How to Bias a Fender Bassman (AB165)
The 6L6 amplifier I chose is a 1969 Fender Bassman head (AB165). I rewired the gawd-awful bias balance circuit into a true bias adjust circuit. I did not change any resistor value in the bias voltage divider circuit, which may have been a mistake. We’ll see why later. The rest of the amplifier was ‘stock’. A tested set of NOS GE 12AX7’s and Westinghouse 6L6’s were installed first. I then swapped the output tubes for a pair of Svetlana 6L6’s. Are we ready? Let’s go. An excellent website to check how your bias currents ‘measure up’ (pun intended) is over at Weber VST. You can check out the ‘Bias Calculator’ by CLICKING HERE. I use the numbers obtained here for reference only, and to see if I have made a gross error in judgment. Numbers obtained from this website will be quoted below, again as a reference only. I am taking a guess that Mr. Weber assumes a 25-watt plate dissipation for the 6L6GC. There are many bias ‘calculators’ available ‘on line’ for you to investigate. For the curious, or for those that wish to compare the parameters selected for the different calculators, you may visit the following alternatives.
- Tube Amp Bias Calculator. I haven’t checked out the other pages to this web site, but so far it looks interesting.
- Dreamtone has a lot of interesting pages to their web site, but this link is strictly for the bias calculator.
- Duncan’s Amp Pages has a plethora of information available; a tube data base, tone stack ‘calculators’, and this ‘anode load calculator’.
If anyone knows of any other similar web sources, a ‘heads up’ my way would be most appreciated. Now, for those that insist on measuring plate current (or what you believe to be plate current), please consider the following:
WARNING! For those of you who insist on measuring plate current, be advised of the following. Measuring by way of the ‘transformer shunt method’, the indicated plate current (or what is purported to be the plate current) will be lower than if you insert an ammeter in series with the plate connection, or use a 1-ohm resistor in the cathode of the output tubes. In one experimental setup, the transformer-shunt method indicated 33mA of idle plate current. Inserting an ammeter in series with the plate lead yielded a reading of 36mA. Measuring the voltage drop across a 1-ohm resistor in the cathode gave a reading of 31.5mVDC, translating to a current of 31.5mA through that particular tube. Carefully measuring the resistance value of the purported 1-ohm resistor, I ‘discovered’ the value was slightly higher than 1-ohm. Hopefully, you see the pitfalls with this method which is highly touted as being the only ‘accurate’ method.
There is no one correct setting. There are only limits of not less than and not more than. I would say not less than 10mA and not more than 50mA in most cases. Any setting that gives you the tone you like within those parameters is correct.
Gerald Weber, ‘A Desktop Reference of Hip Vintage Guitar Amps’
I set up the Bassman head with NOS Westinghouse 6L6GC’s first, and applied the test signal. The Volume and tone controls were all set at ‘5’, and the Volume control was reduced until the waveform showed no ‘clipping’. The bias was set as ‘high’ as it would go, -55.6VDC, and displayed a cross-over notch (although not always as severe as I had hoped for on each oscilloscope). This is seen below, with an RCA WA-504A signal generator feeding a Heathkit 0-10 oscilloscope. I did not change any capacitors in this oscilloscope after acquiring it, but I did check all tubes, and changed a few that tested poorly. I mentioned before how I am ‘against’ using such oscilloscopes as your primary unit, and we will now ‘see’ the many pitfalls. However, I will go through this setup in the interest of science, and also knowing this is very likely an oscilloscope close in age and ‘quality’ to what Leo may have used. I still get a ‘kick’ out of servicing tube guitar amplifiers in a similar fashion to how the ‘old-timers’ may have done it; with ‘vintage’ test equipment. Back to the oscilloscope on hand; note that the trace is very ‘readable’, and appears fairly sharp. The measured idle plate current was 19mA. Right from ‘the start’ this number falls ‘between 10mA and 40mA’, so according to many Internet ‘gurus’, nothing further need be done. But that would make for a boring ‘Lesson’, so let’s have a little fun while we have all this test equipment at our disposal.
