Altec Lansing 1594A - 1967 [100W Solid State Mono Power Amplifier] BOM + Manual

[TelRay]

now feeding the amps with 0.8 VRMS 1,000 Hz Sine Wave (2.26 Vpp)

On UNIT#1 (the "old one" still at Rev 4) I was able to go to full VOLUME without seeing any distortion (either peak or crossover).
I got the unit delivering 52 V and drawing 2.5 A from the mains


On UNIT#2 (the "new one" at Rev 5) I get this going to full VOLUME


SCOPE using 10x probe and set to 1 V/div

I got the unit delivering 60 V and drawing 2.8 A from the mains (very close to the 3 A rated fuse)

[Leo_Gnardo]

TelRay a couple days ago you mentioned getting a thermometer. May I suggest something similar to what I got maybe 10 years back, a laser infrared thermometer. Sounds hi tech, and it sort of is. Expensive? Not so much. Mine was only $25 or so from Parts Express. I'm sure there's a number to choose from and lots of places to buy them. Aim the unit, press the measure button, set the laser beam on whatever it is you want to measure, and bingo the temperature shows up on the screen. One tiny drawback to mine, temp is expressed in Centigrade / Celsius. But I've been dealing with C / F conversions since the late 60's so it's no big deal for me. The battery draw is also miniscule. Heck I don't know if I've ever changed the batteries in mine. Maybe once. IIRC it takes a couple of AAA cells. VERY handy for checking output tubes, especially sets of 4 or more. You can easily identify which one(s) are running excessively hot or cool within a set. I just worked on a '68 Showman head - one of the 6L6's was running some 30 degrees hotter than the rest, indicating excessive current draw, so it got replaced with one more compatible with its mates before delivering the amp to its owner. All sorts of uses for this kind of thermometer outside the workshop too. Very affordable, very available, very useful. Get yourself one!

[TelRay]

...I suggest something similar to what I got maybe 10 years back, a laser infrared thermometer. Sounds hi tech, and it sort of is. Expensive? Not so much. Mine was only $25 or so from Parts Express

yes! that's exactly what i have in mind. I became aware of these laser IR things back in 1996 or so when i started working in the automotive industry. Our maintenance guy at the office has one and I was intending to borrow it
regarding deg F to deg C conversions, I feel you... back in high school i had an "applied physics" professor (the "smelliest" on i ever had.. .OMG) that hardwired us in all IMPERIAL to METRIC system conversions. I still remember by heart things like 0.4536 Kg to 1 Lb and multiplying deg C by 1.8 and adding 32 to go to deg F
moreover... i was thinking about getting a thermal camera

[Helmholtz]

..I got the unit delivering 60 V...

60V RMS, peak or peak to peak? For power calculations you want RMS voltage. 1Vrms = 2.83Vpp for sine signals. Convention is that AC voltage is given as RMS value unless otherwise noted.
What OT tap did you use for measurement and what was the load? 100W into 8R is produced by 28.3Vrms.

Otherwise clipping looks symmetrical. Some 120Hz ripple modulation of the tops at full output is normal. The slanted tops are probably caused by the limited bandwidth of the OT.

[TelRay]

thank you for the reply!

60V RMS, peak or peak to peak? For power calculations you want RMS voltage. 1Vrms = 2.83Vpp for sine signals. Convention is that AC voltage is given as RMS value unless otherwise noted.
What OT tap did you use for measurement and what was the load? 100W into 8R is produced by 28.3Vrms.

Otherwise clipping looks symmetrical. Some 120Hz ripple modulation of the tops at full output is normal. The slanted tops are probably caused by the limited bandwidth of the OT.

Peak to peak ... i'm lazy on the square root calculation but happy to adapt to a more proper language. Duly noted
I tapped into the 4 Ohm one as this is what the manual suggests for P2 (bias or balance) adjustment.
I used 2x 8 Ohm loads in parallel which shows as 3.8 Ohm resistance on the MMT (4.7 Ohm - 0.9 Ohm lead resistance)

So, if I follow what you are saying: 60 Vpp translates into 21.2 Vrms (drawing shows 20 at that tap) and with the loads I am using this is going through 3.8 Ohm and then P=21.2^2 / 3.8 = 118 Watt?

Should I look for a similar "clipping" on the other unit as well?

thx,

[g1]

Does this value mean that maybe this amp is biased to hot?

In post #27 I suggested some measurements that will help tell how hot it is biased (we can compare to the spec given for the "C" version of amp).

[Helmholtz]

So, if I follow what you are saying: 60 Vpp translates into 21.2 Vrms (drawing shows 20 at that tap) and with the loads I am using this is going through 3.8 Ohm and then P=21.2^2 / 3.8 = 118 Watt?

Correct, if you measured voltage directly across the resistors, this is the power the load resistors see/dissipate.
0.9R lead resistance looks very high. Sure you calibrated your Ohmmeter? What do you read if you measure a 1R resistor?

If you actually had 0.9R lead resistance, the RMS voltage measured directly at the amp's terminals should be 5V higher than at the load and the leads would "steal"/dissipate 28W (!).

