Using a OT to change a reverb tanks impedance

[jeff]

 
Instead of plugging into an 8 ohm reverb tank, can I plug into the 8 ohm secondary of a 3K:8 OT. Then connect the 3K primary to two 1475 ohm tanks, wired in series for 2950.

(The OT is "really" a 1K5:4, but that's the same as a 3K:8, right?)

Anybody see any problems with this?

 I know this isn't the cheapest way to do this but will it work? I'm stuck with two reverb tanks with 1475 inputs and I have a 1K5:4 OT(3K:8) that I'm not going to use any time soon.

 Backstory: I got a bunch of reverb tanks from an amp repair guy for free years ago. The reason was that the input coils were blown. At the time I wrote to Accutronics and the guy there was nice enough to send me replacement coils....for free even. Great deal. I asked him to send me some "A" coils(8 ohm) and some "F" coils(1475 ohm). At the time I was building a post speaker reverb and my schematic listed the tank as a 4F. I didn't realize that this was not the same as Accutronic's 4Fxxxx tanks so I have some tanks that have 1470 ohm inputs. I tried writing back but Accutronics is now owned by someone else and doesn't sell replacement parts. I assume this could work but I don't want to blow a irreplaceable coil if I'm overlooking something. It seem a shame to throw these tanks away because I can no longer get replacement parts, hopefully I can make it work.  I know it's not the best, or most economical way to do it, but I have the parts and don't think I'll use them for anything else.  Maybe I'm just cheap and don't want to "spring" for a new tank, but you wouldn't throw away a guitar if the pickup died would you?

[RicharD]

I sorta understand what you're asking but sorta don't.  It seems to be a better solution would be to design a driver circuit that can drive a 1k5 load.  I think a transformer driving a another inductive load is begging for funkiness, not to mention driving a pair of inductor/ transducers in series.  How are you gonna sum the returns?  Seems like a can of worms.  There's no law that says you have to use a reverb tank driver as a plate load.  I'm thinking hang what you got off a cathode follower.  I'm also drinking 151 and Mexican Coke so hopefully someone else might chime in. 

DWI = Designing while intoxicated.   

[jeff]

I probally should just use a circuit that needs a 1475ohm tank, but I wanted to use it as a speaker driven reverb and need an 8 ohm tank.

I was hoping I could use the 3K:8 OT to transform the 3K tank(s) to a 8 ohm tank.

Are two tanks in series a problem? My total would still be 8 ohms.

As far as return I was going to connect each side of a pot to each tank and take the signal off the wiper for an A tank/B tank blend.

[DummyLoad]

pitch the transformer - use a coupling cap ~~.5-1uF. anode load w/ ~~5K it'll need to be a BIG resistor; 10W or so. 12AU7 would work well w/ 2 triode in || 270ohm Rk w/ 350V B+.

tubeCAD shows a smattering better symmetry w/ 220ohm Rk, 4Kohm Ra w/ Ibias of ~~23mA @ 300V B+.

the down side is it sucks more current so you'll need to reconfigure the B+ chain.

magnatone used this plan in model 10a. others have done similar w/ 12DR7 w/ power triode as driver and osc. triode for tank recovery.

http://www.el34world.com/charts/Schematics/files/magnatone/magnatone_m10a.pdf

--DL

[RicharD]

I don't have a definitive answer because I've never tried anything like this.  I must say I do sorta like the idea.


> speaker driven reverb
I don't think you want an 8 ohm tank for this anyway, otherwise your tank is gonna dissipate as much of the load as the speaker (if the speaker is 8 ohms).  I think you actually want a 1k5 tank.  Plug -n- pray.

[jeff]

 Usually there's a resistor to keep from loading the main amp

[RicharD]

> Usually there's a resistor to keep from loading the main amp

And that resistor would be in series yes?  Where's the difference?  What do you have to lose, a reverb tank that's collecting dust?  If you're worried, estimate current and add a series current limiting resistor.  If you don't wanna dew the math, stick a 100k in there and try it.  I think your risk factor is nil.  Clip leads are your friend.   

[jeff]

ahhhhhh

In the past I've used a 47~500 ohm 10W resistor and a 8 ohm tank and it sounds great.

[RicharD]

Right on!  Think of it like like you're doing a line output off the speaker jack. 

[jeff]

This is what I had in mind.

[jeff]

Thing is, I'm not sure about impedance matching but I know an 8 ohm tank, wired like the first schematic works.
How could you wire it to get a 1475 impedance signal to drive the tank off of the speaker? I don't know if I could drive the 1475 tank.

 Don't know about using two tanks in series but by using the transformer, I'd need two in series to get a reflected 8 ohms. Using one would give me 4 ohms, plus if I can I'd like to use them both, I have no need for a 1475 ohm tank.

Not sure what value pot or if it will even work, but I'll experiment.

[sluckey]

Why not just use a cap coupled driver?

[jeff]

Aside from being a waste of money doing it this way, is there anything wrong with doing it this way? Would it work? I'd try it but don't want to damage anything. Do you see any glaring errors?

Why not just use a cap coupled driver?

Because I have no idea what that is. Could you please explain? Do you have a schematic or an example of an amp that uses this to reserch?

I came across this: Haven't read the whole thread but is this what you mean?

[sluckey]

I wanted to make a post speaker reverb

Sorry, I overlooked that point. There are many amps that use a hi Z input tank. Look at the Ampegs, specifically Gemini II, or Magnatones, specifically M10A.

[jeff]

I must have posted and changed my reply as you were posting.
I shouldn't be hell bent on post speaker reverb.
This was just what I could come up with at first with the availble parts I had on hand.
I'm up for suggegtions to use these tanks in a better way.
I check those amps out. Thanks.

[jeff]

  I like post speaker reverb so much because it's like I getting something for nothing. Yeah, you have to use two amps, and your reverb controls aren't on the main amp but, build one unit and you don't have to add reverb to every amp. No need to buy and wire up extra tubes, circuitry, and transformers. I wire my little amps with the first schematic I posted putting the output of the tank on the shorting jack. This way plugging into the input, I can use the little amp as is or, unplug the input and use the other jacks to convert it to a reverb unit.
 Also I really dig the fact that the dry guitar is coming from one speaker and the reverb from another vs using a pedal or FX unit.

[RicharD]

Your 8 ohm speaker is your load.  Normally the series resistor is much larger that the tank input impedance.  This serves two purposes, a it makes it so that the total additional load is much greater than the speaker impedance.  This network is in parallel so you want it to be a higher impedance so that it is negligible to the output loading.  What I've drawn is 101500 in parallel with 8.  The total load is effectively 8 ohms.  You may have to play with the 100k value, that's why I suggest starting with a pot.  This resistor also forms a voltage divider with the tank's primary so that the tank gets a fraction of the signal.  Hang this in parallel with the speaker.  Dial the pot until your signal level is right.  Replace with a fixed resistor.

(edit)
Fixed grammar and added schematic.

[jjasilli]

Could you drive the OT with a 12au7?  I think that's only about 6500R per plate = about 3K with both plates in parallel. 

Or for "cathode driven", check the specs for an Ampeg say a YGM-3 guitarmate.  That uses I think both halves of a 12ax7 through a cap to a reverb tank.  Don't have the tank specs handy but can probably dig them up.

[octal]

I know the OP is thinking about speaker driven reverb, but since there's also been some discussion of cap coupled drive for the reverb, I'll mention that I've had good results with Merlin's cap coupled reverb driver circuit http://www.freewebs.com/valvewizard2/reverbdriver.html  scroll down to "cheap current source drivers." I usually fiddle with the voltage divider/input filter circuit to get more drive. It sounds great, no transformer required. I would definitely prefer this to using a 12ax7 based cap coupled driver circuit.

[jeff]

Could you drive the OT with a 12au7?  I think that's only about 6500R per plate = about 3K with both plates in parallel.

Thanks, This is interesting but not really what I'm going for. This would kinda be like chasing my tail for my application.
I want to use the transformer to change my 2950 tank to an 8 ohm tank.
If I use the transformer as an OT for the 12AU7, it's output would be 8 ohms. I'd need an 8 ohm tank so I'd need another transformer to take the 8 ohm signal out of the OT to match my 2950 tank.
Instead of that maybe just use the 2950 tank as the plate load?????

But this isn't really what I'm trying to do, I want speaker driven reverb.

[sluckey]

But this isn't really what I'm trying to do, I want speaker driven reverb.

Well, you need a more appropriate tank. Or be prepared to use a lot of circuitry to make your high impedance tank work.

[jeff]

OK thanks
Just to be clear(I'd try it but just want to be sure because like I said you can't get replacement parts for tanks anymore).

Plugging your reverb send into a 8 ohm tank is not the same as plugging it into a 8:1K5 transformer plugged into a 1K5 tank.

I know it's not ideal but I've got the parts.

(numbers are simplified for example and are not what I have)

[jeff]

Butterylicious, thanks for the reply and schematic.

That is exactally what I've been doing, but I've been using an 8 ohm tank. In mine I'm using two jacks so the amp plugs into one and the speaker to the other and the jacks are parallel, but electronically it's the same thing.
It sounds good with the 8 ohm tank.
Have I been using the wrong tank all along? I'll try it with the 1K5 tank.