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Old 2nd August 2012, 01:20 PM   #11
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Quote:
Originally Posted by Osvaldo de Banfield View Post
OK, then, the voltage at the rectifier's output, is the same?
Yes, more or less the same as before.
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Old 2nd August 2012, 01:27 PM   #12
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Originally Posted by Positron View Post
I would install parasitic suppressors (grid stoppers) to the first two stages. If nothing more than precaution, but I would suspect an RF oscillation problem. I will not comment on the design or changes to lower gain.

Cheers.
If my understanding is correct, you referring to the 1st and the 2nd stages of the power amp, (the 6SN7 tubes)... I have not updated the schematic but in fact I do have the grid stopper resistor on both the tubes.

I just found one thing that I'm not sure if it prove the "too much gain" situation... when I disconnected the interconnect from the input jack of the power amp., everything alright. However, there is lot of noise when the interconnect plug in, but the other end of the cable not connect to anything... is that caused by the over gain of the amp so the cable picked the noise? (the cable is 1.5m lenght only).
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Old 2nd August 2012, 03:30 PM   #13
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Originally Posted by Angelpat View Post
Hi,

I have the hiss issue on my clone CAT SL preamp. The power supplier have been changed to 80 rectifier tube with EL84 and EF86 for stabilizer.

Everything going well with it but the very noticable "hiss" from the tweeter. I can hear the "hiss" from 5 feet distance.

The heater of the ECC82 and ECC83 have been grounded, and the 6DJ8 have been hanged at 150Vdc.

This kind of "hiss" noisy never happend when I replace the preamp with a passive preamp so I petty sure that it was came from the preamp.

Can someone help to advise where is that kind of "hiss" noisy came from and how to fix it...

I honestly don't see an issue of "too much gain" in this circuit, because it's a low gain circuit.
However, you can install a capacitor, about 7 pf 1000V silver mica, between the plate and grid of V1A (between pins 1 and 2). Install this directly on the tube socket...
This produces a small amount of negative feedback which may help kill the hiss.
Also, selecting the correct tube for V1, there are many ECC83 (12AX7) that will hiss because they are high gain tubes, and you can select one that is low noise (7025), that has much less hiss.
Also, you can install a small capacitor, between the input, pin 2 and ground. I suggest you try a value 39 pf or smaller, and I suggest you use silver mica capacitors, which have much less noise than ordinary ceramic disk capacitors.
And speaking of noise, I suggest you use silver mica to substitute any ceramic disk in the circuit.
And speaking of HISS, I suggest you install metal film or bulk metal foil resistors instead of the traditional carbon composition resistors.
(bulk metal foil has the lowest noise) Use 1 watt resistors for the plate and cathode, and use 1/2 watt for the rest of the signal path.
And so, there are a few pointers to reducing HISS in a hi fi circuit.
The quality of the parts makes a pretty substantial difference, and using a selected 7025 tube, in place of 12AX7, makes quite a heap of difference too.

I suggest you use Bugle Boy, Wing C or or Groove Tubes, selected low noise 7025 or 12AX7...

Now, GROUND the input of the amp, with a shorting plug.
Is the HISS still there?
The reality being that the hiss may actually be coming from the preamp, and is being amplified by the power amp.

Last edited by soundguruman; 2nd August 2012 at 03:42 PM.
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Old 2nd August 2012, 04:01 PM   #14
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Originally Posted by soundguruman View Post
I honestly don't see an issue of "too much gain" in this circuit, because it's a low gain circuit.
However, you can install a capacitor, about 7 pf 1000V silver mica, between the plate and grid of V1A (between pins 1 and 2). Install this directly on the tube socket...
This produces a small amount of negative feedback which may help kill the hiss.
Also, selecting the correct tube for V1, there are many ECC83 (12AX7) that will hiss because they are high gain tubes, and you can select one that is low noise (7025), that has much less hiss.
Also, you can install a small capacitor, between the input, pin 2 and ground. I suggest you try a value 39 pf or smaller, and I suggest you use silver mica capacitors, which have much less noise than ordinary ceramic disk capacitors.
And speaking of noise, I suggest you use silver mica to substitute any ceramic disk in the circuit.
And speaking of HISS, I suggest you install metal film or bulk metal foil resistors instead of the traditional carbon composition resistors.
(bulk metal foil has the lowest noise) Use 1 watt resistors for the plate and cathode, and use 1/2 watt for the rest of the signal path.
And so, there are a few pointers to reducing HISS in a hi fi circuit.
The quality of the parts makes a pretty substantial difference, and using a selected 7025 tube, in place of 12AX7, makes quite a heap of difference too.

