Solid State Amp Advice

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Solid State Amp Advice

Post by Slowy »

This thing is causing me problems. It's not really worth anything but it's a brilliant sounding stage amp; so good that I care about it even though I don't really have any use for it.

Currently, It has quite a loud hum when idling. It's not reverb and it's unaffected by knob twiddling. It can't be heard when playing but in between, it causes looks from bandmates.

Any suggestions on how to fix it? There's a preamp valve for the drive section but as I said, the hum is there regardless of whether the drive channel is selected or not. If it was a little quieter, I might use it more. I have a jam at an outdoors party tomorrow and it would be the perfect amp for this.




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Re: Solid State Amp Advice

Post by NippleWrestler »

Dying filter caps in the power supply maybe.

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Re: Solid State Amp Advice

Post by Slowy »

NippleWrestler wrote: Sat Feb 25, 2023 3:02 pm Dying filter caps in the power supply maybe.
How does one determine this?
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Re: Solid State Amp Advice

Post by MikeC »

Slowy wrote: Sat Feb 25, 2023 3:29 pm
NippleWrestler wrote: Sat Feb 25, 2023 3:02 pm Dying filter caps in the power supply maybe.
How does one determine this?
Replace them and if the hum goes away... You can test them but need some gear to do so. And by the time you open the amp, remove the circuit board to get at 'em and disconnect one end you might as well just replace them all and know for sure.
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Re: Solid State Amp Advice

Post by NippleWrestler »

Slowy wrote: Sat Feb 25, 2023 3:29 pm
NippleWrestler wrote: Sat Feb 25, 2023 3:02 pm Dying filter caps in the power supply maybe.
How does one determine this?
Sometimes visually. They do have a shelf life of about 20 years (+/-5) and begin to dry out or leak or bulge or otherwise go wonky. If you notice a farty soft bass that's also a telltale sign.

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Re: Solid State Amp Advice

Post by Slowy »

NippleWrestler wrote: Sat Feb 25, 2023 4:15 pm
Slowy wrote: Sat Feb 25, 2023 3:29 pm
NippleWrestler wrote: Sat Feb 25, 2023 3:02 pm Dying filter caps in the power supply maybe.
How does one determine this?
Sometimes visually. They do have a shelf life of about 20 years (+/-5) and begin to dry out or leak or bulge or otherwise go wonky. If you notice a farty soft bass that's also a telltale sign.
Nope. Bass is like Tyson.
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Re: Solid State Amp Advice

Post by Jay »

Why leave it so late? Could have checked it out for you in a timely manner :D
Still happy to help off course
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Re: Solid State Amp Advice

Post by Conway »

Needs a new set of tubes. :moresarc:
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Re: Solid State Amp Advice

Post by Slowy »

Jay wrote: Sat Feb 25, 2023 4:24 pm Why leave it so late? Could have checked it out for you in a timely manner :D
Still happy to help off course
It only just came home.
And thank You!
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Re: Solid State Amp Advice

Post by Slowy »

Conway wrote: Sat Feb 25, 2023 4:27 pm Needs a new set of tubes. :moresarc:
Just one but It's Ok; new JJ.
But yeah, wrong on so many levels....
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Re: Solid State Amp Advice

Post by RectifiedAmps »

Make sure all the pot and jack nuts and any other potential grounding points are tight. You can also plug into the return jack and see if the hum disappears - if not it's the power amp causing problems.

There's not much to the power amp in these things - a couple mosfets, an opamp and then the filtered +/- power supply (schematic attached in case it helps). Assuming it's the power amp causing the hum, it'd either be the output transistors are unbalanced (you'd see >1VDC across the speaker out jack) or the filter caps are failing (AC ripple in the DC power supply).
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Re: Solid State Amp Advice

Post by Slowy »

RectifiedAmps wrote: Sat Feb 25, 2023 9:39 pm Make sure all the pot and jack nuts and any other potential grounding points are tight. You can also plug into the return jack and see if the hum disappears - if not it's the power amp causing problems.

There's not much to the power amp in these things - a couple mosfets, an opamp and then the filtered +/- power supply (schematic attached in case it helps). Assuming it's the power amp causing the hum, it'd either be the output transistors are unbalanced (you'd see >1VDC across the speaker out jack) or the filter caps are failing (AC ripple in the DC power supply).
Thanks for that. Tell me, Is it feasible to extract the drive channel and put it in a (probably large) stompbox?
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Re: Solid State Amp Advice

Post by RectifiedAmps »

By ‘extract’ do you mean remove the actual drive channel components, or just reproduce it in pedal form? It’s just a TL072 stage driving both halves of a 12AX7 in succession, followed by a tone stack, so it should be easy to clone as a pedal at least.

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Re: Solid State Amp Advice

Post by AiRdAd »

Slowy wrote: Sun Feb 26, 2023 10:37 am
RectifiedAmps wrote: Sat Feb 25, 2023 9:39 pm Make sure all the pot and jack nuts and any other potential grounding points are tight. You can also plug into the return jack and see if the hum disappears - if not it's the power amp causing problems.

There's not much to the power amp in these things - a couple mosfets, an opamp and then the filtered +/- power supply (schematic attached in case it helps). Assuming it's the power amp causing the hum, it'd either be the output transistors are unbalanced (you'd see >1VDC across the speaker out jack) or the filter caps are failing (AC ripple in the DC power supply).
Thanks for that. Tell me, Is it feasible to extract the drive channel and put it in a (probably large) stompbox?
His Realtube pedal is the preamp in pedal format
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Re: Solid State Amp Advice

Post by FuzzMonkey »

The power filtering caps are probably the cause of the hum given the age of the amp. But it is always good to check those other things first as hardware can work loose over time etc.

The pedal versions of the preamp run off Mains power that is rectified down to 12VDC if memory serves. The pedal themselves can be convert to use a wall wart power supply instead of the cumbersome AC power cord by removing the internal transformer but you still need to use a 15VAC power supply with it instead of something like a 9VDC.

A charge pump / voltage inverter would allow you to use a 9VDC power supply but it depends on the current of the circuit.

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