Plate follower is enough, no need for cathode follower buffer (feedback again)Choke supplies are better than resistor PI filters. Stepped attenuators, especially shunt types are better than ANY pots, hovever expenceive. Ladder Stepped Attenuators rule.... Indeed. And use ONLY Battery-Bias (Grid sounds better). Cathode Bias with Electrolytic Bypass Cap's is definitly a No-No (oops, now I have excluded practically any commercial Valve-Pre from Picture - naughty me....)Vintage guitar amps often produce a tone that is described as "round" or "mellow," and they can produce a desirable overdrive sound. Some of these amps are low-powered, others have lower supply voltages than later models, but many use "cathode-bias" for the output tubes instead of the more efficient "fixed-bias" method. Cathode-biased output tube tone can be "round" if no bypass capacitor is used, or it can be "crisp" if a cap is used - sounding quite similar to a fixed-bias output stage. Modern players have the advantage of an exremely wide "acceptable" tone range to experiment within. They are not limited to the relatively clean sounds that were demanded in the fifties. A greater variety of basic amp tones allows the musician to keep his system simple, but occasionally places him in the position of requiring more than one guitar amp for his range of music styles. Depending on individual requirements, it is often desirable to be able to have both cathode-biased and fixed-bias tones available from one amp, at the flip of a switch. Bias switching is a simple task, but demands the use of reliable components - if you buy surplus parts, confirm their integrity with an ohmmeter. One consideration that is not critical for every player, or on every amp, is whether a tremolo feature is present (and used). If a tremolo is used, then we must not disable the fixed-bias supply completely, as it is tied into the tremolo as a "mute" control. In this case, we simply want to disconnect the bias supply from the output stage when cathode-bias is desired. To accomplish the bias switch in the simplest way, a double-pole double-throw (DPDT) switch is required. This should be CSA or UL approved, to assure the quality of the device. It can be a miniature type, though, as the voltage and current stresses are negligible. In Figure 1, we see a typical Fender bias-balance supply with the new cathode-bias option and switching added. To add the cathode-bias components, you must unsolder the ground straps from pin-8 of each output tube socket. Two 1-ohm 1/2 watt resistors are used to tie the cathodes together, and to the bias resistor (Rk). This resistor will be a high-power wirewound - 10W to 20W, try 500 ohms for a pair of 6L6s, or 800 ohms for a pair of 6V6s. The bypass capacitor (Ck) can be plastic or electrolytic, depending on the value you choose. Its voltage rating should be twice the value of the anticipated DC bias. For electrolytics, make sure that the positive lead is attached to the 1-ohm resistor bridge, and that the negative lead is tied to ground. This cap should be kept well away from Rk as Rk will generate a lot of heat. Note that Ck can be switchable also. Place a 10k half-watt resistor in series with the negative lead of the cap, and connect a second mini-switch across the resistor. With the resistor shunted by the switch, the cap allows full output power, and a loud, bright tone. With the resistor isolating the cap (switch open), the power is restricted but the tone is mellower and breaks up sooner. To switch the cathode-biasing components in and out of circuit, contact-A of the DPDT is wired as in Fig. 1. The other pole isolates the fixed-bias supply from the grid-leak network for the output tubes, as appropiate. The second pole is series-connected between the raw bias supply and the bias pot. Lift the wire that goes to the tremolo off the bias pot, and move it to the supply side of the switch contact. For fixed-bias operation, the switch is up. Contact-A is shunting out Rk and Ck so that the 1-ohm resistors are tied to ground. Contact-B connects the bias supply to the grid leak network, for stock, fixed-bias operation. Flipping the switch down allows Rk and Ck to come into play, where the current flowing through the tubes generates its own bias voltage across Rk. Contact-A shorts the tap of the bias pot, and thus the grid-leak network, to ground, so that the cathode-bias arrangement will be complete. Contact-B opens to isolate this newly grounded point from the bias supply. The supply maintains its usual level, so that the tremolo can still be turned on and off at will. An alternative method using a single-pole double-throw (SPDT) switch and a transistor is given in Figure 2. The SPDT performs the functions of contact-A in the previous circuit, while the transistor performs contact-B's function. In cases where there is both no tremolo and the bias supply is plate-derived (not from its own winding), then an SPDT can be implemented as in Figure 3. Here, a very large resistance - typically 150k - protects the bias supply diode and source winding from excessive dissipation when the bias supply is shunted to ground. If the bypass capacitor is used, its value can be determined using the capacitive reactance formula*, and the value of Rk. You must decide what the frequency response of the cathode-bypassed circuit will be. The formula tells you where the response is down by 3db. Maximally flat response occurs starting at a frequency ten times higher. For guitar amps, response to 70Hz is typical, although if response is down at 70Hz, the perceived crispness of low notes will be enhanced. *f = 1/(2pi x Ck x Rk) and Ck = 1/(2pi x Rk x f) As an example, for a 500-ohm cathode resistance and a 70Hz 3db down point, Ck is under 5 microfarads. For flatter response, a value of 50 microfarads or even 500 microfarads should be considered. The Parts Connection 2790 Brighton Road Oakville, Ontario, Canada L6h-5T4 1-800-769-0747 caps Also, notice the very apparent non-linearities associated with the ceramics. All of these are bad enough to be acoustically displeasing. The characteristics also change quite radically with frequency. It's been pointed out that sometimes, these characteristics can be used to advantage to add "crunch" to guitar amps. Because of the effect of voltage, it ought to be possible to fine tune the sound by using different voltage rated parts (assuming they are rated high enough for your amp.) 6L6 - 30 to 35 ma 6V6 - 22 to 27 ma EL-34/6CA7 - 35 to 40 ma., sometimes even higher! 