It wouldn't.
What are the voltages surrounding that gains stage in question (plate voltage, cathode voltage)?
Sorry, but I would completely disagree - if you feel the need for a grid stopper it needs fitting close to the pin - if not then don't bother
I wouldn't expect either to improve the hiss - but try connecting a 68K from grid to chassis (where you shorted it out previously), does that reduce the hiss?.
You might also try plugging an unconnected jack plug in either of the sockets, and confirm that it makes things even worse (proving the switches on the sockets are working).
Assuming you have plenty of gain?, you might also try disconnecting one of the cathode decouplers (on pin 3 of the first stage) and see how that affects the gain, and what the hiss is like then.
A agree with Malcolm though, that you don't want a 12AX7 in place of the 12AT7.
Most common problem in such designs are the anode loads (1ooK) though, I can't remember if you've changed them or not?.
Just to clarify my views on an input stage grid stopper:
If you have it connected at the tube socket, it will be able to supress RF that is picked up on the wire from the input jack, which will include interference generated by the amp itself within its own chassis. This is a very good thing and will help to prevent the amp oscillating at ultrasonic frequency.
If the grid stopper is further away, e.g. at the input jack, it will not be able to do the above. But, it will still be effective at suppressing RF which is being picked up by the Guitar.
If you have it connected at the tube socket, it will be able to supress RF that is picked up on the wire from the input jack, which will include interference generated by the amp itself within its own chassis. This is a very good thing and will help to prevent the amp oscillating at ultrasonic frequency.
If the grid stopper is further away, e.g. at the input jack, it will not be able to do the above. But, it will still be effective at suppressing RF which is being picked up by the Guitar.
Why does it need to be original? even the original design has major issues.... Pi input is ridiculously large ....and couplings too small...reason they always sounded far too "bloated"..... .01-.022 input and go .047/.1 couplings is far better
and heck of a lot less negative feedback then you will be good to go xD
and heck of a lot less negative feedback then you will be good to go xD
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The black material that the eyelet and backer board is made from can absorb moisture if the amp is stored in a humid environment and not turned on for a number of years. This can cause noise and DC leakage. Some people call this "Tweed Disease". The cure is to gently heat the board with a hair dryer or expose the board to sunlight for several hours on a dry day.
A lot of guitar amps have poor circuit designs and part choices. When repairing things, it's not rare that I have to correct the circuit design so it won' just blow up again soon. What good is a vintage amp if it's no fun to actually play through? I say if it doesn't show from the outside, then changes aren't going to ruin the value of the amp. Power supply caps are the first thing to look at and probably change in a vintage amp. The AC cord may be a 2 prong which can lead to getting severe shocks when the guitarist touches the microphone with his lips. I'd re-do that if it was my amp. All grid stopper resistors should be as close to the grid pin of the tube as is practical or can cause oscillations. All resistors in the front end circuit should be metal film for less hiss noise. Those apparently didn't exist in those days. It's rare but I've heard of old caps being noisy. Tubes can be noisy. The higher the resistance of any resistor, the more noise it will generate, so where possible I reduce those values (unless it damages the sound). The 12AX7 that was plugged in in place of a 12AT7 would have a few problems. The circuit part values would be wrong for a 12AX7, so bias may be pretty far off, plateload being too low would generate significant distortion, and the higher gain would amplify noise more. Check the schematic to see if the 12AX7 is being used for both channels. Some amps have one more tube in the high gain channel that is not used for the low gain channel. You always want to have short leads on all wiring to minimize the chance of oscillation. If it oscillates at super sonic frequencies, you might not realize it, and that could cause audio band noise generation. Usually more of a sputter sound than a clean hiss. Input jacks should definitely short the input when the plug is pulled out. Many of the parts in vintage amps are often way out of tolerance (especially pre 1960's). It's arguable that most of them should be replaced. Since guitar amps often have gains of over 1000, some hiss will exist. Hope this helps.
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Sorry I've been MIA for a few days.
Back at it.
Swapped in a 12AT7 in the driver position, no difference in hiss on the normal channel.
Also tried a shielded cable from the 68K's to the grid, no go.
Also added a 20K immediately at the grid, no go.
Normal channel pin voltages are as follows:
Plate (1): 210v
cathode (3): 1.5v
Now I'm really starting to suspect the input jacks. The phenolic insulators look...well...wet, to say the least. Maybe they're shorting slightly? I'm soaking them in iso overnight and will see what happens tomorrow when they're dry. Why do the inputs make any noise at all when nothing is plugged into the channel? They should be shorted, thus shorting the grid via the grid stopper/leak resistors, and we should theoretically hear nothing at all. Why is this contrary to reality?
Back at it.
Swapped in a 12AT7 in the driver position, no difference in hiss on the normal channel.
