Foreplay III XFMR Resistance Check Problem

[Deafer_than_a_doornail]

Hi All, and Happy New Year!

As a busy guy, it took me a year to complete my Foreplay III (not Extended yet) construction, little-by-little. I purchased the kit on Jan 1 of 2011. I worked as meticulously as I could, though the last time I soldered anything was in about 1979.

I am finding resistance values that are way off from the list, as follows:

Power XFMR terminals: 
6,7,9,10:   Increasing, starting about 0k. It takes about 30 seconds to reach 100k when I hold the probe on a terminal, and keeps going up. All four of these terminals act in the same way. The list shows these should be infinite R.

I've completely rebuilt and resoldered the components at terminals 1~10 on the two terminal strips next to the transformer, and I am unable to find the cause after many hours of going back through the instructions and rebuilding even slightly suspicious-looking joints, with no change to this result.

4,5: Increasing very slowly, starting about 29k. They reach about 32k after 1 minute, and seem to want to keep increasing, very slowly. I'm not sure whether this is right, because the values don't fluctuate, but instead just slowly increase. In addition, all four of terminals H1,H2,H3, and H4 show a simlar reading around 28~32k, perhaps increasing more slowly.

I really hope someone can help me with this. I've had my heart set on hearing my unit in 2011. Thanks!

-Deafer

[Doc B.]

Well, if they are climbing, in theory they would eventually reach infinite resistance. So everything is OK.

[Deafer_than_a_doornail]

Thanks for the quick reply, Doc!

I kept the probe on each terminal (XFMR 6,7,9,10) for about 2~3 minutes this time, and find the increase decelerates, and they level-off at about 130k.

Is that ok?

XMFR 1,2 read infinity immediately.

[Laudanum]

Meters can be funky with those measurements.   Like doc stated, my meter climbed as yours did and I didnt wait to see where it was going to stop but was confident that it was ok.  I moved on to the rest of the checks.    What about the rest of the resistance checks?  Are they in line with the manual within +/- 15%?

[Deafer_than_a_doornail]

Thanks for the reply!

All of my other resistance checks were spot-on, within less than 5% for sure, except one spot where I had to replace a long piece of already bound hook-up wire and 5 welds from RlI to the center switch, because of a short somewhere within the hookup wire itself, probably at the point where I cut the main insulation and foil, I guess. Once I replaced that, all was well except these XFMR terminal values. The way the readings go, with the decelerating increase in R values, it looks like a charging capacitor reaching saturation or something. It certainly never goes "infinite," though, just approaches about 132k...

Do you suppose it's safe for me to plug it in now and continue with my live tests, even though those 4 terminals all read what amounts to 132k Ohm once the meter settles?

It sure would be nice if I could get this finished tomorrow, Dec 31 here, and having taken precisely 1 year, though I'd hate to plug it in and blow some components. 

[Laudanum]

Not having the best meter myself and not waiting to see where it ended up, satisied with the climbing readings, maybe ignorance was bliss in my case as I didnt end up obsessing over the transformer measurements. Everything else measured to within spec and in all my bottlehead builds when I fired up for the first time everything worked, voltage checks passed.   
Personally, I would just do a quick check and make sure that the label on the transformer is oriented correctly.  There was problem yesterday with one kit builder where the label was put on backwards by the transformer maker on one of the transformers so obviously the transformer wasnt wired correctly during the build (Crack kit I think).  Another member mentioned him having a reversed label as well.   So I would check just to make sure the numbers on that label match the numbers embossed on the transformer terminal insulators just to be sure it's oriented and wired in correctly in your build.    If that's ok and the rest of your checks are ok as you mention, I would, again, if it were me,  fire it up and check voltages.   Now, Im not the most tech savvy person by a long shot so what I would do isnt the final word by any stretch of the imagination.    I just know that meters can be strange with some measurments and that is the case with mine.  So if not comfortable with this, Im sure that one of the smarter members will come along with either confirmation, or suggestions/input on the measurements of the transformer.

[Deafer_than_a_doornail]

Thanks again, Desmond.

This looks like "sound advice" 

I had at one point also wondered whether the transformer might be upside-down, but didn't notice the embossed numbers on the transformer casing, and only compared the label with the instruction pictures. The label numbers and embossed numbers match-up in my case.

As soon as I get the room warmed-up (over a meter of snow where I'm at...), I'll go ahead and plug my FPIII in and start my voltage tests. If you don't see a post from me within 24 hours, call the fire brigades!  :0

[Deafer_than_a_doornail]

Ok, so I just plugged it in. What a great feeling to watch the tubes light up and the little LEDs as well.

However, I finished my voltage checks, and the result baffles me.

To summarize, all my DC measurements seem fine, but NONE of the AC measurements at the XFMR terminals are even close!

Therefore, both my Resistance and Voltage measurements at the XFMR terminals are far off.

My XFMR terminal voltages were as follows:

Terminals
4, 5 (list, 3.3 VAC): 58 VAC, 28 VDC
6 (list, 165.5 VAC): 418 VAC, 189 VDC
7 (list, 0 VAC): 418 VAC, 189 VDC
9 (list, 165.5 VAC): 418 VAC, 189 VDC
10 (list, 0 VAC): 418 VAC, 189 VDC

I measured these using both the AC and DC selections on my meter, just in case there was some reason to do so, as the instructions say to switch the meter to read DC, just before the measurement list begins, which seemed strange to me.

