Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: plumcrazyfx on February 11, 2023, 11:40:56 am
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I have a late 61 6G6A Bassman the has a history of farting out or having the sound receed then come back when hitting the strings hard. I've decided to finally track down this ongoing issue and started by comparing my voltages to those from the original layout. A further note- this has had a cap job replacing the filters and cathode electrolytic (F&Ts and Spragues in the last 2-3 years) All of my voltages are dead on (like 2-4v within schematic) except the following. Is there anything that directs me to the issue with the following voltages: 242v on Pin 1 of V3 instead of 260v, 220v on Pin 6 of V3 instead of 235v, and 1.19v on Pin 2 of V2 instead of 1.3v. All of the coupling caps are original blues except for a couple .25ufs that are original Astrons. Thanks for any help.
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Posted in the wrong board
Moving
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Power off, measure Ohms from each grid to ground. Which one is way off?
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On the V3 12AX7 or all of them?
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Here's what I have:
V1 Pin 2 33.3k, Pin 7 no reading
V2 Pin 2 3ohm, Pin 7 991k
V3 Pin 2 33.3k, Pin 7 58ohm
V4 Pin 2 967k. Pin 7 997k
V5 Pin 5 260k
V6 Pin 6 265k
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I have a late 61 6G6A Bassman the has a history of farting out or having the sound receed then come back when hitting the strings hard.
Does this happen on BOTH channels?
242v on Pin 1 of V3 instead of 260v, 220v on Pin 6 of V3 instead of 235v
What is the voltage at point "A" on the schematic? That 27K 1W resistor under the doghouse may have increased in value.
and 1.19v on Pin 2 of V2 instead of 1.3v
Pin 2 should be zero volts. I suspect you mean pin 3? Anyhow, 1.19 or 1.3 is so close it doesn't even raise a brow.
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Mostly on the Bass channel but lately been noticing it on both but definitely worse on the Bass channel.
I had to look under the doghouse but I did replace the original dropping resistors with modern 3w metal film resistors. Point "A" on the schematic listed at 390v measures at 379v.
Yes I meant Pin 3 for the 1.19v reading.
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Contemporaneous actual measured mains and heater V AC provide essential context to circuit voltage measurements.
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The wife decided to replace a ceiling fan so I had to flip the breaker. Unless I want to move this whole rig I'll have to get the voltages later. Before shutoff, I did have it plugged into a Brown Box and the reading on the meter was 120v on the power cord.
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Contemporaneous
Damn! Had to look it up... :dontknow:
adjective
existing or occurring in the same period of time.
"Pythagoras was contemporaneous with Buddha"
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Well I knew what contemporaneous means. But I was damn astonished by a wife who replaces a ceiling fan while the husband fools with a guitar amp!
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Okay, rechecked my voltages now that the fan is up:
119v out of the wall
Red wires on power transformer to ground 372v/369v
After rectifier diodes- 452v vs schem 470v
Heaters 3.30v/3.28v
V1 Pin 1-157v, Pin 6-252v vs schem 160v/250v
V2 Pin 1-167v, Pin 6-166v vs schem 170v/170v
V3 Pin 1-239v, Pin 6-218v vs schem 260v/235v
V4 Pin 1-313v, Pin 6-304v vs schem 330v/320v
Point A- 375v vs schem 390v
Test Point at jct of 100k/100k on V2- 254v vs schem 250v
Did I miss anything.
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I don't see any alarms with your voltages. V4 voltages are approximately 15v low but that's because point A is 15v low. Not really a problem IMO but if you want the voltages to be closer to the schematic values, replace that 27K in the doghouse with a lower value, maybe 24K or 22K.
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Hmmm, so what are next steps to track down the huge sag on loud notes? Especially on the Bass channel.
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I did some additional checking. I have 21v on the Treble pot of the Bass channel.
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I did some additional checking. I have 21v on the Treble pot of the Bass channel.
That's normal for that amp.
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So, my next step is probably to start testing coupling caps for leakage?
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Did I miss anything.
Possible conductive board.
Probe board material in different areas looking for stray voltage where there should be none.
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Hmmm, so what are next steps to track down the huge sag on loud notes? Especially on the Bass channel.
Huge sag may indicate a problem with HT supply to the output stage.
I suggest to measure the voltages at the reservoir and screen grid HT nodes, at idle and with the amp putting out full power.
The reservoir cap voltage can be measured at either standby switch terminal, or on the circuit board.
The screen grid HT node voltage is noted on the schematic as ‘B’, it can be measured at the appropriate terminal on the circuit board, or at terminal 6 of either output valve socket.
https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_6g6a_schem.pdf
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The thing that is unusual is that that huge sag is primarily on the Bass channel. I don't have the experience to not say it's something like blocking distortion. You keep the volume lower or hit the strings softly, you don't get the sag as much. It's worse when you hit the string hard, or feed it something with bigger bass content. It's almost like a compressor pedal turned way up that it cuts the attack and then releases. The Normal channel has far less of this behavior. I did consider the possibility of a conductive board but would like to rule out other potential issues before settling on that.
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Just updating this in case someone in the future searches this. I did some further investigation and it appears that this behavior might be normal on 6G6 and 6G6A Bass channels. It appears the fix may have been implemented in the 6G6B which added a .0005uf cap after the 470k mixing resistors, before the PI which limits the signal from the Bass channel from overloading the PI. This happens more with the Bass channel as it has 2x 12AX7'S vs the Normal channel's 1x 12AX7. This will be my next step.
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FYI- Made these changes and added 1.5k grid stoppers and it fixed 95% of the issue.