So, on to the basic parameters of what I'm planning to build. My primary concern is the preamp, because that is where most of the tone-shaping takes place, at least in the way I use guitar amps. So what I'm designing will be amenable to a preamp-only realization (e.g., a rack mounted unit), or to the typical guitar arrangement of preamp and power amps in a box (the "head"), with separate cabinets for the speaker(s). But for my own use, I want a small and portable unit, with limited power -- but unlimited flexibility. So, my remarkably-complicated preamp will be wedded to a very small and simple power amp and speaker setup, in a single case: a "combo" amp.
My ideal amp will be small in size and power, but it will be stereo. I find that stereo is crucial for reverb and certain delay effects. Unlike the wide stereo image which is the final result of a mix in recording or live sound, stereo as applied to a single instrument such as guitar, serves a quite different purpose; as such, it is not usually important or even desirable to have a wide distance between the stereo speakers. The stereo effect creates a doubling or chorus-like ambiance, similar to the double unison strings on harpsichords or the upper strings of a 12-string guitar. The two side-by-side speakers in a "twin combo" guitar amp are ideally suited for guitar-stereo; it's strange to me that there are not many examples of this (if any?) available on the market. (*) But then, that's why we are here! (A number of twin-combo amps would be good candidates to adapt to stereo: especially those with four output tubes, which could be divided into two push-pull pairs, by adding a second output transformer and a few other components. But here, we are designing from scratch.)
I am considering several different combinations of power amps and speakers. For sonic reasons as well as size, I'm interested in speakers smaller than the traditional 12". Probably for the combo amp, I'll try a pair of 8" speakers, similar to what's in the Fender Champ -- but with a closed box. For the separate-head version, a twin-10" stereo-wired cabinet might be the thing.
For the power amps, I am considering three main possibilities. If I end up manufacturing and selling these, I will likely offer all three options, and perhaps others. To wit:
* 2 x 6V6 stereo (single-ended amps, a la Fender Champ).
* 4 x 6V6 stereo (push-pull, a la Fender Princeton and others).
* 4 x EL84 stereo.
In all cases, the power tubes will be cathode-biased, with selectable negative feedback; tube rectifiers will be used, for their voltage "sag" and slow power-up characteristics. (If it wasn't completely obvious, the aim here is anything but "high fidelity".)
Given the number of preamp tubes I am planning to use (13 in the current design), it is hard to find a single power transformer which can supply all the required 6.3vac heater current for preamp and power amps, while also supplying the right high-voltage (many transformers I've seen which have enough heater current, are seemingly made for non-audio applications, and have inconveniently-high HV values, often 500vac or higher); and to employ tube rectifiers, the transformer must also provide one or more separate heater windings for the rectifiers, at 5vac or 6.3vac, since these must "float" at the high DC voltage level of the rectifier output. One obvious solution would be to include a second, beefy filament transformer, to feed all the hungry preamp heaters. However, given the need for two transformers anyway, I've decided to split up the power supplies more completely, so that there can be a separate power switch for the preamp, and one for the two power amps. (The main complication here is that now two (or more) rectifier tubes are needed. In fact, certain configurations might require three: one of the power amp designs I'm looking at (from the Eico HF-81 stereo amp) uses a pair of EZ81s for its quartet of EL84 output tubes, instead of a more regular choice like a single GZ34.)
The system will be set up so that either the preamp or the power amps can be operated independently. This has certain subtle implications. E.g., the low-voltage 5vdc used for the bypass relays, LEDs, and the footswitch, is controlled by the preamp power switch. When only the power amps are turned on, there's no 5vdc. The issue is that the left and right inputs to the power amps are the effects-return jacks, when the preamp is operating; hence, the inputs are only enabled when the stereo effects loop is switched in by its corresponding bypass relay. In order that the effects loop is "in" rather than byassed when preamp power is off, this bypass relay is wired "backwards" relative to the others: it is by default switched "in" when off. Sending power to the relay puts it into the "bypass" state. (Even though the relay is "in" by default, the LED will only light up if the preamp is turned on.)
Instead of the usual "standby" switch, I plan to fit a "mute" switch, which simply grounds the power amp inputs. (Chatter on the Internet has convinced me that standby switches lack benefits and are possibly harmful to tube lifespans. A mute switch addresses the usual use of a standby switch, with fewer downsides. One industry example I'm aware of: Matamp.)
I have found that negative feedback in the power amp section makes a significant difference in the tone, but there is not a single "best" configuration; different amounts of NFB sound good for different playing styles, speaker cabinets, etc.; so, a switch will be provided to turn off NFB entirely, and "presence" and "resonance" controls will be provided, to shape the spectrum of the NFB when it is turned on.
So, to summarize the controls of the power section, or what I'm calling the "master" section:
* preamp on/off.
* power amp on/off.
* mute.
* master volume.
* NFB on/off.
* resonance (LF boost).
* presence (HF boost).
All of these controls are active when the power amp is on, independent of the preamp. Other controls which I'll speak about in upcoming posts, are only active when the preamp is on.
(*) Note: a quick check of the Internet suffices to show how wrong I was: there are actually quite a few stereo, all-tube, guitar amplifiers available today. The first three I happened across were Magnatone, Orange, and Blackstar: all three are making nice, low-wattage, tube stereo amps for guitar, with the right provisions for stereo effects, etc.. So, it should be easy these days to "audition" the sound of stereo effects as applied to guitar, through closely-spaced speakers, if you're not sure whether you want to undertake the extra effort and cost of building a stereo amp. Indeed, one of these existing amps may fully satisfy you, and if you've got the bucks, then I guess you're all set.
But nobody else has my preamp! Hee, hee.
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