overview of the project
As mentioned, this preamp will be housed in a 19" rack compatible box, without the actual rack-mounting "ears". I.e., the width of the box will be under 17", probably 16" or 16.5".
When I design tube circuits in rack format, I often intentionally don't make the box height an exact multiple of the standard 19" rack "unit height". This is to discourage tight vertical spacing of tube gear in racks. Of course, when carefully set up by a professional, the rack should have enough directed airflow that units get plenty of cooling even when tightly packed in. But failing that, even when slapped into a closed-box rack in a haphazard fashion, my devices will obtain some measure of cooling for themselves by enforcing small airspaces between units. If compactness is an issue, don't use 19" racks in the first place! So anyway, the height of this preamp will probably be about "2.5 U", so to speak.
To a large extent, this project just consists of re-packaging a Dynaco PAS-2. I have made the decision not to substantially alter the functionality of the circuit, even in places where it might be tempting: such as adding to or altering the EQ and tone-control facilities, adding additional gain controls in useful positions, etc.. More to the point, I will keep the two Dynaco circuit boards, PC-6 and PC-5, exactly as originally designed; my changes will be limited to a new set of wiring and controls around the outside of those two boards. My aim is not to provide every possible tonal variation (though this would have been my desire, back in the day). Rather, I want this preamp specifically to provide those tones which nothing else can provide, i.e., the "Dynaco tones".
The front panel control layout emphasizes the structure of the original PAS-2, by providing separately-delineated control regions, corresponding to the two Dynaco circuit boards, PC-6 and PC-5. Another region encompasses global controls, some of which were not part of the original Dynaco unit, such as the "patch" switch and the master volume knob.
Instead of the rotary input selector switch, input always comes from the single 1/4" guitar input jack, and toggle switches determine which parts of the circuit are in the signal path. The two channels, formerly "left" and "right", are now simply numbered "1" and "2", and independent switches and controls are provided for each channel throughout. For each channel, there is a three-way switch to select the EQ curve of the low-level stage (PC-6): RIAA (phono), "special", or NAB 7.5 (tape). A separate "enable" switch controls whether the PC-6 circuit is in the path, or bypassed.
The line-level section, PC-5, provides gain, bass, and treble knobs for each channel, along with two switches: loudness and "filter" (high-frequency LPF). Because I'm not using the original Dynaco stereo volume pot with the center-tap for the loudness circuit, the "loudness" switch instead simply connects the same filter components to the output (wiper) side of the standard gain pots: more like the wiring of the "bright" switch on many guitar amps. This will not really have the same behaviour as the original Dynaco loudness circuit, but it will impart some of the same tonality at certain gain settings (near position 5), and otherwise can just function as an "interesting" tone modification, available when wanted. The "filter" switch, on the other hand, is implemented exactly the same as on the original Dynaco unit. I found this switch to frequently be useful, in shaping guitar tones.
The "global" section of the panel includes the output jack, power switch, power LED, a new master volume pot which I have added to the circuit, and the "patch" switch. This latter controls whether the guitar passes through only one channel of the PAS-2, or through both channels in series. Since each channel also has its own enable/bypass switch for the PC-6 section, this leads to six different modes (at least one PC-5 section is always in the path), with from one to four 12AX7 tubes in series (i.e., 2-8 individual triode sections!). This would seem to be complete overkill, and indeed the full chain can produce enough distortion that it is difficult to manage, this becoming an art in itself. But I now have come to realize that Hafler's negative feedback loops are greatly reducing the gain in each stage, so that four stages in a row aren't as audacious/unusable as they might be, if they were raw open-loop 12AX7 stages. The total amount of distortion might be the same as that produced by other amps with fewer stages, but by producing it incrementally and in constant tension with the negative feedback, it seems that a tone is produced with rather unique character. Nobody would build a guitar amp this way -- therefore the tone is like no other!
The power LED will be my first trial of an idea which, if successful, I also plan to use in my own guitar amp design (see my other blogs on that subject). The LED is a common-cathode RGB type, with just "R" and "B" used. The red is powered from the filament supply, which comes on immediately at power-up. The blue is powered, via suitable dropping resistance, from the high-voltage supply, which takes a little while to warm up. So, on cold power-up, the LED starts out red, and then progressively shifts to a purple shade once the preamp is ready. On power-down, the red quickly fades out, but the blue takes longer, giving a visual indication that the high-voltage caps are not yet discharged.
Since the original Dynaco volume control was located at the input side of PC-5, it was logical to split this stereo control into two separate input gain controls (eliminating the "balance" control and the "blend" switch). However, this still leaves no final volume control for the unit, which is strongly needed because one may wish to turn up the gain controls quite high to get distorted tones; the final volume needs to be determined separately as an obvious matter of convenience. So, I added the master volume pot, a simple passive divider right before the output jack. (Back in the day, I didn't have a master volume knob on the unit as such, but I usually followed it with a volume pedal.)
I am primarily interested in using this unit as a monaural guitar preamp, of course; hence the single input and output jacks on the front panel, and the "patch" switch. However, it makes sense to also provide a set of RCA jacks, giving access to the circuit in a way that allows it to still be used as a stereo hi-fi preamp. These would go on the back panel; I may not implement them initially, but consider them to be part of the "official" design. Just four jacks are provided: inputs and outputs for channels 1 and 2 (L and R). The inputs are either low-level or line-level, depending on the switch settings in the PC-6 control section. The outputs are pre-master-volume, i.e., same as the original Dynaco circuit: the channel gain knobs now become the volume controls. The "patch" switch must be in the disconnected position for stereo operation (i.e., "1", not "1->2"). So the convenient input-switching of the original Dynaco unit is lost, but by patching cables and setting switches, any particular individual stereo source can be played through the system.
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The starting point:
This is all that remains from my old guitar preamp. At a certain point, I had ditched the pedalboard box, and I just had the components laid out on this board, as you see. PC-6 is on the left, PC-5 on the right. How do you like my wiring of the rectifier tube? Also note, I had a cute little neon-lit power switch, but ummm, no fuse... I will be doing things a little differently, this time. I won't even try to power this thing up, until I've made a few changes.
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