Ampeg B15N 1969 I can't trace that hum...

[boroman]

It's another silverface B15N I had on my bench at it has little hum. Hum raises up along with tubes warm up (e.g. when I hit npower and standby at the same time there is no hum, it's begins to be heard when tubes catch their working properties).

I've already
- changed & checked PS caps
- bias cap & resistor (>variable)-
- made grounded power cable
- checked hum balance (but it's not that type of hum and this one works good)
- checked tubes (in other B15N they are completely quiet)

When I pull out 1st and 2nd 6SL7s hum is still there. So this would tell that it's not in preamp.. right?
When I pull out 3rd 6SL7 (phase inv/driver?) hum is gone. I see no burnt resistors, bad preamp caps, etc. Grounds are good.
What's off, the 1st channel volume control raises the hum after 1/3 turn (the same preamp tubes fitted to other B15N does not do it). I am a bit confused here...
cc

[Jon Snell]

If the hum is twice mains frequency, it could be smoothing issues but the Ch1 volume hum level change is interesting.
There is not much to look at except to guess where it may be coming from.
If it was the control circuit, it will have no effect without the 6SL7 fitted.
I would find the two 270k feed resistors from the trebl controls, what is the DC voltage on the junction? It should be xero +-a few millivolts but no more.
If the 0.1uF anode feed capacitor was leaky, I would expect a DC level through the 220k, bass pot to slider through the 120k and on to the 270k grid feed. That will cause a hum.
Removal of the preamp stages is by shorting the Ext Amp socket, that should stop any hum. That will isolate the pre amps completely and let you know exactly where to point your thoughts, proving the phase splitter section is good.

Is the -ve bias voltage good on the 6L6s?

[nickb]

Some ideas...

Put a shorted out jack into the external amp input. If still humming then try replacing V3.
Does it sound normal aside from the hum? If not, it might be oscillating.
If not oscillating and still humming with the shorted jack you might have a bad ground connection.

[g1]

Attaching actual Ampeg factory schematic.

[boroman]

Bias is OK (hum goes just a hair more louder when bias is set hotter, so it deafinitely amplifies)
When shorted out EXT AMP jack, amp is dead quiet
No oscillation.
No change when 6SL7s are swapped.
That 1st channel hum build up after raising the volume might be just something else related to preamp, that's my first thought.

I would find the two 270k feed resistors from the trebl controls, what is the DC voltage on the junction? It should be xero +-a few millivolts but no more.
If the 0.1uF anode feed capacitor was leaky, I would expect a DC level through the 220k, bass pot to slider through the 120k and on to the 270k grid feed. That will cause a hum.

Amount of knowledge found here is absolutely unearthy! Thank you. Will check that today.

[nickb]

I note that the B-15N and B-15-N portaflex have the volume and tone control sections transposed. So it's worth asking the question, are we sure we have the correct schematic for the version you have?

The ext jack short test tells us the problem is before V3 and we also know that pulling V1 and V2 didn't help so that just leaves the power supply and the tone stack (/volume control dependent on version). You mentioned you replaced the filter caps. Did the hum start after that or was it unchanged? Sometimes, new caps are bad so it might be worth tacking another one across the 40uF that feed the V1 and V2 supply, just as a test.

[g1]

I note that the B-15N and B-15-N portaflex have the volume and tone control sections transposed.

Can you elaborate?

[nickb]

Actually it's the '62 vs '68 versions of the B15n that differ. I just have them named rather poorly in my library.

Ampeg_b15n_portaflex.pdf

Ampeg_B-15-N_(12-68)_Portaflex_Schematic.pdf

PS: I really miss the old "manage attachments tool"

[catalin gramada]

Some service manuals included :

https://www.vintage-blue.com/schematics.html

[glebert]

I've seen some posts in the past about noisy plate resistors when you run out of other ideas for tracking down noise. I always wondered if you could jumper around the plate resistor with a cap to see of that changes the noise.

[boroman]

I surrender on this one, unless I find time later this year to resolder resistors one by one.. I got it to 2 good techs and they still dont know what it is.

----

Not to hit another topic, because I have similar B15N here, a little later board (1970 I believe) and amp is making random hiss and crackle. Not volume dependent. It stops when I pull out phase inverter tube. But it does not stop when I pull out two preamp tubes and have phase inverted in.

- I've tried new set of tubes - the same
- replaced 100k/2W resistor that had 135k reading
- replaced bias resistor for proper bias
- replaced all tube sockets (they were breaking in hands)
- it was re-capped when I got it but it's good values/components beside little messy, cap job.

The amp has worked well before, clean, absolutely no noise. One day it started to hiss and crackle. I see no burnt resistor, no leaking caps, everything seems to look good... Still, dont want to replace resistors one by one, but maybe it's a known problem?

[Jazz P Bass]

Have you checked the PI to output tube coupling capacitors?

[boroman]

Have you checked the PI to output tube coupling capacitors?

Not really, but they look good.
Would bad coupling caps produce hiss/cracke/pop (I thought rather they do low end hum when they're bad?)_

[boroman]

Have you checked the PI to output tube coupling capacitors?

EDIT: You were 100000% right. I desoldered original ones and put there new and amp is dead quiet now! Thanks so much!

I will try to do the same to the one mentioned earlier in the topic, maybe hum is also there, as it's near phase inverter area too.
By the way, the 25uf/25V cap could also be the issue?

[drewl]

Hiss and crackling is "usually" caused by plate resistors, especially carbon ones.