Tech Consultations

[Fred Cook]

Some questions for our technically capable members. All refer to a 1969 (June 19 to be exact) Sceptre with orginal 4x12 speaker.

-- The beast has a low, but constant hum. 60 Hz I would guess - audible even when the volume is at 0. Level drops a bit when the Reverb gets past 5??

-- Acquired an original foot switch. Replaced the wire and RCA jacks. Tremelo seems to work fine. Reverb is too noisy to use. Problem goes away when the switch is not used.  

-- Speaker jacks are marked 16 Ohms and 8 Ohms - Extension. Which should I use with factory cab?  

-- Finally, as amp is run louder, past 5, distortion increases. (Duh!) Trick is that it sounds more like solid state distortion than classic tube overdrive. Bias? Should have more or less bias current? Plate voltage is 550V, tubes are JJ KT-88. Rectifier is original RCA. Any thoughts?

Many thanks!

[JoeArthur]

A 60 cycle hum (just slightly higher in pitch than the "A" string of a bass) could be feeding in from the power supply for the reverb recovery amp.  This power supply only features 1/2 wave rectification which would give a 60 cycle tone just waiting to be injected into the signal, and it would be louder when the reverb control is zero... and not dependent on volume control settings.  The first thing I would do is replace the two 1000mfd/35 volt caps (locate point "D" in the power supply section of the schematic located on this site - http://www.sunnamplifiers.com/schematics/sceptre.gif )

If you can find larger than 1000mfd that won't have trouble physically fitting in the chassis, by all means use them... this will increase the filtering ability which is real important in 1/2 wave rectifiers.

What kind of noise are you hearing with the footswitch?  The footswitch connection inside the head goes to the input of the reverb recovery transistor... along with the output of the reverb tank.  If the noise is scratchy sounding, then you could have a bad solder joint in the switch, the switch could need replacing, or the switch jack in the head could be dirty.  I would clean the switch jack in the head first, if it has been unused for some time, then it can build up crud that won't make a good contact.  If you cannot switch off the reverb with the switch, then I would suspect the switch or a solder joint at the switch terminals.

These older amps won't have a lot of classic tube overdrive sound - the circuit is no where near what creates what most people associate with classic tube overdrive.  The power amp is a hi-fi ultralinear design, with KT88/6550 tubes.  The poweramp input is a pentode feeding a split load phase inverter (sometimes called a concertina). Totally different from the run-of-the-mill Marshall or Fender poweramp design.

Calling it "solid state" sounding doesn't clarify the sound for me.  Solid state distortion has a range of sounds, and not all of them are unpleasant.

[Fred Cook]

Joe,

Many tanx!

The reverb sound is a scratchy noise. The jacks on the amp are spotless, but I cleaned them anyway. Will try contact cleaner on the foot switch and a resolder job.

Gotta get tech help to follow up on the 60Hz hum, but at least I know where to start looking.

Solid State sound - well, I guess I would say it sounds raspy rather than "growly." Is that scientific enough? Wanna take a guess at the optimal bias current or static dissapation of KT-88? I get a wide range of answers.

[JoeArthur]

It's not wise to guess... This link has tube charts.  Both KT88 links show 35 watts plate dissipation as "design maximum".  Guitar amp makers usually run higher than this.  Current flow and static dissipation really depend on circuit configuration and supply voltages.

http://www.triodeel.com/tubedata.htm

Me... as I have posted elsewhere, I just set the bias voltage and run with it.

These amps were really built for clean sounds... and chances are that the distortion you don't like is being created by the 6AN8 stages being overdriven.  

I'm not recommending hacking this amp up...  but this circuit just screams for a master volume control between pin 8 of the 6AN8, to allow only overdriving the second half of the 12AX7 preamp without placing the 6AN8 stages into clipping.