Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tubenit on September 30, 2018, 06:42:34 pm
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I am pondering an idea and haven't decided whether I will pursue this or not?
I am wondering IF using a mosfet like this would work for a cathode biased tremolo using 6L6's?
Any thoughts on this idea? Anything that would need to be changed?
The point of using the mosfet would be to free up an extra tube triode for the reverb.
With respect, Tubenit
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I think that should work just fine. It's just replacing a cathode follower. Haven't you already successfully done that in other circuits?
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An LND150 will work fine in this application.
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I think that should work just fine. It's just replacing a cathode follower. Haven't you already successfully done that in other circuits?
Yes, I've done that a handful of times using the CF to drive a tone stack. I didn't know IF trying to drive a tremolo with 6L6 tubes would make a difference?
I looked at the data sheet of the LND150 vs. data sheet of IRF820. I can't make any sense of those enough to determine if one is "better" for this application then the other. IF they are just different without one being better, then I'd prefer to use the IRF820 since I am familiar with it and Hoffman carries those.
With respect, Tubenit
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FWIW: www.geofex.com scroll to 02/05/02 MOSFET Heresies Again! Make a solid state analog of the Fender tremolo (http://geofex.com/Article_Folders/sstremolo/sstremolo.htm) circuit for adding tremolo to an non-tremo amp, or free up a tube for more audio gain.
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I remember Ken Fisher talking about a Vox(?) amp with a SS LFO. He made a joke about the SS LFO, something like, "nuclear radiation caused a mutation in this amp and it sprouted a transistor." (But it might have been a Marshall or even a Selmer?) Any way, somebody did back then.
And they use SS LFO's in pedals for trem/vibrato all the time now.
I thought it was included in the Gerald Weber books, I can't find it. :dontknow:
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Sunn also used a ss trem circuit. Not enough signal to wiggle the output tube grids though.
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I looked at the data sheet of the LND150 vs. data sheet of IRF820. I can't make any sense of those enough to determine if one is "better" for this application then the other. IF they are just different without one being better, then I'd prefer to use the IRF820 since I am familiar with it and Hoffman carries those.
The only things that are particularly important on the data sheets for this Source follower are the maximum voltage and the maximum power. The subject MOSFET is idling at around 0.068W as drawn. The LND150 maxes out at 0.74W and the IRF820 at 50W. But, they are going to work virtually the same in this application. It's just that it is not necessary to use a power MOSFET here. I know what you mean about the IRF820 - - the availability and, especially, what I have on hand always has a profound effect on what I design.
I wonder if you even need a voltage follower here when you are driving a reasonably high impedance. If the load impedance is too low for the LFO, then a 470K Source resistor seems too large for such a load. Maybe there is some other reason for this voltage follower?
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Your doing it with a lot less parts, but I built that Aussie elec stomp box circuit and thought it sounded remarkably like a fender amp.
Also, when we had the store, a lot of Vox transitor amps came in and some of those had a beautiful trem, it could get downright hypnotic if u wanted it to.
Maybe you were asking more if that would work, more than about inserting transistor circuits, but hey, who knows, just trying to be helpful!
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Guys,
Thanks for the posts and comments! I truly do appreciate it. As it turns out, My chassis is really small and the layout board doesn't lend it self well to the conversion using a mosfet unfortunately. I'd have to double deck one board over another and I'm not too keen on that idea. I'm afraid I did not think thru the logistics of this idea well enough. :dontknow:
I will either leave it the same or do a "one tube reverb" with a one tube tremolo.
Thanks, Tubenit
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There are a few Gibson amps that have 1 tube trem. that wiggle the 1st preamp tube.
The Les Paul GA 40 is 1 of them, it uses a 6SQ7 for the LFO, but I know that I've seen a (later) model or 2 with a 12AX7 for the LFO.
They say the trem is very strong sounding on these. As it takes less to overcome the signal the earlier in the tube chain you go.
