SKYLINER build - inspired by ODS #124 and #060

[Stephen1966]

While I still remember this and we are talking about switches, I want to point something out that I don't think anyone has mentioned before.

Here's a photo of the #124 front panel from Rob Livesey's collection.

ODS_124099.jpg

It's been covered elsewhere that by placing the Rock/Jazz switch in a central position (neither Rock nor Jazz) you get the PAB. This switch, to me, looks to be latched in a central position, making it a DP3T (on-off-on) switch. The 1988 layout and 1.6 schematic don't appear to include this though it's been used in many amps subsequently and is well understood. I almost didn't use a DP3T switch since though it will give you the PAB it wouldn't be indicated by the LED. Couldn't resist it though. What can I say, I love switches

[Stephen1966]

Moving on to the 'Hardware chassis back and bottom' layer in the layout this was me, trying to figure out how I was going to bolt in the OT after fitting the boards which, by necessity, cover the heads of the fixing bolts for each other!

SAM_7550.JPG

Basically, it's impossible to fit the OT after the boards or vice versa. Yes there's room to get the spanner under the boards once they're in place but I couldn't get the nuts to catch the thread on the OT screws. The solution meant dispensing with my nice shiny spacers here and bolting the board screws tightly in place before mounting that heft of iron called the OT. There are more elegant spacer solutions out there but a fixed bolt with a judicious helping of washers performs the same task.

SAM_7555.JPG

Now, you would never have noticed it, if I hadn't pointed it out. Follow the layout as described you would discover it soon enough though The lesson here: use threaded spacers that can be fixed in position for the boards before the OT gets installed. Use threaded spacers on all the main boards. And only the sides need to be countersunk (clearance for the head case).

[Stephen1966]

When fixing the can cap, it needs to be insulated both electrically (because the case is the GND) and mechanically (because of vibration).

You don't need to go mad with the gunk.

ODS_124020.jpg

SAM_7559.JPG

[Stephen1966]

At around this point in the build then, the bottom should look something like this:

SAM_7556.JPG

If, like me, you don't find much about the placement of the reverb transformer at that odd angle, then here is my thinking... the magnetic field is off-axis from the most sensitive parts of the preamp and placing it between the tube sockets might have been even better. I don't expect it will give us much trouble here though.

[Stephen1966]

That's it from me for today then. Tune in tomorrow for another exciting installment of "How Stephen Burned His House to the Ground."



Real heroes

[bepone]

That's it from me for today then. Tune in tomorrow for another exciting installment of "How Stephen Burned His House to the Ground."

it is a standard procedure

[Stephen1966]

Okay, back to it...

I've been following the discussion about the lead dress 'Lead Dress: OD V2a Grid wire' https://ampgarage.com/forum/viewtopic.php?f=4&t=34462 and in this photo you will see I have my plate (blue) and cathode (yelllow) wires figured out, but the OD1 grid (green) wire is pressed down into the base next to the cathode wire. I call this triode, V2b in my schematic because the signal enters the OD circuit at pin 7; OD2 the second stage triode is what I call V2a. This naming convention is probably wrong but the circuit is indentical with #124 here. So, too is the lead dress... except for that darned grid wire.

SAM_7578.JPG

In this picture of the #124 OD1 grid wire, you can see it flies in from above at practically a right angle with the board.

ODS_124064.jpg

So there was nothing else for it but to fix that...

SAM_7718.JPG

And erring on the safe side I repositioned the grid wires for the reverb (V5) and phase inverter (V6) as well

SAM_7721.JPG

[Stephen1966]

While looking through these old photos, I also noticed that the signal wires leading from the CL preamp to the OD relay and then into the top of the trimmer for the first OD triode are also flown over the edge of the board into the relay.

ODS_124081.jpg

At the bottom of the image, the blue wire is the CL preamp out, the green wire is the OD preamp in. My relay is further off to the left but I thought it wise to reflect this

SAM_7710.JPG

The blue wire sits under the board quite separately and emerges through the shielded cables to the second eyelet from the right in that line of eyelets. The wire protruding out is a test point. Clipping this to the master pot (wiper) [correction: pin 3, pin 2 goes out to the FX loop] bypasses the reverb circuit. Testing the amp without reverb and setting the main amp up will provide a baseline for when the reverb is in circuit. When the testing is done and it all works as expected, I only need to trim that wire back.

[Stephen1966]

Perhaps someone could explain this to me?

Here is a picture of the current limiting resistor installed prior to forming the caps.

SAM_7708.JPG

The red wire leads from the rectifier and the other leg of the resistor leads to the first bank of reservoir caps.

I'm following bepone's procedure somewhat and there have been no fireworks, nothing to report, it all seems pretty uneventful. So far...

However, I got a 140VDC voltage drop with this resistor, so today, I'll be forming the caps again without it, with full power.

The voltage drop didn't occur after the resistor however, but before it, at the tail of the rectifier.

First thoughts: bad transformer but then, because the voltage range was around 320VDC coming off the rectifier and one side of the secondary is about the same, I thought I might have a bad diode in the bridge. I tested the diodes. Nothing wrong there. So now, I've pulled out the resistor and voila! I get 460VDC with no tubes. This is some kind of Kirchoff balancing act I'm sure, a consequence of the current the resistor was drawing (but didn't measure, so don't ask). The installation of the 100k resistor, is something I would do again since it offers a measure of protection from inrush current that, at the moment of powering it up for the first time adds peace of mind if nothing else. But now, I should do the procedure again to bring the caps up to the operational voltage. I also measured the output of the secondary HV lines: ~326VAC each which told me that the transformer was okay but after the bridge at the test point, it's DC. Is it possible the voltage drop occurred because of impedance in the DC line??? I understand the ability of impedance to affect the voltage and current flow upstream, but if this occurs with DC as well, appparently, not very well!

[bepone]

all normal , you have bleeder resistor to the ground from the cap Ub+ to the GND, forming divider..

[Richard1001]

If you tie a high value resistor between the output of the rectifier and 1st capacitor and measure at the rectifier, the DC voltage will not be smoothed at this point by a reservoir cap.
So your DC voltmeter will measure the effective voltage of the non smoothed DC voltage.

[bepone]

now is time to check if the bias is ok, around --45-50V , if is like that you can start the amp,

THE BEST NOW IS TO REMOVE PI TUBE to check only 6L6 power section! (if youll leavePI inside , and NFB connected, this can lead to oscillation if anode wires are swapped)
so or remove PI, or disconnect NFB

im doing like power on, standby off, variac on full, then after heating of 1 min, i put variac to 0, fast engage standby to ON, raise variac to half of primary voltage, current will start to raise slowly, check the current through the cathode of output tube , if is minor, slowly progressing to the full voltage. at full monitoring current through the cathodes of output, if is around 40mA, output section is ok!

next step is....put PI.. and..

[Stephen1966]

@Richard1001 @bepone

I get it. I was trying to imagine what the DC would look like before any smoothing but the resistor as a divider makes sense when you look at it that way too. Anyway, onward and upward.

Today, I'm just going to form the caps without any tubes in. I'm giving it 50V increments in 1 hour intervals and it's on the first 50V right now. Then I'll power down, discharge the caps and start it up again to 100V. And so on until I hit 240V with the variac. I understand Ken Fisher used to form his caps over a day so this incremental warming of the electrolytic over several hours seems about right for my set up.

[Stephen1966]

now is time to check if the bias is ok, around --45-50V , if is like that you can start the amp,

THE BEST NOW IS TO REMOVE PI TUBE to check only 6L6 power section! (if youll leavePI inside , and NFB connected, this can lead to oscillation if anode wires are swapped)
so or remove PI, or disconnect NFB

im doing like power on, standby off, variac on full, then after heating of 1 min, i put variac to 0, fast engage standby to ON, raise variac to half of primary voltage, current will start to raise slowly, check the current through the cathode of output tube , if is minor, slowly progressing to the full voltage. at full monitoring current through the cathodes of output, if is around 40mA, output section is ok!

next step is....put PI.. and..

It is perhaps a bit different the way I will need to do it.

After forming, and at full voltage I''ll need to check on the dropping resistors in the power supply. I have my target node voltages but because the PS was only simulated and now we are in the real world I fully expect I will need to swap out some of the resistors for different values. I have my decade box to help with that. It's only really relevant with all the tubes in so it will be back and forth fast on fast off. I've done a preliminary test on the negative voltage at the bias filter and it's just a little over -60VDC which is on the money given there is no load yet and the tfmr tap is rated for 60V.

The start up procedures in circulation all seem to point to installing all the preamp tubes first and taking their voltage reading for confirmation, then it seems to be a good idea to turn the bias pot all the way down to its coolest before then installing the output tubes. Is this the time to remove the PI?

[bepone]

It is perhaps a bit different the way I will need to do it.

After forming, and at full voltage I''ll need to check on the dropping resistors in the power supply. I have my target node voltages but because the PS was only simulated and now we are in the real world I fully expect I will need to swap out some of the resistors for different values. I have my decade box to help with that. It's only really relevant with all the tubes in so it will be back and forth fast on fast off. I've done a preliminary test on the negative voltage at the bias filter and it's just a little over -60VDC which is on the money given there is no load yet and the tfmr tap is rated for 60V.

The start up procedures in circulation all seem to point to installing all the preamp tubes first and taking their voltage reading for confirmation, then it seems to be a good idea to turn the bias pot all the way down to its coolest before then installing the output tubes. Is this the time to remove the PI?

yes! ok for bias, you can put to -50V..
you have only one main concern to clarify now.. assuming that all is ok ofcourse, you need to assume something in the life from time to time you can go forward.

output tubes will oscilate or not ,after applying the 450V on them?? chances are 50-50% . oscillations are on full power , so load need to be connected for this stage!
there will be full powered loud noise on the speaker! if you dont connect the speaker and amp is oscillating you have bad chances to burn the output transformer (arc of few kV inside the layers in OT!) it is loud noise so the best to do this test when all your neighbours go to bed, @cca 23.00h

to reduce chances to 0%, you need to or remove NFB (without feedback oscilations will not happen 99%) OR to remove PI (without phase inverter, output tubes have no any loop closed).

without PI tube you can play with the amp all day, adjusting, adjusting the bias , measuring all voltages, without any trouble.