Princeton with a VVR problem….can anyone help please?

[MikeJH]

Hi everyone,

Quite a few months ago a number of you helped me with some questions I had regarding my first scratch build – a Princeton using the 5F2-A circuit with solid state rectification. Many thanks for all your help. When I built it I also fitted a VVR using Dana Hall’s current circuit. 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 have tried a replacement pot and that didn’t fix it)

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 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

[Steve Conner]

The VVR should not be installed before the first filter cap. There it's working with pulsating DC. It should be between the first and second caps, where it has a steady DC voltage.

Do you have a link to the schematic of the VVR circuit you're using?

MOSFETs should always have a gate stopper resistor located right at the gate pin. If you brought the MOSFET off the board and mounted it on the end of long wires, then the gate stopper has to come over to the MOSFET end of the wires too. Otherwise it'll oscillate, which could explain your symptoms.

As for MOSFET replacements, there are loads of better choices than the NTE junk. We recently had a thread on this, and the FQPF6N80CT and STP4NK80 were recommended. You can get the first one from Farnell and the second from Rapid (I think). I haven't tried either of them, but from what you've said there, the NTE part isn't really proven to work reliably either.

One part I have been using is the BFC62, that has worked for me reliably for 10 years, regulating B+ in a 50W amp. But it's an old device, I don't know if you can get it any more.

The above are all 800 volt rated parts, for use in higher powered amps. If your amp runs comfortably less than 500V, which the 5F2A Princeton certainly does, then the IRFP460 is another possible choice. You can pick those up on Ebay.

[MikeJH]

Hi Steve,

Thanks for your quick and helpful comments. I am something of a beginner when it comes to understanding the VVR, so I have to admit to building it by simply following the schematic without truly understanding how it works. I don’t have a link to the schematic, but it’s Dana Hall’s current design and I am using the component values that he uses on the ones that he sells. The Mosfet is mounted directly to the VVR board on the one I have built.

I understand what you are saying about locating the VVR between the first and second filter caps; its current insertion point could explain why the Mosfet expired after only a few months. I don’t particularly want to start hacking my turret board about and spoiling the component layout; would it be OK to add another filter cap directly across the B+ input to the VVR board. Something like a 16 or 22uF 500V? (The power supply caps consist of 2 x 47uF followed by 2 x 22uF).

I’ll take a look at the other Mosfets you have suggested – anything has to be an improvement on paying $10 each for the NTE’s and having them shipped from the USA.

Thanks

Mike

[Steve Conner]

Well, since Hall doesn't provide a schematic, I can't offer any advice about his circuit specifically.

Yes, it should be OK to add another filter cap before the circuit, I guess. (But doesn't adding another capacitor spoil your layout more than moving two wires would?)

When the VVR is turned down, the voltage at its input will increase, because the amp circuit is drawing less current. If you put a 500V cap there, you need to check that it doesn't exceed the 500V rating when the VVR is down all the way. (You'd also need a higher rated MOSFET than the IRFP460 in that case.)

NTE essentially buy up lots of surplus semiconductors that aren't good enough to be sold under the proper spec, and rebrand them.

[MikeJH]

Hi Steve,

Schematic below: I built this circuit on a small piece of turret board.



I know what you mean about attaching an extra cap being messy, but I was only going to do it on a temporary basis - if it fixed the problem I would then do the job ‘properly’ and insert the VVR between the first and second caps. I think I may have been a little confused as to what to do with the resistor between the first and second caps – as it only drops about 5 volts I can probably just remove it. Probably simpler all round if I just do it ‘properly’ anyway.

Interesting comment on the NTE products – I guess you live and learn.

Thanks again

[Steve Conner]

Another thing just occurred to me... Does Hall say in the instructions that come with the kit, how it should be wired into the amp?

If it were me, I would put it AFTER the last filter cap. That's how I built my first one years ago.

If I were putting it between the first and second caps, I'd leave that resistor downstream of it. The way Hall's circuit works (I think, as I've never seen it) it doesn't do any ripple rejection, so the resistor will help with that.

Edit: OK, I saw the layout, and it doesn't do any ripple rejection.

[MikeJH]

Dana Hall suggests between the first and second caps, although in an exchange of e-mails I had with him, he did say it should be OK to position it as I have done on a low power amp of this type.

How important is it that this circuit doesn’t have the ripple rejection (I did say I was new to this, didn’t I )?

I suppose the best thing I can do is to insert my VVR between the first and second caps, retain the resistor that is between them, and see if fixes the buzzing problem?

Thanks

[Steve Conner]

I'm surprised he said it was OK to do that. It just doesn't seem right.

The ripple rejection doesn't matter. All it means is, it's possible to design a VVR circuit so that it smooths the voltage out and gets rid of hum. So the VVR module can replace some of your existing filter components: you may only need one filter cap. (Again, that was how mine worked.)

But Hall's circuit doesn't do that, the output voltage is a faithful copy of the input, hum and all. So you need to leave in all of the components that were originally in the amp for smoothing purposes, such as that resistor. And I suggest putting the resistor on the output side of the VVR.

And yes, what you said sounds like the best course of action.

[MikeJH]

Steve,

Many thanks for your help. I'll make the changes in the next day or two and post the outcome back on this thread.

Mike

[MikeJH]

Steve,

Changes made; VVR now inserted between the first and second filter caps, and all buzzing noises have disappeared.

Thanks

Mike

[deadgroovy]

mikejh
I'm thinking of fitting a vvr on my home built Ampmaker PP-18. Could you upload a schematic of your amp with the vvr in place?
Cheers
Steve

[MikeJH]

Hi Steve,

Unfortunately I don’t have a schematic I can upload for you (mine is pretty much hand drawn); I simply inserted the VVR between the first filter cap and the resistor that follows it. The best visual representation I can give is by pointing you to the ‘Ampmaker’ website. Take a look at the schematic for his WF-55 amp. The power supply setup in my Princeton is the same as the one in the WF-55. Using this as your reference then the VVR is inserted between C7 and R12.

For your PP-18 you could take a similar approach. However, I know that many people who use a VVR on the 18 watt amp often apply it to the Phase Inverter and Power amp sections only, as opposed to whole amp. You can get much more information on this from the 18watt.com forum – there are a lot of postings there about VVR implementation. Take a look in the ‘downloads’ section – there are a number of schematics there showing the implementation of the VVR in the 18 watt Lite IIB, which is effectively the same as your pp-18.

Have you spoken to Barry at Ampmaker about this? He’s extremely helpful, but I don’t know if he’s used VVRs himself; but worth asking anyway.


Mike

[deadgroovy]

Cheers, Mike. I've ordered a VVR kit from Hall Amplification and Dana has emailed a pretty comprehensive fitting guide, Plus the info at 18WATT, I think I'll be OK.
I'll post a thread here when it's fitted and let you know how it sounds.

[JoeK]

I've installed a Hall VVR on my 18watt for just the PA PI. I made a small turret board with the extra components and it was based on the info from 18watt. Here's my amp and layout. 18 Watt S111 pictures by kersanov - Photobucket