On this page:

1. The series/parallel layout of tube filaments in Heathkit transceivers (balancing filament lines with #44 and #47 pilot lamps).
2. The tubes in the HW-101: Originals vs possible replacements (replacements with similar aftermarket tubes).

← HP/PS-23 Power Supplies and Heathkit transceivers - CW operation & sidetone - Mods & Customization
(Number of wires actually needed in patch cords for Heathkit transceivers, sidetone options for the HW-101, service bulletins and customization).

This design is very common in Heathkit transceivers. Regarding pilot lamps, the HW-101 manual calls out #47 pilot lamps (.150 A, referenced in the replacement parts list as #44), but every HW-101 has #44 pilot lamps (.250 A). Service bulletin HW-101-20 refers to this. To make the total of 12.6 volts the HW-101 tubes are connected in series-parallel using two 6.3 lines in series as shown above. Some tube filaments can be connected to 12.6 volts: V16 (12AU7), V17-V19 (12AT7). The rest are 6.3 volts only: V1-V12-V5-V15 (6EA8), V2-V3-V4-V20 (6AU6), V6 (6CB6), V7 (6CL6), V8-V9 (6146), V10-V11 (6HS6), V13 (6BN8), and V14 (6GW8). Note above that V10-V11 are labeled 6AU6 because HW-100 has 6AU6 for V10-V11 instead 6HS6; this part of the schematic was never updated.

Filaments configuration: Tube designation and (filament Amp). 4 groups of 2 tubes in series at left, global group of tubes in series/parallel at right.

A to Ground	V2 (.3) & V16 (2 x .15)	V3 (.3) & V4 (.3)	V17 (.15)	V19 (.15)
A to B					V8 (1.25)	V15 (.45)	V10 (.45)	V7 (.65)	V12 (.45)	V5 (.45)	#47 + # ?
B to Ground					V9 (1.25)	V14 (.66)	V11 (.45)	V6 (.3)	V13 (.6)	V1 (.45)	V20 (.3)

The theory:
• Sum of the 4 'A to Ground' independent tube groups at upper left: 0.9 amps.
• Sum of the 6 'A to B' combined tubes (except pilot lamps) at middle right: 3.7 amps.
• Sum of the 7 'B to Ground' combined tubes at bottom right : 4.01 amps.
• Difference to match lines 'A to B', and 'B to Ground': .31 amps.
• And this is where the pilot lamps come into play. Pilot lamp #47 is .150 amp each, thus total is .3 amps to add to line 'A to B' = 4 amps.
• The total filament amperage draw 'A to B' would be 4 amps (4.01 amps for 'B to Ground') of the combined groups + 0.9 amps total of the independent group = 4.9 amps 'A to B' and 4.91 amps 'B to Ground'. That is, by design .01 amps unbalanced, which is negligible. In these conditions the voltage on each line should be the same (about 6.3 volts) , thus each 6146 would have same voltage and amperage across filaments.
 
The practice:
Table above shows pilot lamps loads the 'A to B' circuit, only. When balanced, if totally possible, each circuit 'A to B' and 'B to Ground' will have same voltage, and this means that both have the same amperage. The 'A to Ground' circuit does not count for this as it loads the entire filament line. Checking filament voltage in a HW-101 is very easy as the test points are the two terminals of the front pilot lamp and ground; terminal with white lead is line A, terminal with brown lead is line B (if they have been connected as indicated in the manual).

• To check 'A to Ground' (12.6 volts) = White and Ground.
• To check 'A to B' (6.3 volts) = White and Brown (the two terminals of the pilot lamps).
• To check 'B to Ground' (6.3 volts) = Brown and Ground.

If after 5 minutes of warm-up, the 'A to B' and 'B to Ground' voltages are fairly equal (difference is less than .1 volts), your HW-101 is fine regarding pilot lamps configuration and tube filaments consumption, it doesn't matter if there are #47 or #44 pilot lamps installed, you're done.
If not pull out the pilot lamps, power on, and measure voltages from 'A to Ground', 'A to B', and 'B to Ground'.

