Loss of power

[goldtop5]

Hello,

I'm trying to work on an amp that has a problem with power loss. I replaced the tubes and tried to set the bias. Next thing I know power dropped by almost 80%. I have this problem with 3 amps: a Marshall JCM900, a Soldano SL-60, and a Fender Tremoloux. The Marshall and Soldano were working before I replaced the tubes. Any hints or suggestions?
Thanks!
Eric

[R ski]

Loss of power

The bias, will cause loss of power with respect to the output tubes. If the preamp tubes are all 12ax7's then they will be fine.

Need to understand what setting the bias really means. My ampagers have different views on this setting. The DC current flowing through the tubes, determines how much the power tubes are conducting. Whether they are 6L6's or EL34's, the bias current should be set at 25 -35 MA. Normally various methods are used to determine the actual current flow. Some amplifiers have variable bias pots. Some recent amps install seriies resistors to determine the actual current setting,. For example 100 ohm resistor @ 25 MA would require a reading of 2.5 VDC across that resistor. E = IxR (100 x .025 = 2.5 V)

If a resistor is not installed. Allow the amp to discharge. Measure the resistance on the output tranny lead, found on the power tube socket and the center tap, found on the power supply after the filter choke. Average out the reading from center tap to each power tube tranny lead. So if the reading was 50 ohms ......... Then E = IxR (50 x ,025= 1.25 V) would yeild 25 MA....35 would be slightly higher.

On fixed bias output tubesm a power resistor determines the current flow. Same math applies. Only thing, is that the power resistor has to be changed to get the desired current flow.

[Enzo]

I don't think bias being off some will result in an 80% drop in output level. But it wouldn't be a bad idea to adjust the bias while the amp is playing to see if it affects the volume level in substantial fashion. In most amps, if you turnit to the coldest extreme, it just makes the amp sound sterile.

The amps worked before you changed tubes, so put the old tubes back in and see if it is as simple as bad new tubes. Make sure the heaters are glowing in ALL the tubes. And make sure the tubes aer indeed all the right type.

Otherwise...

The most common reason for this complaint is effects loop return jacks. In amps that have them, of course. COnnect a cord from effects send right back to the return. If this restores sound, then the return jack needs service or replacement. ANy amp that has preamp out and power amp in jacks in addition or instead should be treated the same. Those jacks are the same thing as efects loops for test purposes.

In all three cases - different sorts of amps - you mentioned changing tubes. If that was done becuase the older tubes failed, then there may be damage to the amps. Remove the power tubes, and with the amp running, probe the tube socket pins with a volt meter. You can do this from the outside of the chassis, just remember to count the pins counterclockwise rather than clockwise from outside.

Make sure there is B+ voltage on both pins 3 AND 4 on EVERY socket. Make sure the bias voltage is present on pin 5 of every socket, whatever the setting might be. Verify pin 8 is grounded in each case.

If your Tremolux is old enough, it might have cathode biased power tubes. In that case, the grids would have zero volts DC, and the cathode would have a resistor to ground like 250 ohms. Check that with power off.

In any case, failed power tubes often burn out the screen resistor, and that would leave pin 4 on that socket without B+.

[goldtop5]

Do you think installing newer tubes that may be somewhat defective could have something to do with the power loss? All the voltages and current check ok on the tube sockets, but it seems like it didn't solve the problem. Where else could I look? On the soldano, I installed a pair of 6L6s which I know are good. The amp worked fine for about 30min, then made a bid drop in volume.

[Enzo]

If it is not a bad tube, then it is somethin gin the circuitry. We then get out the schematics and trace the signal through the amp.

[goldtop5]

Hello,
Here is an update on the Marshall JCM 900. After tracing the problem and finding weird voltages and signal loss to the output, I found that someone else has done modifications to the amp. There were wrong resistor values. I've replaced them with the right ones which the schematic calls for. This did not solve the problem though. The problem was the output transformer. Since the amp was working before, the problem may have been caused by installation of bad power tubes. I'm going to take a guess at that since that was the last thing I did before the problem started.