Tips for running AC tube heaters on a PCB

[TimNJ]

Hi all,

I'm trying to recreate Fender's 5F6-A "Bassman" circuit on a PCB. Want the classic sound, but want to see if I can do it with modern construction. I've seen tube amps that keep the tube heater supplies off board and are wired directly to the tube's pins via twisted-pair elevated off the board. The rationale behind this is to keep the board cool and to reduce differential-mode radiation. First, I do not think the board would get all that hot...takes a pretty hefty current to get a significant temp rise (I think). I was thinking about running the heaters as essentially a differential pair, perhaps one on top and one on the bottom. On top of that, try to keep it from the signal traces as much as possible...

Do you guys think this is reasonable? Any other considerations I may have missed? Thank you.

[floobydust]

The filament wiring radiates hum in two ways.
The electric field is 6.3V/2 or about 3VRMS. This is why the separate cable.
The magnetic field is due to AC current flow, for the output tubes 0.9A each or 1.8A there, and 0.3A for each 12A_7 tube. This is why the twisted-pair.

I would suggest keeping the filament traces as a tight pair on the PCB, and I would have a (grounded) guard-band alongside the filament traces between any sensitive high-impedance nodes like the plates or input tube's grid.

A PCB ground-pour is difficult with tube circuits because the impedances are high and high-voltage spacings, you get capacitive loading and leakage currents that the original point-point wiring never had.

Most important is the multiple single-point grounding technique Fender used. Study the layout carefully, every ground wire in a certain spot despite looking arbitrary.
High current ground (plate xfmer center tap, O/P tube cathodes, filter caps) is separate from low-level ground. This is very important on your PCB layout.

[oldway]

Use 6.3 Vdc instead of 6.3 Vac......

[TimNJ]

Thank you!!

For the guard traces, those would be earth-ground referenced? How would you terminate those? At a single point on the board? And then tie that point to chassis earth ground? I know sometimes having funky shaped, stub-like, traces can be problematic for RFI.

I was thinking about essentially doing point-to-point on the PCB, with two start grounds, one for high current (power amp section) and one for low current (pre-amp/filter network), as you suggest. I've never designed a PCB for high voltages. I am going to do quite a bit of reading up to see what's recommended..though if anyone here has anything to say about that, I would greatly appreciate it.

Thanks again.

[TimNJ]

Use 6.3 Vdc instead of 6.3 Vac......

This is a possibility, but I am concerned that the Fender replacement power transformer I intend to use will  not be able to handle it. Should be able to rectify and create 12.6/6.3V regulated from the heater winding. Would have to drop about 2.5V or so in the regulator, not sure if the transformer can handle the extra power.

I feel like that's kind of going backwards though...because the 60 year old design worked just fine on 6.3VAC!

[TimNJ]

Mistakenly made this reply.

[Someone]

Use 6.3 Vdc instead of 6.3 Vac......

This is a possibility, but I am concerned that the Fender replacement power transformer I intend to use will be able to handle it. Should be able to rectify and create 12.6/6.3V regulated from the heater winding. Would have to drop about 2.5V or so in the regulator, not sure if the transformer can handle the extra power.

I feel like that's kind of going backwards though...because the 60 year old design worked just fine on 6.3VAC!

Rectifiers, regulators, and additional capacitance were expensive so its a cost cutting measure to use AC on the heaters. But even with DC you should still maintain good grounding techniques to minimise any capacitive coupling.

[TimNJ]

Use 6.3 Vdc instead of 6.3 Vac......

This is a possibility, but I am concerned that the Fender replacement power transformer I intend to use will be able to handle it. Should be able to rectify and create 12.6/6.3V regulated from the heater winding. Would have to drop about 2.5V or so in the regulator, not sure if the transformer can handle the extra power.

I feel like that's kind of going backwards though...because the 60 year old design worked just fine on 6.3VAC!

Rectifiers, regulators, and additional capacitance were expensive so its a cost cutting measure to use AC on the heaters. But even with DC you should still maintain good grounding techniques to minimise any capacitive coupling.

Thanks. Right, I understand why they did it.

I  may try to see if I can get a transformer with a higher heater winding current rating and use that instead. Maybe like this: http://www.antekinc.com/as-2t350-200va-350v-transformer/

Only problem I foresee, and I haven't really looked much into it, is getting the -50V (or so) grid bias voltage. I don't know much about tubes, but I think(?) the grids are high impedance and voltage driven. Perhaps I would be fine deriving that negative voltage from one of the high voltage (350V) windings with a resistor divider and filter? They used a 50V winding and a silicon diode in the design from the 50s. Perhaps they used a separate winding because the breakdown voltages of silicon diodes were lower back then?

