valve tone control circuit build

Question

Here I have built the following schematic but have encountered some problems. The treble was crackling and the bass was dropping out. I have somewhat fixed the treble issue with grounding the pots but I still have an issue with them. I think what has happened the heat from the soldering iron has damaged them. I was very quick in soldering but I think it's just cheap potentiometers. I can't seem to find these value potentiometers dual gang linear 1M and 500k.

There is a 50 Hz hum present with the tone control circuit wired in my valve amp. My question is what is causing these problems (mainly the hum) and how do I resolve it? Also what would happen if I changed the bass and treble potentiometer form 1M/500k potentiometers to 100k/250k potentiometer would I loose the bass and treble adjust range or would it have no effect?

UPDATE

Here I found this tone control circuit on the internet. It looks like to me it would be better than the other circuit. As there is no DC on the potentiometers that caused the previous problem with my last circuit but can anyone tell me if I should go ahead with this one? is there any improvements I can make?

Answer 1

Let's start with hum. If your heater supply is rectified, then 50Hz hum usually gets inside your audio tract via capacitive coupling from the power lines (all the wiring in and outside of your house is basically a huge antenna). If not, see below.

Also, a TV sync signal somehow detected inside your amp can present itself as a slightly "phasing" hum. It is not immediately obvious what comes before the preamp in your schematic, but the lack of any series resistance with the grids usually invites radio interference. The 100Hz hum usually comes from your power supply via a number of common means:

1. Insufficient ripple filtering. Can be mitigated by adding a choke (if not present) and/or enlarging filtering capacitors.
2. Magnetic interference directly from the power tranny. Some power transformers operate near their core saturation intuctance levels and therefore can in some cases (not ideal sine in the mains supply, etc) have heightened inductive leakage which can affect sensitive low-voltage circuits. This can usually be hinted at by a hot transformer and distorted sine waveforms on its secondaries.
3. Heater supply. If your heaters are not rectified, you can experience 50Hz leakage onto the cathodes. This can be diagnosed by adding a switch to the heater supply and switching it off while the amplifier is operating. If the heater supply is the culprit, the hum should go away immediately. Rectified heater supply can also induce this kind of hum, but on 100Hz. If this is the case, you can consider elevating your heaters by a DC bias and therefore saturate the leakage current so it does not fluctuate anymore.
4. Grounding errors. Ground loops, insufficient grounding of high-current parts of the device etc. can cause parasitic voltage drops inside sensitive parts of the tract. There is usually no universal solutions other than following grounding rules (see "star grounding" and "bus grounding") I usually use a mix of the two.
5. Lead dress. Having pulsating current loops inside your device near sensitive circuits can couple hum into control wires. Usually diagnosed by identifying potential offending conductors and physically fudging them about listening to changes in hum. If this is the case - you can permanently move those away from sensitive circuits, twist loops in pairs etc.

Crackling in the pots can be a sign of DC voltage present on them. I'd carefully measure if there is any DC on any of the leafs which can indicate a faulty blocking cap. If this is not the case, I'd assume faulty pots as well.

As for the pot resistance. It is hard for me to judge the effect, so I'll give you these network analysis plots - the first one is plotted using the original values, the second one uses 100k for treble and 250k for bass. I'll leave it up to you to decide if this is appropriate. Also keep in mind that you will load down the previous stage more if you use smaller values for the pots.

Cheers.

Answer 2

The hum is due to poor electrical hygiene.

A little of the power line signal is getting onto your audio signal somehow. There are many ways it can do this, so we can only guess at best. The power transformer needs to be far enough away from the sensitive input parts. The 250 V power to the first stage or two should be properly filtered. The chassis should be properly grounded, and grounding carefully considered thru the whole design. Most likely, the hum is getting onto the signal in multiple little ways.

I see that there is also no global feedback around this circuit. The first stage is just a open loop common cathode amplifier. Whatever external noise it picks up gets irreversibly added to your signal. Note that the first stage is also very susceptible to noise on the 250 V power supply.

This is not a great circuit to begin with.

Answer 3

This is not a full answer, since Messers Maxwell and Lathrop have already analyzed in a exhaustive way the questions in the OP. However, there is a point, recalled also in the comments to the answer of Mr. Lathrop, which I'd like to precise: the balancing of heater supply respect to the signal ground.

Balancing the heather AC voltage respect to the signal ground is very important in low noise tube circuits since, if obtained, means that the hum feedthrough from the heater to the cathode is minimized, ideally nulled. Referencing to the schematic below, where

• \$C_{hk}\$ is the (distributed) heater-cathode capacitance,
• \$V_{\mathrm{Fed}_{hk}}\$ is the heater-cathode feedthrough voltage,
• \$V_{h01}\$ and \$V_{h02}\$ are the voltages of heater wires respect to the signal ground,

the optimal balancing condition is not $$ V_{h01}=V_{h02},\tag{1}\label{1} $$ but is $$ V_{\mathrm{Fed}_{hk}}\approx 0,\tag{2}\label{2} $$ which may or may not be implied by \eqref{1}, depending on the physical structure of the heater and on its coupling with the cathode and the surrounding (electrical) environment.

^{simulate this circuit – Schematic created using CircuitLab}

As far I remember, balancing was not done by using a centre tap winding in the power supply transformer, nor by using two fixed resistors connected between the wires and the signal ground, since those design choices perhaps can imply \eqref{1}, not necessarily \eqref{2}. A design choice I've seen very often in such circuits is the following one

^{simulate this circuit}

By moving the centre tap of the heater balancing potentiometer \$R_{h_\mathrm{BAL}}\$ (\$10\mathrm{k\Omega}\$ is a typical value, and perhaps a better name would be "noise nulling potentiometer") you can measure or identify "by hear" the condition \eqref{2}.

Answer 4

I built this circuit (the first schematic) into the front end of a Williamson based amplifier design I built, and it performs absolutely superbly.

Just the usual things, well decoupled DC power supply, twist filament supply wires, run at right angles to signal path wires, components. Ground filament supply with 2 100 ohm 1 watt resistors across it, centre of resistors grounded close to input end of amp.

Try star connecting grounds of components back to a common point, near input end, to cut down earth loop hum pick up.

Plenty on web sites about construction requirements in this regard.