Cathode Follower Reverb Circuit

[printer2]

Talk about project creep. Started out as a Champ with reverb. Late night back of a napkin idea, not too up on FETs so don't know if there is something obvious I am not seeing. Think the reverb tank has an 800 ohm input. Not too concerned about the power supply values yet. Also think I may have to put a resistor in series with the tank to limit current along with adjust the coupling capacitor to limit the low frequencies.

[John]

I *think* you want the reverb "a" point coming off after your tone stack, and then the "b" point being injected before your PI, not after. With a resistor in between points "a" and "b".


Others on here know lots more about reverb than I do though.

[jazbo8]

The name of the thread is bit mis-leading, since the FET's are doing the actual work, perhaps calling this a "source follower" reverb circuit is more appropriate. Also the FET is so easy to drive, there is no reason why you couldn't just use the plate output before the cathode follower (or the output of the T/S as suggested above) as the input for the reverb circuit, just me 2c...

[jojokeo]

Talk about project creep. Started out as a Champ with reverb. Late night back of a napkin idea

Yes, "source follower reverb" is technically correct. Could make it a bit easier(?) or traditional by switching V1 to a 12ax, at, or ay and utilizing the extra stage for the reverb circuit?

[printer2]

Sorry to mislead you guys, have not been thinking about amplifiers for a year or two and brain is not up to speed yet I guess. Originally I was going to drive it directly from the cathode follower and changed to a MOSFET as I was curious as to how well it would work if I wanted to use it on different amps that are limited in the number of tubes available. My original schematic had a FET instead of the input triode, changed it on a whim. Also went back and forth as far as the PI, was thinking of a cathodyne with a MOSFET as the splitter and in place of the triode of the cathode follower. With a bass and treble stack you could shoehorn it into a Champ chassis. Like I said this sort of grew from being a simple Champ.


Thought of where to take the signal from, wanted to try the pre-EQ signal as it might have more mids and less bass and highs as after. Also did not want to do the mixing thing with a resistor as I did not want to reduce the main signal. The reverb return point that I am using can be used as another input to the PI to mix the signals. So you can just hang the MOSFET of the volume control? I'll have to try that. OK, almost looks nothing like the previous circuit but an option I am thinking of, nothing cast in stone. Maybe even use a 6U8 with the pentode in one triode position and the triode in the other. Going to have to do some breadboarding on this one, too complicated to just wing it in a chassis. Kind of funny, just want a little amp which will give me a Tweed or Blackface sound with reverb.

[2deaf]

You can't adequately drive an 800 ohm load with a source-follower that has a 100K source resister.  You will get a very small, severely clipped signal into a resistive load and a very small, mangled signal into the inductive load of a reverb tank.  BTW, a tank with 800 ohms at 1KHz is 400 ohms at 500Hz which makes matters even worse. 

The ideal source resistor for an 800 ohm load would be 800 ohms.  Let's say that the 10/24 schematic has 100V DC on the cathode of the cathode-follower which would also be 100V DC on the gate of the IRF820.  The IRF820 is going to allow enough DS current to flow so that the voltage across the source resistor biases the device to an equilibrium.  This voltage is going to be a little bit different than 100V, but for ease in this example, let's call it 100V.  Now you have 100V across 800 ohms which is 125ma.  Nobody is going to want to set aside 125ma for a reverb driver.  The datasheet makes it look like 125ma would be no problem, but I'll bet that it will get real hot and requires a good heatsink.

I don't know if you have played with this kind of MOSFET packaging before, but the drain is connected to the part with the hole in it, so watch out.

[PRR]

> You can't adequately drive an 800 ohm load with a source-follower that has a 100K source resister.

+1.

> The ideal source resistor for an 800 ohm load would be 800 ohms.

In this case, because we have hundreds of volts of power and only need a few volts in the tank, we "could" cheat-up quite a bit.

The real deal is to figure out how much *current* the tank wants for good drive. I suspect 6mA peak will be ample.

We also have the "800r" tank driven with a 0.22u cap. On the face of it, bass below 900Hz is cut-off. It is actually more complicated: we may have a big resonance in the 500-900Hz range. Best look at any transformerless tube drivers you can find.

There's a reason we like transformers for tubes driving electromechanical stuff. (However the readily-available RTs are scaled for low-Ohm tanks.)