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I've used FET source followers to buffer LFO bias-vary trem in quite a few amps. No problem. IRF820 works great and subs straight into a conventional CF stage circuit. gate=grid, drain=plate, source=cathode. Run a 12V zener between the gate and the source
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I came across an example of Fender using an IRF820 CF on their tremolo. It has 6V6's, but it can easily be tweaked for 6L6.
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Thanks for sharing that! Very helpful. I anticipate trying that out on a build some point.
With respect, Tubenit
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Given the Keen full SS circuit, why "waste" a tube stage if there's sand in the trem path anyway? Any thoughts on this?
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Given the Keen full SS circuit, why "waste" a tube stage if there's sand in the trem path anyway? Any thoughts on this?
I agree. I don't see the tremolo as part of the audio path. There's no way I could tell what device generated a LFO.
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I re-designed the MOSFET tremolo a little after it killed an LND150. I moved the footswitch and installed a Gate-stopper resistor suspecting that high frequency oscillation may have destroyed the LFO MOSFET. It has been working fine in an actual amp ever since.
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Thanks for sharing that schematic Clean Clam! I like that design.
Any chance you have a YouTube or sound clip of the amp? I'd love to hear the tremolo and reverb on this one.
With respect, Tubenit
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Any chance you have a YouTube or sound clip of the amp? I'd love to hear the tremolo and reverb on this one.
I'm not very savvy with computers and I would be a little hesitant to even strum a chord in front of you guys. The tremolo sounds the same as the bias-wiggle ones in the Hoffman library. It may be a little over-the-top at max with the 820K resistors, but you don't need to turn it up that far if you don't like it. It does turn off completely with the knob fully CCW whereas the Hoffman one doesn't.
The reverb is medium density and unremarkable. Whereas Fenders were designed such that they attained full output before the strapped 12AT7 reverb drivers would clip, this driver will not clip no matter how high you turn the volume up. In order to get decent current into the tank at lower volumes, however, I have the IRF820 idling at 8.3mA.
The amp originally had a one-tube reverb that was also unremarkable. It came out of an effort to design a single 12AT7 driver that operated within the maximum ratings.
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What am I doing different that causes my attachments to be full-size like that?
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I use png file type and they're sized right. Don't know if that's why yours are large.
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Caution, an IRF820 in TO 220 package only has a max diss of 1W. You'll want to heatsink and force cool it if you're going to drive it hard
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Caution, an IRF820 in TO 220 package only has a max diss of 1W. You'll want to heatsink and force cool it if you're going to drive it hard
It probably would burn up in the atmosphere, but it doesn't get any warmer than 40 Deg C bolted to the chassis.
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2deaf,
Thanks for the description of the tremolo and reverb. I appreciate your taking the time to do that.
With respect, Tubenit
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I know I'm a little late to the game, but I just wanted to report that I successfully built a LND150 oscillator to wiggle the Bias of my 2x 6V6 AB763 Deluxe style amp. I etched a PCB for it and included a 1/2 speed switch and safety zener-diodes on the gates of each LND150. It's pretty much the same circuit like the one 2deaf presented but with some value tweaks. I also included the option for an intensity pot in the case of auto-bias amps. The resistor values were chosen to provide center-biasing of the oscillator stages and provide enough amplitude to wiggle the bias strongly, but not to cause any undesirable pumping. In my build I used a 100k resistor in place of the intensity pot and took the output of the PCB ("OUT") to the intensity-pot in my amp.
I just wanted to motivate others to use this circuit! I had an actual tube driven tremolo in the amp before and I don't miss anything, the sound is just as good and even provides a little more flexibility.
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Thanks for sharing your success! It is appreciated.
With respect, Tubenit
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I looked at the data sheet of the LND150 vs. data sheet of IRF820. I can't make any sense of those enough to determine if one is "better" for this application then the other. IF they are just different without one being better, then I'd prefer to use the IRF820 since I am familiar with it and Hoffman carries those.
The only things that are particularly important on the data sheets for this Source follower are the maximum voltage and the maximum power. The subject MOSFET is idling at around 0.068W as drawn. The LND150 maxes out at 0.74W and the IRF820 at 50W. But, they are going to work virtually the same in this application. It's just that it is not necessary to use a power MOSFET here. I know what you mean about the IRF820 - - the availability and, especially, what I have on hand always has a profound effect on what I design.
