VVR question

[Wayne]

...for all you guys using VVR in your amps - what is the practical limit for this? I like the idea that the structure of the amp stays the same as you reduce all the voltages in proportion but isn't there a point after which the tubes don't "work" any more, or at least don't behave predictably?

It seems to me that if you start with, say, a 450 V B+, dropping it to 225V would give you 1/4 the power. What happens to the preamp tubes when they're only working with a (guessing) 100 or so volts?

W

[UR12]

...for all you guys using VVR in your amps - what is the practical limit for this? I like the idea that the structure of the amp stays the same as you reduce all the voltages in proportion but isn't there a point after which the tubes don't "work" any more, or at least don't behave predictably?

It seems to me that if you start with, say, a 450 V B+, dropping it to 225V would give you 1/4 the power. What happens to the preamp tubes when they're only working with a (guessing) 100 or so volts?

W

Unfortunatly 1/4 power doesn't correspond to 1/4 as loud

It depends on what preamp tube you are talking about but having said that, if you look at the plate characteristic curves of say a 12ax7 it has a linear range of operation down to very low voltages down to below 50vdc. Of course this is for a given bias. Since most preamp tubes are operating with a cathode bias the bias is directly porportiopnal to the current flowing through the tube. IMHO I don't think the sound of the amp suffers because the preamp tubes are starved for voltage as much as the speakers aren't moving any air at these reduced voltages. To prove this point you can take an amp with a VVR installed and install a line out on the speaker jack and amplify the reduced signal coming out of the amp through a pa system or other amp that doesn't color the sound and you will find that the reduced signal coming out of the amp sounds pretty much the same as it did at higher volumes.

I think the lower limit on a preamp tube is hit when the bias is so low that the signal coming from the guitar pickup is overdriving the input to the grid. At this point you could raise the bias but that would move the operating point of the tube to a non linear region on the plate curve for the given plate voltage.

The lower limit can be adjusted on the VVR by changing the resistor R1 (R1&R5 on the fixed bias board) The board comes stock with a 100k resistor and that limits the voltage 10% of the b+ voltage going to a 220k or 330k will limit it to about 20-30% of the b+ voltage and this may put you in a range that doesn't go so low that the speakers are running at whisper levels but levels reduced enough to play a smaller venue or practice levels. For exampe, if we had an amp running at 400v B+ and you switched R1 to 220k you would put the range at 400v - 80v where as the 330K would give you 400- 120v. By changing the values of the node resistors in your power supply you can tailor the amp (Preamp) to run on any range you want. You hear a lot of guys talk about some amp designs that are designed with "Power scaling" in mind and this is just one way you can do that while retrofitting the VVR into an existing amp.

[billyz]

My experience was that by putting 330k Resistors in, I dropped the voltage to 95 VDC from 400VDC ( maybe it was 405) . This sounded the best to me at the lowest voltage setting. I estimated the wattage to be around 4 watts at the low setting using a pair of 6V6 power tubes.

[Wayne]

Excellent! I had hoped my question would raise a response from "the master". Or wait a minute - maybe that shoud be "non-master" because if it really works, you wouldn't need one... never mind!

I know that 1/4 power isn't 1/4 as loud. I also know that 100W is way too loud for anything I'm doing with the possible exception of outdoor gigs. Even 50W is a bunch, although I wouldn't feel like I was "prepared for anything" with much less.

I would imagine that like any other means of reducing volume, VVR probably works best when it is making the least difference (e. g. 50W -> 40W probably OK; 50W -> 5W probably gonna sound funny).

At any rate, it's another trick I can try when the build happens. Right now, owing to time and $ constraints, it's all just a paper exercise. Hopefully, some time over the winter I can free up some of each and start building something (not even sure what, yet). When I do, I hope to be able to try VVR, PPIMV, and any other means of sucking volume without sucking (much) tone.

Tks,

W

Edit - didn't see billyz's post when I first replied - if you say that you could get all the way down to 95V & still sound good, that's encouraging!

W

[dehughes]

Kinda off topic....different question, same subject:

I've just ordered the parts to install a VVR on my AC30 (thanks to all who contributed info on my other thread, and MANY thanks to Dana...) as I'm going to build an Airbrake to use for the other amps (non-master...whereas I have a MV spot in my AC30 already...).

As such, I'm looking for any recommendations/suggestions/helpful hints as far as building the VVR into the amp. I'm going to build it off a turret strip (already using them in the amp...), so it's mostly a question of where to situate the MOSFET, how to mount it properly as to cool it, and such.

[jaysg]

hmm...you're using the VVR, but not Dana's board? k

Disclaimer...this is not my work and I'm not asking permission to post it...this one is easy to suss out. There should be a heatsink on the other side of the chassis. In correspondence with KOC, he explained that the AC30 configuration -- 4x el-84 with cathode bias is harder on the circuit than a normal 50W fixed bias amp. Also, fwiw, his Sag control is pointless, since it drops B+ under increased load and the Rocket/Liverpool/AC-30 output doesn't play that way.

[edit]I keep crashing the server when I try to load the pic....see your email/pm

[dehughes]

Yup, it came through. Thanks! Looks good...

Yeah, I've PM'd Dana and he's shared the info and parts list, so I'm going to just put it together on one of the many turret boards/strips I have lying about. Should be easy...just wanted to get a head's up from those who have gone before...figure there are all sorts of things I'd mess up.