The Model 0-10 comes from a long line of 4.5Meg bandwidth oscilloscopes available from Heathkit. There is also a Model 0-11 and a Model 0-12, all similar in appearance and ‘quality’. I tried to get a Model 0-12 working ‘right’ for this experiment, but at present we will have to utilize the Model 0-10. What you may notice is that the sawtooth generator is not perfectly linear. That can be corrected to a certain degree, but doing so is not high on my list of things to accomplish on a lazy Sunday afternoon. Ditto for the astigmatism problem. The ‘better’ oscilloscopes utilized push-pull drivers that reduced the nonlinearities of the sawtooth to a bare minimum. You may also see that the waveform just barely shows a hint of the cross-over notch. It does look a little more extreme ‘live’. However, I forge ahead, and adjust the vertical gain, along with the amplifier controls, until I get a waveform that fills the CRT, and shows even less of a cross-over notch. I get the waveform seen below.
The waveform looks surprisingly ‘decent’, so I take some measurements. The idle plate current has now increased to 34.8mA with a bias voltage of -49.7VDC. The measured Plate voltage was 433VDC, and a trip to the Weber Bias Calculator suggest an idle Plate current of 48.4mA, so I am barely better than 50% of maximum plate dissipation. Visiting the Tube Amp Bias Calculator, the numbers are identical, so what do I do? I measure the output wattage into my dummy load, and find out I am pumping out approximately 35-watts. The tone is ‘OK’, and the amplifier surprisingly sounds like a Bassman. Keep in mind many amplifiers left the Fender factory biased in a similar fashion, and seemed to have done well in the real world. I know for a fact I have worked on more than one ‘brown’ Super amplifier, ‘blonde’ Bassman head, and other Fender amplifiers with no adjustable bias. These amplifiers often came to me with original tubes, and sticking in NOS ‘black-plate’ 6L6’s revealed that the idle Plate current was indeed this low. Do you remember back at Biased Opinions we ‘heard’ someone mention that his research indicated Leo biased his 6L6 amplifiers to 50-55% of maximum plate dissipation? This would show how that was possibly achieved. However, I think I may be able to do ‘better’, so I move along to another oscilloscope. This should help me set up the amplifier for the ‘modern’ guitar player who will sacrifice a little tube life for a lot more tone! This is what experimenting is all about. Next I set up a Telequipment D61a oscilloscope, and stay with the RCA WA-504A signal generator. I adjust the oscilloscope to give a fairly ‘large’ reading, yet still within the CRT display area, so that I could clearly see the ‘peaks’ of the waveform. Simple math will tell me that I will be viewing a signal measuring less than 40VAC peak-to-peak, so the controls were set accordingly. Unlike the Heathkit oscilloscope, more ‘modern’ oscilloscope have a calibrated graticule, making time and voltage measurements possible. The Telequipment D61a is a good, basic oscilloscope. It does has a very limited bandwidth of 10Meg, really making it useless for anything but audio work. Therefore, you shouldn’t be paying outrageous prices for this unit. For my purposes, however, it does an excellent job. The Bassman bias voltage is set as high as possible again; -55.6VDC. The waveform obtained is seen below.
I take notice that the cross-over notch is more apparent, and that the tubes do not appear to be well matched. This is evident by observing that the cross-over is not ‘dead-center’ of the wave form. Forging ahead, the bias voltage was reduced (less negative), until the cross-over notch just seemed to disappear. This occurred at a bias voltage of -46.3VDC. The oscilloscope trace is seen below. Again, it should not be too difficult to determine exactly when the notch just disappears with a good quality oscilloscope. Does the 10Meg Telequipment meet this requirement? Let’s find out. Keep in mind that the waveform will appear to increase in size as the bias voltage is reduced (less negative). This is normal, and the oscilloscope settings will have to be adjusted if you want the entire waveform to fill the CRT.
The measured plate voltage was 425VDC, and the measured plate current was 39mA. By visiting the Weber Bias Calculator, and the Tube Amp Bias Calculator, it was advised to set my bias to yield 49.4mA of idle plate current. Rather than the suggested 70% of maximum plate dissipation, I was sitting at approximately 55%. The measured output into my dummy load was just under 40-watts, or as much as I had expected. I left the amplifier idling while I enjoyed a coffee with a visiting friend, and checked everything once again. Perhaps it was the NOS Westinghouse tubes, but the idle plate current hadn’t changed much. I then played the amplifier for about a half of an hour, into the stock 2X12″ cabinet. The amplifier sounded ‘normal’, so I rationalized I did a good job biasing the output stage. I rechecked the bias voltage and oscilloscope waveform. It was as I had left it a half-hour ago. Now, the plate current when strictly using the oscilloscope display was admittedly a little low when compared to ‘guru approved’ numbers, but the amplifier still sounded quite ‘normal’. Many Bassman heads left the factory biased either ‘hot’ or ‘cold’, but this is all relative to today’s accepted ‘norms’. I have modified many Bassman heads from the awful bias ‘balance’ to a more conventional bias circuit, just to get the idle current to fall within a range more acceptable to me. What bias range suits you is another matter, and won’t be discussed further. I have a hunch the bias circuit will have to be tweaked for my tastes, and perhaps that even this Telequipment oscilloscope is misleading me. We’ll leave the bias circuit alone for now, but let’s switch oscilloscopes again, shall we?
The oscilloscope is an instrument which can provide a host of misleading indications if due recognition is not given to its limitations…..
Servicing and Calibrating Test Equipment by Milton Kiver
I switched over to using a Leader LB0-508 oscilloscope. This is a definite ‘step up’ for oscilloscopes, compared to the previous Telequipment D61a. The Leader LB0-508 is a well made, dual-trace 20Meg unit, and has a trace that seems a little sharper and clearer. Below is the same Bassman head, using the same output tubes, and a maximum measured bias voltage of -55.6VDC. The tone control settings were not altered in any way.
The trace shows a remarkable similarity to the waveform seen when utilizing the Telequipment oscilloscope. This tells me the quality should be similar between these two oscilloscopes. I adjust the bias again until the crossover notch just disappears, and I get the following waveform.
Now things get a little interesting. I think about whether or not the signal generator is producing a perfect waveform (an unlikely source of my ‘trouble’), or if the 6L6’s aren’t really very matched (a more likely cause). Other possibilities include the oscilloscope calibration (unlikely), or the probe (quite possibly). Oscilloscope probes do often need to be compensated, and there is a slight chance the probe I used was set up ‘right’ for a 10Meg oscilloscope, but not for a 20Meg oscilloscope. However, when compensating any probe, a high-frequency square wave is used; I did not believe a 1kHz sine wave could possibly make that much difference. Many technicians will also probably have test equipment that is not in 100% operating condition. That will be an underlying point to this whole exercise, and the test equipment will be used exactly as I bought it at the local Ham Radio Swap Meet. Anyway, the measured plate voltage is 420VDC, which is a little lower than the previous example. The bias is at -43.4VDC, and plate current is now measured at 43.8mA, which at first appears to be closer to the target suggested by the Weber Bias Calculator and the Tube Amp Bias Calculator. Remember, however, that our plate voltage decreased a little. A return trip to the Weber web site suggests we now aim for an even 50mA. Now, using a ‘better’ oscilloscope, we have biased this amplifier to approximately 62% of maximum plate dissipation, rather than the accepted ‘norm’ of 70%. It cannot be overstated; using a ‘good’ oscilloscope did not bias the output tubes ‘extremely’ cold, nor did it cause huge cracks to appear in the Earth’s surface and giant boulders to fall from the sky. I lived to convey the results to you now, and I suspect I will be alive again tomorrow. Again, leaving the amplifier idle for a refill of my coffee, and another half-hour playing test, changed nothing. And, since even the most brazen ‘guru’ will state the idle plate current can be anywhere between 10mA and 40mA, how is what I am doing ‘wrong’? Using an oscilloscope, I can also check for phase inverter and/or output transformer symmetry while I am at it.
There are only two things to worry about in biasing a guitar amp. The first… is not to set the bias current so high as to exceed plate dissipation at any portion of the tubes operation… The second is to not set the bias current so low as to sound bad…. There is no single ‘correct’ bias point.
Randall Aiken, aikenamps.com
I switched over to a brand new set of prematched Svetlana 6L6’s, purchased through a reputable tube vendor. Do note that the waveforms look a lot ‘better’, making me believe the Westinghouse 6L6’s are not as matched as my tube testers had suggested. I even include my ‘mistakes’ here, all in the name of education. You can thank me later.