I wonder if you even need a voltage follower here when you are driving a reasonably high impedance. If the load impedance is too low for the LFO, then a 470K Source resistor seems too large for such a load. Maybe there is some other reason for this voltage follower?
for depletion mode FET with slightly higher power ratings, there are other options.
https://www.mouser.com/datasheet/2/268/DN2540%20B060313-965103.pdf (https://www.mouser.com/datasheet/2/268/DN2540%20B060313-965103.pdf)
--pete
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With respect to my circuit posted earlier in this thread I have to admit that it has now failed. :sad2:
I don't know yet what is wrong but the oscillator won't oscillate and the voltages on the LND150 are off (e.g. 45V on the drain of Q1). I assume the LND150s are fried. Now I'm wondering what caused it. I might try Q1 without Zener protection as has been proposed by 2deaf and I will also use only a 12V Zener on Q2. I don't know if this will make the circuit completely reliable but as 2deaf didn't report any failure ever since I was hoping that it is still reliably working.
2deaf, if you're around, could you confirm this or report on any long term experience with the circuit? This would be great!
Using only one 12V Zener on Q1 as is proposed by RG Keens example doesn't seem to produce an oscillation at all when simulating the circuit in LTSpice.
Currently I am using a 10k gate resistor on Q1. Maybe this needs to be increased to suppress high frequency oscillation?
However, I will take the LND150s out of the circuit tonight and test them to see if they both failed.
Any more ideas are highly appreciated.
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One more update for those who are interested:
The LND150s tested fine in a low-voltage chinese components-tester (running from a 9V battery) but I had to replace both of them to get the oscillation going again. The Zeners tested fine as well. For reliability reasons I replaced the 18V Zeners with 12V types back to back on Q1 and only one from source to gate on Q2. Additionally I increased the Gate resistor on Q1 to 100k and reduced the supply voltage to 330V (B3+ in my amp). It is working nicely now but who knows for how long.
Do you guys have any idea what could have caused failure of the LND150s? Maybe those 18V Zeners were too optimistic and I should have went with lower voltage ones in the first place? But then I'm wondering why I see designs around which don't use any zener at all on Q1, like the circuit posted by 2deaf...?
Unfortunately I'm not smart enough to look at this circuit from an electronics engineering point of view, so if anybody knows more about this and could tell me what would have to be considered to prevent the LND150s from frying, I would be really glad.
Thanks!
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With respect to my circuit posted earlier in this thread I have to admit that it has now failed.
I don't know yet what is wrong but the oscillator won't oscillate and the voltages on the LND150 are off (e.g. 45V on the drain of Q1). I assume the LND150s are fried. Now I'm wondering what caused it. I might try Q1 without Zener protection as has been proposed by 2deaf and I will also use only a 12V Zener on Q2. I don't know if this will make the circuit completely reliable but as 2deaf didn't report any failure ever since I was hoping that it is still reliably working.
2deaf, if you're around, could you confirm this or report on any long term experience with the circuit? This would be great!
Using only one 12V Zener on Q1 as is proposed by RG Keens example doesn't seem to produce an oscillation at all when simulating the circuit in LTSpice.
Currently I am using a 10k gate resistor on Q1. Maybe this needs to be increased to suppress high frequency oscillation?
Everything that I still have that uses LND150's is still working fine. The only time I had trouble was when I connected a long cord and footswitch directly to the Gate. I have taken to putting two 12V zeners butt-to-butt between the Gate and Source lately in things like amplifier stages and effects loops even though there was no trouble without them.
I didn't feel the zeners were necessary on the gain stage (Q1) because the gain is high enough that the output signal will max out long before the Vgs reaches 20V. It's not going to hurt having the zeners there and it might prevent some voltage of unknown origin from destroying the LND150.
