Valco 8418 - Champ 5C1 Hybrid

[bobloblaws]

So I picked up a 50's Valco 8418 awhile back and I'm just getting around to tweaking it up. It really was an 8418 in name only as it had been someone else's project and they seemed to use whatever parts they had lying around, and I've gone and turned it into something resembling a cross between the original 8418 circuit and a Fender Champ 5C1. It has a SS rectifier and newish 40/40/20/20 can cap courtesy of the previous owner and I bought a Hammond 1760C output tranny for it. It is sounding OK but is only putting out just over 2 watts which seems rather low. I was hoping you guys might have some tips for potential improvements. Resources I'm uploading are 8418 and 5C1 schematics as well as my own with DC voltages and the specs for the OT and power tube. Based on the 6V6 datasheet I wasn't sure which OT primary tap to use since my plate voltage is somewhere in between the 250V and 315V examples listed in the datasheet corresponding to the 5000 ohm and 8000 ohm taps, respectively. My plate voltage at 292V is closer to 315V than it is to 250V and the amp does seem to have more output using the 8000 ohm tap. Does using a larger tap always result in more power/volume? Is there a risk of undue stress on the power tube if the wrong tap is used? Anyway, any feedback on the circuit as a whole is much appreciated, thanks.


- Bob


Valco Harmony 8418.pdf1760C.pdfchamp_5c1_schem.pdf6V6 datasheet.pdf

[Dave H]

I'm guessing it's way off centre bias on the 5k tap with its 470R cathode resistor and 18k screen feed resistor.

I have a similar circuit which uses a 300R cathode resistor, 4k7 screen feed resistor, 5k plate load and it puts out about 4.5W.

You may also be able to get more than 2W by increasing the screen voltage and using the 8k tap and existing 470R cathode resistor Try changing the 18k screen resistor to 4k7.

[bobloblaws]

I'm guessing it's way off centre bias on the 5k tap with its 470R cathode resistor and 18k screen feed resistor.

I have a similar circuit which uses a 300R cathode resistor, 4k7 screen feed resistor, 5k plate load and it puts out about 4.5W.

You may also be able to get more than 2W by increasing the screen voltage and using the 8k tap and existing 470R cathode resistor Try changing the 18k screen resistor to 4k7.

I'm still pretty green regarding class A operation (among other things). My understanding is that for class A it is biased so that the tube is conducting (ideally) 50% of maximum current at idle (which I assume allows for max power and efficiency and linearity due to the signal having the full 50% available on both the positive and negative swings). When you refer to "centre bias" do you mean just that, how close the idle current is to 50% of max?

In the meantime I will try your suggestions regarding current resistor values and adding a plate load on the 6V6.

Edit: I think I initially misunderstood what you said about a 5K plate load, I thought you meant an additional 5K resistor in series with the plate but I don't see anything similar in any SE schematics so I assume you were referring to the OT primary.

[Dave H]

Centre bias is when the bias point is at the centre of the load line and peak current is twice bias current.

Yes, I meant the OT primary impedance.

[g1]

Single ended class A is most commonly run at around 100% plate dissipation, not 50%.

[Dave H]

Single ended class A is most commonly run at around 100% plate dissipation, not 50%.

Yes, that's true but we were discussing current not power

[g1]

Yes, that's true but we were discussing current not power

Just wanted to make sure. It seemed a bit vague. But I should have left out the 'not 50%' part.
Have never biased for 50% max. current and am not sure if I could.

[Dave H]

Have never biased for 50% max. current and am not sure if I could.

Same here. Reading it back it does look like Bob was saying to bias at half peak current. I meant centre of the load line which is also centre of the current and voltage swings.

I'm guessing this amp will have a max peak voltage swing of about 250V at the plate and the OT is 5k so I'd bias the 6V6 at about 12.5W (250^2 /5000) for centre bias which is 45mA plate current. Hope I got that right. I've just worked it out on a scrap of paper

[Tony Bones]

Same here. Reading it back it does look like Bob was saying to bias at half peak current. I meant centre of the load line which is also centre of the current and voltage swings.

I'm guessing this amp will have a max peak voltage swing of about 250V at the plate and the OT is 5k so I'd bias the 6V6 at about 12.5W (250^2 /5000) for centre bias which is 45mA plate current. Hope I got that right. I've just worked it out on a scrap of paper

The only problem is that to get the 12.5V grid bias requires a 270R cathode resistor instead of the 470R shown.

[Dave H]

The only problem is that to get the 12.5V grid bias requires a 270R cathode resistor instead of the 470R shown.

Thanks, I thought of that in #2 but forgot to mention it in #8

Here's a plot of my Valve Junior (with 6V6) at 4.5W, 285R cathode resistor.

[bobloblaws]

Same here. Reading it back it does look like Bob was saying to bias at half peak current. I meant centre of the load line which is also centre of the current and voltage swings.

I'm guessing this amp will have a max peak voltage swing of about 250V at the plate and the OT is 5k so I'd bias the 6V6 at about 12.5W (250^2 /5000) for centre bias which is 45mA plate current. Hope I got that right. I've just worked it out on a scrap of paper

Fascinating stuff for this greenhorn. I'm sort of keeping up although I don't know the first thing about load lines.

So to be clear, when i say "how close the idle current is to 50% of max" and you say " peak current is twice bias current" we are essentially saying the same thing ,no?

Also, how did you derive the 250V max peak voltage swing estimate?

When I have time this weekend I'll get into measuring the plate current and calculating the dissipation etc. In the meantime I did change the screen feed resistor to 4K3 and cathode resistor to 300R and I'm getting close to 3.5W output, so things are looking up.

[Dave H]

So to be clear, when i say "how close the idle current is to 50% of max" and you say " peak current is twice bias current" we are essentially saying the same thing ,no?

Also, how did you derive the 250V max peak voltage swing estimate?

Yes, it's the same. I probably should have used max plate current (90mA) to avoid confusion with peak signal current (45mA)

I estimated the peak signal voltage swing by subtracting the cathode voltage (12.5V) and tube saturation voltage (30V) from the 292 plate voltage on your schematic.

What level of distortion did it have at 3.5W? I think max power output for SE amps is measured at 10% THD which looks like the plot I posted.

[bobloblaws]

What level of distortion did it have at 3.5W? I think max power output for SE amps is measured at 10% THD which looks like the plot I posted.

For the record my input test signal is 105mVAC.

3.5W was with no clipping (NoClip.jpg). SoftClip.jpg looks something like the plot you uploaded in #10 in which case I'm getting 4W. HardClip.jpg shows the volume knob cranked for 4.8W. I can get it higher yet by increasing the amount of input but of course the waveform gets pretty ugly.

I'm also uploading updated schematic with voltgages.