Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TurboGuitarMelton on July 25, 2019, 01:07:24 pm
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Hey guys,
I built a 5e3 with a triode/pentode switch. It works great, however there is a weird thing that happens when I power the amp on in triode mode. Does not happen when it's in normal pentode mode.
About 3-5 seconds after I switch the power on the amp starts to hum (sounds like 60hz) its a mellow hum... Then it goes away as quickly as it came. It crescendos and then decrescendos until its silent.
I have 470 ohm screen resistors on the screens but I just want to make sure Im not killing these tubes.
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The problem is that in 5E3 (really, most guitar amps) the plate supply is not well filtered; it contains a lot of ripple. The screen supply, with its additional RC filter, is better filtered with less ripple.
Here's the thing: the plates are far more tolerant of ripple than the screens, so everything works OK with the stock circuit. But, when you switch to triode mode the screens are fed from the poorly filtered plate supply. Hum results.
Options:
1) Live with it.
2) Install additional filtering for the plate supply (i.e. right at the output of the rectifier.)
Choosing #2 might get complicated, expensive, and will likely change the sound and feel of the amp in pentode mode.
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I can totally live with it. I just didnt want to be hurting my tubes or something. Makes a lot of sense. Thanks!
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The start up hum may also be due to different warm up rates of the 2 p-p power tubes. HT ripple will largely cancel out in the OT due to p-p action, provided that the primary currents aren’t too far apart, eg with ‘matched’ power tubes. But matching will only be done on tubes that have thoroughly warmed up.
Plate current is controlled by the voltage differential between g1 and g2. In triode mode I think that your g2 Vdc will be a fair bit higher, which should lead to the plate current increasing. That should be reflected in the cathode voltage increasing in triode mode (compared to pentode).
So you may want to check that idle dissipations haven’t gone too high.
Folks that replace the 5E3s 4k7 g2 HT dropper by a (low resistance) choke may notice the same thing.