LA2A Build - Resistor Question

[StormsofVogue]

I'm a little concerned about the physical size of the 1/2 watt metal film resistors I chose for my LA2A build. They're sized more like what I'd expect 1/4 watt resistors to be. I know they're technically rated for it, but I'm a bit concerned about what their operating temp might be given their small size - not sure I really want a component running hot regardless of it's ability to handle it. Should I be worried about this? Data sheet is linked below. Plus photos for reference. Thanks!


https://www.vishay.com/docs/28722/sfr16s25.pdf (SFR25H)

 

[beatnik]

1/2 power rating is fine, regardless of size - although on the La-2a some resistors on the HT line have a higher wattage, it should be marked on the schematic.

The voltage rating could be a concern , looks like they are rated at max 350V, this can potentially be a problem if during power up the HT voltages exceed this rating, or perhaps in the event of a fault or if removing some tubes. It is a lot safer to use 500V rated resistors for tube builds

 

[StormsofVogue]

1/2 power rating is fine, regardless of size - although on the La-2a some resistors on the HT line have a higher wattage, it should be marked on the schematic.

The voltage rating could be a concern , looks like they are rated at max 350V, this can potentially be a problem if during power up the HT voltages exceed this rating, or perhaps in the event of a fault or if removing some tubes. It is a lot safer to use 500V rated resistors for tube builds

Thanks! I'll go ahead and source some 500v rated replacements. I wasn't aware that they'd need such a high voltage rating. I do have 2w resistors where noted in the schematic. Really appreciate your help!

 

[dmp]

They have made higher power resistors in smaller sizes by making the materials survive at higher temps. The surface area is smaller in the smaller size resistors, so the temperature has to be higher to dissipate the same energy. And rightfully you may not want hot resistors in your build.
But many of the resistors in a la2a are not operating near the 2W. Like beatnik pointed out, some old gear had high power ratings for resistors just for the voltage rating.
To understand the power rating, the LA2A schematic has nominal operating voltages labeled throughout. Take the voltage across a resistor and you can calculate the power from ohm's law and P=V*I. For instance, r13 (220K) has voltages on either side of 205v and 120v. This gives a power, P=V^2/R=0.032W
So then you know the resistor is not seeing near 2W and won't be actually operating hot.
But there are a few that are operating at higher power, like R34 (~1.5W), also R29 and they other power supply filtering resistors.
You may want to consider your part selection for those with that in mind.

 

[Rob Flinn]

I always use 1w resistors for this sort of build partly because I don't like the look of the size of modern 1/2W but also because I have found them more reliable. Particularly the Anode resistors, which I have found prone to failure even if they are within the power rating of the static, no signal voltages. The point being that the voltage across the anode resistors can swing around quite a lot when a signal is applied, so the static voltage doesn't necessarily mean a great deal. I have had professionally made audio gear on the bench that used quite weak Anode resistors, that I had to replace because they went open.

 

[StormsofVogue]

Thanks for the input, guys! As a bonus - as @Rob Flinn mentioned - the new properly rated resistors look much better. They’re CCF60 series if anyone finding this thread later wants to know.

A couple of more questions - Is there a reason C2 and C3 are usually bent toward the center instead of just straight across the turrets like I did it? I’m wondering if I’ll be risking noise with my configuration.



I also moved R39 down to the turret board next to R15 since there was room for it. Not sure if that was a good idea either.

 

[kags]

Great thread! I’m in the midst of refurbishing a couple L2as that have been hidden in a studios repair shop for at least a decade.

Is it best practice to convert older 2-wire power cords to a 3-wire grounded and fused IEC receptacle or simply a 3-wire cord with the ground tied to a new chassis bolt? Any concerns or cautions with that update? These are 120volt and are usually racked in grounded rails, but sometimes move around in portable racks.

Thanks!
Adam

 

[StormsofVogue]

Great thread! I’m in the midst of refurbishing a couple L2as that have been hidden in a studios repair shop for at least a decade.

Is it best practice to convert older 2-wire power cords to a 3-wire grounded and fused IEC receptacle or simply a 3-wire cord with the ground tied to a new chassis bolt? Any concerns or cautions with that update? These are 120volt and are usually racked in grounded rails, but sometimes move around in portable racks.

Thanks!
Adam

I'd convert it to 3 wire. I don't think you'll find there's space enough to fit an IEC receptacle in a vintage unit even if you wanted to; which works out since it's sacrilege to cut holes in an original chassis. There should already be a fuse installed even on a vintage unit. I wouldn't add a chassis bolt for ground either. Use an existing bolt. There's a few to choose from. Here's an example of a vintage unit with a chassis ground added:

 

[Rob Flinn]

I wouldn't add a chassis bolt for ground either. Use an existing bolt. There's a few to choose from. Here's an example of a vintage unit with a chassis ground added:

I believe if manufacturing new equipment any bolt for the Chassis Earth should be dedicated for that function only. For example in the pic you posted the Earth lug is clamped down on to a bit of paxolin which is not going to be a tight as bolting it down on to metal.

 

[Ike Zimbel]

I believe if manufacturing new equipment any bolt for the Chassis Earth should be dedicated for that function only. For example in the pic you posted the Earth lug is clamped down on to a bit of paxolin which is not going to be a tight as bolting it down on to metal.

This is correct. The reasoning is, that way you would never, in the course of servicing a unit, have to remove the chassis ground connection to replace a component.

 

[StormsofVogue]

I believe if manufacturing new equipment any bolt for the Chassis Earth should be dedicated for that function only. For example in the pic you posted the Earth lug is clamped down on to a bit of paxolin which is not going to be a tight as bolting it down on to metal.

Yeah, but the guy above is talking about converting a vintage unit from a 2 prong AC cable to a 3 prong. I just don’t see a reason to drill a new hole into an original vintage chassis.

 

[kags]

Yeah, but the guy above is talking about converting a vintage unit from a 2 prong AC cable to a 3 prong. I just don’t see a reason to drill a new hole into an original vintage chassis.

thanks for the image and I get your point about the lack of space for an IEC inlet. I think are a couple other nearby bolts that go through the metal body, like Robb Finn mentioned. I also don’t mind drilling a hole for a proper ground lug. Our studio is not keeping gear (or selling them) as museum pieces and it would be done cleanly so as not to mar the finish in other ways.

I’ve seen an La2a where the fish paper gets pinholes and turret board can short to chassis either blowing a circuit breaker (hopefully) or creating a dangerous condition so I think I’ll go ahead and add the three-wire cord/grommet to these units.

Thanks for the input!

 

[StormsofVogue]

thanks for the image and I get your point about the lack of space for an IEC inlet. I think are a couple other nearby bolts that go through the metal body, like Robb Finn mentioned. I also don’t mind drilling a hole for a proper ground lug. Our studio is not keeping gear (or selling them) as museum pieces and it would be done cleanly so as not to mar the finish in other ways.

I’ve seen an La2a where the fish paper gets pinholes and turret board can short to chassis either blowing a circuit breaker (hopefully) or creating a dangerous condition so I think I’ll go ahead and add the three-wire cord/grommet to these units.

Thanks for the input!

I’d use one of the screws for the power transformer as chassis ground. That’s what the UA reissue does as well as the replicas using the Analog Vibes chassis (and others). No reason to go drilling holes unnecessarily. Ground for a 2009 UA reissue pictured below.

 

[Rob Flinn]

Yeah, but the guy above is talking about converting a vintage unit from a 2 prong AC cable to a 3 prong. I just don’t see a reason to drill a new hole into an original vintage chassis.

Personally if it was going to give me an improved "safety" earth then I do.

 

[StormsofVogue]

Personally if it was going to give me an improved "safety" earth then I do.

Fair enough.