6550 - 40 to 50 ma EL-84/6BQ5 - 22 to 27 ma Class A currents will be higher. Example is 50 ma for a 6L6. Don't try to run an amp designed for AB1 in pure class A, it will overheat and probably blow. To handle the higher idle currents, Class A amps usually run at lower plate voltages. D. Matched output tubes - do you need them? Do I always have to buy matched pairs of output tubes? The issue of "matching" output tubes, either by buying carefully matched pairs or by tweaking the bias levels and drive signals per output tube is not a settled one. It used to be common wisdom to simply buy matched tubes. A few people noticed, however, that they had a favorite pair of output tubes, which made their amp sound much better than others. The common assumption was that these tubes were better matched somehow. When these tubes get measured, though, it usually turns out that they are NOT matched, at least not matched for AC gain characteristics. The concept of matched output tubes comes to us musical amp types from the hifi community, where they are trying to get the lowest possible distortion. This was true from the start, when Fender was trying to build low distortion amps and copied hifi circuits. The concept has simply clung to us, largely through inertia. It is relatively well accepted even in the hifi circles now that even-order distortion is euphonic, sounds good to our ears. It is very likely that the even-order distortion produced when mismatched output tubes are used sounds better than perfectly matched tubes. If you have modified your amp so you can independently set the DC bias and the AC drive signal, you can tune almost any pair of tubes into AC and DC matching. You can also tune in a selective amount of AC drive mismatch to experiment with the selective mismatching sound. There are technical reasons for matching. Getting enough turns of wire on the primary of an output transformer to get the right primary inductance and still using as little iron and copper as possible to do the job properly is an engineering problem that almost always results in Class AB output transformers being smaller for proportional power outptu than a Class A output transformer would be. The (relatively) smaller transformer and wire size makes a class AB (most guitar amps) output transformer susceptible to burning out if one of the half-primaries carries too much current. If one side's a class AB (most guitar amps) output transformer susceptible to burning out if one of the half-primaries carries too much current. If one side of the transformer carries significantly more current (like double) than it would otherwise in "normal" operation, it is possible it will overheat or open, effectively killing the transformer. Tubes that are so mismatched that to get the right total current for a pair means that one is carrying more than 50% over the nominal DC current for a matched pair is getting into the region where you ought to worry about output transformer damage. If you mismatch, try to get the DC current the same in both sides of the output transformer, and an imbalance in the AC gain of the tubes. The logical limit of this AC mismatching is to remove all the AC drive from one output tube, which is a technique used by at least one commercial amp maker. This effectively keeps the output transformer happy with respect to DC, and gives you a single ended output stage; this also costs you a large amount of your available output power, but, hey, we're after tone, right? Note that the commercial tube suppliers have good reason for wanting to sell us matched sets at a premium. I would expect their opinion to be that matched sets are absolutely crucial. As in all musical matters, let your own personal ears be your guide. If you have a set of tubes you know are not matched, or if you have modified your amp to be able to set the bias and drive levels on each output tube separately so you can either match or not match the tubes at will, you might want to try un-matching them and see how it sounds to you. Consider putting a small fan in your amp to cool it. Try a 240vac fan running from the 120 vac line supply, which will run much slower and quieter than a 120vac one. Install small cathode resistors and independent bias adjustment for each output tube to make biasing easy. Open the feedback from the power amp output to it's input for more power amp gain, more and earlier distortion. Or better yet, put in a spst switch and you can pick the characteristics on the fly... For the adventurous, add a separate filament transformer/rectifier/ filter capacitor to make 9-12VDC at several amps and then use a three terminal rectifier to regulate this down to 6.3VDC, and feed this to your preamp tube filaments. Hum from filaments will drop right through the floor. Lotsa work, though. Put 1500Volt, 1A silicon diodes in series with the two sections of your rectifier tube (if you have a rectifier tube) so that if the rectifier tube shorts, the silicon will save the output tubes, and power and output transformer. Gerald Weber advocates using a 270K/27K resistor divider from B+ to raise the filament windings in a DC sense above ground. This keeps electrons from the filament from hitting the plate, another source of hum. Put 1500Volt, 1A silicon diodes in series with the two sections of your rectifier tube (if you have a rectifier tube) so that if the rectifier tube shorts, the silicon will save the output tubes, and power and output transformer. The B+ will go up about 50V when (if!) the rectifier tube shorts, so the amp will have a little more power and run hotter.This can still hurt modern manufactured power tubes if it goes on too long, so check the rectifier tube frequently. Bill Webb's favorite tone mods for Fender amps at the Vibrato channel's second gain stage, change the ceramic 0.02uF coupling cap to polypropylene or polystyrene replace the coupling cap at the input of the phase inverter with a better cap (polypropylene -> polystyrene-> mylar in order of preference); change its value to 0.001 to make the amp "sparklier" and to 0.01 to make the amp sound bigger and more midrangy The 3.3M resistor which mixes the dry and reverb at the output of the 3rd gain stage, vibrato preamp, is paralleled by a 10pF ceramic cap. Change this to silver-mica to make the amp sparklier The power amp feedback loop resistor is usually 820 ohms; insert another 820 ohm resistor. This reduces the feedback, increases the power amp's gain, and softens the onset of distortion. Remove the single bias adjust pot in your amp and put in two, connecting one to each output tube. You can now set the bias voltage on each tube to be different, which can match the DC currents for un-matched tubes, or un-match matched ones for more even harmonic distortion. Tinker the driver circuit to let you adjust the relative amount of AC drive to each output tube. This lets you match/unmatch output tubes in an AC sense just like the bias mod lets you change the relative DC points.