If zobels and conj. filters aren't a bandaid.......

[daz]

Ok, tried that too. Possibly a tiny bit better, but not enough to be sure i'm not experiencing placebo effect. Screens have always been lower by about 15-20V.

[Chuck H]

Running a 4K primary OT set for 8 ohm into a 4 ohm load effectively sets the primary impedance to 2k for the EL34s. It will sound different from the standard impedance setting, but I wouldn't want to crank it too much like that.

FWIW it's generally agreed that one of the smoother tones achieved with EL34s is the Trainwreck Express. IIRC that amp uses a 6.6k primary OT.

But it's a 30 watt OT. I talked to a tech at Hammond once re OT selection. He said that in order to get a proper tone in a guitar amp using their off the shelf OTs (not the newer guitar amp OTs) you have to use a model rated for half the wattage your amp will produce. So it seems that a controlled amount of OT saturation is needed to achieve the typical high/ low rolloff heard in a guitar amp. I think thats where some builds go wrong. Too much iron and/ or too cool bias. Either of those things can cause thin scratchy tone. Both together is horrible for a guitar amp. I can see where some builders or designers would want to use a CF instead of trying to use underatted iron or biasing hotter than normal. The amp would certainly run cooler amd more reliably for it. So I guess it's not a bandaid, but a choice in method of operation. The CF helps to simulate what happens to an OT as it saturates. I haven't had trouble with harsh amps, and I haven't used a CF in any of my builds. So I'm just stabbing at possibilities here. But I hope it helps.

Chuck

[Steve Conner]

As far as I'm concerned, Zobel networks are for keeping the output stages of solid-state amps stable.

If you want to tame high end in a tube amp, turn the treble and presence down, take the bright caps off, try a different speaker, use more negative feedback, etc. Some amps have an undersized coupling capacitor to deliberately roll off the low end, which you can try changing out for a bigger one too.

And yeah, the hearing thing. It's a neat explanation for the existence of Vintage 30s: they were designed by deaf musicians for deaf musicians

[Bruce / Mission Amps]

As far as I'm concerned, Zobel networks are for keeping the output stages of solid-state amps stable.

If you want to tame high end in a tube amp, turn the treble and presence down, take the bright caps off, try a different speaker, use more negative feedback, etc. Some amps have an undersized coupling capacitor to deliberately roll off the low end, which you can try changing out for a bigger one too.

And yeah, the hearing thing. It's a neat explanation for the existence of Vintage 30s: they were designed by deaf musicians for deaf musicians

I always thought true Zobel filters were used on the speaker side of the OT and used only to smooth out the effects on the OT's primary Zed by the variable impedance of speakers from 20Hz to <8KHz, and in hi-fi gear, not guitar amps.

[MattT]

FWIW it's generally agreed that one of the smoother tones achieved with EL34s is the Trainwreck Express. IIRC that amp uses a 6.6k primary OT.

I've tried a CF in one amp and it didn't do much...it dulled the tone a bit but not in the right way. But I had to comment on the above quoted comment. My limited experience with the Express circuit is pretty much exactly the opposite. They are very bright, very 'crunchy', and very aggressive. Smooth is not a word I would ever use to describe their tone (sustain...yes...smooth...no).

[mooreamps]

I always thought true Zobel filters were used on the speaker side of the OT and used only to smooth out the effects on the OT's primary Zed by the variable impedance of speakers from 20Hz to <8KHz, and in hi-fi gear, not guitar amps.

Bruce, I don't know this for sure, but you are probably right. I only looked at the network itself, and discerned it's function with respect to the swept responce curve of a loud speaker. So, yes it would definantly would have to be inserted on the "speaker" side of the O/T ; where a conjuctive filter is inserted on the "tube" side of the O/T.

-g

[Alex/Tubewonder]

Bruce, I don't know this for sure, but you are probably right. I only looked at the network itself, and discerned it's function with respect to the swept responce curve of a loud speaker. So, yes it would definantly would have to be inserted on the "speaker" side of the O/T ; where a conjuctive filter is inserted on the "tube" side of the O/T.

-g

The conjuctive filter or Zobel network preceeds semiconductors, it was originally designed for tube amps. Its job is to make life easier for the output transformer by making the loudspeaker look like a more or less constant resistive element. Its pretty tough job for a tube amp to drive a highly frequency dependent complex load such as loudspeakers inductance by through a transformer which by itself is not presenting a resistive load to the tubes.
By virtue of making the loudspeaker look like a resistive load the distortion characterics of the amp+speaker system will be affected.

Load line analysis of resistively loaded amp is straight forward task, when you include inductive component of loudspeaker the load lines depart from straight line and become elliptic. You can think of it as if for multi frequency signal the load presented to output tubes by the OT+speaker is different for each frequency.

Have you seen an otherwise perfectly designed and properly biased amp suddenly red plate on a specific note? Complex speaker impedance can do this sometimes if the OT has high leakage inductance - which many guitar amp OTs have.

One other thing you can achieve with a CF/zobel is calming down an amp that has a tendency to spurious self oscillations.
If smartly used it can ease your way in search for a power amps sweet spot and getting this sought after musical regenerative feedback.

[Chuck H]

I guess it a matter of interpretation. One mans "smooth" is anothers "muted" etc.

But what I meant was that the Express is not a harsh biting amp under overdrive conditions. It's also worth noting that many home brew clones are harsh and biting. So this comment wasn't about the circuit in general.

Point is, I don't think a low primary impedance is a needed ingredient for good non spikey overdrive.

Chuck