Especially the high non-zero measurements at 7 and 10 worry me.

My mains voltage is 100 VAC, and all but two of my DC voltages were proportionately lower than on the instruction list, which Doc made at 118 VAC.

I don't know whether this means anything, but one of the tubes lights up much more slowly and has much less light in the lower area than the other. The large tube glows sort of purply-white.

Since the DC voltages are all close (terminals 15 and 31 are a little higher, all others a little lower than Doc's), is it safe for me to try my FPIII with a source and amp, even with my XFMR voltages being so far off?

Could it damage my amp or something else?

[Doc B.]

If the DC measurements indicate that everything is fine you can hook it up. There may just be some issue with the meter with respect to the AC measurements.

[Paul Joppa]

Just a caution - Bottlehead products are designed for 120vAC +/-10%; I recommend +/-5% for more reliable performance. A 100v mains is 20% low, and that is too low for proper operation. It may work anyhow, especially with fresh tubes, but it is not recommended.

The biggest risk is inadequate voltage to the tube heaters, which exposes the cathodes to damage by ion poisoning. You can measure the heater voltage between H1 and H2 (across the 10000uF cap). You can increase that voltage by reducing or shorting out the 1.2 ohm resistor.

The second risk is too low a high voltage to keep the regulator tube lit. You can reduce the series resistance by adding a third 20K resistor in parallel with the others, to help with this.

[Deafer_than_a_doornail]

WOW!!!!!!!!! What GREAT sound!!! It's totally different than I expected! It's got a sort of "sweet" quality, but rich and powerful.    And so far there is absolutley no hiss or bad sound, even at full blast, in either channel.

I'm playing an assortment of favorite music now, from flac files fed by a Mac Mini via Hegel DAC to the FPIII, then to a McIntosh MA6800 feeding one pair of B&W 805Ns, one pair of Yamaha NS1000X, and an Entec subwoofer in a large room in an old farmhouse. It's 3am.

Everything sounds better, at this first, excited listening than it did without the pre-amp. I can't wait to try it in my other systems in my main house!!

Thanks Doc for the kit, and for all who helped me get through my testing phase.

(....I think I'll give my meter a toss.    )

Happy New Year All!!!!

[Deafer_than_a_doornail]

Hello Paul

The unit sounds great. I'll do the modifications you suggest, to keep it going longer, but I have a couple of questions about it.

The regulator tube glows pretty brightly, with a purplish-white hue. Does this seem ok, or might it gradually go bad? Also, I plan to do the Extended upgrade, kit already purchased. Should I modify the series resistance to teh tube that will be replaced anyways?

The two power tubes light-up very, very faintly.

What should the voltage be between H1 and H2?

I don't see a "1.2 Ohm resistor" to short out. Do you mean the 0.62 Ohm resistor attached from H2 to H5?

[Paul Joppa]

Yes, sorry, I did mean the 0.62 ohm resistor. The heater voltage should be 6.3 +/-10% (i.e. 5.7 to 6.9 volts; the closer it is to 6.3 the better). The very dim glow of the 12AU7 heaters is because that voltage is low - though they are never very bright.

As long as the glow tube glows, everything is OK in the high voltage section. There is a good chance it will last, at least long enough to get around to the C4S upgrade. The upgrade shunt regulators should have enough compliance to work at 100v - just follow the voltage checks when you get to it, and post if there are any problems.

[Deafer_than_a_doornail]

Hello Paul

As you said, I measured only 5.18 V across the capacitor from H1 to H2.

I therefore soldered a wire from H2 to H5 across the 0.62 Ohm resistor to short it, leaving the resistor in place. This brought the voltage from H1 to H2 up to about 6.04 V.

To get closer to 6.3 V, should I try taking the 6.02 Ohm resistor out completely, leaving just the straight wire in place, or is this not necessary or not a good idea for any reason?

Also, should I run back through all the resistance tests again?

BTW - one of the tubes (right side, viewing with power switch in front) takes much longer than the other to light up, and never gets quite as bright. Same for its LED underneath. Is this normal variation among tubes, or might it indicate a bad solder somewhere?

I'm sorry about the dumb questions: as you can see, I'm a total novice.

[Paul Joppa]

The wire has "shorted out" the resistor, so removing the resistor will not make any difference - that's all the voltage you can get with the stock circuit. 6.0 volts is fine, since the tubes operate well below their maximum current rating - it might even extend their lifetime.

The resistances will not have changed, no need to do the checks again.

Be aware that adding the eXtended upgrade includes another tube, which will pull the heater voltage down a bit further. That may or may not be too low; hard to say until you try it. The recommended solution is to get a step-up transformer and revert to teh stock circuit, of course - I assume these are readily available; they are in most of the world. The alternative would be to change the heater supply to a voltage doubler plus regulator. That's a fairly simple circuit - maybe 8-10 small parts - but it's not a kit. You'd probably have to get a small perf-board of 1 or 2 square inches to mount the regulator and heat sink. I can draw up a circuit for you if you want - could be an educational adventure!