I suggest you use Bugle Boy, Wing C or or Groove Tubes, selected low noise 7025 or 12AX7...

Now, GROUND the input of the amp, with a shorting plug.
Is the HISS still there?
The reality being that the hiss may actually be coming from the preamp, and is being amplified by the power amp.

are you referring to the preamp circuit.. the schematic that I post is the power amp.

the preamp have the ECC82 input stage and the SRPP ECC83 driver for the 6DJ8 cathode out.

You suggested about the 7pF silver mica, should it be at the ECC82 input stage or the ECC83 dirver?

I using the Mullard long plate square gitter ECC83 and all resistors are RMG 1W, the feedback from the cathode of the 6DJ8 to the ECC82 (cathode) is a 20K ohm 2W RMG as well.

I did tried lot of different tubes at all stages but did not found any signficant improvement... The Brima 13D3 replaced to the ECC82 seems the best so far.

Thanks!
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Old 2nd August 2012, 04:39 PM   #15
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Quote:
Originally Posted by Angelpat View Post
are you referring to the preamp circuit.. the schematic that I post is the power amp.

the preamp have the ECC82 input stage and the SRPP ECC83 driver for the 6DJ8 cathode out.

You suggested about the 7pF silver mica, should it be at the ECC82 input stage or the ECC83 dirver?

I using the Mullard long plate square gitter ECC83 and all resistors are RMG 1W, the feedback from the cathode of the 6DJ8 to the ECC82 (cathode) is a 20K ohm 2W RMG as well.

I did tried lot of different tubes at all stages but did not found any signficant improvement... The Brima 13D3 replaced to the ECC82 seems the best so far.

Thanks!
I suggest the 7pf at the first stage of the power amp input, between plate and grid. You can reduce this value, to provide "just enough" and not too much feedback.
Any carbon resistor is noisy, compared to metal film or bulk metal foil.
So, you have eliminated the tube as the cause.

Also tweeter level. Is the level of the tweeter turned up too high possibly, or is it "flat?"

FYI:
Carbon resistors are made of particles that are glued together.
The leads are also glued into the resistor.
This can lead to cracks or voids, inside the resistor.
Metal film resistors have welded on leads. Does not depend on glue. Therefore, much more reliable and less noisy...

Last edited by soundguruman; 2nd August 2012 at 04:45 PM.
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Old 2nd August 2012, 05:08 PM   #16
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When I was building systems, line stage and amps, and if you check the way gain structure is on 50's and 60's standard tube audio gear is,

The Volume control should be almost 3/4 way up for full power out of the amp

I never built more than 12Db gain, with volume all the way up, so at 1/2, it is 6DB gain into a line stage as the output level of mose CD players, tuners etc. is .700 volts, and most power amps are set to need just .700V in for full power.

You can take a sign wave, test signal into the line stage, put a dummy load on the amp, then go from in to out and see how much in level, with volume set at 3/4 you need for full power before sign wave distortion.

The reason for volume at 3/4 up is so the series resistance of the pot is at minimum.

I also, NEVER bypass preamp/line amp triode cathodes. I found the bypassing cathode Thang a complete myth. You get a "puddle" effect and hiss from the cap, as the pool of electrons in the cap are not quiet, like the water in a lake.

Also, the cap has a tendancy to make the gain stage more high freq gainy. I mod vintage guitar amps, a lot of Fenders etc., and I put 68 to 330 or more ohm resistors in series with the cathode caps to bring down Hsssssssssssss

I built all my line stages, 6sn7, 12au7, #76 or #37 etc. with no bypass caps, much quieter, and more real true response. Most great preamp circuits, like Fisher, show a variety of ways to make triode gain and line stages, that have no bypass, or a bypass with feedback on the cathode to control freq response, and make a real smooth tone stage. Good to look at like Fisher and other greats circuit design for example.
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Old 2nd August 2012, 05:13 PM   #17
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Quote:
Originally Posted by ElectricMan View Post
When I was building systems, line stage and amps, and if you check the way gain structure is on 50's and 60's standard tube audio gear is,

The Volume control should be almost 3/4 way up for full power out of the amp

I never built more than 12Db gain, with volume all the way up, so at 1/2, it is 6DB gain into a line stage as the output level of mose CD players, tuners etc. is .700 volts, and most power amps are set to need just .700V in for full power.

