I know, another one of my stupid question, but still I ask so please keep reading.
I see some amp setups that shares the PI with a gain stage on the same tube, where some do not.
I can understand the economics of sharing a preamp tube with the PI but what sacrifices does this create? Is it better to give the Phase Inverter the entire tube for itself or does sharing 1/2 of the tube with a gain stage sacrifice tone or performance in some way? What can be gained but giving the PI 2 stages of a tube over just one stage? Please explain. And does it make it more difficult for the PI to use the tube when it is being used as a gain stage at the same time?
Thanks for your help. It is much appreciated.
Share PI or not to share?
Moderators: pompeiisneaks, Colossal
Re: Share PI or not to share?
Well, there are several types of phase inverters.
There are the "long-tailed" pair, "Isodyne," transformer, "cross-coupled," "Schmidt," "floating paraphase," and "split-load" or "Cathodyne" phase inverters to name more than a few.
I suppose each have their strengths and weaknesses.
Most modern amps use the Long Tailed Pair.
I borrowed this write up from amp tech Hasserl since he is smarter than me:
The phase inverter (PI) is part of the power amp in a push-pull type amp. The signal is supplied to the PI from the preamp. The power amp in a push-pull type amp is divided into two sections, each section amplifying part of the signal. The two parts of the signal are summed back together in the output transformer.
It is the job of the phase inverter to split the preamp signal into two halves, each half a mirror image of the other. The signal is an ac signal with a positive and a negative part, one half of the signal from the PI is the positive part and the other half is the negative part. These halves are sent to the power tubes, one to each section.
There are different types of phase inverter circuits, each type performs its function in a different way, so the resulting sound from the amp is different. The most common types used in guitar amps are the cathodyne, the paraphrase and the long tail pair (ltpi), with the most common probably being the ltpi (most fenders since the 5F6A Bassman use this type, as do all Marshall and Vox amps and their derivatives). The earlier tweed Fenders used both the cathodyne and the paraphrase. The Princeton Reverb used a cathodyne type PI, as do the Peavey Classic series amps. The PI used in the amp plays a big part of the sound of the amp. Think about the sounds of each of those amps and you'll begin to understand the effect each type of PI has on the sound.
As for balancing of the PI tube, there is not a lot of consensus on that. Tube dealers like to promote it as being very important. Many amp techs poo poo it as snake oil or fairy dust. It may be important to have a balanced PI, but it is very difficult to say whether the circuit itself in most amp provides a balanced signal, so adding a balanced tube may do no good at all. In fact, it may actually make the balance worse in some cases. A very few amps have trimmer controls that allow you to balance the PI. In such an amp it may be a good idea to use a balanced tube, then dial it into balance and from them on all you need to do when replacing the tube is replace it with another balanced tube. Another problem with all this though is that tubes drift over time, and just because a tube is balanced when you buy and install it, doesn't mean it will be balanced somewhere down the line. Still other people think having some unbalance in the PI may create some interesting asymmetrical signals that improve the sound of the amp. I'm not sure I buy that, but I'm not going to argue with them, for them it might be true.
There are the "long-tailed" pair, "Isodyne," transformer, "cross-coupled," "Schmidt," "floating paraphase," and "split-load" or "Cathodyne" phase inverters to name more than a few.
I suppose each have their strengths and weaknesses.
Most modern amps use the Long Tailed Pair.
I borrowed this write up from amp tech Hasserl since he is smarter than me:
The phase inverter (PI) is part of the power amp in a push-pull type amp. The signal is supplied to the PI from the preamp. The power amp in a push-pull type amp is divided into two sections, each section amplifying part of the signal. The two parts of the signal are summed back together in the output transformer.
It is the job of the phase inverter to split the preamp signal into two halves, each half a mirror image of the other. The signal is an ac signal with a positive and a negative part, one half of the signal from the PI is the positive part and the other half is the negative part. These halves are sent to the power tubes, one to each section.
There are different types of phase inverter circuits, each type performs its function in a different way, so the resulting sound from the amp is different. The most common types used in guitar amps are the cathodyne, the paraphrase and the long tail pair (ltpi), with the most common probably being the ltpi (most fenders since the 5F6A Bassman use this type, as do all Marshall and Vox amps and their derivatives). The earlier tweed Fenders used both the cathodyne and the paraphrase. The Princeton Reverb used a cathodyne type PI, as do the Peavey Classic series amps. The PI used in the amp plays a big part of the sound of the amp. Think about the sounds of each of those amps and you'll begin to understand the effect each type of PI has on the sound.
As for balancing of the PI tube, there is not a lot of consensus on that. Tube dealers like to promote it as being very important. Many amp techs poo poo it as snake oil or fairy dust. It may be important to have a balanced PI, but it is very difficult to say whether the circuit itself in most amp provides a balanced signal, so adding a balanced tube may do no good at all. In fact, it may actually make the balance worse in some cases. A very few amps have trimmer controls that allow you to balance the PI. In such an amp it may be a good idea to use a balanced tube, then dial it into balance and from them on all you need to do when replacing the tube is replace it with another balanced tube. Another problem with all this though is that tubes drift over time, and just because a tube is balanced when you buy and install it, doesn't mean it will be balanced somewhere down the line. Still other people think having some unbalance in the PI may create some interesting asymmetrical signals that improve the sound of the amp. I'm not sure I buy that, but I'm not going to argue with them, for them it might be true.
Tom
Don't let that smoke out!
Don't let that smoke out!
Re: Share PI or not to share?
Thanks for the reply Structo. I guess there is no easy answer to my question.
Re: Share PI or not to share?
The cathodyne (split-load, concertina) PI provides no gain but uses only one triode. Voltage is uneven, but current draw is (should be) the same on the cathode and the plate. Because it provides no gain, and there is a free triode, builders sometimes take advantage and use that half for a gain stage. Because a split load PI doesn't offer the voltage swing of the other types, some prefer the LTP or Paraphase, both of which drive the finals much harder.MBD115 wrote:I see some amp setups that shares the PI with a gain stage on the same tube, where some do not.
LTP and Paraphase provide gain, but also use two triodes.
I hope this sheds some dim light on the question. I cannot get in too deep on technical details, but I have fooled around with each type enough to have a sense of what they do.
Re: Share PI or not to share?
This reminds of the scene in Roger Rabbit...MBD115 wrote:What can be gained but giving the PI 2 stages of a tube over just one stage?
What you gain is GAIN.
Sorry, I couldn't suppress it any longer.
Re: Share PI or not to share?
Thanks for all the replies guys
Amps like the Express (for example) uses the long-tailed pair and is the most common type of PI, and amps like the 5E3 uses the split load or cathodyne type. I think I've seen the Paraphase in some schematics but I can't remember which right now.
Thanks again for all your help.
Amps like the Express (for example) uses the long-tailed pair and is the most common type of PI, and amps like the 5E3 uses the split load or cathodyne type. I think I've seen the Paraphase in some schematics but I can't remember which right now.
Thanks again for all your help.