EF86 with morph control in PP gigging amp

[tubeswell]

So much did I enjoy the EF86/morph control in the SE amp that I built a couple of months back, that I thought I'd take the same concept into more powerful gigging amp. Did the chassis and combo cab in only 2 days. (I'm really diggin' my new brake)

[tubeswell]

I have done a schematic for this. A few specifics I wondered about.

1) not sure whether to use a CF stage or not after the EF86. The obvious reason to use one would be impedance buffer for tone stack. This would be different to a Route 66 approach (which doesn't use a CF stage - and has a different stack). Any comments?

2) If I do use a CF stage not sure whether it should be a 12AU7 with 22k Rk or a 12AX7 with 100k Rk. The one drawn on the schematic is direct-coupled to the EF86 and a 12AU7 might steal a lot of current (from the EF86) but this could have a nice assymetry under overdrive conditions. (or not?) Any comments/ideas?

3) Tone stack. Shown a couple of variants (both using 1M pots). Which would be better for a hump/scoop control? Any bets? Or would I be better off with a typical Fender TB tonestack or one of those brown fender tonestacks? or...? (Reason I am thinking hump/scoop control is to better cope with the higher gain I would get with the EF86 morph control in 'pentode' mode. whilst retaining treble-bass balance of a single tone control for when in 'triode' mode - hence the (modified) big-muff concept TS in the actual circuit).

4) Tone stack recovery stage. (Borrowed from a 6G3 driver). Would this stage really be necessary in this amp? The reason I put it in is because when the EF86 morph control is in 'triode' mode, the amp ought to behave more like a 6G3 (methinks) (or not?).

5) I think I'll go for an LTP here. This is again borrowed from the 6G3. Alternative would be to not have the TS recovery stage (in 4) above) and have a cathodyne stage here. (Gaining a spare 1/2 a stage). The NFB insertion point would have to change. Methinks the LTP would give better drive for the output stage given the 'lowish' HT voltages expected.

6) Not sure whether to use a 12AX7 or a 12AT7 in the LTP. I have tried 12AT7 as LTP tubes before and they do have a nice mojo. Any comments?

7) the LFO stages are borrowed from a 5G9 and work well with fixed-bias 6V6s at those voltages. If I went with one triode and a mosfet follower (as I have done in other maps) I could get another spare triode. (And if I went with a cathodyne PI, then I could also probably to a reverb - because I'd have a whole spare pre-amp tube.)

I have put the standby between the 2 reservoir caps. I can't see the harm in doing that. Can anyone else?

TIA

[Steve Conner]

Hi Pete, well I see you have an odd number of 12AX7 halves, so it makes sense to use the left-over one for the CF.

What does the morph control do for you tone wise? I also see you're running the EF86 at quite a high current compared to the datasheet example. Did you try the datasheet conditions and find that you preferred it hotter?

Lastly it's OK to put the standby between the two filter caps, as long as there's some impedance in series with the switch to limit the surge. Otherwise the switch is dumping a full capacitor into an empty one, and the contacts will arc heavily and burn out prematurely. If the impedance is a choke, you have to add a snubber network and catch diode to avoid the mother of all pops at turn-off (and possibly burnt contacts again)

[JoeK]

It's quite interesting that I've seen 2 other new amps with their own take on EF86 into 6V6 in the last couple of days. Must be a good combination. Nice looking cabinet and chassis. What thickness ali can you bend with the brake?

[tubeswell]

Thanks Steve.

Re your comments about the EF86 - I am aiming to set the EF86 up for lowish (around 100) gain with HT around about 180-220 and get the load line just at or just below the Vg0 knee. The plate should be sitting around 90-110 with the screen (respectively) at 60-80 and the cathode at about 1 V (.9 - 1.4) - which runs the tube current at between 1.3 to 2 mA. When I did this with the little SE amp recently, I got some nice tones from cleans - http://www.nzguitars.com/forum/downl...le.php?id=8232 , through to good blues tones, http://www.nzguitars.com/forum/downl...le.php?id=8258 , and rock tones with grit with the morph control in pentode mode http://www.nzguitars.com/forum/downl...le.php?id=8347 . The voltages I took for that amp are attached. (The morph control is like a gain control that adds a little (but not tooo much) grit and changes the texture).

Regarding the CF stage idea, on its own the EF86 attenuates highs a fair bit through the tone stack. I tried this with a 5F2A stack and with a big muff tone stack. With the latter configured for a scoop there was a better high-low balance albeit at the expense of a bit of gain. I am thinking that the CF stage will give more flexibility to drive whatever tone stack I use.

I was also thinking that if I used this kind of front end with a 6G3 type driver, PI and output stage, that I could get the tones I was getting out of the SE amp, but with more power (and better trem). With the morph control in triode mode, I am anticipating it will behave more like a 6G3. And with the morph control in pentode mode, I expect to get a nice amount of feedback on tap (without having an adverse effect on the EF86).

I'll have to have a hunt for a standby snubber and catch diode schematic.

[tubeswell]

What thickness ali can you bend with the brake?

up to 2mm Ally or 1mm steel. Its not a big brake. That chassis is 1.5mm ally and is very strong in that configuration.

[mooreamps]

Regarding the CF stage idea, on its own the EF86 attenuates highs a fair bit through the tone stack. I tried this with a 5F2A stack and with a big muff tone stack. With the latter configured for a scoop there was a better high-low balance albeit at the expense of a bit of gain. I am thinking that the CF stage will give more flexibility to drive whatever tone stack I use.

That's the way I did mine.. The plate impedance is so high on the EF-86, it probably requires the CF or SF....

-g

[Chuck H]

SF=???

[tubeswell]

source follower

[tubeswell]

Similar idea to the EF86 but with a 6U8, and a BFPR driver and PI, 5G9 Trem (using a source follower), and an ECL86 for reverb. This probably wouldn't have as much output power as the EF86 version I posted above (methinks?)

[tubeswell]

Finished the (Ef86 concept) version of this amp tonight. Didn't get a chance to really blast it yet, but it sounds very promising.

Happy to say no buzzes or rattles or hums. Tone is EF86tastic! (Need to do some soundbytes). Tone controls need tweaking to get the scoop/hump concept improved upon.

Schematic of as-built version with measured idle voltages attached below. I think it needs a voltage tweak in the power rail. I'll chuck a 5AR4 in and see how that goes (or maybe drop the first supply resistor to 4k7).

[JoeK]

Looks like a great amp! Are you going to leave the cab raw or has it got a seal on it already? Sound clips would be great when you get around to it.

[tubeswell]

Thanks Joe
I haven't done anything to the cab yet. Was umming and ahhing about it this time. Usually I'd cover it in fabric, but this time I might stain and seal it or...

I just had it pointed out that the schematic I posted earlier had an error in the location of the bias TPs - please refer to the corrected schematic here

[Alex/Tubewonder]

Why don't you try feeding the morph control from the CF output? Just a tip...

[tubeswell]

Why don't you try feeding the morph control from the CF output? Just a tip...

You mean like this?