So, mount the MOSFET and then on the other side of the chassis affix a heat sink directly above where the MOSFET is mounted? Is that correct? Does the MOSFET screw directly to the chassis, or must there be some sort of thermal paste and/or insulator in between?

THANKS SO MUCH!

[Colossal]

Excellent! I had hoped my question would raise a response from "the master". Or wait a minute - maybe that shoud be "non-master" because if it really works, you wouldn't need one... never mind!
W

Wayne,

For what it's worth, I added both a VVR and a PPIMV to my 6V6 Plexi build. Since I had no idea what to expect from either, I gave myself the option to try both. The VVR is WONDERFUL at controlling the overall sound pressure (I use a 2x12 with Greenbacks) of my mostly bedroom jamming situation while retaining the character of the amp very well. The amp still gets really LOUD, but the VVR takes the painful edge off in close quarters while still giving the Plexi punch and warmth. I don't use the PPIMV except for very low level playing. So, for example, the PPIMV does not sound very good anywhere but all the way up when the VVR is opened up, but when I turn down both the VVR and the PPIMV to about 9 o'clock, the distortion becomes "nice" once again and is satisfying for late night noodling.

Regards,
Dave

[dehughes]

Thanks for the replies, all! Much appreciated...

Anyone have a recommendation for a heat sink from Mouser? They seem to have TONS....and I wouldn't know which one to choose. I think I'd like to find one that has a screw mount so that I could mount it with the same screw that I'd mount the MOSFET to the chassis...don't think I'd trust thermal tape...

[UR12]

Thanks for the replies, all! Much appreciated...

Anyone have a recommendation for a heat sink from Mouser? They seem to have TONS....and I wouldn't know which one to choose. I think I'd like to find one that has a screw mount so that I could mount it with the same screw that I'd mount the MOSFET to the chassis...don't think I'd trust thermal tape...

Honestly, if you have any old computers laying around, the processor heat sinks work really good (and they are free). If they have a fan mounted on them, just remove it from the heat sink, Drill a hole to put the screw to hold it and the mosfet and your ready to go. I haven't look at Mousers offerings because I ripped out a bunch of sinks from computers they were throwing away at work.

[dehughes]

Thanks for the replies, all! Much appreciated...

Anyone have a recommendation for a heat sink from Mouser? They seem to have TONS....and I wouldn't know which one to choose. I think I'd like to find one that has a screw mount so that I could mount it with the same screw that I'd mount the MOSFET to the chassis...don't think I'd trust thermal tape...

Honestly, if you have any old computers laying around, the processor heat sinks work really good (and they are free). If they have a fan mounted on them, just remove it from the heat sink, Drill a hole to put the screw to hold it and the mosfet and your ready to go. I haven't look at Mousers offerings because I ripped out a bunch of sinks from computers they were throwing away at work.

GREAT IDEA!!!! My goodness man, you should be the patron saint of this place. Thanks so much...again...

[rawnster]

Thanks for the replies, all! Much appreciated...

Anyone have a recommendation for a heat sink from Mouser? They seem to have TONS....and I wouldn't know which one to choose. I think I'd like to find one that has a screw mount so that I could mount it with the same screw that I'd mount the MOSFET to the chassis...don't think I'd trust thermal tape...

Honestly, if you have any old computers laying around, the processor heat sinks work really good (and they are free). If they have a fan mounted on them, just remove it from the heat sink, Drill a hole to put the screw to hold it and the mosfet and your ready to go. I haven't look at Mousers offerings because I ripped out a bunch of sinks from computers they were throwing away at work.

GREAT IDEA!!!! My goodness man, you should be the patron saint of this place. Thanks so much...again...

Hey David, I think I have several old processors w/ heat sinks on them laying around in the garage. I'll dig'em for you if you're interested.

[dehughes]

Honestly, if you have any old computers laying around, the processor heat sinks work really good (and they are free). If they have a fan mounted on them, just remove it from the heat sink, Drill a hole to put the screw to hold it and the mosfet and your ready to go. I haven't look at Mousers offerings because I ripped out a bunch of sinks from computers they were throwing away at work.

GREAT IDEA!!!! My goodness man, you should be the patron saint of this place. Thanks so much...again...

Hey David, I think I have several old processors w/ heat sinks on them laying around in the garage. I'll dig'em for you if you're interested.

I am very interested! Thanks Ron!

[Structo]

Sorry I don't mean to butt in but, why can't you mount the mosfet right to the chassis side wall with some heat sink compount between it and the chassis?

[UR12]

Sorry I don't mean to butt in but, why can't you mount the mosfet right to the chassis side wall with some heat sink compount between it and the chassis?

You can, but when you install it in amps producing 50 watts the chassis by itself isn't may not be enough and you have to add additional heat sink. All chassis aren't created equal. Some are steel and don't dissipate heat as good as aluminum. The aluminum chassis come in different thicknesses and a .090 will sink more heat than a .060 so there are a lot of different variables that have to be considered. I always bolt it to the chassis, turn it down to about 1/2 or 12:00 on the power control and play the amp for a while and check to see how hot the chassis is getting. If you can't leave your hand or fingers on the hot chassis then you need more Heat sink.