Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Diverted on October 13, 2019, 05:18:56 pm
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Hi all,
Just (finished) wiring up a single channel Fender Vibrolux Reverb circuit, single channel. I eliminated the normal channel.
https://schematicheaven.net/fenderamps/vibroluxreverb_aa964_schem.pdf
I am having two issues:
1. The treble pot seems to have taken over volume control duties. With treble at 0, I can crank the volume up and still get very little output. But turning up the treble eliminates that. With treble at 10 the volume control is very responsive and gets loud at 1 or 2. Also, the bass and mid pots are non-responsive regardless of where treble and volume are. Bright switch is working. I have gone over the wiring for this section five or six times and all looks OK. Have also swapped a few tubes and still it persists. Any ideas on where to look?
2. The tremolo is non-responsive. I have never done a blackface-style tremolo before (have always build bias-shift tremolo amps) so I'm not sure how to start troubleshooting this section.
Other than that, the amp sounds great! Very clean, super loud and the reverb is good. I've got 400 on the 6L6 plates with bias right around 42ma per tube.
Thanks,
Ted
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Maybe a few photos of the tone controls will help? I pulled out the wires from where they were ticked so you can see what’s going on. See anything I don’t see? Thanks!
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Work on 1 problem at a time.
On most Fender amps, if you turn the tone controls all the way off, very little to no volume at all. All the signal is sent to ground.
But you say mid/bass aren't working correctly, probably a wiring error, or a bad solder joint.
Here's a great way to double check wiring;
http://el34world.com/Forum/index.php?topic=17701.msg178630#msg178630
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Yes, thanks👍👍 I thought it was definitely a wiring error as I reflowed all solder joints in the area and on pots. Apart from that I just can’t see the error. I’ve traced it out a few times on schematic and layout and all looks OK. Kinda baffled to be honest.
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Bad coupling cap, bad solder joint on eyelet board for mid/bass pots?
Bad mid/bass pot?
Wrong value mid/bass pot, coupling cap?
Bad wire, that you can't see where it's broken inside the wires insulation?
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You must have a footswitch plugged in for this tremolo to work.
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Ahh, 100K slope R!
All the mids/bass have to pass through that 100K slope R. The treble will still pass through the 250pF treble cap and treb pot to the volume pot even if the mids/bass are not getting to their pots.
Does the amp have any bottom end or is it just all top end?
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Found the issue with the tone stack. Somehow, and I have no idea how, the lug that creates the junction of the .1 into the bass pot wiper was grounded underneath the board. Weird; I pulled the pot lead and the .1 and put a turret in that spot on the eyelet, and all is good (see pic).
As for the tremolo I did end up grounding the tremolo. I am now hearing a very quiet thumping when the anp’s At idle so I know it’s oscillating, but adjusting the intensity pot doesn’t have any effect and there’s no tremolo happening. Hmmm.
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Still trying to locate the issue with the tremolo. I am getting some very weird numbers but not having seen one of these tremolo circuits before I don't know what's what. Here's what I'm getting.
1. With tremolo on I can hear the tremolo pulse quietly, but it's not going over to the guitar signal. It dies out when I ground the circuit at the footswitch. When on I can see the neon pulsing with the rate of speed.
2. When I rotate the intensity knob clockwise, the amp's overall volume drops until I get to 10, when it goes silent. At 1, the amp is loud.
This is the case with tremolo on OR off.
Voltages are weird:
Tremolo ON
P1: Fluctuates from 237-255.
P2: Steady 325 or so
K1: 1.7
K2: Low, around 90mv
Tremolo OFF
P1: 385
P2: 340
K1 and K2: Getting minimal, minimal voltage
Scratching my head here.
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When on I can see the neon pulsing with the rate of speed.
That means that 99% of the circuit is working just fine. The only things left are the INT pot, LDR side of the roach, or wiring error around these two components.
When I rotate the intensity knob clockwise, the amp's overall volume drops until I get to 10, when it goes silent. At 1, the amp is loud. This is the case with tremolo on OR off.
This tells me that the wiper of the INT pot is grounded. Look for a short at the roach LDR wires or at the INT pot, or the wire connecting the pot to the board. Of course the LDR could be bad (shorted) but I've never seen on that failed shorted.
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Sluckey,
Some day this will become intuitive enough to me that I will be able to determine these kinds of things by looking at the symptoms.
Until then, again, you are a lifesaver. Stray strand at the eyelet that I could not see was grounding out the other half of the resistor side of the bug. Sucked out the solder, reattached leads and it’s operating properly. Again,
Many thanks for your invaluable help👍👍👍
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> the lug that creates the junction of the .1 into the bass pot wiper was grounded underneath the board.
> Stray strand at the eyelet that I could not see was grounding out the other half of the resistor side of the bug.
You gotta keep a close watch on those stray wires and accidental touchings.
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> the lug that creates the junction of the .1 into the bass pot wiper was grounded underneath the board.
> Stray strand at the eyelet that I could not see was grounding out the other half of the resistor side of the bug.
You gotta keep a close watch on those stray wires and accidental touchings.
Got that right. Hours and hours of torture for a little whisp of wire :laugh:
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Hours and hours of torture
next time just up the B+ til you see sparks :icon_biggrin:
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next time just up the B+ til you see sparks :icon_biggrin:
:huh: :l2: