motorboating magnatone

[sdorer]

Friends,
A few years back I bought a magnatone M7. It looks like something between a television and a portable room huimidifier. It was during the Estey period. Regardless I didn't like the sound of it and since it wasn't exactly a great vintage priceless jewel I Leo'd the circuit. I changed the pre amp, volume tone, and phase inverter into the 5e3 circuit. Now this thing screams. However it motorboats when the volume is at 12 oclock and the frequency of the motorboating decreases as the volume increases. Any ideas where the I should begin the next stage of my quest. It does sound good as is and I don't think I will ever play it beyond 10 oclock. However your thoughts are appreciated. It does have 7189 power tubes. Solid state rectification.

[Leo_Gnardo]

Now this thing screams. However it motorboats when the volume is at 12 oclock and the frequency of the motorboating decreases as the volume increases. Any ideas where the I should begin the next stage of my quest. It does sound good as is and I don't think I will ever play it beyond 10 oclock. However your thoughts are appreciated. It does have 7189 power tubes. Solid state rectification.

You WHAT the circuit??? Don't worry, I've done it too, with no name amps and luckily none have become collectors items.

First suspect when you have motorboating is insufficient power supply filtering. Which can mean failing hi voltage filter caps or their rating is too low in microfarads. Freshen up them filters & see if it doesn't start behaving.

[xtian]

Lead dress can also be an issue. Photos?

[Structo]

Could be power related but:

Try higher value grid resistors on the power tubes.

You have 1K now?

Try 2K2 or higher.

[Structo]

Could be power related but:

Try higher value grid resistors on the power tubes.

You have 1K now?

Try 2K2 or higher.