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Hi everyone,

My first post on the Trinity forums - I’ve been reading/lurking here for some time. Lots of interesting and useful content and some really helpful people. I am hoping some of you might be able to help me with a problem I am having?

While you were all waiting for the Tramp to become available I was already building my own small Tweed amp – a Princeton, using the 5F2-A circuit, but with solid state rectification. I am still new to amp building, this being my first scratch build.

I have fitted a VVR/VRM to the amp. I inserted it directly between the standby switch and the first filter cap and am now having a few problems with it.

When the VVR was first installed I had a few noise/buzzing issues, due to some poor routing of some high voltage wires. I sorted it out and it then worked fine for some months. A couple of weeks ago my son told me the VVR wasn’t working and that the amp was making some ‘other’ strange noises. The Mosfet had blown (NTE 2973), no idea why, so I replaced it. Everything is now working OK again, well almost. The amp now has the following strange symptoms and I cannot work out what is wrong.

When the VVR potentiometer is rotated from Minimum to Maximum there is a loud buzz for approximately the first half of the rotation. For the last part of the rotation there is no buzz. The buzz only occurs while the pot is being rotated; if you stop rotating the pot then the buzz stops.

Rotating the pot from Max to Min produces no buzz at all

Rotating the pot from Min to Max VERY slowly prevents the buzz.

All leads/wires are positioned as they were when the original Mosfet was fitted and the VVR was working properly

If anyone has any idea what is causing this, and how to fix it, I would be really grateful to hear your comments.

I would also be interested to know of any Mosfet other than the NTE2973 that is proven to work reliably. I live in the UK and have to source the NTE2973 from the USA which proves to be rather expensive.

Sorry for such a long first post. (If I can get this problem understood and fixed, then I am hoping to progress onto building a Tweed Deluxe)

Many thanks

Mike H

Could be the control pot was underated? If all else is the same, really points to the pot. For a replacement MOSFET, try the IRFPE50pb.

All the controls wire are twisted?

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

All wires are twisted. I hadn’t really considered the pot; the one I am using is made by PEC which I believe is rated at 2 watts, so it should be OK. I’ll try another pot anyway and see what happens.

Thanks

Mike

I tried a new pot (Alpha 1M Linear) and it made no difference. Can anyone think of anything else that might be causing this buzzing problem? The VVR is inserted directly between the standby switch and first filter cap, I am assuming this is OK – or could it be causing the problem?

Thanks

Mike

Got me on that one! See if Dana Hall has any ideas.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Fixed It! I repositioned the VVR between the first and second filter caps and that cured it. When it was in its original position, between the standby switch and first filter cap, I can only assume the unsmoothed DC was causing the buzzing noise.

Interesting. Yet it worked before? Good to know.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps