Amp sounding like an oscilator???

[Satamax]

Hi everybody!

Well, still struggling with my L50 conversion. The amp doesn't have the tone controls working anymore, there's interaction between normal and bright volume, bright volume turned down, and bright input used, if i move the normal volume, i have sound comming out???? And the hardest part, the amp is "charging and discharging" like an oscilator. When i feed 1000HZ in the amp i sometimes, but not all the time get bleeep, bleeep, bleeep, raising and decreasing. What could that be??? I've been tracing the signal with teh scope, but it's verry weird. I'm using this schemo http://www.schematicheaven.com/barga...dcity_100b.pdf but it's made in a 50W PA.

Thanks a lot and best regards.

Max.

[Amp Kat]

Being that both volume pots are tied together theres no way they can't help but interact just like the topology of a 5E3 does. I'd pay close attention to the voltages in the squares and make sure they are all there present and what the schematic shows mainly those cathode voltages of V2. Put your scope on the two plate junction of 1 & 6 on V2 or safer after the 500pf coupling cap and inject your 1khz signal and adjust your voltage down on the scope until you get the signal back in the screen from it's amplitude changes. When or if you get a good clean PP signal you can measure going in the tone controls at the 100k slope resistor and coming out entering the tone recovery stage at pin 7 of V3. Note the tone stack will slam that signal pretty good so be prepared for some decrease in signal there. If no signal is present find where it drops off and check your connections and make sure they are all there and all the grounds are tied where they should be. Usally these problems are ground related some how or another are just plain wired up wrong.

[Satamax]

Originally Posted by Amp Kat

Being that both volume pots are tied together theres no way they can't help but interact just like the topology of a 5E3 does.

Huh??? They're isolated from each other by the grounding point in the midle! Huh??? Anyway, thanks a lot.

Max.

[Amp Kat]

Pins 1 & 6 of V2 are tied together so no matter which pot you turn simultaneously one effects the other. Both outputs go to the same place. Did you check those voltage points ?

[Jag]

Please don't take this wrong, I'm just confused by your explaination Amp Kat. How would the plates being tied together affect the interactivity of the volume controls of the grids on the same tube?

[stokes]

Thgose volume controls are not tlike the 5E3 at all,look at the schem.On the 5E3 the pots are tied together at the grid they feed,like 2 grid resistors in parallel with each oher.In the Sound City,they are tied to gether at ground,there should be no interaction like a 5E3's interaction.I also dont see the plates causing any interaction here either.As well as what AmpKat says to check,I would also look for leaky caps through the whole circuit,but in particular around V2 those volume controls interacting are suspicious.

[Amp Kat]

If you are playing in the Brilliant channel and turn the normal pot which is tied to the control grid it's going to change the resistance to the grid resulting in a tone change. Since pins 1 & 6 of V2 are tied together any change to one pot will effect the tone of the other. Even though theres no signal from one channel of course unless both channels are used won't you hear the tone change from grid resistance change or am I loosing it here ?

Oh crap I am loosing it here doh your right guys. Ok well Satamax theres your problem. The pots aren't grounded together causing interaction. That's about the only thing that will cause them to interact.