Also tried a shielded cable from the 68K's to the grid, no go.
Also added a 20K immediately at the grid, no go.
Normal channel pin voltages are as follows:
Plate (1): 210v
cathode (3): 1.5v
Now I'm really starting to suspect the input jacks. The phenolic insulators look...well...wet, to say the least. Maybe they're shorting slightly? I'm soaking them in iso overnight and will see what happens tomorrow when they're dry. Why do the inputs make any noise at all when nothing is plugged into the channel? They should be shorted, thus shorting the grid via the grid stopper/leak resistors, and we should theoretically hear nothing at all. Why is this contrary to reality?
Just to recap: if you ground the 'grid end' of the 68k resistor the noise stops, but if you ground the 'other end' of the 68k resistor the noise continues? If that is what happens, it would be natural to assume that the noise is coming from the 68k resistor. However, we seem to have already gone round that loop !
The 'tweed disease' mentioned by Loudthud, could create another resistive path to the grid from almost anywhere. This resistive path would create noise which would be supressed when you ground the grid, but not eliminated when you ground the 'other end' of the 68k. So, if I am right about the symptoms, the tweed disease does seem to fit the bill !
The 'tweed disease' mentioned by Loudthud, could create another resistive path to the grid from almost anywhere. This resistive path would create noise which would be supressed when you ground the grid, but not eliminated when you ground the 'other end' of the 68k. So, if I am right about the symptoms, the tweed disease does seem to fit the bill !
Now I have the 68K right on the screen grid, then grounded. The noise persists, so this rules out the input jacks.
So either the resistor (which is new) is faulty, or I somehow have a resistive connection into the grid-pin? But "tweed disease" is affecting some area of the circuit before the input grid, with my new setup this points to the tube socket being faulty?
So either the resistor (which is new) is faulty, or I somehow have a resistive connection into the grid-pin? But "tweed disease" is affecting some area of the circuit before the input grid, with my new setup this points to the tube socket being faulty?
Tubes and resistors will always generate some noise. Metal film resistors do it the least.
I'd replace every resistor and cap in the input stages (plural) with all 5% or better metal film resistors and polypropylene dielectric caps (the cheaper orange drop caps are fine), making sure that the new plate load resistors have high enough wattage ratings (probably 2 or 3 watt), and the new caps should have more than a sufficient voltage rating since during turn-on (before the tubes start to conduct) the B+ power supply voltage will be significantly higher than when the unit is fully warmed up. And I'd replace the input jacks if they seem at all suspicious or discolored. The plating could be oxidized thereby compromising it's surface conductivity. Pots could be shot too. I've had pretty good luck using Alpha brand pots, which are only about $2 each. I'd spray clean the old ones first with an appropriate "contact cleaner" type product - that might be enough. If they make noise when you rotate them, they definitely need cleaning or replacing. Antique Electronics in Arizona is a good place to get probably all of these parts. Mouser, Digikey, Allied, Newark and others will have them too.
I have seen old tube sockets be bad.
If my above recommendations didn't make a significant difference, I'd consider replacing the tube sockets. It's really hard to be sure my advice is right without me seeing the amp in person. Changing tube sockets can be a bit difficult. You might want to get an opinion from a local service tech to see if the hiss level is really overly loud, and then consider swapping the sockets yourself if the tech recommends it.
Above recommendations are good but I have played and worked on lots of AB165's and the original design should not be causing your excessive hiss.
My advice would bypass the circuitry on the black fiber board (just for the first stage) to see if it is the board leaking current. I have heard of nightmare stories because of "tweed disease".
My advice would bypass the circuitry on the black fiber board (just for the first stage) to see if it is the board leaking current. I have heard of nightmare stories because of "tweed disease".
Sorry if my previous post was slightly confusing! I forgot that the 68k resistor is nowhere near the black fibre board! You are right about the tube socket being a suspect though. The tube itself could also be noisy - I expect you already tried substituting it, but I don't think you mentioned that so far.
I agree with Malcolm to check/replace the first stage tube. Type 7025 or 12AX7WA
is lower noise version and in that position they have been used in Fender amps. For a non-MV type amp the problem is more common, because the subsequent preamp stages are all running full gain and will amplify the hiss in the whole preamp fully, while even if the volume control/signal is low. I noticed a significant drop in hiss level once I went to that type. I also added a master volume on my amp, which really got the noise/hiss level down to a rvery eaonable level at lower playing volumes.
is lower noise version and in that position they have been used in Fender amps. For a non-MV type amp the problem is more common, because the subsequent preamp stages are all running full gain and will amplify the hiss in the whole preamp fully, while even if the volume control/signal is low. I noticed a significant drop in hiss level once I went to that type. I also added a master volume on my amp, which really got the noise/hiss level down to a rvery eaonable level at lower playing volumes.
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