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A while back I built a TC-15 and installed a VVR (VRM) using a pot with a switch mounted where the power switch used to be. Pics and details here...

http://trinityamps.com/phpbb/viewtopic. ... ght=#11234

Just a heads up to anyone who has (or might) try this. I've blown 2 FETs so far. Not sure why. Both were NTE2377. My hunch is that it is a heat issue. I've ran it hard for several hours with no problems before I had a cab to mount it in. I've noticed that when installed in the cab the FET is located in a corner, behind the PT and as far away from the vent panel on the back as possible. Both times the FET failed were a similar situation. Played for and hour or so, flipped the standby to change guitars, flipped the standby again...nothing.

I'm going to try 2 things. First, I added a cap before the VVR to hopefully help smooth the inrush when the standby is flipped. Second, I'm adding a small fan. Will mount it between the rectifier and power tubes - either screwed to the inside of the back panel or mounted to the chassis itself with some sort of bracket.

I'm open to and would very much appreciate any comments or suggestions.

Thanks!

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Deric®

That's too bad Deric® That was the slickest install of the VXX I've seen

Is the chassis hot around there? Maybe lift the MOSFET off the chassis and use a floating stand-alone heatsink instead. Are you getting good conduction between the MOSFET and the chassis, as in not insulated from the chassis. Too much grease can be a great insulator. Maybe change to one of the other MOSFET's... Vent holes around the MOSFET??
Sorry, just thinking out loud...

Joe G

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Some assembly required,
Brain sold separately...

Are you using some thermal grease?

Another thought Deric®...
Do you have the VXX before or after the standby switch (sorry, not allowed to view pictures from here). I'm not sure what would be worse for a MOSFET, the sudden load from coming off of standby, or the sudden inrush of B+ voltage. Maybe something like a MOV or Transorb would take the spike out...
Still thinking

Joe G

_________________
Some assembly required,
Brain sold separately...

FET is bolted to the chassis. Using the insulator that came with the FET. When I swapped the FET last time I reused the insulator. Will use a new one this time.... just cuz. I am using a very small amount of thermal grease - will make sure and clean up any excess from the previous install when I make the swap. I think I have a small TO-220 heat sink I could add to the outside of the chassis where the FET is bolted. Will also try a different FET (NTE2973) as that's what I have on hand.

Joe G.,
It was set-up like this: rectifier, VVR, standby, first filter cap. I've added a cap between the rectifier and the VVR. I had the same thought - would it better to have a cap before AND after??? Never thought about a MOV...hmmmm... Still, I wonder what part (if any) heat is playing...

FWIW, I've used the same set-up with the pot mounted switch in a Lite IIb and a BN Dirty 18 with no problems. The Lite IIb gets use a LOT and has never had a problem. It's in a combo with (tubes down) but has lots of ventilation.

Thanks for the input guys!!

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Deric®

Use IRFPE50 and our circuit; See resource section for circuit. IRFPE50 Datasheet

Use suffifient thermal grease. I use more than some I imagine.

We recommend: rectifier > standby > VRM > first filter cap

Never had a problem AFAIK. Prototype is still working in lbets 18W. bgroup has one in his TC-15 and so does jrock.

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

Thanks for the reply!

For now, I added a fan, changed the FET to a NTE-2973, added a 22uf cap before the VVR(M) and moved the FET an inch or so further away from the PT.

I noticed that the FET was VERY close to the PT - added about .5" of wire to the leads of the FET to give it a little more breathing room.

The more I think about it the more I'm convinced the problem was heat...

Someone over at Ampgarage suggested using the Thermo-Pad's instead of mica/grease. I had some to try but they are the wrong size....

For now I'm up and running again. Played it pretty hard for about 2 hours randomly flipping the standby... No problems so far. Fan is nice and quiet too.

I need to do a Mouser order so I'll order a couple of the FETs you suggested just in case...

Any thoughts on adding a small heatsink to the outside of the chassis?

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Deric®

What are the pros and cons of the VVR and VRM? Am i right in thinking that the VVR drops too much voltage? What if i want to add a VXX partly to bring the B+ down more than 3v if the plates on my TC-15 EL84 tubes are too high (currently measuring 359v, last night it was 365v, this afternoon 355v).

The orignal VVR dropped aabouyt 30 volts so we modified the design and useda different FET to drop less. Only a few volts on the Trinity version. VRM.

Have not had a problem with the FET we use either , FWIW. I think its higher rated than on the VVR.

BTW, 350 is good for a TC15.

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

So if if i'm measuring 359 and i fit the VRM and drop to 356 that is an acceptable voltage.

Any idea what would cause a drop on Pin 1 of the EF86? the layout states 80v and i'm measuring 45V. All other voltages look ok.

It's a fantastic amp.

356 is high , but manageable. Dont put NOS tubes in there. Install the VVR and dial the volatage to a max of say 350 or drop a 10 - 20 V 5W zener in there to permanently keep the B+ to a max.

45V on the EF86 is a bit low. Check some of the other voltage readings in the resource secion.

How is it sounding?

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

Or should i use one of these rectifiers i tested yesterday which give me lower voltages:

Hallicrafters 5V4: 345v
Raytheon: 5U4GB: 332v
Raytheon 5R4WGB: 326v
Osram U52: 323v

How is the balance maintained though, because i find if i drop the voltage on the EL84 plates, correspondingly the voltage on pin 1 of V1 drops to around 118v. When the EL84 plates are 359V with the Mullard GZ34 pin1 V1 measures 132v - which is the lowest voltage recorded on the voltage charts in the Resources section.

I am not sure what is causing he weirdness of the voltages.

1. My Trinity MT measures 301v on the red secondaries and i am 240v mains voltage in Sheffield, UK
2. I used 22uF@63v instead of 25uF@25v

Is the zener often referred to dropped in line a regular zenor diode, or the bolt in chassis type? I'm really sorry to ask this as it will reveal the depths of my ignorance, but which way round does it go?

So, my choices:
Different recto?
VMR?
or
Zenors

I have no idea what is causing the low voltage on Pin1 of the EF86. I will do complete tests today.

How does it sound? Utterly fantastic, the best amp in my camp. It does not do clean though.