London Power ad

Search the Forum

(Advanced Search)

SV1, SV2, SV-TT update
Hi Guys

Back when I designed SV1, SV2 and SV-TT it seemed like "a good thing" to add some tiny caps to reduce the bandwidth of the otherwise wide-bandwidth regulator sections. The idea was that noise on the supply line that might be impressed across the Power Scale control could be amplified through the regulators and be heard through the amp. Of course, C1 across the pot output would clean this up all on its own.

In most installations these caps do not present a problem and everything works fine. However, some installations will show some oscillation and removing the small feedback cap 56-68pF will eliminate the oscillation. In SV1 and SV-TT, remove the 270pF from the bias regulator. Do this in TBS if there are issues. The bias regulator cap can cause a secondary oscillation.

Over the life of these kits, the 330k-1W feeding the Power Scale pot was split into 150k + 180k and a filter cap to ground was added at their junction. This cap helps reduce hum, ripple or other noise getting to the pot. Some installers found that C1 could be removed without any audible effect, although the main purpose for C1 is to keep DC-related pot-scratchy sounds from happening.

Techs report that he removal of these caps makes the amp sound better. It should, since these are very wide-bandwidth regulators which present a low-impedance voltage source to the amp allowing any frequency restrictions to be only those imposed by the amp itself.

I annotated the kit notes but if you have old notes or ones without the update, at least the information is accessible here on the forum.
Hi Guys

SV1 and SV2 are now much smaller than they used to be by about one-third. This allows them to be placed in smaller chassis or into tighter spaces. We used cord-wood construction to achieve this, with all the power resistors standing on end with one lead folded back to the board.

The revision also changes how the Power Scale control is wired, improving low-level resolution of the control. This rewiring is actually the original way I wired the PS pot in my amps, but then changed it for the kits to allow easy ground-referenced selection of multiple PS pots. Although that was a useful feature, only two builders ever took advantage of it.

That multi-PS selection optimisation did a quirky thing to the PS sweep which was nonoptimal yet still allowed dialing power to zero. The new/old wiring is nearly ideal in its pot sweep and the power range is from 100% down to 0% as it always has been.

In the Japanese naming style adopted for the Power Scaling kits, the Super Budget SB-series used a very simple circuit that required a very expensive pot. The pot price got to be prohibitive and it was eventually replaced with good-quality 16mm types that are far less expensive, but required a more complex regulator circuit. In my amps, I called this the Super Design SD-series, and with the modified PS connection, the kits became Super Versatile or Super Value SV-series. The kits now conform to the SD form.

The kit evolution also allowed an increase in the voltage range for SV1 and SV2, eliminating the need for SV-TT and SV3. SV84 was originally designed for use in the ValveJr, which uses a single-ended EL-84 PA with a single supply node for the output stage. SV84 has been redesigned as above and its voltage range has been extended to 750V from 450V. The product description will be updated when the new PCBs arrive.
Hey Kevin,

Great news about the SV updates!

I used the ground-referenced, multi-PS pot circuits in at least one Island Amp model back in the day. Sadly it's been so long I can't remember which one(s) actually made it past the prototype phase. Few players at the time seemed to grasp the concept of multiple channels each with independent control over power amp variables including Scale, Drive Comp, Body, NFB, etc.

Was I one of the two builders? =)
Hi Dante

I guess there were three Big Grin

It is kind of funny about Human interfaces for tech items. Some players dream of a 3-knob amp, or even less, while others want an amp they can control from their phone. I like versatility and default to having sometimes too many controls - most that are for voicing and end up being set as the player needs then forgotten, while a few controls remain "daily users". Then the player thinks a 3-knob amp would suffice.

The closest I came to a 3-knob amp was a variation of STUDIO with 3-knobs plus some switches, but no one ever ordered one. The next was AURORA with 6-knoibs and no switches - slightly more popular - haha.

Since Power Scale and Drive Compensation are "unusual" for most players, i can see that having two of them would be more intimidating - then add "master" versions of them for global control. Yikes!

have fun

Forum Jump:

Come in where it's warm!
A warm welcome to tube amp modding fans and those interested in hi-fi audio! Readers of Kevin O'Connor's The Ultimate Tone (TUT) book series form a part of our population. Kevin O'Connor is the creator of the popular Power Scaling methodology for amplifiers.
Please remember these three principles: respect, sharing, community.
Not familiar with The Ultimate Tone book series? See discussion topics, or click here to visit London Power/Power Press Publishing.

Tube Amp Forum Hosted by London Power
London Power logo