More distortion from EF86's rather than headroom?

[nedhogan]

Can anyone suggest how to get more distortion from the pre amp section of this amp because at the moment it hates OD and fuzz pedals. It seems to get most of its distortion from the PI and the power section. By the way It now has a master volume after the PI.

basically I either need the signal to somehow be higher or I need to lower the clean headroom of the EF86s.

Any ideas people?

[tubeswell]

More distortion?

bypass cap on V2?

[loudthud]

Disconnect the 10Meg resistor that is acting like feedback around V1. If that is too much gain, raise the value to something higher, you may have to use a couple of resistors in series.

[nedhogan]

What on the cathode? Something like 25uf 12V?

I'll do that, didn't realise that would add more distortion, I thought they acted the opposite way round to that but I've never experimented with them.

And yeah I'll remove that neg feed back loop on V1 too.

[nedhogan]

Without the feedback resistor up front, it likes my pedals much more but its very bright sounding now. I think that feedback resistor was the reason it didnt like my OD pedals for obvious reasons though!

Any way to darken it back up again? ....or just tone stack modifying.

[nedhogan]

On a different matter the triple 16uf filter cap is very old looking. Is it possible to just completely remove it and just have the 2 32uf's? I don't see why there has to be so many filter caps.

[tubeswell]

What on the cathode? Something like 25uf 12V?

I'll do that, didn't realise that would add more distortion, I thought they acted the opposite way round to that but I've never experimented with them.

And yeah I'll remove that neg feed back loop on V1 too.

Adding a cathode bypass cap (Ck) to a cathode-biased stage boosts the gain of the stage by helping to hold the cathode at a constant voltage - thereby increasing the 'average' plate-to-cathode voltage. Increasing the capacitance of Ck increases the amount of bass freq in the signal. Seeing as how you haven't got any CK on V2 at the moment, anything you add will boost gain. 25uF should give you pretty much full bandwidth gain boost (which, if you want things to be 'darker', may help a bit). You'll probably even get there with a lesser amount like 10uF. If you starting cutting down to 1uF, you'll notice the boost is more of a treble boost.

[loudthud]

On a different matter the triple 16uf filter cap is very old looking. Is it possible to just completely remove it and just have the 2 32uf's? I don't see why there has to be so many filter caps.

The schematic looks like not all the circuitry is shown. The filtering is overkill for a guitar amp. Point "C" doesn't really go anywhere. The 16uF at that point could be removed. The 68K dropping resistor could be removed (replace with a wire) and both EF86 stages could be connected to one 33uF cap.

[trobbins]

The amp shows it's hifi design origin.

You could consider changing the bass/treble tone pots to input volume pot and also a tone pot circuit of some type - and perhaps reduce the coupling cap from 100N to say 10N.

If you still have output feedback, then you could consider switching it out, or reducing it significantly, or using a presence pot.

You may want to add a grid stopper to the PI input, given that you could easily be driving that hard.

The PI to output stage coupling caps are also relatively high if you are overdriving the output stage, and you could increase the EL34 grid stoppers.

The hi-fi'ish dc coupling of the PI with the earlier EF86 may or may not be advantageous for overdriven signals - you could check the influence by alternatively taking the EF86 screen to a typical HT dropped and filtered bias voltage, and adding a coupling cap and PI input stage DC bias divider from HT, and removing the output stage feedback (as you could head into oscillation).

[nedhogan]

Think Im already in oscillation territory, Every so often the output will "switch" to a "muffle" and stay like that for a while. With all the resistors being 40 years old I keep thinking maybe its one of them but its probably the low grid stoppers, I put a Ck of 10uf in to try and stop it but no luck. Any ideas??

When it isnt doing that it sounds great right now though! Possibly better than my AC30..... but very different.

Referring to the "C" one of you mentioned, I think thats just the B+ socket!.... Thats on the back of the amp, strange scary idea. That is definitely the whole circuit.

I like that you can tell its a HiFi, its a cheap old thing that Brits used to buy and mod if they couldn't afford an AC30.

Thanks to everyone for the great suggestions.

[nedhogan]

The schematic looks like not all the circuitry is shown. The filtering is overkill for a guitar amp. Point "C" doesn't really go anywhere. The 16uF at that point could be removed. The 68K dropping resistor could be removed (replace with a wire) and both EF86 stages could be connected to one 33uF cap.

The filter at that "C" bit is 32uf isnt it? So am I right that I could disconnect that filter and use that for the two EF86's with no drop down resistor between them? Thus just needing a filter for the PI which could be 10uf as I have some of those kicking about.

That would boost the output of V1?... but wouldn't that over load the signal in the rest of the circuit beyond belief?

Also am I right in thinking that I must have a filter between every HT drop down resistor in this circuit? I thought it was just when the drop down stages are in parallel like my ac30, not when they are supplied in series like this one.

Sorry if that's dumb, Im just a moderate novice!

[tubeswell]

Hifi was evidently the goal because of the way the gain stages are set up to have as-wide-a-bandwidth-as-possible whilst keeping the signal clean. Hence it has local feedback on V1, and no cathode bypass cap on v2, and bigger than necessary coupling caps for what you want in a guitar amp. For a geetar amp, it sounds better if the signal distorts, but in adding more gain, you probably also want to loose some of the resulting bottom-end flab that you will get when you boost the gain. Hence you should try different values of cathode bypass cap on V2 and also consider reducing the size of the coupling caps like Mr Robbins suggested.

[nedhogan]

Right can anyone suggest how to get more distortion out of the pre amp and more headroom from the PI because at the moment it gets plenty of dist but only from the PI.
I want V2 to distort a bit more and as far as the PI goes I dont understand them no matter how many times I read about them, if I lower its cathode res Rk will I get more headroom?

Here's a pic of what I've done so far...

[trobbins]

You may want to add a grid stopper to the PI input, given that you could easily be driving that hard.

Have you checked the headroom balance on the PI - you may be able to adjust the anode resistor on the 2nd triode for better balance.

[tubeswell]

For more distortion from the pre-amp, you could decrease the Rk on v1 to something like 1k (tube will run hotter), and for headroom from the PI, try put a 12AT7 in the PI slot.