[TelRay]

0.9R lead resistance looks very high. Sure you calibrated your Ohmmeter? What do you read if you measure a 1R resistor?

mmm... bought this Fluke 117 MMT brand new about a year ago and until now I was completely unaware it needed to be calibrated. Those bastards!
I was looking into the documentation and got into a "service mode" in which I could see if it was calibrated. According to the counter (001) that indicates it has been calibrated at the factory.
and... I don't have a 1R

In post #27 I suggested some measurements that will help tell how hot it is biased (we can compare to the spec given for the "C" version of amp).

yes, I measured the resistance (with the resistor attached to the circuit, this thing is a mouse trap and I have no intention to go through the soldering process again) and voltage
results are after 15 min idle

UNIT #1:
R40 138 mV/2.1Ohm = 66 mA
R50 151 mV/3.8 Ohm = 40 mA

UNIT #2:
R40 103 mV/2.6 Ohm = 40 mA
R50 120 mV/2.1 Ohm = 57 mA

I am guessing I am not able to take these results seriously because of the fact that the accuracy of the MMT is in doubt at this low Ohm range and that I have not measured the 0.5 Ohm resistor out of the circuit (with the accuracy issue this value could not be trusted either)

The next thing I did was to evaluate the TRANSISTORS TEMPERATURE with an IR Thermometer

UNIT #2:

First 15 min idle temperature no input no load vol at 0

Q7, 8: 105F
Q9,10: 113F
Q11,12: 118F

After that 15 min more of operation with 1,000 Hz SINE 0.8 VRMS input 4 Ohm load and delivering 10 VRMS at the output (50% of the 20VRMS it delivers according to the schematic)
Measurement taken at 5 min intervals:

Q7, 8: 129, 132, 135F
Q9,10: 144, 145, 149F
Q11,12: 148, 149, 152F


I guess the transistor (RCA 40934?) should work at a Tj well above the max value shown above is 67C so I guess that for the operation at the volume of this experiment should be OK

wdyt?

[Helmholtz]

I was looking into the documentation and got into a "service mode" in which I could see if it was calibrated. According to the counter (001) that indicates it has been calibrated at the factory.

Factory calibration typically only calibrates the meter itself. But meter leads and the 4 contacts involved add resistance (sometimes up to 1R total depending on contact quality). To test short the meter tips (or measure a short piece of wire) and read resistance.This residual resistance will add to all your measurements. If your meter doesn't provide a user calibration mode that includes the leads you need to do the final calibration yourself by subtracting the residual resistance from the reading. Don't expect final accuracy to be better than 0.1R with 2-lead measurements.
And check you meter battery. Low ohm measurements are demanding on meter/battery current.

[Leo_Gnardo]

I don't have a 1R

FWIW I keep a supply of 1% 1R 3W resistors in case I'm inspired to install them as cathode current sensors in tube output circuits. Cheaper by the hundred as we expect, but treat yourself to some next time you order from Mouser or similar supplier. If you're doing tube amp work you may find the 1R's come in handy from time to time. Get at least one so you can confirm what your DMM probe wire resistance is.

"Can they be trusted?" Well, geeze, I don't have a milliohmmeter so have to go "on faith" that these R's are what they're supposed to be. Made by Dale so it's not some fly-by-night outfit. And I'm always on the lookout for a surplus milliohmmeter. One of these days I'll find one. Eventually...

[Helmholtz]

The specified idle power consumption of 20W suggests an idle current of around 160mA per power transistor. Calculation: 0.16A x 31V x 4 (transistors) = 19.8W.
That would result in a voltage drop of 80mV across each 0.5R emitter resistor.
Did I miss anything?

[TelRay]

To test short the meter tips (or measure a short piece of wire) and read resistance.This residual resistance will add to all your measurements. If your meter doesn't provide a user calibration mode that includes the leads you need to do the final calibration yourself by subtracting the residual resistance from the reading. Don't expect final accuracy to be better than 0.1R with 2-lead measurements.
And check you meter battery. Low ohm measurements are demanding on meter/battery current.

that's exactly how I got the 0.9 Ohm value i mentioned before (shortening the meter tips)
will check that battery too, thx

The specified idle power consumption of 20W suggests an idle current of around 160mA per power transistor. Calculation: 0.16A x 31V x 4 (transistors) = 19.8W.
That would result in a voltage drop of 80mV across each 0.5R emitter resistor.
Did I miss anything?

I guess there are a couple of things in play on my (probably incorrect) calculation above (the fact that i did not desolder the 0.5R to measure it and that the MMT error has not been substracted from the in circuit measured resistance)

[Helmholtz]

that's exactly how I got the 0.9 Ohm value i mentioned before (shortening the meter tips)

Oh I thought you meant the resistance of the leads/wires that connect the load to the amp. Forget what I wrote about power loss in the "leads".

[TelRay]

Oh I thought you meant the resistance of the leads/wires that connect the load to the amp. Forget what I wrote about power loss in the "leads".

no problem

[J M Fahey]

"Can they be trusted?" Well, geeze, I don't have a milliohmmeter so have to go "on faith" that these R's are what they're supposed to be. Made by Dale so it's not some fly-by-night outfit. And I'm always on the lookout for a surplus milliohmmeter. One of these days I'll find one. Eventually...

Easier done than said

Kludge a constant current 100mA supply (IŽll draw one for you when I have some free time), feed it from any 9 to 12V wall wart you have, apply test clips across "mystery resistor" and read voltage drop.

1 ohm will give you 100mV , very easy to read.

I made mine to check my DIY made 0.1 ohm ballast resistors out of 0.50mm Constantan/Kanthal wire and also mystery speaker cable, guitar cables, even jack contact resistance (jacks are terrible connectors, originally designed for 600 ohm telephone lines), etc. ; values which can not be measured with any precision by a multimeter.