5F1 distortion and low volume proble with kit build

[leevc5]

I have checked all the wiring switched out the tubes and cannot figure out what is wrong. When I turn the volume control up I hit a spot at about 75% of the way up where a get a sort of squealing sound then no output after that. below that point I have output but distorted and very low volume.

Any suggestions would be greatly appreciated.

Lee

[eschertron]

Sounds like more than one problem. If this amp has a NFB loop, disconnect. That may affect the squeal at higher volume levels. If the squeal goes away with the disconnected NFB, then the loop wires need to be reversed. Try that first, then we'll see about the distortion.

[TomCarlos]

Similar problem with a Gibson build. Is the speaker jack grounded? If no jack, is the (-) side of the speaker grounded?

[Chuck H]

The squeal and subsequent loss of output at higher volume knob settings is pretty much "tell tale" parasitic oscillation. This can happen due to layout and/or lead dress and/or grounding scheme.

The low, distorted output at lower settings is likely a consequence of the oscillation OR a miswire in the first preamp stage affecting triode operation. That is, bias, plate voltage or some other operating parameter for the tube itself rather than the signal chain.

My reasoning on this is because the amp turning to oscillation at higher volume settings is indicative of the second preamp stage accurately allowing an increase in overall amplifier sensitivity. Since there isn't much distortion possible in the first stage of the preamp of that design it seems likely that the first triode could be working outside of it's operating ideals to create those distortions.

This is just a first draft diagnosis though. The symptoms could likely also be explained by grossly bad operating parameters for the power tubes.

Voltage readings at every tube pin with the amp at idle would be a good place to start for a proper diagnosis.

P.S. eschertron's suggestion is a good one, BUT... Output phase for single ended amps with NFB should, ideally, be a less sensitive matter than it is for push/pull amps. That doesn't mean that an oscillation caused by component proximity or lead dress (that involves the output transformer) which is effectively a positive feedback loop couldn't be rendered a negative feedback loop by reversing the OT leads, and a negative feedback loop would diminish the offending frequencies to a similar order that a positive feedback loop accentuates them and causes oscillation. A layout, lead dress or ground scheme induced negative feedback loop has the drawback of unpredictable results due to phase shifts during highly variable operating conditions. Causing variable levels of instability and reduced efficiency at frequency. Still, many guitar amp designs are OT phase dependent for this reason due to less than ideal design (and sometimes sound really good as a side effect). To this end, it can't hurt to try reversing the OT primary leads.

[mort]

If there's one thing I've learned about building amps is that if it sounds all weird when first fired up, swap the OT's primary leads. Always a 50/50 shot at it not being phased correctly on start up. Sometimes the symptom of it being backwards is loud squeal or hum and sometimes it's low output and badly distorted guitar sound.