Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: cobrabari on September 27, 2018, 08:53:03 am
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HI. can someone help me?I have bought a power board and 3 relay boards (Hoffman) to change the low to high channel in my marshall jcm 800 4010. When the whole thing is fed, the channel change is successful, but I have a strong hum with activated relays. The power I take from the V1 of pre. Can it be that the noise is due to too much absorption on the valve? Thank you
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The power I take from the V1
filament power?
does your relay board have a totally isolated ground, (not connected to the amp in any way)?
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clean build, nice.
this is the schematic I'm looking at
https://el34world.com/charts/Schematics/files/Marshall/Marshall_jcm800_4010_preamp.gif
to change the low to high channel
you're switch input's?
un-solder the fil wires from V1, hum gone?
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Hum is also present without an inserted jack
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without an inserted jack
As I would expect
your purpose for switching is switching between inputs, yes? like an A/B/Y footswitch?
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Exactly. It works fine, but i have Hum only when the relay are on!
In my opinion, the transformer can not withstand 120mA (40mA for 3 relays)
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Your inputs are the most sensitive signals in the amp, putting relays at the input just invites cross coupling problems.
I would free up the boards, move them around, shorten cables, use 2 input shielded wire (if you didn't).
I would also consider just an ABY switch for simplicity, keeping designs as simple as possible keeps the gremlins off guard :icon_biggrin:
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I would also consider just an ABY switch for simplicity, keeping designs as simple as possible keeps the gremlins off guard :icon_biggrin:
An external AB/Y switch will not work with this amp. If you look closely at the schematic you will see that if you plug into the high gain input then you cannot plug anything into the low gain input. The high gain signal travels through the normally closed switch on the low gain input jack.
Exactly. It works fine, but i have Hum only when the relay are on!
In my opinion, the transformer can not withstand 120mA (40mA for 3 relays)
I would be very surprised if the transformer cannot handle an extra 120mA of filament current. The "grnd" terminal on the power supply board must not connect to the amp chassis, either directly, or through a switch, or through a footswitch jack. It must remain completely isolated from chassis. IOW, the 5V supply must float. If you connect that "grnd" terminal to chassis you will destroy the bridge rectifier. It only takes about 17mS to destroy the bridge. So, if this is your case, you already need a new bridge rectifier.
If the "grnd" terminal has not been connected to the amp chassis, try this... Disconnect the green and blue wires from the tube socket. Leave them connected to the power supply board. Now connect a 9V battery to the dangling blue and green wires. Polarity does not matter. Does the hum go away when the relays are energized? If so, you will need to use a separate small filament transformer to power the relay circuit.
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look closely at the schematic
:think1: I see it!
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I will try to use the relays after v1a, turning off the v1b by connecting the grid to ground and connecting directly v1a to v2a to have a clean sound.
(jumping Central VR1 -> C7)
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fwiw
You may have "popping" when switching, consider a 1 to 2meg R across the contacts to help/eliminate popping
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Nevermind didn't read the 4010. I'm thinking 2203.
I have 2 original 800s - the volume difference between the two channels is significant. I don't know if just switching will allow for performance volumes on both without adjustments.