Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: PharmRock on April 22, 2022, 12:05:16 pm
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As most of you know, I've been working on this 1974 18-watt circuit with an EF86 channel in place of the normal channel (the tremolo channel is carried over essentially the same as the original).
For my tone circuit on the EF86 channel, I used a Carmen-Ghia style single tone control, which is sort of like a mid-sweep control. I actually like how it works in this amp...I can really dial things in. However, the Carmen Ghia uses a 12AX7 as the input tube...the tone control in that amp sits between V1a and V1b, when then goes to the PI after that. On my amp, its basically the EF86 pushing the signal through the tone control which then goes straight to the PI.
While the EF86 channel sounds pretty good (I may tweak it for a little more gain), it is much quieter at full volume when compared to the tremolo channel at full volume.
I'm wondering if I am allowing too much signal to go to ground through the EF86 tone control circuit. Would increasing the 180K resistors in the circuit help alleviate this issue? If I do that, I suspect I will need to make some changes to the capacitors in order to preserve the current midrange sweep I have (and like). I'm not quite sure how this tone control does what it does, but I know it works and I like it.
The schematic is attached below. I appreciate the help.
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The EF86 usually does not like to drive a tonestack. Easy to find out. Just disconnect the tonestack (2 resistors, 2 caps, and a pot). Now connect the .0047 directly to the volume pot. Any better?
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Yes the EF86 stage has high output impedance, which means there will be high signal loss if the following load (tone stack) is a heavy load. Compare with the Vox AC4 where the tone control is simple.
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The EF86 usually does not like to drive a tonestack. Easy to find out. Just disconnect the tonestack (2 resistors, 2 caps, and a pot). Now connect the .0047 directly to the volume pot. Any better?
I have a few things to tweak on the amp today, and that will be one of them. If the output increases, I'll probably end up replacing the existing tone circuit with the AC4 tone control, or perhaps just duplicate the one already in the trem channel. I was thinking the 6-position cap switch looks like a good option, but I have another amp that already has that, so looking for something a little different.
The Carmen-Ghia tone control is pretty cool though. When I was trying to see if there is a way to make it work with an EF86, it seems Dr. Z uses that tone circuit in his Stang-Ray amp, but instead of using 180K resistors, there are 330K, and the cap values are slightly different. In any case, I'll do some tweaking today and see how that goes.
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PharmRock, take a look at this six-way tone selector in my AC15. I borrowed it directly from sluckey's AC15 project. I like it better than the single tone control in my AC4. It really has a wide tonal range. I've also attached my AC4 schematic, if you want to compare it to the single tone pot.
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I have a few things to tweak on the amp today, and that will be one of them. If the output increases, I'll probably end up replacing the existing tone circuit with ...
Add a MOSFET source-follower between the EF86 and the tone circuit you choose.
Ask Tubenit how to rig one up (or look at any of his numerous amp projects using a MOSFET).
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Found this thread with some great images on how to implement. I'll definitely look into it. It almost looks too simple.
https://el34world.com/Forum/index.php?topic=21873.0 (https://el34world.com/Forum/index.php?topic=21873.0)
thanks for the suggestion.
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Found this thread with some great images on how to implement. I'll definitely look into it. It almost looks too simple.
Tubenit saw info somewhere a long while back about MOSFETs performing the role of a tube stage in an amp, and asked whether he could add a MOSFET follower rather than a vacuum tube cathode follower for isolation, because adding another tube/socket wasn't feasible.
I think he thought "it can't be that simple" but tried it anyway. And it's become something he uses frequently because it's just so simple to implement.
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The first place that I saw mention of mosfets used as source followers was at R.G. Keen's website in the "Mosfets Follies" article. http://geofex.com/article_folders/mosfet_folly/mosfetfolly.htm (http://geofex.com/article_folders/mosfet_folly/mosfetfolly.htm)
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I've ordered the parts from Mouser to do the MOSFET follower..Getting the IRF820 MOSFET that he uses. There are a couple of places in the chassis where I can mount a little board for the components since there is zero room on my main board.
One place is close to the B+ source for the input tubes as well as being next to the tone control. The problem is that it's pretty close (less than an inch) to the input jack for the EF86 channel. I can use shielded wire if needed but wasn't sure if that's a good place for it. That spot is also about half an inch from the end of the chassis, which is open on either end.
The other place, and this has a little more room, is by the EF86 socket. The wire runs would be a little longer here but not any different than if I had an actual tube doing the same purpose.
