Adding a Master Volume to the Traynor Bassmaster

(Usual tube amp disclaimer: These amplifiers contain voltages that can kill you if you do not understand what you're doing, so please understand what you're doing if you decide to attempt anything described on this page.)

Today, I want to discuss the addition of a master volume control to the Traynor YBA-1. When I first heard about amplifiers that lacked a master volume, I was actually pretty confused because I thought this meant the amplifier had a fixed gain and no volume control. Fortunately, this is not the case, and non-master volume amplifiers have only a single volume control, and it is located in the preamp stage of the amplifier. Below is the general signal flow from guitar to speaker in one of these setups:



So here you can control the volume of the output by just one switch. Some people at this point might be thinking: now why would you need TWO volume pots for a device that has a single output? This is a reasonable thought, since a single volume control can control the output. However, each tube has a slightly different "flavor" of potentially desirable distortion. With one knob, the guitar player can certainly create distortion but distortion from the tubes will only occur at a specific volume. Your amp will need to be quite loud to create nice juicy distortion, potentially annoying all of your neighbors/roommates/bandmates. Master volume helps with this problem by pulling out some of the power right before the signal reaches the output stage. This allows only the preamp (and Phase Inverter) stages of the amplifier to be run "all out," while some of the signal is drained to ground prior to entering the output stage, thereby allowing a quieter sound from the speaker yet maintaining the distortion. The distortion characteristics of so-called "preamp distortion" certainly differ from "all out" distortion where the output tubes and speakers are involved as well.

Common Master Volume Method

On a push-pull output amplifier, a typical place to put the Master Volume is right after the phase inverter (PI). The 6D30 I just built has the master volume there, and I will implement the MV in simulation below in the Push-Pull EL84 example from Duncan Amps using LTSpice (free simulation software). If you're thinking about learning circuits, I can't say enough times how nice it is to have a good simulator handy. I think if you can read schematics that the tutorial here should be enough to follow along with the master volume implementation I will show. I start with the Duncan Amps push-pull EL84. A picture of the full schematic is shown below:


I have circled the individual stages of the amplifier as a guide to understand what's going on. Working from left to right, the RED box indicates the input. The next green box is the preamp stage. In this case, it adds about 30 dB of gain. The next stage (yellow) is the power output stage, followed by what I consider the "actuator stage." The actuator stage is basically where the signal goes from being an electrical current to a sound. The top area consists of the power supply. Since the master volume resides at the power output stage, let's focus on that. It is cut out below:


Basically, the resistance between the two points on the left side will determine the amount of signal passed to the 6CA7 tubes. Right now, the resistance between A and B is infinite. If you choose a large valued resistor and make it variable, it can function as a master volume here. A 1 MegaOhm resistance seems pretty common as a value that functions as an approximation to infinity in this position. Let's see how this works:

The gain at 1 kHz BEFORE the addition of R14 is 53 dB, and with the addition of R14, and after the addition, it is still approximately 53 dB. Therefore we have successfully added something, that does not affect our circuit (at least as far as gain is concerned). Dialing R14 down to from 1000k to 10k (two decades), the gain at 1 kHz is reduced to 50 dB (-3 dB => half). This trend continues at 1k where the gain is reduced to 38 dB. Given the very wide range of values required for this effect to take place, it is worth mentioning that you would only want to use an audio-tapered potentiometer in this position for it's logarithmic response (smooth over wider range). So there you have it, one method for implementing master volume!

Another MV Method Implemented On Traynor YBA-1
On the Traynor YBA-1, I implemented a slightly different MV control. The output section looked a little weird to me, so I opted for tapping some of the voltage off to ground before it made it to the output section. If you are interested in YBA-1 schematics, I have posted them here. The schematic for this mod is shown below:

The area on the left is the output from the preamp/phase inverter stages, and the 1 Meg potentiometer is added in sequence. Physically achieving this on the stock amplifier was not a terrible challenge and required no drilling, since I had an extra hole on the chassis where I had removed the 'ground switch.' As far as I could tell, the two-pronged plug with a ground switch was basically just a device for sending 120VAC from your arm to whatever was grounded. People always say to convert 2-prong tube amps to 3-pronged with proper grounding. Well, this amplifier made me a believer. A photo of the modded amplifier below:

The bright yellow wire that is heat-shrinked to the dull yellow wire running from the front panel to the back shows where the mod is connected. Basically, it works pretty well, although it is a little annoying that the MV is at the back of the amp. Ah well, there is no room on the front regardless.

Recap
Okay, today we have gone through two methods of implementing master volume on tube amplifiers that lack this feature. As always, these new 'features' do not come for free. The first method mentioned adds a HP filter that changes with the volume level, and you might find a similar effect with the second method, depending on the tone circuit in the preceding stage. You might try experimenting around with these MV methods in LTSpice to choose the one you want to implement if you're thinking about it. Or just plug them in and see which one you like better. Another MV method I did not talk about but will probably implement on the Traynor at some point is one I've heard referred to as the Ken Fischer #3. From what I understand, this is a master volume that drains the gain from all stages of the amplifier at the same time via a "ganged" potentiometer. This is a pretty interesting idea that I believe would change the overdrive characteristics by a good bit.