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LX-72A Sanity Check for Noooobie?
LX-72A Sanity Check for Noooobie?
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Old 23rd October 2006, 09:35 PM   #1
Vespasian is offline Vespasian  United States
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Join Date: Sep 2006
Default LX-72A Sanity Check for Noooobie?

First, thanks to all that have helped me out so far...

I'm new at this so please forgive me if I butcher any terms, seem to have things confused or in general, appear to be an idiot.

You may recall that I am hoping to restore Dad's scott amp and after replacing the selenium rectifier with a radio sh*ck 4a 400v, the the heaters started glowing in the 12ax7's. I have two good 7591's so with them running the right channel I started to do some measurements. CD player in gives me sound, no audible distortion, just a slight hum at 0 volume. Tone controls for right channel working fine, volume control is little scratchy. <smile>

I do have replacement caps on the way including a couple JJ cans.

I'm hoping that if I provide some numbers, you could take a quick look and point out anything that is just not right.

I added a 10ohm resistor in series (before R210) for the new bridge rectifier to bring the voltage down a bit as I was getting -46.5 and the schematic shows -45 (see detail below). It is now putting out -44.6 and this potential is showing at pin4 (heater) of the first 12ax7 and the the center of the pots for the 7591's. Pin 5 (heater) is reading -33.4.

The transformer is delivering 6.86vac directly to the heater (pins 2,7) of the first 7591. This is supposed to be 6.3vac?

I'm getting 462v after the 33ohm 10w (R206) instead of 425. After R205 316; R204 274; R203 261; and after R202 I'm reading 160vdc.

453 is the plate voltage on both 7591's while G2 (pins 4, 8) are at 414.

Plate voltage on the 12ax7 is 174 and the triode plate of the 6gh8 is reading 250 (pin 1), while the pentode plate is at 55.4 (pin 6).

Full LK-72a schematic can be viewed : http://hhscott.com/pdf/fs/LK72A.JPG

Please, any comments, questions, warnings etc are greatly welcomed...

Cheers, Ed

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You think it is dirty? Should have seen it last week!
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Old 23rd October 2006, 09:37 PM   #2
Vespasian is offline Vespasian  United States
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Default Duh... LK-72a

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Old 23rd October 2006, 09:53 PM   #3
Vespasian is offline Vespasian  United States
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Default From the assembly manual...

Wanted to post this one too...
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Old 23rd October 2006, 11:52 PM   #4
gingertube is offline gingertube  Australia
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Default Long - Gingertubes Rave / Tutorial

G'day from Oz. Dads amp looks remarkably clean and is well worth a resurection job.

You've got off to a good start at diagnosing any problems with all of those voltage measurements you've done and posted above.

First you need to remember that you are running without one pair of output tubes at the moment and so less current that normal is being drawn from the High Voltage Supply. Thats why you are seeing that 462V instead of the 420V shown as typical.

Another BIG caveat on tube amps is that due to component tolerances, mains supply variations etc + or - 10% is the tolerance you are working to. You will want to watch things like , say, if that HV supply on the anodes of the 7591 are 10% high than ideally you want the bias volts (the -45V supply) to be 10% high as well but in general within 10% is close enough.

When you fit the second pair of 7591s the HV will drop, you may have to adjust that 10R series resistor you fitted to the bias/preamp heater supply to get it back to -45V and because you are then loading the power transformer more heavily you will probably also find that that 6.86 V AC you measured on the 7591 heater will come down a bit too.

The next thing in Gingertubes diagnostics tutorial is that you need to think in terms of currents as well as voltages - where is all the current going is a valuable way of looking at the circuit. In general circuits which are working right pass the right amount of current, circuits which aren't pass either not enough current or too much current. The voltage measurements are what allows you to work out the currents and because we are talking about voltage drops across resistors (a differential measurement if you like) than all those measurements you did above are quite valuable.

