Has anyone run into a guitar oriented circuit that uses all of a pentagrid?
I've found one circuit (Playmaster PM103) using one as a tremolo modulator, but with a separate LFO. So some of the grids are being "thrown away"
Now basic circuits (modulator + mixer) do 100% modulation of the signal, where as tremolo we'd like to manage the depth. It's too early in the day for me to have a serious think before even contemplating tinkering on a bread board.
In my case I've also got a dual-diode + triode tube, so could use the AM detector diodes to built a compressor.
It's an AM radio chassis (Airzone Cub 458) with 6A8G + 6G8G + 6V6G + 6X5G whose bakerlite is smashed: I can either build a new chassis or convert into something interesting.
On the basis of "there's nothing new under the sun" I thought I'd ask
I've found one circuit (Playmaster PM103) using one as a tremolo modulator, but with a separate LFO. So some of the grids are being "thrown away"
Now basic circuits (modulator + mixer) do 100% modulation of the signal, where as tremolo we'd like to manage the depth. It's too early in the day for me to have a serious think before even contemplating tinkering on a bread board.
In my case I've also got a dual-diode + triode tube, so could use the AM detector diodes to built a compressor.
It's an AM radio chassis (Airzone Cub 458) with 6A8G + 6G8G + 6V6G + 6X5G whose bakerlite is smashed: I can either build a new chassis or convert into something interesting.
On the basis of "there's nothing new under the sun" I thought I'd ask
Seeing there is nothing new under the sun, at least tube wise, and how the radio tubes have been everywhere (comparably) you would think there would be more than a few designs out there. But not much to be seen. After all, no shortage of regular tubes that fill the job well enough. Also I think I read that the more elements in the tube the higher the noise.
These tubes (6SA7 and 6BE6) were designed to be the local oscillator using the cathode, G1 and G2 as the plate, and the mixer using the electron stream flowing through G2 as the cathode, G3 as the mixer grid, G4 as the screen, G5 as the usual G3, and the plate as the mixer output. Your 6A8 is a little different, but uses the same concept. This was cheap and effective, but far from perfect.
With two of the five grids tied together, and one tied to the cathode, there are really only three grids available to play with. There are also several types of dual control pentodes designed so that G1 or G3 can be used for controlling the plate current. The 6AS6 is one, and there are several newer versions.
A couple years ago I set out to design a Variable Gain Amplifier (half of a compressor) for use in a vacuum tube music synthesizer. I set lofty goals using the variable gain cell from the Fairchild 670 compressor as my bench mark. I built a prototype using cheap Edcor transformers and some genuine 6386 tubes. I took data on that setup and then set out to match it's performance with cheap tubes and the same Edcor transformers. The 6BE6 pentagrid converter, some 6AS6's and other dual control pentodes, several variable Mu triodes, and remote cutoff pentodes were all tried before I landed on some semi-remote cutoff pentodes intended for TV IF amplifiers. They gave me consistent performance that was good enough for my synthesizer, so I set the project aside to work on the Voltage Controlled Filter. I'm still messing with that design.
Those experiments start in post #25 of this thread:
Modular Synth build
The pentagrid converter VGA with a 6BE6 tube gave me 20 to 30 dB of gain control range, which is probably good enough for a guitar compressor. The 6386 tubes got 70 dB, with the 6KT6 TV tube getting about 60 dB. I then added some mosfet buffers in the circuit which fixed from some frequency response issues, and boosted the range of all the tubes by 20 to 25 dB. At that point I went on to my filter.
Take a look at the Dogzilla guitar amp. It uses a variable gain amp on the input driven by a rectifier tube down stream to make the compressor. Both use radio tubes, but not a pentagrid tube. The VGA uses a 6K7, which is a remote cutoff pentode. I used 6BA6's in my experiments, which is a smaller newer version of the 6K7. They worked a bit better than the 6BE6.
DOGZILLA - Input Stage
With two of the five grids tied together, and one tied to the cathode, there are really only three grids available to play with. There are also several types of dual control pentodes designed so that G1 or G3 can be used for controlling the plate current. The 6AS6 is one, and there are several newer versions.
