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Old 17th May 2012, 06:08 AM   #41
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Quote:
Originally Posted by tomchr View Post
<snip>

I just happened to notice that just about every supply in the amp comes from a voltage doubler -- including the main B+ supply. I suggest monitoring all the supply voltages on an o'scope. I'm a little skeptical about pulling hundreds of mA from a voltage doubler, but I haven't worked that much with those. Of course, if all supplies are solid, the issue isn't with the supplies.

~Tom
Tom, you would be right to be suspicious of this. Voltage doubler is a big compromise in feeding a power stage. Look at it as a series-stacked half-wave rectified supply- as it really is.

In practice, the source impedance of the supply increases where a doubler is used. Compared to a bridge rectified supply, the voltage drop will be nearly doubled, for a step change in current demand, all else being comparable. The 18R in the rectifier feed will add to this effect. Using this supply for a high power amplifier, with high gain voltage amplifier is likely to be trouble.

doublers are OK where you have a constant current-demand, and large capacitors.

But for the power stage, I think the sound will be disappointing with a doubler, even if the instability can be fixed.
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Old 17th May 2012, 11:06 AM   #42
DF96 is offline DF96  England
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Quote:
Originally Posted by tomchr
I doubt it's a stability issue within the main feedback loop.
I agree, but I was concerned about the coupling between the first two stages which is where the oscillation seems to be coming from. Feedback is via the high impedance supply rail. I am concerned that the 'CCS' and the interstage coupling all have time constants in the same region of a few Hz so they will all have lots of phase shift in the same frequency region.

I agree with your concern about voltage doubler supplies. I had not spotted that. The last thing you need for LF stability is high impedance supply rails!
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Old 17th May 2012, 02:22 PM   #43
MelB is offline MelB  Canada
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R56 to R61 are also providing some negative feedback to the B+ possibly 180 degrees out of phase to the B+ supply??? They should cancel unless they don't quite balance?
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Old 17th May 2012, 03:53 PM   #44
tomchr is offline tomchr  United States
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Originally Posted by MelB View Post
R56 to R61 are also providing some negative feedback to the B+ possibly 180 degrees out of phase to the B+ supply??? They should cancel unless they don't quite balance?
Actually no, though I can see how you reached that conclusion.

Although, it looks like R56~R61 and C31 are forming a feedback loop back to B+, they're actually just connecting from B+ to ground. C31 is just a local supply bypassing cap. I suspect R56~R61 and specifically the diodes connected in parallel with R56~R61 are to protect the OPT in case it's operated without a speaker load. That's my guess... I don't think those are part of the motor boating issue unless the amp is currently operated without speakers (or dummy load) connected.

~Tom
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Last edited by tomchr; 17th May 2012 at 03:56 PM.
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Old 17th May 2012, 05:49 PM   #45
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Thaks a lot for all your help, this is realy apreciated! i get some parts tomorrow and hopefully can get to test a bit more.
for now i merged the two drawings from the first post to make it easier to see the whole thing.

i detached the ccs on V2 and replaced it with a resistor for testing, there is only one ccs @V1 and the one in the LTP.
the amp is connected to a 8r resistor.
Feedback is diconnected-
the second channel 320V b+ is disconnected.
i drive it from a pc or a freq. gen, (el cheapo) no difference,
set to 40Hz. I can see a blury (looks like its there twice) wave on the oscilloscope when i measure on the cathode of the output tubes. the Led on the protection circuit flashes, my guess its about 2 Hz, as soon as the Vol. pot is turned up and the amp produces about 5V into the load of 8r.
if i detach the protection circuit from the output tubes there is no change on the oscilloscope.

