Getting Started on SSS 002

[Vertigo]

I put in all the tubes now and I'm still not getting a voltage on the test points connected to the cathodes. If I probe the points on the driver board that are connected to pin 1, I was able to get a reading of ~-60V and dialed the bias pot to the largest negative voltage, which landed around -70.

Edit: Voltages look much more normal now though
Power Board
B+ STB1 - 443
SCRN - 440
B+ RVB - 390 (this one is a bit hot, but the tank isn't connected)
B+ CAN - 410

Preamp board B+
345, 345, 341

RVB Board B+ - 385

[martin manning]

That -70 sounds reasonable, but would put the power tubes in cut off, so no current indicated at the cathodes.

[Vertigo]

I'm finally getting a bias reading. The lowest reading I can get on the cathode with the pot reading 1R is 44mV. The lowest I can get on the control grid is ~ -40V. I assume that I'll need to tweak the 10K resistor on the bias board. If I swap the 10K resistor for something closer to 5K, am I in the ballpark?

It sounds OK but is completely intolerant of my sloppy playing. I've never heard my position changes quite this clearly. SQUEAK!!

I have some reverb troubleshooting to do. There's not much of it, and I'm getting some 60 cycle hum when it's on. When the MID switch is on (near the master) the reverb rings in a bad way. I swapped the RCA cables on the terminals and get no 'verb at all

Everything else seems to work just fine .

edit:

If I'm looking at this correctly, the bias board has two parallel resistors...the 10K and the 20K pot. So the total max resistance is 1/10K + 1/20K = 6.6K. Right now, I have the pot fully open, so the parallel circuit has a resistance of roughly 1R. So if my pot is already letting the full amount of current through, going to a lower fixed resister on the board does practically nothing since it's already wide open. Do I need to look at the 220K resistors on the driver board then?

[martin manning]

If I'm looking at this correctly, the bias board has two parallel resistors...the 10K and the 20K pot. So the total max resistance is 1/10K + 1/20K = 6.6K. Right now, I have the pot fully open, so the parallel circuit has a resistance of roughly 1R. So if my pot is already letting the full amount of current through, going to a lower fixed resister on the board does practically nothing since it's already wide open. Do I need to look at the 220K resistors on the driver board then?

No, leave the 220k's, they are the cathode load resistors. To get a more negative bias voltage you need to increase the resistance from the driver tube grid leaks (the 1M's) to ground. Do you have a schematic of the driver and bias section?

[Vertigo]

If I'm looking at this correctly, the bias board has two parallel resistors...the 10K and the 20K pot. So the total max resistance is 1/10K + 1/20K = 6.6K. Right now, I have the pot fully open, so the parallel circuit has a resistance of roughly 1R. So if my pot is already letting the full amount of current through, going to a lower fixed resister on the board does practically nothing since it's already wide open. Do I need to look at the 220K resistors on the driver board then?

No, leave the 220k's, they are the cathode load resistors. To get a more negative bias voltage you need to increase the resistance from the driver tube grid leaks (the 1M's) to ground. Do you have a schematic of the driver and bias section?

I have the MHartman schematic

These two? How much more resistance would you suggest?

Grid Leak.png

[Vertigo]

Either those two aren't the ones, or I didn't increase the value by enough. I put some 1.2M in there and nothing changed. The amp seemed a little bit louder but the cathode voltage was shifting between 44 and 46mV

[Vertigo]

That looks correct. Grounding the relay with the pull switch energizes it, and it routes the signal through the FET. The foot switch does not function, but its FET LED will stay on (and I'd call that a feature, not a bug). When the FET relay is not grounded by the pull switch, the foot switch controls it. I'd draw it like this:

Thanks for the help with this Martin. It worked perfectly.

