Hi again,
I've got an AB165 bassman on the bench. I've re-seated all the grounds and cleaned the jacks, pots, switches, but there is a hiss on both channels when nothing is plugged in, plus the tone switches pop loudly when flipped (despite their caps being good). Pulling V1 or V2 (or V3) stops the noise in their respective channel.
There aren't many parts in these first gain stages, but which should I suspect first?
thanks!
I've got an AB165 bassman on the bench. I've re-seated all the grounds and cleaned the jacks, pots, switches, but there is a hiss on both channels when nothing is plugged in, plus the tone switches pop loudly when flipped (despite their caps being good). Pulling V1 or V2 (or V3) stops the noise in their respective channel.
There aren't many parts in these first gain stages, but which should I suspect first?
thanks!
I realize there are components that must be replaced if they're causing issues, but would some people pull their hair out knowing that preamp parts have been replaced, forever ruining the value of a vintage amp, such as this? Are there any arguments to be made for specific caps or resistors that should be left alone?
In short yes. For these people you have to go the extra mile. For example todays capacitors are much smaller than the ones of old. The old can types are easy to empty out and stuff with modern caps, put back together and you have vintage look with modern reliability and performance. Resistors get replaced with carbon composition and bypass caps can be found on the bay or if you're like me that have done complete cap jobs for people save the old molded coupling caps that test good to use in these situations.
I leave the coupling caps alone if they are not leaky or way out of spec. Other than that all electrolytics are replaced along with plate resistors. I almost always end up adding grid stoppers and for the real old models that don't have screen resistors on the finals I install them. Non adjustable fixed bias models get a diode replaced and sometimes a bias resistor change depending on power tubes. Some times you get a doozy where you have to rearrange the grounding or lead dress to get noise to acceptable levels.
My friend is a vintage amp/guitar collector and I have done all of his amps with not one complaint yet. In nearly every case there was significant improvement in sound and performance.
Unfortunately I've replaced the 68K grid resistors on the Normal channel, plus the 1M ground shunt resistor and I can't get the hiss to go away. Grounding the input grid still silences the hiss. I'd suspect the jacks, but what's to go wrong? The ground shunt arms are tensioned and cleaned, plus I can jumper the two tips to ground manually but it doesn't help.
From what you describe, it does sound like the 68k grid stoppers are the source of the hiss. (The 1M grid leak gets shorted out when you ground the tip of the input jack.)
Vintage amps do hiss, of course. Are you sure the hiss is too high to be acceptable? Are you very close to the speakers when you are testing? (Sorry if I am stating the obvious here.)
Merlin Blencowe covers noise from the input grid stopper very well in the second edition of his Preamp book - not sure if you have a copy.
You can reduce the noise with a lower resistance grid-stopper. Merlin recommends to stay above 10k, if I remember correctly. In your amplifier, I think two of the 68k grid stoppers are connected in parallel for one of the input jacks? That would suggest you could take them down to 20k, but it would mean making the amp even more non-original, unfortunately.
Vintage amps do hiss, of course. Are you sure the hiss is too high to be acceptable? Are you very close to the speakers when you are testing? (Sorry if I am stating the obvious here.)
Merlin Blencowe covers noise from the input grid stopper very well in the second edition of his Preamp book - not sure if you have a copy.
You can reduce the noise with a lower resistance grid-stopper. Merlin recommends to stay above 10k, if I remember correctly. In your amplifier, I think two of the 68k grid stoppers are connected in parallel for one of the input jacks? That would suggest you could take them down to 20k, but it would mean making the amp even more non-original, unfortunately.
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Grid stop resistor value is dependent upon which valve is being used.
The 68k grid stopper wont add any noise. Since using a grid stop resistor is effectively creating a low pass filter to cut down on oscillation....too high a value will cause higher frequency attenuation.
No need to lower the 68k value, shouldn't be a problem. Double check all wiring and solder connections from grid to jack.
The 68k grid stopper wont add any noise. Since using a grid stop resistor is effectively creating a low pass filter to cut down on oscillation....too high a value will cause higher frequency attenuation.
No need to lower the 68k value, shouldn't be a problem. Double check all wiring and solder connections from grid to jack.
"forever ruining the value of a vintage amp" in my book means painting Tweed black with a corner store bought spray can , pulling the speaker cloth and installing an old piece of curtain, drilling holes above every jack and knob and adding dancing Leds, stuff like that.
Replacing a **bad** resistor which hisses, a leaky coupling cap, a worn/cracked tube, a failing switch, a rusted/worn jack, a blown fuse
, is doing proper maintenance.None of those bad parts will make it sound good, or vintage, some of them won't let it even *work* .
Famous records in general (there *may* be an exception, definitely was not the rule) were recorded with properly maintained amps, doubly so in a Studio environment where Producers and Engineers would have yelled when hearing annoying hum/buzz/hiss.
Probably quite a few backline amps sometimes were not in the best condition, sh*t happens on the road, but they definitely didn't help, quite the contrary.
Another possibility is that the amp has more overall gain than it should have. The schematic calls for a 12AT7. Has someone replaced that with a 12AX7, which would increase the gain significantly? Is the global negative feedback circuit in place and working correctly? If this has been removed or has a fault the overall gain would also be increased.
I am sticking to the proposition that input grid stoppers are a noise source - will explain why later (I've run out of time for now).
I am sticking to the proposition that input grid stoppers are a noise source - will explain why later (I've run out of time for now).
Those 'resistors' could be the cause, but they aren't particularly grid-stoppers, they are simply a switched attenuator on the input.
The only grid-stoppers in the design seem to be on the outputs?.
Using the low sensitivity input, we have the two 68k resistors forming an attenuator (as you say). One of them now also functions as grid leak, the other acts as a grid stopper (it is needed to suppress radio frequency interference).
Using the high sensitivity input, we now have a 1M grid leak and the two 68k in parallel acting as a 34k grid stopper.
Any resistor, sitting at a temperature above absolute zero, generates a noise voltage. This is called Johnson noise. No current through the resistor is required for this.
A 68k resistor at ambient temperature (300K) generates approximately 3.4 microVolts of Johnson noise in the audio band [reference: Merlin Blencowe, 'Designing Tube Preamps for Guitar and Bass', 2nd edition, 2012, page 65].
Whether this small noise source is a problem, or not, depends on the overall gain of the amplifier. For high gain amps it can be a serious problem, although I wouldn't classify a Fender Bassman as high gain.
A 68k resistor at ambient temperature (300K) generates approximately 3.4 microVolts of Johnson noise in the audio band [reference: Merlin Blencowe, 'Designing Tube Preamps for Guitar and Bass', 2nd edition, 2012, page 65].
Whether this small noise source is a problem, or not, depends on the overall gain of the amplifier. For high gain amps it can be a serious problem, although I wouldn't classify a Fender Bassman as high gain.
I would disagree - just because a resistor is in series with the grid doesn't make it a grid-stopper - if they wanted a grid stopper they would have fitted one - as far as I'm aware (based on similar designs, and never having seen this specific model) the 68K's are mounted close to (or even ON?) the jack sockets, preventing them working as a grid stopper (which needs to be close to the valve pin to be effective).
I agree with you that a grid stopper is more effective if it is fitted at the tube socket. But it is still pretty effective if it is mounted at the jack. You still have the low pass filtering of the resistor combined with the Miller capacitance of the grid. You now also have a tiny bit of series inductance due to the wire from the jack. This bit of wire can also act as an antenna of course, but sometimes the connection from the jack to the tube socket is screened to reduce interference pick up. I'm not sure about the exact layout in this particular amp.
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