Problem with motorboating?

Guest · Dec 2, 2004

I need help!

I have bulit a 30W EL34 Push Pull amp (http://www.lundahl.se/pdfs/claus_by...ifier_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

Rob Flinn · Dec 2, 2004

Normally the motorboating is because you have insufficient PSU decoupling caps on one or more stages in the circuit. The decoupling caps are the electrolytic ones that go between the HT & ground. There is normally one per stage in the circuit with a resistor between them, with the highest voltage on the output stage, dropping in voltage to the input stage. Try tacking another one in parallel with the existing decoupling caps one at a time & seeing if the motorboating goes.

Gus · Dec 2, 2004

Not seeing the build makes it hard to help. You might have wires that are to long and causing osc. Or the way the wires are run are causing feedback. or....................................

dale116dot7 · Dec 2, 2004

I've found this happens if you are using insufficient filtering or regulation to the driver/preamp tubes. If you are feeding screens with driver/preamp voltage it'll do this as well.

PRR · Dec 2, 2004

Yes, sure sounds like motorboating.

Are you built mono or stereo? The power supply resistor values are different when you feed two amps from one supply.

Are you sure you have all the several 47uFd caps in the power supply? The 410V and 160V outputs MUST be well bypassed to keep low frequencies from leaking from output back to input stages.

Is it possible to temporarily throw a 100uFd 400V cap from +410V output to ground? 10K and 23.5uFd does not seem to me to be enough decoupling for a very wide-band high-gain amplifier. If that cures it, replace the two "47u 385V" caps with 220u or 470u, 300+V caps.

The 2x150K and 47K-trim in ECC83 plate must be balanced. For initial tests, set the 47K trimmer wiper exactly centered. (Later when it is stable, you can fine-trim it with a distortion meter.) It would be good to check these resistors with an ohm-meter, be sure you did not get a mis-marked resistor. Or that your trim-pot hasn't gone bad. (That is a lot of power for a modern miniature trimmer.)

> The Choke in the PS is reversed 180 deg to the maintransformer. Could that cause the LF noise?

The position should not be critical but..... Hmmmmm...... 10H and 100uFd will resonate at 5 Hz and 300 ohms. And the total load is about 440V/0.2A or 2,200Ω. Seems to me it will really-really want to oscillate at a few Hz.

Temporarily short-out the choke. Does it stop motorboating? Try a 100Ω 10W resistor across the choke; does that cure it? I have a suspicion that this choke/cap ratio is not a good choice. While Claus took great care to ensure HF stability, it isn't clear that low freq stability was checked, or even that he built the choked version: "If you insist on stereo stages... choke." So I would try without the choke (or choke heavily loaded) and get it stable, before worrying about fine details.

Hey! There is no bleeder on the HV supply? That may be acceptable for commercial gear, but not in DIY. Get 4 to 10 of 470K 1W resistors and solder them solidly from the +450V line to ground! ANd even then, wait a good 60 seconds after power-off before sticking your nose in there. Even then the voltage has only dropped to about 24V. And keep a voltmeter on the B+ any time you are modifying, and be VERY SURE to look at it every time. 400V is bad, but also 0V is suspicious and may just mean the voltmeter wire fell off (there's always a volt or so on any electrolytic that has been powered-up today, unless it has a heavy bleeder).

The 0.47uFd 390K coupling to the EL34 grids looks wrong. The roll-off is 0.37Hz, very low, and too-close to the 5Hz power supply resonance and the transformer's variable cut-off. (To get full power to 30Hz, the iron's small-signal roll-off is probably a few Hz. Ultralinear operation reduces that to a fraction of a Hz.) Also, isn't 390K awful high for fixed-bias EL34? 100K is the limit for most power tubes in fixed-bias. With 390K grid resistors, I would expect trouble if the tubes are a little bit gassy. And if using about 470K grid resistors, 0.047uFd is a more likely choice of capacitor. As a test-rig, try paralleling the 390K with 100K. That will stabilize the grid leakage, shift the bass cutoff. If that "works", you still have to re-think, because driver distortion with 150K plate resistors into 100K grid resistors will be high. It wants the current in the ECC83 increased, maybe 47K plate resistors and 27K cathode resistor.