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This amp came to me with reports of a low level hum and a nasty feedback when the previous owner connected a high gain harp. Here is the sequence of events that lead to this post.
Have attached the schematic and pic of switch.
This amp has an off/on/polarity rotary switch.. and has a 2 prong power cord ...this will be relevant shortly...
1. Initial powering on confirmed the hum which was totally unaffected by the volume control and there was a very faint guitar sound, the hum steadily increased in intensity along with some sound resembling feedback so shut off quickly. This was regardless of whether I took either of the power tubes out (suspecting a microphonic power tube), there is still sound with only one power tube but same issue.
2. All the preamp tubes had lit up and none were microphonic - using pencil tapping.
3. Powered off and in order to troubleshoot components without going deaf (don't have a spkr load to hook up) took both the power tubes out. (tubes are pre = 3x6EU7, 1x7025, 2x12AU7, Pow 2x7591 ).
4. With amp on, checked that the filter caps (this has all spragues!!!) were not passing AC. Checked the other caps in the system to make sure that none were leaking DC where not supposed to. All checked out fine.
By this time the amp had been on for 30-40 mins. So I decided to try again - powered off and reconnected power tubes and then turned it on - incidentally remember that this has a 2-prong power cord - had earlier found that one of the 2 settings resulted in about 10VAC on my guitar and the other showed none, so obviously used that setting from there on - will redo a 3-prong cord later. When I repowered it, the hum was gone (!!!), just barely some sound that indicated that the amp was on. Reconnected the guitar and there was the most heavenly tube warmth - played for an hour or so! Everything worked perfectly, tremolo, reverb, volume/tone controls, both channels and all 4 input jacks.
THEN... as I was turning it off, this has 3 positions OFF/ON/POLARITY, had been on the polarity setting, so it had to pass thru the ON setting before reaching OFF, the moment it touched the ON, to my dismay, the loud hum came back momentarily. From then on everytime I turned it on, all the old loud hum was back on both on settings, ON and POLARITY.
Deduction I made - bad switch, so disconnected power and turned it on and off rapidly thru all its three settings and then tried again with power and I would intermittently get good sound sometimes but each time as I turned off the hum would return it would take several attempts to get good sound again.
Now about the switch, cannot seem to find the identical piece. The closest is a 4 pole 3 position rotary switch from Gama Electronics.
http://gamainc.com/cgi-bin/products.cgi?item=21143NS
Could use tips on some things:
1. Is there any other explanation on why the switch is good and something else may be the problem given the details I reported?
2. Is there any way to confirm this by bypassing the switch completely, and if so how? Didn't want to solder something wrong and blow it up!
3. Is the switch that I found, a good replacement or alternately can anybody point me to the right replacement switch.
4. If the Gama switch will work then how do I connect the wires, that one has 2 lugs on one deck and the lower deck has lugs all around it, while the original one also has 2 lugs on top but the lower one has only 3 (maybe the others were snapped off intentionally during manufacture?).
Thanks in advance!!!
Have attached the schematic and pic of switch.
This amp has an off/on/polarity rotary switch.. and has a 2 prong power cord ...this will be relevant shortly...
1. Initial powering on confirmed the hum which was totally unaffected by the volume control and there was a very faint guitar sound, the hum steadily increased in intensity along with some sound resembling feedback so shut off quickly. This was regardless of whether I took either of the power tubes out (suspecting a microphonic power tube), there is still sound with only one power tube but same issue.
2. All the preamp tubes had lit up and none were microphonic - using pencil tapping.
3. Powered off and in order to troubleshoot components without going deaf (don't have a spkr load to hook up) took both the power tubes out. (tubes are pre = 3x6EU7, 1x7025, 2x12AU7, Pow 2x7591 ).
4. With amp on, checked that the filter caps (this has all spragues!!!) were not passing AC. Checked the other caps in the system to make sure that none were leaking DC where not supposed to. All checked out fine.
By this time the amp had been on for 30-40 mins. So I decided to try again - powered off and reconnected power tubes and then turned it on - incidentally remember that this has a 2-prong power cord - had earlier found that one of the 2 settings resulted in about 10VAC on my guitar and the other showed none, so obviously used that setting from there on - will redo a 3-prong cord later. When I repowered it, the hum was gone (!!!), just barely some sound that indicated that the amp was on. Reconnected the guitar and there was the most heavenly tube warmth - played for an hour or so! Everything worked perfectly, tremolo, reverb, volume/tone controls, both channels and all 4 input jacks.
THEN... as I was turning it off, this has 3 positions OFF/ON/POLARITY, had been on the polarity setting, so it had to pass thru the ON setting before reaching OFF, the moment it touched the ON, to my dismay, the loud hum came back momentarily. From then on everytime I turned it on, all the old loud hum was back on both on settings, ON and POLARITY.
Deduction I made - bad switch, so disconnected power and turned it on and off rapidly thru all its three settings and then tried again with power and I would intermittently get good sound sometimes but each time as I turned off the hum would return it would take several attempts to get good sound again.
Now about the switch, cannot seem to find the identical piece. The closest is a 4 pole 3 position rotary switch from Gama Electronics.
http://gamainc.com/cgi-bin/products.cgi?item=21143NS
Could use tips on some things:
1. Is there any other explanation on why the switch is good and something else may be the problem given the details I reported?
2. Is there any way to confirm this by bypassing the switch completely, and if so how? Didn't want to solder something wrong and blow it up!
3. Is the switch that I found, a good replacement or alternately can anybody point me to the right replacement switch.
4. If the Gama switch will work then how do I connect the wires, that one has 2 lugs on one deck and the lower deck has lugs all around it, while the original one also has 2 lugs on top but the lower one has only 3 (maybe the others were snapped off intentionally during manufacture?).
Thanks in advance!!!
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