Running iPod into 6AU6 input...

[disorder]

I picked up an old Voice of music 162 amp (5 watt with 6AU6 input and 6AQ5A power). The input runs through a pot as voltage divider and then heads directly to grid of 6AU6. When I run my iPod into it (just one channel, not summed) it sounds choppy and gaty and weak in low end from 0 to halfway. From halfway to full it starts sounding better and with volume pot cranked it sounds awesome it's just too loud. Turning the volume down on the ipod results in the same issue once you get the volume to a more reasonable level.

I tried multiple tubes and all voltages seem to be reasonable.



Can any point me in the right direction? All I can think of at this point is as you change the volume you change the input impedance of the amp. So perhaps as you get a lower input impedance the ipod starts to freak out? Yet I get the chopped gated sound when leaving amp on full and turning ipod down.

[Steve Conner]

It should sound better than that. Try checking C1, C2 and C3 for DC leakage (might be easiest to just replace them with known good ones)

[disorder]

I'm trying to comprehend what is going on with R6, R7 pot, and C3 that connects preamp tubes cathode to power tubes plate. Is this a form of feedback? If so, how does it allow R7 to work as a tone control? And won't this affect the bias of the preamp tube as more signal gets put on the cathode?

[disorder]

Took some voltage readings... in the process I noticed there is a simple RC high pass filter at the input. Probably not the issue though.
Two things stand out to me... First is the grid of the power tube has a constantly rising voltage on it. That then settles around 3.3V and still fluctuates some after that (3.6 after thirty minutes and still trickling up). Next is the voltage on the preamp tube grid. Is this normal? Luckily the input jack doesn't see this thanks to the RC filter at the input that isn't in the schematic. Any ideas? I'm looking at C2 being leaky?

[lowell]

Not immediately familiar w biasing a 6au6, but if it's like most of the other common tubes, looks like its under biased. Should the grid voltage not be negative relative to the cathode?

[Steve Conner]

I posted a reply to this thread but it got deleted somehow. You might have a bad coupling capacitor to the power tube grid or an old, worn-out power tube with excessive grid current. I usually replace that coupling capacitor in old tube equipment even if it's not faulty, because it probably will be soon.

The input stage voltages do look a bit suspect, so check all the resistor values and try a known good 6au6 if you haven't already.

[disorder]

Went through and measured all resistors, all are within +/-20% - nothing too crazy for carbon comps. Replaced C2 with a brand new film cap and that seemed to stop the voltage leaking onto the 6AQ5 grid. Also replaced the 6AU6 with a known good NOS RCA type. The preamp stage voltages pretty much stayed the same. My next step is replacing the other caps and swapping out the power tube (have a NOS RCA 6005/6AQ5W on the way).

Also, the input high pass filter is really strange (and not on the schematic). Input goes to R1 pot, wiper to .01uF cap, which then connects to 4.3Meg resistor to ground and to the 6AU6 grid.

Thanks for the help so far guys. I have another question in the mean time... whats causing the 0.5V on the 6AU6s grid?

[lowell]

"whats causing the 0.5V on the 6AU6s grid?"

that's what we're trying to figure out here. Does the bottom of R1 have 0v? This is ground, but lets make sure it is ZERO volts.

Also, R1 measuring 2meg across it?

What resistance do you get from 6au6 grid to ground with R1 all the way up, and then with R1 all the way down?

[disorder]

So I was reading a dc voltage at the input grid of the 6au6 due to the volume pot. Once I bypassed the pot and put a 1MEG to ground at input grid, the voltage went away. I also was able to source the issue for the shitty sound... the speaker. So now that this thing is up and running I want to make it quieter (there is HUM). Before I go ahead and just shotgun the old cap can in there, I'd like to understand this power supply. So the first cap (reservoir cap) is probably 40uF as it seems this is the max value for a 6x4 rect tube. Usually the next few caps for the following stages are the same capacitance or even larger than the first. In this amp they decrease with each stage. Is there a reason for this? Or did the designer just use what was available? Could I just three 40uF across the board and end up with a more filtered supply?

[Steve Conner]

Yes you could. I think back then designers were limited by the large size and high cost of electrolytic caps. They've got much smaller and cheaper.

[disorder]

Thanks steve. What gets me about this design is how the power tubes plate and screen grid share the same supply only separated by the transformer primary.

[Steve Conner]

Well, there's nothing stopping you from adding an extra resistor and capacitor to filter the screen supply. That would certainly get rid of some more hum.

[bob p]

I don't know jack about ipods, but I doubt that they're good voltage sources. It wouldn't surprise me if they misbehave when the load Z changes wildly.

[lowell]

I would try adding a bypass cap to V2 cathode and run the filament center tap to the top of that cap instead of ground. I've found that raising the voltage potential to the filament center tap in SOME amps, can really reduce the filament hum quite a bit.

[disorder]

Well, there's nothing stopping you from adding an extra resistor and capacitor to filter the screen supply. That would certainly get rid of some more hum.

This would also change the behaviour of the tube itself right? It currently uses the same B+ line for the anode and the screen grid of the power tube. Adding another cap and resistor to better filter the screen grid is how I've seen most pentodes run... the Fender champ for example. This amp baffles me with the screen grid connected to B+ and the lower primary and the plate connected to the top of primary.