> Let's say that the 10/24 schematic has 100V DC on the cathode of the cathode-follower which would also be 100V DC on the gate of the IRF820.

Note that the new plan in Reply #4 shows gate biased to ground. On most MOSFETs this will give dead-zero current flow. (Which does help with the heat problem.) The LND150 in the recovery stage is different; and may be a convenient choice for the driver?)

> set aside 125ma for a reverb driver.

That's about double the current and power that the two 6K6 will draw. PT almost 3X bigger than needed for output.

I also see the reverb recovery coming off a grounded reverb level pot, but fed into a point (call it V1b plate) with high DC.

I have doubt that the reverb recovery level will be strong enough to flog the output stage vigorously.

I also predict that V1b plate will sit at 70% of supply voltage. Cathodyne (Source-odyne?)) wants grid (gate) sitting a lot closer to 1/4 of supply voltage.

GE data suggests 2*6K6 use a 400r cathode resistor, not 1K. Sure the tubes will "work" but the optimum load should rise from 12KCT to around 30KCT (and max power output down near 4 Watt). With 1K bias and available 10K-12K loads, power output could be a couple Watts.

[printer2]

Yeah, know that a cathode follower would not be an efficient method of power transfer but thought of it more like a current source rather than having the source and load matched. Also realized that I do not need hundreds of volts to drive the coil. Not that you want constant power at all frequencies, need to roll off the lows so things do not get muddy. Did not realize they rate the coil at 1 kHz, oh well.


I knew the second circuit was not workable and could work with a LND150 driving it, have a circuit somewhere in my files. I was showing the rest of the amp as a jump from a Champ, to P-P then to the first schematic. Nothing really nailed down other than the power transformer and the output tubes.


Used 480R for a couple of 6K6's before, seemed to work fine. Now where is that thread?


http://el34world.com/Forum/index.php?topic=18645.msg190933#msg190933


Guess I have to dot my I's and cross my T's before I post anything. Was only a few half baked ideas (should have made that more clear) and wanted to know if anyone has done something like it and got it to work. My shop is set up for building acoustic guitars right now and I have no room to breadboard a circuit but recently got a radio chassis and I am debating what to build out of it. More into acoustic guitars right now, my latest build.





Still need to do a fret job on this guy but strung it up to see what it sounded like, another 'what if' build. A fun little guitar.

[2deaf]

In this case, because we have hundreds of volts of power and only need a few volts in the tank, we "could" cheat-up quite a bit.

We could put a sizable resistor in front of the tank and a proportionately sized source resistor.  We can tolerate a lot of voltage drop across the resistor portion of the resistor/tank voltage divider because we have a large signal available.  Now we have a much more constant current situation because the voltage rises at the tank as the impedance rises with frequency while the voltage of the signal at the source remains about constant.

We also have the "800r" tank driven with a 0.22u cap. On the face of it, bass below 900Hz is cut-off. It is actually more complicated: we may have a big resonance in the 500-900Hz range. Best look at any transformerless tube drivers you can find.

That's a pretty good estimate.  My Accutronics 8E is dead-on 900Hz with a 0.22uf cap.  It moves down to about 300Hz with a 2.2uf cap.  It takes 22uf to get it low enough that it doesn't cause any trouble.

The LND150 in the recovery stage is different; and may be a convenient choice for the driver?)

The LND150 is too wimpy for a direct driver.  The things will barely do 1ma at 500V.  They will do a real impressive job of driving a transformer, however. 

[PRR]

> only a few half baked ideas

I did not see that.

Did not want you soldering-up something that needs more think-time.

Sorry if it sounded critical.

[DummyLoad]

use the microchip D2450. 


--pete

[2deaf]

Guess I have to dot my I's and cross my T's before I post anything.

I don't think anybody was being critical, least of all PRR.  I hit these things cold, armed only with the datasheets.  I spent a lot of time on them and I would have loved it if somebody could have given me some crucial information the way PRR just did in a single post.

I like your thread.  It introduces some different ideas and sparks some interesting discussion. 

[printer2]

> only a few half baked ideas

I did not see that.

Did not want you soldering-up something that needs more think-time.

Sorry if it sounded critical.

