From the frying pan into the helicopter

[RC_cola]

Im puzzled with my results tinkering on the Lectrolab.

I now have a condition where I get helicopter type sound at a volume setting at around three quarter full with strumming the geetar with the tone control at full rotation. Turn the tone all the way out and the chopper sound deminishes. Let me back up a bit, its a tremelo type sound with a geetar strumming.

With no strumming, volume up full with tone all the way down, no chopper. Turn the tone up to almost full and the chopper sound begins.

I put three new radial caps in to replace the can original filter cap to see if the hum would go and it did, one 20 uf and two 10 uf's. I used 22uf caps for the cathode. I replaced the carbon 1 watt 270R on the cathode of the power tube with a 3 watt 270R metal oxide type.

Coming off the tone control I replaced an older .01 uf orange drop with a new .02 uf orange drop.

Any input on this is appreciated.

[jjasilli]

Amplifiers, and their attendant issues, predate the helicopter, which apparently arose (literally) in the late 1930's.  It seems earlier 20th Century attempts at the helicopter were called gyroplanes, and predate amplification.  Hence your issue might have been called gyroplaning!

However, a more familiar term was adopted:  motorboating.  See:  www.geofex.com > -Tube Amp Debug Page > Putt - putts like a motorboat

[RC_cola]

Cool story!

I read the putt, putt so just answer this, Which caps are the decoupling caps?

[jjasilli]

Coupling caps are the capacitors that go between the plate (output) of one tube to the signal grid (input) of the next tube.  They are usually about .01uF - .1uF.  They do 2 things.  For AC (signal) operation they filter frequencies to shape the tonal response of the circuit.  For DC (power) operation, they block the DC on the plate from the signal grid of the next tube.  For this reason they are sometimes called blocking caps, but this term has mostly fallen into disuse (like gyroplanes!)   

[RC_cola]

The Lectrolab is working great! There is virtualy no hum and very little hiss at full control settings.

A very big thank you to all who made suggestions to my questions, those responses were very beneficial!

[Frankenamp]

Coupling caps are ...  They are usually about .01uF - .1uF.  They do 2 things.  For AC (signal) operation they filter frequencies to shape the tonal response of the circuit.  For DC (power) operation, they block the DC on the plate from the signal grid of the next tube.  For this reason they are sometimes called blocking caps, but this term has mostly fallen into disuse (like gyroplanes!)   

OK, now you have me inter'sted... My Buckeye {7B4 (x2) => 6SF5 (same thing in a metal jacket) => 6SN7 (PI) => 6L6G (2)} has .02uF coupling caps in every stage except the .05uF's on the 6L6's. I was about to make the generalization that I see more .02uF caps octal (earlier) designs, and .01uF caps in later amps with the ubiquitous 12A_7 miniature tubes. Is this a function of plate resistance forming an RC network ? If so How does that affect or effect the freq response (and tone)? And as a followup, earlier amps commonly have a .02~+/- mica or ceramic cap on the input grid in lieu of the more modern 30K-68K input resistor; I assume it has a similar function? Is there a simple equation that determines this?

[jjasilli]

Is there a simple equation that determines this?  What we have is an RC network.  The equation is Freq = 1 / (2 X Pi X R X C).  For AC (signal) operation, the coupling cap is connected to the plate resistor of the prior stage, which resistor is in parallel with the grid leak resistor of the next stage.  That plate resistor is also in parallel with the plate impedance of the prior stage.  The coupling cap may also be affected by the internal miller capacitance of the next stage; or that my be cut-off by a grid stopper.  Though the formula might appear straightforward enough, it may be difficult to apply in practice.  I think that this level of theory / technology is beyond the reach of DIY.  Hence it is necessary to plagiarize.

[sluckey]

And as a followup, earlier amps commonly have a .02~+/- mica or ceramic cap on the input grid in lieu of the more modern 30K-68K input resistor; I assume it has a similar function?

That is probably part of a gridleak bias circuit. The entire circuit is a grid cap and BIGGG grid return resistor, usually 5Meg or more, and the cathode will be connected directly to ground. That was a common way to bias a tube way back then.