Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dbishopbliss on December 28, 2020, 09:51:28 am
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I started building a Sluckey AC15 five years ago but then had an accident that severed the tendon in my finger so I put the project on hold and then life got in the way. Now that things are working again and I have free time I would like to finish up the project. When I stopped progress I had the turret board completed and the transformers, jacks, pots, etc mounted on the chassis but no wires connected. When I started looking at the schematic and compared it to my project I realized I have a number of differences so I thought I would reach out for some guidance. I think my questions will be very simple but its been a while. :-)
Here's a photo of what my chassis looks like now:
(https://i.ibb.co/HqwpNqN/C82-BB75-C-F43-A-4482-9-A6-C-38-C2-B5-C5-EF33.jpg) (https://ibb.co/QMSj8M8)
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Glad to see the project back on. So, what questions do you have?
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Here is what I think I should be doing:
- Connect a wire from the ground tab of the power input module to the chassis ground tab (obscured by the blue capacitor).
- Insulate and hide out of the way Yellow and Gray power transformer wires because they aren't used.
- Connect a wire from the power input module to the fuse.
Question 1: Does it matter which tab on the power input module and which tab on the fuse?
- Connect a wire from the fuse to a tab on the power switch.
Question 2: Does it matter which tab on the power switch?
- Connect black wire from power transformer to the other tab on the power switch.
- Connect white wire from power transformer to the other tab of the power input module.
Question 3: I did not include a standby switch, do I need the 250R/10W resistor show in the schematic?
- Connect red wires to tabs 1 and 7 of V3 - EZ81 Rectifier socket
Question 4: Where should I ground the red/yellow center tap wire?
- Connect the green wires from the power transformer to the power indicator bulb, connect the wires running to the filaments to the bulb as well.
Question 5: Where should I ground the green/yellow center tap wire? Would it make sense to elevate the voltage and connect it to the cathode of the preamp or power amp tubes?
- Connect a wire from tab 3 of V3 - EZ81 Rectifier socket to one tab of the dual can capacitor
- Connect the wires from the choke L1 to the tabs on the dual can capacitor
Question 6: Where should I ground the dual can capacitor?
Thanks for your help!
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Question 1: Does it matter which tab on the power input module and which tab on the fuse?
Question 2: Does it matter which tab on the power switch?
Is your power switch a DPST (4 lugs)? If so, refer to the schematic. Connect a white wire from the IEC socket NEUTRAL lug to one pole of the power switch. Connect a black wire from the IEC socket LINE lug to the tip of the fuseholder. Connect another black wire from the side of the fuseholder to the other pole of the power switch. Connect the two transformer primary leads to the other lugs on the power switch (Doesn't matter which lead goes where.)
Is your power switch a SPST (two lugs) If so, connect a white wire from the IEC socket NEUTRAL lug directly to one of the transformer primary leads. Use heat shrink tubing. Connect a black wire from the IEC socket LINE lug to the tip of the fuseholder. Connect another black wire from the side of the fuseholder to the power switch. Connect the other transformer primary lead to the other lug of the power switch.
Question 3: I did not include a standby switch, do I need the 250R/10W resistor show in the schematic?
no
Question 4: Where should I ground the red/yellow center tap wire?
Question 5: Where should I ground the green/yellow center tap wire?
Question 6: Where should I ground the dual can capacitor?
see the ground scheme
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see the ground scheme
I had the other pages printed out with the project, but I didn't have that one. That clears things up. I told you the questions would be simple.
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I am using the Cliff-style jacks for my speaker connections. I am not wiring mine as an internal speaker and external speaker jack like Fender. I want to wire them so that one jack is for the 8 ohm transformer tap and one is for the 4 ohm transformer tap. To do that will I wire something like the following (crude character based drawing)?
4 Ohm 8 Ohm
O----O--------O----O---- to Ground Buss ---->
| | | |
| | | |
O O O O
| |
+ to 4 Ohm + to 8 Ohm
(https://i.ibb.co/L8nz7Wg/6-C58-AF0-D-3-F01-4-C8-E-91-B3-4-E5-A2-FD7-A0-A1.jpg) (https://ibb.co/drG0Yqp)
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Those drawings only work with fixed width fonts.
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Those drawings only work with fixed width fonts.
Yeah, I tried using the Font Face/Font Size feature but it didn't seem to work. Neither did the Tt (teletype) feature, but I had to run to an appointment. Thanks for the diagram. That's what I thought I should do, but I wanted to verify.
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Before I install the turret board and start soldering leads to the sockets I have a couple more of questions:
Question 7: Should I test voltages at different points before installing tubes and hooking up the turret board? That is, plug in to the wall and turn it on, or is that bad?
Question 8: Where does the ground for the footswitch go? I'm using a Cliff jack which is not grounded to the chassis.
Question 9: Is it better to cut unused leads or is it fine to heatshrink the ends and coil them up?
Question 10: How does my lead dress look (see picture)? Anything I should change before I start cutting leads and soldering?
(https://i.ibb.co/ByYJJ0f/F6-A7-BD48-0-F23-417-E-A591-7-B82-B94470-BE.jpg) (https://ibb.co/wS2qq5d)
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Before I install the turret board and start soldering leads to the sockets I have a couple more of questions:
Question 7: Should I test voltages at different points before installing tubes and hooking up the turret board? That is, plug in to the wall and turn it on, or is that bad?
Yes. All you can check are AC voltages but this will tell you the PT wiring is correct. Be sure the power switch, fuse, and pilot lamp work. Check for correct voltages at the rectifier socket. Plug the tubes in and be sure every filament lights. Knowing all this is right can save some headaches later on.
Question 8: Where does the ground for the footswitch go? I'm using a Cliff jack which is not grounded to the chassis.
On the board. Connect to the negative lead of that 33µF filter cap on the end of the board.
Question 9: Is it better to cut unused leads or is it fine to heatshrink the ends and coil them up?
That depends. But looking at your OT wires... Don't coil them up like that. They're right under the turret board. Now referring to your pic, take the blu/wht, brn/wht, orn, and black wires up to the edge of the chassis, turn left and run them all toward the pilot lamp. Cable tie to your choke wires. The black wire will connect to the power buss bar. Put shrink tubing on the other 3 wires and neatly tuck under the pilot lamp. ***CAUTION*** There is B+ on the blu/wht and brn/wht wires! Refer to this pic...
http://sluckeyamps.com/VAC15/big_guts.jpg
Question 10: How does my lead dress look (see picture)? Anything I should change before I start cutting leads and soldering?
Install your pot buss bar before you put the board in. And if you have wire pass thru holes on your board, solder all wires to the board BEFORE you install the board. Refer to these pics...
http://sluckeyamps.com/VAC15/13.jpg
http://sluckeyamps.com/VAC15/16.jpg
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I fired up the power transformer and voltages on all the filaments was correct as was the voltage across the secondary so it looks like I'm good to go.
I re-routed the blue/white, brown/white, and black wires as you suggested, but not the orange wire. If I ever decide to swap the 16 ohm output tap (orange wire) for the 4 ohm output, I would like to have easier access to it.
Question 11: Would it be better to run the orange wire towards the front of the amp but just don't zip-tie it down in as many places, OR can I run it along the back of the amp?
Question 12: In the "Big Guts" shot of your website there are 1 Ohm resistors attached to tab 3 of the power tubes, connecting to the cathode resistor and bypass cap (130R/10W), but I don't see these in the layout diagram. Are they necessary? What are they used for?
Question 13: Do you wrap your leads when connecting them to the tabs or just push through and solder them in place. Its hard to tell from the photos?
Here's what my chassis looks like now.
(https://i.ibb.co/Vp02cmp/9892-D2-AF-3-C0-F-4-F47-9878-CE56-E7200-A13.jpg) (https://ibb.co/S7DR4r7)
Here is my turret board loaded with leads waiting to be connected.
(https://i.ibb.co/879RPW9/C93-C05-F2-28-A7-4-E6-B-8685-9-D0-DC75-BB6-B3.jpg) (https://ibb.co/1mTpRDT)
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Question 11: Would it be better to run the orange wire towards the front of the amp but just don't zip-tie it down in as many places, OR can I run it along the back of the amp?
Route it with the other two secondary wires to the speaker jacks. Leave it full length. Or, you could just install a third spkr jack between the existing two jacks and be done with it now while you have easy access. If I had the same OT that you have my amp would have 3 speaker jacks. I suggest routing all 3 secondary leads between the EL84s.