A couple years ago I set out to design a Variable Gain Amplifier (half of a compressor) for use in a vacuum tube music synthesizer. I set lofty goals using the variable gain cell from the Fairchild 670 compressor as my bench mark. I built a prototype using cheap Edcor transformers and some genuine 6386 tubes. I took data on that setup and then set out to match it's performance with cheap tubes and the same Edcor transformers. The 6BE6 pentagrid converter, some 6AS6's and other dual control pentodes, several variable Mu triodes, and remote cutoff pentodes were all tried before I landed on some semi-remote cutoff pentodes intended for TV IF amplifiers. They gave me consistent performance that was good enough for my synthesizer, so I set the project aside to work on the Voltage Controlled Filter. I'm still messing with that design.
Those experiments start in post #25 of this thread:
Modular Synth build
The pentagrid converter VGA with a 6BE6 tube gave me 20 to 30 dB of gain control range, which is probably good enough for a guitar compressor. The 6386 tubes got 70 dB, with the 6KT6 TV tube getting about 60 dB. I then added some mosfet buffers in the circuit which fixed from some frequency response issues, and boosted the range of all the tubes by 20 to 25 dB. At that point I went on to my filter.
Take a look at the Dogzilla guitar amp. It uses a variable gain amp on the input driven by a rectifier tube down stream to make the compressor. Both use radio tubes, but not a pentagrid tube. The VGA uses a 6K7, which is a remote cutoff pentode. I used 6BA6's in my experiments, which is a smaller newer version of the 6K7. They worked a bit better than the 6BE6.
DOGZILLA - Input Stage
Thanks. Good to hear from @tubelab_com : I know both those articles well. Compressor or external LFO it is,
Them's the ones. In my case it's a 6A8G doing oscillator + mixer duty for a superhetrodyne radio.
I think it is a left pond versus right pond thing, like lift or elevator, tyre or tire. Always been pentagrid converter where I was.
Check this
Davoli A6 DTE534 Ampl/Mixer Krundaal; Parma, build 1960 ??
The Davoli A6 from Italy. I found one at a flea market about 20 years ago and bought it because it looked good. I remember it sounded surprisingly big and tough for its size. I was young and silly then and eventually tore it down for parts because it was poorly built and electrically unsafe, the pcb and tubes were mounted on the control panel at an angle and rattled loudly.
The good news is, I had the good idea to trace the key parts of the schematic and I will post it when I find it from my files.
It has a straightforward single ended EL84 output stage and a clever pentagrid (heptode for Europeans) preamp with neon lamp oscillator modulating the pentode section through one of the grids. If I remember is actually a quasi textbook implementation of the Philips datasheet.
Davoli A6 DTE534 Ampl/Mixer Krundaal; Parma, build 1960 ??
The Davoli A6 from Italy. I found one at a flea market about 20 years ago and bought it because it looked good. I remember it sounded surprisingly big and tough for its size. I was young and silly then and eventually tore it down for parts because it was poorly built and electrically unsafe, the pcb and tubes were mounted on the control panel at an angle and rattled loudly.
The good news is, I had the good idea to trace the key parts of the schematic and I will post it when I find it from my files.
It has a straightforward single ended EL84 output stage and a clever pentagrid (heptode for Europeans) preamp with neon lamp oscillator modulating the pentode section through one of the grids. If I remember is actually a quasi textbook implementation of the Philips datasheet.
trying again, hopefully it will work this time
The preamp tube is an ECH81.
Grid bias is used, probably to reduce component count and cost. It did work ok, as far as I can remember.
The neon lamp is a big one, probably a 10mA one, but I have not checked yet.
I was surprised to see G3 grounded but it is actually the recommended operating conditions for AF amplification as per the Philips datasheet.
This prompted me to attempt a rebuild of the original, but in a safe (earthed) and sturdy chassis.
The preamp tube is an ECH81.
Grid bias is used, probably to reduce component count and cost. It did work ok, as far as I can remember.
The neon lamp is a big one, probably a 10mA one, but I have not checked yet.
I was surprised to see G3 grounded but it is actually the recommended operating conditions for AF amplification as per the Philips datasheet.
This prompted me to attempt a rebuild of the original, but in a safe (earthed) and sturdy chassis.
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