[Negatron]

Quote:

Originally Posted by Tyne /img/forum/go_quote.gif Hey, I've been enjoying my Max for quite some time now, but I've recently been having a problem with the left channel. Usually when I turn the amp on only the right channel works. I have to turn the amp off and turn it back on to get them both on. Sometimes after a few hours of listening the left channel will go dead again. Any advice on what might be the problem? I went with BJT Diamond Buffer for the output stage btw.

The problem may well be as simple as a bad solder joint. Just look at the bottom and I/O connectors (if you have screw down headers check them too) Use a magnifying glass and look for grey colored solder joints (that are not shiny). Solder joints and very small solder bridges from splashes that heat expansion could be bringing into play may well be the cause. If you see a suspect joint reflow it with flux. Hopefully it's an easy one.

One more thing. In the absence of an occiloscope you can use a DMM on millivolt scale and being carefull not to short out anything, trace an input signal to where it stops. http://www.diyforums.org/MAX/schematic/MAXsch75.jpg
It's usually simple. (That does not always mean fast.

)

On the otherhand, if your luck is like mine, it is a problem that one on earth has ever seen before and evil spirits from outer space are wagering on the outcome of your sanity.

 

[amphead]

Quote:

On the otherhand, if your luck is like mine, it is a problem that one on earth has ever seen before and evil spirits from outer space are wagering on the outcome of your sanity.

Ha Ha.....so true!
Good luck Tyne, we will get it fixed eventually. If you don't see anything you could post some pics. Edit: try swapping tubes from left to right, eventhough that probably won't be your problem.

 

[tomb]

Quote:

Originally Posted by amphead /img/forum/go_quote.gif Ha Ha.....so true! Good luck Tyne, we will get it fixed eventually. If you don't see anything you could post some pics. Edit: try swapping tubes from left to right, eventhough that probably won't be your problem.

Actually, that's usually my first suspect - bad pin connections on the tube or the socket. There are up to five pins on the tube that if loose, would cause you to lose a channel completely. Two of them are the heater connections - if you see the tube glow (other than the LED's), then you can dismiss that. However, the plate and grid pins could have a bad/intermittent connection. If you can measure a bias, then you can dismiss the plate pins. You can refer to the tube pin diagram on the MAX website:
MAX Tubes
Pins 3 & 4 are the heater connections, pins 2 and 7 are the plates, and pin 1 is the signal input (pin 7 is also the signal output). Remember that these designations are reversed from how you see them on the board - the pin diagram is referenced by looking at the bottom of the tube. You might try wiggling or canting the tube slightly if anything is amiss with your measurements. That can confirm some bad pins. Often, either a cleanup or bending the pins out slightly can fix the situation.

I've also had intermittent issues with a signal-input terminal block on one MAX, but it came down to the wire being flattened out so bad that the screws were clamping down on the insulation (it's a well-used prototyping board).

It might be a good idea to stop and check those things the next time the Left channel is not working.

If those things all check out, then follow ruZZ.il's and Negatron's advice and start looking for some bad solder connections.

 

[naamanf]

Seeing that it works if you cycle power I would also take a look at the one mechanic part on the board. The relay.

 

[jimmyjohn]

Another troubleshooting question:

I bought a used unit a couple of weeks ago, and it's been working fine. However, yesterday I checked the bias voltages (normally 14.5V), and one channel was 9.9V while the other was around 20V. There was a pretty equal voltage change in opposite directions. Any ideas as to what would cause this?

Over the weekend I noticed that most of the nuts for the volume knob, headphone jack, and input jacks were loose so I tightened everything. Could a loose ground connection at the chassis have caused my original bias adjustments to be incorrect?

Also, how stable should these bias settings be? I have adjusted them to 14.5 volts several times, but they always drift about +/- 0.5V when I check them again.

Thanks,
Jim D.

 

[Negatron]

Quote:

Originally Posted by jimmyjohn /img/forum/go_quote.gif Another troubleshooting question: I bought a used unit a couple of weeks ago, and it's been working fine. However, yesterday I checked the bias voltages (normally 14.5V), and one channel was 9.9V while the other was around 20V. There was a pretty equal voltage change in opposite directions. Any ideas as to what would cause this? Over the weekend I noticed that most of the nuts for the volume knob, headphone jack, and input jacks were loose so I tightened everything. Could a loose ground connection at the chassis have caused my original bias adjustments to be incorrect? Also, how stable should these bias settings be? I have adjusted them to 14.5 volts several times, but they always drift about +/- 0.5V when I check them again. Thanks, Jim D.

Power supply and ground are on the board so probably not.
Did you by any chance remove the tubes and mistakenly put them in the opposite sockets?

 

[jimmyjohn]

Quote:

Originally Posted by Negatron /img/forum/go_quote.gif Power supply and ground are on the board so probably not. Did you by any chance remove the tubes and mistakenly put them in the opposite sockets?