Question 12: In the "Big Guts" shot of your website there are 1 Ohm resistors attached to tab 3 of the power tubes, connecting to the cathode resistor and bypass cap (130R/10W), but I don't see these in the layout diagram. Are they necessary? What are they used for?
They are used to monitor the cathode currents of each EL84 separately for bias calculations. Not necessary.
Question 13: Do you wrap your leads when connecting them to the tabs or just push through and solder them in place. Its hard to tell from the photos?
I ALWAYS wrap. Look closely at the tube sockets. You'll see the wrap. Same for switches, pots, jacks, everything.
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It's alive!!!
The amp passed the smoke test and all voltages seem to be pretty close to those on the schematic. Plugging in, the amp sounds great. Amazing really. I have never had an amp that was so touch sensitive. Goes from clean and chimey to dirty just by digging in or playing harder. That said, there are a few things that need to work out:
On the EF86 channel, if I turn up about 3/4 of the way there is a high pitched squealing sound and lots of noise. Not like guitar feedback, sounds more like an old dial up modem if you know what I mean. When I turn the volume back down it goes away. On the 12AX7 channel, if I turn up to 1/4 and turn up (down maybe the amp was upside down, the direction that cuts out the bass) the cut control there is lots of noise as well.
Any thoughts on how to diganose where the noise is coming from?
Do I need a footswitch to turn on the Tremolo/Vibrato? Those controls are doing nothing.
Obligatory gut shot:
(https://i.ibb.co/196f0DY/A3-F8485-D-4-B87-44-A4-AE91-21-D4-D27-FB590.jpg) (https://ibb.co/LC5z0LD)
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Very neat build!
You need a ground jumper on the footswitch jack to enable the vibrato. Like this...
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Its hard to see in the picture, but I have the footswitch jack wired like the speaker jack. That is, the two lugs closest to the chassis share a common ground wire (call them top-left and top-right). The lug on the bottom-right goes to the board.
You are saying run a wire from the top-right to the bottom-left. Should I keep to top-left and top-right connected or should I remove that?
Some more follow up on the noises. I hooked up the speaker output to a Captor X (load box, ir-loader) so I could control the actual volume I hear. What I found is that if I turn the 12AX7 volume up to 75% it gets very noisy, but if I go past that to 90 or 100% the noise goes away. Does that mean its a bad pot? I can make a recording if you want to hear it.
One other thing... there isn't much clean on the amp at all. Pretty much anything past 10% has a lot of dirt. I was thinking things would start breaking up around 30 or 40%. I am using a guitar with Filtertrons but my Tele is also pretty hairy. Don't get me wrong, I love dirt and it sounds great but I wanted a little headroom. Is that just how the amp is?
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You are saying run a wire from the top-right to the bottom-left.
yes
Should I keep to top-left and top-right connected or should I remove that?
doesn't matter
I'm not very good at guessing noises or how an amp should sound, especially over the net. Got bad ears. I've never had mine turned up past about half way. About all I can say is i prefer the normal channel. Sounds like voxy beatles. And it's stronger than the vib channel. I like the sound of the vibrato effect but turn it off and the vib channel just sounds hohum to me. Blending the channels together sounds good when the vibrato is on. I've never considered my amp to have a dirty sound. I've only played a strat with it. Been several years since it's been turned on.
Maybe Ed Chambley will chime in. He has good ears, has built hundreds of these AC-15s, and is familiar with how they sound in a band situation.
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For those who care to listen to samples, here are a couple just to get an idea of what the amp sounds like:
Sample of the brilliance control: https://soundcloud.com/dbishopbliss/1960-vox-ac15-brilliance
Sample hacking my way through The Who's Substitute: https://soundcloud.com/dbishopbliss/1960-vox-ac15-with-celestion-blues
Sample with a little more gain comparing a Two Notes Captor X with an SM57 recording a WGS Black and Blue in a 2x12 cabinet: https://soundcloud.com/dbishopbliss/1960-vox-ac15-captorx-vs-black-and-blue
No comments about the sloppy playing please. :-)
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Nice sounding amp!
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I finally got around to making a recording of my AC15 Tremolo Channel that is having the issues.
