Blues junior's plate voltage acting strange. Problem solved!

[Stevem]

Here's what the bottom line is with a lot of oscillation issues.

Let's look at one gain stage feeding another.

The output from the up stream plate decoupling cap is a lower impedance then the down stream ( higher impedance ) grid that it's feeding. The length of the wire feeding the down steam grid, be it preamp tube or output tube can act like a antenna or a transmitter, but either way that's a problem.

This shows that we want wires feeding grids of any tube to be as short as possible.

Achieving this means that the decoupling cap from the up stream stage should be right at the grid or as close to that physical situation as possible.

In this way the up stream wire being of a lower impedance can be long without making undue oscillating issues.

[pdf64]

I'm mystified why it's become internet lore that it's essential to measure the output valve anode voltage. The probe, lead and meter then becomes a transmitting antenna and instability can easily result, invalidating the reading. Just measure the voltage at the OT CT, it'll be as near as makes no difference.
If the anode voltage must be measured, removing the valve in the immediatly preceding stage, eg phase splitter, circuit, will eliminate the likelihood of oscillation.

[BurnedFilament]

I recall there is a note in the BillM mods about oscillation in the PI (Blues Junior Disease). A 100pF cap in parallel with R30 (100k) on the Cream board and later should fix this problem. Hopefully this helps. I am seeing this on 2 BJr today in my shop. One I swapped V3 with V1 and the oscillation was gone. The other amp needed the 100pF cap as so.

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