[rfeenstra]

Another project!  This will be on the shelf for a while.  It belongs to my sister who bought it and a matching tuner in approximately 1965.  This was a Scottkit that I assembled while in, I think, 9th grade (she purchased the tuner assembled since she didn't trust me to get that right).  It has not been used for a decade or more since it "hummed" a lot.  A look at the bottom side shows that she listened to it a bit too long with it humming! She had a cabinet custom built (3 pieces) that also housed a Girrard turntable and a pair of Jensen SG-300 12" Triax speakers.  I don't remember what the system sounded like but it will be kind of fun to resurrect it.  I see you guys talking about the Fisher stuff, how does this compare?

   
   
   
   

[Phlogiston]

It is, at least, as good as a Fisher.

[rfeenstra]

Before I opened the bottom of the amp, I had a bit of apprehension about my work as a teenager.  But, after close examination, I was fairly pleased with my layout and solder connections way back then!  I read a review of this amp a couple of hours ago and they reported as being very warm but not very bright.  It rolls off the highs a bit and works best with speakers that accentuate the the high end.  We'll see.  I tested all of the tubes and the 12AX7's, the 5AR4 and the 6U8's test ok, if not very good.  The output tubes, 7591s, are all very weak.  All tubes are original.  The 12AX7's are Telefunken and the 7591's are branded as Scott.  Not sure of the others as the identification markings are gone. Unfortunately, my sister lost the original assembly manual with the balancing instructions.  I found the schematic on line and hopefully will find the rest of the info I should have.  When I get to this project,  I'll re-stuff all of the capacitors, check all of the resistors, and replace the outputs with matched 7591's.  When this is finished, I'll tackle the tuner.

[morzh]

Well...this is an integrated amp. Compared to it, my Eicos are the epitome of simplicity.

[rfeenstra]

Well, it didn't sit on the shelf. It went to the bench.  Replaced all the tubular and lytic caps, several resistors  that were actually within spec, but did it for grins and giggles cuz they were in bias and balanced circuits, one open power resistor, 4 matched output tubes, and one bias supply selenium rectifier. I had to add a bit of resistance to the the bias supply to compensate for the silicone rectifiers. I didn't re-stuff the old caps as the new caps were actually larger than the originals (physically), but they didn't look all that different anyway. I got a kit of lytics from Hayseed Hamfest. They had a twist-lock kit for this amp. They look different but I'm looking for performance, not original appearance for this amp. I set bias and balance for the the 7591A's ( boy do those things generate heat!) and ran a sine wave signal through the amp.  I noticed that the sine wave was slightly miss-shaped on one channel.  Turns out I didn't do quite as good a job building as I originally thought.  Found a slight wiring error in the preamp section that accounted for the slightly distorted waveform.  I didn't have a scope or signal generator in my grade-school days!  Once that was fixed, I started doing some power/distortion tests when I started seeing some very strange wave-forms on one channel on the scope.  A 6u8 died.  Now I have to wait for a matched pair of 6u8's. Original 12ax7's and 6u8's were Telefunken's.  Don't know what I'll be getting for replacements of the 6u8's.  I replaced the 7591A's with new, Russian made TungSol's.  Review's seem ok, but we'll see.  At least they are the same physical size as the original tubes. I'll hook this amp up to my DAW  in my sound studio to see how it compares to more modern stuff.  I suspect I will be disappointed, but we'll see!

[morzh]

Jim McShane speaks highly of the Russian Tung-Sols; I got those for my EF-22s.

>>>I noticed that the sine wave was slightly miss-shaped on one channel. Turns out I didn't do quite as good a job building as I originally thought. Found a slight wiring error in the preamp section that accounted for the slightly distorted waveform. I didn't have a scope or signal generator in my grade-school days!
>>>

I'm sure this was what accounted for that extra warm tube sound

[rfeenstra]

My sister probably thought she had something wrong with one ear.

[Electrothaumaturgist]

"Deaf in one ear and 'dumb' in the other" was said about my father- by my mother... 

[morzh]

....as later was discovered, all that famous therapeutic quality of the air at the resort was due to the unknown to everyone leakage from the main sewer pipe

[rfeenstra]

Restoration complete.  Had to purchase a matched pair of 6U8's.  Matched Sylvania's are what came.  The channels matched up quite nicely.  Set the bias for the outputs at 65 ma, a fuzz low,  and set the balance.  Frequency response is very smooth from about 50 to 20K.  Starts rolling off below 50.  Hooked it up to my Legacy Classics in my studio and powered it up.  Sounds great!  Better than I expected, even comparing it to my mono blocks.  Very pleased.

[rfeenstra]

Been listening to this thing for nearly 2 hours now - love it!  Need to shed a layer, though.  My studio is well insulated and I think I could use this amp as a furnace in the winter!  7591's generate a bit of heat.

[Phlogiston]

I put a small 12V fan on the back of my tube receivers/amps and run it at 6 - 9V. It keeps the whole thing cooler and at reduced speed (6 - 9V) does not make much noise. The room will still get warm.

Sort of hard to make out here. The wall wart is plugged into the switched outlet so the fan turns on and off with the amp. This is a fisher X-100.

   

[rfeenstra]

I had put a small 110 volt fan in the cabinet that this was in when my sister had it.  The tuner was above the amp and I wanted to get rid of the heat. That fan needs to be taken apart, cleaned and re-lubed.

[rfeenstra]

Question for you tube amp gurus:  A bit of background before the question. Normal suggested bias for the 7591A pair is 70ma combined cathode current (35ma/tube). The amp generated fair heat and the trannies would get fairly warm to the touch.  Reduced current to 65ma and noticed a reduced heat factor, without affecting the sound - plenty of power in my studio.  I've now reduced the cathode current to 56 ma (required a bit of circuit change to get the bias voltage where it needed to be for the reduced current), and the amp still sounds great, though I'm sure my max power output is down, but still way more than I need.  How far can one reduce idle current before a degradation in sound at lower listening levels?  I would assume that getting the current too low would put the tube in an operating range where linearity would be affected.  I'm just looking for maximum tube life without sacrificing sound quality.  Power supply voltages run about 15 volts high with the reduced output idle current.  A problem? Well within the voltage ratings of all caps.

[codefox1]

Shelf space was more important than tube life way back when. The "vent" panel on top of amps and combos was minimal. Yeah, a "whisper" fan or one from a discarded PC power supply helps a lot, and the 7591's will go on for a while. Probably could sub some bigger and more commonly available pentodes if width allows and height no longer a problem when it becomes necessary.