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Untapped Potential: Heath's W-5M

Just re-reading some of the first page in this thread. I discovered a mistake I made for setting bias. Dave said to short the 0 and 16 output taps and no input. I had a 16 ohm load resistor on the terminals. I’ll try it again when I get home.
 
Both units with the 16458 outputs are working. Taking them to the owner today. One curiosity in both amps is that I had to have the the bias pot turned all the way down to get the 0.061V bias setting and that is after adding a 10k resistor in series with the 4.7k from the balance pot tap. I'm getting about 32V bias at the 6L6 grids with 36V from the raw supply. Seems like I need a bit more bias voltage to have some range. Wish I had purchased 28V transformers instead of the 24V versions.

Any ideas why this is happening this way? Other than this they seem to be working perfectly.
 
I would agree because you’re also running out of voltage drop for good filtering. Usually a raw supply of about double the actual need works well after the filtering and adjustment networks are in place.

Dave
 
The 24 volt transformer can work perfectly as well: just use a voltage doubler rectifier and you’ll have all the bias voltage you need. I did this in one of my own units and it works just fine.

Dave
 
Got it. I'm taking them back to the owner today but if he ever runs out of bias on the low current end I'll have him bring them back for some rework. I thought about a doubler but I achieved the correct bias even though it's with the bias pot at full minimum current. This was the case on both pair of W5's I've rebuilt. The first pair is so full I will probably recommend changing the transformer for a higher voltage. The recent pair has a lot more room so I could build a doubler and fit it in somewhere.
Thanks
 
I'm going to take on the task of building Dave's circuit into two W-5M units for a customer.

Will the end user need to check and adjust the bias setting or once it's set it should be good to go?
 
rd48sec said:
I'm going to take on the task of building Dave's circuit into two W-5M units for a customer.

Will the end user need to check and adjust the bias setting or once it's set it should be good to go?
Click to expand...

You could set it up on the bench and then number the tubes for the customer so they're inserted in the correct order. Otherwise, the customer should check the bias him or herself. This is something he or she should know how to do anyway for the future. You might include an inexpensive digital meter and instructions. That's what I do when I build amps for for non-technical friends.
 
It will need periodic checks. It shouldn’t drift too quickly but it will drift over time. Dave’s new build is a bit of a project but worth it. I’ve done four of them so far and expect to do another pair.
 
nerdorama said:
Hello Dave,
Thanks for writing. I'll be starting on the second unit today. Stripping and cleaning first.

I have a safety related question. On this amp I drilled out the RCA jack opening and installed an isolated jack. So far I haven't connected the audio ground to chassis. There is a power cord chassis ground to the green wire. Do I need to connect audio ground to the chassis or is it ok to let it float? I've seen articles about using a bridge rectifier with a resistor and cap as a ground break. Since these are for a friend, I can't be certain how they will be used in future and want them to be safe but not a cause for ground loop problems as well.

Thanks,
John
Click to expand...
John, what did you decide to do? I have the same problem, i.e., insulated phono jack.
 
So far I haven't changed anything. The audio common bus is floating from the chassis. Chassis is connected to power ground wire. Amps are dead quiet but I wonder about whether this is a safety issue. Seems like it should be ok with the chassis grounded. Maybe one of those plug-in GFCI units would be wise. That would also provide for brown-out or power-out episodes requiring a reset.
 
Would it be possible to use a Heathkit WA-P2 Pre-Amp with the Gillespie modified W-5M amp? The W-5M will need to supply the heater 6.3VAC@1amp and 300VDC@10mA. Where would the 300VDC supply come from in the Gillespie design?
 
You would still have just the 1A you need for heaters. This pushes the filament winding to it's limit of 4A. The 300VDC and 10mA could easily be provided if you keep the octal socket connections available but you will likely have to adjust the dropping resistors in the modified power supply schematic. The original schematic show the 400V section which feeds the driver stage also going to the preamp octal socket so you should be able to work it out as the driver stage in the modified plan is still at 400V. I don't know the WA-P2 so can't give more specific recommendations. A potentially larger issue is the DC elevation of the filament CT. In the original the filaments were referenced to ground in the preamp I believe. This feature alone may make the use of the Heath preamp with the modified W5 unworkable.
 
so there is no connection at all between signal ground and the earthed chassis mains? Might want at least a cap in there for RF bleed reasons, but other solutions involve a pair of back to back diodes across those points to provide DC isolation while still providing a path to mains ground in case something dumb happens.

Earthing (Grounding) Your Hi-Fi - Tricks and Techniques

Earthing (Grounding) Your Hi-Fi. Detailed explanations of the safety and sonic aspects of proper earthing (grounding) techniques
sound-au.com sound-au.com

the 'zero volt line' is the signal/B-.


wouldn't be a big deal to unhook the hum balance pot in the Heath preamp to fix the elevation thing.
 
nerdorama said:
Back in the saddle. Got 3 more W5’s in the last few days. The pair from this weekend included the mono preamps and have the original KT66’s. Hope they test well.
Click to expand...

Sounds like you've found your niche. By all means, rescue them all! ;-) And that's not snark!
 
16309's on the recent 3 amps. I got one a month or two back with a bad 309 and found a 458 online. I'll match it up someday or find another 309 and sell the 458 to a friend who wants a spare.
 
Hi Dave, I have been restoring a pair of these but have run into an issue. Both of these W5-M have AC measured between 170 and 240 VAC from chassis to the mains ground. The power trans are fine looking, with no tar coming out holes of the trans, and both amps working great. Not sure what is going on here and/or if this is a problem. What do you suggest? Thanks!
 
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