Phase inverter with ef86 question

Winny_Pooh

Hi gents. I have a clone of a 15w Matchless Nighthawk (original Matchless OT) that I'd like some advice on. It sounds really good but breaks up before noon on the dial and I wish that the breakup were later and smoother like my super lead. The hardness and fizz ismore noticeable when recording direct with a reactive load.

Would an ecc81 (lower gain more current?) Or ecc82 in the PI position reduce and smooth the PI clipping somewhat?

Current valves are: Mullard EF86, JJ ECC83S & 2x JJ EL84

RiftAmps

The issue here is not the PI, but that an EF86 is a sharp-cutoff pentode. Smooth OD is not a characteristic of the EF86, but sharp and aggressive is.

You could try an E80F, which is an excellent audio valve, they’re pin compatible so just plug and play

Winny_Pooh

Thanks @RiftAmps did some googling on that valve. I think I will just go the whole hog and put in a (edit: spitfire pre with one 12ax7) in there and keep the rotary tone control (runs off the volume pot). Unless anyone sees potential issues?

Dodge

The Lightning preamp won't get you past noon without breaking up. Perhaps look at the Spitfire? That has a single tone control.

ICBM

I've never understood the fashion for EF86s in guitar amps - I've never heard one that doesn't sound too hi-fi - flat-sounding clean and harsh overdriven.

I would replace the EF86 with a standard cascaded (not paralleled as in the Lightning) 12AX7 with the gain control after the first stage, which will enable you to keep it cleaner.

The plate and cathode resistor values in that Lightning schematic are nuts for a paralleled 12AX7, it's no wonder it breaks up early.

Winny_Pooh

Thanks @Dodge , @ICBM ; so given the spitfire layout could I get away with running the current tone selector off the plates of the 12ax7? 

ICBM

Yes, it should be fine - but if you want less early breakup, as before I would definitely reduce the plate and cathode resistor values for the first 12AX7 - the standard values for a *single* half 12AX7 are 100K and 1.5K, so for a paralleled one you want 50K and 750 ohms - 47K and 820 ohms are the usual preferred values.

Alternatively you could use the first half (with 100K/1.5K values) as a gain stage feeding the volume control, and the second half as a cathode-follower feeding the tone selector.

Winny_Pooh

Thanks @ICBM ;So if I understand correctly, with a single 12ax7: have a 50k plate resistor and connect pins 1&6, then change the resistor off pin 3 to 750/820ohm.
Then keep the tone control connected to the volume pot and the "output" of the tone selector to the 12ax7 plate?

ICBM

Is there a tone control as well as the selector, or am I reading that wrong?

You have the resistor values right, yes.

Winny_Pooh

@ICBM I meant the rotary selector from the creme brulee layout when I said "tone", sorry. 

ICBM

Yes, that can be connected to the input side of the volume control as normal.

gjonesy

I think you could adjust the gain on the EF86 - iirc the 330k and 2M2 is the area to look at
People more knowledgable than me should be able to inform you better (and keep you safe/alive if your digging around in the guts of your amp)

Winny_Pooh

gjonesy said:

I think you could adjust the gain on the EF86 - iirc the 330k and 2M2 is the area to look at
People more knowledgable than me should be able to inform you better (and keep you safe/alive if your digging around in the guts of your amp)

It's ultimately more about the quality of the breakup. I was assuming maybe the PI was being driven too hard but ICBM suggests its more to do with it's nature. 

ThePrettyDamned

Winny_Pooh said:

gjonesy said:

I think you could adjust the gain on the EF86 - iirc the 330k and 2M2 is the area to look at
People more knowledgable than me should be able to inform you better (and keep you safe/alive if your digging around in the guts of your amp)

It's ultimately more about the quality of the breakup. I was assuming maybe the PI was being driven too hard but ICBM suggests its more to do with it's nature. 

I've got an ef86 ac15 and it works really well for me, but it's a very snarly, gnarly kind of break up with some hi-fi ish qualities in the top end. 

I love it - it's not "rough" as such, but slightly wild. 

Winny_Pooh

Kudos to @ICBM thanks for the recommendation! I changed over to a 12ax7 on the weekend and the amp is so much better! 

I've used the resistor values from a Spitfire and popped one 12ax7 in (the power supply is less filtered than the Spitfire) kept the switcheable coupling caps (brilliant actually) and everything else.