Potato Cakes
Well-known member
Hello, Everyone,
I've recently been enjoying building point to point guitar amps and thought I had a pretty good handle on making sure the output transformer and speaker selection would work well with the amplifier circuit. My third amp build is a Vibro Champ. This is a 5W single ended amp using a 6v6 for the output tube. The transformer I bought was from Mercury Magnetics and listed as a replacement for a Tweed Champ, but this also uses the same schematic and output tube minus the vibrato section, so this transformer should be fine. It also has a single 8 ohm output which is going to an Eminence 10" speaker that is 8 ohms and rated for 75W. After getting the initial build errors corrected, I got this amp fired up and was enjoying playing through it for a little less than an hour when it started intermittently crackling very loudly, to constantly crackling, to no guitar signal and just louder than normal amp hum. I did the whole routine of changing out all of the tubes and rechecking solder connections but nothing seemed to fix the problem. Further testing found that that my B+ voltage would fluctuate greatly and then stop at around 100V (should be about 355V). After more troubleshooting, I found that this problem would persist with no tubes, which I would expect a very high B+ voltage with no load. I disconnected the output transformer primaries and then the B+ voltage behaved as it should. This is the same amp I mentioned in another post that was squealing very loudly because the negative feedback connection was backwards, so I don't know if this damaged the transformer, it was faulty from the manufacturer, or if a mismatch of components caused the failure. I don't believe the higher speaker wattage rating has anything to do with it as I am using this same model with another 5W single ended build and a 15W push pull amp without any issue.
I am measuring 255 ohms across the primaries, which I think is an indicator of a bad transformer since I usually see less than 1 ohm on any other transformer that I've measured this way. On the secondaries I get 0.7 ohms.
In the meantime I've ordered a bigger transformer for higher wattage amps using the same output tube. For the future I would like to know if there is anything else besides wattage rating of the transformer and the impedance of the speaker being used that need to be taken into consideration when ordering components for tube guitar amplifiers.
Thanks!
Paul
I've recently been enjoying building point to point guitar amps and thought I had a pretty good handle on making sure the output transformer and speaker selection would work well with the amplifier circuit. My third amp build is a Vibro Champ. This is a 5W single ended amp using a 6v6 for the output tube. The transformer I bought was from Mercury Magnetics and listed as a replacement for a Tweed Champ, but this also uses the same schematic and output tube minus the vibrato section, so this transformer should be fine. It also has a single 8 ohm output which is going to an Eminence 10" speaker that is 8 ohms and rated for 75W. After getting the initial build errors corrected, I got this amp fired up and was enjoying playing through it for a little less than an hour when it started intermittently crackling very loudly, to constantly crackling, to no guitar signal and just louder than normal amp hum. I did the whole routine of changing out all of the tubes and rechecking solder connections but nothing seemed to fix the problem. Further testing found that that my B+ voltage would fluctuate greatly and then stop at around 100V (should be about 355V). After more troubleshooting, I found that this problem would persist with no tubes, which I would expect a very high B+ voltage with no load. I disconnected the output transformer primaries and then the B+ voltage behaved as it should. This is the same amp I mentioned in another post that was squealing very loudly because the negative feedback connection was backwards, so I don't know if this damaged the transformer, it was faulty from the manufacturer, or if a mismatch of components caused the failure. I don't believe the higher speaker wattage rating has anything to do with it as I am using this same model with another 5W single ended build and a 15W push pull amp without any issue.
I am measuring 255 ohms across the primaries, which I think is an indicator of a bad transformer since I usually see less than 1 ohm on any other transformer that I've measured this way. On the secondaries I get 0.7 ohms.
In the meantime I've ordered a bigger transformer for higher wattage amps using the same output tube. For the future I would like to know if there is anything else besides wattage rating of the transformer and the impedance of the speaker being used that need to be taken into consideration when ordering components for tube guitar amplifiers.
Thanks!
Paul