#124 Build Questions

[martin manning]

Do you have a lightbulb current limiter? A Variac does not limit current, and the bulb brightness behavior gives you some indication of current draw vs. time.

[lgehrig4]

Do you have a lightbulb current limiter? A Variac does not limit current, and the bulb brightness behavior gives you some indication of current draw vs. time.

No, but if you recommend using one I can make one. The hard part will be finding a high wattage (or any wattage) incandescent bulb.
edit: I'll probably have to overpay and purchase a reptile heat bulb

Could you please confrm if I wired the IEC/Fuse/Switch correctly? I already made one error and now I'm paranoid.

IEC
Common to fuse tip
Hot to power switch

PT/Relay transformer
One primary of each to other side of switch
One primary of each to fuse side lug

[GAStan]

Could you please confrm if I wired the IEC/Fuse/Switch correctly? I already made one error and now I'm paranoid.

Page 10 and page 40

Guitar Amp Wiring Notes 1-1 (1).pdf

Actually the entire document is a good read.

[martin manning]

Multiple 40W oven bulbs is one way around it. There is a pdf on limiters in the Tech section, sticky post. You could just start at very low voltage, say 12 VAC using your Variac, and look for DC voltages that are ~1/10 of normal. Start with no tubes installed and check for heater AC, and DC on the relay supply (regulator input), all of the B+ nodes, and bias.

The safest way to wire the primary is AC hot to fuse tip, fuse side terminal to main power switch, and PT primary lead to the other side of the main power switch. The other PT primary lead goes to AC neutral.

[lgehrig4]

Could you please confrm if I wired the IEC/Fuse/Switch correctly? I already made one error and now I'm paranoid.

Page 10 and page 40


Guitar Amp Wiring Notes 1-1 (1).pdf

Actually the entire document is a good read.

Thank you!

[lgehrig4]

Multiple 40W oven bulbs is one way around it. There is a pdf on limiters in the Tech section, sticky post. You could just start at very low voltage, say 12 VAC using your Variac, and look for DC voltages that are ~1/10 of normal. Start with no tubes installed and check for heater AC, and DC on the relay supply (regulator input), all of the B+ nodes, and bias.

The safest way to wire the primary is AC hot to fuse tip, fuse side terminal to main power switch, and PT primary lead to the other side of the main power switch. The other PT primary lead goes to AC neutral.

Great! This should get me on my way. Thank you!

[martin manning]

Interested to hear how it goes.

Here's the rest of a quick start-up guide:

• Start with no tubes installed and at very low voltage, say 12 VAC using your Variac, switch power on and look for DC voltages that are ~1/10 of normal.

• Check for heater AC, and DC on the relay supply (regulator input), all of the B+ nodes, the positive end of the electrolytic on the FET board, and bias.

• If the low voltage measurements look good, bring the input voltage up to 60 VAC and check the voltages listed above again, this time look for something about half of normal.

• If that result looks good, you have AC and DC voltage where you should, and you probably don't have any serious shorts.

• Make sure that you have negative bias voltage on the power tube sockets (pin 5's), and set the bias trimmer for the lowest (most negative) voltage there.

• Turn power off and dial the Variac down to zero volts.

• Install the tubes, connect a load to the speaker output, switch power on, and slowly bring the input voltage up to 120 VAC. Look, listen, and sniff as you go.

• Check voltages again. B+ voltages will still be high, but you should now see the supply node voltages dropping as you go down the line.

• Next, see if you can set the output tube bias. There is a new doc in the FAQ section on that... you can be the first test subject ;^)

You should get some noise in the speaker as the bias is dialed in, and then you can check the function of all the switches and controls.


PS You might want to edit the title in the first post in this thread to something like "#124 Build Questions" so it can be found by others traveling this path.

[lgehrig4]

This is perfect!

I rewired the AC section so now I'm ready for testing. Heading out to Home Depot in a bit to pick up parts for the bulb limit and just realized that I don't have a quad of 6L6's Thought I did


...and I changed the title to #124 Build Questions

Thanks again!

[lgehrig4]

I completed the first 5 steps. The only thing I was unsure of was the bias on the FET board so I just measured the resistance on the trimmer, which was 3.85K. Larger voltages over 100 were rounded.

Measurement | @12V | @60V

Heater AC - .68 | 3.51

Main Power Supply
B+1 - 43 | 223
B+2 - 43 | 223
B+3 - 42 | 219
B+4 - 37 | 193
B+5 - 33 | 169

Rectifier Bias Supply
AC in - 37.9 | 166
Bias (red/blue wire) - 5.44 | 28

ODS Relay Supply
AC in - 1.375 | 7.04
Reg Input - .915 | 7.65

FET Board
100u cap positive - 28.7mV | 28.3mV


There was negative voltage on pins 5 of the power tubes. The lowest I was able to get this to was -17.55V (with variac set to 60V)

So the only two questions I have at this point is:

1. How do you check the FET bias?
2. What reading should be on the Relay Supply 'Reg Output'? I got nothing on this but you didn't mention this in your steps so I guess that is why. At least I hope that is why.

[martin manning]

The heater and B+ voltages look good, except for the FET board electrolytic, where you should have seen 4-5V with the Variac at 60V.
Check to see if the jumper from B+5 on the preamp board going to the FET board is connected.

FET biasing is covered in this .pdf: https://ampgarage.com/forum/viewtopic.p ... 91#p211891

I think you would need to see around -25 VDC on power tube pin 5's with 60VAC input. It's possible you will need to change a resistor in the bias supply to get that down lower.

The regulator output voltage won't come on line until you get enough input voltage, so don't worry about that yet.

I would try running the input voltage up to 120VAC and see what you get, Nothing bad happened, so it's probably ok.

[lgehrig4]

The heater and B+ voltages look good, except for the FET board electrolytic, where you should have seen 4-5V with the Variac at 60V.
Check to see if the jumper from B+5 on the preamp board going to the FET board is connected.

FET biasing is covered in this .pdf: https://ampgarage.com/forum/viewtopic.p ... 91#p211891

I think you would need to see around -25 VDC on power tube pin 5's with 60VAC input. It's possible you will need to change a resistor in the bias supply to get that down lower.

The regulator output voltage won't come on line until you get enough input voltage, so don't worry about that yet.

I would try running the input voltage up to 120VAC and see what you get, Nothing bad happened, so it's probably ok.

I think I misunderstood what is considered down with negative voltage. I turned the bias pot all the way in the other direction and the reading is now -24.1V

The B+5 is connected to the FET board. The reading at the connection is 169V and after the 150K R its down to about 8.8V and then at the electrolytic + node its 26mV

At 120V, the reading at the connection is 330V and after the 150K R its 27V. Still no change in the electrolytic reading

[martin manning]

Ok that sounds better for the bias voltage. The voltage after the 150k on the FET board is what I was looking for, and that’s where the 100u electrolytic positive lead should be connected. If you can verify that I think you can move on.

[lgehrig4]

Ok that sounds better for the bias voltage. The voltage after the 150k on the FET board is what I was looking for, and that’s where the 100u electrolytic positive lead should be connected. If you can verify that I think you can move on.

I'm happy to hear this!

Yes, the 100u cap us connected to this resistor via short jumper in the back. Why isn't the reading the same then if they are both connected? Resistance between these two points is 1.7MOhm, which I don't understand. Cold solder joint or just electrical theory that I am not aware of yet?

Screenshot 2025-04-05 at 9.03.00 PM.png

[martin manning]

Strange. Look on the underside, and reflow the solder joints? Don't worry about FET bias yet. That can be sorted out after you get tubes in and have the power tubes are biased.

[lgehrig4]

I got that sorted out. That short jumper broke off the electrolytic on the back. Not sure how that happened.

I remembered that I had a fresh quad of JJ 6L6GC in one of my Mesa's so I popped those in.

Initial Vak was 425, then dropped down to to 410 after inital bias.

1a = (%PaMax * PaMax) / Vak
1a = .60 * 30) / 410
1A = 44mV

Earlier in the thread we discussed test jacks. All P tube pin 8's have a 1Ohm restistor to ground, and each test jack is connected to a pair of of pin 8's. The first one connects to V4/V5 and the 2nd to V6/V7. Does this mean that the 44mV I'm reading is technically 22mV per tube and that I need to bias up to 88mV? The tubes look fine. No smells but there is good heat so visually it looks like it should be where it is. Or at least close to it.

B+ voltages after installing tubes and biasing
B+1 | 411
B+2 | 411
B+3 | 380
B+4 | 281
B+5 | 217


I couldn't resist plugging a guitar in and upon initially hearing this I think something is wrong.
1. First the volume is not there and the amp sounds thin
2. Volume ans Master seem to do very little until you get to to about 6 and even at tht point there isn't much volume or body.