Webb 6-14-E Steel Guitar Amp

[CJ]

kind of a rare bird, 1980 something transistor amp ,

discrete design has developed some low level cracklin and poppin,  these steel guys are picky, sent it out to a tech but he fixed everything but the problem, so we will give it a shot,

 

[CJ]

15 inch K-130 changed out to some type of rare earth element magnet type, neuridium or something like that,

Eminence Telonics it says, TSNEO-15-4

 

[CJ]

what a coincidence, circuit looks familiar, a lot like the Peavey Series 300 bass amp we just worked on,

Peavey Session 400 has similar circuit also,

 

[CJ]

preamp

 

[CJ]

eq section

 

[CJ]

one section comes out the front, one out the back, interconnects help, but one phone jack has to be unscrewed from the back panel,

reverb pan has junction plate so the pan can stay in the bag,

 

[CJ]

RCA 1B04  7G51  , schematic misprint says 1804,  probably in the 3055 range,

sheet metal screws thread into solder chunks on back of the board,  thought it was locktite making removal so hard, but no.

going to check hfe and put on new washers,

if the transistors are nice, we will use grease,  ;D

 

[CJ]

transistors are spread out in gain, but nothing too radical,

do old transistors sound better? i do not want to find out so i will leave them,  hoping they don't pop.

steel players should be limited to 50 watts anyway, with 100 watts for 5 second solo so duty cycle is low, 

 

[PRR]

> what a coincidence, circuit looks familiar, a lot like the Peavey...

SO similar, they maybe both came from Jack at RCA.

 

[JohnRoberts]

> what a coincidence, circuit looks familiar, a lot like the Peavey...

SO similar, they maybe both came from Jack at RCA.

Jack did a lot of RCA's early transistor application notes before leaving RCA to work at Peavey. I don't think he wrote any guitar amp app notes.

When Hartley had trouble keeping his amps from blowing up he called RCA for help, and the rest is you know...

JR

 

[PRR]

> I don't think he wrote any guitar amp app notes.

It's not a guitar amp (until Hartley or Webb put their mark on it). It is an all-purpose audio amp.

It's not in the RCA lit I have handy. But who the heck needs over a hundred Watts of audio??? Yet Jack could have been working on it in Somerville (RCA supported much odder research). Info handed out on "need to know" basis, radar-servos, shaker-tables, and other strange users. Even those guitar amp folks.

 

[CJ]

there is a lot of hype on these amps, "the Holy Grail of all steel amps" "the Rolls Royce of all steel amps"  and prices reflect this, 1500 to 2500 smackers,

made in Antioch CA, nothing there but an oil refinery and wind surfers,

we are going to fix a problem with the heat sink holes not matching the PC board holes, resulting in micro clearance between hot and ground,

and the board was glued to the heat sink, which means the board bows when you tighten the  transistors, so we are going scrape the glue off the board and heatsink and drill a couple of holes for screws to hold the board in place,

 

[CJ]

drill and camfer the holes

 

[CJ]

plenty of clearance now Clarence,

you can see where the pins were close at the bottom there,

 

[CJ]

old solder removed, re solder without tapping the solder blobs for #6 screws, instead using nuts with built in internal locks,

glue gone, so the board sits flush which will keep tension equal due to non bowing from glue bridges

 

[CJ]

original heatsink had two diodes incased in glue and crammed down between fins, kind of kludgy, so we borrowed a dual diode package from a Marantz carcass and drilled a hole for it,  this should improve heat tracking,

 

[gridcurrent]

original heatsink had two diodes incased in glue and crammed down between fins, kind of kludgy, so we borrowed a dual diode package from a Marantz carcass and drilled a hole for it,  this should improve heat tracking,

there has been some discussion at another forum that tight thermal bias tracking in a quasi-comp output stage is not recommended.

looking through RCA "Audio Amplifier Manual",  APA-551,  dated 9-79, circuits appear to be refinements of those in the RCA Transistor DataBooks. 
Nearly all  those published  designs include an output inductor, some include a Zobel, but never is the Zobel placed before the inductor which is modern convention.

 

[JMFahey]

Nice amplifier and excellent job  That output configiration was classic 1970´s ; not trusting puny drivers to handle final transistors hungry bases  (your ~80 Hfe is measured at a few mA; at high current it could drop as low as 10 or so  ) so an intermediate transistor , usually same type as final ones simply because "they already had some"  eased the driver´s work, which then could be as weak as MJE340/350 or similar.
You´ll find same trick on early Acoustic and Randall amplifiers ... besides Peavey of course.

1B04 were high voltage and dissipation beasts , way higher class (and price) than humble 2N3055; only problem being tbhey diodn´t handle much current.
IF I don´t vremember wrong, around 8A or so.

Steel players needed high power amps and efficient speakers,not to play that loud but for clean heaqdroom.

And in the early days of terrible PA systems, they needed 200W and JBL speakers to match "clean" band members playing Fender Twin full tilt or Acoustic 260/270 , while Bass players often used Acoustic 360/361 so as not to depend that much on PA.

Not sure about ventilation windows or cutouts but that heatsink might benefit from a sideways blowing small CPU fan.
Even a tiny 2" one would move air somewhat, better than nothing.