@melB i shorted R1 ( was suggested before) and there was no change.

i was suspicious of the doublers as well, but the guy who designed the circuit i used mentioned that he does that all the time with success..
as i wondered in the original post, would a regulator in the b+ help with the stability of the b+ even if its positioned after a Doubler?
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Old 18th May 2012, 02:32 AM   #46
tomchr is offline tomchr  United States
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Quote:
Originally Posted by yurgs View Post
i drive it from a pc or a freq. gen, (el cheapo) no difference,
set to 40Hz. I can see a blury (looks like its there twice) wave on the oscilloscope when i measure on the cathode of the output tubes.
Are there any issues when the amp is driven by other frequencies or the input is grounded (no signal)? Your most recent post seems to indicate that the problem goes away when the volume is turned down. Is this correct?
If the problem is only there during sine wave testing at very low frequencies and goes away when the signal is removed, then my bet is that you're looking at supply sag and excessive supply ripple. That would also explain why you're seeing a "double trace" on the oscilloscope. If this is the case, you should see the problem get worse at even lower input frequencies and higher input signal amplitudes.

LF sine wave testing is pretty hard on an amp. The voltage doubler supply makes this even harder as the supply caps are charged at 50 Hz not 100 Hz as they would be with a "normal" supply. The reason you're seeing the bias LEDs blink is then probably that you have the 40 Hz signal beating against the 50 Hz mains. If this is the case, I'd expect the amp to be able to reproduce 40 Hz at low output powers but not at higher output powers. Is this what you're seeing in your circuit? If that is what you're seeing in your circuit, the only solution I can think of is to increase C8, C9. Or accept that you can't reproduce <40 Hz at full output power.

Have you looked at the supply voltages with an oscilloscope yet? What were the results?

Also measure the B+ with an AC (yes AC) voltmeter. You'll probably have a few volts of AC (ripple) on the B+ with no signal. Apply 1 kHz input signal. Note the ripple voltage. Decrease the frequency. Note the change in ripple - especially below 50 Hz. What do you see?

Quote:
Originally Posted by yurgs View Post
the Led on the protection circuit flashes, my guess its about 2 Hz, as soon as the Vol. pot is turned up and the amp produces about 5V into the load of 8r.
The protection/bias monitoring circuit in Post #33 has five LEDs in it. Which one of them is flashing?
One of them is labeled "RED flashing" implying that it has a built-in circuit that makes the LED flash when power is applied (yes, that kind of LED does exist).
If the LED in series with R23 and the SCR is flashing at 2 Hz then it's because the SCR is triggered and the internal circuit in the LED is designed for 2 Hz flashing. Once the SCR has triggered the only way to turn it off is to kill its anode current. I'm guessing that's not the LED that you say is flashing as it would presumably turn off the amp via the relay marked K2.

It also looks like the diode to the right of R25 should be reversed. Otherwise that 3 second time delay won't work. Unless I'm misunderstanding something. It's a bit hard to read that schematic, honestly.

Quote:
Originally Posted by yurgs View Post
i was suspicious of the doublers as well, but the guy who designed the circuit i used mentioned that he does that all the time with success..
as i wondered in the original post, would a regulator in the b+ help with the stability of the b+ even if its positioned after a Doubler?
Doublers are often used in PC power supplies for 115 V operation. So the circuit can be designed to work. But in case of a PC, the doubler is followed by two regulators - first a power factor correction circuit (boost regulator) then the buck converter to the voltages used in the PC. Turning 115 V AC into 400 V DC and then back down to 12 V DC is a completely different story than creating a good B+ from a voltage doubler.

In your case, you should use a high-voltage regulator such as my 21st Century Maida Regulator after the doubler circuit. So the regulator input would connect to the top of C8, the regulator ground to the bottom of C9. The output of the regulator would then feed the OPTs and the rest of the power supply circuit. The regulator would reduce whatever ripple is on C8+C9 to practically nothing -- as long as the regulator always has about 20 V across it. So it wouldn't solve your issues at 40 Hz, full output power but I bet the amp would be a lot quieter and sound better with the much, much lower output impedance of the regulator.

Let's figure out what's going on with your amp before changing the supply, though.

~Tom
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Old 18th May 2012, 08:21 AM   #47
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To clearly illustrate the unsuitability of voltage doublers in high-power AB1-amps, let us explore the calculations.

