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RR763 BLACKVIBE Guitar Tube Amp

By Rob Robinette

Have comments or corrections? Email rob at: robinette at comcast dot net


Update 2/2019: Deleted the bias board by moving the bias circuit to the main board.

Design Philosophy

The 1963 Fender AB763 blackface Vibroverb is my favorite amp of all time. It will be forever linked to Stevie Ray Vaughan's amazing tone but building a blackface Vibroverb is a daunting task, even for an experienced amp builder. I designed the RR763 BLACKVIBE amp for builders who don't want to deal with the complexity of the AB763 circuit's reverb, tremolo and dual channels but still want its authentic Normal channel tone. Throw a Strymon Flint reverb & tremolo pedal in front of the BLACKVIBE and you have a Vibroverb Vibrato channel too.

Removing the reverb, tremolo and Vibrato Channel drops the preamp tube count to only two but the tone stays 100% Normal Channel Vibroverb. The rest of the amp stays true to the Vibroverb. This amp uses authentic Vibroverb power and output transformers and choke so voltage levels and power output are all blackface Vibroverb.

BLACKVIBE (6L6)

BLACKVIBE 6V6

BLACKVIBE MICRO

BLACKVIBE

It's very much a single channel Vibroverb. Download the pdf here and the DIYLC file here.


Circuit Tweaks

The AB763 Vibroverb's reverb, tremolo and Vibrato Channel were deleted.

The Vibroverb has no Mid tone control on either channel but I added one to the BLACKVIBE to enhance tone shaping. I used a 50KA pot for the Mid/Raw control versus the AB763 standard 10KA so we can dial out some mid scoop and boost preamp gain. This added gain is enough to surpass the Vibroverb's Vibrato Channel preamp gain. With the Mid/Raw control at full "down" you get the Vibroverb's standard 6.8k of mid resistance so keep the Mid/Raw control full down for true Vibroverb tone but turn the control up and mid frequencies and gain are added. Near the end of the knob turn we're approaching tweed territory.

I deleted the separate bias board and moved the components to the main circuit board. I also modified the bias circuit to add a Balance pot and increase the bias range. The Balance pot allows us to set the same bias current in both power tubes even without a matched pair. It also allows us to dial in some intentional mismatch to add sweet sounding even order harmonic distortion to thicken up the clean tone. The bias circuit 50k trim pot can bias 6L6GC tubes hot and yet can still cool the bias for a pair of JJ6V6 too. See Adjustable Balanced Bias for details. If you want to stick with the "normal" AB763 bias circuit just reference the Vibroverb layout at the bottom of this page.

The filter cap "doghouse" has been eliminated by moving the filter caps to the main circuit board. The deletion of the reverb and tremolo freed up lots of circuit board real estate.

A Fender style dual ground bus is used. The power tube screens, phase inverter and preamp triodes are grounded to the chassis at the input jacks. You must use a star washer on the input jack with the chassis ground to ensure a solid chassis ground connection. The power tubes are grounded to the chassis at the "Power Amp Ground" located near the power transformer. The transformer high voltage and 6.3v center taps are also grounded there. The bias circuit is grounded to the Bias Balance pot so use a star washer on the pot for a solid ground connection.

Although the BLACKVIBE is a single channel amp I kept the Vibroverb's 220k channel mixing resistor because it has an effect on the phase inverter's overdrive tone.

I added rectifier "backup diodes" to protect the amp from a rectifier tube short failure. The diodes also make life easier on the rectifier tube so it should last longer.

I increased the watt rating for many resistors to promote longevity and reduce speaker hiss.

The layout also shows the use of metal film resistors on the input jacks in an effort to reduce hiss.

The build can be simplified by eliminating the Bright Switch, Bright Cap and Standby Switch, none of which are really needed.

Many AB763 blackface modifications apply to the BLACKVIBE.

It's a very simple amp compared to the Vibroverb below. Download the pdf here and the DIYLC file here.

