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Discussion in 'Amp Tech Center' started by itsa68, Mar 16, 2016.
Will inspect again when I return home. Any guesses on which part of the circuit?
That sounds like a grounding issue to me as well. Could be a bad joint, could be a ground loop. Could you post up some high res gut shots? The tone is likely a combination of the 12ax7 in V1 and the lower voltages. Personally, I'd sort out the hum first, then work on the voltages.
I'll snag some good shots tonight. I have heard the term "ground loop" but am unsure of what exactly it means, so I will do some research.
Thanks very much for all the input.
I'm having some trouble understanding the ground loop concept. Could somebody give me an example of how a single amplifier could have a ground loop?
My scheme puts most things to chassis and then connects the chassis to ground prong.
EDIT: this off has been hugely helpful. I've just read through the whole thing and will be updating ground scheme this week. I now see the error of my ways lol
Shots of the amp internally. This week I will be moving the filter caps so that they're geographically closer to their part of the circuit, as well as employing a bus grounding scheme and totally rewiring the heaters. Currently, the striped heater secondary lead is grounded and the solid green is powering the heater pins, which are in turn grounded themselves. After reading, I now realize that this is a terrible way to wire tube heaters lol
A few things jump out immediately- the heater wire is parallel with the signal wires in the preamp section- that will cause hum. Get those separated. Looks like you're using the old fender heater wiring scheme as well - no center tap. You might consider doing parallel heater wire runs with a virtual tap if the PT can support it. That will also help with hum. Make a preamp ground bus and set the grounding point at the input jack. The power section can be grounded where the filter cap grounds are made, and keep those as close to your 'safety' ground as possible. That should get you started. I'm sure there will be a lot more folks chiming in as well, but those are the things that jumped out immediately to me.
Thanks for the confirmation. First thing to do will be employing a virtual tap and correctly winding heater supply. Then providing a better grounding scheme.
In the valve wizard PDF which I linked above on grounding, the author suggested that the best way to layout an amplifier is to place the filter caps as close to their intended stage as possible and to ground ALL components to a central ground bus which connects to the input jack. In your opinion/experience, would it be worth moving my power supply as suggested?
In either case, I intend on rebuilding the heater supply and then stopping to see what (if anything) has improved, for the sake of better understanding practical heater effects
this is a good learning experience for you.
i applaud your effort and success (may not feel like it).
you've committed several layout "sins", it would be very lengthy to go into.
next amp, think of the design and execution as flowing logically from power to output to phase inverter to preamp.
the genius' worked all this out for us many decades ago and we simply execute thusly and enjoy the results.
if you keep chipping away with your plan, that is a good effort (the heaters etc).
if those caps (filter caps) are rated for 450 (they look small thou) try the 5u4 and maaaybe the gz34, but with the gz34 and SS rect you will be possibly riding very close/ over their limit.
edit* was thinking of your unloaded b+ gz34 and loaded 450v caps with some instantaneous voltage buffer are fine i bet
Thanks for the encouragement! My perspective is that this is a learning experience. This amp will be the first of many, and it's a growth experience more than a tone hound thing.
The filter caps are rated at 500v. I definitely checked that out before considering the 5u4
oh what the heck, just to spark some discussion and maybe some differing opinions...
if that were mine, i'd put the rectifier where you have that empty socket by the PT, build the power section in that area. move the 6v6s over two slots to the left world. cover the old 6v6 slots and drill holes and put your tone controls and vol way closer to the 12ax7s maybe in the spots of the old 6v6s. see what i'm sprayin' re: layout??
running the wire to the tone and vol way over by the pwr section is asking for noise.
theoretical et al, and my opinion
i am going to do a ptp ultra high gain preamp in a small box next from the great Merlin B, probably going to be a headache!!
giant kudos to you
I'll be looking out for your build thread!
I appreciate the layout suggestion. I may end up doing so...
Spent some time and totally redid the heater and grounding scheme. Heater is now dual wire with a virtual ground. All grounding flows through a central grounding bus. Noise is reduced CONSIDERABLY
However, I'm still getting some "flarp" when I hit full chords. The sound becomes detuned a bit and sounds like a dead battery with lower strings and neck pickups.
Here's a clip, shot by my lovely wife:
And here's how the amp looks now. Still a few things to edit, but I'm feeling good about today's progress
I'm assuming the problem is a power issue. What should I do now? I also noticed that when I flick the power off and keep playing, the amp doesn't slowly fade out like all other tube gear I've owned. Instead it drops out very quickly
Couple questions, what is that thing the power green is going to? You are running all your grounds to the input jack. Best way to twist wires for filaments is to put the wires in a cordless drill and hold the ends, comes out perfect. Whats that blue switch?
Green power lead is soldered to a circle ulnar connector and attached to chassis. And correct, my bus is grounded by the input jack according to the model provided by the pdf I linked earlier in the thread. The blue mini switch toggles between different coupling cap values.
Now that the grounds are redone, could we get an updated voltage chart? Flarping™ can be a number of issues but lets start with the basics now that a major wiring change has happened.
I went to measure voltages a moment ago and discovered the 2A slow blow fuse had burned. Local hardware place is closed today so I'll pick up another box of fuses tomorrow.
Blown fuse? Do I detect... A clue???
What? You don't have any aluminum foil or chewing gum wrapper handy?
Will a finger do in a pinch?
Edit: this is a joke, don't any dummy try this
Yes, but you have to lick your finger first to aid conductivity.
P.S. Thanks for the disclaimer