JPB-100 (Rock Troll)



Specifications

Input impedance:

1 Megohm

Output impedance:

8 Ohms.4 Ohms

Preamp tubes:

Three 6SL7GT (NOS)

Power tubes:

Two KT88

High voltage:

560 Volts

Filament heaters:

6.3V A/C

Output power:

100W RMS into 8 ohms or 4 ohms

Operation voltage

115V A/C 60Hz

Internal speaker:

Eminence 1518  15”

Internal speaker impedance:

8 Ohms

Internal speaker power handling:15

150 W

External speakers:

None

Height:

22” (24.5” with casters)

Width:

24.5”

Depth:

9”

Weight:

61 pounds

Covering:

Blonde Tolex *

Cabinet cleaning:

Moist cloth, dish-washing liquid



This one is a BEAST!

I often hear people say things like: 'Nobody needs 100-watt amps anymore...'.

As I discuss on the Home page, many people tend to project their own experiential limitations onto others, often elevating them to the status of some fundamental law of physics.

If you ever have occasion to play with a band that includes an acoustic drummer, and want to produce a clean tone that will cut through the mix, then no 22-watt, and few 50-watt amps are going to cut it. 

Clean headroom, is the operative concept here.

However, when 100 Watts is a bit much, this amp offers the ability to reduce that, in incremental steps, all the way down to 25 watts! No attenuators, no tricky gizmos -- just good, simple, sound engineering.

This is a unique one-off amidst my basic line of Cascodyne-Architecture amps (see Cascodyne Architecture). 

The basic architecture of Rock Troll is best described as quasi-Marshall. The underlying topology is very similar to the JTM-45 (Blues Breaker), with some significant modifications:

  1. Preamp and phase inverter all use toneful octal 6SL7GT tubes (all vintage, but tube is in current production). These bottles sound way better than their 9-pin miniature counterparts, and are less prone to microphonics, particularly in high-power combo cabinets.
  2. The first preamp stage uses two triodes in parallel, not unlike the Matchless Chieftain, but in this context think of it as the two channels of a vintage Marshall jumpered together, as many Marshall aficionados are inclined to do.
  3. The tone stack is in the classic Marshall position, following a cathode-follower stage (more on this in a moment), and before the phase inverter, but this tone stack is a modified Baxandall for less control interaction, and less signal loss. The Middle control is a Steve Bench design, utilizing a tiny inductor, and is completely independent from the other tone controls.  Finally, a pull control implements a frequency shift that affects the Bass and Treble controls.
  4. The second preamp stage and cathode follower are an improved design that includes bootstrapping for maximum gain, plus additional components to guard against arcing at power-up. Filament supply is DC-elevated for additional cathode protection.
  5. The Phase Inverter is a standard long-tail-pair (LTP), but with component values specifically tailored to the 6SL7GT, which is considered by many experts top be the ultimate tube for Phase Inverter service.
  6. The fixed-bias power section makes an honest 100 watts from a pair of KT88s. A pentode/triode switch reduces the output by approximately 50%. 
  7. A 'Power' control introduces a continuously adjustable degree of cathode load, softening response and lowering the power by and additional 50%, for a minimum power level of 25 watts.
  8. Combo cabinet contains a massive 15" Eminence 1518 speaker rated at 150 watts.
  9. Separate Gain and Volume controls allow serious drive at moderate volumes. 
  10. Individually adjustable bias for each power tube, plus external (BNC) test probe ports for adjusting bias without removing chassis.
  11. Unique standby-indicator blinks when power is on but amp is on standby. (Stops when standby switch is set to 'play'.)
This is a complex sounding, touch sensitive amp capable of delivering clean tones at stage volumes, but also perfectly willing to crunch and drive with the best.

Price: $1nquire