You can take a sign wave, test signal into the line stage, put a dummy load on the amp, then go from in to out and see how much in level, with volume set at 3/4 you need for full power before sign wave distortion.

The reason for volume at 3/4 up is so the series resistance of the pot is at minimum.

I also, NEVER bypass preamp/line amp triode cathodes. I found the bypassing cathode Thang a complete myth. You get a "puddle" effect and hiss from the cap, as the pool of electrons in the cap are not quiet, like the water in a lake.

Also, the cap has a tendancy to make the gain stage more high freq gainy. I mod vintage guitar amps, a lot of Fenders etc., and I put 68 to 330 or more ohm resistors in series with the cathode caps to bring down Hsssssssssssss

I built all my line stages, 6sn7, 12au7, #76 or #37 etc. with no bypass caps, much quieter, and more real true response. Most great preamp circuits, like Fisher, show a variety of ways to make triode gain and line stages, that have no bypass, or a bypass with feedback on the cathode to control freq response, and make a real smooth tone stage. Good to look at like Fisher and other greats circuit design for example.
Yes, the amp sensitivity.
Too much sensitivity = too much HISS.
Whet level sine wave applied to the input will produce a "full" clean output?
This is the sensitivity of the amp.
Now, Electric Man knows what he's talking about. Thumbs up.
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Old 3rd August 2012, 01:42 AM   #18
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Quote:
Originally Posted by Angelpat View Post
... when I disconnected the interconnect from the input jack of the power amp., everything alright. However, there is lot of noise when the interconnect plug in, but the other end of the cable not connect to anything... is that caused by the over gain of the amp so the cable picked the noise? (the cable is 1.5m lenght only).
Are you saying that when the input cable to the first stage is disconnected, then there is no hiss ? Does the input to the first stage use a socket that shorts the input to ground when there is no plug inserted?
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Old 3rd August 2012, 01:53 AM   #19
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You cannot test the power amp noise until the input is grounded.
Any unterminated wire to the input will cause hum and noise.
Seems like the hiss only occurs when the preamp is plugged in?
Then the noise is in the preamp and the power amp is amplifying the preamp noise.
Seems like we should be looking at the preamp, not the power amp.

Plug a CD player directly into the power amp input.
Don't use the preamp.
Don't play a CD, but just turn on the power amp and the CD player.
Do you hear HISS?
I thought you would not.
It's the preamp that's hissing.

Last edited by soundguruman; 3rd August 2012 at 01:56 AM.
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Old 3rd August 2012, 08:57 AM   #20
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Default Hsssssssss could be oscillation

Hi

The Hsssss could be the triode in stages going into high frequency oscillation as the fat cathode bypass caps make them unstable, and triodes have a tendency to become high freq sensitive or peaky when bypassed.

Another reason why I never bypass cathodes.

Solution A. Add about 56 to 330 ohms "R" from each cap to the cathode. Also use 12K to 33K grid resistors, all line stages. This adds a tiny bit of linear negative feedback, and makes the triodes stable.

Solution B. Take the caps off and run free of the puddle effect, and the non linearity of the cap. A resistor is much more linear than a big cap, and the transit response of a cap is pretty bad, varies with waveform and frequency.


A small amount of neg feedback, with series grid and cathode resistors makes the tubes stable, quiet, and much better tone.

Also, SRPP, or Mu stages, if you look at schemos of gear from the past, you will find small PF value caps, and more resistors added to make the stage stable.


Look at the channel A AF amp and Cathode follower in the schemo. The AF amp is a great classic design, and has great tone.

I used to mod these and sell them, one of the popular preamps of the day, also bypassed the EQ.

Change the 12at7 to a 12au7, and drop the 56K to 12K for lower impedance out.

Lots of good design work here.

Also, if you want to leave all as is, some series resistance, maybe 33K and a 47PF cap from first grid to ground, or from plate to grid might do it.

Scope helps
Attached Images
File Type: jpg Fisher_400C_schem.jpg (302.9 KB, 123 views)

Last edited by ElectricMan; 3rd August 2012 at 09:00 AM.
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