Printed circuits restrict freedom on replacements/mods turning technological devices into throwaway appliances. There will be a problem if a tube mounted on a printed circuit cannot be replaced by another of the same type and same pin architecture. In this case the affected printed traces must to be interrupted and re-circuited with external wires, which can cause issues, for this reason when working on PCBs we need to know tube socket equivalences, not tube replacement chances, this can only be done on chassis mounting. Therewith, as shown in the previous paragraph, since filaments are in series in 4 independent groups, and series/parallel in 1 two-line global group, when replacing one tube with another that has different filament amperage implies to replace associated tube/s and recalculate the group. It is important that you replace/correct LOAD on both sides (A/B lines), and/or take existing values into account to avoid unbalancing filament voltage. The original HW-101 tube 'line-up' is:

• Most of these tubes come from ordinary TV and radio circuitry; some of them are getting scarce.
• If originals are not available, most of these tubes are fully substitutable with similar ones, also coming from the same type of circuitry, and in circuit will do the job just as effectively. The problem that arises is pin matching, the alternative is to interrupt the traces to tube sockets and make jumper connections, but that is always best avoided. Another important point is the filament consumption of the new tube and its insertion in the correct place of the series/parallel system, that's why it is described above.
*Ic-eqv : In-circuit equivalent → to apply only to the particular circuit.

Bi-directional 6HS6 / 6AU6 replacement

Original 6HS6 Miniature tube at left, and possible/known equivalences at middle / right. (Tube graphics: De Muiderkring Tube Handbook, 1966) The 6AU6 to 6HS6(A) change was implemented in the HW-100 due complaints on sensitivity on 15 and 10 meters. The 6HS6 mod did not come from Heathkit engineers: A Ham who was employed at Heathkit, Jim Isham (W8TXX), did some tests with other tubes noting that a HW-100 with 6HS6 tubes worked better on sensitivity, then Heathkit checked it out and implemented 6HS6 in future production. That's why it's fun to see that currently there are some claims about go back to 6AU6 by the same reasons which it was originally included in HW-100. BTW, the receiving section of the HW-101 constitutes one of the best designs of a receiver integrated in a transceiver at that time, especially with the 6HS6 change to do the job of V10, which is the tube specifically involved in sensitivity. The performance of the HW-101's receiving section is similar to that of many of standalone receivers operating in T-R configuration. I suspect that 6HS6 gives higher gain not only because it has higher transconductance, but because its internal resistance better adapts to the real impedances of the existing coils (relatively low, VERY similar to tube-TV coils designed with impedance scaling techniques). When trying to do the same with higher impedance coils, what is gained with transconductance is lost with mismatch. The two 6HS6 can be replaced by two 6AU6, making this tubes a 'back to the future' experience because they were the original tubes in the main design of this transceiver (HW-100). 6AU6 has .3A filament, 6HS6 has .45A filament, each A / B line has one 6HS6 tube, when replacing one tube, the balance of both filament lines should be considered. It is possible to replace V10 only (which is on the 'A to B' line) if .150A of less is compensated (.450 6HS6 vs .300 6AU6) by replacing one pilot lamp or both with another/others of more amperage (#44). Now the two pilot lamps should draw .150A more than before to balance with the 'B to Ground' line. Returning 6AU6s to each line, both lines remain balanced, and total amperage relaxes .150A. Tryed with 2 new 6AU6 tubes, and I can confirm that the original change from 6AU6 to 6HS6 is correct, my 'Hot Water' warms the water more, it has better sensitivity on all bands. I have not reviewed in detail other considerations, but it is common that when the sensitivity increases other aspects, such as cross modulation, get worse, particularly in simplified receivers, like those of all the transceivers of that time. IMO to get the best of both worlds (sensitivity - cross modulation) is to combine 6HS6 (.450A) for V10 (RF Amplifier) and 6AU6 (.300A) for V11 (First Mixer), it works perfect, but filament balance needs to be readjusted by removing .150 amps from the 'A to B' line by pulling out a #47, or by sharing .250 only on two pilot lamps: .150 (#47) + .100 (#?). Remember that this amperage distribution is for my particular HW-101 (YMMV, read above). The solution in this case is to return to the #47 + #47 configuration = .300 amps, so only .050 amps will be misadjusted, which is acceptable. There are other options about replacing V10 and V11, questionable but possible. 6BA6 (EF93) is compatible with 6AU6, and can be temporarily installed replacing V10 or V11. The most important difference between those two is that 6BA6 is a tube designed specifically for AGC, in the rest it is like 6AU6. Combinations 6AU6 + 6BA6 and 6BA6 + 6AU6 can be made because those two types have the same filament amperage (.3), they all work OK, although the best would be 6AU6 as RF amplifier because it has a little more 'S' (transconductance). 6BA6 is less "hot" than 6AU6, it will work well on reception, but would not be suitable on emission. And for the same reasons 6HS6 can be replaced by 6HR6. It is not initially advisable to replace V2, V3 (6AU6) with other types since they are associated to the Xtal filters and both tubes are used on transmitting.

Some 6EA8 replacement options

Original 6EA8 Noval tube at left, and possible/known equivalences at middle / right. (Tube graphics: De Muiderkring Tube Handbook, 1966)

Don't know why, but 6GH8 (or 6GH8A) is often assumed to be a direct replacement for 6EA8, this is not correct, they are similar but not equal. 6EA8 has direct replacement (same specs), but with different pinout, in the 6EU8 tube (mouseOver 6EA8 graphic).

6EA8 is a two section tube that has a pentode section with a medium Ri which is better for a transmitting layout, so more attention should be paid to this replacement. There are two options at right: filament voltage and amperage are identical, the biggest differences are in the pentode section that both shows a reception-friendly layout (Ri). 6GH8 triode section is quite similar to that of the 6EA8, the biggest differences are in the pentode section (as in 6U8), but 6U8 triode section is identical to 6EA8, so it can be an alternative (Collins Radio used 6U8 on their transmitters).

First off, the tube replacement search process consists in analyzing parameters and possible behavior of the new tube in the involved circuit, but when using printed circuits the pin configuration must be taken into account seriously, this adds more difficulty to the selection process. When making these types of modifications, attention should be paid to design performance ratios and maximum construction ratios, actual applied voltages should never exceed the latter. When working above operating voltages, it is strongly recommended to increase cathode resistor values to provide safe bias for a "good tube health", and to work at a more correct point on the expanded load line. Many circuits already work preventively with increased cathode resistors values.

The tube that needs the least analysis to replace is V15 (tone oscillator). It is a simple AF oscillator (pentode) with a bias switched amplifier circuit at its output (triode), for such a simple job any tube of above to the right will work fine, others may need a higher value of cathode resistors or a decrease in B+ on the plates, many not even that. 6U8 (ECF82) is a great candidate because triode sections of 6EA8 and 6U8 are practically identical (I have made this change on my HW-101). BTW, a 6U8 has close replacement in 6AX8; and the triode section of the 6LM8 is equal to the triode section of the 6GH8; also, tweaking resistor values a 6BL8 could also serve to replace V15. The second simplest is V1 (speech amplifier), priority here is good microphone amplification and very good adaptation to the balanced modulator, therefore, it is important to have a triode section similar to the 6EA8 triode section, and a pentode section with performance. Referring to V12 (second receiver mixer and relay amplifier), the triode section must match the relay load. And finally V5 (first transmitter mixer), the pentode section must adapt perfectly to the circuit, the triode section does not matter as it is not used here, so this tube could be replaced by a Noval type pentode that have a medium Ri for example, but in this case the pin coincidence would be impossible, and here we have a good example that printed circuit boards restrict the ability of making mods.

To be continued... on the fly, with the rest of the HW-101's TV and radio tubes.

In order to work on a HW-101 it is essential to have the complete schematic on a large sheet, it is the only way to see how the stages interact. It is also highly recommended to have the manual.

HW-101 schematic, 3000 x 1400 px.

Usual equipment that may be required to check and adjust these rigs: A tube tester to check current condition of the tubes, especially those used on transmitting; a VTVM or DVM for testing voltages and resistors, a capacitor tester to check the electrolytics of the HP-23 power supply in order to evaluate a possible reform procedure (if these capacitors are of good manufacturing quality, have not been "punished" by start-ups, and have not dried, it is perfectly possible to reform them); a dummy load; a receiver or a digital frequency counter are also useful for checking output frequencies via proximity coupling; you do not need modern gear at all, the frequency counter is the exception.