When is it advisable replace PI valve?

[Kuga]

I heard from some techs when you need replace power valves It's advisable replace PI valve too. What do you think about It?

 

[PelliX]

I heard from some techs when you need replace power valves It's advisable replace PI valve too. What do you think about It?

When it's knackered. Get a valve tester or go by ear.

 

[Kuga]

When it's knackered. Get a valve tester or go by ear.

I built a ECC83 valve tester. I checked all my ECC83 valves. Some ones on PI position looks like suffer more stress.

 

[PelliX]

Some ones on PI position looks like suffer more stress.

That can well be the case, depending the amp. The PI is often hit relatively hard, depending on the topology.

 

[Pete Farrington]

I built a ECC83 valve tester. I checked all my ECC83 valves. Some ones on PI position looks like suffer more stress.

You know what you test rig is showing, whereas I'm not sure.
Can you explain what's being shown, and how analysis of the readings has led you to come to that conclusion?

I don't think most common LTP circuits put their triodes under particularly stressful operating conditions.

See https://www.aikenamps.com/index.php/technical-q-a

"Q: Do I need to replace my driver tube every time I replace my power tubes?
A: No. This is an "old wive's tale" propagated by the tube vendors who want to sell you more tubes. The "driver" tube, or phase inverter as it is more appropriately known, is under no more stress than any of the other preamp tubes. The output stage of a normal guitar amplifier is AC-coupled, class AB1 or class A1. The "1" suffix indicates that the tubes do not draw grid current during any portion of the input cycle. A "2" suffix indicates grid current during some portion of the cycle, as in class AB2. Since almost all guitar amps are class AB1 or A1 (it takes a DC-coupled cathode-follower or interstage transformer to be able to drive the power tubes into the positive grid region), the phase inverter does not have to supply any current to the grids (it can't really source current anyway - it only sinks current through the plate load resistor, which is quite large, and inherently limits the plate current to a few mA). The AC coupling (capacitor coupling) between the phase inverter and the output tubes precludes any flow of grid current anyway. The output tube grids will merely act as a grid clamp, shifting the bias downward as the output is driven harder. This in no way stresses the phase inverter tube. In addition, the plate load resistors and the bias current in a typical phase inverter are identical to the preamp stage values, indicating that they are operating in the same area, dissipation-wise, so they cannot be "wearing out" any faster. In fact, the reverb driver tube on a typical amplifier is dissipating more power than the phase inverter, and should be replaced more often, if anything. Don't be misled by the higher plate voltage on the phase inverter either, because the cathode is usually sitting somewhere around 30V - 100V above ground, which lowers the plate-to-cathode voltage by that amount. The plate-to-cathode voltage is what determines the power dissipation of the tube, not the absolute plate voltage."

 

[21fret]

A: No. This is an "old wive's tale" propagated

yeah 6.3 megatons of the old bags’ hatred towards your favorite pastime. might need more tubes, (or break out that 100w).

 

[Kuga]

You know what you test rig is showing, whereas I'm not sure.
Can you explain what's being shown, and how analysis of the readings has led you to come to that conclusion?

I don't think most common LTP circuits put their triodes under particularly stressful operating conditions.

This valve tester works prety same condicions as a valve amp. It has same power suply like a Madamp J5MK1. I did many test with ECC83/12AX7 valves. New ones, bad ones, balanced, etc... It test anode voltage and anode current on each triode. I test each triode with about 350Vdc. You can see on B+ display. You can see anode voltage on each triode on anode display using a toogle switch. I test anode current on each triode. I use this schematic from this youtube video where he explain It. He talks just about anode drop voltage on each triode but you know a drop voltage has relation with current. If not a drop voltage on anode no anode current. No voltage and current on cathode too. I realise If you have less than 1mA on one anode a valve sounds weak. A new ECC83 valve has about 1,25mA on each triode If you use this schematic. A balanced ECC83 has same anode current and same anode voltage on each triode. I tested many ECC83/12AX7. Current production ones and NOS ones. This tester don't show If a valve sounds good. It don't show If a valve it's noisy. It show how a valve is working. It show how fast anode get max current on each triode. A good condition valve quickly is working. A tired valve needs some seconds to show max current on each triode. You can see how quickly is a valve working with analog current meters. I can use a DMM to test current on each triode as well. I get a super acurated readings with a DMM. It has two toogle switch to open the circuit then you close the circuit using a current meter to get current readings.

It has three transformers. One for HT. One 12Vdc 500mA for valve heater. One 5Vdc cellphone battery charger for screen displays.

 

[Matthews Guitars]

Replace it when a new one makes the amp work and sound better. Every so often, try out a new one. If you don't hear a difference, put the old one back in and keep running it.

 

[Kuga]

Replace it when a new one makes the amp work and sound better. Every so often, try out a new one. If you don't hear a difference, put the old one back in and keep running it.

You are right. But with this tester I know and I can see why a valve sounds weak and I need replace a ECC83/12AX7 valve. I can know If a preamp valve it's ok faster than by turning on and playing an amp.

 

[Pete Farrington]

It test anode voltage and anode current on each triode

There's meters under the following labels
HTVol
B plus
ANODE
Could you explain what they're measuring?

 

[playloud]

If the amp sounds like "Mod 7", then one half has failed...

 

[nortiks]

I have a number of NOS and new current stock 12AX7, 12AT7, 12AU7 and a mismatch between the two triodes is the rule. The exception is when they are pretty close...which of course you can pay extra to get. So that needs to be considered when measuring a mismatch in a used tube. If you don't know 100% they were matched when new, which is improbable, then you can't jump to the conclusion that one side is taking more wear and tear when you measure the mismatch in a used tube.

 

[Kuga]

There's meters under the following labels
HTVol
B plus
ANODE
Could you explain what they're measuring?

HT voltage display show when HT voltage it's on. When it's on 257-259 Vca I know I'm testing at 230Vca. I use a variac and I use an isolated transformer with It. B plus display show valve suply. Anode display show anode voltage. If anode display voltage it's same like B plus display voltage on one triode I know this triode don't works.

First I put a valve on the socket then turn on switch on one side. The switch on the side turn on 12Vdc heater suply and turn on 5Vdc screen displays. After obout 10 seconds I can see on current filament display a consume about 0,150 A. A heater use more than 300mA when filament is cold. Then I can turn on HT voltage switch to get mesuraments. If a triode it's ok I can see max anode current in two seconds after I turned on HT voltage switch. If needs 5 seconds to show his Max anode current this triode it's worn out. When a triode it's worn out show less than 1mA

 

[Kuga]

I have a number of NOS and new current stock 12AX7, 12AT7, 12AU7 and a mismatch between the two triodes is the rule. The exception is when they are pretty close...which of course you can pay extra to get. So that needs to be considered when measuring a mismatch in a used tube. If you don't know 100% they were matched when new, which is improbable, then you can't jump to the conclusion that one side is taking more wear and tear when you measure the mismatch in a used tube.

I tested current production balanced valve from TAD(two). They show same anode voltage and same anode current on each triode. They are used phase inverter valves. Also I tested an older than me green logo Philipps ECC83 that tested great. This valve sounds awesome. Some current production valves I tested worn out really fast (7025 12AX7 from TAD) Less than three years of use on V1 position. I tested very used Marshalls ECC83 from '90s. Some ones still ok.