Pulling 2 tubes in a PittBull 45 for lower output?

[Jason G]

Hi everyone,

I haven't found the answer to this question, although I've owned and known of plenty of amps out there with a quad of output tubes that this would work in... i.e. pulling the inner or outer pair of output tubes to reduce the max output level and allow the user to overdrive the amp just a bit easier at a given volume.

On a similar topic, I'd also like to go with some type of attenuator, but would like some opinions concerning the numerous makes/types that now inhabit the market. The Dr.Z Brake Lite looks nice and I've heard good comments about the Weber VST products. My only experience is with the Hot Plate and while it did the job and gave a "preamp/power-driven out" off the unit (big plus!), it did seem to squash the tone and feel rather rudely and seemed a bit of a compromise for what it gained?

I'd like to hear your comments.

take care,

Jason G in Amarillo, TX

[schwimbo]

You can pull 2 tubes in a Pittbull 45 (I pulled the inner 2) but need to do a couple of things to make the tubes run OK (without overheating, etc.). First, lower the speaker impedance selector by 1 click (if you are running into an 8 ohm speaker load, set the speaker impedance selector to 4 ohms), or you can run into a 16 ohm load keeping your selector switch at 8 ohms. This should be all you need to do for Class AB operation. However for Class A (cathode bias operation), read the next paragraph.

If you want to use the Class A (cathode bias) setting, you need to hook up a cathode resistance with a value twice that of what you would use if you were running all 4 output tubes. This will make sure that you have the same voltage drop across the cathode resistor(s) when you cut the cathode current flow in half (by using only 2 tubes). If you don't do this, the cathode voltage drop will be half of what it is when running 4 output tubes, and your tubes will run very hot. I first selected resistors that were double the resistance value of the cathode resistors that are used for 4 tube operation, and then hooked up a switch that lets me add an identical set of resistors in parallel so that if I do decide to run 2 tubes, I just use the first set of resistors, but if I want to run all 4 tubes, I switch in the second set of cathode resistors in parallel to get the resistance value back to what it was originally (for 4 tube operation).

[support]

In general, cutting the impedance in half to compensate for pulling 2 tubes is not a bad idea, but since it's only an approximation, you don't hurt anything by not doing this.

Don't change the cathode resistor value though. You will end up with a significant amount of crossover distortion.

Dave

[schwimbo]

Dave,

You only could introduce crossover distortion (assuming there was none present before) if you increased the cathode resistor value while still running 4 tubes, thus increasing the voltage drop over the cathode resistors, and making the tubes run colder. If you doubled the value of the cathode resistors only for 2 tube operation, then there would be the same voltage drop (1/2 cathode current x doubled cathode resistor value = same voltage drop over the cathode resistors). This is done in some amps that run in cathode bias when someone decides to "split the cathode resistor". You are probably familiar with this, but for those of you who aren't, it works like this: the original cathode resistor value (Rk) is multiplied by the number of output tubes (let's say 4 as in the case of the Pittbull 45), and a resistor of value 4Rk is attached between the cathode of each tube and ground. Then each tube delivers 1/4 of the total cathode current to a cathode resistor having 4 times the original cathode resistor's resistance, giving the same voltage drop - over each of the 4 cathode resistors (that are attached between the output tubes' cathode and ground) that was there originally when all 4 tubes' cathode currents were fed into 1 cathode resistor. This would let one run 4 tubes, 2 tubes, or even 1 tube without having to change anything, since each tube would have its own dedicated cathode resistor of the correct value. While my solution was different (in that I didn't attach a cathode resistor to the cathode of each tube), I followed the same principle, in that I grouped 2 tubes together to feed a cathode resistor that has twice the value of the original cathode resistor, but if I decide to use all 4 tubes, then I bring another identical cathode resistor "online" (and in parallel) to handle the doubled current. I've done this and it works fine (no crossover distortion at all).

[support]

Again you ignore effective plate impedance...which you can't do and claim to be correct at the same time. Perhaps you have found a critical flaw in ohms law that only you know about...

[schwimbo]

Dave, I'm not sure where I have "found a flaw in Ohm's Law". Please clarify that. I have a quote here from Randall Aiken's website about the subject:

"Checking the bias current in a cathode biased amplifier is easy, just measure the voltage across the cathode resistor and divide by the resistance value to obtain the cathode current. Note that if the output tubes share a common cathode resistor, you must divide the current reading by the number of tubes sharing the resistor.......You can also use individual cathode bias resistors on each tube. The value of the resistor will be double that of the common resistor if two tubes are used, or four times that of the common resistor if four tubes are used. Each resistor would also have to be bypassed with its own electrolytic bypass cap."

If you think Randall has found a flaw in Ohm's Law (that only we know about), let us know where this is. I don't see where plate impedances factor into this discussion about splitting the cathode resistor.

[schwimbo]

Follow up question for Dave:

If you recommend not doubling the cathode resistor value for 2 tube operation, are you saying that running 2 tubes in the Class A position would not overheat the tubes??? I've tested this (by accident several years ago), and the tubes did run way too hot, and VHT support at the time advised me that this was not a good operating state for the amp (running 2 tubes with the existing cathode resistor in the Class A mode). If you choose to set the switch to Class A, you had better not run the amp with just 2 tubes (or you risk burning them up), unless you double the cathode resistor value.

[Jason G]

Wow, thanks for the feedback! So I'll go ahead and try pulling the inner 2 tubes and select the 4ohm tap, while keeping it in Class "AB" power amp mode.

Regarding the cathode bias mode, I'd like to hear more about this if I can. I'm not enough the guru on such matters, but it would be just great to be able run the amp with a single pair of output tubes with the amp in Class A.

For schwimbo, do you currently own and play a PB45? I'm curious about your cathode resistor mods with switch. With the switch in place, are you able to "cut out" a pair of tubes from the circuit path with just the switch? I.E., you don't need to physically remove the "pulled" tubes to transition from 4-tube into 2-tube operation?

Also, is there any need to drop the output impedance "one click" in cathode bias class A using a single pair of tubes... or 2 "cut out" via switch?


And if anyone else can chime in(Stevie?) regarding the reworking of the cathode resistance for 2-tubes/class A operation... I'd sure be appreciative! And I imagine there are other PB45 owners who would like clarification on this.

thanks again and all the best,

Jason G in Amarillo, TX

[schwimbo]

Jason,

I do currently have a PB45 that has the modification to allow me to run in cathode bias with 2 tubes pulled. The switch (the way I wired it) doesn't cut out the 2 tubes (you have to pull them out physically) but it doubles the cathode resistance so that you are running them at around the same dissipation level as for 4 tubes with the stock cathode resistance. I also changed the stock value upwards a little because with the stock value, I was getting dissipation readings above tube manufacturers' recommended 12 watts idle dissipation per tube. I suppose I could have looked for EL84 tubes that had been selected for a lower cathode current rating, or made sure that the input AC line voltage was at 117 volts (it is high in my house at 123 volts), but given my situation, the larger cathode resistor value did help lower the cathode current and overall dissipation to the 12W level. I didn't bother to change the cathode bypass capacitor for 2 tube operation but I think that the effect of the capacitor is one of tone (which I found to be fine leaving it alone) rather than tube longevity.

I do still drop the speaker impedance selector by one click for 2 tube operation (from 8 to 4 ohms).