Yes, that's certainly a possibility, since I removed the tubes to get the case open when I tightened everything.

What about the bias voltage drift? Is it a concern? Like I said, it's usually +/- 0.3 - 0.6 volts.

 

[Negatron]

Quote:

Originally Posted by jimmyjohn /img/forum/go_quote.gif Yes, that's certainly a possibility, since I removed the tubes to get the case open when I tightened everything. What about the bias voltage drift? Is it a concern? Like I said, it's usually +/- 0.3 - 0.6 volts.

Just for curiosities sake, swap the tubes back and see if you get the same thing. Bet it goes right back. As to the drift, no big deal. as the transistors in the CS change temp there will be a slight current shift and the tubes will change some as they stabilize also .3 volts is not much.

 

[slowpogo]

I don't think this is mentioned on the Max site...is there any benefit in paying for matched tubes? Or does adjusting the tube bias basically "match" them?

 

[Negatron]

Quote:

Originally Posted by slowpogo /img/forum/go_quote.gif I don't think this is mentioned on the Max site...is there any benefit in paying for matched tubes? Or does adjusting the tube bias basically "match" them?

I have a Hickok 600A and I never bother to match them. Just adjust the Bias.
They are just $2 tubes, no need to worry abour small stuff.
Matching is generally for push pull amps where there is a need, and the anal retentive. If you meet neither qualification just listen and enjoy.

 

[rockcod]

while we are on the subject of tubes, I have problem sourcing 12AE6/A in larger quantity. Most of the usual places (tubedepot, tubeworld, the tubestore, vacuumtubes.net, etc.) offer only singles, or at the most, single pairs. Are there any other vendors that'd offer more, say, 5-10 pairs of the same type?

 

[slowpogo]

I would suggest trying Consolidated Electronics:

Consolidated Electronics, Incorporated

The website looks kind of low-rent and shady, but I've dealt with them before and they're pretty good. The guy who runs it is a nice guy and easy to deal with, and I know they have 12AE6's; and I suspect they have a bunch of them. Just a suggestion.

 

[04BluMach]

Quote:

Originally Posted by 04BluMach /img/forum/go_quote.gif Snip ..............While I am at it - will pick up values for Plate and Cathode to try out after it gets well burned in to see if my burned out ears can identify a Riken difference............

Quote:

Originally Posted by Negatron /img/forum/go_quote.gif They (Rikens) will be a bit of a pleasant surprise on RB14. Regarding the Rikens on the plate though, there is really no true plate R and replacing RA8 with a Riken will be overridden by the following signature of the CS. and the cathode is an VR so no go there. However, with the AMB jfet mod w/MOSFETS, replacing RB2,3 with 1.2K Rikens is another place you can go with benefit that you will hear. JMHO Edit: Though not tried, there may be a gain from using 100R Rikens at RB8,9 though you may find once used there is a law of diminishing return unless RB14 is jumpered. So it is likely a waste if already replacing RB14.

How stupid on my part, well thats what happens when you are too tired and just blathering trying to think of something cute to say versus intelligent.....

Of course with a CS on the plate - the resistor is buried under the CS signature. And the cathode resistor is a trimmer (DUH!!!).
[It certainly would provide some impovement replacing the trimmer by a Riken - but wouldn't allow much adjustment range for bias afterwards....LOL]

 

[rockcod]

Quote:

Originally Posted by slowpogo /img/forum/go_quote.gif I would suggest trying Consolidated Electronics: Consolidated Electronics, Incorporated The website looks kind of low-rent and shady, but I've dealt with them before and they're pretty good. The guy who runs it is a nice guy and easy to deal with, and I know they have 12AE6's; and I suspect they have a bunch of them. Just a suggestion.

Thanks! I will check them out.

 

[04BluMach]

Quote:

Originally Posted by Negatron /img/forum/go_quote.gif I have a Hickok 600A and I never bother to match them. Just adjust the Bias. They are just $2 tubes, no need to worry abour small stuff. Matching is generally for push pull amps where there is a need, and the anal retentive. If you meet neither qualification just listen and enjoy.

Yeah, no benefit matching these triodes in SE circuit.

Speaking of matching - if you are considering implementating the JFET Mod be aware that you need to match the JFETS Complemenatery pairs preferably as a fully matched/balanced "Quad" set.

So, either -
plan on buying a substantial quantity to roll your own QUAD sets by matching IDSS values to maintain desired current levels and keep the channels balanced (and store all the leftover unmatched JFETS in a jar ???)
...or
head over to the AMB store site to order your two sets of JFET complementary pairs and the additional item for JFET Quad matching service.

The price for the JFETS and the service are very reasonable. I couldn't imagine spending all that time to match them (assuming you already have the test jig setup) and only charging $4. Maybe he has Elfs come in at night....