No problem, I came here because you guys are knowledgeable and helpful. Been too busy getting the house ready for winter and did not have much time to think things out, just did some cut and pasting to come up with a schematic. Might have to go low voltage for the drive section, need to look at what I have to use for a power supply. 

[printer2]

Replied to another thread about putting a triode in front of a 6SJ7. First order of business would be reducing the gain of the pentode and going from grid leak to cathode bias. Remembered I had a circuit with a switch to do that using a three pole mini toggle switch. So I added the triode to the front of it then I thought, 'why not build this one?' Still wanted to have the Tweed volume and tone control, once I adjusted the values for a Treble Bass tone stack to be used behind a pentode. Might as well put in a switch to go from Tweed to BF. May have to adjust some values as I was guessing about the plate resistance, should be in the ball park.





Forgot to put the first volume control on the panel, may change yet. And this afternoon I was debating doing a low voltage build with 12L6's. Way too many ideas, more tubes than I will ever use and to little time. The values for the filter nodes are just place holders, didn't bother working them out yet.

[printer2]

Changed again. Did an amalgamation of some past ideas. In tweed position of the first switch you get a 5E3 type input and the second switch goes between a Tremolux 5E9A (paraphase PI) or 5G9 (Long Tail Pair fixed bias). Aren't four pole switches wonderful? The other position for the first switch would be for a Blackface amp, LTP for a cleaner sound, Paraphase for a little unbalance. Power supply values need to be worked out, originally was going to go 6W6GT for the power tube but may do a 6K6GT. Guessing the reverb as shown might be a problem for the Tweed position and anticipate putting it across the 1M resistor between the two switches for BF mode. A little more complicated amp than I originally was going to do, I wanted to replace the BF/Tweed Champ I gave to my brother and have reverb added.

[2deaf]

On that voltage doubler, how about hooking the bias supply to the other leg of the transformer and putting a 0.22uf cap. between that leg and the voltage divider?

[printer2]

On that voltage doubler, how about hooking the bias supply to the other leg of the transformer and putting a 0.22uf cap. between that leg and the voltage divider?

Well that is interesting. I copied the bias supply from a schematic of a 12AB5 amp I built, seems the cap was left off that schematic but I have one in the amp. Good catch. I wanted to take the bias from the other leg but I did not have the room to do it without making things look messy. Caught on that one also. You guys are good. I tend to do a rough idea of a circuit that I want to do and adjust as need be to make it workable as I am building. Really should note on the schematics when they are an as built rather than a work in progress.


The doubler would be used if I do a 12W6 version of the circuit and I would take the half supply to feed the screens since they have a significantly lower max voltage than the plates. If I make the amp with 6K6's I probably will go with a full wave supply. Leaning more to the 6K6's and the 12W6's for the previous schematic but without reverb or the first triode. Might make that one first just because it is simpler. I need to get going on one of them before I get sucked back into working on another acoustic. Need to build a Telecasters also. Sure did not realize I was jumping down the rabbit hole when I started my first amp.

[2deaf]

I not sure if you agree with me about taking the bias supply from the top leg or not, but I'm pretty sure it won't work if you take it from the bottom leg.  The top leg version has an AC path so that there will be AC across the voltage divider to rectify.  The bottom leg version has a straight-wire path back to the transformer that is a much better path than going through the voltage divider.

The high voltage with a voltage doubler is full wave, but the mid-point voltage is half wave.  If you are going to use the mid-point for pre-amp tubes, it will require more filtration. 

[printer2]

No, I do agree with you. Just was too lazy to rearrange the schematic. Taking voltage from the mid-point of the doubler was to feed the screens of the 12W6's as they have a maximum voltage of around 160 V. The preamp tubes will be fed from the full supply. The doubler will be used in the 6Sj7 amp. The schematic with all the switching will be a 6V6 amp and use a full wave supply. I feel the 6V6's will make the amp more versatile.  Actually going to be 12V6's, got a bunch of them. Sorry for all the flipflops. Was trying to work out what I actually wanted the amp, now two of them, to do.


Which leaves the 12W6 amp as the practice amp. Still not 100% sure of the circuit yet. Without the input triode I may have to go with a 12SL7 in the phase inverter spot. Hope it is enough gain. The 12W6 (12/25/50L6's) have a greater sensitivity, maybe it will be fine.

[DummyLoad]

[b >]I'm pretty sure it won't work if you take it from the bottom leg.[/b]

i ran a sim on it. it will not work if you take the bias from the tap that connects to the caps . only works if you use the diode junction tap. IOW, the transformer tap that connects to the two diodes.

--pete

[printer2]

Thanks again. Yes I know it will not work as shown, don't need to do a sim, just looking at it you can see it is wrong. Just quickly did a copy and paste from another schematic of an amp I built and would have put the connection on the top leg if I had open space on my schematic. Was going to do a dashed line over the bottom line to the top but slipped my mind by the time I made a image file of it. Also since deciding not to use 12W6's in this circuit there is no need to do a doubler circuit and I will go full wave as I have transformers for that.


The doubler will go on the 6SJ7 amp and it will be cathode biased. I will refrain from printing another schematic until I get closer to building the amp as I might get another idea I might throw in. Will be making the 6W6 amp first, still not sure about whether I want to put a triode after the 6SJ7 and before the PI. I may have to breadboard the circuit and hear what it can do.

[printer2]

There really is a good reason why I have not been working on my amp. The neighbor across the lane was having an oak cut down, I asked the guys if I can have a section of the trunk to build guitars (acoustic) out of. Cut it up into slabs a while back, dry enough to process. Spent today cutting up and sanding down to thickness a section of it.


Looked intimidating.





Can't believe the work to get to this.





Going to build a couple of parlor guitars as gifts for Christmas if all goes well.

[PRR]

Oak? Don't you want spruce?

Come up here. I have a 33 foot length ready for your truck, some 4 footers you can drag out of the woods, and if you hang around I'll put a saw in your hand and you can take out about 40 more (before they fall on my power wires).

[printer2]

Maine, now if you were a state or two closer...


Just wanted to make a guitar or two right from splitting the log to finishing. Not that oak is a bad wood to use, just not many people want their guitar looking like their furniture. Pretty sure I'll be using a spruce top. Did make a walnut guitar with walnut top. Similar in sound to an all mahogany, sweet sounding guitar. Built a walnut and spruce body but never get around putting a neck on it. Cutting up some maple today, have four requests from work for guitars. Want to prototype one and if it turns out alright make a few more.

[printer2]

OK, few ideas on what I want to do. I reconnected with a few people that took part in a $100 amp build challenge, it got me to re-look at a simple design, going to go with that one. Will post it when I know the thing will work. There was another two designs I posted a long while ago, strange that I re-looked at them also and combined them.


As simple as the other one is this one will be, much more. Want to add reverb or at least an effect loop to add reverb. Schematic is a work in progress. This one will be built as a head using 12W6 tubes for the output for a home friendly roughly 8W. I am going with the 12W6 rather than use 12BK5 seven pin tubes like I originally planned. The reason being the 12W6 is an octal tube and it will be easier to convert this to 6V6 power (actually 12V6, I have a few of them). Real excited about this one, sort of a barn burner.


I have some acoustic guitars I need to make but I am going to fit building these two in along side the guitars. Hope to have them all done before Christmas. We'll have to see how that will go, some R&D needed to be done for all of them.


Plexi-JCM-Trainwreck





See, that is what happens when we don't have any snow. Just realized the date, maybe in the New Year for the build. Snow is two weeks late, not that I am complaining. If it wasn't for the house reno it would be reasonable. I'm two weeks behind.

[PRR]

As shown, S2 will "pop".

6V6 can be run in the same conditions as 6W6 or 6BK5, at a small fraction of the heater power. (The 6W6 family shines for high cathode current with low supply voltage.) But if you have an oversupply of heater power and hungry-heater tubes, that's fine.

[DummyLoad]

12bk5 is a 9pin.


good tube. very easy to drive and takes 300V B+ with ease. downside is .6A heater draw: 1.2A with 6bk5.


--pete

[printer2]

As shown, S2 will "pop".

6V6 can be run in the same conditions as 6W6 or 6BK5, at a small fraction of the heater power. (The 6W6 family shines for high cathode current with low supply voltage.) But if you have an oversupply of heater power and hungry-heater tubes, that's fine.

