6av6 (or any 1/2 tube) tremolo with SE 6L6

[munkeyboy]

I have a Single ended 6L6 champ conversion with an extra 7 pin tube available.  I've have so many 6av6's laying around.  I tried using it as an extra gain stage, but I didn't like it at all (Allen 5F1+).   I'd like to add a tremolo to this.

However, I'm still learning about tremolos and not sure how to go about this.  I tried with the Gibson GA-5T circuit, but nothing really happens.  I've read Rob Robinette's 1/2 tube article and seems that his circuit won't work with cathode bias or large tubes.  Both is what I have and guess this is why the Gibson GA-5T circuit won't work.  Any other circuit that could work with a single 7pin tube and this cathode bias 6L6 SE amp?   If not, I'm would be open to replacing socket with 9pin.

BTW attached is the tremolo circuit I'm using (replaced two 220ks with pot for intensity).

[kagliostro]

Here you can find some tremolo circuits used in different SE amp and also info and explanations on the posts

https://el34world.com/Forum/index.php?topic=20727.msg218834#msg218834

Franco

[DummyLoad]

poke the 7pin out to a 3/4" hole for a 12AX7 and graft in the termolux 6G9-B ckt. you don't need the bias power supply parts. 


--pete

https://el34world.com/charts/Schematics/files/Fender/Fender_tremolux_6g9b_schem.pdf

[kagliostro]

As a 6av6 is half a 12ax7 I think you can give a try also to the Trem-O-Nator circuit of our friend Sluckey

https://sluckeyamps.com/trem/trem.htm

Franco

[tubeswell]

I tried with the Gibson GA-5T circuit, but nothing really happens.  ... BTW attached is the tremolo circuit I'm using (replaced two 220ks with pot for intensity).

A 6AV6 is 'equivalent' to 12AX7 triode, but the triode in a 6BM8 'equivalent' to a 12AU7 triode. A 12AU7 is lower gain - but higher current - than a 12AX7, so a circuit designed for a 12AU7 will (all other things being equal) tend to sink too much current with a 12AX7 in it. So you need a LFO circuit designed for a 12AX7 triode, e.g. a Fender Vibrolux.

Also, the GA-5T trem insertion point occurs at the output tube grid, and the output tube is cathode bias. So, not only do you need a higher-current LFO driver in that circuit, but the cathode bias resistor on the output tube is 'fighting' (auto-biasing) the output tube in order to try and stabilise the output tube gain. (So the trem in that particular amp is not the most well-thought-out design). You could adapt the trem in the Fender Excelsior circuit (which is cathode bias, and has the LFO insertion point between the pre-amp (PI) plate and the output tube grids). Or you could use the trem in the Fender Vibrochamp circuit (which works by changing the bias voltage on the 2nd pre-amp stage).

[munkeyboy]

Thanks guys.  I tried the Kalamazoo Two circuit.   But I'm not sure what I need to change value-wise to get is working correctly.   For giggles I attached to v1a or v1b cathodes .  V1a didn't produce any noticeable changes, v1b did work but sounded pretty bad.  Static/distortion on each "pulse".

I'm using the allen layout below, but removed that extra gain stage (now that 6av6 is open). Sorry, I haven't bothered with creating a schematic, since its so close to the champ.

FYI, this is a converted Bell Pacemaker PM10.  I will probably endup replacing that 7pin for a 9pin so I can put in the vibro champ trem.

[PRR]

...A 6AV6 is 'equivalent' to 12AX7 triode, but the triode in a 6BM8 'equivalent' to a 12AU7 triode.....

More like say 12AT7? Mu = 70?
https://frank.pocnet.net/sheets/049/6/6BM8.pdf

The C-R-C-R-C-R oscillator needs a gain of 27(?). This may need careful optimization for Mu=70. Heck, we know not all 12AX7s reliably oscillate when we want them to.

[munkeyboy]

Ok until more parts come in, I was still playing around with what I have on hand so I can learn.  I found one that works ok... at least it sounds better than the cathode wigglers I tried.  It is the circuit from the Garnet Gnome.   I increase a few caps to slow down the trem a little and added red LED instead of C/R.   I am using b500k pots right now because they have switches on them that I'm using for bright and trem on/off.   Once I change the pots to 1m, I'm hoping for better results.   The reason this didn't work as good before was my bias was way to hot.  Like an idiot, I forgot I was using a 6L6GB not the 6L6GC.

Two questions on this.   I know this circuit is fighting the cathode bias of the output tube.  But is there anything else I can alter (cap/res/pot) that can help deepen the tremolo?  If I can get it just a little bit more depth, it would be worth keeping.

Second question.  This amp has a MV right off the output grid.  This of course effects (or negates) the tremolo effect when turning down the master volume.  Not the end of the world, but would be curious if there is a way to correct for this?

Again, I know there are potentially better solutions if I change parts (full 12a*7, roach, or mosfet).   Just playing with what I have on hand for now.

[tubeswell]

...A 6AV6 is 'equivalent' to 12AX7 triode, but the triode in a 6BM8 'equivalent' to a 12AU7 triode.....

More like say 12AT7? Mu = 70?
https://frank.pocnet.net/sheets/049/6/6BM8.pdf

Yep good point - I was focused on gm being around 2,500 to 3000 UMhos

[tubeswell]

...I know this circuit is fighting the cathode bias of the output tube.  But is there anything else I can alter (cap/res/pot) that can help deepen the tremolo?  If I can get it just a little bit more depth, it would be worth keeping. ...

You could use a current buffer - like an IRF820 - I've built a SE amp with this type of trem, and the trem does work (although it needs to be almost fully cranked to work well).

[kagliostro]

To have a good tremolo why don't consider the Trem-O-Nator ?





For you is a must to have a tremolo that don't acts on the signal path ?


Franco

[munkeyboy]

Yep, I like those options.  Was just learning with what i have on hand.   I have to order either the IRF820 mosfet or the optocoupler, plus the special pots.

[munkeyboy]

To have a good tremolo why don't consider the Trem-O-Nator ?





For you is a must to have a tremolo that don't acts on the signal path ?


Franco

Ok so I got the parts for Sluckeys Trem-o-nator.   For a Champ circuit, where it the best place to insert the signal?   For now, I have it inserted at the power tube grid (but before MV).   It's a bit weird.   Increasing intensity cuts volume significantly.. more than other tremolos.  Also adds some sort of blocking distortion (fizz) and noise, but my bug is a bit far from the controls, so I prob need a shielded cable.

[sluckey]

I would connect to the volume pot. And read the second paragraph...

     http://sluckeyamps.com/trem/trem.htm

[munkeyboy]

I would connect to the volume pot. And read the second paragraph...

     http://sluckeyamps.com/trem/trem.htm

Thanks I had just done this 15 min ago.  It does sound better there.   I think the fizz is actually coming from the MV for some reason.   Stupid question.. what difference does the reverse audio pot make.  I don't have any other RA pots to test with.  Everything else is normal linear or log pots.

[shooter]

fwiw;
try wiring a log pot "backwards", poor mans "fix-ish"

[sluckey]

Any taper pot will work but the reverse audio pot spreads out the change effect across the knob numbers more evenly, ie, turning from 1 to 2 will produce about the same change as turning 5 to 6 or 8 to 9.

[kagliostro]

@ munkeyboy

Please if you go on experimenting, let us know results

Tanks

Franco

[munkeyboy]

Will do.   It is still on my todo list, but occupied with a few other projects ATM

[munkeyboy]

@ munkeyboy

Please if you go on experimenting, let us know results

Tanks

Franco

@kagliostro

6.8k from INT pot to signal (into tone pot) seems to work best for me.

[kagliostro]

Many Thanks

Franco