Screen grid regulator

[Ian444]

I recently bought an old guitar amp (Vadis LB60 Extended Range) cheap, gave it a good looking over and fired it up. Here's an example pic and schem.I brought it up on a variac and noticed a fairly high B+ over 470V and 460V on the screens. The outputs are 2 x 6CA7, I was thinking the screen grid voltage was a bit on the high side, especially considering there are no screen grid resistors - none at all!

What to do, the B+ itself is not a problem, but the screen grid voltage is a worry for tube life. I built up this circuit based on Pete Millett's DCPP hifi amp screen grid regulator. If you go to this web page and scroll down to "power supply" you will see the circuit I used, the "B+ ripple filter" circuit, not the "G2 regulator" circuit. In the B+ ripple filter, I changed R56 from 47K to 180K, and R69 from 1M to 540K. This causes the output to regulate at 75% of the B+, lowering the screen voltage to around 350V. As the B+ sags, so will the screen voltage, staying at 75% of the B+ at all times. I mounted the mosfet to the side of the chassis, its a plastic package, and it just gets a little bit warm.

Its also similar to a VVR except that its fixed and only controls screen voltage, and the B+ for the preamp tubes. I am very happy with the result. Thought I would share as it may help people with amps that have a B+ that is a bit too much on the high side.

I did another little mod that might be of interest. Since the amp has two near identical preamps, I converted one of them into a single gain stage, the output of the tube going straight to the volume, and output of the vol going to the 2nd input jack, which is now an output. Then I can connect that output into the other channel's input with a patch lead, to give a boost. It makes the topolgy a little bit similar to a VoxAC30/59 Bassman/JTM45 and gives a lovely dynamic yet cleanish sound. The amp can still be used in its original form by just plugging into the other untouched preamp.

[triode3]

I recently bought an old guitar amp (Vadis LB60 Extended Range) cheap, gave it a good looking over and fired it up. Here's an example pic and schem.I brought it up on a variac and noticed a fairly high B+ over 470V and 460V on the screens. The outputs are 2 x 6CA7, I was thinking the screen grid voltage was a bit on the high side, especially considering there are no screen grid resistors - none at all!

I would not worry about screen voltage "for tube life" with 470Va/460Vs.
No screen resistors is a small concern... stick a 1.5k 3W or 5W on each one.
Other than that, those voltages (unless you are running them at or above maximum current) is well within spec.

[Ian444]

Yeah, I guess I need to HTFU. But it does sound good with the lowered screen voltage... Just worries me having an old amp running near the limits, it will get cranked up to near max at a gig and I don't want it to drop its bundle I guess the confidence comes with experience.

[Cygnus X1]

Well it's certainly something else to throw into the mixing pot and think about.

I appreciate it.
Sometimes we think in a closed loop and forget about what other ideas can teach us.
I too would feel a bit nervous in that territory but I tend to play it safe.

Thanks!

[Timbo63]

I have had a look around for details on any Vadis information and found very little, so before I gave this Vadis BR20 amp to my eldest son Oscar, I have taken some photos and drawn a tracing as best that i could see,to assist any one repairing or thinking of building one of these Northern Beaches (Brookvale) Australian Harp-Guitar-Accordian Amps.

[Timbo63]

I have added the Tracing to assist any amp repairs or builds.

[Colossal]

Nice hand drawing

[Ian444]

Yes, very nice drawing, there looks to be a few minor errors but the gist is there I think. I do like hand-drawn schematics. Can't quite decipher the tone controls. I have never heard of that model, looks like a barrel of fun!

Just as a quick update to the original post, that screen regulator seems to be doing a fine job, as I use this amp a lot, and I really love the sound. Here's a (digital) schem of the reg, (the cap sizes may be overkill as the circuit was modified from a hifi amp circuit).

[desu boi]

This is not a regulator, it is a capacitance multiplier. A capacitance multiplier has no regulation.

On the original web site, http://www.pmillett.com/dcpp.htm there is a proper regulator for the screens, named "G2 regulator". Then there's this filter, which is a capacitance multiplier, named "B+ ripple filter" on the original.

[Ian444]

Yes, the original DCPP ripple filter has a 47K/1M voltage divider feeding the gate of the mosfet, but this variation has a 180K/540K voltage divider which results in 3/4 of the B+ voltage to be filtered and fed to the screens and preamp stage. The experiment was simply to lower the screen voltage in proportion to the B+ voltage, rather than just fix the screen voltage at some nominal lower value with zeners. If B+ drops under heavy load, the screen voltage will also drop in proportion, i.e. the screen voltage will "track" the B+ but at a lower voltage. So not a regulator in the strict sense, more a dynamic regulator.

[desu boi]

So not a regulator in the strict sense, more a dynamic regulator.

It is not a regulator in any sense. It is a capacitance multiplier.

"Dynamic regulator"?? Did you make that up on the fly? Please do tell how you define this concept.

[John_P_WI]

It is not a regulator in any sense. It is a capacitance multiplier.

Why not a regulator? The voltage is set by the ratio of the two resistors. As the supply voltage sags or rises the screen voltage will track OR REGULATE to the ratio set by the resistors. Using a set of zeners to set the voltage to a given value would then be a regulator with a set output voltage.

Is this a capacitance multiplier or simply an active device with a very fast response time???

[tubeswell]

I was going to post a long-winded explanation about how you could do a primitive shunt regulator with power resistors, but I thought better of it and changed my mind - so,... as you were.

[surfsup]

interesting that there is no cap on the heater filament elevation point. (on the millett amp)...

[tubeswell]

interesting that there is no cap on the heater filament elevation point. (on the millett amp)...

I think he's counting on the regulation in the B+ line to keep the supply voltage on an even keel. And maybe it does. Maybe.