Troubleshooting guide

Knucklehead Reverb 1.0 p 36
When the tubes have cooled, remove them. Replace all power tubes. (Don't throw
away good tubes from the old set, though—save them as spares!) Install the rear grille.
General information about tube types.
Your Knucklehead Reverb has been shipped from the factory with either 6L6GC or EL-
34 Output Tubes. On the rear Grille, the factory fitted Tube Type is labeled.
It is also indicated in the last 2 digits of the serial number.
The Tubes are graded and matched. The value is labeled on the base of the tube. If
you order a replacement set of Rivera tubes of the same value, you will not have to re-
bias the amplifier.
In the 55 Version, we use a different Output Transformer for the EL-34 or 6L6GC
version. This is to optimize the amps output stage for the particular tube type. In the
100 Version, this was not necessary as the Tom Reichenbach Designed Output
Transformer works well with either Tube Type.
EL-34 is the most common of all European high power tube types, used in Hi-Fi, P.A.,
and Musical Instrument amplifiers for over 50 years. Philips/Mullard, and Telefunken,
built great ones as well as the best coming from G.E.C./Gold Lion from England (KT-
77). However, as we write this manual, there are five factories in the world building this
tube; Svetlana in St. Petersburg, Russia, Reflector (Sovtek) in Russia, JJ/Teslalovac in
the Czech Republic, EI in the former Yugoslavia, and a factory in China. NOS (New Old
Stock) supplies from various other factories are still available, albeit in short supply.
Unfortunately due to the conflict of NATO and Serbia, the Yugo/EI EL-34 is sporadically
available, and it’s quality was never robust. Out of all that are currently made, we have
had the best success with the Svetlana, and some of the JJ/Tesla.
6L6GC is the most common Output Tube used in American-Made Guitar amps.
Fender has been thought of as the one Guitar amp manufacturer that made the 6L6GC
famous. Even Marshall was founded on a Fender Bassman 4-10 design that utilized a
military 6L6 version called “5881”.
RCA was one of the original manufacturers that set the standard for this type tube.
Originally available in a metal housed tube, then later on in various glass envelope
versions, it was a rugged and relatively low cost Beam Power tube. In the 70’s and into
the 80’s, Sylvania worked hard to create the best version ever made, called the STR-
387. Ed Jahns, chief Fender tube amp engineer from the CBS-Fender era was
responsible for pushing Sylvania to create this special 6L6GC. (ed. note., Paul Rivera
worked with Ed Jahns from early 1981 to late 1984 during his days at Fender) Mesa-
Boogie used them as well. Ruby Tubes markets a Chinese copy of this STR-387 tube
and it seems to be ok. NOS GE are somewhat available and are a good tube. In the
60’s through the early 80’s, a version was produced in East Germany and Russia, easily
identified with a “Pinched Neck” sealing inside the tube. This was the worst version
produced that we have ever seen, and was marketed in Europe and the USA under
Siemens-ITT, RFT, Schaller, Raytheon, Westinghouse, Radio Shack, Amperex, and a
host of other brand names. The screen grid as well as the plate dissipation of this tube
was very small and never could hold the specs of the real 6L6GC.