6G15 Reverb + vib - single extra 12AX7?

[Papernut]

I'm new to the forum as a member, but have browsed it quite a bit over the last year or so and found some very helpful info.
I'm hoping to get some advice on a tech issue with a 6G15 Reverb project.

I've got the tube bug and built several of these since last summer: mostly straight 6G15 spec with different valve/cap/tank combinations, but recently a variation using a different, on-board amp style OT, 12AT7 instead of 6K6 etc, plus a Garnet Reverb clone too...They are turning out great, and now I want to try one with added Vibrato.
I have plans for the Tore-Vibe etc, but first I wondered if I could just add the simple Vib section as per the old Vibro Champ, using a single 12AX7 and connecting the intensity control to the 12AT7 preamp tube.
I've put it together and it kind of works, but here's the thing:
When I test the unit plugged in to my little Smokey cigarette packet amp it works fine – although the speed/intensity relationship isn't consistent (almost like 2 tremolos beating against each other at certain points) - but you can clearly hear the vib working and being adjusted.
But when I plug the output cable into a normal amp, or recording interface – NO VIB at all!!
If I connect through the Smokey, in then out to another amp, the vib is there!!

I'm guessing there's a reason why those Rev/Vib combo units use 2 extra 12AX7s???
I'd be grateful for any advice, or schoolboy errors being pointed out!! 

[kagliostro]

Nice story, interesting question .....

a schematic will help all to understand better what is happening

K

[Papernut]

Thanks K,
Here are a couple of pics that might help.
Vibro Champ schematic with the vib section circled, and point where it connects to the 12AX7 preamp tube.
The 6G15 schematic showing the point I've connected the vib section to the 12AT7 - have tried pin 3 and pin 8 with the same result.

[kagliostro]

Here what Univox do in the U45



Read here - may be replay #9 interest you

http://www.el34world.com/Forum/index.php?topic=15338.0

K

[Papernut]

Ah! Will have a look, thanks......

[HotBluePlates]

You may need to remove the 25uF cathode bypass cap on the stage where the trem is injected (or significantly reduce its value to something like 1uF). The bypass cap may tend to reduce the trem signal strength.

Second, I hope you gave your new added trem components their own B+ dropping resistors and filter caps, perhaps even adding a "spur" to the B+ line kind of like how Vox did multiple spurs in their B+ supply (as opposed to Fender, that typical had a single straight B+ line).

The thought is if you tapped power for the new 12AX7 trem tube from the same filter cap as the input 12AT7, the two points with pulsating current due to trem may be fighting each other. Notice in the VibroChamp schematic you posted, Fender sourced B+ for the trem circuit one node upstream from the stage at which trem was injected; they may have figured out you have to do that due to the current relationships between the two (I don't know this for sure, as I haven't tried it).

Of course, there could also be a simple wiring issue or intermittent associated with jacks which explains why the trem worked, then didn't.

[Papernut]

I think you have hit 2 nails on the head there!

1) The last permutation I tried was injecting the tremolo at the recovery tube stage - the last 12AX7 in the circuit - that's the only time I heard a hint of very faint trem through a normal amp - and there's a 250uF cathode bypass cap there in the 6G15.

2) You're right, the power supply is tapped from the same B+ point for both tubes - I'm using the Triode 6G15 layout for the power supply, so I can try tapping power for the trem earlier in the B+ line. I'll also check out Vox option....

That's very helpful advice, 2 things to try - will keep you posted, thanks!

[HotBluePlates]

Well, the "Vox option" will require adding a dropping resistor and filter cap, and feeding the trem circuits just from the newly-added filter cap.

Maybe you don't have room to add the parts neatly. In that case, try moving the trem circuit over the the 2nd 40uF cap in your power supply. That is, the one which is attached to the 6K6/6V6 and reverb transformer.

[Papernut]

Thanks - Only just had a chance to get on to this again - taking the power for the trem circuit from earlier in the B+ line seems to make a difference - through a normal amp I can hear the trem oscillating faintly in the general noise of the unit - speed & intensity seem to be doing the right things - but still not coming through and affecting the actual signal.
Tried removing the bypass cap on the 12AT7 - no difference, just got a bit more noisy. Tried lower value bypass cap too - no difference. I guess it's a question of finding the best place to inject the trem signal.
Weirdly, through the smokey amp now I only hear the trem momentarily as I move the intensity control.....
I'll keep trying - and post if I work it out!!

[eleventeen]

Just as a complete drive-by WAG, maybe you are loading down the trem oscillator too much and stopping it...? I repeat, a WAG. Can you stick a scope probe on it, or..even an analog multimeter might show wiggling at lowest speed.  HBP had some neat suggestions.

[kagliostro]

@ Papernut

May be you are interested on reading this

http://www.valvewizard.co.uk/trem1.html

http://www.el34world.com/Forum/index.php?topic=15087.0

I hope this can help you

K

p.s.: Sluckey's site is offline, here you can find the Ampeg J128 documentation - on the top right of this page there is the download button http://www.mediafire.com/view/?j6a5b1bpsm44p3m

[HotBluePlates]

Weirdly, through the smokey amp now I only hear the trem momentarily as I move the intensity control.....

Eleventeen's suggestion to use an analog meter to check the oscillator function is a good one. Set it to measure d.c. volts on the plate; there is a slow a.c. output there, but with an analog meter you will see that as the needle moving back and forth around the d.c. plate voltage.

As always, triple-check wiring as any new build is likely to have wiring/solder problems. You new addition to an existing good circuit counts as a "new build". Your mention of intermittent operation sounds very much like an intermittent solder or ground connection.

[Papernut]

Thanks for the help & suggestions Everyone,
I don't have an analog meter - but I did check out K's links and Sluckey's Ampeg schematic, and I've put an LED in instead of the cathode R/C. So now when I power up I can see the LED start to flash.
The speed control changes it, but at slowest speed the oscillations die away and the LED just stays on. Speed it up a bit and it flashes again.
It does this with or without plugging into an amp....but I guess it tells me the trem circuit itself is basically working...?

I'm still not hearing any trem - so I'm guessing the problem is still the way I'm injecting the trem signal, or where, or something I've not got right elsewhere, as HBP suggests.

I'm posting up Tore's schematic, which has the trem going to a second extra 12AX7 etc - will keep thinking, but maybe I should just do this!!

[HotBluePlates]

Maybe you have to. I don't know, as I've not built a Tore-Vibe, and I didn't add trem to the reissue 6G15 I used to own.

Notice C13, C14, C15 and R18, R19, R20/R21; these form 3 high-pass filters to help get rid of the trem signal pumping.

[Papernut]

  Just had a major breakthrough! I found this....

http://www.el34world.com/Hoffman/revibe_origin.htm

...and tried bringing back my trem signal at the point it does in the diagram there - so it's connected to the mixer pot and via the 0.1u cap to pin 8 of the reverb recovery tube.
Success! Nice deep trem with speed & intensity working fine.
It is a bit thumpy - so I see now why the HPF section is there.
I'll have a look at that next.....But for now I'm pleased I've got it going - thanks for all your help and advice!!!!

[Willabe]

Could you please post a drawing of what you did? I'm not sure about what your saying.


             Brad     

[Papernut]

Here's a diagram showing what I've got so far......

[Willabe]

Ok, now I see. Thank you.


             Brad       

[Papernut]

**Update**!
The way I had it the thumping was too much - went back to the cap & resistor instead of the LED to see if it would tame it.
Now I'm injecting the oscillator signal to the grid of the reverb's preamp tube - pin 2 on the 12AT7, see new schematic pic below!
It sounds great this way, hardly any thump.....really nice with the trem signal hitting the springs I reckon...