THE EXPERIMENTER’S CHASSIS
I came up with this idea after getting tired of keeping a log of modifications and circuits I wanted to try the next time I had an amplifier open on my bench. Also, any old-timer who was acting as my tube teacher and mentor would seem to have the same answer when I asked if an idea would work and/or work well; ‘Build it and find out!’ The lesson I was being taught was I would learn more by doing than by having the answers given to me.
We start by buying/building a large chassis enclosure. My enclosure measures 24″ long, 3″ high, and 12″ deep. This is more than enough to accommodate any circuit variable I want to hear first hand, without the need to sacrifice any vintage amplifier in my collection just for the sake of science. Here is where I will tell you that I used a lot of switches to accomplish the task of letting me know how different configurations would sound. Typical rotary switches today have what I call the ‘Rule of 12’. That is to say if the switch has a single pole, it usually has 12 ‘positions’. Two pole rotary switches usually have 6 positions, three pole switches have 4 positions, and four pole switches have 3 positions. This will come in handy to remember. On more than one occasion I used a switch that was ‘wasting’ a section or two, but it was the switch I had on hand. You may come up with a ‘better’ arrangement, or a more elaborate one. The ideas presented here were used by myself as a springboard, and the possibilities are really unlimited. So let’s get started, shall we? I decided to build a simple tweed style 2X6V6 amplifier, without any frills such as Reverb or Tremolo. This lets me hear the tone of the amplifier circuits, and not any ‘coloring’ I may have added.
CAUTION: Place the amplifier in the STANDBY mode to do any switching and listening tests, OK?
I started off with the power supply, and a solid state/tube rectifier switch, where the solid state side of the switch also ‘featured’ a SPST sag resistor option. This lets others hear if they like the tube tone better than a diode with/without a sag resistor. There is more than one way to skin the proverbial cat, and here are just a few ideas I came up with to achieve rectifier switching;
The circuit above uses different filtering ‘levels’ to really hear the difference in tonal responses filter capacitor values can make. Any value I list from this page is just a rough suggestion. As an example, 100uF filtering for the diode rectifier option may not be practical from a cost or ‘space’ perspective. Try a 16uF and 47uF for the two filter options. The point is to have enough of a ‘difference’ between the two filter values. Mesa Boogie use a switching arrangement similar to that shown below. Filtering remains the same on each rectifier choice, and saves the cost of one filter capacitor. CAUTION: If you go with a 20uF input filter capacitor, make sure you use a choke/capacitor filter/decoupling circuit to feed the screen grid supply. You may get a little ripple hum if you don’t, so use that choke or ‘bump up’ the tube diode’s filter to 30uF or higher, until hum isn’t a problem. Hey, isn’t this what experimenting is all about? And, since we are experimenting, why not try different rectifier tubes? You can go so much beyond the 5Y3/5U4 route. With only a slight rewiring of the octal socket, you can audition the 6AU4, 6AX4, 6AX5, 6BL4, 6BY5, and I am sure you can figure out a plethora of other suitable tubes with a tube manual in your hands. Be forewarned that these tubes all draw over 1 ampere of heater current, and you may have to add a separate 6.3VAC transformer. However, simple filament transformers are pretty cheap compared to full-fledged power transformers with high voltage taps as well as filament windings. As an example, one supplier up here in Montreal sells a 6.3VAC/3A transformer for a piggy-bank busting $3.50. Of course they have a minimum order of $100, but you can buy an armful of resistors, capacitors, pilot lights, and the like, and easily reach $100 before you know it.
Moving up to the input jack section, you can use the typical ‘Fender’ input circuit, at least for now. Following the input, I used a four pole/3 position switch to select from;
- a single stage from a 12AX7. The other section is shorted to ground (pins #6, #7, & #8).
- a single stage from a 6SL7. The other section is shorted to ground (pins #4, #5, & #6).
- an entirely separate 12AX7 paralleled up to achieve less tube noise.
This lets me hear three different set ups right at the input. Many players can’t hear a great difference, some hear none at all. What you’ll more than likely hear is different tubes having different frequency responses and dissimilar ‘linearity’. In some instances the tube selected could be noisier or more microphonic compared to the others. I sometimes compensate for the frequency response of each tube with a careful selection of the coupling capacitor. I don’t label any switch setting, to avoid hearing with my eyes. This also doesn’t allow any player I let try out the chassis to automatically go to a setting he’ll say he prefers. You should put some kind of grid resistance on each tube, so no grid is allowed to ‘float’, even when switched out of the circuit path.
