Tube Preamp - NE555

Thread Starter

rpschultz

Joined Nov 23, 2022
416
Could someone please explain what is going on in the below circuit?

9VDC in. 250VDC out to power a pair of 12AX7 tubes for guitar preamp. What does the NE555 do? Or, how does the NE555 boost the voltage?

1707245331462.png
 

BobTPH

Joined Jun 5, 2013
8,659
The 555 pulses the MOSFET on to charge the inductor, then when it is it goes off, it rises to a high voltage and charges the capacitor through the diode. In other words the circuit is a boost converter. The voltage divider and transistor provide feedback to control the output voltage.
 

crutschow

Joined Mar 14, 2008
34,028
It's a rather crude converter as the Q1's base-emitter voltage is used as the reference voltage for output regulation.
But that's likely sufficient to power the tubes' plates.
 

MisterBill2

Joined Jan 23, 2018
17,783
The 555 timer IC is serving as the controller in a switch-mode boost supply..When the mosfet switches ON , energy is stored in the inductor, then when it switches off, the energy is added on top of the supply voltage to charge the "C1" capacitor. rather brilliant although crude.
 

Thread Starter

rpschultz

Joined Nov 23, 2022
416
OK, interesting. I'm trying to learn more about tubes used in guitar amps and pedals as gain stages. So below there are 2 different DIY pedal tube circuits that use the above 9v power supply arrangement. From Conspiracy to Commit Electronics – Tube-based effects for the DIY community (c2celectronics.com)

This one has one 12ax7. The first half V1A is for gain? and the second half V1B is going into the tone stack? Not sure exactly. This pedal has Volume, Tone, Gain and two switches (lows and mode).
1707337738123.png

This next circuit has two 12ax7's. Seems like the first V1A is an input buffer. V1B half gain. Then a cascading of V2A and V2B into the tone stack?!? This pedal has 5 knobs: Gain, Vol, Treble, Bass and Distortion Vol. Seems odd to put the distortion knob at the end.
1707337845480.png

Finally. How is the tube gain adjusted? Like for the V1A, is it some ratio of R9, R14 and R20?
 

Ian0

Joined Aug 7, 2020
9,489
First circuit: Both halves are used as simple gain stages. Just the usual common-cathode amplifier stage. Works just like a common-source amplifier with a JFET. The output impedance is high, so the tone control has to have a very high impedance.

Second circuit: V2B is a cathode follower. Same as a JFET source follower. It gives a low impedance output to drive the tone control stage.

Guitar people are a bit obsessed with the 12AX7 (aka ECC83). The ECC81 and ECC82 have much lower output impedances and therefore are much more suitable for driving the tone contol stages.

I would thoroughly recommend Merlin Blencowe's book
https://www.lulu.com/shop/merlin-bl...cover/product-1rev2w64.html?page=1&pageSize=4

The ECC83 is still made, but there are vast quantities of new-old-stock valves designed for American televisions available for $1 each, which still makes them cheap even when imported across the Atlantic, and there are also loads of Russian ones, but them might be covered by sanctions at the moment, although there are supplies in Eastern Europe.
 

LadySpark

Joined Feb 7, 2024
38
This next circuit has two 12ax7's. Seems like the first V1A is an input buffer. V1B half gain. Then a cascading of V2A and V2B into the tone stack?!? This pedal has 5 knobs: Gain, Vol, Treble, Bass and Distortion Vol. Seems odd to put the distortion knob at the end.
1707337845480.png


Finally. How is the tube gain adjusted? Like for the V1A, is it some ratio of R9, R14 and R20?
This one has two gain controls, the one labeled 'CL VOL' is clean volume for bypass and 'Dist Vol' is the volume when the distortion is engaged.
 

LadySpark

Joined Feb 7, 2024
38
Hmm. Complicated. I might do better simulating those circuits with Spice.
you should get their board to build upon. Because its not out of the ordinary for schematics have typos with kit builders.
I don't understand why they are using 250V in a pedal for that effect since many are 12-30V on the tube.
But they are trying to do a guitar preamp in addition to..
I would try a space charge tube like a 6GM8 and not use 250V and drop everything to 12V if you want to make it a little different.
 
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Thread Starter

rpschultz

Joined Nov 23, 2022
416
I'm a rookie in this area, but my experience with preamp tubes is that they all generally run in the 200-300 volt range, for reasons I don't understand.
 