I recommend using an AB763 Pro chassis for the build because it's the exact same size as the Vibroverb chassis, the power transformer will drop right in and it has only 4 preamp socket holes versus 6 for the AB763 reverb amps. The Pro chassis also has a rectifier tube socket hole. You will have extra control panel and preamp tube holes though. The open tube socket holes will aid in chassis cooling. A custom faceplate will cover the unused control holes. You might want to consider adding a presence control and master volume to fill those empty holes ;) See the BLACKVIBE MICRO layout to see a simple but effective Trainwreck Type-3 master volume.

The Classictone AB763 Vibroverb transformers are as follows:

Power Transformer 40-18029 355-0-355V 200ma

Output Transformer 40-18006 40 watts

Choke 40-18003 4H 90ma

Doug Hoffman can custom make the BLACKVIBE circuit board. He will also populate it for you if you wish.

There are many cab makers that can build you an AB763 Vibroverb cab that the Pro chassis will slide right into. I'm a big fan of Mergili cabs. Mojotone.com also sells excellent cabs.

The Vibroverb came with a 15" speaker but you can build the BLACKVIBE with whatever speaker compliment you wish or simply build it as a head. The Classictone 40-18006 Vibroverb output transformer comes with 4, 8 and 16 ohm secondaries for very flexible speaker options.

Stevie Ray Vaughan always used an EVM 15L with a distinct, yet American voice. Unfortunately they are out of production but re-cones do show up on ebay occasionally.

Weber California 15 in ceramic and AlNiCo is an excellent choice. The Weber California 15" was designed to mimic the JBL D130 which was a Fender Vibroverb OEM speaker (along with the Jensen C15n in the next paragraph). The 80 watt, 8 ohm ceramic version is much less expensive but louder than the AlNiCo version. I highly recommend the paper or "LargeH" dome option over the aluminum dome. The "LargeH" dome (large hole cloth) was designed to tame the brightest amps so use it if you prefer a darker tone.

Jensen C15n AlNiCo is the modern equivalent of what the AB763 Vibroverb came with from the factory and it gets goods reviews. American voice and relatively inefficient so not as loud as other speakers.

Eminence legend 1518 is a popular 15" speaker. It has a loud, efficient and punchy American voice.



RR763-V BLACKVIBE 6V6

The BLACKVIBE 6V6 is designed for those that don't need the big 40 watt output of the 6L6 BLACKVIBE. The BLACKVIBE 6V6's 20 watts of output is enough for most club gigs and still sounds great in the studio. Although the BLACKVIBE 6V6 uses the AB763 Deluxe Reverb power and output transformers we use higher phase inverter and preamp supply voltages to keep the tone closer to a 6V6 Vibroverb. Voltage dropping resistors and the bias circuit are changed to suit the 6V6. The NFB tail resistor was increased from 47 ohms to 100 ohms to compensate for the lower output transformer secondary voltage of the 6V6 power amp.

Although the GZ34 rectifier tube is specified, power supply filtering is slightly reduced compared to the 6L6 BLACKVIBE to support the 5U4GB rectifier in case we need to use that rectifier to lower the amp's voltages.

Download the pdf here and the DIYLC file here.

We'll use the AB763 Deluxe Reverb's power and output transformer and choke.

The Classictone Deluxe Reverb transformers are:

Power Transformer 40-18016 330-0-330v 120ma

Output Transformer 40-18002 6.6K:8 ohm 20 watts

Choke 40-18040 4H 50ma

I recommend using an AB763 Deluxe Reverb chassis for the build because the transformers will drop right in. The Deluxe Reverb chassis also has a rectifier tube socket hole. You will have extra control panel and preamp tube holes though. The open tube socket holes will aid in chassis cooling. A custom faceplate will cover the unused control holes. You might want to consider adding a presence control and master volume to fill those empty holes ;) See the BLACKVIBE MICRO layout to see a simple but effective Trainwreck Type-3 master volume.

Download the pdf here and the DIYLC file here.

The build can be simplified by eliminating the Bright Switch, Bright Cap and Standby Switch, none of which are really needed.

Many AB763 blackface modifications apply to the BLACKVIBE 6V6.



RR763-M BLACKVIBE MICRO

This is the one watt, 12AU7 push-pull power tube version of the BLACKVIBE that is perfect for true bedroom level practice and studio work. The preamp and long tail pair (LTP) phase inverter are 100% AB763 Vibroverb but the power amp uses a single, cathode biased 12AU7 power tube wired in true push-pull.

