Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Archives of favorite topics => Topic started by: tubenit on June 06, 2009, 09:31:35 pm
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PLEASE scroll down this thread to the information about Power Scaling and Kevin O'Connor. This is information that I learned 12/31/11 and wanted to be sure that the Hoffman forum guys had.
I added a VVR to my 56T reverb. It works fantastic! Not very hard to do but took a little time. Maybe about 2-3 hrs because of moving some parts over and drilling new holes, etc.
It sounds fabulous to me. It really does keep the tone at lower volumes & maintains the sweet harmonics and touch sensitive nature.
I'm very pleased with it.
This is what I finally came up with. You can compare it to the original 56T reverb to see how the VVR was inserted and what changes are needed on the B+ rail ........... such as adding some more filter caps and another dropping resistor and 1N4007 diodes.
with respect, Tubenit
I also included some other schematic examples.
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Here is a link to a website that sells VVR's for cathode and fixed biased amps:
http://hallamplification.com/main.html?src=%2F#2,2
Based on what I read ........ it appears that a 12v zener is recommended over the 6v zener. And NTE 2973 is recommended.
I read different explanations about whether the VVR needed to used with a heat sink or insulated from chassis or attached to a aluminum chassis.
As typical worst case dissipation will be around 30W or so, the Mosfet can be chassis mounted using an insulator (mica + thermal grease, or, better, composites such as Keratherm or Silpad w/o grease) and insulated hardware. Using a dedicated heatsink, say a 3°C/W model or better, would be safer.
you do have to isolate the case (the drain) from the chassis ground with a mica insulator
I didn't find these devices to generated all that much heat even after playing for a while but since heat kills these things, you still need some heat sinking to get rid of any heat generated. The aluminum chassis works well enough; on the steel chassis, I used a 2"x3" aluminum finned sink recycled out of a auto stereo
I have put fixed bias VVR to two Plexi Lead amps and bolted mosfet straigth to aluminium chassis with isolator only and without any heat sinks. They still work smoothly without problems.
As well as the mica, or other, washer you need to use either a nylon nut/bolt or the appropriate mounting kit - usually a little plastic bush that stops the bolt from touching the face and inside of the hole through the tab. Depending on the manufacturer/supplier the mosfet may come with the washer and bush or you may have to buy separately.
I always use some Loctite (thread glue) on the steel nut and bolts, (with bolt spacer and mica pad), that I use to fasten the VVT MOSFET to the amp chassis.
It is important that that bolt remains tight enough to avoid moving and shorting the MOSFET. Also, I always try to remember to check that no swarf, stray solder or wire that inevitably finds its way into a chassis, is present so that it cannot short across the side of the MOSFET to the grounded chassis
I like to put the compound directly on the transistor and use a toothpick to spread it to a thin and even coat. Then put the mica insulator on the transistor, sliding it slightly to get good contact with the transistor. Then I put another thin and even coat of compound on the clean side of the mica insulator. I don't actually put any compound on the heatsink. In fact, I clean off any old compound. Now just install the 'buttered' transistor to the heatsink. Using this method produces good heat transfer and the cleanest install for me. There are plenty of other ways to do this though.
I like the white silicone paste compound best. It can be messy if you aren't careful.
I found some schematics for a VVR and cut and pasted them into one page below. Check for errors!
With respect, Tubenit
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I did not use the "VVR" design but KoC power scaling design. I just bolted it to the aluminum chassis. Had it in 4 amps with lots of gigging. The aluminum chassis worked fine as a heat sink. Yes, I **had** to mount an isolator (with thermal grease) or it would be very bad news. I wired it to the B1 and B2 of the power section and phase inverter. Worked fine.
The second schematic looks pretty good for controlling power section and PI for cathode bias.
I think LC uses his on a bit more than the power section/phase inverter, I am sure he'll chime in here.
ToN
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More explanation I found about heat sinking and isolating the VVR. However, I don't understand the part about isolating and using conductive grease ????
The easiest, cheapest, and most rugged method is to mount the mosfet to the chassis. Orient / align the chip where you want it on the chassis. Sand, grind, scrape any paint or powder coat from the chassis where the chip will be mounted. Paint and powder coat do not conduct heat well. Bare metal will serve as a better conductor.
Remember... the metal back side of the chip (on most mosfets) will be live with the B+ voltage so it needs to be isolated from ground and other components.
Drill the mounting hole for the chip. File or sand any burrs or edges raised while drilling. A sharp metal spur can pierce through the mica and short out the chip (= smoke = smell = 8 more dollars). It may also cause some buzzing issues if it doesn't cleanly short out, so sand it flat.
Use the mica insulator that comes with the chip and some conductive grease. Fasten the chip to the chassis an appropriately sized bolt, nut, and lock washer. Plastic fasteners are nonconductive and reduce the risk of a short.
Wire the chip into your circuit.
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electrical isolator
thermal conduction grease
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I got these thermal pads from Mouser. They are an electrical insulator and an thermal conductor. I can't remember what they are called but they solve all the problems with ease.
I use a small heat sink on my VVR/Powerscale projects push/pull amps.
