Corrective filter in 4xEL84 amp

[Gaz]

I remember Paul Ruby did a article about buzzies or fizzie, in the 18w amps.

This PDF shows scope shots and good info how to cure this stuff.

thanks for sharing that article, I have a pretty well known booteek amp that looks like this on the output and have been wondering where to start with it. Lifting the cathode bypass cap helps considerably. It is also biased super cold but didn't make much difference with a more appropriate cathode resistor.
[IMG577]http://i1233.photobucket.com/albums/ff388/randalp3000/amps/IMAG0481_zps009f95dd.jpg[/img]

ugly isn't it?

Das' nasty.

[Gaz]

So, I got to try the amp at louder volumes today with the 5k/.0047uf rigged on a switch, and I've gotta say that while it gets rid of the fizziness, it's a pretty extreme roll off. I'm going to have to play around with the cap value a bit to see if I can find a middle ground.

[darryl_h]

The original ( RCA ) purpose for the corrective/conjunctive filter was to compensate for the peaky treble response of small speakers and inadequate output transformers in mantel radios. The resistor and capacitor values were chosen for this application. In guitar amplifiers the filter is not for wholesale removal of high frequencies, but should be for taming just the extreme highs.

Try experimenting with a much lower resistor value, so the network becomes something like 1k/.0047. It's counter intuitive, but the filter then only removes extreme highs without muffling all the treble.

[Gaz]

I totally understand all that, but again, the Carmen Ghia and Badger both use the 10k/.002uf values (which is the same as 5k/.0047uf). I think it's better the change the cap value rather than the resistor for power dissipation reasons.

[teemuk]

[IMG577]http://i1233.photobucket.com/albums/ff3 ... 9f95dd.jpg[/img]

ugly isn't it?

Das' nasty.

That's what most tube power amps usually output when overdriven: Hard clipped signal with plenty of crossover distortion.

That's what makes them sound how they sound like.

Better just stop listening with your EYES.

[Gaz]

I dunno, I've personally never seen one where it rises like that at the crossover points. I also don't have a digital scope.

[Tillydog]

... both use the 10k/.002uf values (which is the same as 5k/.0047uf)...

I don't believe that those values are equivalent - they may have the same (nearly) frequency response, but the second one has half the impedance of the first, so will have a much more noticeable effect - Considering both the frequency response and the impedance produces unique solutions for R&C values.

I've tried predicting values for conjunctive filters by calculation for a couple of amps, and never really got anywhere.

I moved to using 'Zobel' type networks on the secondary so I could experiment at lower voltages and I ended up making a quick and dirty R / C substitution switch which I could plug into a parallel speaker jack and adjust while trying the amp out over an extended period. It's still in my junk box, and may be the ugliest thing that's ever been posted here, but it was really useful in finding the best compromise. (It has resistors between 10R and 100R and 0.68u / 1u / 2u2 capacitors.)

For some reason, Zobel networks seem less socially acceptable than conjunctive filters in guitar amps, but achieve the same thing AFAICT ...?

[matt h]

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[Tillydog]

Hah! I hadn't even seen that thread - Jinx!

Will follow up there.

[roberto]

Yep. Keep it up and you may win the zobel prize. Thank heaven for little coils.

[img:20:20]http://www.ps4ita.com/forum/public/styl ... /rotfl.gif[/img] [img:20:20]http://www.ps4ita.com/forum/public/styl ... /rotfl.gif[/img] [img:20:20]http://www.ps4ita.com/forum/public/styl ... /rotfl.gif[/img]

[SeanF]

For some reason, Zobel networks seem less socially acceptable than conjunctive filters in guitar amps, but achieve the same thing AFAICT ...?

not exactly. They're both low-pass filters, but the Zobel's position on the secondary means it's nearly invisible to the power tubes; it's just dumping top end before the speaker. They're typically used in hi-fi speakers to flatten a woofer's response. The conjunctive filter (as I think of it) appears in parallel with the OT's primary, effectively raising the power factor of the primary very close to unity, and preventing top end from ever getting into the OT, where it can create a lot of distortion.

http://www.aikenamps.com/zeroz.pdf

[matt h]

(deleted)

[gary]

unsurprisingly, there is a lot of info on the 18 watt site about taming the fizz associated with heavily overdriven el84s.

I can sympathise with the poster's comments about the swirly fizz at low voltages. I tried all the suggested fixes for dealing with fizz in the 18 watt circuit, including the conjunctive and zobel filters. In the end, the only way I have achieved good results with low vvr settings ( low voltages ) was to remove the bypass cap on the el84s' cathode resistor. Evidently this reduces the gain in the el84 enough to remove the annoying fizz on the decay.
If you are usually running your amp 'flat out' but at low voltages ( e.g home / hobby player ) I can recommend lifting the bypass cap. There is no shortage of distortion available on my lite iib. I would probably put it on a switch if I was using the amp at performance levels too.

[gary]

just remembered there was also a thread about the feasibility of a variable zener approach similar to Paul Ruby's fix for inclusion in a scaled/vvr'd 18 watt. I believe the outcome was that the modder ended up with testing a succession of zeners and settling on a low value zener which suited the whole sweep of the vvr , which he found counter-intuitive.

[Structo]

I'm pretty sure you have tried this but how about a cut control, to dial down the treble artifacts in the output?