How - From a couple of your measurements and comparison with values taken off the schematic:
Vdrop R205 = 154V across 8K => is 19.25mA
Should be 125V => 15.7 mA
Vdrop R204 = 42V across 4K7 => is 8.9mA
Should be 33V =>7mA

That straight away tells us that approx 8 mA is drawn from the end of R205 (ideal value). Checking the circuit you see that this is the supply to the Triode section of the 6GH8 which is a concertina phase splitter. You can also see that this correct because that current causes 60V drop across the 15K anode and cathode resistors. That is 60/15K = 4mA for the concertina splitter in each channel.

That voltage drop looks a bit high and normally it would mean that the voltage at the end of R205 is a bit low. You won't really know untill the second pair of 7591s is fitted and the real HV is reduced.

Assume however, for the sake of arguement, that the voltage at this point is too low then:
First check WITH THE POWER OFF that R205 is still the right value (8K +/- 10%).
Then check the voltages across those 15K anode and cathode resistors to see that the 6GH8 triode section is pasing the right current (also check that they are the right value and the same value - this affects AC balance in the push pull output)
If that voltage drop is correct it means that the extra current which is causing a too low a voltage is either being drawn from the front end via R204, which you can check by measuring the voltage drop across R204 OR the HV filter electrolytic capacitor C204 is leaky and needs to be replaced .
You get the drift!!

Across R203 = 13V should be 12V, across 2K2 => 5.5mA
Across R202 = 100V should be 65V across 47K => 1.4mA

From the voltages you measured , that last one looks a bit sus, check that 47K, look to see if that stage is drawing too much current (by checking voltage drop across anode or cathode resistors), if not then suspect C201 might be leaky.

One last point before I finish the rave. Some of the voltages shown on the schematic around V5 (the 6GH8) are WRONG. The anode volts on the triode section should be 240V not 275V as shown (4mA => 60V drop across 15K from 300V rail) and the voltage on the grid of the triode should be 50V not 85V as shown. Note that it is shown as 50V on the anode of the pentode section to which it is connected. The fact that there is supposedly a 35 volt INCREASE in voltage along a piece of wire shows that one or the other of those voltages is wrong. The triode section needs the grid to be lower in voltage than the cathode for correct biasing (Grid volts with respect to cathode volts (Vgk) must be negative) so the 50V is right and the 85 v is wrong.

Given the +/-10% we need to keep in mind, thats saying that the 6GH8 voltages you measured are indicating that that stage is operating correctly.

So all you need to diagnose - resurrect the amp is a multimeter and a few calcs using ohms law.

Finally once thae amp is running - set those DC balance pots (R43 and R143) simply by sticking your ear against the speaker, with no input, and adjusting for minimum hum.

Hope that this little tutorial / rave is of help. Best of luck with "dads amp".

Any problems you run into - there is always someone here who will help.

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Old 24th October 2006, 12:32 AM   #5
Vespasian is offline Vespasian  United States
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Join Date: Sep 2006
Default Thanks Ian...

Your comments / analysis are exactly what I was hoping for! Thanks so much for taking the time to respond in that fashion. I will be carefully reviewing your post.

Cheers, Ed
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Old 24th October 2006, 01:20 AM   #6
Tubelab_com is offline Tubelab_com  United States
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Location: West Virginia panhandle
The higher than normal voltages in an old amp are common. The less than normal load (2 tubes) will cause some of this. The bigger reason is that old equipment is designed for 110 or 115 volts AC. Modern power lines are closer to 125v AC.

I am testing a freshly built amp that is running with its (Hammond 115 volt) power transformer heavilly loaded (I stuck KT88's in where 6L6's belong) and my filament voltage is still high at 6.58 volts. There is not much that you can do about this, and it has not yet caused me any grief.
Tubelab, it's 5 year mission. To explore strange new tubes, to seek out new circuits and topologies, to boldly go where no tube has gone before......
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