[vk6zgo]

Use 6.3 Vdc instead of 6.3 Vac......

This is a possibility, but I am concerned that the Fender replacement power transformer I intend to use will be able to handle it. Should be able to rectify and create 12.6/6.3V regulated from the heater winding. Would have to drop about 2.5V or so in the regulator, not sure if the transformer can handle the extra power.

I feel like that's kind of going backwards though...because the 60 year old design worked just fine on 6.3VAC!

Rectifiers, regulators, and additional capacitance were expensive so its a cost cutting measure to use AC on the heaters. But even with DC you should still maintain good grounding techniques to minimise any capacitive coupling.

Unless your tubes have bad heater/cathode leakage (which some secondhand ones may have), there is no need for dc heaters.
In this case, dc isn't a "cure-all", as leakage will still cause changes in tube bias with dc.

DC heaters for indirectly heated tubes were rare as hens teeth during the heyday of tube equipment, except in a few highly critical instrumentation applications.

Large Transmitting tubes had directly heated cathodes, & used ac with special balanced (centre tapped) filament transformers, & in later times dc filament supplies, but "Receiving" type indirectly heated tubes almost exclusively used plain old ac heater connections.

An exception was portable & other battery equipment, which mostly used directly heated tubes of the "1",& "3" prefix series like ID8, 1T4, 3V4, etc.

To someone who "cut their teeth" on the British "Mullard", & Oz RTV & H  "Playmaster" series, then went on to work on PMG " type 3" amplifiers, some of the techniques used by Fender are a bit weird, so I have a bit of trouble getting my head around them.

Re: Tubes used with PCBs---
I've seen a lot of these "cooked" from the early days of phenolic boards.
FR4 material is better, but it still can deteriorate over time.

The heat source isn't due to high resistance in the tracks, it is the heater element in the tube.
Heat travels out via the tube pins, & associated socket, eventually causing damage to the  solder connections.
Increased resistance of these joints causes voltage drops & associated heating, so the thing has a cumulative effect.

[calexanian]

There are a few proper ways to do this. Many companies elect to run the filaments as flying twisted wires above the board. It fits common convention but adds hand labor. Another way to achieve common mode rejection is to run the traces in parallel or on opposite sides of the board. This works reasonably well. A better solution is to do one of these in conjunction with raising the filament reference voltage at an elevated voltage. Say 20 or 30 volts. This reduces internal tube hum coupling. Most amp companies did not do this do to there not being a convenient relatively low voltage DC supply available. Cathode biased amplifiers occasionally would tie the filaments to the cathode therefore creating a pseudo supply, however this is only advised when using a transformer with a center tapped filament winding. Fender power transformers typically do not have a center tap on the filament winding and use a pair of 100 ohm resistors located near the pilot light to ground reference the filament supply. Also, commonly overlooked is the phase of the filament supply on each preamp tube can be reversed for minimum hum.

Good luck!

[TimNJ]

Use 6.3 Vdc instead of 6.3 Vac......

This is a possibility, but I am concerned that the Fender replacement power transformer I intend to use will be able to handle it. Should be able to rectify and create 12.6/6.3V regulated from the heater winding. Would have to drop about 2.5V or so in the regulator, not sure if the transformer can handle the extra power.

I feel like that's kind of going backwards though...because the 60 year old design worked just fine on 6.3VAC!

Rectifiers, regulators, and additional capacitance were expensive so its a cost cutting measure to use AC on the heaters. But even with DC you should still maintain good grounding techniques to minimise any capacitive coupling.

Unless your tubes have bad heater/cathode leakage (which some secondhand ones may have), there is no need for dc heaters.
In this case, dc isn't a "cure-all", as leakage will still cause changes in tube bias with dc.

DC heaters for indirectly heated tubes were rare as hens teeth during the heyday of tube equipment, except in a few highly critical instrumentation applications.

Large Transmitting tubes had directly heated cathodes, & used ac with special balanced (centre tapped) filament transformers, & in later times dc filament supplies, but "Receiving" type indirectly heated tubes almost exclusively used plain old ac heater connections.

An exception was portable & other battery equipment, which mostly used directly heated tubes of the "1",& "3" prefix series like ID8, 1T4, 3V4, etc.

