paraphase bias wiggle again

[andrew]

It was from a new pack of NTE brand 10M ohm.

[andrew]

The 10M resistor was from a new pack of NTE. But I found a fix! I replaced one of the .01 caps with a .047 and no more helicoptering. Thanks for helping me think through this.

[d95err]

Good to see you got it to work!

So - how does the tremolo sound? Does it sound different from the tremolo in your Rocket builds?

[andrew]

The Rocket EL84 bias wiggle sounds wobbly and fatter. The PI bias wiggle is tighter and cleaner. I am using a 6SL7 in the PI bias wiggle with both cathodes sharing a 1K resistor as in the schematic. I found another power tube bias wiggle circuit on a Matchless Hurricane. I may try it someday but for now the Rocket trem is fine. Thanks for the inquiry.

[andrew]

Wow. I swapped out the 6SL7 ( it is fitted with a 9 pin adapter) for a 12BZ7 and the trem was so strong it made me dizzy. The BZ7 tone was too brutal though. A 5751 was not as strong but was sweeter.Both tubes make the 6SL7 trem sound anemic but I do like the tone of the 6SL7. One question though why does it take a few seconds for the trem to activate when you hit the footswitch? Thanks

[tubeswell]

How is your trem RC network designed? If you have all the resistors going to ground potential, then it is normal for the oscillator to take a few seconds to start working. But if you take (say) the middle resistor and attach it to the cathode (instead of to the ground), that should 'kick-start' the oscillator when you hit the trem footswitch.

[statorvane]

I found another power tube bias wiggle circuit on a Matchless Hurricane.

That is not wiggling the PT bias, but the grids. I know it's hard to see, but take a closer look - Hurricane is cathode biased.

[andrew]

I am using the 5E9A layout. I tried running either of the 1M to the cathode of the trem tube but there was no change when I tried it with one of the resistors and the trem didn't work when I tried the other. Also how can I eliminate the loud pop when I footswitch it off?
Thanks for clarifying the terminology on the grid wiggle.

[tubeswell]

I am using the 5E9A layout. I tried running either of the 1M to the cathode of the trem tube but there was no change when I tried it with one of the resistors and the trem didn't work when I tried the other.

That is puzzling. Did you run it to the cathode of the LFO stage? (and not to the CF stage by mistake?)

[andrew]

Yes to the LOF side. Thanks.

[tubeswell]

Hi Andrew

Are you able to post a schematic of your amp (with the HT and cathode/screen/plate idle voltages) please?

[andrew]

Hi, The amp is pretty much the layout of the GA20 lite and the tremolo section of the 5e9a. I'll get the voltages out for you. In the 5e9a layout there are two .01 caps in the trem. I had bad helicoptering and I replaced the one on the right with a .1 cap and the helicopter went away.I had the slow trem activation problem and I took out the .1 and replaced it with the .01 again. Now the trem activates very quickly BUT I have the helicopter again. I raised the cap up to .06 and there is a faint helicopter and the activation is a little slower but not as slow as the .1 cap. Thanks again.

[tubeswell]

The helicoptering is likely the result of marrying a trem designed to be used in one circuit with one set of voltages-tubes etc, with another circuit. I think you should persevere with aiming for getting the trem output voltage swing at the right strength.

In the case of the 5E9A, Fender basically turned the depth pot into a variable cathode resistor for the CF stage, with the bottom end of the pot hooked up to the paraphase cathode. Adjusting the resistance of the pot changes the strength of the voltage swing from the CF stage. On your amp - the strength of the voltage swing from this is turning off the paraphase stage too well.

So to reduce this, you could possibly re-configure the depth pot to be more of a classic voltage divider that follows on from an independent CF stage (i.e.: adding a separate cathode resistor and blocking cap to the CF stage, and having the depth pot input coming off the other side of the blocking cap, and wiring it as a voltage divider/level control with the output lug going to ground and the pot wiper going to the cathode of the paraphase stage).

[andrew]

I can do the 500k pot plan but the second method is above my understanding. I would need a drawing to figure it out. Tomorrow I will try the 500k pot. Thanks!

[andrew]

Is this correct? on the cathode side of the 12ax7 attach a cap on pin 1 and from the other end go to the input of the pot. Attach a resistor to pin 3 and go to ground. The pot wiper goes to the cathode of the PI and the remaining lug on the pot goes to ground.