Either of these have an advantage over the other?
In Tubenit's implementations, he has a 12V Zener coming from the source ("cathode") to the gate ("grid", also connexted to the EF86 plate). In reading RG Keen's article, the Zener is not mentioned. What is the purpose of the Zener here, and is the 12V value critical?
thanks!
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The 12v zener between Gate and Source is an insurance mechanism to prevent overvoltage which would otherwise destroy the insulation layer between these. The IRF820 will ‘work’ without it, but may not last long. Best to keep it there. Note that the IRF820 is a 500V rated device, so will work directly in place of a 12AX7 triode without needing any ‘extra’ components (apart from the 12V Zener). The TO220 package is rated for 1W without any heat sink, so it will be able to handle the power that a 12AX7 sees if used in a stage intended for a 12AX7.
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In Tubenit's implementations, he has a 12V Zener coming from the source ("cathode") to the gate ("grid", also connexted to the EF86 plate). In reading RG Keen's article, the Zener is not mentioned. What is the purpose of the Zener here, and is the 12V value critical?
Having a zener diode is critical IMO and 12v is right for the IRF820 MOSFET. It is there to protect the MOSFET's gate.
It is used so often that sometimes folks drawing diagrams think, "Well, everyone knows that it must be there."
Kinda like sketching a preamp circuit & not-drawing the details of the power supply: "But everyone knows the tubes won't work without a power supply. So they know they will have to tack on a power supply, even if not-drawn..."
Notice that where R.G. Keen draws a MOSFET source follower drive for output tubes (http://geofex.com/article_folders/mosfet_folly/mosfol5.gif) that he includes a 12v zener.
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Thanks for the info/advice everybody. Parts are ordered so I hope to tackle this next weekend.
The only other experience I have with a MOSFET was with VVRs. Those required a chassis mount/heat sink. Glad to know I won't need to do that for this application.
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If you adopt the SF then you can try different Tone Controls like James, TMB or whatever you want, without limitations
Franco
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yeah it will definitely open up a lot more possibilities. the limiting factor may be front panel real estate...I'm runningoutofroom. Maybe I should be looking into stacked/concentric pots. Once I get it installed I'll check into expanding the options.
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There are a couple of places in the chassis where I can mount a little board for the components since there is zero room on my main board.
Maybe something like this to plug into your board could be an option. I plug these into the bias resistor spot on Univox boards to add bias control. No board mods required.
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The G-S voltage limit tends to be +/-20V, so rather than just a single zener, a series pair of 12V zeners in reverse polarity to each other seems a good idea, being as however unlikely the occurance of the gate being >-20V below the source, for the cost of another zener, device destruction is something worth avoiding.
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Maybe something like this to plug into your board could be an option. I plug these into the bias resistor spot on Univox boards to add bias control. No board mods required.
I think I can come up with a pretty small footprint like that. Thanks for the pic and suggestion.
The G-S voltage limit tends to be +/-20V, so rather than just a single zener, a series pair of 12V zeners in reverse polarity to each other seems a good idea, being as however unlikely the occurance of the gate being >-20V below the source, for the cost of another zener, device destruction is something worth avoiding.
So as long as the cathode bands are not going in the same direction, then I'm good, correct? Cathode tied to cathode, or anode to anode doesn't matter.
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...a series pair of 12V zeners in reverse polarity to each other seems a good idea...
A single standard 20V Zener breaks-over at 20V one way, 0.6V the other way. Since we have no NEED to force the Gate negative, this is just what we want.
Agree that form modern MOSFETs in tubey circuits, 12V is way ample, even 7V (BJT BE junction) is usually enough to pass the desired currents.
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PharmRock, take a look at this six-way tone selector in my AC15. I borrowed it directly from sluckey's AC15 project. I like it better than the single tone control in my AC4. It really has a wide tonal range. I've also attached my AC4 schematic, if you want to compare it to the single tone pot.
It might be my eyes but you seem to have the signal grounded after the volume pot on your schematic. Regards Joe
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The G-S voltage limit tends to be +/-20V, so rather than just a single zener, a series pair of 12V zeners in reverse polarity to each other seems a good idea, being as however unlikely the occurance of the gate being >-20V below the source, for the cost of another zener, device destruction is something worth avoiding.
So as long as the cathode bands are not going in the same direction, then I'm good, correct? Cathode tied to cathode, or anode to anode doesn't matter.
Correct, but I think PRR found a flaw in my thinking :think1: