[Maxx134]

Haven't seen any schematic for this to judge.

 

[Haidar]

Haven't seen any schematic for this to judge.

Hi Max, There is a schematic here:
https://www.head-fi.org/threads/any...une-appj-pa1502a.780178/page-24#post-13480908

 

[baronbeehive]

I've had the opportunity to compare my speaker APPJ with my other amp, a Little Dot MkVI+ now that I've more or less finished the mods on it.... and now that it's working again. I think my speaker APPJ must be very similar to the headphone APPJ because it seems to be performing very similarly to what Maxx134 has said about his.

First I would say that you would expect the Little Dot to be good, it is a balanced amp, heavily modded to match with the headphones you are using and to optimise tube operation, and has expensive tubes - too expensive.

I found that the Little Dot is slightly better in resolution but the APPJ is very close, the LD is also slightly better in soundstage, probably due to the resolution which puts the instruments accurately in the soundstage, easy to hear through headphones, however the APPJ is again not far behind, it has a deep, wide soundstage, very realistic sounding through speakers, great detail and depth. I still feel that the APPJ has a slightly tighter bass, but the mods on the LD have put it very close with excellent detail. Also the APPJ has slightly better textural detail, I feel this is partly due to the frequency range which allows the APPJ to stand out in some ways, for example cymbals really sparkle without being too sharp. Recent mods however have pulled up the LD and it is now very close. Guitars ring out beautifully on both amps, also pianos have weight and attack on both amps.

I think we've come a long way with this amp simply by installing some quality components to make the difference. I have to say that I'm very happy with them both, they're both performing at top level..... perhaps not at the $100K level, but then I wouldn't know about that would I!!

 

[jekjek]

I've had the opportunity to compare my speaker APPJ with my other amp, a Little Dot MkVI+ now that I've more or less finished the mods on it.... and now that it's working again. I think my speaker APPJ must be very similar to the headphone APPJ because it seems to be performing very similarly to what Maxx134 has said about his.

First I would say that you would expect the Little Dot to be good, it is a balanced amp, heavily modded to match with the headphones you are using and to optimise tube operation, and has expensive tubes - too expensive.

I found that the Little Dot is slightly better in resolution but the APPJ is very close, the LD is also slightly better in soundstage, probably due to the resolution which puts the instruments accurately in the soundstage, easy to hear through headphones, however the APPJ is again not far behind, it has a deep, wide soundstage, very realistic sounding through speakers, great detail and depth. I still feel that the APPJ has a slightly tighter bass, but the mods on the LD have put it very close with excellent detail. Also the APPJ has slightly better textural detail, I feel this is partly due to the frequency range which allows the APPJ to stand out in some ways, for example cymbals really sparkle without being too sharp. Recent mods however have pulled up the LD and it is now very close. Guitars ring out beautifully on both amps, also pianos have weight and attack on both amps.

I think we've come a long way with this amp simply by installing some quality components to make the difference. I have to say that I'm very happy with them both, they're both performing at top level..... perhaps not at the $100K level, but then I wouldn't know about that would I!!

Agree. This amp is amazing!

 

[baronbeehive]

Agree. This amp is amazing!

Ha ha, yes indeed!

I'm considering some changes to the circuit. If one were to remove all the negative feedback, what would the output impedance be?

This is an interesting idea, and one which has been mentioned on another thread.

The 6V6 is operated in tetrode mode and this is why NFB is required to keep impedance down. If NFB was removed the 6V6 should go triode mode and then some other changes would be required all of which is beyond my skill level atm.

It's shocking to think that this baby amp can be improved yet still.

 

[Maxx134]

The output sockets are wires for triode operation, so no change there.

I am thinking the removal of feedback would change the output volume, sound, and impedance of the "circuit", but the secondary of output transformers are fixed,
So their impedance to the headphones should be the same.

The output tube impedance match to the transformer primary should change and may or may not be an issue.
Would have to calculate the output tube impedance and also find info on acceptable range to match.

Although may be easy to disconnect, (I haven't checked) my concern would be both volume and distortion if it was a large NFB..
Most likely moderate if the resistor is a high value.

Haven't check just commenting.

 

[Haidar]

The 6V6 is operated in tetrode mode and this is why NFB is required to keep impedance down. If NFB was removed the 6V6 should go triode mode and then some other changes would be required all of which is beyond my skill level atm.

The output sockets are wires for triode operation, so no change there.

