I'm still quite the newbie and am climbing the steep learning curve, so please bear with me.
I've been struggling to eliminate a nagging low-level 60hz hum in the power amp section of a 1960 Motorola console (left and right channels are SE, the bass PP). So far I've replaced almost all the components except the transformers and power supply resistors. Most significantly, I've reworked the original chassis grounding into a star/busbar scheme, and installed a 3-prong plug for a safety ground. (yes, still hums with ground lifted)
The hum is better, but still there. However, the amp will now motorboat under an odd array of causes:
Going inside the amp, most voltages check out, however the motorboating often commences when the meter probe touches pins in the left channel circuit (most often the plate and cathode of the left el84, but sometimes also the grid of the 12AX7, and rarely the left input jack itself). The motorboating usually stops as soon as I remove the probe, but on occasions where it doesn't, it is sometimes stopped by simply contacting the busbar ground and the chassis with the multimeter probes.
So my suspicions lay with a bad ground and/or a stray, wandering capacitance in the left channel. However, all the grounds appear to check out, the socket pins have been cleaned and retensioned, a suspicious wire to the left OT replaced, tubes substituted, resistors and caps check out. There is no short in the OT. Anything else I should check out that could be the cause? Do I need a scope and/or variac to diagnose?
Schematic and photo below. Left and right channels on the left, the right channel is closest to the left edge.
Thanks for any help!
I've been struggling to eliminate a nagging low-level 60hz hum in the power amp section of a 1960 Motorola console (left and right channels are SE, the bass PP). So far I've replaced almost all the components except the transformers and power supply resistors. Most significantly, I've reworked the original chassis grounding into a star/busbar scheme, and installed a 3-prong plug for a safety ground. (yes, still hums with ground lifted)
The hum is better, but still there. However, the amp will now motorboat under an odd array of causes:
- when no input (separate rca cables) is connected
- with shorted inputs (shorted rca cables)
- when the left and right input cables are separated by any distance near the input jack (which is inexplicably next to the PT, which can induce hum)
- does not motorboat when a long combined input cable is connected to the amp but unconnected at the other end.
- whenever the left channel power tube (el84) or the wire to the left channel OT is wiggled.
Going inside the amp, most voltages check out, however the motorboating often commences when the meter probe touches pins in the left channel circuit (most often the plate and cathode of the left el84, but sometimes also the grid of the 12AX7, and rarely the left input jack itself). The motorboating usually stops as soon as I remove the probe, but on occasions where it doesn't, it is sometimes stopped by simply contacting the busbar ground and the chassis with the multimeter probes.
So my suspicions lay with a bad ground and/or a stray, wandering capacitance in the left channel. However, all the grounds appear to check out, the socket pins have been cleaned and retensioned, a suspicious wire to the left OT replaced, tubes substituted, resistors and caps check out. There is no short in the OT. Anything else I should check out that could be the cause? Do I need a scope and/or variac to diagnose?
Schematic and photo below. Left and right channels on the left, the right channel is closest to the left edge.
Thanks for any help!
Attachments
Which amplifier is motor-boating? If the main amplifier is, replace R5 with 10k and replace C3 with 1M2. That will give the 1st part of the ECC83/12AX7 some grid bias as it is floating at present and will be unstable.
It's the left channel amp (at the bottom of the schematic) that's the apparent problem. The bass PP amp appears to work fine, though it does pick up the 60hz hum once warmed up.
One other thing about that left channel: when the 12ax7 (v1) is removed, there is no motorboating, but there is a nasty hissy noise from the left speaker.
Another small detail: I've been using 5751's (JJ) instead of 12ax7's (previously Sovtek LPS), in attempting to tame a rather loud amp. Has no effect on the motorboating.
One other thing about that left channel: when the 12ax7 (v1) is removed, there is no motorboating, but there is a nasty hissy noise from the left speaker.
Another small detail: I've been using 5751's (JJ) instead of 12ax7's (previously Sovtek LPS), in attempting to tame a rather loud amp. Has no effect on the motorboating.
Just add 100k resistors in parallel with each RCA input jack, it will solve the problem with bias of the bass channel.
Oscillations can be caused by wires from output transformers too close to input wires.
And since the amp is with feedbacks, you can't tame the loudness changing tubes.
Oscillations can be caused by wires from output transformers too close to input wires.
And since the amp is with feedbacks, you can't tame the loudness changing tubes.
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Sounds like high frequency oscillation. Add grid stoppers.
Replacing almost everything probably changed the stray capacitance enough to tip it over the edge. Changing the grounding may or may not have helped too. Better to diagnose a fault before fixing it.
Replacing almost everything probably changed the stray capacitance enough to tip it over the edge. Changing the grounding may or may not have helped too. Better to diagnose a fault before fixing it.
I'm confused: is my problem actually in the bass channel, when all the motorboating symptoms are in the left channel? Or are suggestions for the bass channel addressing only the hum?
I did try some 470k grid stoppers on the 12AX7 grids; it motorboated immediately upon power-up. Should they be on the el84's instead? what resistance?
I did try some 470k grid stoppers on the 12AX7 grids; it motorboated immediately upon power-up. Should they be on the el84's instead? what resistance?
470k is not a grid stopper, except possibly in a guitar amp; it will create enough phase shift to create new stability problems. Try 4.7k or something similar on all valves.
A 10k is more suitable as a 12AX7 grid stopper. The 470k would be suitable from the 12AX7 grid to ground.
Output tubes use more like 1k grid stoppers.
Sometimes it is better to add small caps from grid to ground instead of resistors-stoppers. Try something like 33 pf.
If it is motor-boating is that not a low frequency? It looks like the right and left el84's have different cathode bypass caps. The schematic has 8 uF as a value there if you went higher in value it would increase that channels low frequency response. Try disconnecting the cathode bypass cap on the left channel.
Symptoms which vary as you move wires around or change what is at the far end of an input cable are a clear sign of RF instability. RF in an audio amp rarely shows itself as RF (particularly as most audio people don't know what to look for) but shows itself as bias shifts, hum, distortion etc. I suspect that what he is seeing as motorboating is actually squegging of an RF oscillation.MelB said:
Thanks, Wavebourn, the 33pF's did the trick.
The 60hz hum is still there in the bass channel... I'm afraid in my frustration to banish it I was wishfully thinking the motorboating was a related issue, since I suspect a scope would show it on the B+. Back to the drawing board with that...
The 60hz hum is still there in the bass channel... I'm afraid in my frustration to banish it I was wishfully thinking the motorboating was a related issue, since I suspect a scope would show it on the B+. Back to the drawing board with that...
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