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My Homebrew Experience (EF86 and UL stuff)

8/6/2000 3:56 AM
mitchell	My Homebrew Experience (EF86 and UL stuff) I've tinkered around now for a couple of months with a homebrew amp design and I thought I might pass on what I've learned so far. I started with an EF86 input tube and I've stuck with it so far. It's fed from a 300V B+ connection and uses a 220K plate resistor and a 1M screen grid resistor. 2.2K resistor on the cathode bypassed with a 25uf cap. I'm using a Svetlana EF86 and I've had no problems with noise or microphonics. It is hard to say how it sounds compared to a triode like a 12AX7. I would say it sounds warm if not a tad bit sterile. It reacts with pedals better than any 12AX7 based amp I've heard; no mush or compression. The bad thing is that the plate impedance is so high that a normal FMV-style tone stack just won't do. Right now I have the EF86 coupled to a volume pot which feeds a 6SL7 in a long-tailed pair PI configuration. If I need to roll off some highs I do it with my guitar's tone knob. My output section is a Magnequest UL OT made for EL84's or 6V6's. I'm currently using 6V6's. I started with a pi-filter feeding the OT, as I had seen many hi-fi amps and also the Allesandro guitar amps using this topology. However, I has also read a post here by Dr. Z saying that he did not use a choke in his Route 66 UL amp as he found that it "choked" the sound. Well, initially I was not at all impressed with the UL sound when fed from the pi-filter. It sounded rather dull. After getting some advice here (thanks guys) I tried taking the OT's B+ right from the first cap after the rectifier. I was worried that I would get alot of hum and ghost notes. Well, using 60 uf of filtering I get no more hum than I had with the pi-filter and the sound is much, much better. In fact, it's fantastic. I hear a very slight ghost note from time to time but it's not annoying at all; in fact, I think I like it! UL seems like the perfect connection for tetrodes and pentodes because you can ditch the negative feedback loop but still get a low output impedance with tight bass. The transition from clean to dirty with this amp is much smoother than with a traditional amp with feedback. My next amp will have EL34's in UL with a 12AX7 based preamp and PI. Thanks to everybody who has helped me so far here. This is truly one of the great places on the internet. Morris Mitchell One last thing: the note that Jimi hits in "Machine Gun" (you know the note) is the greatest thing ever!

8/6/2000 3:56 AM

mitchell

My Homebrew Experience (EF86 and UL stuff)

I've tinkered around now for a couple of months with a homebrew amp design and I thought I might pass on what I've learned so far.

I started with an EF86 input tube and I've stuck with it so far. It's fed from a 300V B+ connection and uses a 220K plate resistor and a 1M screen grid resistor. 2.2K resistor on the cathode bypassed with a 25uf cap. I'm using a Svetlana EF86 and I've had no problems with noise or microphonics. It is hard to say how it sounds compared to a triode like a 12AX7. I would say it sounds warm if not a tad bit sterile. It reacts with pedals better than any 12AX7 based amp I've heard; no mush or compression. The bad thing is that the plate impedance is so high that a normal FMV-style tone stack just won't do. Right now I have the EF86 coupled to a volume pot which feeds a 6SL7 in a long-tailed pair PI configuration. If I need to roll off some highs I do it with my guitar's tone knob.

My output section is a Magnequest UL OT made for EL84's or 6V6's. I'm currently using 6V6's. I started with a pi-filter feeding the OT, as I had seen many hi-fi amps and also the Allesandro guitar amps using this topology. However, I has also read a post here by Dr. Z saying that he did not use a choke in his Route 66 UL amp as he found that it "choked" the sound. Well, initially I was not at all impressed with the UL sound when fed from the pi-filter. It sounded rather dull. After getting some advice here (thanks guys) I tried taking the OT's B+ right from the first cap after the rectifier. I was worried that I would get alot of hum and ghost notes. Well, using 60 uf of filtering I get no more hum than I had with the pi-filter and the sound is much, much better. In fact, it's fantastic. I hear a very slight ghost note from time to time but it's not annoying at all; in fact, I think I like it! UL seems like the perfect connection for tetrodes and pentodes because you can ditch the negative feedback loop but still get a low output impedance with tight bass. The transition from clean to dirty with this amp is much smoother than with a traditional amp with feedback. My next amp will have EL34's in UL with a 12AX7 based preamp and PI.

