Aria Pro AG30 tube amp...anyone familiar with

[Helmholtz]

What is the pin 9 voltage?
If it's a triode-pentode it can't be an LTPI which needs a double triode.
Please post pics showing the tube interior.
Might have missed it, but is the amp working?

Your your Ohmmeter to see where wires land.

[Pedro Vecino]

Could be a Chinese 6N2. Similar to a 12AX7 but with the filaments on pins 4 and 5. Pin 9 is a screen between triodes that is not connected internally.

[Helmholtz]

Again: What is the voltage on pin 9 (or pin 6)?
Amp working?
If there's something wrong with the amp, voltages might be wrong and misleading.

If pin 1 and pin 8 are the PI outputs (please verify by following the wires) they would be plate and cathode of the same triode section, meaning a cathodyne. That would fit with the measured voltages.

[TomCarlos]

Helmholtz and Pedro…. Thanks for the replies.

Yes, the amp is working. I believe it is all original and no rewiring has occurred. My concern is that “if” V2 should fail, what is it? If it happens to be a 7199 (which is very possible), can you find one without paying the super high prices I am seeing on the net. It looks like the 6U8A is a substitute, provided that you rewire three of the pins. I have the instructions for that.

To the best of my ability, I took photos of the tubes. It is hard to see where the wires land. But I hope you can take an educated guess.
Yes, Pins 1 and 8 have the coax cables connected to them.

IF this tube is a 7199, Helmholtz would be correct. The image below is of the 7199. And you can see that Pin 1 is the Triode Plate and Pin 8 is the Triode Cathode. So that would make this a Cathodyne Phase Inverter.



I also found a thread here on MEF, titled "7199 as Phase Inverter." So it seems like Sunn and Ampeg used the 7199 long ago.


Voltages with full power applied to tubes, all readings with respect to chassis ground.

Pin 1: 235 volts
Pin 2: -.135 volts (yes, that is a minus)
Pin 3: 54.5 volts
Pin 4: Heater
Pin 5: Heater
Pin 6: 83 volts (has green jumper wire to pin 9)
Pin 7: .9 volts
Pin 8: 88 volts
Pin 9: 83 volts

[TomCarlos]

Update - When I test this tube on my B&K 707 tester. if testing as a 7199, it is a no go. But if I test it as a 6U8A, it looks good. So given that and the rest of this post, that is my vote. Does anyone agree?

The real acid test is to buy a 6U8A and drop it in the amp.



This is a real struggle....

The voltages I posted in the previous post seem to make sense for the Triode. But they look wrong for the Pentode - if we are talking about a 7199. HOWEVER, it this amp has a 6U8A tube with the pinout shown here, do the voltages make sense?




The following diagram comes from a Sunn amplifier that uses a 7199. I changed the pin numbers according to the conversion chart below.


Pin 1: 235 volts
Pin 2: -.135 volts (yes, that is a minus)
Pin 3: 54.5 volts
Pin 4: Heater
Pin 5: Heater
Pin 6: 83 volts (has green jumper wire to pin 9)
Pin 7: .9 volts
Pin 8: 88 volts
Pin 9: 83 volts

This seems to make more sense. The conversion of the 7199 to the 6U8A include the following:

[Helmholtz]

Yes, voltages make sense with the 6U8A.

[Pedro Vecino]

It´s strange that an amplifier made in China has an American or European tube. I just saw that there is a tube called 6F2 that is advertised as compatible with the 6U8/ECF82.
https://es.aliexpress.com/item/10050...yAdapt=glo2esp

[TomCarlos]

Ok... quick follow up.

First, I am confirming that V2 is a 6U8A. I purchased an NOS tube, dropped it in the amp and it works just fine. So thanks for help me solve the mystery on the V2 tube. For good measure, I did replace the power tube sockets as the pins were loose and worn.

Next, I noticed something odd (odd to me) on the heaters. There are two green wires for the filament circuit. One would think they would provide the typical 6.3vac voltage. But I noticed that on V4, one side of the heaters is connected to the chassis with a black jumper cable. So after removing all tubes and removing the jumper, I confirmed the filament windings on this transformer produce approx 12.8vac. So grounding one side of the filament windings gives us the 6.3vac. Again, I have not seen that before. So I am guessing this transformer has been used in other amps.

This amp uses green wire for other connections - very confusing. I replaced those with wires of other colors.

I have written to Aria hoping my email will find it's way to an old amp guy who can provide me (us) a schematic. And if I get that, I will come back and post it here.

[Helmholtz]

Not sure if it's the case with your amp, but if the PT has a 12.6V heater winding with a grounded CT, each of the outer winding ends will have 6.3V wrt ground but with opposite phase. So heater load can be split between both sides.
(You surely know that actual heater voltage is always measured directly between heater pins and not to ground.)

[g1]

Maybe the heater winding rose to 12V because you removed all the tubes. Grounding one side of the winding should not matter. Recheck with tubes removed and jumper re-attached.

[TomCarlos]

Gentlemen.... I can confirm that with all tubes installed and the connection to ground, I am measuring 6.3vac across the heater pins on all tubes. So my guess is that there is no center tap for the heater circuit. Thanks !!

[g1]

What I was trying to say was that it's not the connection to ground that drops the voltage down, but the loading of the heater winding. The 12.8V reading was unloaded (no tubes installed).

[Helmholtz]

Gentlemen.... I can confirm that with all tubes installed and the connection to ground, I am measuring 6.3vac across the heater pins on all tubes. So my guess is that there is no center tap for the heater circuit. Thanks !!

CT would not change tube heater voltage. It's just a way to distribute the load.

[Helmholtz]

What I was trying to say was that it's not the connection to ground that drops the voltage down, but the loading of the heater winding. The 12.8V reading was unloaded (no tubes installed).

I have never seen a transformer winding voltage sag by 50% with designated load.
That would mean huge transformer losses and heating.

[g1]

I have never seen a transformer winding voltage sag by 50% with designated load.
That would mean huge transformer losses and heating.

I've never seen that either, but can't think of anything else that would explain the readings.