6H100 Build Issues

[Phil_S]

From the Weber website: WS1 8-pin Copper Cap. Diodes and Limiter only, No Sag. Vdrop: 1V max @ Imax: 1000ma
I also can't imagine how you can see 700v at pin 8. There should only be rectified DC voltage at pin 8. If input is 340-340, the maximum output would be (sqrt(2)*340)-1 = 480VDC. Maybe something is wrong with the copper cap?

Is there an independent test i can do on the cap that would tell me if it is bad?

I see I'm late to the continuation of this party.

I guess you could breadboard a replica of the power supply and see if the copper cap performs or not, but that seems like a waste of time and resources. Just go for the 1N4007's. You can buy a small package of 10 on eBay for about $2. If you filter out foreign sourced, you should have them in a few days. Example: https://www.ebay.com/itm/10pcs-1N4007-R ... SwVHpfNG2p

I see there is a raging debate over whether the PT is bad or not. FWIW, I am in the camp that believes the PT is OK, but that's neither here nor there and I don't see the mileage in debating it at this point. It seems to me that the simplest course is to get the 1N4007's and see if they fix the problem. There are at least two reasons to go this route. One is that the Weber website says the WS1 is out of stock, so you won't want to wait around until they get more of them, if they ever do. The other is that, if this works, then you can just move on to complete the project, and since it is such a simple thing to do, why not just do it? I see no down side to this. Also, what PDF64 suggests is easy to do, so just do that, too.

As Stevem points out, nodes A-E are all at the same B+ because there are no preamp tubes. I asked for these readings as confirmation. If there was a voltage drop, it would point to something wrong. There is nothing conclusive here. When you put the preamp tubes in the amp, you may find something. However, we still have the strange behavior of the basic power supply. This leads me to think that is where we will find the problem.

[pdf64]

I switched to DC then checked post 8 and got 706v.

I would check that again. The theoretical maximum DC you can get from that 340-0-340 PT is 480Vdc...

If the HT fuse was blown, or there was otherwise a bad connection between the HT winding's CT, then with standby open (and hence no current draw) the 0V reference would move to the HT leg connected to the bias supply, and the arrangement would resemble 680VAC at a very high source impedance, which would then be half wave rectified. Hence in that scenario, the VDC at the rectifier output can be much higher than normal.
There's been a recent thread with this same symptom and root cause (ie a blown HT fuse in the CT to 0V return of the HT winding), but I'm struggling to find it.

[Stevem]

Pdf64, I don’t think what you’re discribing here as a possiblity that pans out.

If that was the case then in one of my previous post where I had a Bassman right in front of me with its standby switch wired in like his fuse in the PTs V+ center tap did not make for high voltages like he is having when the standby switch was open ,and I should have by what your last post is presuming!

[Phil_S]

There is obvious disagreement about the PT. Having slept on this, I see no harm in spending $2 for some diodes (assuming they need to be bought) and seeing what happens. If the diodes fix the problem, then we know the copper cap is defective. If they don't fix the problem the answer lies elsewhere. There isn't much to lose except a little bit of time and very little money, and this is easy to do. Why not just do it and eliminate the copper cap as the potential problem?

[pdf64]

... I had a Bassman right in front of me with its standby switch wired in like his fuse in the PTs V+ center tap did not make for high voltages like he is having when the standby switch was open ,and I should have by what your last post is presuming!

There are several dozen different variants of Bassman designs, containing just about every possible permutation of valve amp circuitry. So for me to have any chance of helping you, please link to the appropriate schematic

[csjoyner]

Just to make sure I do this right I have included the rectifier layout I will build. Please let me know if if there is an issue with it. Also, any suggestions on bulb watt for the lightbulb current limiter? There seems to be some debate.

[pdf64]

That looks ok.
As you’ve not got balancing caps across them, they should all be from the same batch.
Purchased from a reputable supplier, ideally one that’s listed on a manufacturer’s website. Not cheap fakes from ebay etc.
The 640VAC terminals look a bit close together.
Use a proper board material that’s rated for high voltage electrical applications, eg garolite. Not the crap black compressed paper akin to vintage Fender boards.

[Stevem]

Yes, that’s exactly what you want for the diode set up!

I don’t think you need to go thru having to make a bulb limiter, as your wiring seems to have been good all along, what I would do while ordering up the parts is to get some .200 to .250 amp fuses to use in place of that .500 amp fuse just for first time start up purposes.

Just be sure to set the negative bias voltage as high as it will go before taking the amp out of standby, in fact do this before even popping the output tubes in there sockets, and make sure it’s higher then the needed -55 volts.

If you have not ordered any parts a nice improvement over those standard 1N4007 will be to order up modern UF5408 fast type recovery diodes.

These are more expensive, but will reduce power supply noise hash from getting into the audio stream.

Another thing you could do when you get the chance is to place a .01 uf 1000 volt disc type cap across every one of those six 1N4007 diodes which will help out in terms of cutting down on that noise hash issue.

That’s the one really nice thing about a tube recto is that they do not produce noise hash!

Also if you wire like I posted before, that standby switch into that center tap red/ yellow wire it will last far far longer then having all of that DC voltage on it and also not make popping sounds out thru the speaker when turned on.

[csjoyner]

I would still like to build the lightbulb limiter for the future. What watt bulb should I use?

[pdf64]

A selection of bulb wattages is best, eg 40, 60, 100, 150.
Low values for initial power up with no valves, and for low power amps. Higher once the wiring is proved and you’ve got valves in, but there’s a concern about shorts manifesting under load.
So work your way up.
I rarely do any amps over 50W, so my 150W bulb, which I use for load checking 100W amps, has hidden itself somewhere.
Obviously, the bulbs need to be filament type, incandescent or halogen, not fluorescent or LED equivalents.

[Stevem]

If your building a limiter you can make it work for higher power amps when needed by just adding another bulb and socket and a 120 rated SPDT ( single pole double throw ) toggle switch and wiring it like this

[Phil_S]

I'm an admitted amateur, so I'd like to know more. Why do we need 6 diodes? Each one is rated 1000V and 1A. It seems to me that just 2 diodes will do the job. What am I missing?

[Stevem]

Yes, your fully right, in terms of today’s diodes and there greater reverse break down voltage as compared to what Fender had to work with in the late 50s and 60s in terms of state of the art!
But it does not hurt to have 6 of them, and only reduces the DC output voltage to be had by less then 2 volts.

[pdf64]

...Why do we need 6 diodes? Each one is rated 1000V and 1A. It seems to me that just 2 diodes will do the job. What am I missing?

640Vrms = 900Vpeak
That may be seen to be pushing 1kV diodes too close to their limit; consider that transformer regulation will cause secondary voltages to rise when unloaded, and then there’s the mains variance upper limit to factor in. Hence series pairs (ie 4 1kV diodes) are the minimum for good engineering.

[Phil_S]

640Vrms = 900Vpeak That may be seen to be pushing 1kV diodes too close to their limit; consider that transformer regulation will cause secondary voltages to rise when unloaded, and then there’s the mains variance upper limit to factor in. Hence series pairs (ie 4 1kV diodes) are the minimum for good engineering.

Thank you for the explanation. It makes perfect sense.