The first set of chords is 1/4 of the way up, then 1/2 way up, then 3/4 of the way up. You can really hear the funny sounds here - then all the way up - notice the funny sounds are gone. The final scale is play 3/4 of the way up. Again, notice the almost transistor like sound, but there are not transistors in this amp.
The amp was plugged into a TwoNotes CaptorX, into a Focusrite Saffire Pro 24 into Garage Band.
Any thoughts?
https://soundcloud.com/dbishopbliss/ac15-12ax7-channel-problems
Another question... I am finding that I have almost no clean at all with this amp in the EF86 channel. Really, as soon as I turn it up so that it is louder than my guitar acoustically I start to have breakup (let's call that 1). I have seen videos of other amps using the same circuit and it seems like they have more headroom. Is there something I can do to give me more clean (other than just rolling back the volume on my guitar which seems to kill the highs)?
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Another question... I am finding that I have almost no clean at all with this amp in the EF86 channel. Really, as soon as I turn it up so that it is louder than my guitar acoustically I start to have breakup (let's call that 1). I have seen videos of other amps using the same circuit and it seems like they have more headroom. Is there something I can do to give me more clean (other than just rolling back the volume on my guitar which seems to kill the highs)?
What are the HT, plate, screen and cathode dc voltages for the EF86?
And is the filter cap for the EF86 HT node grounded? (I.e. do you measure DC continuity between the filter cap ground lead and the chassis?
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I finally got around to making a recording of my AC15 Tremolo Channel that is having the issues.
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The amp was plugged into a TwoNotes CaptorX, into a Focusrite Saffire Pro 24 into Garage Band.
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What is the amp's sound like when plugged into real speakers, and recorded with your cell phone?
I am guessing you have issues with the connections to the mess of outboard stuff, or have a level issue that's causing something after the amp to distort. So verify the amp is actually the issue by getting rid of everything other than the amp/real-speaker.
Another question... I am finding that I have almost no clean at all with this amp in the EF86 channel. Really, as soon as I turn it up so that it is louder than my guitar acoustically I start to have breakup (let's call that 1). I have seen videos of other amps using the same circuit and it seems like they have more headroom. ...
I own a couple vintage AC30s, and a '65 AC10 (which has an EF86). Opinions vary, but I want distortion from the EF86 in the AC10, and set the controls to cause the EF86 to distort early.
If you really want the EF86 to stay clean (and there are no wiring errors), make the screen voltage higher by reducing the value of the resistor from B+ to screen.
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What is the amp's sound like when plugged into real speakers, and recorded with your cell phone?
I am guessing you have issues with the connections to the mess of outboard stuff, or have a level issue that's causing something after the amp to distort. So verify the amp is actually the issue by getting rid of everything other than the amp/real-speaker.
Worse than the recording I linked to. The distortion is definitely in the amp. Could it be a bad pot? Seems very dependent upon where the pot is turned and I can hear it go in and out as I move over a specific range of the pot.
I own a couple vintage AC30s, and a '65 AC10 (which has an EF86). Opinions vary, but I want distortion from the EF86 in the AC10, and set the controls to cause the EF86 to distort early.
If you really want the EF86 to stay clean (and there are no wiring errors), make the screen voltage higher by reducing the value of the resistor from B+ to screen.
Don't get me wrong... I am liking the distortion but maybe its just my guitar have high output - Filtertrons, A5 bridge pickup in my Telecaster, P90s. Maybe I won't need more headroom if I can get the 12AX7 channel cleaned up.
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I wonder if it becomes unstable at certain settings?
Have you got a scope?
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Worse than the recording I linked to. The distortion is definitely in the amp. Could it be a bad pot? Seems very dependent upon where the pot is turned and I can hear it go in and out as I move over a specific range of the pot.
I don't know a good way to diagnose without a "listening amp" (aka, a signal tracer) or a scope.
I wonder if it becomes unstable at certain settings?
Are you thinking over-long wires to/from the phase inverter or output tubes?
DBishopBliss, what's the story on those very long Green & Yellow wires that seem to appear from under the middle of the board & run over to the speaker jacks? And that Orange wire that turns left instead and is just hanging out over at the preamp area?
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...I wonder if it becomes unstable at certain settings?
Are you thinking over-long wires to/from the phase inverter or output tubes?
