adjustable screen voltage

[M Fowler]

Tone King has a JTM45 style amp out for 2014 that has 6v6 tube used for regulating screen voltage and is adjustable on back panel see 1:55 on video.

Anyone do this? http://www.youtube.com/watch?v=juRvduBxhvA

[tubeswell]

Yeah I've seen it somewhere- maybe the pentode voltage supply regulation was described in Merlin's Power supply book. I'll dig my copy out when I get home and check it out.

(Or it may've been something Steve Lucky had done over at the Hoffman forum a while back?) I know I've seen it somewhere.

[VacuumVoodoo]

Sure, but use a tube for this? Waste of good vacuum...

[Cliff Schecht]

Sure, but use a tube for this? Waste of good vacuum...

But delicate harmonics that thrive in a vacuum sealed glass envelope are not able to exist in a silicon lattice..

[ToneMerc]

OC3 regulator

https://tubeamparchive.com/viewtopic.php?p=105858

https://tubeamparchive.com/viewtopic.php?t=11404

TM

[VacuumVoodoo]

Sure, but use a tube for this? Waste of good vacuum...

But delicate harmonics that thrive in a vacuum sealed glass envelope are not able to exist in a silicon lattice..

In a series voltage regulator? This could possibly, hypothetically, contain tiny grain of truth in a shunt regulator.
BTW it's nothing new, it's tube archeology. It has been used together with grid bias to position load line just below the knee on anode current/voltage characteristics while at the same time lowering screen voltage. So, no, not a new invention at all. An innovation in a guitar amp, certainly.
In this application you can position the load line above/on/under the knee to your taste. Of course, if you drop screen voltage below certain level you get output power reduction. So using this for tone adjustment together with a separate power attenuator makes some sense. Pity it's only a set and forget adjustment only - it's trim range is limited by the acceptable variation in idle plate current it introduces. This and bias trim are interdependent.
No no knobs for you!

[stephenl]

I agree with Mr. Niemand...waste of a tube. I used a zener/mosfet circuit with current limit, out of Merlin's book, initially to try to preserve screen life.

I have it installed on a PP KT66 output stage. From a tone perspective, the bass is super tight and punchy - it might annoy your bass player This PA already had pretty good bass performance, it might be a little much for some with the regulated screens. It *might* be a good thing to try on an amp with lack of or flabby bass

Some other options for HV regulators...search Pete Millet's site for his Maida Regulator- he sells pcb's for it on his ebay site. John Broskie also has one that uses an LD1085 and a IXCY current regulator for the pass element. The PCB solutions are nice and compact.

I tried powering the preamp from a regulated supply as well. It was quiet but I decided I like a little modulation and compression from the PA. It would make a good "chunka chunka" machine. I ended up feeding the preamp pi filter from the plate supply.

I do like the punchiness of the regulated screens but I think I may try adding some sag resistance in front of the rectifier to round off the sound a little.

Edit... If you want the screen voltage adjustable, you can use an LP powerscale or vvr. According to KOC, you can adjust the screen supply down to about 2/3. The PS and VVR won't give the effect of regulating the voltage, just adjusting it.

[funkgang49]

If you want the screen voltage adjustable, you can use an LP powerscale or vvr. According to KOC, you can adjust the screen supply down to about 2/3. The PS and VVR won't give the effect of regulating the voltage, just adjusting it.

Isn't adjusting the screen voltage not regulating it what's being done in the Majesty amp?

And then why use an aditional tube to do it?

[VacuumVoodoo]

They are regulating the screen voltage but not stabilizing it. Screen voltage is not set to a fixed output voltage by an independent stiff reference. It follows the B+ supply variations but keeps the difference between B+ and screen voltage constant. This way screens follow B+ sag dynamics.

[FUCHSAUDIO]

Standell, Guild, and a few other amps did this in the past. Audiophile stuff (Audio Research for example) has had it as well. Today, Mosfets are a simpler solution than a tube to get that job done. A combination of screen voltage adjustment and bias adjustment can be used as a variable wattage control (the Marshall AFD amps do this with a computerized circuit ).

Mark Bartell is a pretty smart dude, so I bet it could be a cool circuit.

[Andy Le Blanc]

the relationship between the screen and plate voltages also holds a relationship with the static current draw, its best to set your screen voltage as you set your bias, it too will vary with the effective characteristics of the tube your working with. It's that or you set it up with a range of tube characteristics and bias points in mind... then the burden will be on the end user to maintain bias and use select tubes.

I like the bias and screen voltage at a certain spot, if you get someone in there monkeying around it compromises my design intentions. One good thing about control'ed regulation of these variables could be a more plug and play view toward tubes, no matter what you put in the tube sockets it'll sound the way you programmed it to, maintaining the voltage gradients thru the tube no matter what.

[tubeswell]

Sure, but use a tube for this? Waste of good vacuum...

But delicate harmonics that thrive in a vacuum sealed glass envelope are not able to exist in a silicon lattice..

[M Fowler]

Thanks for all the information guys.

[talbany]

I don't see much use in lowering the screen supply on a 6V..Changes the character of breakup (tone get's sterile harsh) Pretty cool if you do it to a larger tube and 6550 KT88 for real clean tones (those needed to be regulated)..I don't see the tonal advantage in a tube like the 6V that already sounds great run into saturation (what they are known for)..The OC3 regulator was also used in the Hammonds to regulate the screen's on the 6550's where they really needed to be lowered and regulated to keep the screens from spiking up when the grids were pushed..

Tony

[FUCHSAUDIO]

I don't see much use in lowering the screen supply on a 6V..Changes the character of breakup (tone get's sterile harsh) Pretty cool if you do it to a larger tube and 6550 KT88 for real clean tones (those needed to be regulated)..I don't see the tonal advantage in a tube like the 6V that already sounds great run into saturation (what they are known for)..The OC3 regulator was also used in the Hammonds to regulate the screen's on the 6550's where they really needed to be lowered and regulated to keep the screens from spiking up when the grids were pushed..

Tony

Agreed. Plush did the OC3 regulator in a bunch of 6 x 6L6 and 6550 based amps also.