011, 06:59 AM

thanks for your assumptions.

I also thought of restoring to original cap values, and the 80uF primary supply cap is definitely too high. and the recapper-guy did ground the ps-caps to chassis and this looked strange to me. restoring to original routing hopefully removes my problem.
I'll have time to do work on the amp tomorrow and then report back here...

beans-on-toast · #4 03-22-2011, 07:37 AM

Quote:

Originally Posted by vinylivo

the recapper-guy did ground the ps-caps to chassis

That's your problem then.

With the 80uF cap you will strain the tube rectifier. Adding an additional diode in line with the high voltage tap will help fix the problem but a smaller valued cap should be used.

See this schematic at the 5Y3GT rectifier for an example as to how Fender addressed this issue in the reissue Deluxe. The diodes serve to extend the peak inverse voltage and protect the rectifier tube.

BassmanPaul · #5 03-22-2011, 09:11 AM

According to my RCA tube manual the maximum capacity a 5AR4 can handle is 30µF not 60µF. That was the value designed into the amp originally. Yes the 5V4 has a larger voltage drop and, IIRC, a larger heater draw, 3A.

vinylivo · #6 03-24-2011, 10:27 PM

Hi portaflex fans,

just wanted to report my results back.

after rewiring the whole power supply back to original the amp is pretty silent. there's just a very very slight buzz when you turn up the volume to max. ...I discovered another mod, the feedback was reduced dramatically from the original divider of 10kOhms/220R to 34k/120R. no wonder there was so much gain. I'm gonna restore that to original as well.
replacing the rectifier tube to 5AR4/GZ34 gave quite some more power, something around 50W or more. no wonder if you see the datasheet compared to the 5V4G. because I couldn't get 30uF in time I installed a 50uF/500V F-T-cap for the primary power supply filtering. the amperex datasheet I have still says 60uF max cap, or 750mA peak and 250mA average current ratings. GE datasheet even states 825mA peak. this is not exceeded with this amp as it draws only 120mA in idle and less than 250mA on full power. BTW I added a trimpot to the bias voltage circuit to be able to adjust bias current.

finally I just wanted to say that this ampeg thingy just has a very nice sound. not too much power but enough for studio sessions or easy live events.

greets froma switzaland

beans-on-toast · #7 03-25-2011, 06:22 AM

Great! I think that the amp sounds even better when you increase the value of the first power supply cap so at 50uF it should sound very nice.

These spec sheets can be confusing when it comes to this maximum capacitance issue. It is the peak current that is important and this is affected by the capacitor, the transformer impedance, and any series inductance or resistance. If you are not exceeding this peak, you can increase the capacitance. With the test conditions that they used, they would specify a maximum of 60uF. The other important consideration is the surge when you turn on the amp.

Make sure that when you turn the amp on there isn't an arcing or flash inside the 5AR4. This can happen with larger sized capacitors in place and it can cause the tube to blow. If you have this problem just install the diodes that I mentioned in my previous post.