Steel String Singer #005 build

[captaincoconut]

Okay guys, here's where I am at the moment. I have pretty much wired up a large portion of the chassis with the preamp board being the only missing part:



Here's the EQ section with a close up shot on the Jazz/Rock, Deep and Bright toggle switches:




For the FET Boost wiring I followed Martin's proposal posted here. Using the footswitch you can simply switch between FET and normal when plugged into the FET-input. Steel String Singer #005 has the benefit of a front panel mounted FET level potentiometer. Pretty neat!



The preamp board is waiting for its turn, but I will do another thorough inspection of the wiring before continuing on first.



I'm just missing out on a few capacitors, so another parts order is due. I will also have to find a solution for the footswitch, but already have a plan.

[martin manning]

Looking great! I would be testing this in sections as you go to make sure that all that careful work won't need to be torn up if something is miss-wired.

[captaincoconut]

Looking great! I would be testing this in sections as you go to make sure that all that careful work won't need to be torn up if something is miss-wired.

Thanks! Right now, without the preamp board installed everything is easily reachable. In fact, I had the amp already powered up at different points to check if there are any shorts and if voltages look right (up that point in the building process). So I'm positive but will check wiring a few times anyway. Just have to find the time...

[martin manning]

Sounds like you are on it. That’s exactly what I was referring to. At this point you could check all power supplies and relay operation, and bring the power amp up and test it.

[beasleybodyshop]

Looking good!

[captaincoconut]

I'm a little confused with the instructions to set the PI balance in @beasleybodyshop's schematic.

Three questions:
1. Power amp input = the point right before the .02uF cap right after the master volume output?
2. I don't really get point 3 (see picture). Where am I supposed to set the output level?
3. Is this the most effective way to set PI balance? I have a scope on hand, so...

[martin manning]

Three questions:
1. Power amp input = the point right before the .02uF cap right after the master volume output?

Top of the MV pot, or if you have an FX loop, the return/power amp in is an option.

2. I don't really get point 3 (see picture). Where am I supposed to set the output level?

You must have a dummy load connected to the output, so measure RMS voltage there and adjust to 50% power calculated as I^2 R

3. Is this the most effective way to set PI balance? I have a scope on hand, so...

Hook one channel to each cathode (at the top of the 1R current sense resistors and match the voltage peaks.

More discussion here:
https://ampgarage.com/forum/viewtopic.p ... 89#p416389
And lower down, here:
https://ampgarage.com/forum/viewtopic.p ... 19#p467019

[captaincoconut]

You must have a dummy load connected to the output, so measure RMS voltage there and adjust to 50% power calculated as I^2 R

When determining the actual RMS power output of one of my builds, I would inject a sine wave into the preamp, hook a scope probe across a dummy load and crank volume until the wave starts clipping, then P = U^2/R.

But here's what I don't get: How am I supposed to adjust the power stage output to 50% when I inject the signal into the power amp through the FX return? After the master pot there is no other control that would allow me to dump the input signal down. Am I supposed to lower the output of the signal generator itself until I only see 50% output or am I missing something?

Hook one channel to each cathode (at the top of the 1R current sense resistors and match the voltage peaks.

Might be naive, but why wouldn't I just inject a signal into the phase inverter, hook one channel to each output of the phase inverter/input grid of the power tube and adjust the trimpot until both sides are 180° out of phase?

[martin manning]

How am I supposed to adjust the power stage output to 50% when I inject the signal into the power amp through the FX return? After the master pot there is no other control that would allow me to dump the input signal down. Am I supposed to lower the output of the signal generator itself until I only see 50% output or am I missing something?

Yes, or use the MV to dial it in. You will need something like 2.5V rms. The reason for injecting the signal at the power amp input is that you want an undistorted sine input, which may not be the case if you go through the preamp.

...why wouldn't I just inject a signal into the phase inverter, hook one channel to each output of the phase inverter/input grid of the power tube and adjust the trimpot until both sides are 180° out of phase?

If you balance the PI output (you would match the amplitudes of the two output signals) you are not allowing for the possible unbalance of the power section (power tube gm and output transformer).

Is this the way HAD did it? Who knows, but it is typical Hi-Fi practice, and equivalent to the procedure described by Ampeg for their SVT.

[captaincoconut]

Yes, or use the MV to dial it in. You will need something like 2.5V rms. The reason for injecting the signal at the power amp input is that you want an undistorted sine input, which may not be the case if you go through the preamp.

Ok, just to make sure I got that right:
1. I connect a dummy load to the speaker output.
2. I connect both multimeter probes to the power tubes' cathodes (one probe for each tube pair). Do I actually have to connect the cathodes in each pair or is the 1 ohm resistance small enough that it doesn't matter?
2. I inject a signal into the phase inverter input strong enough to yield half of the amp's expected rms output. With 4 x 6L6 and a 100W Twin Reverb iron this should be about 50W, so with an 8 ohm load I would expect to see around 20V.
3. I turn the phase inverter trimpot until I see 0V on the multimeter.

Correct?

If you balance the PI output (you would match the amplitudes of the two output signals) you are not allowing for the possible unbalance of the power section (power tube gm and output transformer).

Is this the way HAD did it? Who knows, but it is typical Hi-Fi practice, and equivalent to the procedure described by Ampeg for their SVT.

Got it, thanks!

[martin manning]

Ok, just to make sure I got that right:
1. I connect a dummy load to the speaker output.
2. I connect both multimeter probes to the power tubes' cathodes (one probe for each tube pair). Do I actually have to connect the cathodes in each pair or is the 1 ohm resistance small enough that it doesn't matter?

Yes for a 4-tube output connect the cathodes on each side together so you are seeing the total current on each side. Remember the DMM is measuring DC voltage in mV.

2. I inject a signal into the phase inverter input strong enough to yield half of the amp's expected rms output. With 4 x 6L6 and a 100W Twin Reverb iron this should be about 50W, so with an 8 ohm load I would expect to see around 20V.

Yes, and I should have said V^2/R, so you are correct (50*8)^0.5=20V

3. I turn the phase inverter trimpot until I see 0V on the multimeter.

Correct. Expect to see +/- a few mV range.