Variation on 5D4

[heisthl]

I'm not a harp player but people keep asking me about building one, I came up with this variation on a 5D4, sounds good with my limited harp abilities - I'll try it out at some jams this week and hear what real harp players sound like through it. Open to suggestions from those of you who've been down the 5D4 road on possible changes.

[MWJB]

I doubt that 500pf bright cap on the tone control is doing anything beneficial for harp.

For the FAT switch, how about parallelling a larger "hi cut" cap value, like being able to switch from 0.005uf to 0.01uf? Won't that 10meg increase gain & therefore feedback drastically?

Those 10uf cathode bypass caps at V1 could undoubtedly go up to 100-220uf.

If you're going to stick with 12AX7 (I would also try 5751 in V1 & 12AT7/12AY7/12AU7 in the PI, sometimes you just stumble on the magic combination - ideal set up may vary somewhat with different mics), I'd bump up the voltage a little, say 130-140v min on the 12AX7 plate?

I like the switchable effects loop.

What's your plate current, I'd be looking for 50mA min with 2x10" & 6L6 in cathode bias (sorry voltages are a little difficult to read - looks like 430 something?) OR try 6V6 at 30-40mA...this bit is a bit counter intuative - in my own 2x10" design, running similar voltages to yours, I found the 6V6s gave a better hi end cut but with less feedback, perceived as louder, when compared to 6L6. 6L6 at those lower voltages can end up with the hi end dulled off somewhat, which sounds nice & fat in testing, but might not work best on stage. Incidentally, I found the same thing when swapping tubes about in a Mini Meteor (preferred 2x6V6 with the bias switch set to 6L6 over actually using 6L6).

I'd be interested to try running the NFB loop from the "B" cathode of the PI tube (22K dropping resistor) as you have already split the cathodes, maybe with a 5E7 style presence control & compare to the stock 5D4?

I'd generally be trying to lessen dependence on that PI "unbalance control", nice to have in an emergenc but you really want to make sure that the amp will produce gig volume & reasonable fidelity with the PI balanced & just use the pot to introduce grind if necessary.

[MWJB]

Also try A/Bing the difference between your cathode bypass cap on the power tubes & an unbypassed cathode resistor, maybe put the bypass cap on a switch?

[heisthl]

Thanks for the feedback (feedback is not a good word on a harp forum)
I left the treble cap (500pf) and the FAT switch (tone bypass) so it could be used for guitar if needed. I'm aware of the effect of higher bypass caps on V1 and will be subbing in larger alternative (100uf or so) values when my product tester harp buddy starts playing while I tweak. The amp has tons of low end, but maybe more is better.
The low preamp plate voltage is to accomodate the more standard 12AX7 tube type. I intially had 150 volts on V1 plates with a 12AY7. Then I used a decade box to arrive at the new dropping string to make a 12AX7 sound like a 12AY7. I built this with a 250 ohm cathode bias with 22uf bypass but went up to the 500 ohm/27uF so the tubes would run cooler at 35ma per tube. I know conventional harp amp wisdom is to run them hotter but my enclosure is a rear ported almost solid back and I don't want to use a fan. I'll post a picture later today.
I'm curious about the feedback/presence recommendation - generally they reduce lows, am I missing something?

[heisthl]

I did a little more tweaking and did up the V1 voltages by about 20 volts using a 68k dropping resistor - it does add some treble but it also adds some low end fidelity. To compensate for the tonal changes I upped the cathode bypass on V1A to a 470uF. One thing to keep in mind is the girth the FX loop adds especially with this style of PI, really nice overdriven sound without fizz or raspiness. Which probably makes up for the lower idle current on the output tubes. It's going into the cabinet now until it get's a real harp players critique for final tweaking.

[MWJB]

"Then I used a decade box to arrive at the new dropping string to make a 12AX7 sound like a 12AY7" I don't see how that can quite be achieved, the 12AX7 will always have lower lows & higher highs, the 12AY7 will have better fidelity & more mid focus in the same circuit (which can be a good thing if you find a 12AX/5751 a bit "woofy"). Typically I'd suggest 90-120v on a 12AY7 plate, BUT I'm not sure that in your circuit a 12AY will be best, I'd definitely try 12AT7/5751 in V1.

"I built this with a 250 ohm cathode bias with 22uf bypass but went up to the 500 ohm/27uF so the tubes would run cooler at 35ma per tube. I know conventional harp amp wisdom is to run them hotter but my enclosure is a rear ported almost solid back and I don't want to use a fan." I think at 35mA you're going to find that a lot of the necessary highs are going to be lost from the 6L6s in cathode bias, they may sound rather stodgy & you may not be benefiting from the typical evelope of attack/compression that people like about cathode bias. I'd seriously be considering 6V6, between 30-40mA, if you want to keep the bias cooler & the temperature down.

"The amp has tons of low end, but maybe more is better." Good low end with reasonable fidelity & note seperation is what you really want. You need the low end of a A or G harp to come out naturally, especially the 2 draw bends, otherwise the amp is in danger of sounding monotonic.

"I left the treble cap (500pf) and the FAT switch (tone bypass) so it could be used for guitar if needed." In that case I'd make the 500pf cap switchable (in for guitar/out for harp) & leave the 0.005uf grounded.

Presence can be great for tightening up the amp's response & helping keep some "front" on the note without having to dial in excessive highs & skew frequency response/make the amp brittle. I'll admit that I haven't tried presence control with a paraphase PI, but I'd be temped to give it a go.