Thanks to everybody who has helped me so far here. This is truly one of the great places on the internet.

Morris Mitchell

One last thing: the note that Jimi hits in "Machine Gun" (you know the note) is the greatest thing ever!

8/6/2000 5:55 PM
Dr Rico	Hey, sounds cool! Have you considered splicing a AC-30/4 or D/C-30 style tone stack in there? That would give you some more control at the amp without the plate impedance issues with a more standard tone stack. I'd love to see a schematic of your amp, if one's available. Hasta -> Rico P.S. RE: "the BOG MG note". Heck yeah!

8/7/2000 4:25 AM
mitchell	Rico, The D/C-30 has no tone stack on the EF86 channel; it's just a rotary switch that changes the coupling cap value from the plate of the EF86 to the volume control. It basically just rolls off bass as you switch to smaller coupling cap values. The "cut" control is part of the power amp section (post-PI). I don't think the Vox AC-30/4 had a tone stack on the EF86 channel either. Just a "cut" control like the Matchless after the PI. What I have done is just tinker with the coupling cap off the EF86 until I found what sounded good with my guitar (with single-coils). It would probably be too bassy with a humbucker-equipped guitar. Morris

8/7/2000 4:25 AM

mitchell

Rico,

The D/C-30 has no tone stack on the EF86 channel; it's just a rotary switch that changes the coupling cap value from the plate of the EF86 to the volume control. It basically just rolls off bass as you switch to smaller coupling cap values. The "cut" control is part of the power amp section (post-PI). I don't think the Vox AC-30/4 had a tone stack on the EF86 channel either. Just a "cut" control like the Matchless after the PI. What I have done is just tinker with the coupling cap off the EF86 until I found what sounded good with my guitar (with single-coils). It would probably be too bassy with a humbucker-equipped guitar.

Morris

8/8/2000 1:20 AM
Dr Rico	Yeah, that's what I meant, Morris. Sorry if I worded that unclearly. ;^) So, as I was saying, wouldn't that sorta "whatever you wanna call it" give you more control so you could dial it for buckers AND SCs? Hasta -> Rico

8/8/2000 6:46 AM
Steve A.	Morris: What I have done is just tinker with the coupling cap off the EF86 until I found what sounded good with my guitar (with single-coils). So what value coupling cap did you end up using? And did you use a treble-cut control in the power output section... if so what value cap and pot? Your amp sounds interesting! Thanks for telling us about it! Steve Ahola P.S. The UL taps in Hammond OT's are usually at 40%, I believe. Does your Magnequest OT use 40% UL taps, or something else?

8/8/2000 6:46 AM

Steve A.

Morris:

What I have done is just tinker with the coupling cap off the EF86 until I found what sounded good with my guitar (with single-coils).

So what value coupling cap did you end up using? And did you use a treble-cut control in the power output section... if so what value cap and pot?

Your amp sounds interesting! Thanks for telling us about it!

Steve Ahola

P.S. The UL taps in Hammond OT's are usually at 40%, I believe. Does your Magnequest OT use 40% UL taps, or something else?

8/6/2000 8:34 PM
Randall Aiken	Regarding the high output impedance of the EF86: you can easily DC-couple a cathode follower to the plate of the EF86 to get a nice low impedance output to drive whatever type of tone stack you want. Of course, it takes another 1/2 of a 12AX7, though. Randall Aiken

8/7/2000 5:15 AM

dutch

quote:
"Regarding the high output impedance of the EF86: you can easily DC-couple a cathode follower to the plate of the EF86 to get a nice low impedance output to drive whatever type of tone stack you want. Of course, it takes another 1/2 of a 12AX7, though."

Randall-
I've found one type of tone stack that doesn't like the low impedance drive of a cathode-follower: the tweed Deluxe volume/tone pair. When the tone's turned all the way down, it doesn't roll off high end with the standard value caps--it only boosts high end via the other cap (assuming the volume's not all the way up, that is).

I guess the low impedance output from the cathode-follower overcomes the loading of the treble rolloff cap when the tone's turned down. So, if you want to drive this type of tone circuit from a CF driver, I guess you'd have to put in some series resistance (33k would probably be enough) to emulate the output impedance of a common-cathode stage.

C ya,
Dutch

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