Worked first, now fuzzy/compressed sound, overdissipating valves

[dmeier]

Hi,

I built a Hiwatt DR103 clone that worked for a few hours just fine but at a last session after a few minutes of power off and then turning it back on it audibly changed after the first few notes to a much lesser power level.

If I play notes very lightly it sound as supposed but as soon a harder note is played it changes to a fuzzy and compressed sound. It could also be noticed that the valves start overdissipating while playing even at very low biasing. I had modified the original bias circuit with a voltage doubler so I can adjust within a wide range. The voltages are in the allowable range.

That's what I already tried/ruled out:

-) visibly nothing is burnt
-) change tubes -> same result
-) use an external DC-source for bias -> same result

Anyone which had to track down a similar problem? Any clues to investigate further?

Thanks, David

[MWJB]

When you say it is a "clone" (hate that phrase), did you copy the HiWatt layout exactly?

Sounds like a parasitic oscillation that is drawing current from the power tubes under load & causing them to red plate.

Post some pictures of your build &/or compare to a real HiWatt & copy that as closely as you can.

[dmeier]

It is an exact copy except the voltage doubler for the bias (after layouts from MHuss). I do also own several DR103s so I can confirm it is a "true copy" and the transformers are original Partridge copies. The wiring is done carefully and I tried my best to wire it as neat as the originals are.

What I do not understand is the fact that the amp ran perfectly for a few hours. Before it broke down I played it for 15 minutes or so, shut it off, turned it back on after a minute and after hitting the first chord it sounded alright for half a second and since then I have this problem. There was no popping sound or smoke but it broke down from one moment to the other - just like when a component fails.

If it is a parasitic oscillation shouldn't it exist ever since?

Do you have hints on how to track down the problem?

[MWJB]

"If it is a parasitic oscillation shouldn't it exist ever since?" it will depend on how hard the amp is driven (not necessarily on pot settings, but signal voltage & low end).

OK so you seem pretty confident that layout is good, so please list dc voltages, "allowable range" is too vague, quantify "very low biasing".

[dmeier]

Finally, I got some time to continue troubleshooting. It seems the OT is shot. I hooked up a 50W Hiwatt OT with only two tubes running and the amp works. Re-hooking up the existing one reproduces the error. However, measuring the impedance ratio with a 1V 1kHz input signal I calculated a primary impedance of about 1900Ohm which is not too far off spec (1750Ohm).
Anyway, I will have get me a new OT then...