Hi Kevin et al.

On the Custom Special schematic in TUT3 there is a zener diode (100v, 1w) straddling Vb and ground.

My amp building experience is limited, I'm used to seeing resistors here.
I believe it's purpose is to provide a stable fixed bias voltage relative to ground.
So if the plate voltage varies for whatever reason, the bias remains the same.

My question is - Why is this better than the bias following the ups and downs of the plate voltage?

The amp I am building will hopefully be gigged.
It has a universal power transformer, wired as Kevin suggested at the back of TUT3.
So different countries, different mains voltages and hopefully lots of festivals with dodgy generators and clubs with terrible mains supplies.

Regards.

Hi Kevin et al.

I'm laying out the eyelet board for my TUT3 Custom Special build (again).  Great fun.

Checking the suggested layout against the improved schematic in TUT3 I see a few discrepencies.
Some of them I can work out, some of them I can't.

Is this a good place to ask about them?

Regards.

Hi all.

I am part way through a build of Kevin's Custom Special amplifier from the TUT3 book.  Great circuit.

Checking my current needs for the 6.3VAC secondary, I calculate 4x 1.5 A for the 6CA7 valves and 3x .3 A for the 12AX7 valves.
Which comes to 6.9 Amperes.

The rating for the specified Hammond 278CX transformer is 6 Amperes on the 6.3 volt secondary.

15% over the rated capacity seems a lot to me.  Am I being overly conservative?

Hi all!


I am plannng to build the Hood Amp, it is in Tonnes of Tone.

Is it necessary to use the hefty diode bridge or can discrete diodes be used?



Warm regards,

Strelok

Hi Guys

In the Precision Power Scale thread I discussed using a "build-out" resistor to improve the control resolution at the quiet end of the Power Scale sweep. This method can be applied to all previous Power Scale installations. Ideally, we want the "audible transition" of sound-to-silence to occur exactly at the end of the PS sweep. The value required depends on the supply voltage in the amp, the kit type, and how quiet you wish the minimum loudness to be.

Classic-PS and the SB-series both have a build-out resistor already. The main purpose of the inclusion of this resistance was to keep the mosfets 'on' at the full-CCW end of the Power Scale pot sweep. However, if the sound disappears before full-CCW, then adding a bit more resistance in series with the pot-0 lead will be effective.  In these Power Scale forms, it may only take a few kiloOhms, maybe up to 10k, to achieve this AT alignment with the pot sweep.

There were quite a few DC-PSK variations, and then the SF-series. The DC-PSK may require a build-out resistor up to 33k.

The Super Value SV-series followed, with many variations as I tried to use component values within the kit to eliminate dead spots of the pot sweep. Build-out resistor values of 10k to 33k are typical. Dead sweep at the CW or CCW pot ends shift with the supply voltage, requiring a tweak of two resistor values in the kit to optimise it for a given B+.

The Precision Power Scale Zh also needs a build-out resistor of 10k to 33k. I was too quick ordering these Zh has no loud-end dead spot but will have a quiet-end dead spot without the build-out R.

The Zp Precision Power Scale circuit has an on-board AT pot allowing the installer to set the audible transition EXACTLY. Zp has no dead spots and I believe it represents the ultimate performance in Power Scale kits.

The Precision Power Scale kits are still labeled as SVn but with a Zh or Zp suffix, as SVn-Zh and SVn-Zp. The SV2-Z forms incorporate VCK, since VCK is always needed in Power Scaled cathode-biased amplifiers. VCK is still available separately as it has other applications.

Hi Guys

The bias regulator in SV1 and TBS Tracking Bias Supply have a feedback resistor that controls the proportion of bias voltage to screen voltage. In absolute numerical terms, we describe this as a percentage, where the bias voltage available to the bias pot is expressed as a percentage of the screen voltage at the tube.

 Most large bottle power tubes (and 6V6) used in musical instrument amplifiers have about the same voltage gain and end up with very similar grid-voltage to screen voltage proportion, where |-Vb| is around 10% of Vs. This is the "target" control voltage. Some amplifiers are designed to only have this much bias voltage, for example Hiwatt, and there is no leeway to run the tubes cooler or to accommodate tubes with higher transconductance - these will red plate. As TUTs recommend, the bias pot should have closer to 15% at its 'cold' end to be able to turn off most tube samples.

In our bias regulator, the feedback works against 330k, so we get these percentages for different Rfb values:
56k2 provides 15%
47k5 provides 13%
36k0 provides 10%
30k1 provides 8.5%

The lower values are suitable for EL-84 and 8417.

RBX Raw Bias Auxiliary Supply has a limited output, which is still higher than most stock bias supplies, but its applicability is reduced with higher percentage bias range combined with higher Vs, as follows:
56k2 limits RBX to amps with Vs=560V
47k5 limits RBX to amps with Vs=640V
36k0 limits RBX to amps with Vs=840V
30k1 limits RBX to amps with Vs=1kV

hey dudes

I was wondering how much power you think is good for guitar?
how much for bass?

A couple of bass players I know think 100W is good enough for jamming and small gigs. I also know a guy who insists he needs his SVT. Ugh all these guys are using old ampeg tube heads but the 100watt guys have way lighter amps than the SVT. I can see why they like the lower power just on the basis of weight  lol

bass seems to need a lot o power to sound as lloud as a guitar with a lot less power.

i was at this outdoor party last summer. The band had all small amps one guitar had a 20W openback tweedy lookin thing but you could hear him a couple of km away. Not sure how much power the bass had but he was using a tiny cab and everyone was keeping up with the drums which fortunately werent too loud.

My 100w Marshall was way to freakin loud so I got power scaling kits from KOC and now it is a sweet amp to play. sweet like rippin but not rippin my head off. I havent tried to measure how much power I use but it probably isnt even 20W with a drummer. just on my own its probably way on the down low man

So whaddaya think/ how many watts do you have?

peace (so we can hear the power chords)

New member here. Specifically joined this forum to get some suggestions on a new project. 
To qualify, the last few years I've built about a dozen diff amps ranging from 18w to 50w.
Currently I've picked up a 1979 70w Pro Reverb that appears to have been dormant for quite awhile. Rusty chassis straps, dank grill cloth and it appears to have all of the original tubes. I don't want the 70w vanilla PA type sound we get from this amp. I'm looking for ideas to modify. I considered gutting the thing and building a 6G16 amp. I believe I can use the same PT and OT, chassis and cabinet. Or I could try to bring this 70w PA sounding amp to the 40w early 70's version with maybe more breakup at a lower volume.
I'm getting ready to disassemble it. Clean up the hardware. Grill cloth and tolex. I could just restore it as it is and resale it. I paid $600 for it and reverb and tremolo work great. Speakers are great. I really would like more of a project then just restoring.
Appreciate the feedback

Hi Guys

Power Scaling has been around for decades and has undergone continuous improvement and change as far as the kits go, but the goal and the performance overall has always been the same. We have been forced by parts availability issues to make wholesale circuit changes, such as the switch from the SB Super Budget series that used an expensive mil-spec pot and a very simple circuit, to the SV Super Versatile kit series, when the special pot became exceedingly expensive. So, we designed it out and the kit became a little more complex to accommodate its absence. We tried to use the budget of the old pot as a guide for the new circuitry cost.

Along the way, many amp builders, techs and hobbyists have tried to copy what we do. Most have since disappeared. One tech, Dana Hall, saw the original Classic-PS circuit in an amp he was to design a PCB for and decided to market his own kit. He called it "vvr" for variable voltage regulator. It is a Human foible that when we see how something is done, we inevitably say, "Oh... I knew that". One might have been aware of the basic circuit yet never did they apply it how we did.

Dana made his circuit simpler and changed the active current clamp to one that is hit and miss, depending on an unpredictable mosfet specification. It would have saved a component to leave this feature out. He also made a design choice that we described in TUT4 (The Ultimate Tone volume 4) as being "not preferred", because in some cases indirect control is preferred over direct control. For Dana this allowed another simplification and he sold his kit for a very low price benefiting a lot of players who wanted to make their amps quieter. He also sold his kits to a few amp builders who incorporated it into their products.

One of the problems with Dana's interpretation of Power Scaling, was that in most cases the whole amp is controlled. There are a few problems with doing this. One is that the amp tone changes with the power setting. This is distinctly not Power Scaling. The other problem is that every Volume pot that connects to a tube grid becomes "scratchy" when rotated. This is due to the changed DC current through the pot. The fix is to add a coupling cap to isolate the pot from DC, but this further requires that a grid-leak resistor be added for the tube grid as most guitar amps use the pot for that function.

The scratchy pot problem extends all the way back to the input where the guitar is plugged in. Most tube guitar amps do not have a coupling cap at the input; rather, they have a direct DC connection. The guitar pot is suddenly behaving quite rudely!

With Classic-PS as offered originally, none of those issues existed. Only the output stage was Power Scaled. Classic-PS had different issues that mostly made installation a little trickier, but once that was done correctly the tone stayed the same as one dialed the controls down. For the player, the main imposition was that there were two panel controls, Power Scale and Drive Compensation. Both had to be set about the same to retain the amp tone. They could be used independently to achieve three alternate performance ranges. Dana did not include the Drive Compensation control either out of further simplification or of simply not knowing why it was needed?

Other things, such as Power Dampening, were a copy of Mesa-Boggie's Limit control from a specific bass amp model. This varied the bias to the Schmitt splitter and thus limited drive to the output stage and subsequently of output power. This approach has the scratchy pot problem which we fixed in our SL-MV Splitter Limit Master Volume kit. We added three components to the existing one, quadrupling complexity, but making the approach actually useful for anyone that might need to change the control setting more than once per performance.

Marshall introduced a two-thirds form of Power Scaling on its Slash and Yngwie models. They followed the concepts presented in SSH Secrets & Secret Holders, but made an interpretive error in the execution. Their error is pretty common for techs and engineers not used to dealing with mosfets in power control positions in tube amplifiers for musical instruments. Yorkville Sound made the same error, although not in a variable power circuit, rather, in an active hum filter. In the Marshall amps, techs reported that if they disconnected the "Electronic Power Attenuator" that the amp sounded as it should, but once reconnected the amp sounded "stifled". Marshall combined the power control and drive compensation on a single control and that part worked inasmuch as the stifled sound was consistent over the loudness sweep.

In electronics there are countless ways to achieve the same goal and every tech, engineer or hobbyist will try to re-invent everything to put their own mark on whatever they are attempting. Sometimes, the best and/or easiest ways have been found. It was said once with respect to our Power Scaling kits, "Kevin O'Connor likes complicated circuits". It is not that I like them so much as I believe in Einstein's wisdom: "A thing must be made simple enough to achieve the goal, but no simpler" So, for me I do not want to sacrifice performance niceties, such as "smoothness of control" or player ergonomics just to save pennies, or to have an aesthetically simpler circuit.

All of the above is explained in much greater detail in TUT4 and TUT6.

Hi Guys

The bias regulator used in our Power Scaling kits requires a higher raw bias supply voltage than many stock bias supplies provide AND a lower impedance. Bias supplies derived from the plate supply through high-value resistors DO allow an outrageous voltage to be attained, up to the absolute value of the B+. However this is at high-impedance. To make the bias regulator happy, the dropping resistor has to be made much lower in value, usually by paralleling many more resistors. This leads to an excess of heat in the chassis. The output of this supply needs a zener clamp as a minimum to protect the pass element of the bias regulator.

Other situations allow the stock bias winding to be separated from ground and a voltage doubler circuit to be implemented. This works reasonably well in Hiwatts, some Fenders and some Marshall amps.

Using an auxiliary transformer wired backwards and powered by the heater supply eliminates all of the concerns with the other methods, while providing a high-voltage at medium- to low-impedance. Our RBX Raw Bias Auxiliary Supply kit provides -84V with a typical bias-set network attached to the bias regulator output. With no load its output can pop up to -110V, but this is okay for the BJT in the regulator.

Readers of the TUTs (The Ultimate Tone series books), this forum and our FAQ will know that it is always a good idea to have excess sweep of the bias controls to allow complete turn-off of every tube sample. Usually, having 15% of the absolute screen voltage is sufficient. For example, if Vs=500V then -Vb should extend to -75V; for 400Vs, -60Vb, and so on. The target bias voltage for large-bottle tubes and 6V6 is around 10%, but we do not want to make the mistake of some amp companies of ONLY providing that amount of bias voltage as some tubes will red-plate and some will be stone cold.

From the numbers above we can see that there is a limit of compatible B+ for RBX. The bias regulator BJT can go to having zero volts across itself with -84V output. This would correspond to a screen voltage of 560Vs. For screen voltages higher than 560V RBX is inadequate for the task and the tubes will be overbiased and likely red-plate. The solution is to use a larger PT in the RBX format, i.e. go from 6VA to 12VA, then the output voltage will rise to almost double. An amp with 750Vs needs -113Vb with standard tubes to assure a proper control range of the tubes. Even at 600Vs, as in a Marshall Major, we need -90V for adequate controls, so RBX is not quite good enough there.

Note that the RBX PCB is sized only for 6VA transformers, which is suitable for the majority of amplifiers. For Vs>560V you would need a separate PT and RBX-LT with higher-voltage caps than usually provided.

Going to the larger auxiliary PT allows us to reconsider the decision of wiring the new PT forwards or backwards.The forward wiring has no loss per se and the output voltage will be as one would expect, allowing the use of the same 6VA PT provided this will support the bias-set network et al. We would need a 115Vac or so secondary. The primary can be a single or duals - with the 229-series from Hammond every PT has dual primaries and secondaries out of necessity of the design. The primaries are wired in series or parallel, as required for your mains, and the secondaries are wired in parallel. We now have enough bias voltage for Vs=800V. For higher Vs we wire the secondaries in series and regulate it down to a reasonable range less than 200V so as to protect the bias regulator BJT

Have fun