DBishopBliss, what's the story on those very long Green & Yellow wires that seem to appear from under the middle of the board & run over to the speaker jacks? And that Orange wire that turns left instead and is just hanging out over at the preamp area?
It was a general comment, based on the behaviour described; the audio clip doesn’t make it cut and dried obvious either way, but given the concern, it’s worth further investigation.
I didn’t reread the thread first, but certainly those wires mentioned are worth reviewing.
Looking at it now in the light of the current concern, something else that flags a concern for me is the power valve grid stoppers. My understanding and experience is that it would be better for the EL84 control and screen grid resistors to be mounted at the socket terminals, rather than on the board. I seem to remember a recent Vox model crops up on the forums with this same issue (ie instability due to board mounted grid stoppers) that gets resolved by fitting the grid stoppers to the socket terminals.
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Unfortunately, I don't have a scope.
The yellow, green and orange wires are coming from the output transformer. The orange wire is for the 16 ohm tap that is not used now. The first OT I was going to use only had two secondaries, but I decided to replace it and since I already had a profession back plate made I just insulated it. I think I'm going to replace one of the jacks with a switch that will allow me to use all 3 - although all my speakers are 8 ohm so not really necessary.
I followed the layout by Sluckey and I don't think others have had issues but I guess that doesn't mean its not possible.
I will try to get some voltage readings soon.
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Unfortunately, I don't have a scope.
The yellow, green and orange wires are coming from the output transformer. The orange wire is for the 16 ohm tap that is not used now...
Yikes - get it away from the preamp :icon_biggrin:
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Yikes - get it away from the preamp :icon_biggrin:
You talking about the orange wire or the output transformer? The orange wire is along the rear of the chassis.
If you are talking OT... I really can't imagine that is the issue since many people have followed the Sluckey layout with no issues.
I'm still thinking it might be related to the pot. I guess I could try to simply swap out the pot. Or, another possibility would be to measure the value of the pot where the noise happens. Then disconnect the wires and clip in a resistor of the same value. If the noise is still there then I would know its not the pot.
Again, it is only when the pot is about 3/4 of the way up. If I turn it all the way up, the noise goes away.
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I was referring to the orange wire that runs back over the length of the back panel, towards the input end of the chassis. Note that the 16ohm output will carry twice the signal voltage / has twice the gain, as the 4ohm output.
I haven’t checked Sluckey’s layout, but I’d be surprised if he suggested doing that. Running inputs and outputs close together is generally contrary to the principles of good layout / lead dress.
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I was referring to the orange wire that runs back over the length of the back panel, towards the input end of the chassis. Note that the 16ohm output will carry twice the signal voltage / has twice the gain, as the 4ohm output.
I haven’t checked Sluckey’s layout, but I’d be surprised if he suggested doing that. Running inputs and outputs close together is generally contrary to the principles of good layout / lead dress.
Originally, I had it coiled up under the board and he recommended doing that. It's easy enough to move around to see if it makes any difference.
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I was referring to the orange wire that runs back over the length of the back panel, towards the input end of the chassis. Note that the 16ohm output will carry twice the signal voltage / has twice the gain, as the 4ohm output.
I haven’t checked Sluckey’s layout, but I’d be surprised if he suggested doing that. Running inputs and outputs close together is generally contrary to the principles of good layout / lead dress.
Originally, I had it coiled up under the board and he recommended doing that. It's easy enough to move around to see if it makes any difference.
You were supposed to keep the orange wire together with the other secondary wires, such that it ends up by the speaker jacks, NOT THE PREAMP AREA. Maybe I was not clear on that.
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You were supposed to keep the orange wire together with the other secondary wires, such that it ends up by the speaker jacks, NOT THE PREAMP AREA. Maybe I was not clear on that.
I just re-read about routing them between the EL84s. Missed that. I will start by moving the orange wire and then see if I can re-route the others. I may not have enough lead. If not, should I splice length on, or just leave it? I don't hear any noise in the EF86 channel, but maybe that's because the wires are closer to the 12AX7 channel. But, if it as the OT wires, wouldn't I hear the noise when the amp was all the way up?
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...I don't hear any noise in the EF86 channel, but maybe that's because the wires are closer to the 12AX7 channel. But, if it as the OT wires, wouldn't I hear the noise when the amp was all the way up?
