Balanced or Un-Balanced Phase Inverter Valve?

[mascis]

I'm trying to get the best possible sound from my Twin Reverb Reissue, I've replaced the speakers and now I'm going through the valves but I'm not very experienced with different NOS valves.

I have a very nice sounding 1965 Mullard ECC81 that I could use as a phase inverter but it's not balanced. Should I use it anyway or is it worth getting something balanced?

I was looking at a balanced General Electric ECC81 that I thought would be nice. I can use the Mullard as a reverb driver in my Deluxe Reverb

Any recommendations?

Thanks

[martin manning]

Balanced is the way to go, but how unbalanced is the one you have, and how do you know this?

[mascis]

Hm I don't really know... The dealer I use always seems to state if the valves are balanced, this one wasn't described as balanced, so I'm presuming it's not, I have no way of testing though...

Hm.. Might go for the GE ECC81 then

Thanks

[matt h]

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[mascis]

Ah ok, this is very helpful, I didn't know that. I think some experimenting is in order!

Do you know why people particularly seem to recommend a balanced valve at the phase inverter position? I've never heard it recommended in any other position...

Many thanks

[martin manning]

It kinda depends on what you like. "balanced" is a concern that wasn't so, uh, prominent back in the day (you know, when all those great recordings were made).

Maybe because the average twin triode was more likely to be well balanced back then?

Certainly, experiment if/when you can to find what your ears tell you that you like. But there's no shame in either preferring a more or less balanced tube.

To figure this out you're going to need a balanced and maybe a couple of unbalanced tubes, which will likely be different in other ways as well.

If you have a multimeter and a sine wave signal source you can measure AC voltage across the PI outputs to see how well balanced the output is. The two plate resistors in the PI are mismatched on purpose to compensate for the imbalance of the circuit, but their values have been chosen assuming a balanced tube.

[mascis]

Yeah okay I'm curious, I have a multimeter and I can use a sine wave sound from my computer.

So I need to measure the AC voltage. Can you tell me where the PI outputs are where I'd need to put the connectors from the multimeter?

Thanks!

[matt h]

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[matt h]

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[mascis]

Ah so that's why people say it's good to have a balanced phase inverter! I get it. Very well explained Matt, even I understood it!

As for working inside valve amps...I'm comfortable with very easy stuff and I know not to go anywhere near the filter caps, I don't go poking around. If it's a case of just taking the chassis out and knowing where to put the multimeter, I'm happy with that. If I need to clip wires and my soldering iron out etc I'd rather not.

Is it something someone can clearly explain how to do here?

[matt h]

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[ToneMerc]

Here's some reading in of PI balancing in the context of having a PI trimmer. I did not really consider PI balancing until I started doing Dumble circuits, although since have incorporated the PI trimmer into other circuit topologies.

https://tubeamparchive.com/viewtopic.php?t=18048


When building I generally will chase a balanced PI tube because in a perfect world, that's what should be there and I have the entire amp documented. FWIW, I have gone to using Gil's method of cathode balancing I call it.

IMHO, I see it as another parameter which may or may not be tangible depending style, tastes, dynamics, current circuit component values, etc.

Looking back I have played lots of old and modern designs, with old and new tubes and most likely plenty of them were unbalanced and they sounded great, it's all relative, so your mileage may very.

TM

[vibratoking]

I'll apologize up-front in case this has already been mentioned.

Some guys like a balanced PI, others don't. Some say it sounds sterile, etc...

I think it ultimately depends on the amp, whether it is overdriven, and where it is overdriven. With preamp overdrive, a balanced PI sounds very good IMO. Dumble for sure. With output section OD, including PI, then unbalanced sounds good IMO.

There are a lot of ways to balance. Here is one link: https://tubeamparchive.com/viewtopic.php?t=18048
There is another long one if I'm not mistaken. I like the FFT method. Let's me see the second harmonic, but that takes equipment. You can't really use almost any signal. You have to make sure you are not clipping anywhere. If you change the amplitude without clipping you will most likely see that the outputs become unmatched unless your PI tube is matched perfectly. That ain't gonna happen. So what is the right amplitude? This is not a perfect science. There are a lot of variables that haven't yet been mentioned. If you test before the transformer then you omit it's imbalance. Another reason why I like the FFT method.

What is balanced, anyway? Are we talking DC balance or AC balance or both? IME, very few tubes are balanced for both. At least I have not seen many. Conversely, I am cheap and don't spend a lot on tubes. My view on spendy tubes is very different from most here. I spend my money other places.

[mascis]

Ok, thanks everyone! I'm going to have a good think about this and when I feel totally confident I can do it without turning myself into fried chicken, I'll do it. It may be a weekend job so that I can really take my time and make notes like you say Matt.

I'll let you know how it goes

[martin manning]

Yeah okay I'm curious, I have a multimeter and I can use a sine wave sound from my computer.

So I need to measure the AC voltage. Can you tell me where the PI outputs are where I'd need to put the connectors from the multimeter?

Thanks!

You must have a load on the speaker output when you operate the amp, so unless you have a dummy load you'll have to listen to an obnoxious howl while you measure. You can measure the AC voltage across resistors R44 and R45, and there is only about -50V there. Test points 19 and 20 are the PI plates, but there will be 300+ volts DC at those locations. The '65 Twin schematic is available on Fender's web site, and the AC voltages shown on it differ by 5%.