Using octal tube for a 'one tube' reverb

[smackoj]

I have an old radio chassis that I converted to a tweed deluxe type amp (not exact copy). I still have seveal octal sockets open for a reverb and I would like to make it a 'one tube' variety. Can I copy the one tube reverb in the Archives and use a 6SL7 or 6SN7 rather than a 12AX7?

TIA amigos

 

[kagliostro]

May be you can try with a 6SL7 that has a gain factor of 70

6SN7 has a mere 20 as gain factor, 12ax7 has 100 as gain factor

K

[smackoj]

good point. I thought the 6SL7 was the same as 12AX7 gain but I see now it is less. I still am not sure about using the octal tube by itself? Mainly because I see in the Archives under 'one tube reverb' that there were amps using one 12AX7 aka Twin Triode but most of the other examples shown are like the 6MB8 which is a pentode and a triode in the same bottle. The 6SL7 is a high-mu Twin Triode tube.

thx

 

[kagliostro]

Looking for octal preamp tubes used as one tube reverb

I've find this old tread by our friend Tubenit

http://www.el34world.com/Forum/index.php?topic=5388.0

Very interesting  

K

[jeff]

Check out some ampeg schematics, You could use a 6SN7 and the right tank and not even need to buy a transformer. I don't know if it could be a one tube jub but if you got the holes...

[smackoj]

thanks, I got the sockets avail....I know Ampeg did an octal tube tremolo also. No luck finding a octal reverb setup using a rev. tank though....maybe I'll just try one of the one tube layouts in the Archives and switch the 12AX7 pins for the 6SL7 pins?

 

[sluckey]

http://www.el34world.com/charts/Schematics/files/ampeg/Ampeg_R12R_Reverb_Rocket.pdf

Same amp, better schematic...
http://www.el34world.com/charts/Schematics/files/ampeg/reverb_rocket_12r.pdf

[kagliostro]

Tubenit has draw a 2 x 6SL7 version in the link I posted

http://www.el34world.com/Forum/index.php?action=dlattach;topic=5388.0;attach=6441

if you want to have a one tube reverb I agree that you can try this other Tubenit schematic




K

[kagliostro]

Hi Sluckey nice report !

which kind of reverb tank do you think Ampeg used in that transformerless units ?

K

[sluckey]

I'm pretty sure they used Gibbs. The cap coupled input would need to be hi Z, probably 1.5K to 2K. And 2K to 2.5K would be fine for the output.

[DummyLoad]

I'm pretty sure they used Gibbs. The cap coupled input would need to be hi Z, probably 1.5K to 2K. And 2K to 2.5K would be fine for the output.

the closest to specs above i could find are the three in the links below. otherwise request a custom? was thinking of using the R4FB2C1A - the input jack is isolated.

http://www.tubesandmore.com/products/P-R4FB2A1D

http://www.tubesandmore.com/products/P-R4FB2C1A

http://www.tubesandmore.com/products/P-RMOD-4FB3A1B

[smackoj]

thanks for the input guys....mucho apprecianado... do have question about the rev tank I have to use. It's an Accutronic tank, 3 spring short type with no other markings except the Patent Ofc. numbers....It was taken from a Princeton Chorus 65 watt SS amp, so whatever the input Z and output Z are, it was matched to SS not tubes. How can I tell what this tank wants to see on the input to run right?

 

[kagliostro]

Here you can read something of interesting about reverb tanks

http://www.el34world.com/Forum/index.php?topic=14318.0



As you can see, is interesting to read that to identify the impedance, of an unknown Tank, DC resistance can be used

K

[smackoj]

ok, I checked dcr at the input rca jack. it reads 58 ohms....so that means I have a 'E' model which needs somewhere between 600 and 800 ohms impedence from the driver tube?

 

[HotBluePlates]

Just go ahead and do the 1-tube, and use your 6SL7. Worst case, your reverb will not be as strong as you might like.

Many people who don't want full-surf reverb prefer the lesser drive of the one-tube circuit. For what it's worth, the 12AX7 should be a bad candidate because it won't pass a lot of current through the reverb transformer. Yet it still works.

I've played some of Tubenit's amps with reverb, and have never needed to have it above midway to have more than enough reverb for my taste. The circuit sounds quite nice.

So just do it. 

[tubenit]

Wow!  I totally forgot about Bill's octal reverb.   

I use 1M dwell pots on the 1-tube reverb that I have done.  I also use 1M reverb pots.  With that, the dwell is on "5" & reverb on "3" and it has  plenty of reverb for me. It should be noted that the original 1-tube reverb is from the dumlish clone guys and they use a 2.2M reverb pot which was waaaayyyyy too much reverb for me. ]

With respect, Tubenit

[smackoj]

"So just do it"  HBP....ok, let's get a tweekin' and heatin'.  

I'll be back...Pilgrim!

 

[Jerry garrcia]

Tubenit has draw a 2 x 6SL7 version in the link I posted

http://www.el34world.com/Forum/index.php?action=dlattach;topic=5388.0;attach=6441

A long time since this has been posted but I’ll take a chance.

I’m just starting on the long stretch all metal BR-3 build. Will use the 6SF5’s instead of the loctals.
I just realized that I have a ton of chassis space left since it’s PTP. It will fit a reverb circuit including the two pots.
Do you know if a 6SC7 work in this circuit or will the shared cathode, once again, ruin it? Must be one or two metal tubes for the looks. I do have 6J5, 6N7 and so on.

[PRR]

> all metal BR-3 build.

http://www.prowessamplifiers.com/schematics/Gibson/br3.pdf

[Jerry garrcia]

> all metal BR-3 build.

http://www.prowessamplifiers.com/schematics/Gibson/br3.pdf

That is the one. But will do the octals mentioned as preamp tubes and got two metal 6v6’s. Would love to fit a tube driven reverb in the chassis. But metal tubes. Could do a standard Fender 12ax7 reverb (strange that it functions with the low current tube) with my last two 6SF5’s but would like to spare them if possible. So got metals 6SJ7, 6SC7, 6N7, 6J5, 6V6 and the precious 6SF5’s.

[Jerry garrcia]

Could I do the reverb circuit of the twin reverb but with a parallel 6N7 instead of the parallel 12at7 and a 6SF5 as the recovery tube? It, 6N7, has a bit less gain (35 vs 60) but the plate current and plate resistance are quite similar.
Not good yet with finding out what kind of reverb transformer and tank to choose depending on what type of driver I’m using. I do have a 125A20B transformer and a MOD Reverb Tank 8AB2A1B at home.
It won’t be a one tube reverb but I can probably fit two extra tubes. Or a 6SK7 driver🤔

[Jerry garrcia]

The 6N7 will unfortunately not work due to 0.8mA heater current…
So still stuck. I could use the 6SL7 driven reverb previously shown in this thread and use a parallel 6SC7 as the driver and half of a 6SC7 as the recovery tube? Almost identical as the 6SL7 except for the shared cathode.

[DummyLoad]

Use a 6DR7 - the low mu triode drives the tank and is RC coupled, so use a reverb tank that is suited as replacement for an Ampeg V4 or Reverb Rocket. The hi mu triode is the tank recovery gain stage. use a 4-5K 10W resistor & 1 .5-1uF metal/mylar film coupling cap for the reverb driver. For the recovery amp use typical Fender 100K/1.5K values.

Magnatone used a similar arrangement in few of their amps, however, Magnatone typically used the high mu triode as a tremelo driver.

EDIT: Rk for the lo mu driver with B+ at 350V +/- should be about 330 Ω 1W, bypass it with 10-22uF. 

--Pete

[kagliostro]

Here is how they used the 6DR7 for Reverb and Vibrato (use a circuit similar to Magnatone) in an old italian GEM amp

Pin # 1 - 2 - 9 are the Low Gain Triode

Pin # 6 - 7 - 8 are the High Gain Triode

Franco

[DummyLoad]

My apologies - Use the 6EM7 which is an octal package, not the 6DR7 - the 6EM7 is basically the same thing as the 6DR7. Suspect the 6DR7 was a revised 6EM7 released in 9pin package.


--Pete 

[Jerry garrcia]

My apologies - Use the 6EM7 which is an octal package, not the 6DR7 - the 6EM7 is basically the same thing as the 6DR7. Suspect the 6DR7 was a revised 6EM7 released in 9pin package.


--Pete

Thank you. So the tank is driven without a transformer? I like your and @kagliostro suggestions. Unfortunately my amp “must” be an all metal one. Part of the “lure”.
Would an old metal 6V6 work as a driver (35-47mA plate current) or an 6j5 (plate current at 10) and a 6SF5 (gain 100) as the the recovery tube?
The 6j5 might be too weak. Need to read up on the transformer less reverb.

[kagliostro]

To me you can use a 6V6 as reverb driver, only I haven't seen one driving a tank without a transformer

However I think that also in that way it can have success


One other option will be to use a double triode with the same spec of an ecc82 the way Merlin suggests on his pages, see attached

Franco

[Jerry garrcia]

Thanks Kagliostro. So a Parallel 6n7 or a 6V6 it seems to be. In either way I need to find a nice looking 6.3V, 1A filament transformer to cope with the higher current demand due to the new tubes for the reverb unit. The 6N7 route might be better since I can use my old reverb transformer.

If anyone has the will and time. How do I calculate the primary impedance on the reverb transformer?

[kagliostro]

Transformers don't have an impedance, they have a turn ratio, feed the secondary with a low AC voltage (if you have only a 6.3v transformer you can use it), verify the voltage you have on the primary, then divide this voltage by the voltage you applied to the secondary and you'll have the turn ratio of your transformer

Now you must know the impedance of the input of your tank (some tanks are simply 8ohm)

Multiply the turn ratio by himself, es. for a turn ratio of 25 you must do 25 x 25 = 625 then multiply the tank impedance for it  if it is 8ohm you have 8 x 625 = 5000 and 5000ohm in the reflected impedance at the primary of your transformer with an 8ohm load, simple not ?

Franco

[Jerry garrcia]

Sorry was not thet clear. I do know how to get the winding ratio on a OPT and get the correct speaker for it.
Now my question was how do I get the right load for the driver. Different drivers, different loads I guess. As with power tubes

[DummyLoad]

Yes, that's doable - I actually prototyped an all metal tubes amp that I spun up on one of my breadboards back around 2011, I called it the Metal-Mania.

See attached... No reverb, so if you want to use all metal tubes, 6V6 in metal are becoming scarce, 6F6 (1621 military eq.) is an option and seem to be more plentiful, Fender actually used the 6K6 as the tank driver in the early stand-alone reverb, the 6F6 is is an uprated 6K6, or less beefy 6V6, about 1/2 gm of a 6V6 and 6F6 has beefier screen than 6V6 or 6K6.

If I were doing this, I'd do the following: Since we can mostly agree that the Super Reverb is a superb platform, take a single channel of that, replace the twin-triode reverb tank driver with a 6F6 and use all 6SF5 for the triode stages, as for the rectifiers, the only (mostly) available element is the 5T4 - I used a pair of them to satisfy the current requirements in my prototype. I had another in works, called Rockin' Rosie Riveter (Super Reverb) with glass output tubes since the metal 6L6 output tubes are becoming quite scarce.

Ignore the DC filaments in the schematic attached, I ditched that idea on the breadboard. Amp sounded damn good, but sourcing the tubes is/was a PITA.

--Pete 
 

[DummyLoad]

Rockin' Rosie Riveter with reverb. Plan attached. All iron is sourced from Hammond.


--Pete

[Jerry garrcia]

Rockin' Rosie Riveter with reverb. Plan attached. All iron is sourced from Hammond.


--Pete

I did post a reply yesterday when I was waiting to do a X-ray on a messed up shoulder. Somehow it didn’t post. Probably due to a to large picture of my draft schematic. Will go for the 6V6/6F6 driven reverb.
I do have some extra metal 6V6 to spare.
Just a bit uncertain where to place it. I will redraw a new schematic where the tone stack is placed on the right place.

[Jerry garrcia]

So now I have a schematic. A rough draft
So if anyone sees a fault or something that ought to be corrected please tell me.
Is the reverb circuit correct and placed in the optimal position?
Should I have the tone stack after the second 6SF5? Had that before but now the reverb is there.

[Jerry garrcia]

Wrong V3 designation. Should be a 6J5

[DummyLoad]

Wrong V3 designation. Should be a 6J5

Reverb driver power tapped at node A would likely be a buzz-bomb, also, recovery stage wants clean(er) power than that at node B, ideally as clean as the 1st stage, even better, if possible. Best results would be to tap driver at node B with another RC stage for that and move recovery power tap to node C.


Just my 2cents.


--Pete

[Jerry garrcia]

  Thanks! That’s why it’s called a rough draft. Only built reverb with LND150 before, and buzzing. Been running shielded wires all over that amp and don’t dare to start it up since I’m to afraid of the result.
Will do what you suggested. A 3.1R and a 20 uF cap to driver from node B.
Does it look ok otherwise? In the right placement? Read somewhere that the tone and reverb shouldn’t be after one another so moved the tone stack.

With this 6V6 I’m running quite a bit of current. 120mA when idling. PT Hammond 290CAX is rated for 115mA. Will have to play gentle.

Last question for now. Need 1A of extra filament current. Any idea how to get that? Haven’t found a 6.3V transformer in Europe. A 12V and a resistor?

[DummyLoad]

Some suggestions. Please review attached.

Respectfully, 

--Pete

EDIT: T3 is a Hammond 1750AX - 7.5K:8 3.5W.

[Jerry garrcia]

Hi and thank you. I had already made a small change but your seems more correct.
Started to drill and can’t wait to try the interstage transformer pi.
Thanks once again for all help. Will get back when completed. Found a <15€/$ 6.3V, 1.2A transformer for the filament

[kagliostro]

Now is late to consider it, but as I've just find it in my archive I post it only to share for further future searchers



Franco

[Jerry garrcia]

Nice. But I can’t fit any more tubes 🙂. Two extra from planned is about all I can fit. Have to use a separate filament transformer to get away with the extra current.

Will a 115mA HT transformer handle a 120mA idle current ? Sound strange of course that I asked if 120mA would be handled by a 115mA PT but a lot of times the PT has some “slacks” and will handle the higher current. Usually the PT is more stressed when the volume is cranked up, now I have the reverb… should I up the PT?