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With Loose Change's help ....... I have put together a list of parts to order to make this VVR work.
Note that some of these parts you can easily get from Hoffman when you place your next order like the diodes, pot, resistors, filter cap, terminal strip, etc.......
I have NOT built this ......... so I can not say for sure that all the part numbers are correct. But I'd thought I'd share this for others to either comment on .......... and/or correct. Be sure to thoroughly read this thread and study all the schematics.
Also note that I don't list the quantities on the parts list. For example: three 1N4007's are used, several 220k's etc......
The parts list is for the 56T reverb I built based on DaGeezer's design. See the schematic posted at the start of the thread.
*EDIT: Loose Change has used IRFP460 in his VVR's with no problems. Mouser has'em for $2.92 for the Vishay. Another forum member mentioned using Mouser: 844-IRFP460PBF ($2.92) and Mouser: 512-IRFP460C ($5.44)
DaGeezer referenced:
http://www.mouser.com/Search/Refine.aspx?Keyword=IRFP460
http://www.mouser.com/catalog/specsheets/91237irf.pdf
with respect, Tubenit
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this (http://www.st.com/stonline/books/pdf/docs/9663.pdf) device seems to be a suitable replacement for NTE2973. slightly higher power diss.
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A forum member installed a VVR in a cathode biased 30w HoSo56 type amp and had some oscillation problems. This was a response by another forum member who had experienced that and what their resolution to that was.
Try this, loosen the small nut holding the mosfet to the chassis, just a little and I bet the noise issues go away. I know you used the insulator but don't use any heat sink compound. Sometimes the compound leaks out on the screw maybe and causes a very small short.
But whatever it is, tighting the nut too tight causes these oscillations. I've had this several times before and it drove me nuts. Loosen the nut, just enough to have a flat contact, get it to the point of just eliminating the noise and a little looser. If it still makes noise, just let it hang loose for testing. And go from there.
IN some email correspondance with Dana Hall regarding using his VVR for cathode biased amps, he stated the following:
I don't use any heatsink grease on any of mine. There are different kinds of heatsink grease and some contains metalic particles (conductive) that can short out the mosfet if it oozes out and touches the chassis. I just make sure the mica insulator is installed properly between the mosfet and the chassis and all should be good
I am using his VVR in my 56T reverb without any conductive grease and it seems to be working just fine.
Tubenit
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Here's a link to a thread that has some good "trouble shooting" info & such for the VVR.
http://www.el34world.com/Forum/index.php?topic=7413.0 (http://www.el34world.com/Forum/index.php?topic=7413.0)
Also, it has been suggested (by member FYL) that it is important to place the 100k "gate stopper" resistor as close to the "gate" of the mosfet as possible in order to decrease the chances of oscillation problems.
Some of the designs I have seen have the 100k stopper placed very far away from the gate........
Geez'r
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Here's a pic of Dana's version (populated / bare board). Note where the gate stopper R2 is located.
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For those wishing to design their own boards, here's a schemo and layout of a basic VRM version. The pdf is an X4 hard copy of the board, the zip includes usable sch and brd files.
Misc:
Use Cadsoft Farnell Eagle (the free version is fine), ExpressSCH won't understand the .sch file.
Layo is double sided but can be modified for single side as no tracks cross.
Mounting holes are designed for M3 metric spacers.
Even if 500V design rules have been used, the board should be varnished in order to minimize hazards.
Small R's are 0309/12-sized, with a max body length of 9 mm and 12.5 mm with leads properly bended, OK for Vishay PR01 1-watt MF.
Replace R1 and R2 with 100K models if you want to stick to UR12's schemo and values.
An IRFP460 is spec'ed for Q1 but many others are compatible. Use at least a 500V/8A part suitably heatsinked.
Use a 1ML/500V pot, preferably from RV4 or K series.
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More links to discussion threads w/ good info on the VVR:
***Pot for VVR?>>> http://www.el34world.com/Forum/index.php?topic=7779.0 (http://www.el34world.com/Forum/index.php?topic=7779.0)
***help, trouble with VVR >>> http://www.el34world.com/Forum/index.php?topic=7602.0 (http://www.el34world.com/Forum/index.php?topic=7602.0)
***Mosfet options>>> http://www.el34world.com/Forum/index.php?topic=7709.0 (http://www.el34world.com/Forum/index.php?topic=7709.0)
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I thought I'd post a few photos of VVR installations. Two of these are Dana Hall's VVR on my amps. One is on the 56T
(HoSo56) and the other is on my T-Lite 5879 OD with 5881's. Note that on the T-Lite that I needed to add some wire to position the mosfet correctly.
The other photo is an 18w built out of an old Hammond organ chassis. The VVR is built on to a terminal strip. Again in this photo , there is wiring going to the mosfet.
It appears that the 18W VVR is similar to mine in that the chassis is serving as a heatsink to the mosfet.
With respect, Tubenit
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FYL provided a photo of a turret board VVR. And I also have attached a photo with schematic.