I am thinking the removal of feedback would change the output volume, sound, and impedance of the "circuit", but the secondary of output transformers are fixed,
So their impedance to the headphones should be the same.

The output tube impedance match to the transformer primary should change and may or may not be an issue.
Would have to calculate the output tube impedance and also find info on acceptable range to match.

Although may be easy to disconnect, (I haven't checked) my concern would be both volume and distortion if it was a large NFB..
Most likely moderate if the resistor is a high value.

Haven't check just commenting.

Thanks both of you! To me this is worth exploring, even with some alterations to the circuit.

 

[jekjek]

It's getting quiet here

I am listening to this unknown ultron 12ax7 square getter

It sounds pretty awesome

 

[Maxx134]

@Maxx134 i was backreading and CZ4A said that for kt66 tube, tungsol is recommended just because the draw is .9 a. Other kt66 tubes pull 1.3a.

https://www.head-fi.org/threads/any...une-appj-pa1502a.780178/page-15#post-12521319

If those genalex kt66 work great and won't damage the amp, then im down to try them

I found out that the Genalex tubes I am using are running a bit hotter than all previous tubes I used so far, so they making the amp a bit hot after on a while, but I am sticking with them anyways as the amp seems to not be having issue with them.

It probably is not recommended to run tubes with heaters that high but so far I am liking these tubes most overall.
Resolve is just as good with other tubes uses but these do have the largest soundstage.
The Tung-Sol KT66 was nice but not have the resolve of the others.
I believe it needed more burn in but I was not impressed.
The Gelanex was impressive from the start.

The KT66 type have the largest soundstage on this amp circuitry..
More than the 6L6 which was another favorite for large stage.
The other tubes I tried excelled at clarity, like the 6BG6G (RCA)..
All output tubes..

 

[baronbeehive]

It's getting quiet here

I am listening to this unknown ultron 12ax7 square getter

It sounds pretty awesome

I can't remember if I had an Ultron or a Hytron, but it was a Tungsol.

Yours looks like it has round plates, does it have any other markings

A pic might help!

 

[jekjek]

Here you go
Anyone can help to identify this?

 

[baronbeehive]

It's getting quiet here

I am listening to this unknown ultron 12ax7 square getter

It sounds pretty awesome

I think Ultron was just a German label of various European manufacturers. I see the plates are the flat ladder type plates, probably the most common type, they're the same whether it be Mullard, Amperex, and others. I think probably not these though, Mullards and Amperexes usually have the 2 or 4 seams on top of the glass and other identifying marks. The getter is common too.

Unfortunately without any codes either etched or printed it would be impossible to identify these, they could be great...… or not so great!!

As your probably know I thought my Philips tubes were either Mullards or Mazdas and it was only by identifying the codes on the glass that the Philips factory could be identified.

 

[jekjek]

I think Ultron was just a German label of various European manufacturers. I see the plates are the flat ladder type plates, probably the most common type, they're the same whether it be Mullard, Amperex, and others. I think probably not these though, Mullards and Amperexes usually have the 2 or 4 seams on top of the glass and other identifying marks. The getter is common too.

Unfortunately without any codes either etched or printed it would be impossible to identify these, they could be great...… or not so great!!

As your probably know I thought my Philips tubes were either Mullards or Mazdas and it was only by identifying the codes on the glass that the Philips factory could be identified.

Thanks baron

My APPJ will sometimes lost the left channel sound completely

I am not sure is it problem with the volume pot or the tube

I am using RCA 6V6 now but still experiencing sudden channel lost

 

[baronbeehive]

Thanks baron

My APPJ will sometimes lost the left channel sound completely

I am not sure is it problem with the volume pot or the tube

I am using RCA 6V6 now but still experiencing sudden channel lost

When did this start happening, and under what circumstances, for example does it happen after a while after switching on without you touching it or does it happen when you move one of the tubes?

First though, swop the 2 power tubes around and see if the problem moves with the tube.

 

[jekjek]

When did this start happening, and under what circumstances, for example does it happen after a while after switching on without you touching it or does it happen when you move one of the tubes?

First though, swop the 2 power tubes around and see if the problem moves with the tube.

It will happen randomly after i turn on the amp for around 20 to 30 minutes

then the left channel will fade off in 2 seconds

Initially i thought is because of the loose screw in the volume pot. I can get back the left channel by wiggling with the volume pot

Last week I have tightened up the volume pot and its tight now. No room to wiggle now

Changed power tube from 6V6 to EL33 with same issue