[Richard1001]

If I'm looking at this correctly, the bias board has two parallel resistors...the 10K and the 20K pot. So the total max resistance is 1/10K + 1/20K = 6.6K. Right now, I have the pot fully open, so the parallel circuit has a resistance of roughly 1R. So if my pot is already letting the full amount of current through, going to a lower fixed resister on the board does practically nothing since it's already wide open. Do I need to look at the 220K resistors on the driver board then?

No, leave the 220k's, they are the cathode load resistors. To get a more negative bias voltage you need to increase the resistance from the driver tube grid leaks (the 1M's) to ground. Do you have a schematic of the driver and bias section?

I have the MHartman schematic

These two? How much more resistance would you suggest?

Grid Leak.png

No, Martin is talking about the voltage devider 130K/820K that sets the grid voltage.

I would suggest keeping the stock schematic and solder a 3K3 resistor on the bias rectifier board (getting rid of the biaspot)
There is no need to use your amp as a heater unless you want to wear it out faster

[martin manning]

Sean, I'm not sure what your bias circuit looks like now. Ryan's eyelet board layout has a 20k 2W pot across a 10k resistor on the bias board like you describe above, but you have a PCB with perhaps some other adjustable bias network (?). An alternate arrangement was discussed here: https://ampgarage.com/forum/viewtopic.p ... 50#p417450

You could do something like that with the 20k pot you have in combination with a 120k resistor, giving you a range of 120 to 140k for that leg of the voltage divider (which is nominally 130k to ground). If you still have the 20k 2W pot paralleled with a 10k on your bias board, you would replace the 10k on the board with something like a 3k3 to get the range centered around where you need it, and move the pot connections to the driver board. You need to understand what circuit is on your driver PCB to figure that out, though.

[Vertigo]

After sleeping on it, I hope I understand it a little better now.

Martin, I have the PCBs from Ryan/Erwin

It looks like I should remove the bias pot from the bias/rectifier board and swap the 10K 3W resistor to a 3k3 3W.

From there, I need to tweak the voltage divider on the driver board. Normally, there would be a 820K/130K divider there, but Jelle subbed them for 1M/118K. I'm going to put them back to their original values and see how the bias is from there. If that doesn't get it to where it needs to be, I can essentially put a pot in series with a resistor, lowered from 130K down to ~118K to trim it, but I'd most likely have to put a couple of eyelets in the board or use some structural goop to keep the connections solid.

Thanks for the assistance everyone

[martin manning]

I agree with swapping the 10k on the bias board to 3k3, but I would be inclined to use the 20k pot you have mounted to the chassis to make an adjustable divider since it is accessible from the outside. Is there a drawing of the PI/driver board that shows the components and traces somewhere? It should be easy enough to make that work without any drilling or cutting traces.

[Vertigo]

I agree with swapping the 10k on the bias board to 3k3, but I would be inclined to use the 20k pot you have mounted to the chassis to make an adjustable divider since it is accessible from the outside. Is there a drawing of the PI/driver board that shows the components and traces somewhere? It should be easy enough to make that work without any drilling or cutting traces.

Thank you Martin.

This is what my board looks like. It's Ryan's layout that I modified to reflect the actual boards I received. The notes in red on the resistors is what Jelle sent. The yellow wire on the top left is what's coming in from the bias board.

Driver board.png

Bias Board.png

[erwin_ve]

I agree with swapping the 10k on the bias board to 3k3, but I would be inclined to use the 20k pot you have mounted to the chassis to make an adjustable divider since it is accessible from the outside. Is there a drawing of the PI/driver board that shows the components and traces somewhere? It should be easy enough to make that work without any drilling or cutting traces.

I'll do that asap.

[Vertigo]

looks like it wouldn't be too much trouble to pull one end of the 118K, solder the lead from the pot input there, and then connect the lead from the wiper directly to the 118K resistor with a bit of shrink tube over top of it.

[martin manning]

Yes, that will work, but it'd be best to have everything securely tied down. You could move the resistor, soldering it to the pot and to a terminal strip, then run lead wires over to the board where the resistor was. Adding an eyelet to the board would also work.