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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: May Sun 26, 2019 6:54 pm 
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Hello John,
I hope you get it figured out


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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: May Sun 26, 2019 8:38 pm 
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radiorich wrote:
Hello John,
I hope you get it figured out




Hi Rich. I will find it. Having much fun on the troubleshooting.
Good to hear from you...... 8)

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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: May Sun 26, 2019 8:51 pm 
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Flip
I don't understand how you could confuse the two amps. One is working perfectly and belongs in the Smithsonian. 8)
The other, new amp has a problem. That is the one we are referring to.
I forget I'm talking to someone with a Spock mentality. :P

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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: May Sun 26, 2019 9:01 pm 
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I am pleased to report that the problem has been found, and repaired.
Had nothing to do with grounding. Now the noise is gone. And the house ground connects to the chassis.
The B- also connects to the chassis, at the inputs only.

I found the B+ conductor leaving the power supply compartment was running parallel and right next to
the AC filament wiring.
I separated both high voltage conductors. Re-routed them through dedicated holes in the firewall.
That's all the problem.
So simple, but complicated.
The AC was affecting the power line, even at such a low impedance.
Was this termed as a ground loop?
Attachment:
lpprob1.jpg
lpprob1.jpg [ 228.43 KiB | Viewed 634 times ]

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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: May Mon 27, 2019 2:59 am 
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john8750 wrote:
I am pleased to report that the problem has been found, and repaired.
Had nothing to do with grounding. Now the noise is gone. And the house ground connects to the chassis.
The B- also connects to the chassis, at the inputs only.

I found the B+ conductor leaving the power supply compartment was running parallel and right next to
the AC filament wiring.
I separated both high voltage conductors. Re-routed them through dedicated holes in the firewall.
That's all the problem.
So simple, but complicated.
The AC was affecting the power line, even at such a low impedance.
Was this termed as a ground loop?
Attachment:
lpprob1.jpg
Glad you found it. :D

I don't think that's a ground loop. Sounds more like induced voltage/current but I can't quite resolve how that would explain the given symptoms.

As for 'confusion'. You need to remember that I 'm not there and don't know a thing other than what you tell me. For example, I still have no idea what input you are/were testing when you found the hum. You didn't tell me.


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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: May Mon 27, 2019 6:56 am 
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Flipperhome wrote:
john8750 wrote:
I am pleased to report that the problem has been found, and repaired.
Had nothing to do with grounding. Now the noise is gone. And the house ground connects to the chassis.
The B- also connects to the chassis, at the inputs only.

I found the B+ conductor leaving the power supply compartment was running parallel and right next to
the AC filament wiring.
I separated both high voltage conductors. Re-routed them through dedicated holes in the firewall.
That's all the problem.
So simple, but complicated.
The AC was affecting the power line, even at such a low impedance.
Was this termed as a ground loop?
Attachment:
lpprob1.jpg
Glad you found it. :D

I don't think that's a ground loop. Sounds more like induced voltage/current but I can't quite resolve how that would explain the given symptoms.

As for 'confusion'. You need to remember that I 'm not there and don't know a thing other than what you tell me. For example, I still have no idea what input you are/were testing when you found the hum. You didn't tell me.



I understand.
It would be so cool if you were here. I wouldn't have had the problem. You would see it right away.
Sure teaches me a lesson about AC filament wiring.

The MP3 input is the only input I was using.
All 3 input and output RCA's are common bussed grounds and connect to the chassis at that point.

I will do some testing on the RIAA input. It has a hum which is controlled with the volume level. About 100hz or so. But not the problem I just fixed
on the other inputs.

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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: May Tue 28, 2019 3:31 am 
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Well- Turned on the power in the shop this morning. Amp is humming again.
OK. Time to get serious.
Went into the house, got the original little Prince.
Hooked it up in the shop. And-humming.

Hooked up the new Little Prince in the house.
Not humming.

Must be a problem with the ground to the shop.

THEN IT HIT ME. Flip, we went through this problem before.
The power inverter I use for the solar circuit messes with the ground.

Fine tuned the filament trim pot.


I still have a slight hum in the RIAA input.
And the treble pot has little effect. Gotta be my fault somewhere.
Attachment:
lp final1.jpg
lp final1.jpg [ 201.11 KiB | Viewed 617 times ]

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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: May Tue 28, 2019 3:35 am 
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Noticed that my 807 tubes have a blue glow inside. Is this normal :?:

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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: May Tue 28, 2019 5:37 am 
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john8750 wrote:
Noticed that my 807 tubes have a blue glow inside. Is this normal :?:
Yes. And you'll probably see it fluctuate with the music, especially the bass. Here's a static shot of the 'blue glow'.
Attachment:
807-3.jpg
807-3.jpg [ 135.35 KiB | Viewed 615 times ]


Here's a video of a fellah who's miffed by the blue glow of his 'new' 807 clone replacements. Good shot beginning around 6:30.
https://www.youtube.com/watch?v=8VCigZ0Fe9o

He's apparently concerned too but it's quite normal.

That's the case 99% of the time. In the rare case the tube is gassy a 'whitish-blue glow' (or purplish glow) will be a soft haze in the interior rather than a distinct patterning predominately on the glass (or other tube structures).

The 'blue glow' is fluorescence as the electrons escaping the structure hit something. If it's gassy they hit the gas, which is why it shows as hazy cloud inside the tube. Otherwise (for the 99% of the time it's a good tube) they hit the glass (or an intervening structure such as the mica supports) creating more of a 'surface' glow that tends to look a little like an inverse shadow, meaning a bright area where the electrons hit rather than 'dark'. When 'music' is played the plate voltage varies so the number and energy of the electrons vary giving it a 'dancing' effect.


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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: May Wed 29, 2019 12:10 am 
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Flipperhome wrote:
john8750 wrote:
Noticed that my 807 tubes have a blue glow inside. Is this normal :?:
Yes. And you'll probably see it fluctuate with the music, especially the bass. Here's a static shot of the 'blue glow'.
Attachment:
807-3.jpg


Here's a video of a fellah who's miffed by the blue glow of his 'new' 807 clone replacements. Good shot beginning around 6:30.
https://www.youtube.com/watch?v=8VCigZ0Fe9o

He's apparently concerned too but it's quite normal.

That's the case 99% of the time. In the rare case the tube is gassy a 'whitish-blue glow' (or purplish glow) will be a soft haze in the interior rather than a distinct patterning predominately on the glass (or other tube structures).

The 'blue glow' is fluorescence as the electrons escaping the structure hit something. If it's gassy they hit the gas, which is why it shows as hazy cloud inside the tube. Otherwise (for the 99% of the time it's a good tube) they hit the glass (or an intervening structure such as the mica supports) creating more of a 'surface' glow that tends to look a little like an inverse shadow, meaning a bright area where the electrons hit rather than 'dark'. When 'music' is played the plate voltage varies so the number and energy of the electrons vary giving it a 'dancing' effect.



Yeah, I notice it dancing with the music. Much more visible inside the cabinet.
I have it connected to the Little Prince 12sl7. Am very happy with the sounds.
The matching chassis looks cool.

Problems to work on:
Treble tone control not very responsive. A tone difference can be heard by turning the pot. But I don't hear a difference from cut to boost.
The RIAA still has a hum, controlled by the volume. The equalizing circuit seems perfect. But riding on a hum in the background.
I remember what you told me about the 12sl7 not being as good as the 20ez7. And needing to shield the tubes. Any ideas?

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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: May Wed 29, 2019 3:58 am 
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john8750 wrote:
Flipperhome wrote:
Yes. And you'll probably see it fluctuate with the music, especially the bass. Here's a static shot of the 'blue glow'.
Attachment:
The attachment 807-3.jpg is no longer available

Here's a video of a fellah who's miffed by the blue glow of his 'new' 807 clone replacements. Good shot beginning around 6:30.
https://www.youtube.com/watch?v=8VCigZ0Fe9o

He's apparently concerned too but it's quite normal.

That's the case 99% of the time. In the rare case the tube is gassy a 'whitish-blue glow' (or purplish glow) will be a soft haze in the interior rather than a distinct patterning predominately on the glass (or other tube structures).

The 'blue glow' is fluorescence as the electrons escaping the structure hit something. If it's gassy they hit the gas, which is why it shows as hazy cloud inside the tube. Otherwise (for the 99% of the time it's a good tube) they hit the glass (or an intervening structure such as the mica supports) creating more of a 'surface' glow that tends to look a little like an inverse shadow, meaning a bright area where the electrons hit rather than 'dark'. When 'music' is played the plate voltage varies so the number and energy of the electrons vary giving it a 'dancing' effect.

Yeah, I notice it dancing with the music. Much more visible inside the cabinet.
I have it connected to the Little Prince 12sl7. Am very happy with the sounds.
The matching chassis looks cool.