To someone who "cut their teeth" on the British "Mullard", & Oz RTV & H  "Playmaster" series, then went on to work on PMG " type 3" amplifiers, some of the techniques used by Fender are a bit weird, so I have a bit of trouble getting my head around them.

Re: Tubes used with PCBs---
I've seen a lot of these "cooked" from the early days of phenolic boards.
FR4 material is better, but it still can deteriorate over time.

The heat source isn't due to high resistance in the tracks, it is the heater element in the tube.
Heat travels out via the tube pins, & associated socket, eventually causing damage to the  solder connections.
Increased resistance of these joints causes voltage drops & associated heating, so the thing has a cumulative effect.

Ahh okay, totally forgot that the heating element itself is basically directly coupled to the PCB. I suppose this could be a pretty big reason NOT to use a PCB though I still think I'm going to do it. Might go for 2oz copper.

Thank you.

[TimNJ]

There are a few proper ways to do this. Many companies elect to run the filaments as flying twisted wires above the board. It fits common convention but adds hand labor. Another way to achieve common mode rejection is to run the traces in parallel or on opposite sides of the board. This works reasonably well. A better solution is to do one of these in conjunction with raising the filament reference voltage at an elevated voltage. Say 20 or 30 volts. This reduces internal tube hum coupling. Most amp companies did not do this do to there not being a convenient relatively low voltage DC supply available. Cathode biased amplifiers occasionally would tie the filaments to the cathode therefore creating a pseudo supply, however this is only advised when using a transformer with a center tapped filament winding. Fender power transformers typically do not have a center tap on the filament winding and use a pair of 100 ohm resistors located near the pilot light to ground reference the filament supply. Also, commonly overlooked is the phase of the filament supply on each preamp tube can be reversed for minimum hum.

Good luck!

Thanks. Are you suggesting to tie the 6.3V center tap to 20 or 30V? What effect would that have on interference? Thank you.

[CatalinaWOW]

Don't know how much truth there was to it, but back in the day it was accepted knowledge that running tube filaments on DC reduced life due to electromigration of the filament material.   If necessary to run DC it was recommended to periodically reverse the polarity.

If you wanted to honor this lore today it would be easy to use set up the power to reverse each power cycle.  Could even be done old school with latching relays.

[xfs]

The heater is in most cases positive compared to the cathode, so it can draw electrons which you hear as humming voltage at 50Hz. The idea behind elevating is that it will still draw that current, but it will completely saturate, therefore 0Hz, ergo no hum.

In the attached picture, D is connected to some point on your high voltage, preferably at a later stage where it is filtered better. What you can also see is, the later preamp tubes are supplied by AC, while the first two are supplied with DC heater, since they are the most sensitive to hum. At later stages, the signal is well above the level of the hum. A regulator really isn't necessary, there is enough room within the tolerances of the heater voltage. Adjust the R for your nominal line voltage, any fluctuations will be fine.

[schmitt trigger]

How good is a tubed guitar amplifier without hum?  ;
That is part of their charm!

[floobydust]

An electric guitar itself is quite good at picking up hum.
In a recording studio, murder to get one super quiet. I spent a day trying to figure it out, ended up being stray flux from fluorescent light ballast in the floor, the ceiling on the floor below us 

Or the hum in the amplifier is out-of-phase with what the guitar is picking up. It all cancels out and things are so quiet.

The Fender mid sections without 100% bypassed cathodes would be the sensitive portions.
OP can try run them on batteries and see if there is benefit.

[klaff]

If you decide to do a DC supply for the heater, consider the inrush needs. Designing a DC power supply for an incandescent bulb is painful because the inrush current can be around 15x steady state.  I'm not sure what the ratio would be for a vacuum tube heater, probably less but still significant.

[kalel]

If you decide to do a DC supply for the heater, consider the inrush needs. Designing a DC power supply for an incandescent bulb is painful because the inrush current can be around 15x steady state.  I'm not sure what the ratio would be for a vacuum tube heater, probably less but still significant.

I'm not knowledgeable at all when it comes to power supply designs or tubes, but would it be possible in such a case to design a power supply that "ramps up" the output to limit the inrush current, or would this damage the incandescent bulb or vacuum tubes? I have to wonder that if it was beneficial and feasible, it would probably already be used in things.

[klaff]

One can do a soft-start approach, and googling for vacuum tube heater filament inrush finds a number of discussions. My point was that in the design, whether implementing  a soft-start or not, the supply will have to start against a (perhaps surprisingly) low impedance load, and if that isn't considered you can end up with a supply which won't ever get to the right operating point.

