The stock values are the best place to start, instead of trying to redesignTime to build another amp using as much as possible parts I have on hand. Transformers I have would lend themselves well to a Fender Harvard, which I also understand makes the basis for a very sweet harp amp. A few questions though, directed especially to anyone with experience with these amps (Goose?):
1. How much of the Harvard's character comes from the 6AT6/6AV6 preamp tube? Some have maintained that these are basically 1/2 of a 5751/12AX7, and since I have plenty of the latter but none of the 7 pin tubes or sockets, I may opt for this if there's no tonal reason not to do so. Also, having 2 input triodes would let me use grid leak biasing on one, with the other set up in the normal Harvard fashion.
The 6AV6 is not the same as 1/2- 12AX7. This tube is used for automatic volume control (compressor type) circuits. It has two extra control grids that are used for a special purpose audio limiting gizmo. The preferred substitute is
6BK6. It definitely has a unique construction, although it also is a hi mu triode, like a 12AX7. 7 pin sockets are easy to obtain and install.
2. The Harvard schematic shows no grid load resistor to ground on the input. I would think this would be a problem for mic elements that want to see a
high impedance, esp crystals. Anyone with any experience here?
It does not work out that way. It can go without a resistor as long as it's connected to something and the wire is shielded, like a mic element, or shorted to ground.
3. Anyone with experience or thoughts on preamp plate voltages for a Harvard harp amp? Stock values are 160, 170 and 207V on the 1st and 2nd preamp stages and PI, respectively. Normally I'd go a bit lower, 100-125V on the first stage.
Whelp, there's always more headroom to be had with higher plate voltages.
4. Coupling caps--I have lots of .1 on hand, which is my default value for a harp amp. Any reason not to use these to couple all stages?
Using .1 for all the stages will result in motor-boating low frequency oscillation.
5. The schematic shows a 25uf cathode bypass cap on the 1st preamp stage, none on the 2nd. I'm inclined to add a switchable 50-100uf bypass cap to the 2nd stage, to provide the option for more punch. Again, anyone with experience/thoughts here?
It will certainly punch more with a bypass on the second stage, but no need to go higher than 25uf. I might use 2.2! This may also cause the audio to overload a bit too much, and sound muddy, but try it anyhow.
I mean there is a master volume? I think you need one if you are planning on doing that modification.
Any other comments from people with experience with this circuit will be appreciated.