Look at the input stage circuit in my 4 tube design in post #34. (GTA_4T_XXXX) The pentode is loaded with a bootstrapped mosfet which presents a near infinite load on the pentode's plate. This in theory provides near infinite gain. The pot and the parallel resistance of R6 and R9 added are the true load resistance on the pentode. This makes the pot a variable gain control capable of blasting the grid of the following triode into overload, hence the knob on the pot is called the "saturator." Note the 47K grid stopper on the triode.....to avoid really blasting the grid, and the overload recovery associated with doing that. The DC voltage at the junction of the two resistors and pot forces the plate voltage to a constant voltage.
The single knob tone control has a combined low cut / boost, and rolling response kink. Rather unusual effect, and the cap values might need tweaking to suit tubes used and personal taste, but it also has an effect on the total amp distortion. I suspect the any typical tone stack could be used that has a similar 10 to 20 db loss.
All of this is connected to a voltage source down stream from the output tubes screen grid, so that the preamp supply voltage drops from about 160 volts at idle to 135 volts at full crank. The intentional voltage sag provides for some long sustain. If the saturator is turned down enough to stay just below clipping, there is no sag, and a good clean tone, but roll into the guitar's volume knob, or switch to a loud pickup, and heavy metal racket ensues.
With the resistor values shown and a 1 meg pot the gain can reach a V/V of several hundred. The limiting factor becomes microphonics in the input tube.
Note that the 18FW6 is an 18 volt 100 mA 6AU6, and the 18FY6A is an 18 volt 6AV6 which is 1/2 of a 12AX7. This combination makes for some serious Soldano kind of gain, which can be toned down with a smaller pot.
I have tried this same concept with some subminiature tubes. So far the best results have been with a 5678 providing a gain of about 100 from a 48 volt power supply (over spec for a 5678, but I have been to 100 volts without blowing a tube.
I also plan to try something like a 6AK5 to see haw far I can push the gain. I'm hoping that smaller tubes are less microphonic.....and I have lots of them.
Traditional iso boxes sound "stuffy" and dead so the silent sister was invented. Basically it's a closed back guitar cabinet coupled to a folded transmission line that's tuned low enough so as to not allow the bulk of the sound generated from the driver to escape the cabinet. The coupling chamber has provision to hold a mic in front of the driver. Because the cabinet is not sealed it's supposed to "breathe" and not sound like a stuffy iso box. The theory makes sense. I was going to build a couple of them for the church I was working at but parted ways with them before getting to do so so unfortunately I don't have any real life validation for this theory.
I made the same guess. I've tried a 6AG5 and a 6AK5 and both sounded good to me. I didn't encounter microphony issues, but I'm kinda going in the opposite direction to you as far as gain structure goes.
I use the pentode (or beam tetrode) as the last stage in the preamp, not the first. And I try to choose an operating point and anode load that only gives a voltage gain of about fifty times, roughly what one expects to get out of a 12AX7 triode stage with the output loaded.
One interesting thing I've noticed is that the traditional way of biasing the screen grid of a small-signal pentode, using a large-value resistor to B+, causes lots of sag to occur when you start to overdrive the pentode. Too much sag, actually, because it then cold-biases the pentode and mucks up the distortion quality, or worse, causes that awful blubbering blocking distortion.
I think it might be better to bias the screen grid either from a voltage divider (between B+ and ground), with values chosen for a Thevenin resistance lower than the traditional screen grid bias resistor, or to use a little LND150 MOSFET voltage regulator (essentially a little VVR) feeding a lower-resistance screen resistor. I haven't tried either experiment, though.
-Gnobuddy
You made this comment before and it got me to thinking (sometimes a bad thing for tubes....they get fried). The pentode has far more "stuff" inside to rattle, and therefore more likely to become microphonic.
My tube experimenting days started at an early age with a Fender Champ 5C1 schematic and whatever tubes I could find rummaging through trash. That amp used a 6SJ7 feeding a 6V6, powered by a 5Y3. 6SJ7's and 6SK7's were very common in old radios. As were 6K6's and a few other octal output tubes. 6V6's were not so common though. I guess that gain structure stuck with me for nearly 60 years. I did feed it with a germanium preamp or fuzztone though.
It would be a good idea to test swapping the stages to see if microphonics improve (likely) and how the swap affects the tone (likely, but unknown). I see several possible ways to accomplish this.
1) Hack up my working amp. Other than microphonics, which are not bad as long as I don't set the head on top of the speaker and turn the saturator up to 11. It is currently my only working amp, and it does get used. I'd rather not mess it up until I have something better.
2) Find and hack up the furball breadboard that was used to create this amp. It was in rough but working shape last time I saw it, so this may be a good course of action. If I can find the furball, I will likely hack it to test the stage swap on the same amp design. The
3) I found the original perf board prototype used to make the first version of this amp. It appears to be still configured for 3 series heater tubes. The other two tube sockets were not used.