Thanks, forgot to put the resistor in, I usually use a 4.7 M for it. I have taken to switching power supplies for the heaters. I usually find old laptop ones for $3 at thrift stores. Like you say these tubes are current pigs, without the bricks it would be harder to use them. I have some 6W6's but only two 6V switchers that are capable of feeding them and a few preamp tubes, the number of them may be asking too much of it. Using the PS makes for a quieter amp and frees up some VA's from the power transformer. I have used 120V:120V/120V:240V isolation transformers for a few amps, usually a 30 VA Hammond with SS rectification. Helps to make for a lightweight amp. 


I have not used the 12BK5 yet, thought this might be a good project but got the feeling this may be an interesting amp and I may want a higher power version of it at some time. I have 30 12BK5's that I got for a couple bucks a piece but they have yet to find a home.

[printer2]

Was part of a $100 amp building challenge a couple of years back, some of us decided to revisit a few ideas we had. I took the Princeton and replaced the cathodyne stage with a mosfet and got the amp down to one 12AX7 stage. Used a 120/240V:120V isolation transformer for the HV supply, a 12V switching wall wart for the heaters. 70V line transformer for the OT.  Left out the tone control section as I was just checking if the mosfet PI would work. The tone control uses a three pole switch to change from a Tweed tone control to a Big Muff (originally Supro Thunderbolt) mid cut or tilting control. Some resistor tweaking and should be good to go.



Breadboard.



Oh yeah, one 12AX7, two 12AQ5's.

[DummyLoad]

simulation says "FET_A_DYNE" will work better with 10K for Rs and Rd as opposed 47K for Rs and Rd you show.



with 300V B+ and fixed or zener bias, the output into 220K loads is about 72Vpk before onset of clipping. in the sim, the clipping was symmetrical with the 20K load.     



--pete

[printer2]

Thanks, I'll try lowering the resistors. Will be running the amp at about 250V. I ran it up higher with the variac but there was not a lot of change. I did have the 12AQ5's biased with a 330 ohm resistor, I usually parallel up another resistor to dial in the current. This is for a small Champ sized cabinet, I keep giving away the ones I make, we'll see how long this one sticks around.

[PRR]

> simulation says "FET_A_DYNE" will work better

Am I missing something?

IMO it won't work at all with 2.2M+2.2M or half-way bias. For 300V of B+, you have 150V across the bottom resistor, 150V across the top resistor, and *zero* across the FET. It can only pass half the signal.

As a first cut, a Cathodyne (any device) should have input bias nearer 1/4 of the supply voltage. 75V across bottom R, 75V across top R, and 150V across FET. For a perfect FET and NO-load, that gives +/-75V swings at both outputs.

Here we have ~~~ 200K load on 50K cathodyne resistors. The resistors can 'only' swing about 200K/250K or 0.8 of the 75V. The FET can swing to its max. (A tube would not.) A best-fit cathodyne bias would be like 81.8V at the cathode(source). Divider network like 0.27. But the 6V6/12AQ5 don't need anywhere near 70V of drive. Anything close to 1/4 will be fine. 3.3Meg and 1Meg, 4.7Meg and 1.5Meg, etc. Merlin shows something near there:

link

Since the 220K load is "much" more than the 47K resistors, and this does not have to be a to-the-max design, 10K seems like overkill. In particular: more supply current, more ripple, which matters because this form of grid/gate bias gives very little ripple rejection.

[DummyLoad]

>> Am I missing something?

no, we/i am?

printer2 has zener bootstrap bias working on his breadboard. i have both working in sim. why 1/2 B+ V divider? you need 25% B+ for source (cathode) and 75% B+ for drain (anode). i used 3.3M and 1.2M for fixed bias V divider. 20K load (R4-R5) seems to work best: output symmetry is best and outputs clip symmetrically and at 300V B+ 72Vp output.

notice gate charge time and time fro bias to stabilize in video capture - link below:
 
https://www.dropbox.com/s/5odeansth9f0ap2/fet-a-dyne-sim-take3.avi?dl=0

click download and play local for better resolution and watch XSC4 window.

the time to bias stabilization is proportional to the values of R6-R7 and C2-C3. if i increase part values then the time to stabilize increases.

--pete

[PRR]

Perhaps not all on the same page. I am looking at the sniplet attached below. Has 2.2Meg+2.2Meg divider.

> why 1/2 B+ V divider?

That was my question. I am advocating 1/4 B+ divider.

As do you: "i used 3.3M and 1.2M for fixed bias V divider" (0.2667)

> video capture - link below:

Sorry: can NOT read that in dropbox's viewer, even full-screened (128081024). When downloaded, the dl fails multiple times. Eventually I get the whole file, but whatever viewer Win7 picked (it needs a good smack-down) will not zoom larger (OR smaller?). I get that you have two variant cathodynes, one with no explicit bias (except Zener leakage), one with a good divider. There's dancing sines on the other side, but too small (and fast) to judge peak output or bias-stabilization. I can't read any DC voltage values.

> time to bias stabilization is proportional to the values of R6-R7 and C2-C3

Audio can start any time and at any size. SPICE normally computes an infinite-time DC bias before transient sim starts. Seems to me the bias should be stable already. Maybe not getting (seeing) what you are saying?

> printer2 has zener bootstrap bias working on his breadboard.

I see that Zener as protection, not boot-strap, not bias.

Seems to me that with conventional (positive gate) MOSFET, the idle current will be zero.

Yes, with LARGE signal the Zener clamps gate-source at -0.6V to +12V, average +6V, which gets the MOSFET conducting. And the coupling cap will integrate charge, perhaps to a point that the stage is just-clipped. But does it play sweetly?

The 47K or 10K concerns me less. 10K gives 16% higher peak output, yes. But either way makes several times more peak output than usual self-bias audio tubes need.

[printer2]

Don't know much about mosfets used linearly, on-off I knew more but forgot it over the years. So remember reading about using a mosfet for a cathodyne PI but when I looked up amps that actually use one I came up empty. Just for the heck of it I though maybe I can run it direct coupled to the preceding triode stage, might be an issue what the voltage splits across the mosfet and the two resistors but I'll deal with that later. As far as I can remember the B+ split across the resistors and not a heck of a lot across the mosfet. What the hey, wasn't expecting to get lucky anyway. But I got audio out the OT. ???

I really must not understand mosfets, I didn't get it. But I was not getting the power I thought I should get out of the amp. Started taking some measurements and the mosfet hooked up to the preceding triode brought down its gain. Also the plate voltage. Pretty sure it was due to the zener, thought the zener would mind its own business as the mosfet would need less than the 12V to find its happy place. Seemed the zener was dropping the 12V from the plate to the lower 47k resistor.

Decided to go capacitor coupled and fixed bias on the mosfet. Have most of my resistors in little envelopes, all out of 1M's, grabbed the 2.2M's and stuck them in. Was going to start there and bring down the value of the lower one (I know I have some resistors somewhere here), but just wanted to change the circuit and make sure it worked. Not the best breadboard in the world and some of the holes are not as tight as they could be. First order of business is to check the dc voltages to make sure I really had a complete circuit path.

Well I did, passed audio also. Still confused how the splitter worked with the resistor values (makes a lot more sense after reading DummyLoad's idea of the zener biasing the mosfet) but at least I got the triode's gain back. Actually sounded pretty good considering the mess of wires. But I just wanted to confirm the PI operation before I laid out an amp with it. After reading your two posts I had to go and measure what the thing was doing.

So as per the schematic I was getting 25V P-P into each 12AQ5 before a hint of distortion appeared. With that determined I changed the lower resistor from 2.2M to 820k (still have not found those 1M resistors). Clean signal before a hint of distortion went up from 25V to 45-50V. Plugged in the guitar and tried both the right way to use the circuit and my wrong way. Other than the little more clean output the circuits did not sound much different to me clean or distorted. Do like the added cleans though.

Yeah I know, I would be shaking my head also. But this was a quick experiment to see if the mosfet would work as a splitter and if it did, fine, if not I will learn some more about it when I had more time and continue on building my workshop (which I really should have been doing). Will be putting up some drywall tomorrow, we'll see when I get back to making a board.

Thanks for your help guys, understand more than I did a day ago as a result of your help.

[PRR]

> direct coupled to the preceding triode stage

The previous plate is probably at 70% of B+.

As discussed, the cathodyne can't work as expected until its input is below 1/2 of B+, and doesn't get "right" until 20%-35% of B+.