Drew provided a close up photo of his terminal strip mounted VVR. He commented that if he had to do it over again, he would consider a smaller PCB version.
With respect, Tubenit
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Terminal strip photo (edited) compared to terminal strip VVR drawing.
EDIT: I added a photo of a VVR in a higher wattage tweed cathode biased amp. Notice how wires were extended to
be able to place the mosfet in an open spot on the chassis used as a heat sink.
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The Zener should be a 12v Zener and I connect it directly to the Mosfet (left side of that big resistor). Hmmm.
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I used the zener that Dana was recommending at the time (which was back when the VVR was just beginning to get popular).
The first of the two attachments to the second post in this thread has my drawing at the 6:00 position, but someone has added "S", "D" and "G" labels to the MOSFET. The "G" leg does connect directly to the Zener (at the first [top] terminal of the tag strip).
I couldn't tell you how these things work if my life depended on it; I just followed the directions that were posted at 18watt.com. The thing does work, as drawn/built; that's all I know.
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I used the zener that Dana was recommending at the time
A 6V zener is OK for a small amp, say a Vjr or a Champ, 12V is required for larger ones.
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18watt w/VVR
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As of 12/31/11, it has come to my attention that Kevin O'Connor feels that the VVR is essentially a copy of his power scaling. Prior to this, my impression was that they were dis-similar enough that there were no concerns about "infringement". Some of my impression of that they were dis-similar was based on comments by KOC, himself.
Reducing the power output of the power stage *without altering* how it handles signals is quite simple, and any approach used to achieve this IS Power Scaling. There are also simpleĀ ways to control power output which result in *alternate tones* as we dial down; these are simply "Variable Power".
I have not seen or studied the Power Scaling schematic, so I am not certain what is accurate? Nor do I know the law well enough to know what would be infringement or not infringement? The information in this thread about VVR's is stuff I have found from various websites and reposted on the forum. Having said that, I feel it's appropriate to post some information about Kevin O'Connor's Power Scaling as some Hoffman forum members may which to use that instead.
This is from Willabe:
TUT4 (chapter 3, 97 pages) and TUT 6 (chapter 1, 78 pages) have info on Power Scaling, --- lots --- of info, you'll have to see it to believe it, the --- thought --- he has put in PS. Every situation/need you could think of, it's in there. This alone to me, shows that Kevin came up with PS.
TUT4 (Sag, chapter 4, 24 pages) and TUT6 (Sustain, chapter 7, 18 pages) have info on his "Sag" by use of power mosfet (compression) and other ways to get this in an amp. Again, --- lots --- of thought put into this.
There was quite a bit of discussion at Kevin's PS site about Kevin's and Dana's method of bias tracking. I would go with HBP's thoughts on this.
IIRC, HBP's pointed out one time here that KOC's PS is fed from pulsating DCV and Dana's VVR is fed from filtered DCV and IIRC KOC said the reason for using pulsating DCV is it's much easier on the mosfet.
Kevin also used a heaver duty mosfet, and his later kits used 2 or 3 in series. He also used a 2w military spec pot for the first PS kit's, 500 dcv ark over to case? Latter kits could use any style/size pot.
With respect, Tubenit
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And here are some links to KOC's information about power scaling for those who are interested.
You will have to join & be a member of the Power Scaling forum to view information here:
http://www.powerscaling.com/phpbb3/viewforum.php?f=3 (http://www.powerscaling.com/phpbb3/viewforum.php?f=3)
And some more information from KOC about his power scaling:
http://www.londonpower.com/powerscaling_vs_vp.htm (http://www.londonpower.com/powerscaling_vs_vp.htm)
http://www.londonpower.com/pscaling.htm (http://www.londonpower.com/pscaling.htm)
Again, I do not know if the VVR somehow infringes on the PowerScaling design or not as I don't have a Power Scaling schematic and haven't had a chance to compare the two. But in all fairness, I felt it important to share this information in a timely fashion to my learning about it.
with respect, Tubenit
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This is what I finally came up with. You can compare it to the original 56T reverb to see how the VVR was inserted and what changes are needed on the B+ rail ........... such as adding some more filter caps and another dropping resistor and 1N4007 diodes.
Tubenit,
What is the idea of the added diodes?
/Leevi
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I am not completely sure why the extra diodes? I copied the circuit.
I am thinking it keeps the voltage going only one direction?
The 1N4007 are on a few cents each so it's not a cost issue, really.
With respect, Tubenit
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Ok, I didn't find any reason for it, maybe it could safe the MOSFET if the rectifier
breaks down?
/Leevi
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This is a response to a thread about blown mosfets on Dana Hall's VVR
Rectifier> VVR> Standby> Filter cap
Blew 2 FETs then added a cap between the rectifier and VVR. Problem went away. A year or so later I did some testing and thought it felt different with the extra cap. Pulled it out. A week later blew the FET. Finally just moved the first filter to before the VVR (instead of adding a cap).
Rectifier> Filter cap> Standby> VVR
Sounds and feels the same ( I think??) and has been rock solid for a couple of years.
I've been using IRFPE50 $3.85 at Mouser
Good luck.