The output stage is KT88 class, AB1, fixed-bias, ultralinear, supply labelled 475V. This is a data-sheet condition for the KT88, and allows us to see the required demand on the power supply.

idle: 100mA [2x 50m]
full-power: 280mA [2x 140m]

We'll allow 20mA per channel for the other stages, but the exact value is not important: it is nearly constant.

The OP's schematic shows both channels hooked to the same power supply, so we must double the current demands.

The PSUD2 snap shows the outcome as the current rises from idle to full power. The degree of problems facing this design are obvious: the B+ sinks from 475V to 375V. говно! 100V sag. Even if only one channel runs from this B+, the sag will still be 70V.

The rms current in the trafo is 1,5A rms at full load, so the trafo appears to be correctly sized - but the effect of the doubler remains.

We can explore whether upgrading the quality of the doubler components might improve performance. Removing the 18R rectifier-protection resistor, and assuming that all copper-losses can be reduced to 7 ohm (referred to secondary) reduce sag to about 50V, but the rectifiers and capacitors would need to be upgraded to withstand the increased stress that this change will force.

As a calibrating remark, many guitar amps have B+ supplies that wilfully sag under load. But typical 50W Marshalls barely sag by 50V, even when played hard. To sag by 50-100V is not HiFi, for sure.

In this design, things get worse because the voltage-gain stages will suffer, when the main supplies fall. This may be enough to cause motorboating, especially if the open-loop gain of the amp is higher than necessary.

A HV Regulator will not be a good solution in this case - if we need 20V headroom (which is typical), the output voltage would need to be set to 350V, which would degrade the performance of the driver stage very badly, as 6SN7 stages like a B+ of 450V or more for a KT88-class driver. The regulator would be stressed with 125V and 30W of idle power, which would be deeply undesirable, if not absurd.

What to do?

Sharing the power supply between driver stages and power output stages is always a compromise, but with a doubler, the performance will be really bad.

The right solution depends on your expectations for the physical layout. If you can accept that a bigger power supply is needed for good performance, then I recommend that you leave the doubler supply to feed the driver and heating circuits, and use a properly specified trafo for the power stage:

primary 230V
secondary 355V with minimum 750mA rms, or 300VA rating
secondary referred resistance: 7 ohm
Bridge rectifier using UF4007 or 3A equivalents
Choke 1.3H 20 ohms DCR

Model carefully with PSUD2 to check the supply meets your design requirements.

A supply built with this, and ordinary electrolytics (Nichicon KX for preference, or LG, or Panasonic TSHA/HB or TSUP) and a 20-DCR 1.3H choke will drop only about 5V to full load and will be a platform for the best possible sound.

Such a Trafo does not need to be high cost. In most locations, industrial Panel Trafos can be bought for low cost, and here is my favourite, for UK builders:

300 VA

Whether this fixes the motorboating is not certain, but such a poor supply will make things very hard for you to debug, and even when you fix it, the performance will not be worth the effort
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Old 18th May 2012, 09:27 AM   #48
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Quote:
Originally Posted by Rod Coleman View Post
The PSUD2 snap shows the outcome as the current rises from idle to full power. The degree of problems facing this design are obvious: the B+ sinks from 475V to 375V. говно! 100V sag. Even if only one channel runs from this B+, the sag will still be 70V.
Your Russian is perfect :-)
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Old 18th May 2012, 11:24 AM   #49
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спасибо! I'm just a poor student of Russian. But my Design Engineering is on solid ground, I trust.
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Old 18th May 2012, 12:16 PM   #50
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Quote:
Originally Posted by Rod Coleman Click the image to open in full size.
The PSUD2 snap shows the outcome as the current rises from idle to full power. The degree of problems facing this design are obvious: the B+ sinks from 475V to 375V. говно! 100V sag. Even if only one channel runs from this B+, the sag will still be 70V.


Thank you for your explanations.
I agree that the doubler is not the ideal choice, yet,
if i connect both channels as it is in the circuit, and i drive one channel to clipping with a 1kHz signal the voltage drops to about 467V.
Thats quite different to the simulation...
and this is with the 18r in the circuit!
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