ODS 124 PCB Build

[martin manning]

Follow-up: I do not want/need a light indicator for this function. Can I just run the resistors directly to ground?

Yes.

[professormudd]

Greetings all.

I wanted to throw this out there and make sure what I am doing will work. I notice that on 124-proper the 120R resistors to ground from the filament are located on V4, on the PCB layout it look not seem to matter. So I came up with this little drawing to combine the 120R resistors to ground and the 330R/Diode to the LED all onto one terminal strip.

Does this look like it will pose any problems for me?

[xtian]

Looks like a valid solution.

[professormudd]

I already have a matched quartet of power tubes (TAD 6L6WGC) on hand, but as for the preamp tubes I have a few options available. Of course I plan to swap out and do my own testing, but I would like others' thoughts on which tubes would be best for each position. I am also interested in any technical justifications you might be willing to offer.

I have the following tube available and would like to start here:

3 Premium selected TAD 12AX7A-C
2 JJ ECC83-S
1 JJ ECC803S

Or are all of these crap?

[xtian]

I'm firmly in the "tubes make very little difference" camp. Here's another detailed tube shootout for 12AX7s:

https://www.youtube.com/watch?v=HEC2Zvz-0G8

[professormudd]

Quick question: the layout has the nfb connected to the 4ohm lead. Should this be connected to the output jack instead?

[pompeiisneaks]

The difference between an impedance selector and a jack are mostly irrelevant here, what is most important is that the NFB resistor is calculated based upon the impedance of the tap it was designed for. In the schematics for the #124 it shows that single lead OT is a 4 ohm impedance OT. This means so long as you keep the NFB resistor the same and connect it to the 4 ohm tap, it will be right. If you connect it to the jack and swap impedances, it will only be right for the 4 ohm tap, and can cause unexpected results.

This is why they used the impedance selector as the connection point to ensure it was always connected to the 4 ohm tap. Even if the tap isn't the 'active' one that portion of the NFB will still come out into the circuit at the right level/ratio.

~Phil

[professormudd]

The difference between an impedance selector and a jack are mostly irrelevant here, what is most important is that the NFB resistor is calculated based upon the impedance of the tap it was designed for. In the schematics for the #124 it shows that single lead OT is a 4 ohm impedance OT. This means so long as you keep the NFB resistor the same and connect it to the 4 ohm tap, it will be right. If you connect it to the jack and swap impedances, it will only be right for the 4 ohm tap, and can cause unexpected results.

This is why they used the impedance selector as the connection point to ensure it was always connected to the 4 ohm tap. Even if the tap isn't the 'active' one that portion of the NFB will still come out into the circuit at the right level/ratio.

~Phil

Thank you.

This right here is why I ask even when I think it's "obvious".

I appreciate the easily digestible explanation.

[professormudd]

Okay... I have completed assembling the innards of this chassis and did a brief check plugged into my light bulb current limiter and without any tubes installed. No smoke! Bulb lit then dimmed! Neat!

I plugged into the surge protector directly (still without tubes) and I checked pin 3 of each octal socket and got 475v on all 4 sockets. Power LED is operable, though kinda dim. I might try a different resistor value.

I also checked the voltage from the smaller PT, I am getting 8.7v there.

I believe I am ready to install tubes and set the bias. This build did not include the installation of tip jacks or bias test points and I am now realizing I do not have any 1% 1R resistors.

[xtian]

Good so far.

Those 1R bias sense resistors are a real time saver. But no worries. You didn't ask, but here's how I'd bias. When loaded and running with all tubes, your B+ at power tubes' plates is going to drop to roughly 450v (you can check this afterwards, and make adjustments if needed). You said you're using 6L6 power tubes. So from this bias calculator, I'd recommend shooting at 60%, which is 40mA per tube. IMPORTANT: We have TWO power tubes on each side, so you need twice the current on each side.

Step 1: With amp off, unplugged, and caps drained, measure resistance from OT primary's CT (usually a red wire) to each side of the winding (usually blue and brown wires, but definitely pin 3 of power tubes). You'll record values of ~50-100 ohms each side. Example: you record 72 ohms for the left side and 80 ohms for the right side. Use Ohm's Law to see what voltage is needed across these resistances to achieve 40mA current. Answer, for 72 ohms is 2.88v, and for 80 ohms is 3.2v. Remember there are two tube on each side, so double these figures, and these will be your target voltages for next step.

NOTE: When you power up in next step, amp may howl or complain loudly because the gNFB is wired in reverse, giving you runaway positive feedback. Power down and swap OT primaries.

Step 2: Clip one lead of your multimeter to the OT primary's CT, and the other to pin 3 on one of the left power tubes. Turn bias pots to coldest settings (most negative voltage possible). Turn amp on and let it warm up, keeping an eye on the power tubes to watch for redplating. It make take 15 minutes or longer for power tubes to stabilize, but there's no need to wait longer than one minute to start this process. What's the voltage difference between CT and pin 3? If less than target value, turn up the bias control on that side. Swap the multimeter lead from pin 3 to the other side's pin 3 and repeat the bias adjustment.

All of this is covered in Paul Ruby's startup guide. Read that too, in case I made any errors.

[professormudd]

...You'll record values of ~50-100 ohms each side....

I recorded 14.5R and 13.5R. Amp off, unplugged, tubes in. This is out of the range you mentioned. Is this a problem?

[xtian]

...You'll record values of ~50-100 ohms each side....

I recorded 14.5R and 13.5R. Amp off, unplugged, tubes in. This is out of the range you mentioned. Is this a problem?

No problem.

[Richard1001]

I already have a matched quartet of power tubes (TAD 6L6WGC) on hand, but as for the preamp tubes I have a few options available. Of course I plan to swap out and do my own testing, but I would like others' thoughts on which tubes would be best for each position. I am also interested in any technical justifications you might be willing to offer.

I have the following tube available and would like to start here:

3 Premium selected TAD 12AX7A-C
2 JJ ECC83-S
1 JJ ECC803S

Or are all of these crap?

The TAD 12AX7-C are indeed crap. The pins are to long and uneven spaced. They will wear out the sockets fast.
Performance wise they are no match for the original Chinese military tube that was used as oem tube by everyone in the 80's and 90's. The original tubes are still available and don't cost much.

I tried a few TAD tubes and don't use them. If anyone wants them, I'll send them for only the shipping costs.

[professormudd]

...You'll record values of ~50-100 ohms each side....

I recorded 14.5R and 13.5R. Amp off, unplugged, tubes in. This is out of the range you mentioned. Is this a problem?

No problem.

I did your procedure as follows:

13.5R, 40mA (13.5 x .004) = 0.054, giving me .108 per side.

I only have one bias pot on this amp. Clipped my common lead to the standby switch, clipped other lead to pin 3. Everything was turned all the way down and I adjusted the pot until I got around .108. I turned some knobs and watched that voltage reading just to around .3**. I never saw any red plating.

I'm not confident that the bias is set correctly, so I'm ordering a probe (I'd like one on hand anyway).

Other than the bias there is a constant hum that does not seem to respond to chopstick prodding.

Meanwhile I went ahead and plugged a guitar in just to hear it and the clean tone is really nice, the eq seems to work, the rock/jazz, deep, and bright switches work, but the overdrive and pab switches (or relays) do not seem to work. I hear no audible change when either is flipped. The PAB seems to work with the rock/jazz in middle position.

I checked that there is 12v going to each relay that cuts to .0xx v when the switches are engaged.

[martin manning]

I recorded 14.5R and 13.5R. Amp off, unplugged, tubes in...

...I did your procedure as follows:

13.5R, 40mA (13.5 x .004) = 0.054, giving me .108 per side.

I only have one bias pot on this amp. Clipped my common lead to the standby switch, clipped other lead to pin 3. Everything was turned all the way down and I adjusted the pot until I got around .108.

You're off by a factor of ten. You want to see 1.08 volts across the 13.5Ω side of the OT.