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Discussion Starter · #1 ·
I was changing tubes in my Marshall SV20H recently and ran into a new question. I found that one of the ECC83's is a Phase Splitter (or Phase Inverter) and is supposed to be balanced. I changed the other two pre-amp tubes and left that one in until I understand what's involved.
I'm not too technical about electronics, as you can tell ... is it as simple as ordering a balanced tube from tubestore (my source)?
What exactly gets verified before a single tube is designated 'balanced'?
 

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I found that one of the ECC83's is a Phase Splitter (or Phase Inverter) and is supposed to be balanced.
The PI tube doesn't necessarily need to be balanced however, the signals feeding the grids of the power tube should be balanced unless you desire harmonic distortion which many guitar players strive for. Usually passive components in the PI, can be tweaked to obtain a balanced condition. An oscilloscope should be used to balance the input and outputs of the output tubes...a visual measurement of the signal's amplitude...the output transformer may be imbalanced too.
 

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Don't sweat it too much.
The output tubes require one signal to be inverted in phase, thus the phase inverter stage is needed. Marshall typically uses a configuration that uses both halves of the ECC83 tube. The signal coming out of each half (at the plate) should be the same amplitude to drive the output tubes the same. So theoretically for lowest distortion, and maximum power these halves should be gain matched. However as Paul alluded to there are passive components, possibly the output transformer and output tubes that can throw this balance off. How badly, probably not very much, and probably not what you want in a guitar amp anyway.
 

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I use a 25 k (balancing) potentiometer to feed stock value resistors for the phase inverter in my builds.
For my taste, instead of 82 and 100k, I use 100 and 110k values.
I tweak by ear and there is an audible sweet spot while playing an open string but the difference is subtle.
I suppose this could be thought of as balancing...or maybe the sweet spot is unbalanced when measured or scoped ?
Same goes with the output section...I use a bias trim pot on each side of the output transformer.
Here, the audible sweet spot is more noticeable .
I should mention that the audible sweet spot very rarely translates to equal values of current draw measured on the output tubes.
Combination of all subtle tweaks start adding up and do make a noticeable difference.
Like fine tuning a hi speed jet on a carburetor....
Or just set everything to 6.....either or will do.
What???? The last line? Joke..... sorry , couldn’t resist .
 

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I used to live in Denver Colorado and Fat Willy, aka Lord Valve, from NBS Electronics was my amp tech. Google him for a good laugh. Anyway, he modified a number of amps to have a pot to adjust the Pi balance as part of an experiment.
 

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Some amp builders deliberately bias preamp tubes off their bias centre point specifically to create a more unbalanced waveform which has the desirable effect of creating more even order harmonics....bad for hifi....good for guitar amps.
Cathodyne PH has long been a desirable circuit to get that uneven output drive signal in guitar amps such as the Princeton Reverb.
 

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I found that one of the ECC83's is a Phase Splitter (or Phase Inverter) and is supposed to be balanced.
Who said this? :) Maybe they were confusing it with matched power tubes?
There is no mention of it in the manual. Marshall does not sell such a tube. Marshall does not equip these amps with such a tube.
Even the tubestore's super ultimate $750 tube set (biting my tongue on this) for this model does not include such a tube.

Sounds like internet lore to me (Marshall Forum? ;) ). Like others mentioned here, a balanced PI tube might be desirable for ultra clean non-distorted HiFi systems, but that is the opposite of what most people want from a Marshall amp.
 

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Discussion Starter · #11 ·
Who said this? :) Maybe they were confusing it with matched power tubes?
There is no mention of it in the manual. Marshall does not sell such a tube. Marshall does not equip these amps with such a tube.
I wrote it down in my guitar log book, with a question mark, no source ..?
Flipping through internet pages again I think this might have been my source:

Based on the information I received here so far I decided not to worry about it. Thanks!
 

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I wrote it down in my guitar log book, with a question mark, no source ..?
Flipping through internet pages again I think this might have been my source:
https://www.ampvalves.co.uk/phase-splitter-valve/
What a load of crap. The guy is a tube seller, and is selling a matching service as well as the tubes.
I don't think there is one correct piece of info on that page.
 

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Discussion Starter · #14 ·
What a load of crap. The guy is a tube seller, and is selling a matching service as well as the tubes.
I don't think there is one correct piece of info on that page.
Right .. when I read this page I noticed that it’s mostly typical salesman lingo. Electronics is foreign to me but I do have engineering background ... yet, I took a note of the balanced phase splitter and decided to check this out.
I came across this UK page while looking for tube recommendations for my Marshall. I’ve been getting my tubes from Hamilton but they recommend their “preferred series” a lot an I’m not sure if these are preferred to buy or maybe sell ..? Anyhow, looks like Tung-Sol 12AX7 is staying.
With EHX controlling most of the tube market these days choices are tricky ...
 

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Balancing Push-Pull Amplifiers
The push-pull configuration uses two tubes with applied input and output signals, 180° out of phase; even order harmonics and combination frequencies are cancelled.
Optimum performance from a push-pull amp requires careful balancing. The physical layout of a push-pull amp is more demanding than that of a single-ended amplifier and special matched components should be used for symmetry.
Tetrodes or triode-connected tetrodes in class AB are commonly used and Class B is acceptable if the small level of crossover distortion is acceptable.
In a push-pull amp, imbalance in the plate load or plate-dissipation is usually due to lack of symmetry. Normally, the tubes are similar enough that imbalance is not associated with the tube or its characteristics. This may be verified by swapping the tubes in the sockets, provided both tubes have the same plate, screen and control-grid voltages…observe whether the unbalanced condition remains with the socket location or moves with the tube. If it remains with the socket location, the circuit requires adjustment. If appreciable imbalance is associated with the tube, it is possible that one tube requires replacement.
The basic indicators of balance are the plate current and dissipation of each tube.
The circuit DC plate, screen and grid-bias voltages to each tube, share common supplies and the plate circuit is mechanically and electrically symmetrical.
Imbalance in a push-pull amp is usually caused by unequal voltages applied to the grids of the tubes or unequal voltages to the plates of the tubes. The grid-bias should be balanced until equal DC plate currents flow in each tube. Then it should be balanced until equal plate dissipation appears on each tube or equal plate voltage.
The balance of plate current is a more important condition than equality of screen or grid-current…tubes tend to be more uniform in plate current characteristics.
The screen-current is very sensitive to voltage balance and may be used as an indicator.
Once the DC plate currents are equal by adjusting the bias, the plate circuit can be adjusted to provide equal plate dissipations; if the tubes have equal screen-current characteristics, balance may be achieved by balancing the screen-currents. If the tubes differ somewhat in screen current characteristics and share a common supply, the final trimming of plate circuit balance may be made by interchanging tubes and adjusting the circuit to provide the same screen-current for each tube, regardless of its location.
For class AB2 operation, the grid-current is not used as an indicator of balance. It is probable that after following the above procedure the grid-currents will be fairly well balanced but this condition is not always a safe indicator of balanced grids.
 

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Yes, I remember many calibration procedures where I was chasing my tail.
 

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Phase-splitter, phase-inverter, it's the stage of a push-pull guitar amp that produces two signals, one in phase and the other, 180° out of phase…phase inverted. These two opposite signals then drive the two output tubes or transistors of the amp.
 
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