Bias Vary Trem, Cathode Biased Amp

[Boots Deville]

The attached partial schematic shows the power amp and trem from an amp I recently built.  The trem circuit is taken directly from a 5G9 Tremolux.

I now want to build the same amp using cathode bias, but I'm looking for input on how to best implement the tremolo.

I seem to remember a thread similar to this in the past, but my searches didn't find what I was looking for. 

My thoughts are that I could do one of the following:
1.  Use the 5E9a tremolo circuit, which is cathode biased, but it uses a paraphase PI.  I would think that in my case the correct insertion point would be the cathode of V2B.  How would this compare to the 5G9 tremolo?
2.  Or I could use the setup I've got simply grounding the leg of the intensity pot that my schematic shows connected to the bias supply.  I think I've seen something like this somewhere, maybe on a Gibson?
3.  VibroChamp trem circuit connected to V2B Cathode, but I really like the sound of the 5G9-style trem I have shown so would like to stay close to this if possible.

Any thoughts on these ideas, or others that might be preferable?

A side question - the 5E9A schematic shows a 10M resistor from the tremolo footswitch jack to the third node of the power rail.  I've never noticed anything like this before - what is the purpose of this resistor?  http://www.el34world.com/charts/Schematics/files/fender/TREMOLUX_5E9A.pdf

[Boots Deville]

It's been three weeks since I originally posted this but I thought I'd bump it with an update and a question.

I've finished the amp going with the "option 2" approach above.  Basically connecting the intensity pot as illustrated in the shown schematic, but connecting the #3 leg of the pot to ground (the one that was connected to the bias supply in the fixed bias version).

Guided by information in other threads I made the following tweaks to get the trem to my liking:
-  The 1M resistor connected to the intensity pot was reduced to 220K.
-  The intensity cap was changed from .1uF to .2uF
-  The tremolo off footswitch was changed so that it now grounds the intensity pot wiper to turn it off.

Remaining issue: "Thump Thump Thump" coming from the speaker, the volume of which corresponds to the intensity setting, the speed of which corresponds to the speed setting.

I found a few posts on this but nothing concrete where someone said they did "x, y and z" and it resolved the thump.

I've got the JJ 6V6's biased at 12W, not accounting for screen current.  Rk=427R (measured), plates: 383V, cathodes 29V.

Any suggestions for things to try to resolve the thump?

[tubeswell]

Guided by information in other threads I made the following tweaks to get the trem to my liking:
-  The 1M resistor connected to the intensity pot was reduced to 220K.
-...


Any suggestions for things to try to resolve the thump?

One way of reducing the thumping is to bias the output tubes hotter, however depending on how how you have them at present, that might not be an option. (The output tube dissipation increases when bias-vary trem is activated - esp on slower trem speeds, so you need to watch it)

Another way of reducing the thumping is through attenuating the voltage swing from the output of the LFO circuit. In this regard, the 1M together with the 250k pot and the rest of the circuit to ground through the bias supply, form a voltage divider that attenuates the tremolo voltage swing. Since you already changed that 1M to 220k, then maybe try incrementally increasing it back to 330k, 390k, 470k until the thumping disappears...

... and then try biasing the output tubes a little cooler, so that you get the trem to the point where you have maximum slam without the thump. JM2CW

[HotBluePlates]

The attached partial schematic shows the power amp and trem from an amp I recently built.  The trem circuit is taken directly from a 5G9 Tremolux.

I now want to build the same amp using cathode bias, but I'm looking for input on how to best implement the tremolo.

... I could use the setup I've got simply grounding the leg of the intensity pot that my schematic shows connected to the bias supply.

That's where I would start. Especially since you have an existing design, and the only planned change is cathode bias for the output tubes. Once you start making other changes like different phase inverters, etc, you might muck up the original good design. Meaning maybe solving one problem creates a new problem to solve.

[jjasilli]

You seem to have unfotunately re-created an inherent design problem with trems.  The good news -- you're in good company with the best desginers, who have sometimes resorted to more complex circuits like opto-couplers, or more tubes and split-phase trem designs, to nullify the trem signal when there's no guitar signal.  But how to fix your thumping?  Some ideas:

1.  On/Off Trem Sw - imperfect, but at least there's no thumping with the Sw Off -- this solution is used by the Univox U-42.
2.  Lead dress?
3.  Diode across intensity pot, sending +trem signal to ground.  (This was recently posted in another trem thread)
4.  Checkout the geofex.com trouble shooting section

[tubeswell]

Another thing I have found with applying bias-vary trem to cathode biased output stages, is the depth control is less effective at wiggling the output tube grids than it is in fixed bias, because (as far as I can tell) the trem voltage swing ends up 'fighting' the tendency of the cathode bias stage to auto-bias/correct itself, so you have to turn the depth pot up a lot further to get it to wiggle. So you can move the LFO insertion point to a pre-amp stage or you cantry playing around with different configurations of depth controls

[Boots Deville]

Thanks for the responses guys, very helpful.

tubeswell, you've set me straight on the output tubes' bias effect on the thumping.  I had it backwards, and was biasing colder trying to reduce the thump (to no avail).  I'll go a little hotter and keep an eye on the plates.  As for bumping the 220K back up, I'll try that.  1M, which was great for fixed bias, gave me barely any trem at all in the cathode biased amp.  220K with .2uF cap sounds good, but I'll mess with that resistor some more, or I might move the insertion point earlier in the signal chain.  I'll have to look closer at the layout and see how intrusive that would be.  Not a lot of elbow room in this amp.

jjasilli -
1. I have a trem footswitch and it works, that may have to be good enough.  Comforting to know Univox thought so!
2. Lead dress - not much room to chopstick, but I did try and heard no change.  On the plus side, aside from the thumping the amp is dead quiet.
3. Diode across the intensity pot.  I came across that fix in another thread and tried it. In my case it didn't make a bit of difference.
4. Geofex - I didn't see anything on his site related to thumping trem.  Ticking yes, but not thumping.

thanks again!