There was only one time that I was pretty sure about how I ruined an LND150. I treated the limiting values the same way I did with tubes, exceeded the 20V Gate-to-Source voltage, and it went belly up. Fast. The other times I never really knew what happened and I never could seem to reproduce the conditions.
You can't use just one zener from the Gate to Source on a gain stage because it will clamp the Vgs in one direction at 0.6V (or whatever the forward voltage is for a zener). You need two zeners in series in opposite directions so that the signal doesn't clamp in either direction until it reaches the zener voltage.
The single zener of the voltage follower (Q2) works because the thing sets its own bias and the forward voltage drop of the zener is large enough to allow nearly all of the voltage swing. The voltage follower has an ugly clip on the bottom side without the zener. The positive side can theoretically exceed 20V without a zener when the current abruptly stops rising through the voltage follower while the gain stage keeps going. See the attached LND150 average characteristics.
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treated the limiting values the same way I did with tubes,
:laugh:
I started tubes with a SS mind, took me awhile to accept tube abuse as a normal condition :icon_biggrin:
somewhere in history I watched video on the junctions inside chips as limits were exceeded, phufff! The Engineer explained but I mighta started day-dreaming :laugh:
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Everything that I still have that uses LND150's is still working fine. The only time I had trouble was when I connected a long cord and footswitch directly to the Gate. I have taken to putting two 12V zeners butt-to-butt between the Gate and Source lately in things like amplifier stages and effects loops even though there was no trouble without them.
I didn't feel the zeners were necessary on the gain stage (Q1) because the gain is high enough that the output signal will max out long before the Vgs reaches 20V. It's not going to hurt having the zeners there and it might prevent some voltage of unknown origin from destroying the LND150.
There was only one time that I was pretty sure about how I ruined an LND150. I treated the limiting values the same way I did with tubes, exceeded the 20V Gate-to-Source voltage, and it went belly up. Fast. The other times I never really knew what happened and I never could seem to reproduce the conditions.
You can't use just one zener from the Gate to Source on a gain stage because it will clamp the Vgs in one direction at 0.6V (or whatever the forward voltage is for a zener). You need two zeners in series in opposite directions so that the signal doesn't clamp in either direction until it reaches the zener voltage.
The single zener of the voltage follower (Q2) works because the thing sets its own bias and the forward voltage drop of the zener is large enough to allow nearly all of the voltage swing. The voltage follower has an ugly clip on the bottom side without the zener. The positive side can theoretically exceed 20V without a zener when the current abruptly stops rising through the voltage follower while the gain stage keeps going. See the attached LND150 average characteristics.
2deaf, thanks heaps for chiming in and sharing your experience with me. That's of great help! :worthy1: I think it's generally not a bad idea to include back to back Zeners on every LND, even though it's not always necessary.
So it's good to see you've been using the LND150s without problems, still I'm wondering why my circuit failed. I have tested the circuit at voltages of 460 V in my amp when there were no power-tubes inserted, just to confirm the oscillator works. Maybe this was too much for the LND150s...? The tremolo still worked after those tests however. In the normal amp it was running from ~415VDC, have you used voltages this high for your LND150 builds?
Well, let's see if my current circuit will stay trouble-free from now on (fingers crossed!). Still, if anybody has ideas on what could have been the cause of the failure or general advice for this kind of circuit I'd appreciate it. :-)
Thanks a lot!
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I've run the things off of the screen node on big amps that were around 450V with no problems. I have a 520V bench supply that I could try if I ever get that curious.
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Well, this leaves me puzzled. But who knows what actually caused the failure. My circuit is almost exactly the same as yours posted in this thread. I've got a larger Source resistor on Q1 (1k instead of 750R), a smaller Gate stopper (10k instead of 1M) and a smaller Source resistor on Q2 (total 110k instead of 250k). I choose each R_source using LTSpice, trying to center-bias the LNDs.
One more thing that just popped to my mind:
My amp doesn't use a stand-by switch and produces a relatively loud "pop" sometimes when switching it off. Maybe this pop is indicative of voltage spikes upon switching off which could have damaged the LNDs?
Really trying to bring all my ideas together :dontknow: