Fender Deluxe Phase Inverters

[Sir Cuitous]

Please help me interpret this article.

In the self-balancing type, a resistor that is common to both circuits goes to ground. If one side is driving too hard, it phase cancels some of the signal developed across this common resistor, thus balancing everything out. This is where I had a happy accident.

At the bottom of the article he descibes the "happy accident" where he replaces a fixed resistor with a potentiometer. Is he talking about the common resistor in the above quote from the article? Is that the 56k resistor found in the 5D3 schematic?

[tboy]

Is he talking about the common resistor in the above quote from the article?

That's how I would interpret it.

Is that the 56k resistor found in the 5D3 schematic?

The 5D3 doesn't use a paraphase inverter, so I'd have to say no.

[MWJB]

Yes, he is talking about the 56K in the 5D3 inverter. The 5D3 DOES indeed use a paraphase PI. It wasn't until the 5E3 that Fender dropped the paraphase PI in Deluxes.

Personally, I'm not a big fan of the loss of fidelity that you typically get with unbalanced PIs, even for harp...but that's not to say it doesn't "work" (taking the edge off feedback & cutting highs, but sometimes it can just sound like a blown OT, YMMV as ever). It's a trick worth knowing about if you get stuck for ideas.

There are various ways to get a similar effect - mismatched power tube plate current, 12DW7 PI tube in a Fender style long tail PI, a pot to cut signal to one half of the OT...even pull the power tubes from one side of the OT winding (making sure that each tube has it's own cathode resistor if cathode biased & that in fixed bias the resulting rise in plate voltage won't kill the tube you leave in place).

[Sir Cuitous]

Thanks! Would you mind taking a guess at the value of pot he used to replace the resistor? 50k or 100k maybe?

[MWJB]

It is 50K.

[Bruce / Mission Amps]

Please help me interpret this article.



At the bottom of the article he descibes the "happy accident" where he replaces a fixed resistor with a potentiometer. Is he talking about the common resistor in the above quote from the article? Is that the 56k resistor found in the 5D3 schematic?

GW is notorious for bloviating and massive over exaggerations.
I've also tried this thing and didn't find it to be anything special... nor was it worth doing in any other amp after that.
IMHO, good, NOS, lower gain preamp tube selections, zero negative feedback
and a toggle switch to reverse the phase of the speaker load works better.

[tboy]

By gosh, you're right MW. Turns out I followed a bad link and looked at the wrong schematic.

[Enzo]

And in talking about the pot, the whole point of using a pot is to vary the resistance. SO the value of the pot is not particularly important. Replacing the 56k with a 50k would mean you are lowering the resistance and have no intention of raising it. (Can't go over 56k with a 50k pot, can we?) A 100k pot would mean we coulg go either way with plenty of room. But you could do the same with a 250k pot too. It might be a bit less convenient, but it would work.

Whether it is worth doing is a separate question.

[Sir Cuitous]

Thanks for the comments. It doesn't sound worth it. Bruce's suggestions sound right. I had a feeling someone on this forum would have tried this.

[MWJB]

"Thanks for the comments. It doesn't sound worth it." If you're using a self balancing PI anyway, there's no harm in having it...you don't have to use it, but the option is there.

If, on the other hand, you are setting out to build a harp amp for gigging with, then there's no reason to stick to a paraphase PI (unless you're looking for an old school, early 50's vibe...which in turn, will bring down your headroom), most popular current production, big, stage amps used for harp use long tailed PI's (with NFB loops, but hey, it's horses for courses).

As Bruce says, Gerald may be overstating the case a little with respect to the control's practical effect...but there are plenty of other builders/tweakers who do similar.

I'd establish what the amp in question is going to be used for & what the taste of the player in question is.

Enzo, a 250K pot would work but would also affect tone & feedback by increasing the power tube grid load value as resistance is increased...fine for 6L6s in cathode bias, perhaps not so good for 6V6s? It might be a pain dialling the pot back to the "stock" balanced setting, especially if you used an audio pot, but I guess if you're dialing by ear that where it sounds best IS where it sounds best.

[Chris / CMW amps]

Side note: I really don't understand "paraphase PIs not clean enough/etc.". I've build some amps with paraphase PIs that did have more headroom/etc. than let's say a 5E3....

Of course it all depends on the whole circuit, used parts, etc.

For me it's just a tone-shaping, overdrive-shaping and splitting circuit.
Paraphase: lot of focus on the mids
Cathodyne: slightly brighter, more "edge" when overdriven
LTP: lots of shaping/tweak-possibilities, more low-end and "sizzle" when overdriven

"Unbalancing PI's": that's possible with about any PI-circuit and it's also fun for guitars.

[Bruce / Mission Amps]

... most popular current production, big, stage amps used for harp use long tailed PI's (with NFB loops, but hey, it's horses for courses).

As Bruce says, Gerald may be overstating the case a little with respect to the control's practical effect...but there are plenty of other builders/tweakers who do similar.

I'd establish what the amp in question is going to be used for & what the taste of the player in question is.

OK, yes, this only my personal opinion here, but I think harp amp builders using this GW "paraphase unbalancing" happy accident method are wishful thinkers, possibly with limited amp building experience ... I haven't had single player be blown over by it.
There are other ways to get this unbalanced sound (and reduced feedback with less output power) in amps using other phase inverters like a LTP PI that work better... and those amps can make more power when running straight.

Even so, one of the best sounding harp rigs I've heard to date are very very slightly modded, "preamp tube and input jack modded", 6G12 Fender Concerts with two Weber 10F150s in the bottom and two 10A100s in the top.
Other players I know have been using the Eminence versions of these two speakers and they sounded pretty darn good too.
The Concert type amps, setup like this, don't seem to feedback much at all, plus they are fairly loud and punchy for a serious player.

By the way, the reason I mentioned the no NFB is because with a OT secondary lead phase reversing switch, in most amps when you reverse the speaker phase you'll get the PA to break into an oscillation.
It wouldn't do that if you put the switch on the cabinet though....
an 'L' bracket with a DPDT switch mounted to the side is OK.

[MWJB]

"Even so, one of the best sounding harp rigs I've heard to date are very very slightly modded, "preamp tube and input jack modded, 6G12 Fender Concerts with two Weber 10F150s in the bottom and two 10A100s in the top.
Other players I know have been using the Eminence versions of these two speakers and they sounded pretty darn good too.
The Concert type amps, setup like this, don't seem to feedback much at all, plus they are fairly loud and punchy for a serious player." - Indeed, (IMHO) a well set up brown Concert is still the yardstick by which big harp amps are judged (the black AB763s are fine amps but don't have the legs of the higher voltage browns)...I've heard stuff that sounds as good, but not necessarily "better"...in fact most of the guys I know still keep to 12AX7 (usually running 160v to 200v on the input plate), though they don't always use all 6.

+1 on the Eminence 102, though I was recently told that they are now out of production?

My experience with the Kendrick amp was that, to get reasonable volume before feedback, you HAD to employ the unbalance control...which would explain why he considers it such an integral feature/USP. The Kendrick TC35 is more a mid powered amp when compared to 4x10", fixed bias, LTPI Fenders. It has a strong "signature" tone that you'll either love, or not. That's why I suggest Sir Cituous do a little homework re. playing requirements...some guys have to have cathode bias/lo voltage/hi current, others are more used to fixed bias, especially for stage work (another reason why I normally default to LTPI, it's simpler to rig up as switchable cathode/fixed).

[Sir Cuitous]

Side note: I really don't understand "paraphase PIs not clean enough/etc.". I've build some amps with paraphase PIs that did have more headroom/etc. than let's say a 5E3....

Of course it all depends on the whole circuit, used parts, etc.

For me it's just a tone-shaping, overdrive-shaping and splitting circuit.
Paraphase: lot of focus on the mids
Cathodyne: slightly brighter, more "edge" when overdriven
LTP: lots of shaping/tweak-possibilities, more low-end and "sizzle" when overdriven

"Unbalancing PI's": that's possible with about any PI-circuit and it's also fun for guitars.

I was thinking about guitar. I have a request for a guitar amp for an existing Deluxe style cabinet/speaker. Like Chris, I was also thinking that the paraphrase 5D3 would have more headroom than the 5E3 (if so, probably not a lot more).

I will probably not use this mod, but I appreciate all the good info about PIs.

[Boogdish]

GW is notorious for bloviating and massive over exaggerations.

I do assembly at Kendrick and can confirm the validity of this statement. I love Gerald, but yeah.