[oldway]

[floobydust]

Old tube gear does not care about inrush from the filaments. Everything is oversized, especially the power transformer which only has to tolerate inrush for a few seconds.
This is the 1950's when iron and copper were cheap.
Fender 8047 power transformer remake

[TimNJ]

Thank you for all of the replies! Taking into consideration all of your suggestions, I think I will try to just run AC on the board and see if I can get acceptable performance like that. If not, I can always respin the board. I may try to impose a DC offset on the heater supply as suggested.

I haven't done the circuit analysis on the 5F6-A schematic much, nor am I fluent in tube circuit design, but in general, what voltages do resistors/caps see in these circuits? Do I usually need 400V+ resistors?

Thanks again.

[vk6zgo]

If you decide to do a DC supply for the heater, consider the inrush needs. Designing a DC power supply for an incandescent bulb is painful because the inrush current can be around 15x steady state.  I'm not sure what the ratio would be for a vacuum tube heater, probably less but still significant.

I'm not knowledgeable at all when it comes to power supply designs or tubes, but would it be possible in such a case to design a power supply that "ramps up" the output to limit the inrush current, or would this damage the incandescent bulb or vacuum tubes? I have to wonder that if it was beneficial and feasible, it would probably already be used in things.

Unless you want to feed  filaments to something like this ,
http://n6jv.com/museum/6166a.html
you are unlikely to run into any problems!

For the amplifier circuit provided by the OP, the total filament current including that of all the tubes is around 1.8A.
Indirectly heated tube heaters operate at a dull red  level of heating, so the difference between hot & cold resistance is radically smaller than incandescent bulbs or classic "bright emitter" indirectly heated tubes.

The tubes used in Audio Amplifiers & much other electronic equipment were designated " Receiving Tubes" by the various manufacturers, & many millions of such devices were operated over many decades, without any problems with heater damage from inrush current.

[vk6zgo]

There are a few proper ways to do this. Many companies elect to run the filaments as flying twisted wires above the board. It fits common convention but adds hand labor. Another way to achieve common mode rejection is to run the traces in parallel or on opposite sides of the board. This works reasonably well. A better solution is to do one of these in conjunction with raising the filament reference voltage at an elevated voltage. Say 20 or 30 volts. This reduces internal tube hum coupling. Most amp companies did not do this do to there not being a convenient relatively low voltage DC supply available.

I cannot see any way this would be beneficial.
Back in the days when all equipment used tubes it was not done, not because of the lack of suitable voltages, but because it doesn't do anything!

Cathode biased amplifiers occasionally would tie the filaments to the cathode therefore creating a pseudo supply, however this is only advised when using a transformer with a center tapped filament winding.

This is a ridiculous idea---- the whole concept of indirectly heated cathodes is to electrically isolate the heater & cathode.
Centre tapped ac filament windings were certainly used with directly heated tubes in large Transmitting tubes,where the hum level is very much lower than the signal level ( filament voltages on these are usually similar to that of receiving tubes)

This technique was also tried in the very early attempts at Mains operated receivers, but was found to be ineffective.
When indirectly heated tubes became available, it was discarded.

Fender power transformers typically do not have a center tap on the filament winding and use a pair of 100 ohm resistors located near the pilot light to ground reference the filament supply. Also, commonly overlooked is the phase of the filament supply on each preamp tube can be reversed for minimum hum.

Good luck!

It would seem that guitar amplifier circles are the repositories of a lot of "voodoo" ideas, pretty much on a par with the "audiophools".

[vk6zgo]

The heater is in most cases positive compared to the cathode, so it can draw electrons

How?
In the case of a tube with a cathode resistor, the cathode is positive w.r.t  chassis ground, & the AC heater is either "floating" w.r.t ground, one side is grounded, or it is centre tapped to ground.
Even if the polarities were as you suggest, the heater & cathode are insulated from each other, the insulation is not a semiconductor junction, either, & the capacitance between the two elements will have a huge reactance at 50/60 Hz.

which you hear as humming voltage at 50Hz. The idea behind elevating is that it will still draw that current, but it will completely saturate, therefore 0Hz, ergo no hum.

In the attached picture, D is connected to some point on your high voltage, preferably at a later stage where it is filtered better. What you can also see is, the later preamp tubes are supplied by AC, while the first two are supplied with DC heater, since they are the most sensitive to hum. At later stages, the signal is well above the level of the hum. A regulator really isn't necessary, there is enough room within the tolerances of the heater voltage. Adjust the R for your nominal line voltage, any fluctuations will be fine.