Magnatone 113, but with an FX loop instead of vibrato circuit?

[Gainzilla]

Hi All,

I'd like to build a Magnatone clone, but include an FX loop in lieu of the vibrato circuit (my logic that I can use it for time based effects, as well as the magnatone vibrato clone pedal I already have... In other words, more versatility). I've re-drawn the schematic, adapted from the ammo can version as I think it would work, but included the ammo can version schem with the vibrato circuit.

My issue is this: I'm not sure exactly where the vibe circuit starts and ends and whether or not (or how much) the circuit influences the core tone of the amp when not engaged. One specific question... The first tube buffer of the vibrato circuit looks like a cathode follower. Should I leave that in and feed the FX loop first stage or is it cool as I drew it. Also, feel free to point out any errors.

Thanks in advance!

Attached: Redrawn Mag with loop & Ammo can schem for reference:

[Gainzilla]

Adding this post for the sole purpose of bumping my post count up to 3. Seems legit. 

[terminalgs]

base circuit:

The circuit you have there is based on the 213.  This was also the Tonemaster 214, a Panaramic 1210, and a bunch of other ODM contractees of Estey in the early 60's.  the circuit became the 413, and then the 96-10041, which was the most advanced that circuit ever got (solid state rectifier, elevated heaters, etc..).  Find schematics and more info at http://magnatoneamps.com/

modification:


If you build it like that:

high gain stage-> volume -> high gain stage -> CF -> trim -> 50% voltage divider (100k/82K) -> low gain stage -> cathodyne.


If you don't use effects, you should have plenty of gain, and it might work. 

If you use effects that have high Vrms sensitivity,  and you can give it a high voltage signal and get the same back (unity) (like a tube reverb),  maybe you are ok.  if the effects have lower voltage input requirements, like a SS pedal might, and you have to trim it way down with the 100k pot,, then low voltage into that 50% divider then on to a low gain 12au7 stage,, hmm...  i don't think you'll have enough gain to give the PI.

[Gainzilla]

Thanks for the info, terminalgs! As a longtime enthusiast of the big 3, I'm trying to branch out a little!

Re:the loop, if I understand you correctly, you're saying that the combo of the voltage divider and the lower gain return buffer stage may not drive the PI adequately (with pedals in the loop, for example)? What if I replaced the voltage divider with a Variable resistor and bumped up the gain in the return buffer stage?

Here is another loop I've used successfully. This one can be switched out of the circuit. I would use a 100k pot rather than the 50k shown below:


Thanks in advance!

[terminalgs]

Yes,  If you make that 100k/82K divider a pot, you should be okay.  If it were me, I'd use a 12AX7 triode as the gain stage immediately before the PI.  the V1b might not be needed, especially if all tubes are 12AX7s.

[Gainzilla]

Good call on the 12ax7. Plus I have a ton of those laying around.

Here's a crazy idea... Any problem Using a switch to lift the ground on one of the 6v6's and run it as single-ended?

Thanks for your help!

Cheers!

[HotBluePlates]

... Any problem Using a switch to lift the ground on one of the 6v6's and run it as single-ended? ...

You can do that, but there's a better way...

You have 0.1uF caps going into each 6V6's grid, with a 470kΩ resistor from grid to ground. Have a switch which disconnects the 0.1uF from the grid and 470kΩ resistor on one of the 6V6's; be sure to keep the 470kΩ resistor running from 6V6 grid to ground.

What this does is disconnect drive signal from that 6V6, but keeps it idling. You're using a shared cathode resistor for the 6V6's, so disconnecting the cathode of one means you also need a new cathode resistor of double the original value to maintain the same bias voltage. Additionally, the 6V6 idle current gets balanced between the driven tube and non-driven tube, so the OT doesn't tend to be saturated the way it might otherwise (e.g., by using a push-pull OT without an air gap in single-ended mode, which normally has an air gap to prevent saturation due to unbalanced d.c.).

[Gainzilla]

Thanks Hotblueplates, that seems like a better long term solution for the life of the amp. A couple questions:

Should separate the shared bias resistor and double the voltage for both?

Re:the transformer, I haven't selected one yet. Do most PP trannies have the air gap in SE mode or is there something specific I should look for?

Thanks again!

[HotBluePlates]

Should separate the shared bias resistor and double the voltage for both? ...

If you do it my way (switch to open the connection from the 0.1uF to the 470kΩ/6V6 gid), then you don't do anything to the existing cathode resistor.

If you do it the way you were originally thinking, you need to give each tube its own cathode resistor, and that resistor should be double the value which is in the amp now.

... Do most PP trannies have the air gap in SE mode ...

A transformer is either designed/built to be a single-ended transformer, or to be a push-pull transformer. Only SE transformers have air gaps.

Why?

A transformer passes power from primary to secondary by virtue of a magnetic field which is transmitted/sustained by its magnetic core. Current causes the magnetic field, so the idle current of a single-ended stage passing one-way through the transformer "eats up" some of the transformer's magnetic capacity.

To cope with this, a SE transformer has an air gap, and also uses a larger core for a given amount of power.

Said a different way, push-pull has the idle currents flowing from the mid-point of the primary out to either end, and so balances/cancels the magnetic flux of the idle current. As a result, it doesn't need an air gap. Further, because the core's magnetic capacity isn't being eaten up by idle current, the core can be smaller for the same power throughput and bass response. Said yet another way, this makes P-P transformer smaller and cheaper for a given power.

You might find a "universal" transformer claiming to be able to be used both in SE and P-P mode; in reality, it is designed for SE operation and doesn't realize the economy or weight savings of P-P.


Which brings us back to my suggestion. It might look odd at first, but allows you to buy a lighter & cheaper push-pull output transformer intended for 6V6's. When you do not drive one side of the output stage, it still idles and reduces magnetic flux in the OT primary (and hum due to power supply ripple), yet you still have true SE operation.

Some may argue you won't realize the best loading of the remaining tube for maximum output power, but I offer that maximum output power isn't why you decide to run SE in the first place.

I'll also acknowledge now I shamelessly stole this idea from Kevin O'Connor. 

[Gainzilla]

Thanks for the great reply! I feel like I have a better understanding of how SE and PP trannies are optimized for their intended purposes (purpii? lol)

Re: the separate bias resistor: [facepalm] Duh. That's kind of part and parcel with switching out the signal rather than the cathode resistor, right? lol

Re: Not caring if I achieve maximum power in SE mode: Exactly right. Looking for flexibility here.

Great stuff!