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Messages - g1

#1
Quote from: saturated on February 02, 2025, 05:43:03 AMAt night in the small town it got quiet with the exception of an audible hum  :grr
Sometimes I would have the windows open I was in an upstairs room at the end of the house.  This (real or perceived) hum/pitch/monotone frequency tormented me to some end.  In fact I made a few nocturnal excursions around town looking for it.  I would drive around to different areas and shut the engine (slant six 66 Dart) off and sit and try to determine where it was coming from.
Spoiler alert: I never did find it or find out anything.
Tinnitus is not always a ringing and can be a hum.  I have it at 104 Hz.
Most of the time I don't notice, but sometimes it gets bad.

Sometimes the hum can be external though.  For a long time there were people driving around in Windsor, searching for the hum, like you.   :)

https://www.detroitnews.com/story/news/local/wayne-county/2020/07/28/windsor-residents-rejoice-after-mysterious-hum-goes-silent-after-decade/5529975002/
#2
Amplifier Discussion / Re: Ampeg PF-500 Fault
January 30, 2025, 05:04:30 PM
Quote from: gerry113377 on January 30, 2025, 11:02:00 AMthe "pad" became separated from the "through hole". Did I say that right?
'Through hole' in general refers to the older system with holes in the boards.  The components are called 'through hole' type, which means they have the type of leads that go through the holes in the board.
The modern technology is SMT or 'surface-mount' where there are no holes in the board and the component leads are flat.

With through-hole system, when the board is double-sided there are the little pipes connecting the traces at top and bottom sides of board.  They can be called 'plated through-holes', 'vias', or 'feed-throughs'.
When the board is single-sided, the pads don't connect through, they only connect to the component lead on the trace side.
#3
Amplifier Discussion / Re: Ampeg PF-500 Fault
January 28, 2025, 02:13:58 PM
Quote from: gerry113377 on January 27, 2025, 03:48:56 PMBoth sides of the board are different circuits, so when I soldered in the mosfets, the solder didn't make it to the otherside
The 2 sides of double-sided boards are connected by 'feed throughs' or 'vias' which are like tiny pipes at the through holes where the component legs go.  They are not just connected by solder wicking through.
What often happens is the vias get pulled out along with the component legs when the parts are removed.  It can be very difficult not to tear them out unless you have a vacuum de-solder station or use something like 'chip-kwik'.  If you look close at the parts that were removed, you may find the vias still on the legs of the old parts.
Sometimes you have to repair the board by running tiny wires through the through holes and connecting to the traces on either side. 
Sometimes you can get lucky with the solder connecting to both sides, like you did.   :)
#4
Quote from: argenta on January 14, 2025, 10:06:11 PMwhen testing for shorts between the transistors and the heat sink I observe the following:
The meter reads short circuit initially, then over a few seconds resistance increases to open. Why is that?
Those transistor collectors are connected to the supply rails.  The heatsink is grounded to the chassis.  When you measure, you are measuring from supply rail to ground. 
What you are seeing is the meter charging up the supply rail filter caps.
#5
Tubes and Hybrids / Re: Another standby switch thread
January 14, 2025, 01:41:37 PM
Yes, maybe it would be better if it was called the 'break-time' switch.   :)

I understand what Merlin says about cathode stripping being a myth, but I prefer to use the switches as prescribed by the manufacturer for turn-on and turn-off.
He gets very adamant about his anti-standby stance and it ends up putting me in the "methinks he doth protest too much" zone.

I have suspicions about tube warm up possibly being involved with tubes going microphonic, but have absolutely no hard data to back that up.
#6
Your meter should have a spec for the AC volts range as far as frequency response.
My true RMS meter says 45Hz to 2Khz.  A lot of them are just for measuring AC volts in the 60 to 100Hz area.
Yes, scope is best bet for higher freqs.
#7
As a general rule, if it's a cap issue the hum should be 120Hz.  60Hz hum would be more likely ground issues, as outlined by phatt above.
#8
Tubes and Hybrids / Re: Another standby switch thread
January 05, 2025, 02:03:14 PM
Quote from: saturated on January 05, 2025, 05:56:58 AMAlso it is toob rectified if it matters
Yes it matters.  With solid-state rectifier you do not want to hit the tube's plates with instant DC voltage before the heaters warm up, so standby is a way to deal with this.
With tube-rectifier, the DC builds up slowly while the heaters are warming up, so you don't really need a standby switch.
#9
The T has the metal tab, the TF is fully plastic encapsulated, so no insulator required.
Not sure if the metal style transfers heat better to the heatsink.
#10
Quote from: Paul Marossy on December 28, 2024, 07:18:26 PMSo despite the notion that any old FET should work, this circuit is very picky about what FET it will accept.
Not sure where that was said.  'Any old' would still require it be n-channel jfet with same pinout.
2N5952 has a different pinout than the PN4093 or J113, so it will not work if installed with the same orientation.
#11
I guess it does not have a 3-prong AC plug with chassis connected to ground pin?
#12
Quote from: saturated on December 24, 2024, 02:00:42 PMAfter a little research maybe it's possibly a TDA2030 or LM1875 ?
TDA2030 was what I was thinking as well.  Here is their G10XL schematic which may be similar.
#13
Sounds like phantom power for condenser mics.  Modern stuff uses 48V for phantom.
#14
One thing I noticed on the layout, the fets have the middle leg bent forward, whereas the small bipolars have the middle leg bent to the back.
Since the fets had all been changed out by someone else previously, make sure they are not installed 'backwards'.
#15
Agree with JM that there may be other issues rather than the fets which are just being used as switches.

A jfet is low resistance (or 'on') in a non-powered state (or when no voltage applied at gate).  These are N-channel so they require a negative voltage at their gate pin to turn them 'off'.
Source to drain resistance should measure less than 100 ohms with amp un-powered.
Removing the fet from the circuit will simulate the fet in it's 'off' state (which is high resistance from source to drain).