Simple ReAmp Mod? Help with Input Impedances, Please!

[slor]

I've just rebuilt a little McGohan M102 PA head. The idea is that it will be used to drive the reverb send in my home studio (the reverb "chamber" is my hardwood-floored living room).

So rather than taking a balanced mic-level or unbalanced phono input (both of which it's set up to do), I'll be taking a balanced line-level signal out of my DAW and running it through the amp. I'm assuming that all I need is an input transformer rated at 600 ohms primary to 50k ohms secondary to drive a 6AU6A (the first preamp tube).

However, studying the schematic more closely, it might make more sense to skip the first gain stage and route the transformer output to the phono stage. After all, I'll be using a +10dbm signal, and I doubt I'll need that much gain.

The physical rerouting of the signal is easy; what I'd like to know is what the second gain stage (a 6EU7) wants to see in terms of impedance. I've searched for a reference for "typical" values but have come up short. Is there a guidebook, or at least a rule of thumb?

Many thanks,
Seth



McGohan M102 Schematic

[Miles Prower]

Originally posted by slor
The physical rerouting of the signal is easy; what I'd like to know is what the second gain stage (a 6EU7) wants to see in terms of impedance. I've searched for a reference for "typical" values but have come up short. Is there a guidebook, or at least a rule of thumb?

There are no typical values for input impedance. The input impedance is, for all practical purposes, the same as the grid resistor. All that's important is to make sure you don't have a tremendously high input impedance since that can cause high frequency rolloff when combined with input capacitance. I have no idea what a "DAW" is, but I'd be surprised if its output impedance was too high.

[slor]

Hmmm....interesting. After bouncing between the schematic and the RCA tube book for the last 30 minutes, I was beginning to think the same thing: It's dependant on the grid resistor.

"DAW"? That's Digital Audio Workstation, I believe, a fancy term for "my computer." The computer will be putting out standard line level: +10db at 600ohms. That's not the issue; what I'm trying to figure out is what the secondary of the input transformer should be.

For what it's worth, the unit came with a 150ohm/50kohm mic transformer. I would use it, but 150 sounds pretty far away from the 600 the computer is putting out.

If the line transformer is putting out 50kohms, does that mean I should change the grid resistor of the phono stage 6EU7 (currently 330K) to something closer to 50K?

-Seth

[korneluk]

Originally posted by slor
...The computer will be putting out standard line level: +10db at 600ohms. ...For what it's worth, the unit came with a 150ohm/50kohm mic transformer. I would use it, but 150 sounds pretty far away from the 600 the computer is putting out.

Better yet, get yourself a pair of Altec 15095 or 15095A (specs are identical, just make sure you get two of the same). These are labeled as a 1:5 step up transformers in an octal package that fits the socket already on the amp. Primary impedances are 150/600 ohms and 15K secondary. This way you can try both settings without having to modify the rest of the circuit.

-- josé k.

[slor]

Thanks much for the replies, gents. Unfortunately, I'm still scratching my head. Specifically:

-Miles, if the input impedance were essentially the same as the grid resistor, why is there no input resistor on the first gain stage (the 6AU6A)? (I see a 1 meg grid-to-ground resistor, but that's different, right?).

-Jose, thanks for the tip; I have a couple of 150/600 primary transformers...I'm more concerned with the secondary and how it interacts with the tube. The unit I'm working on came with a mic input transformer rated at 150/50K (I have a similar one labeled "line input transformer" rated at 600/15K). Why would I use one (15K) or the other (50K)?

Thanks much,
Seth

[Miles Prower]

Originally posted by slor
Thanks much for the replies, gents. Unfortunately, I'm still scratching my head. Specifically:

-Miles, if the input impedance were essentially the same as the grid resistor, why is there no input resistor on the first gain stage (the 6AU6A)? (I see a 1 meg grid-to-ground resistor, but that's different, right?).

Actually, there is. If the 6AU6 has a 1.0M grid resistor (which it does) then its input impedance is 1.0M, at least for audio frequencies. (You don't have to start worrying about it at least until you get to short wave frequencies.) Hollow state comes closer to the ideal of infinite input impedance than BJTs. Unless your source impedance is over 100K, doanworryboudit.

[slor]

Thanks Miles! But wait...still mightily confused over here. What's the relationship between the output of the transformer (50K) and the input resistance of the tube (1 meg)? And "hollow state"? "BJT"?

If I'm reading you right, you're essentially saying: "Use whichever transformer you like, unless it has a huge (>100kohm) impedance....

Help.
-Seth

[Miles Prower]

Originally posted by slor
Thanks Miles! But wait...still mightily confused over here. What's the relationship between the output of the transformer (50K) and the input resistance of the tube (1 meg)?

When you see something like 150R : 50K specified for a xfmr, what this means is a voltage ratio of:

Vsec / Vpri= sqrt(Zsec / Zpri) so:

Vsec / Vpri= sqrt(50E3 / 150)= 1 : 18.26

So this xfmr has a voltage step up ratio. As for impedance, then, yes, a secondary load of 50K will appear as an equivalent source resistance of 150R. As for how this relates to the 6AU6, the total impedance at the grid becomes: Rg || Zsec, which would be approximately 47.6K. As for other impedances, these will be transformed by the same impedance ratio of 1 : 333.

As for which one to use, go for the 600 : 15K. The voltage ratio is lower, and you already have a ton of gain there. Also, the lower the impedance seen at the input side, the harder that load becomes to drive. I'd give it a secondary load of 100K or so and see how that works out concerning frequency response and ringing.

As for myself, I have an iron allergy. I try to avoid chokes and xfmrs as much as possible. There will be those who disagree, but that's my design philosophy.

And "hollow state"? "BJT"?

Hollow state: What vacuum tubes are cuz they're hollow inside.

BJT: Bipolar Junction Transistor.