Running iPod into 6AU6 input...

[Steve Conner]

This was a pretty common hook-up for small low-cost tubes like the 6AQ5 and EL84. The screen grid was supplied straight from the raw B+ to reduce the component count. This caused more hum, but they were typically used with tiny speakers in open-backed table radio cabinets, that would filter out the hum due to a complete lack of bass response.

The screen grid isn't connected to "the lower primary" as such. That node is the B+ supply. The screen grid and one end of the OT primary are connected to it.

Modern schematics are drawn with more positive voltages towards the top of the page, but in old ones they didn't bother. If it bothers you that the plate is connected to the "top" of the primary, you can mentally turn it upside down.

[km6xz]

The high pass filter is probably a 6db/oct low pass filter. The amp was surely intended as a ceramic cartridge amp. The cartridge, being a constant amplitude response, as opposed to the constant velocity response of a magnetic cartridge and cutting lathe.
So low frequency will need to be boosted in a playback system using normal records with a ceramic cartridge. You are getting more hum because of the higher gain needed to reproduce the low frequencies of records cut with Constant Velocity cutting heads.
You are using it with device that does not have an EQ curve to compensate for mechanical realities of records and cutting so it does not need the 6 db/oct roll off starting at about 500hz.
I would reduce the gain a bit, set up a proper match for the iPad and remove the phono filter..
The iPad has a class D amp and the output filtering needs a relatively low z to work as intended but the amp input is very high, 1 megohm. Add a load of a few hundred ohms at the input and feed the top of the volume control with that. You could reduce the pot value to 10-50k and probably lower noise as well. By lowering gain, and flattening the response, the bottom end will not longer be higher gain than the mid and top range so the hum should reduce in relation to the mid and HF which are now being attenuated.
The screen voltage is fine, that was the normal way of supplying screen voltage at that era.

[bob p]

^^^ @km68z brings up some important points that should probably be in a FAQ for people who are trying to re-task old amps that had inputs that were tailored to receive inputs from devices that are extinct today. This type of conversion requires 2 tasks that often get overlooked: impedance matching and filter removal.

Here's another example from and amp that I had been working on recently: (schematic here)



Input Z: I've already made this point but maybe it wasn't clear enough.

If you look at the inputs, the input impedances on this amp are a lot higher than input impedances are today. Today's devices may have an output impedance of a few hundred ohms, and an input impedance in the tens of k up to maybe 100k. Old devices might have input impedances that are orders of magnitude higher; if you attempt to drive them by a modern device that can't act as a good voltage source the results will sound really bad.

The crystal phono input is a great example. If you take a modern device like a CD player (OK, maybe a CD player isn't such a modern device) and plug it into each of the inputs, you'll find that some of the line stage inputs sound just awful... specifically the crystal phono input will sound dead, with very poor frequency response and a profound mid-range hump. The Tuner input will sound a lot better.

There are a couple of reasons for this. One is that the crystal phono stage's input impedance is well over a megohm. That input might actually work pretty well for a high impedance guitar pickup, but it's going to be horrible for a device with a low impedance output like an iPod or a CD player. Notice that the Tuner input has a much lower input impedance. the iPod or the CD player will sound a lot better through that input.

To correct this problem, it's best to re-scale the voltage divider on the inputs so that the input impedance of the amp will be in the range that works well with modern electronics: somewhere in the range of perhaps 20k to 100k. As an example, changing the values of R2 to 100k and R36 to 6k8 (removing the pF cap) will maintain the voltage ratio that's needed by the input while lowering the stage's input Z to ~100k. The result will be that the iPod would have a much easier time driving this kind of input, compared to the stage's previous value of about 2M. I'd expect similar results for your application, where the input Z is variable between a value of 0 and 1M. That extremely high input Z at line levels could only have been designed for a piezo phono pickup that has an extremely high Z.

The other thing to consider is that phono equalization is used to provide de-emphasis/emphasis of specific frequency bands so that it would be easier to fit grooves on a record and get a reasonable result when playing the record back. Unfortunately, phono equalization methods changed quite a bit over the years, so there isn't a standard formula for the types of filtration that were applied until things became standardized with RIAA equalization. There's plenty to read on this subject, more than I can type here. But in a nutshell, you need to look for frequency response shaping filters in the signal path that don't make sense for a line level input that needs a flat frequency response. When you see them, remove them.

Taking these two approaches should make your amp sound a whole lot better.

[disorder]

Should I still add a grid resistor to ground and a series grid-stopper resistor to the 6AU6? These would surely have an impact on the input impedance and I am not sure how to calculate with them present.

[bob p]

I'll walk you through the calculations for the Tuner input in the schematic that I attached:

Zin = input impedance = R33 + (R1 || tube grid input impedance);

where || is shorthand for the parallel resistance: R1*Rgrid / (R1 + Rgrid)

Substituting the second equation into the first, we get the result:

Zin = R33 + R1*Rgrid / (R1 + Rgrid)

The combination of R33 and R1 comprise a voltage divider that is quite "top heavy." That is to say, most of the Z in the divider is attributable to R33 and not to R1. Because R33 is quite large, R1 is small and Rgrid is large, the fractional term involving R1 and Rgrid works out to be much smaller than R33. So in this example Zin is primarily determined by R33.

[Steve Conner]

Does an iPod really sound bad if it's running into a too-high load impedance? What's the theory behind that? For line or headphone outputs, I always assumed that a too-low impedance was bad, but too high wouldn't make a difference.

[bob p]

I agree Steve. I always thought the loading problems only came along with a mismatch to the low side. Some recent tests with cheap electronics have had me scratching my head.

Does an iPod sound bad running into too-high load impedance? I don't know. I don't own one.

Does a CD/DVD Player? Yes, IME cheap ones can misbehave like that. I don't know why.

My comments come from recent experience with a cheap DVD player when refurb'ing the "glad" amp. My input source was an el-cheapo DVD player, the kind that you can find on sale for about US$20 at a discount store. I had already run through the amp, verified that all the parts were in-spec, and verified sensitivity of each input and it's frequency response. There was a noticeable difference in sound quality when the el-cheapo DVD player was driving a 2M2 input vs. a 470k input. I re-scaled the 2M2 input to 100k and the problem went away. (You can ignore those pF caps on the inputs as I removed them).

I understand that the data sort of flies in the face of theory, but they are what they are. I haven't bothered to look at what's inside of the DVD player. Whatever it is, It's got to be the cheap.

Maybe I'm totally wrong on this. If you can explain what's going on, please help me out.

As far as the OP's question goes, I think his amp would indeed behave better when driven by an iPod if he changed the input Z so that it was constant instead of variable between such wide extremes.

[disorder]

I've been looking at popular headphone amplifier schematics and noticed most are in the 10k to 100k range, so that says something. I've also always been of the belief that you want out impedance as low as possible and input as high possible, ie allowing for an output that is capable of driving current and an input that doesn't ask for too much current. The exception to this rule being the guitar world where sometimes loading is a good thing such as the original fuzz face pedals.

[disorder]

Could I replace the 1M grid resistor on the 6AQ5 with a 1M pot as a volume control?

[disorder]

Could I replace the 1M grid resistor on the 6AQ5 with a 1M pot as a volume control?

Any harm in this?