Following this ‘stage’ could be a volume control, or a tweed style tone control circuit with a volume control. I chose another voltage amplification stage, and here is where you can really have some fun. Try switching different coupling capacitor types. I used a 2-pole/6-position rotary switch at the input of the tone control section thusly;
- Position #1: .02uF Orange Drop capacitor.
- Position #2: .01uF Orange Drop capacitor paralleled with a .01uF ceramic disc.
- Position #3: .02uF ceramic disc capacitor.
- Position #4: .02uF Orange Drop capacitor.
- Position #5: .02uF ceramic disc capacitor.
- Position #6: .02uF metalized polyester tubular capacitor.
One thing to point out here is the importance of keeping the voltage rating the same for all capacitors on the switch. It is possible that a .022uF/400VDC capacitor could have less inductance than a larger .022uF/630VDC capacitor, and there may be some phase shifting. Of course, with a ceramic disc capacitor, inductance is not an issue at all. The point here is to not let the tail wag the dog, but to try and ‘hear’ only one variable in capacitors, and that is the insulator material. I have been known to regularly ‘change’ the above switching arrangement, even having 5 positions with the same capacitor (not just the same value and capacitor type repeated 5 times, but keeping the same capacitor in the circuit). To do this easily, I have three prewired switches hidden away and by taking just a few minutes to solder two wires can start the festivities. Very sneaky. Can you hear a difference between a .02uF ceramic disc and a .02uF Orange Drop? If you can’t find a .02uF ceramic disc, parallel two .01uF ceramic disc capacitors instead. This gets many people going when I don’t label anything and have a guitar player say he prefers the ‘tone’ of the Orange Drop at position ‘1’ better than the ‘tone’ of the Orange Drop at position ‘4’. Would you tell him? Try it yourself, have fun with a few friends, and find out if capacitor types make a difference for you. From here I went to a tweed style tone circuit; nothing fancy. I don’t even have any switching in this section!
Next you could place a straight voltage amplifier stage (to make up for the tone circuit loses), followed by a phase inverter circuit. Use either a split load or a long-tailed circuit. I chose a split-load only because it ‘saved’ 1/2 a tube. From the output of the phase inverter to the output tubes you could also put a rotary switch and try different coupling capacitors here, but you should use a 4-pole/3-position switch. This doesn’t allow too much chicanery by duplicating positions, but I still ‘fool’ people by switching in 3 different types.
Output tubes are another source of experimenting fun. Have you tried 6BG6’s in an amplifier? How about 6W6’s, or 6Y6’s? By having 7 pin sockets on your chassis you can try 6AQ5’s, 6AR5’s, and others. You’ll need a good tube manual and an understanding of what tubes can and cannot be used as power output tubes, but this will teach you more than having misinformation spoon-fed to you by typical tube gurus. Build something that doesn’t work and try to learn and understand why your circuit didn’t work. This also applies to circuits you build that have hum, oscillate unintentionally, or are unstable. Practical knowledge is worth far more than any book can teach you.
Lastly, I place a switch on my output transformer (if it has additional impedance taps). I don’t label those either. Other switches on the chassis that aren’t there to compare ‘parts’ but to demonstrate ‘circuits’ include;
- Pentode/Triode option.
- different Feedback resistor values.
- a 6AU6 tube as the input voltage amplifier (could also be on the 1st rotary switch).
- a transformer coupled phase inverter.
- Choke/resistor option in the power supply between the plates and screen grids.
There are many other ideas, and I’m sure you can come up with a few that I didn’t think of. Keep all switch wiring as short as possible, and ‘out of the way’ of other wiring (especially audio and high voltage DC) to avoid extraneous noises introduced in the signal path. You may have too much preamplifier gain for 6V6’s and would have to trim it back accordingly or loose a gain stage. Or switch to 6L6’s. The ideas presented here are just a starting point; don’t use these circuits verbatim. That’s also why I didn’t list many component values (unless they were important to the switching options). Have fun.
Recommended reading for this lesson is to get your hands on ‘The Groove Tube Book‘ (No.4), as well as Gerald Weber’s Desktop Reference of Hip Vintage Guitar Amps, and study every schematic in the book. Try to ignore the horseshit about interleaved output transformers and tag boards. Just study the circuits. Look at how different amplifiers achieve ‘feedback’. Try to reason why Gibson did their tone controls one way, while Ampeg did it another way. Check out the phase inverter HiWatt uses. Do you understand it? Can you guess what the original purpose is behind the 250-ohm resistor in the Gibson GA-8T? What else will happen? (Hint: There is no filter after the resistor!) Dabble with these circuits on your own experimenter’s chassis, and see if you can actually hear any difference. Which circuit do you prefer? This is how you really learn about tube guitar amplifiers. It has been said that knowledge without practice makes but half an artist.
Now go and plug your soldering iron in!