Knight 25 watt HiFi Basic amplifier... how hot is hot?

[primosounds]

Hi, all I just got this mono block amplifier pair running. To do that I installed some 6 henry 160 ohm inductors to replace the missing ones, and a 12uf x 630v Solen caps to replace the 8uf x 800v original C1 caps. All other circuitry and components were left intact as I just wanted to get it running to see if there were any major problems before a major rebuild. The tube line up for this amp is 2 x EL37 outputs, 1 ef86, 1 12ax7 front end, and a 5v4 ( original was 5ar4) rectifier. I am using some GE 6L6GC instead of the EL37. With this tube lineup and the changes I made the voltage specs are very close to original. 480v anode voltage/475 G2, 44v bias, and also similarly close voltages in the driver / splitter circuit. Sound wise the amp sounds very nice, clean and powerful, and plays quite loud without breaking up. There is no hum or other noises, except the pots and switches are scratchy, but no big deal. So, my only worry, is that after 2 hours the power trans gets very hot. Too hot to leave your hand on it for more than a few seconds. I know some amps are designed to operate like that . My EICO HF87 gets that hot and the instruction manual says that is normal. But, for this Knight amp I have no instruction manual and I worry that the amp's power transformer maybe over heating. Maybe, the slightly larger choke is pushing the power trans too hard? Anyway, if somebody has any experience with this amp and can let me know if there is a problem or not , please let me know. Thank you for your replies, primo

 

[cademan]

I haven't memorized or taken a look at the specs recently for the 6L6GC's compared to the EL37's, but if the 6L6GC's draw more heater current than the EL37's, that will make the power transformers run a little hotter. If the 6L6GC's draw less heater current than the EL37's, then I think that the power transformers would be ok even if you think that they are getting overly hot.

My H.H.Scott LK-48's power transformer will get very hot in that you can only touch it for a few seconds but I have had it for years and have played it for hours with no problems.

 

[battradio]

If you're just using your hand it is not too hot 150 to 160 fahrenheit is normal for a power transformer . EL37's draw 1.4 amps of filament current and 6L6's draw on .9 amps filament current your fine .


http://frank.pocnet.net/sheets/129/e/EL37.pdf
http://frank.pocnet.net/sheets/049/6/6L6GC.pdf

 

[gadget73]

Knight stuff also can be a smidge on the marginal side, so it may just run hot by nature.

 

[primosounds]

If you're just using your hand it is not too hot 150 to 160 fahrenheit is normal for a power transformer . EL37's draw 1.4 amps of filament current and 6L6's draw on .9 amps filament current your fine .


http://frank.pocnet.net/sheets/129/e/EL37.pdf
http://frank.pocnet.net/sheets/049/6/6L6GC.pdf

Right,i did check the filament currents for the tubes and was surprised that even with the lower requirement for the 6l6 the power trans was so warm. Of course, it is sandwiched by the 2 output tubes and the rectifier on the other side. Have you ever rebuilt one of these? I hate to work on the circuit board since it is one of those early type brown cardboard? with the super thin tin traces.

 

[battradio]

I have not worked on that particular one , but the Knight kits and Heathkit's all had small power transformers for the output power and ran hot . They were priced to sell and most everyone new at the time they were sold that tube equipment ran hot .

You need a temperature controlled iron and a solder sucker to work on paper boards , always cut the component leeds to be replaced up top and the unsoldering will be much easier and less likely to damage the circuit board .

 

[simondolan]

Hi!
Do you have a schematic for this amp?

Mine is running only 15 watts

500 Va

Thanks

Simon

 

[Brice]

I had the same problem when I had mine.

I used a pair of TungSol KT66 taking only 0.9A heater current. That helped. I also reduced the idle current by increasing the cathode resistor a bit.

But I used to run it with a small USB fan on the side to cool the PT.

 

[larryderouin]

110 to 120*F registers as hot via your hand and can cause 1st degree burns(Sunburn type) if left long enough. 130*F can scald your skin and produce 2nd degree burns. Which is why manufacturers of Water heaters set the temps for 120*F max. Dishwashers that have a "high heat" wash cycle either require 150*F inlet water or have a heater element inside. The best way to check the temps now on transformers is by way of an infrared or laser temp gun/thermometer. 150*F to 180*F is pretty much normal for a lot of transformers but anything over 190*F can be catastrophic for them.

The setup on the HK is somewhat similar to the Sansui 1000A. Right to Left you have the Power Trans, 2 Power resistors, 1 pair of 7591's, output transformer, another pair of 7591's, and the other output transformer. Mine is used 30-40 hours a week and there are a few tricks I've used below.

When I 1st got it temps on the trannies were hovering around 175 for the power and center transformers and 160 for the 2nd output. Moving the power resistors to the front of the unit and using chassis mounted power resistors of 50W brought the temps on the power and the center output transformer down to 155*F. Then I added HVAC Aluminium Duct seam tape to the end bells and laminations facing the tubes to reflect back some of the heat from the tubes. This brought all the transformers down another 5*F. Changing from EH7591's with the large bottle, to Tung-Sol 7591 Re-Issues lowered the temps another 3-4 average. The separator plate between the transformers and the Can caps was removed due to lack of airflow across the one pair of tubes, and transformers. With the top off the temps are right around 150-155*F, and with the top on, about 160*F-165*F. NO FANS Used as they don't do much other than move dust around on this particular unit. Airflow is improved with the removal of the plate, and it's been this way for about 6 years running 30-40 hours a week. No tubes or other parts have been replaced since the overhaul in 2009 other than the output tubes for testing. It sits on the "top" shelf of a TV stand with only the TV above it by about a foot and a 1/2. It's not so great in the summer as it fights the A/C, but in Winter it's a great room heater. It's had all the upgrades to the Power Supply, Bias circuit, grid and screen resistors, and bias is adjustable by 4 pots now. Electrolytics, PIO's, and all films that were in the signal path were changed if they were bad. All power supply cap were changed regardless, and a computer power supply input board with X and Y rated caps was installed. This included and IEC plug so the Aux pwr plugs were removed.

Anything you can do to lower the temps will help the unit overall, but your fingers won't notice the difference. So get an Infrared or Laser Thermometer and just keep an eye on it. Fans are recommended a lot of times, but if you live in a dusty area, be advised you should vacuum the unit at least once a month to remove the dust that is settled. If I use a fan it's usually a household 12" oscillator and it blows across the back on the end of travel. Which is plenty and doesn't get as much dust into it.

 

[Tom Bavis]

Bias voltage doesn't tell you much. What matters is the CURRENT though the output tubes. Though GCs can take 30W dissipation each, that level may be too much for the power transformer. (6L6 needs more negative bias voltage than EL37 for same idle current).