> But I got audio out the OT. ???

Strung-up at too-high voltage, it will "pass signal"; like a diode, to one output in-phase. Depending on some other stuff it may drive the other output but again IN-phase.

A push-pull output stage will "got audio" with quite severe un-balance of drive.

Also now the poor gain-triode has to drive its own load and BOTH cathodyne loads. With effectively less B+ (since the two 47K are splitting it). It will perhaps "work". There are better schemes.

> not getting the power I thought I should

Which is often a sign of un-happy drive.

2.2Meg and 820K is 27% of B+, a fine breadboard operating point. In fact that's probably nearly the best you can get.

[DummyLoad]

pictures are worth 1k words... so then 2 are worth 2k words?

see attached - file name says it all.

--pete


EDIT: note that both oscillograms are with 60Vp drive.

[printer2]

> direct coupled to the preceding triode stage

The previous plate is probably at 70% of B+.

As discussed, the cathodyne can't work as expected until its input is below 1/2 of B+, and doesn't get "right" until 20%-35% of B+.

> But I got audio out the OT. ???

Strung-up at too-high voltage, it will "pass signal"; like a diode, to one output in-phase. Depending on some other stuff it may drive the other output but again IN-phase.

A push-pull output stage will "got audio" with quite severe un-balance of drive.

Also now the poor gain-triode has to drive its own load and BOTH cathodyne loads. With effectively less B+ (since the two 47K are splitting it). It will perhaps "work". There are better schemes.

.

Not touting the arrangement as a scheme, just outlined my thought process.

> not getting the power I thought I should

Which is often a sign of un-happy drive.

More the signals being balanced and only the difference getting through the output stage. Pulled my scope out at this point.

2.2Meg and 820K is 27% of B+, a fine breadboard operating point. In fact that's probably nearly the best you can get.

Debating this one. The whole point of the exerciser is to come up with an interesting cheap to build amp. For the poor guy that has to buy resistors in packs of five (or whatever quantity) I wanted to reuse the same values as much as I could. The tone control 220k resistors double as the output grid leaks. Changed the schematic with the two triodes using the same 47k's for grid stoppers as the splitter. Power supply dropping resistors both 10k, was thinking of a 1M for the lower value and maybe a 3.3M for the upper resistor.

Still need to do some listening but am leaning toward 220k plate and 1.5k cathode resistors on the triodes. Seems more lively.

[PRR]

> cheap to build amp. ...resistors in packs of five... reuse the same values

Ah. Really pinching the pennies.

Then 1Meg and 220K; probably values already on the list. The resulting 180K is entirely fine loading for standard Fender 100K+1.5K gain-stage bias. How it works with your "lively" 220K plate resistor is a matter for test.

> fet-a-dyne-bias-half-Vdd.png

Yes, bad; and if you de-AC-couple the 'scope inputs (DC couple to the cathode-plate/source-drain) you will see that the flat-spots in the middle align.

[printer2]

> cheap to build amp. ...resistors in packs of five... reuse the same values

Ah. Really pinching the pennies.

Then 1Meg and 220K; probably values already on the list. The resulting 180K is entirely fine loading for standard Fender 100K+1.5K gain-stage bias. How it works with your "lively" 220K plate resistor is a matter for test.

Never thought of going lower with a 220k. Will have to try it.

It was a challenge to come up with an interesting amp for under $100 using readily available parts. I figure you could source parts from a number of places but without factoring in shipping the goal is hardly realistic. So I tried to limit suppliers to two. Might sound like pinching pennies but that was part of the challenge. I did not want to go the normal low dollar route using a SE output. Came up with an all tube combination (SS rectifier) getting under $100, not including chassis but from cord to fuse to knob. It was interesting, this amp is just taking one of my ideas to completion.

http://www.diyaudio.com/forums/instruments-amps/190738-hundred-buck-amp-challenge.html

[DummyLoad]

printer2 try a 1M & 330K for Vbias divider. if you don't have a 330K, stack 220K + 100K.

sim shows that 1M & 330K (1/3) V divider with 364V Vdd indicates bias is at 90V at source & 273V at drain. read XSC3 & 4 in the pics and XMM4-5-6 are the relevant.

real world test with 1M and 330K with 364V Vdd indicates bias is at 87V at source and 272V at drain.

