"VVR mod" what's this?

[Sea Chief]

Hi chaps,

read up on a mod that's feasable, & even recommended, for my Vox AC30/6 TB (not the china made CC: mine's a UK made 90's one).

But what are these 3 letters, & what might this 'first base' mod actually do? I only know it to be suggested -instead of- adding a powerbrake attenuator type thing (becuase it doesn't stress the tubes like an attenuator does).

So I like the idea then, but is the VVR is a form of overdrive 'addition'?

Mysterious letters! thanks Capt

[xtian]

VVR = variable voltage regulator. As you drop the B+ voltage, your amp will distort earlier, at lower SPL.

[Greg Robinson]

VVR stands for "variable voltage regulation". Also known as "power scaling" (trademark of London Power).

Many styles of music make use of overdrive, that is, driving the amp until it clips to its power rails. If you get this clipping from the power amp and not the preamp, you need to turn up to the full volume the amp can produce. This is fine, except when you'd like that same sound at lower volume.

The output power of an amplifier is ultimately determined by it's power supply. The tubes and output transformer are actually secondary considerations, they're selected to convert the power rail into audio output, and must be rated to be able to handle the power it can deliver, but there's nothing stopping us from using "over-rated" tubes and transformers to deliver a smaller output power, ie you can get about 18W out of a pair of EL84's with about 300V B+, but there's no reason you can't use a pair of EL84's at 250V for about 11W.

So what if we had a knob that could control the voltage of the power supply? Well then we could turn it down and still get power amp clipping. In principle we can simply "turn down" the voltage to a 100W amp and make it a 0.5W amp if we wanted to. And this is exactly what "VVR" does.

You have probably heard tales of Eddie Van Halen using a Variac to lower the voltage to his amps and help him get his tone (the truth of this is still hotly debated, Eddie was very protective of his "tone", to the point of playing with his back turned to the crowd during some of his early finger tapping days so no one could steal his technique - he often told diversionary lies about his technique and equipment). The problem with this is that a Variac will alter all of the voltages in the amp, including the heater voltages. If the heater voltages are turned down too far, the tubes will not be able to function at all, and this can even cause damage to the cathode coating called interface resistance.
So we want something that can alter just the B+ to the tubes, this is what VVR does.

And yes, unlike an attenuator on the output of the amp, where the power tubes are still being driven to high power and being worn out, VVR takes the stress off the tubes by making them clip at lower power levels. There's no free lunch though, some of that stress is transferred to the voltage regulating element (usually a mosfet) as wasted heat.

There are lots of variations of VVR, some "scale" the voltages to the entire amp, some only scale the phase inverter and output tubes, some only the output tubes themselves, or even just the output tube screen voltage. Each method has its own benefits and compromises.

Cathode biased amps like an AC30 are the "simplest" type to approach, as the bias voltage (cathode voltage) is simply a product of the cathode resistor value and the B+ applied to plate and screen. Fixed bias amps on the other hand are more complicated because the bias voltage must also be scaled in proportion to the B+ voltages. Scaling the preamp to low voltages will cause the grid to go to a positive voltage which can cause pops and clicks with any channel switching, and/or a scratchy volume control on your guitar, but there are ways to mitigate this.

There's also the issue that the transfer function of tubes is not completely linear when scaling plate and screen voltage in proportion to one another, so the loadline moves relative to the knee when turning down, which alters the way the power amp clips. Not to mention the inherent problems with psychoacoustics and volume reduction, thanks to Fletcher Munson (Equal Loudness) curves lower volumes will always result in a reduction in perceived bass volume unless that is boosted to compensate.

Like anything, VVR is a compromise, and in my experience is best used moderately. Promises of a 100W amp retaining the same tone down to fractions of a watt are IME exaggerated.

VVR/Power Scaling were "hot" concepts a few years back, with it popping up in a fair few boutique amps, and has long been a favourite of the DIY crowd. Various kits have been offered for DIY modification over the years. Lots of reading out there.

[Mick Bailey]

I've had a couple of amps for repair that had VVR built in, and also experimented myself. I kept the preamp and heater voltages, but variously experimented with reducing the plate + screen, or additionally the PI as well. My conclusion is that it's OK within a limited range when taking a simple approach (using a single MOSFET), but when taking the voltage down to provide a comfortable listening volume the characteristics change too much for my own liking. The distortion becomes fizzy - very much like when you turn off an amp without using the standby while still playing.

My thought is that to do a better job the voltages would need to be individually balanced as they are reduced, such that they are not all reduced in the same proportion. With cathode bias it is a lot easier, but there's still the relationship between the PI and power tubes that seems to me not to be proportional.

[Helmholtz]

Scaling the preamp to low voltages will cause the grid to go to a positive voltage which can cause pops and clicks with any channel switching, and/or a scratchy volume control on your guitar,...

Are you sure about the positive grid voltage?
I'd expect a negative grid voltage due to increased grid leak current.

[tedmich]

I've always assumed the typical VVR implementation is an offered solution to the many year fallacy of the electric guitar industry: selling over large/loud amps for home/small venue usage. This was the strategy because all of the pro marketing heroes were playing in huge stadiums at crazy high volumes; their amp choices make ZERO sense for these different implementations.

An analogy might be marketing NASCARs to everyday drivers (something which is sort of happening in the US now!)

Many amps, SLO being a well known example, sound fantastic only when crazy loud. VVR isn’t going solve these issues, while modelers do a much better job. Yet another issue also solved by modelers; sounding like an entirely post production guitar (as heard on your favourite album) while playing “live” which is a discussion for another day

[Sea Chief]

Hi chaps, & Greg Robinson in particular, huge thanks for that explanation. I will go over this next week, when I have a few days off work/ will absorb.

That's great, so I know what the vague idea is of a VVR (at this stage/ will go over all replies next week). In fact I have seen a similar 30/6 TB amp to mine, with what must be this VVR mod done (the pot was put in the standby hole).. & undoubtedly a huge success from both the sounds heard (a youtube clip, two US chaps), & from a very experienced player trying out his buddy's amp: both the owner & the player commented on how good an addition it was.

I just didn't know that their mod, was in fact this VVR one. So now I'm interested in at least trying it: & I'm further helped by my pro amp builder here in UK, whom I make cabinets for, who kindly said last year he'd design a small circuit for me. I didn't know this was in fact, a VVR. Tricky bit is now trying to nudge him! Quite a favour, but he really is an amp expert, & would know the best small circuit to fit.

That's great help chaps, grateful indeed- SC

[Greg Robinson]

Are you sure about the positive grid voltage?
I'd expect a negative grid voltage due to increased grid leak current.

Quite right, getting my pre and power tubes backwards. Power tube grids tend to emit electrons due to thermionic effects being in close proximity to a big hot cathode, preamp grids tend to go negative due to collecting stray cathode electrons. Thanks for the correction.

[Mick Bailey]

An analogy might be marketing NASCARs to everyday drivers (something which is sort of happening in the US now!)

A diversion - I couldn't help but reply..... In a nearby village I met a lady last summer who was driving this on the road (it's fully street legal, parachute and all....) The dyno test only went up to 3,000 HP, though it makes far more. Turning circle is limited for rural lanes, but power output is probably more than adequate for the hills around here;

https://santapod.co.uk/entrylistdeta...list&Class=VWP

[sparkies]

Hi chaps,

read up on a mod that's feasable, & even recommended, for my Vox AC30/6 TB (not the china made CC: mine's a UK made 90's one).

But what are these 3 letters, & what might this 'first base' mod actually do? I only know it to be suggested -instead of- adding a powerbrake attenuator type thing (becuase it doesn't stress the tubes like an attenuator does).

So I like the idea then, but is the VVR is a form of overdrive 'addition'?

Mysterious letters! thanks Capt

VVR isn't going to work well in amps other than self biased ones. That is why someone recommended it for the AC30. But I look at this mod it just makes me think someone was just figuring out a way to make a different sound with the amp from under drive distortion. Which, I would do that in a pedal instead of the amp.

The attenuator doesn't stress the tubes. Its driving the signal hard into them. The effect of putting the attenuator on the output is to induce transformer saturation. That most of it gets eq out when mixing. Quite frankly, that sound is better produced in rack equipment on a recorded channel than at the amp.

[g1]

The attenuator doesn't stress the tubes. Its driving the signal hard into them.

The power tubes are putting out more power when used with an attenuator so they will wear out faster.

[The Dude]

Yes, an attenuator or power soak after the OT simply limits signal to the speaker by dissipating some of the output across a resistive load. The tubes are working harder for less audible output.

[Chuck H]

Dude and g1 have it on the attenuator. This is one reason attenuators have been accused of causing amp failures. With the volume under control users feel free to crank the amps. This absolutely does cause a greater stress and wear condition on the power tubes that aren't present for master volume type amps. Power scaling does reduce wear similar to a master volume setup. In that tubes operating under limited voltage/current experience less wear by a similar degree with volume reduction. If you want your amp as it's made to sound cranked at lower volume your best bet is an attenuator. At the expense of cranked power tube wear. If you're happy with preamp tones or any difference from VVR reduced power tube voltage then you can use those options and probably extend tube life.

I use an attenuator. I like it.