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Fixed to Cathode Bias switching problems...
#1
So I built an amp with a fixed/cathode bias switch like that for the Standard project in TUT5. The only difference was that I used a relay in place of the switch and a cap across the cathode resistor, but I've had some problems.

While testing without tubes, I found that in cathode bias mode, 1/2 of whatever the bias voltage was set to showed up at pin 5 of the tubes...fixed bias was fine. Simulations confirmed this behavior, and I found that changing the 2x 30k1 resistors around Q10 to 331k (c-b) and 2.21k (b-sw) ensured the bias didn't leak through anymore. Is that normal?

The second thing is the relay keeps sticking after a couple of switches. I assume this is the contacts welding from the cap discharging when the relay shorts it. Oddly, when I unsolder it and activate it out of circuit with a battery, it no longer remains stuck and works again. This relay looks heftier than the PA66-CR ones, so I expected it to be fine. Is there a way to stop this from happening like soldering a cap across the contacts to take the initial discharge or something?

My solution for now was to just get rid of the cap, but the problem is testing with a sine wave was putting 10W into that resistor (470R 7W). The ones in TUT5 are all 5W, and I realize that a constant sine wave is not like actual playing, but is this a big enough resistor? I'm going to assume that with the cap, all we need to worry about is more or less the idle power...any AC will go through the cap?

Thank you all!
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#2
Hi

You have an early printing of TUT5 - the two 30k1's for each of Q10,11 were replaced by a single 30k1 from C to B on each BJT.Also note that the NPNs drawn earlier was a mistake and that these should be PNPs with C to V- and E to the bias pots. Everything works as it should.

Relay sticking can be caused by various things, including the contact welding you mentioned. What have you done on the coil side?

The coil voltage is zero when not energised, with both ends sitting at the supply voltage. Upon closing a switch for the switched end of the coil, the supply is across the coil and the relay contacts have switched. When you open the coil switch, there will be a very high transient voltage on the free end of the coil caused by inductive flyback, which for a 12V coil could easily be 80Vpk.

Normally a diode is placed across the coil to absorb this energy and clamp the voltage across the coil. However, even though this is the most common snubber method used, it is far from the best as it restricts how fast the contacts can move. Using a zener diode across the coil control element allows one to allow a specific transient height. For example, a 36V zener would allow a 24V transient given a 12V supply.

One can also use a cap or series-RC as a snubber across the control element, but these do not work as well as the zener.

Regarding Rk and Ck: Rk has to be sized for the average heat over the audio cycle. An unbypassed Rk will have transient heat higher than its rating but that is okay if the avrage is within the rating. Personally, i use Rk values that set the idle power to much less than the full rating of the tubes and TUT5 maintains that protocol except when one tube of the pair is pulled, then the remaining tube will run at its full rating. DO NOT TRY THIS IN OEM CATHODE BIASED AMPS UNLESS YOU INCREASE Rk AS IS DONE HERE !!

Ck is there to keep bias stable, and as TUT4 informs, is the first form of "fixed bias".

have fun
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#3
Hi Kevin, thank you for your reply! Yeah I have all the TUT series as well as the speaker book and TOT...love them all.

Ok, saving a component is a better solution than mine! Plus adding that resistor back that opens up an easy way to do mixed bias, huh?

On the coil I used a diode snubber, but I can easily try the zener...I believe I have some 33V. I assume since we are looking at the coil that it means that reduced speed of the contact could be the issue? It's pretty weird when it happens, you can hear the relay click but its far weaker than normal.

So for Rk, how do we determine what average is? What I tried to do was bring the amp to max clean power with a sine, then measured the voltage across Rk. In this case it was 70V and the resistor was 470 Ohm, which calculated as 10.5W. But this is where it gets foggy for me. Plus, that sucker got hot fast, which scared me a little.

That's really smart why you use the higher value for Rk. If one tube died, I'd imagine the other would shortly follow. I can't speak to the merits of warmer bias yet, so higher values are fine with me, lol.
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#4
Hi

In the bias circuit, we are not adding a resistor back unless you meant that you removed all four 30k1s? There has to be a base current path for the fixed-bias position and the R between C and B for each transistor does this. The bias control is split between tube pairs and each BJT switch has to be independent, so maybe for you this means adding back two Rs.

How you measured Rk during the full power test is correct and it is dissipating the power you calculated, so yes it is going to get hot fast and should be a higher wattage part. Maybe the TUT5 Rks should be 10W, but the original player this amp was designed for rarely used the cathode-bias mode and likely never full tilt since Power Scaling is on-board.

I am not actually much of a fan of cathode-biased power stages and use it only for a tone difference. I've used Rk values up to 1k for standard tubes and this certainly keeps the heat controlled for both the tube and Rk.

The unsnubbered coil allows maximum speed for the relay contacts to open and close but places high-voltage stress on the coil control element. The standard diode is a brute force damper and does not allow the fastest relay transitions. Too-slow open/close causes the welding effect if switched under load. The currents in the Standard are low, but it is enough to hold the contacts together.
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#5
So what I meant was, let's call Rc-b R1 and Rb-sw R2. The simulations helped me figure out that R1 needed to be 100x R2, so I went with R1 = 330k and R2 = 2.21k (what I had on hand). But it sounds like you kept R1 = 30k1 and eliminated R2, making it 0 ohms. So you save a part because 30k1 is surely 100x 0 ohms, lol. Those are things that are easy to overlook.

Then my mixed bias comment was that if you kept R2 30k1, it could be any value to suit really, and hooked up a switch to short it, you'd effectively have mixed bias provided you also switched an appropriate resistor in the cathode at the same time. R2 shorted = cathode bias, R2 in circuit = mixed.

If the amp was the same circuit in the earlier TUT5 like I have, then you likely wouldn't need a 10W part because you'd have mixed bias (if R1=R2), which would cut down on the power. I actually thought that's what you meant to do. Since the cap makes it a form of fixed bias, but removing the cap requires a larger part and generates a lot of heat, you could add a portion of the fixed bias voltage back. This would allow the cathode resistor to still bounce around like you'd get with it unbypassed, but you'd get the heat savings because of the fixed bias portion.

I can see it being good for some styles with a lot of expression in the fingers. This was my first experience with cathode bias...I really thought it shined with single ended settings.

Thank you again!
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#6
Hey

Maybe it's a coincidence that your Rc-b =330k and the screen voltage sampling resistor in the bis reg is 330k? The two are actually unrelated as in fact is the case for the Rs around the BJT bias switches. You have to think of the output of the bias reg as a voltage source and the BJTs simply as switches, tying that voltage source to the bias-set network, or isolating the voltage source from the bias-set network.

The Rs around Q9,10 are selected to allow sufficient base current that the BJTs can pull the bias pots as close to the bias reg output as possible.

There are lots of ways to have mixed bias. TUT4 shows swept fixed to cathode bias using RmX methods. Another approach is to have a variable Rk and a tracking bias reg with adjustable tracking ratio. You could also make things completely manual but safe by using much higher Rk values. Older STUDIO amps had such a feature controlled via the Fiixed-Cathode switch and a Heat switch, later combined on one 3-way switch as Tweed-Mix-Modern.
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