The 100 mA series heater tubes that I used in this amp are from the last days of vacuum tube radio. They never really saw large scale acceptance since the transistor crushed the tube out of the table radio market, and shrunk the table radio market severely itself by making portable radios a reality. I'm wondering if their build quality is less than stellar.
There are hundreds of pin compatible tube combinations available with 6 volt heaters. The original design on this board started with a 6AU6 and a pair of 6AQ5's, maybe I will add a 6AV6 and return to that line up so that I can roll through a lot of tubes to see what works the best. I have lots of both 6AK5 and 6AG5.....long term I would like to make a small combo or two to stuff inside one of two dead solid state amps that I have. The Crate has a reverb tank, so that's a new design.
I have experimented a bit with screen grid connections in tube guitar amps, buth in the output and preamp stages. The 390K to B+ used in this amps seems to work well when driven with my $50 Squire Strat. It is possible to drive the amp into farting out blubbering noise when whacked with a hot signal from a pedal or two. This circuit fixes the plate voltage at a bit more than half of B+, which does sag, while reducing the tendency for overload. You must use a smallish screen bypass though.
Other experiments used the usual resistor and zener, or a mosfet follower to feed the screen. The output voltage of the follower can be modulated by a voltage that moves up (cathode of output tubes) or down (screen of output tubes) for different effects when overdriven.
The concept of the original HBAC was to design a minimalist practice amp for low $$$$, and this is a good compromise. I wanted to get the widest variety of "tones" from a small amps without any outboard effects.
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I thought the same thing. Extending that logic, a little beam tetrode should have one less grid to rattle, so perhaps it will be less microphonic than a true pentode?
On inspection with a magnifying lens, both the 6AG5 and the "pentode" inside a 6JW8 seem to actually be beam tetrodes. I can only see four grid support rods, two rods each for the control and screen grids.
Back around 2010, Google turned up a DIY design called the Tube Overdrive Special on a forum called EL34World ( Tweed Overdrive Special ). I liked the sound of the overdrive in the sound clips posted to that thread.
The TOS topology is interesting, it uses three 12A*7 gain stages (with heavy interstage attenuation between 2nd and 3rd 12A*7 stages) followed by a 5879 pentode as the final stage of the pentode.
Here is a soundclip posted by one of the two designers:
SoundClick player
I should probably mention one other topology I experimented with a couple of years ago: triode-pentode-triode, using two 6JW8s, with fixed resistor voltage dividers for interstage attenuation in between stages. I threw that one together while I had no oscilloscope, so it was far from optimized, but it had a little of the same rounded, singing overdrive quality as the TOS.
I think all the above discussion is relevant to this thread, because all these designs use the preamp to produce the signature sounds, feeding a clean power amp. So any of these might turn out to be a good silent stage amp, feeding the PA directly.
Nooooo!
This is a real problem for me, as soon as I come up with something that sounds halfway good, I stop tinkering with it, because I want to play through it instead.
I really like the 6AG5 for shimmery "almost cleans", like a good Blackface Fender. With my humbucker-pickup guitars, all it took was treble boost EQ from roughly 1 kHz to 5 kHz before the 6AG5 stage, and a 6AG5 loadline that ran "above the knee" for triode-like distortion, only more so.
I had a gain stage ahead of the pentode, so it was (too) easy to turn up the gain enough to get those sorts of problems.
Using smaller screen grid and cathode bypass caps solved the blocking distortion, but also took away the low end response. In the end, I went back to a bigger screen grid resistor. If I don't turn the gain up too far, this gives a nice "squish" on initial note attack that sounds almost as though there's a delay pedal (set to fairly subtle) in the chain. But turn it up, and the blubbering and blocking starts.
I guess this stage is definitely ripe for some experimenting with other screen-grid biasing arrangements, and/or interstage attenuation. It would be nice to keep the "squish" and lose the blubbering without also losing the bass.
-Gnobuddy
I wonder what a pair of them in push pull would sound like?
If I didn't give them away when I left Florida, I have a box full of them. Probably a hundred or more. I'll check tomorrow, if so I can smack one with a hammer and find out exactly what's inside.
This thread got me remembering a few things from the HBAC, which lead me to that thread, which got me digging through boxes. I have found several abandoned projects including a guitar preamp and a couple more breadboards.....even my old germanium fuzz box I made in high school. The amp that I have been looking for however remained elusive.
Posting and reading through some of the old HBAC thread sparked up a few dim memories. I have been looking for a PC board. I found pictures of a complete amp in a cabinet. Those things are a bit harder to hide, so I went over to my storage building and within 5 minutes found the old 5 tube Amp 2.0 from the HBAC thread. It was my favorite of the two designs, but Amp 1.3 got updated to become amp 1.5. I will have to tweak on amp 2.0 a bit to see what it can really become. It was right up against the $100 limit as built.
It is still unfinished, but it looks like only final transformer wiring is needed before plugging it in. As I remembered the "L" pad power brake is in there. I posted the pictures in the HBAC thread.
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