i assume that there is SOME parallel loading of the smaller resistor in the divider and hence why 1/3 of Rtotal is needed to set bias at 1/4Vdd for Vs & 3/4Vdd for Vd. that was throwing me off with SIM because originally as i started with 1/4 for small R with 3.3M & 820K and by trial and error, got Vs to 1/4 Vdd for Vs on the sim and i ended up using 3.3M & 1.2M to accomplish that.

link to photos of the experiment below. 10Vpk is all the signal generator had. the input trace (single) has a 10x probe so read 10x scale on vert.

https://www.dropbox.com/sh/3x5vul3jrqc8dj7/AABTrdLyhjflIXwqdHCWe3pMa?dl=0

if it's relevant - Vgs is 3.3V on the breadboard.

this experiment was simply a sanity check to correlate NI multisim with real world: results were reasonably in-line with the real-world and that's what i was hoping for.

--pete

[DummyLoad]

schematic of the breadboard experiment in the previous post.


--pete

[tubenit]

My understanding is that oak is a poor quality tone wood. I played a Peavy electric out of oak once & it did not sound good at all.  just an FYI.


with respect, Tubenit

[printer2]

My understanding is that oak is a poor quality tone wood. I played a Peavy electric out of oak once & it did not sound good at all.  just an FYI.


with respect, Tubenit

A cardboard Strat is making the rounds on the forums recently, electric guitars are more about the parts rather than the wood.  In some circles oak is well thought of, it is just hard to sell because it looks like your grandmother's furniture.

https://www.youtube.com/watch?v=-IzkO0qckFc

https://www.youtube.com/results?search_query=J.+Brentrup+White+Oak

http://theunofficialmartinguitarforum.yuku.com/reply/1804326/Nice-QS-White-Oak#.Vm10_F4dypo

http://acousticguitarforum.com/forums/showthread.php?s=5225f66a318696557a3e9281efdf1aef&t=401428

http://www.acousticguitarforum.com/forums/showthread.php?t=176265

http://www.acousticguitarforum.com/forums/showthread.php?t=350449

[printer2]

 
 

Finished schematic. Am going to remove the Big Muff tone control and add the NFB switch to give the amp a more cleaner mode. Otherwise the amp works well with the parts as updated. Again, the amp run on 250V, it sounds fine but the weak point I think is the output transformer. I am sure it would sound better with an OT that was really designed to be one. I did move some parts on the board. Moved the output cathode resistor and cap under the filter caps, put the rectifiers in its place.
 
 I put 400V caps on the board, who knows, maybe one day I will change out the output transformer, flip the power transformer around and feed it to a doubler. Did that in another amp and puts out about 400V into a pair of 12AB5's, a 9-pin version of the 6V6. Maybe change the power supply resistors though.

[Fresh_Start]

Whatever happened to the MOSFET driven reverb concept?

Chip

[printer2]

Next amp build, this one.



The current one I am building was brought to the top of the heap when some guys sparked up the $100 amp thread. Thought it would be a quick build and I wanted to see how viable an amp it was. I did not anticipate some health problems that have plagued me the last couple of months. A week could go by with me not getting into my basement to work on a project. I think I would like to build a Telecaster before I start on the above amp. I have had the body and the neck cut out for a couple of years but got sidetracked with acoustical guitars. Going to go 12V6GT in the above amp rather than 6K6. I have some chassis and cabinets already built, hopefully it won't take too long. I want to stay pretty close to the respective circuits for this amp, the only place that will need experimenting with is the reverb. Hope it goes well.

[printer2]

Not a 100% physical layout but easy enough to make one from it.

[Fresh_Start]

Thanks for the schematic.  The MOSFET snippets will be useful.  However, I am not looking for a different amp to build. My to-build list is long enough.  Most will have reverb.

Are there any "known-good" reverb circuits which employ a cathode follower in the driver?  I ask in part because of the success Tubnit has had with MOSFET cathode followers in other applications.


Merlin suggests an SRPP as a "modern" approach sans transformer. Looks a bit like a cathode follower but isn't. I have no idea if the second, top triode could be replaced with a MOSFET.

Respectfully,
Chip

[printer2]

The fine folk at music-electronics-forum came through with a SF suggestion.



another interesting picture.