A Trainwreck Type-3 post phase inverter master volume is added to help control how hard the little tube is driven.

Large grid stop resistors are added and the grid leaks are reduced to help the little triodes deal with overdrive grid current.

The NFB circuit has been massaged to compensate for the low output transformer secondary voltage.

The BLACKVIBE'S power supply has been modified to keep the little power tube happy. Since our triode power tube has no screens the screen power node has been deleted. The choke now smoothes the entire amp's power supply.

The rectifier tube has been changed to a 9 pin EZ81 which uses a standard 9 pin socket and uses the same 6.3v heater circuit as the rest of the tubes so a 5v heater circuit is not required. The EZ81 uses 1 amp of 6.3v heater current and is rated for a 50uF first filter cap. If you prefer to use an octal rectifier then wire an 8 pin rectifier socket like in the full size BLACKVIBE above and use a GZ34 rectifier tube.

The 6.3v heater center tap is elevated by connecting it to the power tube cathode resistor (see layout below).

The amp sounds great with full size cabs and speakers but you could use 4 six inch speakers in a cab and call it a Super Reverb Micro. A 10 or 12 inch speaker works great for a Vibroverb Micro.

Download the pdf here and the DIYLC file here.

The BLACKVIBE MICRO uses a Classictone 40-18027 275-0-275v power transformer.

The output transformer is a 5 watt rated Hammond 125B wired for 22.5K:8 ohms (use the orange and yellow secondary wires--do not connect the black wire).

A Classictone 40-18003 4H 50ma choke supplies the entire amp with ripple-free power.

Download the pdf here and the DIYLC file here.

The build can be simplified by eliminating the Bright Switch, Bright Cap and Standby Switch, none of which are really needed.

Many AB763 blackface modifications apply to the BLACKVIBE MICRO.


Add a Presence Control

I'm a fan of late-in-the-circuit tone controls which allow tone tweaks with less side effects and the Presence Control is one of my favorites. To add a Presence Control we'll delete the 47 ohm NFB Tail resistor and replace it with the presence circuit. The circuit is made up of a 5KL pot with a .1uF cap and 4.7k bypass resistor. We also have to adjust the size of the NFB resistor and phase inverter tail resistor (PI Tail in layout) to compensate for the presence control's added resistance (going from 47 ohm NFB tail to 2.4k of tail resistance). With this presence circuit you'll get standard AB763 tone with the presence pot turned full down.

The 4.7k bypass resistor on the presence pot reduces total presence circuit resistance to a max of 2.4k and keeps the amp working if the pot wiper fails (5k in parallel with 4.7k = 2.4k). The .1uF cap sends high frequencies from the NFB loop to ground which boosts highs at the speaker. These boosted highs have less NFB so they are "thicker and harrier" with additional harmonic and intermodulation distortion which adds "presence".

The ratio of the NFB resistor to the NFB Tail resistor determines the level of NFB signal. The ratio of our BLACKVIBE is 820 / 47 = 17.4. Since the NFB tail resistance is being increased to 2.4k we can use the NFB ratio to calculate the appropriate NFB resistor to pair with the presence circuit: 2.4k * 17.4 = 41.8k and 43k is the nearest standard resistor size for the new NFB resistor.

Since the presence control adds 2.4k of tail resistance to the phase inverter circuit we need to decrease the PI Tail resistor from 22k to 20k to compensate.

For the BLACKVIBE MICRO only, use a 12k NFB resistor to compensate for the low voltage output of the output transformer secondary.

We delete the 47 ohm NFB Tail resistor and replace it with the presence pot. The NFB resistor is increased to 43k and the PI Tail resistor is reduced by 2k to make up for the added presence pot resistance.

We can use this circuit to install a presence control in any blackface or silverface amp with a long tail pair (LTP) phase inverter. See the AB763 Presence Control mod for details.


The Original AB763 Vibroverb

Reverb, tremolo and a second channel add a lot of complexity to AB763 amps. Download the pdf layout here and the DIYLC file here.


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