• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

K-12g one channel louder than the other

Well... if its an amplifier that is symmetric (i.e. same tubes on left and right channels), then swap tubes one at a time between the channels, and see which if any have substantially more or less gain. Easy, trivial, informative - even if nothing shows up.

There will only be 2 outcomes: a tube is indicted, or, nothing will change. If a tube is indicted, then just order another one from the SAME manufacturer, or TWO from a different manufacturer. Substitute it/them in. The problem should go away. But though it is more likely that the replacement tube(s) will have higher and correct gain, there's always a chance that the real problem was a tube with TOO high gain! Rare, but possible.

IF NOTHING changes when you swap the tubes initially, then ... follow SoonerOrLater's advice, and check your wiring and hookup carefully. You might have substituted a wrong resistor value someplace that is cutting the weak channel's gain.

There you go.

GoatGuy
 

Lamber

Member
2013-05-05 3:20 am
Thanks for the input. I've swapped the tubes around and haven't noticed a change. I'll go through it and check resistor values later today. Is there anything else I should check? It's consistent in that the sound level on the right channel is always the same amount softer than the left channel.

Could there be a problem in the volume pot? I've read that the volume pot in the K12g kit is low-quality and can cause problems. How would I check if it's bad, though?
 
IIRC the pot is soldered directly into the pcb so not so easy to check out a faulty pot, might be easier to swap inputs to the dc blocking caps at the start of the signal paths to see if the fault moves channel. Before that try swapping source RCA plugs to eliminate them. If no change then eliminate speakers again by swapping. Only then can we assume the fault is in the amplifier. This might seem very basic but it has to be done. Fault finding has to proceed logically from a firm foundation.

The output transformer primary and secondary leads are also ( IIRC ) soldered onto the pcb so I can't think what could go wrong there that wouldn't be far more catastophic :D


Ps Welcome Lamber to the forum :)

pps do you have access to a meter that you could use to check the resistances either side of the wipers on your dual gang pot.
 
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Lamber

Member
2013-05-05 3:20 am
Alright, sounds like I have a good foundation to start checking from :)

I have swapped the speakers and the problem is the same (except it's the left channel that's quieter, as expected).

I don't have access to a meter today, but I'm bringing it in to the lab tomorrow to work on it (this is an independent project for electronics lab).

By the way, I just figured out something weird. I'm not sure if this helps or not, but when I turn down the input volume and max out the amplifier volume, the problem is less pronounced. The lower the volume of the amplifier, the greater the difference in channel output (as far as I can tell by listening). Does this point to the volume pot as the problem?

I'll check things out with a meter tomorrow and see what I can come up with.

Thanks for the help! I'm pretty new to amps so I don't know much yet, but I'm learning.
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
That does point to a problem with the pot but if that were the sole cause then the volume would be equal with it on maximum.

The definitive test for all this is to use a scope and just check the absolute levels at the key points in the circuit. Its literally a minutes work.
 

Lamber

Member
2013-05-05 3:20 am
Alright, I took some measurements with a multimeter, no power applied, as a base for continued troubleshooting. Here's what I found (tubes removed).

With the pot turned all the way CW, and the black lead to speaker black (ground):
1. The front center pin from the pot has a resistance of 103k
2. The back center pin from the pot has a resistance of 104k
3. These values correspond to the resistances measured from pin 2 of Tubes 1 and 3 (with tube 1 being the rightmost tube on the finished board.

With the pot turned all the way CCW:
1. Front center pin reads 7.8 ohms
2. Back center pin reads 8.3 ohms
3. These values correspond to pin 2 of tubes 1 and 3

With the pot at some random spot:
1. Front center reads 32.2k ohms
2. Back center reads 33.3k ohms
3. Correspond to pin 2 of tubes 1 and 3

At another random spot:
1. Front reads 84.5 k ohms
2. Back reads 85.7 k ohms
3. Correspond to pin 2 of tubes 1 and 3


There are three pins on the front of the pot and three out the back. The center pin apparently connects to the wiper, since it is the only one that varies as the pot is turned. The wiper position affects tubes 1 and 3, which are the pre-amp tubes? (from what I can deduce from the schematic DIY Audio Projects Photo Gallery: Click image to close this window). The pot is 100k, so the max value is close enough. Also, it seems that the pot is fine, since the difference in resistances is not nearly enough to cause the issue I found.

Some more measurements determined that the resistances to ground match for all pins of tubes 1 and 3, and tubes 2 and 4 (as they should. When I say match, I mean well within 5% (which is fine, considering that the gold-banded resistors are only guaranteed to 5%).


I think I've eliminated the pot as the problem, and the tubes as well (since I've swapped them around and experienced no change). What should I do next? Voltage measurements with a live circuit? Should I be looking for bad caps?
 
Should I be looking for bad caps

Very unlucky to get a bad cap in a new build. Any fault in the electrolytic caps would be common to both channels. C1 and C4 could be tested by being shorted with a wire clipping providing you don't have any significant dc on your input signal. If your sure you haven't swapped a resistor by mistake the next step that I would do is to look for a short or a dry joint. Only then would I power up ( without valves ) and start checking voltages with respect to ground. That will be when good safety practices will have to be observed as you may be working with voltages in excess of 250Vdc. Psu caps will remain charged after switch off as there is no bleeder resistor.
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
I'm going to suggest two things here.

Post #2 here has two files that you can burn to CDR etc in seconds. Lets get an idea of the actual imbalance.

http://www.diyaudio.com/forums/mult...much-voltage-power-do-your-speakers-need.html

Hopefully your DVM can resolve low AC voltages. Play a tone and adjust the volume for say 1.00 volts across one channels speaker. Then measure the voltage across the other and post the result.

Secondly you can measure DC voltages with it switched on, just be careful. Compare the cathode and anode voltages between channels.

I would also check by measurement that ALL resistors are the correct values and identical between channels.

Edit... make sure the rails are discharged before measuring resistors and if any resistors give a "wrong" reading then lift one end to isolate and measure again.