Ampeg B25 restoration - Bad Hum

[sabredude]

I'm restoring a 1969 Ampeg B25 and have replaced all of the Electolytics but the amp has a TERRIBLE hum - even worse as I turn up the volume.
I replaced the 30 and 40uf caps with 50uf caps thinking that might help a little but no difference. Also, the first 30uf cap that is attached to the leg leading to the output transformer does not exist (never did) in my amp. I added a 50uf cap in it's position and little changed. I also replaced the cap in the bias circuit.
If I listen to the circuit (cap fed guitar cable to an amp as simple tone tester) I hear a ton of 120hz at the output of the bias circuit as well as the first couple of stages of filtering (A, B, C).
When I apply a signal from my generator, I hear that signal but it is almost equal parts signal and hum.
Otherwise the amp sounds marginal with an instrument going through it but at higher volumes, perhaps 25 watts or so, is distorted with a farty sound. This occurs well below rated power.

Thoughts on where to look next? I have tested all of the resistors around the power supply and all seem to be within tolerance.

[Structo]

Did you replace the 100uF/100v bias cap?

Did you make sure to put the anode (positive) towards ground?

Usually 120Hz hum is from the filtering and power supply.


60Hz hum usually indicates a heater supply (AC) problem.

Strange that amp doesn't have a filter cap on the A node.
Straight off the rectifier to the power tube plates?

[sabredude]

Yes, I replaced the 100uf bias cap. Polarity is correct.
As for the first filter, I cannot see any remnants of a cap being hooked up their. Solder joints are un molested and there's no mounting brackets or marks showing that there ever was one. I did add a cap emulating the schematic but it didn't seem to make a difference. I can jumper it in/out of the amp and have tested at various points with no improvement.

[Leo_Gnardo]

I've worked on a couple, if memory serves primary filter was one of those 20 or 30 uF 500V "dynamite stick" caps also found in B-15's. If there's so much hum, it doesn't matter whether the filter cap is there or not, try answer 2: There's a hum balance pot? Bet you it's toast. If neither, then we're running out of cheap solutions.

Non cheap solution A: your 7199 drive tube is noisy, good luck finding a nice quiet new one without breaking the bank.

Non cheap solution B: output tubes way out of balance, one's not working or else open winding on output transformer primary.

[Stevem]

The hum balance would effect 60 hz and he's is stating 120 hz noise, but non the less they do fail a lot so check it!
I do not think that one leg of the OT being open would make for the 120 hz showing up in the preamp section like he has going on, but one thing to check would be if the tang switch in each input Jack is shorting like it should when there is no cable plugged in!
How much AC ripple voltage are you reading on the plate of the first gain stage?
Also note that when the bottom cover plate is not on these amps the Hum level will be higher!

[sabredude]

I replaced the hum balance pot with 2 x 100ohm 1W resistors that I verified were closely matched. I used one of those converter plugs to use a 6u8a tube in place of the 7199. I bought two tubes and two converters at the same time and have swapped both to see if that was the issue. I have a 7199 on order as well in case the adapter is the issue.
Otherwise, all tubes are new JJ's.
I will look into the jacks to see if there is a shorting issue - didn't think about that. Also, the line output sounds fine. I don't seem to get excessive hum on the preamp section. just the section for the output transformer and bias.

AS for measuring the AC ripple, is that as simple as setting a VOM on AC and measuring the DC line? Seems to me that I heard that before and it seems to make sense but that seems too easy.

Thanks for the help guys!

[Stevem]

Yes, it's that easy!

[martin manning]

But, I believe I recall seeing some reports of cheap meters not being capable of this type of measurement.

[JazzGuitarGimp]

I had an Ampeg VT-22 in for service several months ago. It had issues with hum. Fortunately, but quite accudentally, I discovered that the speaker output jack was missing the outter insulating shoulder washer. At some point the nut came loose and fell off, along with the washer, and the owner put the nut back on without the washer, which shorted the jack's sleeve connection to chassis, causing a ground loop as well a considerable amount of hum. My hand accidentally brushed up against the speaker cable plug while the amp was on and the hum briefly went away, causing a bit of head-scratching on my part until I took a closer look at the jack.

[Stevem]

Its nice if the meter reads true rms, but even with a cheap meter if you check the power supply nodes by going up stream to the preamp you can get a feel if the ac ripple is dropping down like it should when going from node to node!

[sabredude]

for troubleshooting, I removed the tubes starting at the preamps, then PI and found the hum continued even with all of the tubes except the output tube removed.

I did the ripple test and found approx. 25v of ripple on the first stage(output transformer). I added the cap that is specified for the first stage but was never installed on the amp and that brought the ripple down to a negligible amount. My VOM is on the cheap side so I ordered a True RMS VOM to get better results. I found with my tester that I would get initial ripple readings then the would go away with a second or two and suspect that something was weird with the tester. The 25v ripple however was constant.

I added the cap before with the preamp tubes in, and the results were worse than without the cap. I will re-insert the preamp tubes and try again.

I have also verified that the output jack is isolated from the chassis - something I didn't even realize existed but the rubber insulators were intact and I also physically removed it from the chassis with no improvement.

Output transformer appears to be working fine as I applied a signal from my tone generator to each side of the primary and got an output. This isn't a test I've done before but my logic was that if either of the primaries were shorted or open, I should only get an output from one side or the other.

For this project, I am trying to keep most of the outside original but making some modern updates to the amp to make it usable for my client. All humming issues were there before I started my mods. I was hoping that in the process of mod'ing the amp, I would replace the back cap, resistor, ground, etc.

Mods I have performed:
replaced power cord with grounded cable and chassis grounded the cable.
removed "death cap" and associated electronics.
moved standby to front where ground switch was previously located.
replaced standoffs that had fallen apart.
Replaced all electrolytics - bias and 4x power supply caps. Raised 30/40uf caps to 50uf. replaced dynamite stick with 50uf cap can.
Added 50uf cap can to first stage filtering (as in schematic).
removed Ampeg Speaker cable (part of checking the output isolation and unwanted by client) Speaker cable was just 2 wires so no safety switching in the cable like some other ampegs.

Grounds in the chassis are:
Power cable (by itself and as soon as it gets in the chassis.
Standby switch which simply grounds the center tap of the power transformer secondary (by itself next to the standby switch)
Star ground near first cap can for everything else.

I have attached gut shots of the amp to see if anybody has any ideas.

[Stevem]

That resistor in that second shot (2 watter may be) that is sitting parallel to that new black filter is over heated and may have changed its value greatly.
At this point I would lift out of the circuit the down stream end of all the coupling caps and any that are leaking the least bit of dc replace them!

[qeopmizwal]

I too am trying to tame B25 hum, was curious if you ever solved your problem.

[Stevem]

What things have you done so far to try and cure your hum issue?

[TUBEDUDE]

Have you replaced the rectifier? Is it possible to for one side to be bad?