MikeC Sixty-One SRT

Amp tinkerers and people who aren't afraid of 700 volts or so in here....

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Marshmallow
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Re: MikeC Sixty-One SRT

Post by Marshmallow »

Hey Mike, What wiring/ voltage rating are you using for your preamp?

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Re: MikeC Sixty-One SRT

Post by MikeC »

Whakanuia o mea kei a koe

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Re: MikeC Sixty-One SRT

Post by MikeC »

An update at last. Tonight I completed the re-build using the new turret board. I've also built/installed the VRM module for power scaling (on the side at the top left). All of the changes, mods and voicing done to "Frankenstein's monster" are now part of this build. I powered her up with only the rectifier & all looks good. Tubes in next - watch this space!
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Re: MikeC Sixty-One SRT

Post by MikeC »

Valves all installed & she's running - yay! Voltages all good too. The only issue is a bit too much hum coming thru the speaker (50Hz) which might just be the VRM. My next job is to sort the hum out.
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Re: MikeC Sixty-One SRT

Post by codedog »

This is exciting stuff!

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Re: MikeC Sixty-One SRT

Post by MikeC »

codedog wrote: Thu Jun 15, 2023 3:49 pm This is exciting stuff!
And frustrating!
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Re: MikeC Sixty-One SRT

Post by codedog »

MikeC wrote: Thu Jun 15, 2023 4:38 pm
codedog wrote: Thu Jun 15, 2023 3:49 pm This is exciting stuff!
And frustrating!
Yeah, I figured it might be for you. But any progress is exciting to hear.

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Re: MikeC Sixty-One SRT

Post by Jay »

MikeC wrote: Thu Jun 15, 2023 3:47 pm Valves all installed & she's running - yay! Voltages all good too. The only issue is a bit too much hum coming thru the speaker (50Hz) which might just be the VRM. My next job is to sort the hum out.
What's the problem with the transformer mounting? I see you drilled the holes but not using them...

Re hum, I feel your pain. Perhaps redo the heater wiring for the two valves near the input? Twist them much tighter and split in the middle of the valve socket, not earlier. Also run them down on the chassis as much as possible.
When faced with quality, I recognise it every time.

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Re: MikeC Sixty-One SRT

Post by MikeC »

Jay wrote: Thu Jun 15, 2023 6:09 pm
MikeC wrote: Thu Jun 15, 2023 3:47 pm Valves all installed & she's running - yay! Voltages all good too. The only issue is a bit too much hum coming thru the speaker (50Hz) which might just be the VRM. My next job is to sort the hum out.
What's the problem with the transformer mounting? I see you drilled the holes but not using them...

Re hum, I feel your pain. Perhaps redo the heater wiring for the two valves near the input? Twist them much tighter and split in the middle of the valve socket, not earlier. Also run them down on the chassis as much as possible.
The chassis is for a stock 18W TMB amp and is pre-punched/pre-drilled from the factory in China. It was the cheapest chassis I could get and aluminium is much easier to work than steel. So I've had to modify it for a smaller PT, an additional cancap and an extra noval valve base. There's really too much going on in the space and I always presumed that I'll need a purpose built chassis.
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Re: MikeC Sixty-One SRT

Post by RectifiedAmps »

With hum, it’s usually down to grounding issues or the heater circuit. You could try elevating the CT of the heater windings by 20-30VDC. Attaching it to the output valve cathode usually does the trick but in your case I’m not sure how the zener would affect things. It’s an easy thing to try in any case.

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Re: MikeC Sixty-One SRT

Post by MikeC »

Some progress... like many of the older Vox amps I've now got a filter choke across the entire power supply. This adds weight & cost so I'm aiming for it to be an option in the final product. There's just too much going on now so it's time for a purpose built chassis.
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Sixty-One SRT Chassis Build.jpg
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Re: MikeC Sixty-One SRT

Post by MikeC »

Rebuild in new chassis complete. Added a "cut" control which is very useful. Final tuning now.
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Re: MikeC Sixty-One SRT

Post by robthemac »

MikeC wrote: Sat Jul 29, 2023 4:51 pm Rebuild in new chassis complete. Added a "cut" control which is very useful. Final tuning now.
Sing out if it needs to head north for some warm-weather testing.
Jops wrote: Sun Sep 18, 2022 7:46 am Spring is the comic sans of reverbs anyway.

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Re: MikeC Sixty-One SRT

Post by MikeC »

robthemac wrote: Sat Jul 29, 2023 7:06 pm
MikeC wrote: Sat Jul 29, 2023 4:51 pm Rebuild in new chassis complete. Added a "cut" control which is very useful. Final tuning now.
Sing out if it needs to head north for some warm-weather testing.
Noted. I'm gonna need a tester to give it a test tickle! Check if it has balls enough :)
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Re: MikeC Sixty-One SRT

Post by MikeC »

Reporting some great progress on the Sixty-One SRT and it's sounding good to my ear. I'm using a pair of vintage Vox Silver Alnico's for testing (thanks Murky). Tremolo, Reverb, Cut and VRM (Power Scaling) installed, debugged and working well. She does 15W RMS clean and dirty's up nicely too.
Have a look at my control panel design and let me know what you think!
Sixty-One SRT Chassis Topside.jpg
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Sixty-One SRT Control Panels.jpg
Sixty-One SRT Control Panels.jpg (114.59 KiB) Viewed 34597 times
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