Problems to work on:
Treble tone control not very responsive. A tone difference can be heard by turning the pot. But I don't hear a difference from cut to boost.
The RIAA still has a hum, controlled by the volume. The equalizing circuit seems perfect. But riding on a hum in the background.
I remember what you told me about the 12sl7 not being as good as the 20ez7. And needing to shield the tubes. Any ideas?
The treble control should work exactly the same as the original. The 12SL7 being 'not as good' was my thinking it would have slightly higher distortion due to the lesser individual tube gain, not that any of the function would behave differently.

Have you shielded all the inputs yet? Ground wire from the turntable to the preamp? Your inverter noise? I'd make dern sure everything is working properly before entertaining the idea of shielding the tubes. If it comes down to that my suggestion is the same as before. Make a perforated top cage around all the tubes like they did in the olden days. Like this modern variant.
Attachment:
pa_one_cage.jpg
pa_one_cage.jpg [ 177.58 KiB | Viewed 601 times ]


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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: May Wed 29, 2019 5:46 am 
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The treble control problem has to be a wiring error. I will find it. The quality of sound is acceptable, although I feel it is not exactly the same as the 20ez7.
Still Hi-FI. I understand it is the exact same circuit.

I don't have anything to shield inside the RIAA amp. Components connect all parts of the circuit. I will try a tin can over the tubes, see if it makes a difference. Will check the circuit closely.

I have the amp connected in the house where the ground works. Only problem is in the shop.

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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: May Wed 29, 2019 5:55 am 
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john8750 wrote:
The treble control problem has to be a wiring error. I will find it. The quality of sound is acceptable, although I feel it is not exactly the same as the 20ez7.
Still Hi-FI. I understand it is the exact same circuit.

I don't have anything to shield inside the RIAA amp. Components connect all parts of the circuit. I will try a tin can over the tubes, see if it makes a difference. Will check the circuit closely.

I have the amp connected in the house where the ground works. Only problem is in the shop.
or maximal effect the 'tin can' should be connected to chassis (earth). Also, don't run it very long without ventilation. They might not seem very warm but confined with no air circulation they'll eventually get hot.


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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: May Fri 31, 2019 10:11 pm 
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I don't know what else to check in the RIAA circuit. It has a hum that is controlled by the volume pot. No other interference.
And shielding the tubes didn't help. I noticed if I thump the chassis, sounds like a drum in the speakers. Tapping the tube makes the loudest sound.
So, remembering your warning about the 12SL7, I don't know if it can be fixed.
Could it be the power transformer transferring a vibration through the chassis effecting the tubes.
Do you think the equalizing feedback circuit could me modified for the 12SL7?


Now, the 20EZ7 version is, IMO, the best.
It has no hum anywhere, and sounds great.
Here is the insides of the 20EZ7.
Attachment:
olp2.jpg
olp2.jpg [ 333.06 KiB | Viewed 569 times ]

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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: Jun Sat 01, 2019 12:23 am 
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john8750 wrote:
I don't know what else to check in the RIAA circuit. It has a hum that is controlled by the volume pot. No other interference.
And shielding the tubes didn't help. I noticed if I thump the chassis, sounds like a drum in the speakers. Tapping the tube makes the loudest sound.
So, remembering your warning about the 12SL7, I don't know if it can be fixed.
Could it be the power transformer transferring a vibration through the chassis effecting the tubes.
Do you think the equalizing feedback circuit could me modified for the 12SL7?


Now, the 20EZ7 version is, IMO, the best.
It has no hum anywhere, and sounds great.
Here is the insides of the 20EZ7.
Attachment:
olp2.jpg
I don't know how microphonic the 12SL7 is, but it sounds relatively sensitive, and if the power transformer is 'humming' that vey well could be transferring it though chassis vibration.

An amusing anecdote. A few years back I designed a 6BQ5 'Mini-Guitar Amp'. It was actually a hybrid but that's another story. Anyway, I knew 6BQ5s were microphonic so I tested a hand selected the best ones for the amp (I've got a whole box of the things). It worked great. In fact, it worked so good that I took it to a local recording studio for testing and review. During setup we ran into a little snag and I didn't bring long enough cables to plug into their system. Anyway, to make a long story short I placed it on a piano bench (not a blessed thing else in the room), flipped on power, and the most horrendous squeal you ever heard blasted out of the thing. :shock: Needless to say I panicked and my underwear wasn't clean any more. After some scrambling I figured it out though. The piano bench was acting like a drum head feeding vibrations to the amp and causing it to feedback. Microphonics.

Any other place I could have put it would have been fine but NOOOOoooo. I had to pick the worst possible scenario. Not intentionally mind you. It just turned out that way.

The point to the story is vibrations can play hell with tubes and, being such high gain (it's more than the guitar amp), the RIAA section is the place it would happen.

You could try isolating the PT from the chassis to see if that's the cause. Can you unbolt it and place some rubber grommets under it? That might not get rid of it, depending on the grommet compliance, but if the hum changes in character in any way that might be a clue that's where it's coming from.

It could also be bad routing of the heater wiring.


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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: Jun Sun 02, 2019 5:09 pm 
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That's a great story Flip. I had a problem with an old record player and a tube amp. Sitting in its console, A heck of a rumble was in the speakers. Worked on it for days. Finally discovered that the motor Barings were dry, rumbling.
Sometimes the problem is so simple.
The little prince just isn't a good idea with the 12SL7. Or, would DC heater voltage be better. I lifted the power transformer and still had hum.
I have checked the circuit wiring forward and back. It is correct, and matches the first Little Prince.

Any way we can get 75vdc from the power transformer? I know 150ma, probably not. Or change to a different PT?
DC heaters might make a difference. Other than that I don't know what else to try :cry:

Thanks again Flip....

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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: Jun Sun 02, 2019 7:47 pm 
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john8750 wrote:
That's a great story Flip. I had a problem with an old record player and a tube amp. Sitting in its console, A heck of a rumble was in the speakers. Worked on it for days. Finally discovered that the motor Barings were dry, rumbling.
Sometimes the problem is so simple.
The little prince just isn't a good idea with the 12SL7. Or, would DC heater voltage be better. I lifted the power transformer and still had hum.
I have checked the circuit wiring forward and back. It is correct, and matches the first Little Prince.

Any way we can get 75vdc from the power transformer? I know 150ma, probably not. Or change to a different PT?
DC heaters might make a difference. Other than that I don't know what else to try :cry:

Thanks again Flip....
Btw, some people might wonder if I knew microphonics was an issue why it came as so much of a surprise but it's precisely because I was aware, in the beginning, that it was. See, that's one of the reasons it was designed as a hybrid with a high gain jfet input stage. The 6BQ7s weren't in any 'sensitive' areas and, of course, I had done extensive testing to see if the 'problem' was solved, and it was. I just didn't make it 'piano bench' proof. :lol: (it was because the amp was so small and light that it was affected. That and I had taken the feet off so it was sitting 'hard' on the bench).

I don't think it's the 12SL7s that are the problem, it's the lack of DC heaters. It really should only be necessary on the 12SL7s in the RIAA amp. Unfortunately. that PT doesn't have enough 12V power to rectify it to DC. A Full wave bridge almost doubles the current pull so (for the whole lot) instead of 1 A you'd need 2A (roughly speaking). Alternately, you might get by with tacking on a small 12VAC 1A for DC to just the 3 RIAA tubes.

Now, there's something else we could try first that theoretically just might work, and that's to positively bias the heaters. Harman Kardon used that trick in their A224 amp. Of course, they had a nice power output stage to create the positive bias from the 6BQ5 PP cathodes but we could perhaps do the equivalent with a divider off the main power supply. I'm debating where to try that without affecting the filtering but, what the heck, we can try a simple version and if that has any effect worry about optimizing it later. So, disconnect the hum pot wiper from ground and then take it to the junction of a 100k and 10k. Then take the other end of the 100k to B+ (270V) and the other end of the 10k to ground. That'll give use a divider for about 24VDC. I'd suggest making the divider first, before swapping the heater ground connection, and verifying the voltage because the heaters can't tolerate more than +-90VDC so if something is wrong with your setup you could pop the heater insulation.

I'm not sure how the impedance of the divider might affect it's effectiveness so, if it does, try adding a 10 uF (50V or more) to the junction and see if that improves the rejection.


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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: Jun Mon 03, 2019 1:22 am 
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That will be easy to try. 8) I have the resistors :D

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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: Jun Mon 03, 2019 1:34 am 
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john8750 wrote:
That will be easy to try. 8) I have the resistors :D
The Borg Queen just didn't know what the heck she was talking about when she said resistors are futile, did she? :lol:

Uh, no, not resistors. The word is resisTANCE. The phrase was "resistance is futile."

Oh, that's different..... Never mind.


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 Post subject: Re: Another Little Prince- tube preamp
PostPosted: Jun Mon 03, 2019 6:43 am 
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Flipperhome wrote:
john8750 wrote:
That will be easy to try. 8) I have the resistors :D
The Borg Queen just didn't know what the heck she was talking about when she said resistors are futile, did she? :lol:

Uh, no, not resistors. The word is resisTANCE. The phrase was "resistance is futile."

Oh, that's different..... Never mind.




Yeah, the Borg. 7of9. Not much metal in that assimilation :roll:
I will let you know if that helps.

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