LadySpark

Joined Feb 7, 2024
38
I'm a rookie in this area, but my experience with preamp tubes is that they all generally run in the 200-300 volt range, for reasons I don't understand.
Its just the schematics you are picking out, which are created by people using the generic high voltage bias charts. If you look at the GE datasheet for the 12ax7 you will see that chart.
A lot of tube theory and info is not out there in plain sight. And some of it conflicts with what some people know about other things. Chandler has something like this, but the guitar preamp, tone stack buffer is op amp based which they run the tube off the +15 -15 supply.
 

LadySpark

Joined Feb 7, 2024
38
You have to run them with the grids positive, so they behave like bipolar transistors. Personally, I don’t see the point. If you are going to make a valve preamp, then do the job properly. Running a valve at 12V seems like a marketing gimmick to me.
That is what was called space charging the triode. That came from people years ago making car tube stereos. But its easier to take a video IF tetrode and tie the first grid to the plate and run the 2nd grid as the control grid. But manufacturers made a series of tubes made like that called "space charged" tubes
 

LadySpark

Joined Feb 7, 2024
38
Blencowe has an article on operation at <12V.
http://www.valvewizard.co.uk/Triodes_at_low_voltages_Blencowe.pdf
You have to run them with the grids positive, so they behave like bipolar transistors. Personally, I don’t see the point. If you are going to make a valve preamp, then do the job properly. Running a valve at 12V seems like a marketing gimmick to me.

There are a few medium voltage tubes that do have operational curves at lower than 50V. 6922 was one of these.
 

Thread Starter

rpschultz

Joined Nov 23, 2022
416
Guitar people are a bit obsessed with the 12AX7 (aka ECC83). The ECC81 and ECC82 have much lower output impedances and therefore are much more suitable for driving the tone contol stages.
This is true, and due to the gain. 12AX7 has a gain of 100, compared to the 12AT7 (EEC81) of 60 and 12AU7 (EEC82) of 20. Some people swap in a 12AT7, but rarely are 12AU7's used for much of anything guitar related. It's all about gain I suspect, and in guitar-speak... gain means ability to produce clipping. Tubes can produce "soft" clipping, whereas transistors are a "hard" clipping. Different types of diodes can do either. Lower gain produces a smoother non-clipped sound and is sometimes desirable.
 

Thread Starter

rpschultz

Joined Nov 23, 2022
416
Second circuit: V2B is a cathode follower. Same as a JFET source follower. It gives a low impedance output to drive the tone control stage.
About 10 years ago I was into modding amps. I modded 3 different Fender Blues Jr's using kits from BillM. Bill has since passed and others have taken over these type kits. But the coolest mod I did was taking an unused triode (V2A) of a 12AX7 preamp tube and creating a cathode follower buffer for the tone stack. I had to do some heavy modifications to the circuit board, which at the time was almost beyond me. But it was super fun and it worked... and I couldn't hear ANY difference. HA.

Many really old Fenders (pre-BF) and most Marshalls use a common cathode stage before the tone stack so that the stack does not "load down" the previous gain stage. This extra stage does not provide any (voltage) gain itself, but by decreasing the load on the previous stage. This may be similar to "gives a low impedance output", as you suggest.

A Mod for the Fender Blues Jr. (mhuss.com)

BLuesjrinputMod.jpg
 

nsaspook

Joined Aug 27, 2009
12,769
That is what was called space charging the triode. That came from people years ago making car tube stereos. But its easier to take a video IF tetrode and tie the first grid to the plate and run the 2nd grid as the control grid. But manufacturers made a series of tubes made like that called "space charged" tubes
LV "Space Charge" operation of tubes is perfect for an effects box because of the distortion they normally produce.
https://forum.allaboutcircuits.com/...cuum-tube-guitar-amplifier.112582/post-877316
https://forum.allaboutcircuits.com/threads/it-pays-to-read-the-manual.110207/post-850985
https://forum.allaboutcircuits.com/...o-they-require-to-operate.133082/post-1105634

They were also used in some of the original complex digital devices where the distortion in the linear range of operation didn't matter.
https://forum.allaboutcircuits.com/threads/some-questions-about-vacuum-tubes.80047/post-568101
The KWR-37 weighed 100 pounds (45 kg) and contained some 500 subminiature vacuum tubes, whose leads were soldered to printed circuit boards. Each flip-flop in the KW-37 required three tubes, placing an upper bound on the total number of stages in any shift registers used at 166. Squeezing so much logic in such a small and rugged package was quite a feat in the 1950s.

Each KWT-37 filled an entire relay rack with five stacked modules. A precision time reference occupied the bottom, three key generators (stream cyphers in civilian parlance) occupied the middle and an alarm panel occupied the top position. The outputs of the three key generators were combined in a voting circuit. If one of the units' output did not match the other two, an alarm was sounded and the output from the two units that did agree continued to be used.
This device was still in service until the middle 80's.
 
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