Anybody else find a balanced phase inverter tube too nice in a Plexi circuit

Glass Onion

Toneful truth seeker.
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Reeves custom lead sounds too smooth and evenly saturated with a balanced PI tube.
Just wondering if anyone else hears it this way.
 

Heady Jam Fan

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9,009
I don't play a Plexi, but...

If I'm buying a PI, I usually get a balanced one, but I've definitely toss a PI in from a box of preamp tubes and I have no idea if they were balanced. I've seen arguments that say its pointless buying balanced because other components/circuitry could very well be off by just as much (an unbalanced PI might actually be better for balancing things). It seems, in a lot of musical equipment there is beauty in imperfection. I can't say for sure, but I'd guess the tone might very more from one tube to the next as much as balanced vs unbalanced...

*bracing myself for all the posts disagreeing with me*
 

Glass Onion

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I am starting to think any one given tube may be perfect just because of what you say. I randomly try a different tube in the v1 spot sometimes just to hear a difference and what may look or be thought of as junk
Can make a better sound. 😁
 

Heady Jam Fan

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9,009
I am starting to think any one given tube may be perfect just because of what you say. I randomly try a different tube in the v1 spot sometimes just to hear a difference and what may look or be thought of as junk
Can make a better sound. 😁
Yeah, unfortunately I enjoy some expensive tubes in my DR (such as my Grey Plate RCA in V2 and Jan-Philips 6V6s). But they came with my amp and I wouldn't spring for NOS tubes unless I got a record deal ;). Otherwise I rolling my Ruby's or JJ's through the preamp spots until I get the best tone I can get.
 

Glass Onion

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I sent a 72 Marshall in to be looked over and did not get back the preamp tubes I sent with it. And the same thing happen ed to a Jubillee Marshall I sent for work. I will be sending all further work with less valuable tubes. I only use NOS if I know the dealer and can trust them. I can say the NOS stuff is way more consistent. 😁 have a good one man.
 

Heady Jam Fan

Member
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9,009
I sent a 72 Marshall in to be looked over and did not get back the preamp tubes I sent with it. And the same thing happen ed to a Jubillee Marshall I sent for work. I will be sending all further work with less valuable tubes. I only use NOS if I know the dealer and can trust them. I can say the NOS stuff is way more consistent. 😁 have a good one man.
Yeah, I'd be smart to buy some now and stash them before they start to become really rare and jump in price. Thats a bummer about not getting your tubes back - I'd be pissed.

Take care.
 

ACETECH

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A balanced phase inverter is only as good a the parts it is hooked up to. Most circuits I have seen have the gain in one channel higher than the other to make up for imbalance in the tubes. The bias in the output tubes is there to adjust for minimum distortion and max tube life. Moving the bias point changes the tube current but it also changes the point that the tube starts to conduct.
Since the phase inverter has minimum gain most tubes should work with no problem.
Harmonic tubes are a different story, so check for harmonic tubes by tapping them with a pencil with the volume up and no input.
Good Luck.

When I had my repair shop I always returned bad parts, still do.
 

Glass Onion

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Put a brand new tube in the phase spot to see if it helped and the volume was way low. I had switched cabs also so I thought maybe it was that but the new cAb had higher efficiency speakers. Turns out the tube was bad from the start. Put another tube in and life is restored. Just gods to show dont mess with anything until you at least try another tube. 😡😁
 

drbob1

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28,606
It really depends what you want to do… If you have a very high quality amp-short wire runs, 1% resistors and caps, really well done transformer and matched power tubes, then a balanced PI may be the one thing you need to get the two sides of the push pull to balance. That will result in the highest clean output available. It will emphasize odd order harmonics, though, which may make overdrive sound a bit harsh.

Unbalanced OT, tubes, wiring and PIs (especially of the inherently unbalanced cathodyne variety) result in an asymmetric output (one side of the sine wave bigger than the other) which cuts clean but makes the distortion character emphasize even order harmonics. Some amps allow you to mismatch power tubes (like my London Power Studio) for even more dramatic even order emphasis (got an EL34 and a 6V6 in there right now).
 

Supercubziggy

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I am using a Sovtek LPS in the 3rd position of my Suhr SL68. It came with ARS tubes which were nice tubes also. In true TGP fashion i immediately removed them all and tried different ones. I have tried NOS Mullard, late 50s GE, Tung sol(new), JJ, ARS, Chinese(really nice actually), and a couple others.
 

Husky

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12,218
There are guys and amp builders who like to mismatch the phase inverter a bit.
Just saying:hide2

You could always put a trim circuit in there ala Humble and adjust to your preference.
I would advise that if you had a scope.
 

Hulakatt

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Tried a balanced Groove Tubes in the PI of my jtm50, bleach... added in a bunch of hiss. swapped in a Ruby and was much happier.
 

Mattbedrock

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4,773
I just ordered 10 assorted 12ax7's from my favorite tube supplier. Included in that were some long plate GE and JJ tubes listed as "above average microphonics" that I plan to try in phase inverters and other non critical spots.

I want to hear what PI swapping really does.
 

Roe

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8,056
There are guys and amp builders who like to mismatch the phase inverter a bit.
Just saying:hide2

You could always put a trim circuit in there ala Humble and adjust to your preference.
I would advise that if you had a scope.
yes, the long tail phase inverter isn't matched perfectly. This is especially the case with little or no negative feedback, a small tail resistor, and PI breaking up. If you look at a 1959 or 2203 with 100k @ 4ohms NFT, it will often redplate on v4-5, but not on v6-7
 

Guinness Lad

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Most likely the rest of the amp isn't balanced. This is the crap they sell to high end stereo guys, you know the kind that place rocks an shelves then gush about how much better the imaging is....:jo
 

velvetgeorge

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703
Roe is spot on. The PI circuit in a plexi, for example, is very unbalanced. The side of the power section nearest the preamp is driven much harder than the power supply side. A balanced PI tube maintains this inbalance. If you want a tube to compensate for this, section A needs to have less gain than section B. but, when it's balanced you lose that plexi character and tone to a degree.

I always put lower spec ( but still matched with about 3mA) output tubes on the hotter side, just to even out the current draw in the output section a bit. But it is still unbalanced.

George
 

velvetgeorge

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703
Most likely the rest of the amp isn't balanced. This is the crap they sell to high end stereo guys, you know the kind that place rocks an shelves then gush about how much better the imaging is....:jo
Exactly! No need for balanced preamp tubes in a Marshall, thats marketing. Though in general you do want tubes with strong a triode on both sides.

Looking at V1. Triode A is the gain stage for the normal channel, triode B is for the bright channel.

V2: triode A is a gain stage common to both channels. B is the cathode follower that drives the tone stack. These stages are in series, not parallel. No reason they must be balanced.


George
 




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