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.

Examination of a 6SN7 Preamp Circuit

Today, I will simulate a cathode-biased amplifier using 6SN7 tube model and compare the simulation to the data sheet. Using the data sheet acquired from the Tube Data Sheet Locater, we can see the schematic for a cathode-biased amplifier circuit and various values for resistances in the circuit. This example is derived from the excellent tube HOWTO at Boozhound Labs. A similar circuit analysis will be simulated here with a test for the accuracy of the onset of distortion in the 6SN7 tube in the simulation.

Input Capacitor

Viewing the circuit in the above chart, you can see Esig, which is the input signal for the circuit. There is initially a capacitor in the circuit for the purpose of coupling the signal to a prior stage or instrument. In case this has not been mentioned previously, the purpose of a capacitor in this position is two-fold:

1) Block the DC voltage from a prior stage

2) Pass the AC signal to the amplification stage

Input Resistor

So the signal travels from a source (such as a guitar or record player) through this capacitor. The next thing in the chain is the input resistor Rg1. The basic requirement for this resistor is that it be large enough to dominate the voltage divider formed by the source impedance. As you can see, we can choose any value in the 3^rd column for this resistor, ranging from .1 to 10 MOhms. Following the example on Boozhound, I am choosing 100k. This is a pretty typical value for a record or guitar amplifier. Now the voltage across Rg1 is our input signal, which exists at the grid of the 6SN7.

Load Resistor

The next decision we must make is based on what we will be driving with this tube. Again, following the Boozhound example, we will be driving the input to a subsequent stage of the circuit. In this case, we are driving a 240kOhm resistance. So far we have

Rg1 = 100k

Rs = 240k

Therefore we direct our attention to the only row where Rg1 and Rs have these values (row 2). Here you can see we have a few options as far as the plate voltage goes: 90V, 180V, and 300V. You can see by comparing the columns for Eo at each voltage that the plate voltage determines how much output the amplifier can drive. For instance, looking at the row we have chosen to work with based on the input and load resistance (again, row 2), if Ebb=90V, the maximum output from this amplification stage is 16 volts RMS before the onset of distortion. Increasing Ebb to 180V allows the tube to produce 33 Vrms before the onset of distortion. I have chosen 180V. After decisions for input resistance, output load, and plate voltage are made, you pretty much just need to match the values to the parts in the schematics and simulate. I have made this file for LTSpice and stored it here. A drawing of the file is here:

Accuracy of Simulation Compared to Data Sheet

According to the first table, we expect the amplifier to have a voltage gain of 15 with the onset of distortion occurring when the amplifier tries to produce a signal with an RMS value of 33V. Using an input signal of 1V for normalization, we can get the following plot to test the transfer function of the equation:

Reading this plot, you can see that the gain is approximately 24 dB. “Argh,” I hear you moan in response to having to compare dB to a simple multiplicative gain. Honestly, I completely agree with you—this is annoying. However, decibels are nice for describing amplification because numbers like 1500X as loud start sounding a little silly. And even though this stage is only 15X the original signal, it will be fed into another stage that will multiply that signal by more, so the overall gain gets out of hand pretty quick. Back to the question at hand of whether the simulation is accurate, we must answer whether 24 dB gain is the same as 15X the original signal. The equation for the output gain in dB is

# dB = 20 * log(Vout/Vin)

Solving for dB, using Vout/Vin = 15 yields 10^1.2, which is approximately 15.8. Therefore, the simulation is pretty true to the data sheet. While we’re at it, I think it would be instructive to discuss the onset of distortion. Distortion is an important topic for both Hi-Fi tube amplifiers and guitar amplifiers, since you generally want to avoid distortion in the former and sometimes you want to create it in the latter. Viewing the data sheet you can see that the amplifier will experience 5% harmonic distortion when trying to create an output with an RMS-voltage of 33V. This time, I will use the transient response capabilities of LTSpice to simulate the scenario where the amplification creates a 33V output signal. I will use a 1 kHz signal for the simulation. From the frequency response, you can see the 1 kHz signal will be amplified by about 22 dB (12.6X the original signal using the above equation). Using the gain and definition of RMS voltage, you can find that a voltage of 3.6V will cause the gain stage to try to drive approximately 33V RMS, thereby introducing 5% harmonic distortion. This is shown below.

You can see a small amount of asymmetry in the negative cycles of the waveform, but this looks pretty nice, so presumably 5% harmonic distortion, although probably not an acceptable parameter for audiophiles with good ears, is not visible to the eye at least. To test if we are near a threshold point, I am running the simulation again with a 6V input signal at 1 kHz (shown below):

Here you can see the characteristic onset of distortion in the negative cycle of the waveform. This is all pretty cool, if I do say so myself, since it seems the simulation and data sheet match fairly well.

Cathode Bypass Gain Switch

I will do an analysis a bit later regarding how this sounds, but for now, I would like to discuss a simple method of adding a gain switch to this preamp stage. This method of adding gain is very simple. All I am going to do is add a switch to the cathode capacitor (Capacitor C1 in the circuit). The tube’s amplification abilities are related to the voltage at the cathode. There is always an AC signal at the cathode related to the input signal due to the nature of the tube. The role of C1, called the cathode bypass capacitor, is to filter this signal so that the node at the cathode is mostly zero. Removal of this capacitor allows the AC signal to exist at the cathode, thereby creating fluctuations in the cathode voltage. These fluctuations result in a reduced gain. The frequency response of the amp without the capacitor is shown below:

Wow! We have reduced the gain from 24 dB to 19 dB. You will also note that this capacitor was acting as a HP filter when in the circuit, since the low frequency response is much more flat. Two things can be learned from this: 1) you can put a switch to connect or disconnect the capacitor C1 to create a +5 dB gain stage and 2) if you want to maintain a flat frequency response, you should be careful in choosing your cathode bypass capacitors, since they are HP filters.

I will wax philosophical for a moment here and would love any input from more experienced builders regarding the topic of cathode bypass. From the above analysis, it seems like this is one place where the goals of a guitar amplifier and Hi-Fi amp might be at odds. Generally, it seems like Hi-Fi amps would want to avoid bypass caps as much as possible to maintain signal fidelity, while guitar amplifiers might want to use the capacitors to achieve higher gain, earlier onset of preamp distortion, etc.. Any thoughts?

Okay amp lovers. Today we have learned

1) How to pick parts for building a cathode-biased preamplifier stage using the data sheet for a 6SN7 tube

2) How to simulate said circuit in LTSpice

3) Made comparisons between data sheet and simulation for amplification parameters and distortion onset

4) Discussed the use of cathode bypass capacitors as a gain switch

That is all for now. Now, go huff some solder fumes, grill some bratwurst, have a beer, or go biking. Personally, I am gonna do all four.