The measured plate voltage was again 420VDC, and the measured idle plate current was 20.2mA when biased at the extreme negative value (as seen above left), and went up to 41.4mA with a bias voltage of -46.2VDC (as seen above right). According to many ‘gurus’, any setting here would have been ‘correct’. The Weber Bias Calculator and Tube Amp Bias Calculator did suggest 50mA, so we are barely at 58% of rated maximum plate dissipation. I chose to play the amplifier ‘as is’ (41.4mA), and it sounded just fine. Decreasing the bias voltage (less negative) did change the tone somewhat; at higher idle currents the amplifier could ‘drive’ easier, and this is probably what most modern gurus are after. But there was absolutely nothing ‘wrong’ with the tone of the amplifier at 41.4mA of idle plate current, regardless of what the Weber Bias Calculator suggests. How educated people derived at the mantra of ‘70%’ is still a mystery to me. How they calculate the maximum plate dissipation is yet another mystery. Another ‘step up’ in oscilloscopes is the Elenco S-1345. This is a 40MHZ oscilloscope, and even the crossover notch seems just a little sharper and clearer. Below is the waveform seen, with the same Svetlana 6L6’s, and the bias set at -55.6VDC. Idle plate current is measured again at 20.2mA.
Adjusting the bias voltage, we achieve the following waveform. Again, the amplifier controls, as well as the vertical gain of the oscilloscope, are also carefully adjusted. This keeps the waveform filling a good portion of the CRT.
The bias voltage is measured at -42VDC, and the plate current is measured at 44.3mA. With a B+ of 420VDC, this is roughly 62% of the maximum idle plate dissipation according to Mr. Weber and the Tube Amp Bias Calculator. Who do I believe? Regardless of who you believe, if you have been paying attention, the ‘better’ oscilloscopes do seem to bias this amplifier a little ‘hotter’ than the oscilloscope with a lower bandwidth and lower CRT accelerating voltage. Coincidence? Absolutely not. But do keep in mind that Leo possibly owned a lowly Heathkit oscilloscope, OK? Next up I decided to give a Tektronix 453 a whirl at biasing up my Bassman. Of course, Tektronix is the ‘Cadillac’ of the oscilloscope world, and as such they command top dollar on eBay. I was lucky enough to procure this unit locally, and certainly did not pay eBay prices. These are not toys, and a steep learning curve is required in order to get the most out of any Tektronix oscilloscope. However, I will certainly not be using this oscilloscope to its full capabilities; I will just be biasing up my Bassman. As a side note, this particular unit was available with Nuvistor tubes in the vertical amplifiers and in the triggering circuit, or with FET’s in their place. My ‘new’ unit has the FET’s, thankfully. The Tektronix 453 is a 50Meg oscilloscope, and is quite suitable for most technicians. Check out the waveforms below.
The bias is again at -55.6VDC, and the idle Plate current is measured at 20.2mA for this round of testing. Carefully adjusting the bias, volume and tone controls, I achieve the following waveform.
The bias is measured at -42.2VDC, in close agreement with the Elenco oscilloscope. So it came as no surprise that the Plate voltage is measured at 422VDC, while idle Plate current was again measured at just over 44mA. I could let the amplifier ‘run’ for a spell, and double check the voltages and current. But I decided instead to try one more oscilloscope. The Hitatchi V1065A is probably my ‘best’ oscilloscope in terms of features and bandwidth. With a 100Meg bandwidth, it is suitable for just about everybody except microwave engineers. The oscilloscope features digital displays of the parameters to the waveform being measured, such as time base, voltage, etc. None of this is really important to the tube guitar amplifier technician, but we should know that having these features means this is a ‘serious’ oscilloscope. So how does the waveform look? See below.
Hitatchi V1065A is a serious oscilloscope. Don’t laugh! I mean it!
The crossover notch isn’t as sharp as I’d hoped for, but you can still see that the bias is not set ‘right’. Let’s adjust the controls, and see what we can come up with.
OK! I take lousy pictures, but you should get the idea. You may also note that the vertical amplifier display shows that I have altered the setting. This is as I mentioned before; you want to fill the screen with your waveform, and adjusting oscilloscope settings will be necessary. What did I end up with? A bias voltage of -41VDC, a Plate voltage of 419, and an idle Plate current of 44.8mA for my efforts.
The obvious problem is when has (the crossover notch) ‘just disappeared’? Most folks do just a bit more than ‘just disappeared’ and get their (output tubes) too hot causing shortened tube life and overheating. Not very accurate or repeatable.
