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I recently finished a build from the SoLow Watt design that has been built by several on the Hoffman and AmpGarage forums. It is a push pull amp. I used 6F4P tubes instead of ECL84. 6F4P is supposed to be the Russian equivalent of the ECL84/6DX8. Schematic attached. All new parts, except the tone caps and PI coupling caps. I got the 6F4P from a member on the Wattkins forum, don’t know anything about them. Output transformer is a Hammond 125E, wired for 17K to 8 ohms.
It worked first time, and sounded great for a couple hours. Now it sounds like a long string of fire crackers going off, kind of like 4th of July in my neighborhood. In retrospect there were a few isolated pops at first. After maybe 2 hours of playing and maybe 10 off/on cycles, the popping got worse and worse. It would start 30-50 seconds after initial turn on. While the popping is going on, voltages are unstable, and one or both of the 6F4P are strongly microphonic. Tapping anywhere in the chassis or amp it is microphonic.
With a guitar plugged in, it makes sound, but sounds awful when the popping is going on.
For now I will skip over my initial attempts to trouble shoot. I now think it is an issue with the tubes. As of last night I found one of my 6F4P is shorted between grid and cathode. With my remaining 3 6F4P, the popping starts after 30-50 sec of warm up. In the dark, the 6F4P blue glow gets brighter as the pops happen. The onset of popping seems to occur as the tubes get warmed up
The popping behavior occurs even with the 12AV7 preamp tubes pulled.
I am going to order up some NOS ECL84/6DX8 tubes.
To reiterate, the amp seemed to work well for a couple hours. But I wonder if the 6F4P just have a high failure rate, or if something about the build is stressing them. SoLow Watt 12A_7 Pullshocks 8 23 2021.pdf
Thanks in advance
Pullshocks / Mark
Pacific Northwest
It worked first time, and sounded great for a couple hours. Now it sounds like a long string of fire crackers going off, kind of like 4th of July in my neighborhood. In retrospect there were a few isolated pops at first. After maybe 2 hours of playing and maybe 10 off/on cycles, the popping got worse and worse. It would start 30-50 seconds after initial turn on. While the popping is going on, voltages are unstable, and one or both of the 6F4P are strongly microphonic. Tapping anywhere in the chassis or amp it is microphonic.
With a guitar plugged in, it makes sound, but sounds awful when the popping is going on.
For now I will skip over my initial attempts to trouble shoot. I now think it is an issue with the tubes. As of last night I found one of my 6F4P is shorted between grid and cathode. With my remaining 3 6F4P, the popping starts after 30-50 sec of warm up. In the dark, the 6F4P blue glow gets brighter as the pops happen. The onset of popping seems to occur as the tubes get warmed up
The popping behavior occurs even with the 12AV7 preamp tubes pulled.
I am going to order up some NOS ECL84/6DX8 tubes.
To reiterate, the amp seemed to work well for a couple hours. But I wonder if the 6F4P just have a high failure rate, or if something about the build is stressing them. SoLow Watt 12A_7 Pullshocks 8 23 2021.pdf
Thanks in advance
Pullshocks / Mark
Pacific Northwest
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