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heathkit w-5m renovations [boondoggle edition] quantum vacuum deep dive

Was wondering the same thing, what is voltage sag under expected load? One reason I have been using 6L6 rather than KT66 is the heater wiring draw with the KT66's being right at the edge of transformer loading, 1.8A for two 6L6's vrs 2.8A for two KT66 tubes. In that configuration I was seeing about 6.4v on the heater wiring all tubes installed btw. Line voltage here 123/124vac.
 
so far, after adding 0.22 resistor 5w in series with the heater tap there is a 0.55v drop across the resistor making the adjusted voltage 5.99v. this is with the full load of heaters attached. mains txf seems to be healthy, loaded or unloaded.
 
heathkit w5m - 13.jpg
installing txf-blk and looking at how many components will need to be removed to install the custom b+ board. also focusing on 6L6g cathode bias resistor ladder and caps.

heathkit w5m - 14.jpg
evaluating physical size vs mounting options for the new 30uf 600v film caps. should only need three of these monsters for the b+ board circuit. c-r-c up front after the v-doubler, darl in the middle and r-c-r in the back including b1 separation. simple circuit as it should be.

designing a what-if protection response system, thats far more difficult plus needing more components. burning up fuses is not a sensible design strategy, gets expensive.

Screenshot 2026-06-24 at 16-54-13 Search results for 30uf 600v Film Capacitors – Mouser.png
 
heathkit w5m - 16.jpg
slowly piecing together the new dc framework. should be obvious what major cosmetic/electrical items are being deleted from future use although still in physical attendance (as is mandatory for vintage beauty).

heathkit w5m - 17.jpg
side profile of the diy circuit design, built from the ground up. r&d test version and not final show pony final production release candidate v1_0_0_0.

also, first version of the overly restrictive output bias ladder. way too high especially at low b+ testing territory.

cathode > 330r 3w (each side) > 1k 5w//1k 5w > 330 3w (each side) > factory pot (each side)

heathkit w5m - a1.png
for easier component value comparison, on this thread page

wonder if bumping up the 270r screen tap resistors would be of any benefit after looking at the dcg-designs separate supply voltage. perhaps 1k/1.5k/2.2k/4.7k magnitude to increase voltage drop?
 
heathkit w5m - a2.jpg
original mains cores that were heated in an oven and then chiseled out. not a pretty process from the sounds of things.

heathkit w5m - a3.jpg
no idea what chemical composition the potting material is but it certainly complicates things.

hmm, one core is wider than the other... interesting indeed.
 
in regards to my heater voltage measurements and perceptions, some specificity should be noted.

loading (on chassis, in-circuit) of the mains txf is with:
ht secondary: using half tap, not full + ct ground
6.3v tap: using full + zener regulated dc offset (33w 1.3w x2 in series + 330k dropper resistor & ?10k output resistor) on ct
5.0v tap: unused/retired unless relay boot-up/cold-surge or safety circuit designed (pending)

so this slow and low b+ starting position/testing is hardly taxing the new/rebuilt mains txf. by deleting the energy guzzling tube rectifier (~15w heater load) how much less thermal stress is the mains txf burdened with. i would guess an increase in service life and longevity...

the low loading would explain minimal/zero sag of the 6.3v even if the tap itself is heavily loaded. txf core probably much happier, flux density vs saturation of the globally significant core (one core with multiple taps, they all affect each other in some small way).

need to dial-in optimal heater-ct offset around 33v-48v max, if all heaters feed off the same global tap.
 
active circuit measurements, most recent as of todays timestamp:

mains 120v, idle 43w (plus 0.2w or whatever from the power strip being used)
b+ = 215v
b1 = 203v
6L6g cat = ~18v
all voltages with reference to circuit ground

*didnt think to measure grid-1 voltage with ref to gnd. need to start measuring from cat-to-g1 in future for -v bias number.

Screenshot 2026-06-24 at 19-13-50 480v 6L6 bias factory.txt - Circuit Simulator.png
better picture of the electrical situation. output tube resistance equivalent based off of real circuit measurements, voltages across resistors +/- real world tolerance variances. voltage drops are a close approximations derived from ideal circuit sim parameters. a resistor doesnt measure perfect out in the real world, no do tubes.

measurements of each side of the bias ladder (pot tuned for balanced dcv between factory test points)
[cathode / cathode]
17.8v / 17.9v
[68r / 68r]
16.2v / 16.2v
[220r 2w flameproof]
5.29v / 5.29v
[68r / 68r]
3.33v / 3.88v
[pot: 150/150 if centered]

*still need to verify actual pot resistances (full vs wiper positions)*

clearly one output bottle needs a little more electron love than the other. if only i had focused on who had the higher number. hmm, things may have been handwritten from left to right although measured inside tube to outside tube (hypothesis pending).

**previous snapshot showed 151r for out-txf voltage drop. it has been corrected with measured dcr.

still need to post a clean & updated version of working schematic in the falstad sim. the sim has its limitations and faults but its better than guessing.
 
i suppose this should be an [open-source] {boondoggle} vs another criminal billionaire pump&dump. minus all the headaches of bs patent-pending manipulation on other planets, who has time for that.

the star/center of the filter design will be revealed soon, including part number (absolute beauty spec's although slightly expensive).

heathkit w5m - a4.jpg
more views of original mains txf's

heathkit w5m - a5.jpg
not much to see but interesting to note core width though in other photos
 
Screenshot 2026-06-24 at 22-20-10 6L6 tubes.png

highlighted the important part from the link posted earlier in the thread. this amp is designed to handle kt66 and possibly higher heater current options. interesting to note the limitations for 6L6/6L6G/6L6GA and how an over-enthusiastic b+ can do damage.

heathkit w5m - 18.jpg
somewhat closer look at the initial updated bias ladder values plus temporary electrolytics (reduced to 47uf vs original 100uf)

difficult to see, pins 1 and 6 of the output sockets are now clean so 7027a glassware can be used. also updated screen grid resistors, will be evaluating higher values vs stock 270r.
 
saggy screen supplies are generally undesirable. Honestly based on Dave's work I suspect you may really want to not use the screen taps at all, and just feed this as a pentode amp. Seems like the transformer doesn't work so awesome feeding the screens.

Why are you only getting 215 volts of B+? It should be easily double that. A 6L6 is a poor match at that low a voltage. I'd probably be looking for something like a 6W6, though the transformer load is all wrong for that. Could run it half impedance mode, that might work out acceptably.
 
gadget73 said:
Why are you only getting 215 volts of B+?
Click to expand...
i will post the corresponding schematic soon which should shed some light on the freak-of-nature circuit that is being developed.

gadget73 said:
saggy screen supplies are generally undesirable.
Click to expand...
yeah, dave mentioned something about high frequency instability and some notes about his design side-stepping some of the design limitations of the original circuit. probably do some listening experiments vs screen resistor value vs another b+ modified output or something.
 
 
Screenshot 2026-06-22 at 16-55-05 Assembling and Using Your Heathkit High Fidelity Amplifier M...png
for future reference, added the voltages listed in the heathkit pdf to the front end of the schematic to help calculate current for each triode. manual indicated that the voltage table on page 27 of 36 is with the preamp connected. which will obviously load the b+/b1 down more than usual.

from left to right:
2.2v/470r = 4.68ma
100v/22k = 4.54ma
13v/2200 = 5.90ma (/2 = 2.95ma)

might look into balancing the front end at 4.25ma+4.25+3.25+3.25 [=15ma total] although thats further down the to-do list vs figuring out a b+ starting point. which is starting to look like 286v after adjust the sim and predicting load outcomes.
 
first version of the test circuit which allowed for somewhere around 4w before overdrive distortion starts to set in. since the amp has never been powered up on the bench there were initial unknowns to evaluate i.e. does anything work. last time these amps received power was over a decade ago, current owners estimate.

Screenshot 2026-06-24 at 23-24-28 t1 - 485ac 215dc built v1.txt - Circuit Simulator.png
ripple noise not accurate due to the sim darlington (two transistors) vce needing to be adjusted to match measured dc voltage drop. the two 47k resistors feeding the base are actually 19.4k each. also the zener's are for combating cold surge over-voltage protection, possible thermal feedback for a protection circuit design.

the revised schematic so far still needs to be built and measured for accuracy. it will be targeting ~14ma for b1 circuit and 286v for b+ which should help the output wattage. another thing to adjust is boost potential vs b+ safety sag, to help the system hit around 18w dynamics comfortably which makes the out-txf just about immortal (hypothetical estimate) even with 130vac on the mains 24/7/365.

falstad data file attached. just load up on the sim site and hit the reset button, watch the numbers fly. side sliders are minimal and outdated vs current sim to be physically/electrically built.
 

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