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Old 2nd December 2004, 06:08 AM   #1
waner is offline waner  Sweden
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Default EL34 Push Pull weirdness

I need help!

I have bulit a 30W EL34 Push Pull amp (http://www.lundahl.se/pdfs/claus_byr...fier_30wpp.pdf)that I can`t get to work as it should.

1.I`ve got a huge low freq noise in the amplifier.
2.The amplifier gets distorted very early.
3.There is a pulse sounding in the speaker (similar to the sound when you tapping on a microphone). The pulse is linear not random (oscilation?). If I measure with a multimeter on the anod of the EF86 the pulse disapear. If I look at the amplifier in the dark the EL34 blinks blue in time with the pulse. One EL34 at the time. The bias voltage is also pulsing (not stable).

I have tried to switch the NFB on the secondary, then the amp really started to oscilate.
I have tried drive the amp without NFB that increased the LF noise.
All the dc voltage seems to be okay

I have an external PS to the amp. All the leads to the amp runs in the same cable (both the high voltage and the AC for the filament) could that have something to do with the LF noise?
The Choke in the PS is reversed 180 deg to the maintransformer. Could that cause the LF noise?

Please please help me

Regards
Mattias
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Old 2nd December 2004, 01:46 PM   #2
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Quote:
3.There is a pulse sounding in the speaker (similar to the sound when you tapping on a microphone). The pulse is linear not random (oscilation?). If I measure with a multimeter on the anod of the EF86 the pulse disapear. If I look at the amplifier in the dark the EL34 blinks blue in time with the pulse. One EL34 at the time. The bias voltage is also pulsing (not stable).
What you are describing is called motor boating. This is unwanted coupling of signal either from the power supply or through grounding, resulting in Positive feedback at some low frequency, usually at 1 or 2Hz. Remember all wire has some resistance! This can be caused by:

1. Bad or miss-wired decoupling capacitor, i.e. C4, C3, C12 on the 5-20 schematic.
2. Miss-wired ground wire(s).
3. Bad layout. i.e. using a common ground return path (wire) for all stages of the amplifier to the power supply.


The input/driver circuitry should have their own ground wire separate from the output stage ground wiring, connected only at (inside) the power supply ground.
The heater supply should have it's own wiring, i.e. not inside the same cable. 16-18ga twisted pair depending on the distance between the amp and PS.
Try changing where the spkr ground/secondary connects to ground.
Go back and re-read the manual and make sure everything is connected and goes to where it's supposed to go.

Quote:
I have tried drive the amp without NFB that increased the LF noise.
This tells me that it definitely has nothing to do with the NFB loop.
Hope this helps and gives you some ideas where to look! If you haven't read this before, it may help.

www.audioxpress.com/resource/audioclass/

Wayne
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Old 2nd December 2004, 02:15 PM   #3
nilrog is offline nilrog  Sweden
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Have you built this amplifier with the same components as described in the paper? I'm going to build the same amp but i'm still waiting for some parts.

Quote:
3.There is a pulse sounding in the speaker (similar to the sound when you tapping on a microphone).
The swedish magazine AoE had the same amplifier as a project last year during the summer. We'll, not exactly the same... They used LL1620 transformers and had added back some of the components that was removed by Claus along with some other modifications.

Anyway, they stated that there was an oscillation of ~.5 Hz caused by the 470nF caps before the EL34s so they lowered the value of these caps.

/Roger
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Old 2nd December 2004, 03:14 PM   #4
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Default Re: EL34 Push Pull weirdness

Quote:
Originally posted by waner
I have bulit a 30W EL34 Push Pull amp (http://www.lundahl.se/pdfs/claus_byr...fier_30wpp.pdf)that I can`t get to work as it should.
Hmm, pretty good circuit. Looks like a cross between two of my schematics actually.

Quote:

1.I`ve got a huge low freq noise in the amplifier.
TURN IT OFF!

Quote:

2.The amplifier gets distorted very early.
3.There is a pulse sounding in the speaker (similar to the sound when you tapping on a microphone). The pulse is linear not random (oscilation?).
Motorboating.

Quote:
If I measure with a multimeter on the anod of the EF86 the pulse disapear. If I look at the amplifier in the dark the EL34 blinks blue in time with the pulse. One EL34 at the time. The bias voltage is also pulsing (not stable).
Because, like any oscillator, it is running between cutoff and saturation. Cutoff is hard enough that the blue glow disappears from the tubes, while saturation is hard enough that grid current appears (from overdriving the output grids), biasing them down. This is class C, 'grid lead biased' (so called from the bias voltage appearing across the grid leak resistor).

Quote:
I have tried to switch the NFB on the secondary, then the amp really started to oscilate.
Alright, then you have a few too much phase shift somewhere forward. If it screams backwards, then you have NFB connected correctly as-is.

Quote:

I have tried drive the amp without NFB that increased the LF noise.
Ok, guess not. Must be PSU as others have said.

Quote:

I have an external PS to the amp. All the leads to the amp runs in the same cable (both the high voltage and the AC for the filament) could that have something to do with the LF noise?
No. Low frequencies like these would never be coupled from wire to wire, they are just too slow to induce any voltage or current in nearby wires.

Quote:

The Choke in the PS is reversed 180 deg to the maintransformer. Could that cause the LF noise?
Nah. Although it should be 90 degrees...

I tell ya what I would do. I'd trim down the power supply, use only enough resistance to decouple the stages. For the ECC83, instead of a 10k, use 1k. Use 4.7k for the next (supplies the EF86). Or even none for the second - the pentode and dual triode can share the same supply rail. You'll have to rebias the pentode stage (adjust the 390 ohm cathode resistor until plate is around 80V). Oh, and remove the zener diode in the PSU.

Tim
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Old 2nd December 2004, 07:05 PM   #5
waner is offline waner  Sweden
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Thank you so much for your replys!!!

I have grounded all of the components to ne point in the amplifier. This point goes trough the cable to the PS and this is grounded at one point in the PS.
The heater wiring has it one wiring but it goes to the PS in the same cable (it`s a cable with 12 leads) as the DC voltage.
The cable is about 10 feet long.

I will have to take a bite in the sour apple and do some heavy ground rewiring.

Thank you so much Wayne.


All the components are the same as in the schematics exept for the outputtransformer. I have used a LL1682/PP 5:5500. I use 6ohms speakers that will give me a primary of 6500 ohm.
I will check the 470 caps

Thank you Nilrog
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Old 2nd December 2004, 07:05 PM   #6
waner is offline waner  Sweden
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Hi Sch3mat1c

I will try to redesign the PS (it seems easier than to do the rewiring).
I have built a stereopair that is driven by this PS. I have already changed the 10k to 4,7k and the 100k to 47k. Do you think this is enough for two channels?

Thank you so much for your help

Regards
Mattias

ps
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Old 2nd December 2004, 09:17 PM   #7
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Quote:
Originally posted by waner
I have built a stereopair that is driven by this PS. I have already changed the 10k to 4,7k and the 100k to 47k. Do you think this is enough for two channels?
Yes (as long as the capacitors have doubled, as well as the power transformer's current ratings). But that may not fix the problem; you need to reduce the number of phase shifts in the amplifier. In this case, it seems to be the output stage bouncing on the first rail, causing the others to bounce along with it. If you reduce the number of stages, by cutting the last one (and rebiasing the first stage as necessary) as I mentioned, that should get it.

Tim
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Old 3rd December 2004, 02:08 AM   #8
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Before you go too far in rebuilds etc. - check the following

Your problem is definitely a power supply decoupling problem causing "motorboating" as described in posts above.

It may well be C4 or C7 electrolytics. Either the electrolytics are stuffed or you have a dry joint on either the high voltage side or the ground side such that the capacitor is not really connected at all.

I had exactly this problem with an EL34 Guitar amp I built. Turned out to be a dry solder joint on the ground side of the electrolytic.

Good Luck with your debugging.
Cheers
Ian
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Old 3rd December 2004, 04:31 AM   #9
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Quote:
If I look at the amplifier in the dark the EL34 blinks blue in time with the pulse.

Cmon guys, why spoil the wonderful Christmas decorations? This guy will only need help in January.
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Old 3rd December 2004, 04:58 AM   #10
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Default One more stab i n the dark

I just re-read the posts and had another look at the schematic.

You mentioned that if you put a multimeter probe onto the anode of the EF86 the problem went away. That is when you added the small capacitance of a multimeter probe and lead.

There is another possibility - rather than your problem being sub-sonic oscillation (motorboating) due to insufficient decoupling of the power supply it may be that you have the opposite problem i.e. bursts of High frequency oscillation (supersonic) and you are only hearing the "envelope" of this.

Check that your 4k7 grid stopper on the EF86 has the body of the resistor as close to the grid pin as the wiring will allow. Also check that you have the EF86 in Triode Mode with the screen properly connected to the anode.

Cheers,
Ian
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