R.G. Keen, geofex.com
What, if anything, can be concluded from all of this? I have a few points to make, and you may come away from this ‘Lesson’ with a few of your own observations. This is all perfectly acceptable, and hopefully this will lead you toward doing your own experimenting. Most 6L6 amplifiers biased strictly by the crossover notch method will not approach the ‘70%’ bogey value of maximum idle plate dissipation, until you start using extremely ‘high-end’ oscilloscopes. Accept this fact, and your life will be a lot less stressful. You should be aware of exactly when the crossover notch disappears. Giving the bias adjustment ‘a little extra’ (just for ‘insurance’) is not a good idea, unless you carefully measure your plate current, and are assured of not exceeding the maximum plate dissipation. The control settings on the amplifier made a tremendous difference in how accurate the bias adjustment can be. It cannot be over-emphasized; we must set the controls for a symmetrical waveform, and maximum unclipped output. The accuracy is also affected by the meter you use, and the method you use to measure idle plate current. If you are using 1-ohm resistors in the cathode, the 1-ohm resistors have to be matched, and be exactly 1-ohm. I opened up the plate connection, to avoid the screen current and insure accuracy. Measuring voltages with a fancy Fluke DMM gave ‘different’ results when compared to voltages measured with a ‘garden variety’ DMM purchased from my local Radio Shack. Therefore, your measurements will absolutely vary from mine. Although the bias voltage is reduced (less negative) until the notch ‘just’ goes away, it returns ‘softly’ when the waveform is seen to start clipping from increasing the Volume control. Also, you can ‘tweak’ the bias voltage, by decreasing it slightly after the notch has just disappeared; beyond a point the waveform will not increase in size, and the top half will begin to flatten out. This is a good time to stop, and increase the bias voltage slightly. Remember to make sure your output tubes plates are not glowing a nice cherry-red, or that you have exceeded maximum plate dissipation. This is how you will learn exactly what you are doing, and the possible consequences. One critic argues that the negative feedback in the amplifier affects the bias adjustment when biasing with an oscilloscope, and removing the feedback loop negates the bias adjustment. I suppose if you cannot dazzle me with technically brilliant arguments, the next best thing is to baffle me with bullshit. I haven’t met a Bassman head yet having a feedback loop that liked to go AWOL when I least expected it, negating my bias adjustment. Oscilloscopes with a lower bandwidth and/or accelerating voltages yielded a trace which appeared ‘fuzzy’, making it difficult to distinguish exactly when the crossover just disappeared. As a theory, in many cases the crossover may not have completely disappeared, resulting in an amplifier biased on the cold side. Truth or fiction? The oscilloscopes with a good, clear trace showed the crossover notch disappearing more accurately, and resulted in an amplifier biased much ‘better’. If you learn to understand the waveform presented to you from your old Heathkit oscilloscope, you can ‘compensate’ for the fuzzy waveform, and make sure the crossover notch has disappeared. This is a criticism to the oscilloscope ‘method’, and I cannot argue that criticism. However, until you own a ‘good’ oscilloscope, you will simply have to learn to make compromises and compensations. Using your ears to fine tune the bias adjustment is another way to make sure you have it ‘right’. In the end, you will almost always have an idle plate current that falls somewhere between 10mA and 40mA, so why criticize how I got my highly accurate number? Also, I have yet to figure out where this ‘magic number’ of 70% of maximum plate dissipation comes from. The ‘dummy load’ you use will only affect your work if you are ‘sweeping’ the audio range, and plotting frequency response. A ‘better’ mousetrap involves a ‘load’ designed to emulate the speakers frequency response and varying impedances. Do you really need this kind of accuracy? Not really, but if your are the curious sort, investigate this circuit by CLICKING HERE. An excellent ‘article’ on this very topic can be found by CLICKING HERE. It is a very brief missive, but still makes some very good points. Recommended reading, to be sure. The whole point of this exercise was to show you that using an oscilloscope to bias your amplifier is not as bad as many gurus will have you believe. However, there are pitfalls that have to be addressed. Back at ARTICLES THAT DIDN’T MAKE THE CUT, you were given a hint that any oscilloscope can lead you astray. Make sure the oscilloscope you use is in good operating condition, and use the appropriate probes. The same carries over to your signal generator. Misunderstanding or misusing these techniques can also result in a very disappointing experience.