Knight KM-15 Monoblock(s) Rebuild

[riverty]

The quick backstory: I picked up a pair of these monoblock amps from a local store about 15 years ago. The sales ticket said they had been "serviced", so I didn't hesitate to put them in use in my living room stereo / TV setup. They worked well and sounded great.

About a year later, while I was playing music and working on something else in the house, one of the amps started to hiss, smoke, and smell. Of course I quickly shut them down. One of the amp's power transformer had just melted down. I'm not sure why. I had other amps to use and simply put these monoblocks away in storage ... until now.

So, I have a pair of monoblocks, one with the power transformer fried, one that worked fine last time I shut it down. What to do? I needed replacement transformer specs. I got the schematic, and it has voltages listed, but there is nothing to tell me current draw (consumption) for B+. Now, I'm sure there are more seasoned folks here that would just "know", but I didn't. So, I decided to do a quick rebuild of the working amp and use it to take measurements for the replacement transformer.

I basically rebuilt the power supply on the working amp which means I replaced the 4 filter caps and power resistors, I am using a new EZ81 rectifier tube (instead of the EZ80), and replaced several (really all) resistors in the amp. All new tubes too! I checked the output transformer and it tests good. I replaced 3 of the tubular caps used in the amp but all other caps in the amp are original disc caps, which hardly ever go bad. (never say never - I know)

The amp seems to work fine.

So why the post? Well, because everything is too high! LOL!

After doing all the parts work, I brought the unit back up slowly using my variac and monitoring voltages along the way and I noticed that I reached running B+ voltage way before I hit 117V in. What I mean to say is at 120V in, everything in the amp is running too hot. Schematic calls for 350V plate supply and at 120V in, I get above 380V on the plates! Not to mention everything else in the amp runs too hot! I found that if I dial the input voltage to around 106V, I can hit my target voltages (for the most part).

So, what gives here? I'm thinking I have a bad/cheap power transformer that is just generating too much voltage for the amp. This might be the reason the other amp's power transformer died? Maybe some kind of breakdown in the power transformer's windings over the years? Your thoughts?

Either way, I'm going to order replacements and would like to know your thoughts before I do. The attached schematic is the KM-15 amp with my voltage measurements in green based on 106V AC in. I didn't want to run the amp at a full 120V (or even 117V) long enough to take measurements for fear of blowing something else up.

Regards,

 

[smoking-amp]

Check the resistance of R-25 and R-15. Then check the voltage across those resistors while powered up and running (no signal input) at the 106VAC. You can calculate the idle running current of the tubes from those measurements. If the tubes aren't drawing nominal idle currents, then the B+ will be high.

The schematic shows 12V across R-25 (220 Ohms). Which would be 54.5 mA for both output tubes. That would be expected with a 10K OT.

Schematic is showing 1.5V across R-15 (1 K Ohms), which would be 1.5 mA for both 12AX7 tubes combined.

 

[Mark Tillotson]

First thing to do is figure out why a fuse didn't blow, and rework the fusing to actually protec t the transformer correctly. Judging from that schematic there isn't a mains fuse. Whoops, bad that someone sold you them in that condition if so.

The transformer may be faulty (for instance shorts between winding layers in the primary can increase the output voltage and cause massive heat generation inside the transformer).

Presumably its behaving different to the other one. It could be the cause of the failure, or a symptom of something else overloading it.

Its a faff but you could try the good unit's transformer in the bad unit, if that's OK then the transformer only is at fault. However figure out a proper fusing setup first, you don't want to repeat the problem.

 

[PRR]

What "wall"-voltage makes 6.3VAC on the heater circuit?

This will suggest if you need to "use a lower wall" (add a 12V transformer to buck 120V to 108V) or just add resistance in the B+ feed to get the fairly bold DC voltages down.

> nothing to tell me current draw (consumption) for B+

It is "on the plan". (Trusting the plan is a different matter.)

R-25 is the cathode current for the big jugs and the great majority of the B+ current. 220r with 12V across is 54mA. This is lower than datasheet suggests. It may rise in tone bench-testing. It probably won't rise with unclipped speech/music. Round-up to 60mA.

R-22 is 2.2k and drops 20V which is 10mA on my thumbs. This is all the small tubes plus the G2s of the big tubes (which we already counted in cathode current). More exactly, consideration of plate resistors and their drops says <3mA for all the small jugs. Split the estimates' difference as 65mA idle.

If you do ever bench-test: datasheet for 7189 says probably less than 105+15mA in big jugs at FULL sine power, so 123mA max.

Some PTs rate for AC Amps, some for assumed DC Amps.

Hint: look at "18W Marshall" replacement transformers. This is the *same* amp only less "polite" than a hi-fi amp. The iron price is very competitive and the hi-fi amp is less strain than a crazed guitar-warrior.