Not necessarily, a system becomes unstable when the output is in phase with the input and the gain is at least unity.
Phase shift is affected by the vol control setting as it alters the source resistance that interacts with the following stage’s Miller capacitance.
The source resistance will tend to be highest (and hence corner freq lowest) at a midway setting.
The noise might be a DC shift caused by the amp working hard when oscillating. You might be able to measure a change in the HT or EL84 cathode VDC either side of the noise; that would be pretty firm evidence of oscillation.
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Sorry, catching up. I am going to assume you used Sluckey's final layout. You have separate inputs for the Wobble Channel. I have had this distortion 2 times, both times due the the the 12Au7.
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Sorry, catching up. I am going to assume you used Sluckey's final layout. You have separate inputs for the Wobble Channel. I have had this distortion 2 times, both times due the the the 12Au7.
I just swapped the 12AU7 with 3 different tubes and same effect each time. While I was at it, I swapped around the 12AX7's and still the same. Looks like I'm going to have to go under the hood.
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I rerouted the OT 16ohm wire and pulled the 4 and 8 ohm taps away from the 12AX7 tube (pretty sure that is the PI tube next to the EL84s) and the distortion does not change no matter how or where I move those wires. They are still under the board in the same spot but no matter how I move them, there is not change in the distortion.
Another observation... the volume of the 12AX7 channel is much lower than the EF86. That is, if I have the 12AX7 channel on 3 and the EF86 on 3, the AX7 is much quieter and much less full sounding. Stays the same all the way up. I have the channels internally jumpered. I can have the AX7 on 5 and then start turning up the EF86 and it will overpower starting around 1 or 2.
I have done a chopstick test and nothing makes any noise or changes when I poke around.
I was reading about the "listening amplifier". Any tried the type where you make a box with just a pot and a probe? Then you output the signal from the box to an audio amp? I have solid state power amp I could use. Maybe that could help identify where the distortion is introduced.
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... the volume of the 12AX7 channel is much lower than the EF86.....
Corgis are much shorter than Great Pyrenees. We knew this when we got them.
(https://live.staticflickr.com/65535/48099984102_7a2d1a24af_n.jpg)
Corgi vs Great Pyrenees (https://www.youtube.com/watch?v=AyKAtv3lXCk)
Pentodes have more gain than triodes. This is not news either.
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Another observation... the volume of the 12AX7 channel is much lower than the EF86.
I noticed that too.
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Another observation... the volume of the 12AX7 channel is much lower than the EF86.
I noticed that too.
Just confirming it was expected.
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I replaced one of the speaker jacks with a speaker selector switch that falls between the two EL84s. I re-routed the output transformer wires under the board at right angles to most of the components and leads above them until they are come out between the EL84 sockets. Then I replaced the speaker output jack that falls between the EL84s with a speaker selector switch that allows for the 4, 8 and 16 ohm output taps to be used. Most of the distortion is gone now. I think there is still some, but at that volume it is hard to tell because there is a lot of natural tube distortion. I can live with "hard to tell" because I probably won't be using the amp on 8 very often.
So the lesson here is, lead dress really does matter. I am baffled when I look at some of these old point to point amps and wonder how/if they were ever quiet.
One other thing, there is a faint clicking going on in the EF86 channel even when the 12AX7 volume is all the way down. I think it is from the Tremolo/Vibrato circuit but the controls don't seem to impact the speed of the click. However, if I plug in a footswitch and turn off the Vibrato then clicking goes away. Is this to be expected?
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One other thing, there is a faint clicking going on in the EF86 channel even when the 12AX7 volume is all the way down. I think it is from the Tremolo/Vibrato circuit but the controls don't seem to impact the speed of the click. However, if I plug in a footswitch and turn off the Vibrato then clicking goes away. Is this to be expected?
I've never noticed any clicking in the normal channel.
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I've never noticed any clicking in the normal channel.
My channels are permanently jumpered. Maybe that is it.
I thought I had read that jumpering the channels this way would allow the EF86 channel to have Tremolo. Mine only has Tremolo if I turn up the volume of the 12AX7 channel. Am I remembering that wrong?
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There's no way to have tremolo on the EF86 channel.
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I brought my amp to a friends this weekend and it sounds great, but again zero clean. I'm not talking edge of breakup sort of distortion, its full on distortion as soon as I get to a level that could compete with an drum set. I like that, but it make the amp less versatile and not really pedal friendly.
I own a couple vintage AC30s, and a '65 AC10 (which has an EF86). Opinions vary, but I want distortion from the EF86 in the AC10, and set the controls to cause the EF86 to distort early.
One thing that occurred to me, I do not have a Hi and Low input on my amp. On all my past amps I only used the Hi input so I left it out. Could I simply add in the 1M resistor (or play with values to see what I like) to lower the signal?
If you really want the EF86 to stay clean (and there are no wiring errors), make the screen voltage higher by reducing the value of the resistor from B+ to screen.
I'm not really sure which resistor you are referring to. The pin-out on the datasheet says 2 and 7 are the screens but they aren't connected to anything. Please clarify and suggest a starting point value. In theory, can I clip a resistor in parallel?
(https://i.ibb.co/CQnLHFq/1960-Vox-AC-15.png) (https://imgbb.com/)
best way to save your pictures (https://imgbb.com/)
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I believe R20 and yes you can || the resistor
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One thing that occurred to me, I do not have a Hi and Low input on my amp. On all my past amps I only used the Hi input so I left it out. Could I simply add in the 1M resistor (or play with values to see what I like) to lower the signal?
If you really want the EF86 to stay clean (and there are no wiring errors), make the screen voltage higher by reducing the value of the resistor from B+ to screen.
How is your input jack wired up? Maybe you wired up the input jack to the tube grid wrong?
Could I simply add in the 1M resistor (or play with values to see what I like) to lower the signal?
This 1M resistor you're referring to here, is it the the EF86 input grid R?
Or the EF86 screen grid R?
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I'm not really sure which resistor you are referring to. The pin-out on the datasheet says 2 and 7 are the screens but they aren't connected to anything.
No, pins 2 and 7, the ~ S ~ on the pinout diagram stands for 'shield' not screen. EF86's have an internal shield for noise surpression that can/should be wired to ground.
Grid ~ 2 ~ is the screen grid.
(Grid 1 = input grid, grid 3 = suppressor grid.)
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the posted clip has G1 = pin9, G2 = pin 1 and G3 = pin 8 (I didn't look up datasheet, it's lunch time:)
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Could I simply add in the 1M resistor (or play with values to see what I like) to lower the signal?
This 1M resistor you're referring to here, is it the the EF86 input grid R?
Or the EF86 screen grid R?
So I was way off in my recollection of how things worked. :-( I just reviewed Rob Robinette's page on how input jacks work (https://robrobinette.com/How_Fender_Input_Jacks_Work.htm). I have the 1M Input Impedance Resistor and a single 33K grid stop resistor mimicing the type 68K resistors in parallel on a Hi input.
Based on his explanation, 1/2 of the guitar signal goes to ground when you plug into the Lo input. On a typical wiring, there is a 68K grid stop and the other 68K resistor acts as voltage divider sending signal to ground.
Would adding in another 1M resistor in parallel with the existing Input Impedance Resistor have a similar effect? I realize this would drop the Input Impedence to 500K but a typical Low input has a value of 136K... at least that is what I'm understanding from Rob's page.
Or... should I play with lowering the G2 Screen Input resistor. (I know... try both). The Screen Input resistor is currently 1M. If I put another 1M in parallel than that will cut the value in half. Would that be too low (as in would it do damage)?
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Do you have two input jacks? If so, just wire them exactly like the schematic you posted.
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Do you have two input jacks? If so, just wire them exactly like the schematic you posted.
One thing that occurred to me, I do not have a Hi and Low input on my amp. On all my past amps I only used the Hi input so I left it out. Could I simply add in the 1M resistor (or play with values to see what I like) to lower the signal?
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Or... should I play with lowering the G2 Screen Input resistor. (I know... try both). The Screen Input resistor is currently 1M. If I put another 1M in parallel than that will cut the value in half. Would that be too low (as in would it do damage)?
I'd play with the EF86 screen R 1st.
This will help you;
http://www.valvewizard.co.uk/EF86.html (http://www.valvewizard.co.uk/EF86.html)
And this;
http://www.valvewizard.co.uk/pentode.html (http://www.valvewizard.co.uk/pentode.html)
What you are asking about the screen R is in there and he talks about using a screen stopper R too, I would definitely try that.
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Do you have two input jacks? If so, just wire them exactly like the schematic you posted.
No, just one. Both channels are permanently jumpered to the single input jack and I wired them as the Hi input.
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See the pic if you want to wire your input jack as a "LO" jack. I think you could just turn the vol knob on your guitar to accomplish the same thing.
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I think you could just turn the vol knob on your guitar to accomplish the same thing.
I noticed my guitar losing a lot of highs when rolling back the volume knob. Maybe I need to add a treble bleed circuit but that is a lot of work since I have a ES335-style (Eastman T386). Not to mention, it seems like every guitar that gets plugged in has the same effect. Zero clean, highs roll off with the volume down.
I played around with clipping resistors in parallel with the screen grid resistor (R20) and really didn't hear much difference. I will keep trying different things.
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> highs roll off with the volume down.
How long is your cable?
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> highs roll off with the volume down.
How long is your cable?
4 feet, or 10 feet. Happens with both.
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I think you could just turn the vol knob on your guitar to accomplish the same thing.
I noticed my guitar losing a lot of highs when rolling back the volume knob. ... Not to mention, it seems like every guitar that gets plugged in has the same effect. Zero clean, highs roll off with the volume down.
The 50% signal loss due to the input jack wiring is the same as turning your guitar's Volume from 10 to 9 (http://www.geofex.com/article_folders/potsecrets/pottaper.gif).
Regardless, the first thing that comes to mind if you're unhappy with treble loss due to turning down your guitar's Volume is to turn the Cut control for more treble. Depending on your speaker choice, you could have more treble than you want anyway.
I wound up trying to work around a 1964 Deluxe Reverb's bright cap by turning the amp's Volume up to 7-8, then turning my guitar's Volume near-off to get the room-loudness I wanted (late-night apartment playing). Worked fine for me with a Tele with what's generally called "50's wiring" (https://www.youtube.com/watch?v=uRUobVmNKko) when applied to a Les Paul (retains highs when both Vol and Tone controls are rolled back).
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I understand about rolling back on the volume, etc. but that doesn't really help me if I want to play with a drummer. If I turn the amp up to where it can be heard (call that 3) then it is very overdriven. If I roll back on the guitar volume then I am below drummer level. I'm not necessarily looking for sparkly clean fender tones... I like a little edge on everything but it would be nice if I could have that tone AND be able to play with a drummer.
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... doesn't really help me if I want to play with a drummer. If I turn the amp up to where it can be heard (call that 3) then it is very overdriven. ...
This is where you have to figure out whether the amp is distorting because the EL84s are distorting (and the EF86 is clean), or because the EF86 is distorting (and the EL84s are clean).
Fastest way to do that is measure the d.c. volts across the EL84 cathode resistor: that's your bias voltage. Now attach your meter to the EL84 control grid, set for a "peak a.c." measurement. Play until distortion just-starts, then back off a hair on volume. You might need to reset the meter's peak-hold function to capture the peak signal driving the EL84s just-before distortion starts.
Compare the measured peak to the EL84 bias voltage. If the peak voltage is about the same as the EL84 bias, you're hearing the EL84s distort; you'll need a bigger output section for louder-clean. If the peak voltage is well below the EL84 bias, you're hearing the EF86 distort (or much less likely, the phase inverter distort); adjust the screen voltage to reduce distortion.
"Which way, or how do I adjust the screen voltage?"
Put a pot somewhere: might be as a variable resistor in place of the resistor from B+ to screen. Or it might be that you keep the screen resistor as-is, and add a variable resistor from screen to ground/cathode (somewhat like the vintage AC10).
You're adjusting by-ear to your-taste, so no one can really help you out with what to do. Just gotta do it, make a choice aligned to your preference.
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Here’s the finished amp in the head cabinet. The logo is just a paper cut out. If you know of a good laser engraver please le me know.
(https://i.ibb.co/tLBD0nC/47487-AD8-A0-D6-42-EF-A25-A-7-F1521-B448-DC.jpg) (https://ibb.co/mNJ4ndz)
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Looks really great! :icon_biggrin: