My journey of building a tube amp (12AX7 and EL84 SE)

[zeh]

i connected the second half of the 12ax7, tried to overdrive it with the first half and then use an attenuator/voltage divider to bring it down again before it goes to the power tube. sounds nice so far but i think i needs improvement. maybe you can help me out further. this is what i have now:
power dissipation seems to be 9W, which is a little too high, it should be max 6W because of the output transformer (at least the OT has writte 6W on it...), i know the EL84 can do 12W...
any suggestions in changes of the circuit to make it better will be highly appreciated, iam looking for a bluesy kind of sound. if i crank up the gain to 50% i want it to start to crunch, before that it should be clean but still have lots of output volume. The state i have now is that i can only get 'really loud' if i give plenty of gain, which leads to lots of distortion... i did not connect the NFB yet, have no idea of how to connect it to be honest, need to read about it first. anyways, it sounds nice so far. my pickups also have quite high output, its a 78 les paul with 2 humbuckers.. i will try tomorro to tame it down a bit and get more clean output power. thanks for help!
cheers chris

 

[zeh]

i will also include a reverb thats not there yet, so ignore the RVB nodes..
i also have another question: i want to add a tone stack of course, will i use this as the 'attenuator' at the same time?

 

[zeh]

i have 2 pots, the 1M and the 5K. is it fine like this? i tried other combinations of voltage dividers. whats the goal here? the RC filter? the maximum grid resistor value from the next stage? what to look for here? thank you guys

 

[zeh]

for those interested i will post my LTSpice file here, you will have to download the dmtriodep and 6bq5 include files from duncansamps, but if somebody asks for it i can also post it here again

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[zeh]

i will also include a reverb thats not there yet, so ignore the RVB nodes..
i also have another question: i want to add a tone stack of course, will i use this as the 'attenuator' at the same time?

i want to use a opamp diver circuit here, but i dont know all the exact values of the '9T33' 'made in japan' sping tank i have laying arround yet... i will have to order a LCR meter (one of those cheap ebay component testers) to get the exact values of the unit.. dunno how to measure it with the equipment i already have, maybe i figure it out...

 

[zeh]

i use a 12AU7 now, this is much better because it has way less gain...

 

[musipol]

Just a couple of thoughts. If you're going after a certain guitar players tone (and what guitarist isn't), find out what amp he/she uses and examine that schematic for design clues like:
1 what topology (how many gain stages, where is the tone stack located, etc)
2 What tone stack is used (BF Fender/Marshall/baxandal/ Tweed Fender, etc)
3 How is the tone stack driven (Cathode follower or from a plate resistor)
4 Master vol or not.
5 Other voicing techniques in preamp (size of cathode bypass and coupling caps, etc)
6 Use this info to decide what topology you should try to help you get that sound
Keep in mind, you could spend a lot of time figuring out what works in terms of tone (bright, or dark, smooth OD vs gritty,fizzy, etc).
Also keep in mind how much a guitarist's recorded sound can be modified in a studio setting.

Good Luck

 

[zeh]

Hi Musipol,
thanks for your reply and thoughts!
iam not too much after a special guitar players tone, i just need a small amp to play at home with low to medium volume...
i dont like high gain and lots of distortion, so i dont need a lot of gain stages and so on, it think i can totally keep it simple, maybe the tone control can even be one of those one knob things. (i played around with the tone stack designer by duncansamps and there are some neat ones in the examples) i think i will leave it for now like it is but will try to add the reverb next, then will think about 5).. i guess i can adjust those filter params a bit.. 3) is an interesting point that i will check... the whole cathode follower thing can be helpful if i need lower gain feeding the next stage right?
kind regards

 

[Wavelength]

Hi there,

iam trying to build my first tube amp and iam having some issues.
if i only connect the power tube and plug in a test signal from my signal generator directly into the grid of the power tube, i hear output on the speaker, if i crank up the voltage to about 4-5V it starts distorting, i think this is normal behaviour since the powertube gets overdriven at this voltage.
if i only connect the input stage (1/2 12ax7, the unused pins of the 12ax7 are grounded and the heater is only using pin 9 and 5, so only the first half should be used) i can measure a clean amplified signal after my coupling capacitor.
The issue is, as soon as i connect the coupling cap output to the grid of the powertube and increase the supply voltage, at some point around 150V, the signal on the grid of the powertube starts 'ringing' with some high frequency and the output is kind of distorted and the output power is also not very high. i have tried to simulate the project as good as i can using LTSpice, but since in reality always everything is slightly different, the voltages will not 100% match, i guess this is due to the series resistance of the power transformer which differs in reality, i tried different values, also measured the DC resistance of the power transformers secondary.
any help will be much appreciated, since i never built a tube amp i dont know how to proceed now, maybe you can guide me into the right direction.
View attachment 120347
at the test point i can measure about 5V AC, so the input signal gets amplified correctly, but ONLY if i disconnect R3 from the grid of the output tube.

in my simulation actually every looks totally fine and its working correctly. i also tried exchanging input and output tube and its the same error..

thanks and kind regards
chris

Hi might be a bit late here... First R4 cannot exceed 300K or you will start to draw grid current which will cause all kinds of problems. Second if your not going to use feedback a 47pF cap across the R4 (now 300K) would limit the bandwidth and control the oscillation.
Single ended amps will get real noisy without good supply. I would also not link the Screen and the 12AX7 HV together. I like to take the B+ and drop the screen with a 2K2 5W resistor to the screen and bypass that to ground with a 10uF cap. You can do the same thing to the 12AX7 HV. If you are going to use a transformer and create the HV without a tube rectifier (not something I suggest).... it still needs like rectifier 47uF ~~ 5-10H choke~~47uF->B+ for good operation. An EZ80/6V4 rectifier would work great here super sounding and can carry that circuit well.

 

[Rusan]

power dissipation seems to be 9W, which is a little too high, it should be max 6W because of the output transformer (at least the OT has writte 6W on it...), i know the EL84 can do 12W...

An OPT wattage rating refers only to the power sent into the speaker load, which will vary somewhat depending on its winding ratio and the load impedance. The important factor you don't want to exceed is its maximum offset DC rating, which will cause core saturation, especially at lower frequencies. But, since your trafo is marked as being specifically for the EL84, that practically ensures it's safe for the 55mA max or so that said EL84 will draw at a hot Class A op point. IMHO, a single ended EL84 in pentode operation sounds its best when dissipating about 12 watts anode + screen at idle. The sim's 35mA plate current is possibly a bit too low, which is maybe 9.5 - 10 watts total at your operating point.

Wavelength beat me to the punch on the screen resistor and power supply filtration. I'd definitely want the screen running a few volts lower than the plate, with an appropriately decoupled resistor between it and the power supply. Otherwise, you're looking at excess screen current and a possible runaway.

I noticed that the above sim shows 41 volts dropped across the OPT primary, which corresponds to over 1.1K primary DCR. Is this your OPT's actual primary DCR? Something like 300 ohms would be the norm here.

 

[zeh]

Hey guys,
thanks a lot for the further explanations and input! i will modify the circuit according to your suggestions and see what i can get.
@Rusan, thanks, thats good to know that i can run higher power with this OT, i also was thinking like 'its quite big', because i have seen way smaller OTs for the EL84.. however, my primary DCR is about 660 ohms, i think i changed this in the simulation to match the voltage that are present in real life more accuratley. i have ordered an LCR meter and will measure all the excact values of the transformers as soon as i receive it. The screen grid thing also confused me a bit, in the datasheet they have two U/I diagrams for the tube, one is for Ug2=250V the other is 300V, so i was thinking this screen grid voltage will stay like this, no matter whats the actual plate voltage.. thanks for the explaining
@Wavelength, sadly i only have a power transformer without a center tap i tried using two diodes to create a virtual center tap and an EZ81 but there i got only half the voltage so i guess for this method the power transformer needs to have 2 times the desired voltage? thats basically the reason iam using this selenium rectifier, which also leads me to another question: it has written 250V on it, does this mean 'per diode (should be 4?)' or is it for the whole thing? then it should have been blown already i guess..
anyways, thanks for the suggestions, i will do the desired changes but first i will read about all the topics you mentioned, grid current, screen grid voltage, try to figure out the exact values of the transformer(s).

thank you guys! kind regards
Chris

 

[zeh]

@Wavelength, what should i read about when talking about R4? what is this resistor actually doing? thanks for your help! i was basically thinking like: i have a voltage coming out of C1, if i put a low resistor here, there will be current start to flow, lowering the voltage so i tried to keep the resistor very large to keep all the voltage of the signal? basically i thought first this resistor was not needed at all, but i needed a pot here at this point, i just used what i found then, was a 1Meg pot so i tought thats good

 

[zeh]

before i continue, what makes more sense, or doesnt it even matter if i use a 12AX7 or a 12AU7 ? i mean the 12AX7 has sooo much gain, is this even useful for this amp? as far as i undestood now, the high amplificated signal gets lowered down again using a voltage divider / attenuator before it hits the next stage, so its basically stupid to bring it up and then down again? what is the difference? why would someone use a 12AX7 and bring the output waaay down instead of using a tube with less gain? thank you guys

 

[fazer]

You could use a voltage doubler circuit

 

[Rusan]

The screen grid thing also confused me a bit, in the datasheet they have two U/I diagrams for the tube, one is for Ug2=250V the other is 300V, so i was thinking this screen grid voltage will stay like this, no matter whats the actual plate voltage.. thanks for the explaining

Although there are amp designs with significantly higher screen voltage than plate voltage, it's not a good idea. Too-high screen grid voltage causes the screen to attract additional electrons that would normally pass through it to the plate.

Tubes like the EL84, EL34, 6550 and KT88 that were designed for ultralinear operation have appropriately robust screen grids to handle its rigors, but that doesn't mean they're indestructible. The UL tap is normally a few volts higher than the plate end of the primary, because it's usually located about 40% (18.5% impedance) of the distance between the plate end and the B+ connection, which of course means less DCR and voltage drop. However, the screen voltage swings somewhat with plate voltage, whereas in standard pentode operation the screen voltage is more or less stable. So, on the negative plate voltage swing, the screen voltage is still more or less at B+. Hence, the resistor that Wavelength suggested will serve to limit screen current draw and prevent it from ever going into a runaway condition.

before i continue, what makes more sense, or doesnt it even matter if i use a 12AX7 or a 12AU7 ? i mean the 12AX7 has sooo much gain, is this even useful for this amp? as far as i undestood now, the high amplificated signal gets lowered down again using a voltage divider / attenuator before it hits the next stage, so its basically stupid to bring it up and then down again? what is the difference? why would someone use a 12AX7 and bring the output waaay down instead of using a tube with less gain? thank you guys

i also have another question: i want to add a tone stack of course, will i use this as the 'attenuator' at the same time?

Conventional Fender/Marshall TMB tone stacks have quite low impedance and therefore very high insertion loss, which will consume a lot of that gain, especially when driven from the high output impedance of a 12AX7. Without a tone stack, you're cascading the full output voltage of the first 12AX7 stage into the second one; the 47K - 1M divider in your sim only reduces the output signal voltage by about 5%.

 

[Wavelength]

Zen,
You can create a full wave rectifier with an EZ80/81 and two diodes and that should work fine but really 47uF would be the max. I just did that the other day and basically VAC*1.424 -Vr (voltage drop of the rectifier) -10% (ripple) = A+ then choke then another 47uF and the B+ should be fine.
SE designs basically work this way. Output tube dissapation/(3 to 4) will yield the maximum output at the speaker terminals. Just remember no feedback and the damping will effect the power output. By that I mean damping is the output impedance which is in series so you loose power.
I would really watch how much power and voltage you run the EL84. I hate to fix other peoples stuff. But I bet I have replaced probably 20 VOX power transformers because those buggers go like fireworks.

 

[zeh]

thanks guys, i watched some stuff about tone stacks, their location, about input and output impedance and cathode followers and lots of other stuff, i understand way more now. need to let all that sink in and then work on my schematic again
@Wavelength, ok cool, i will also try this, need to install the tube socket for the EZ81 again and see if this will work.. can you give a schematic example? also why is this 47u maximum? i mean its fine in my case because i use one of these old dual 47uF caps that are mounted on the chassis, but whats the reason? thanks
i tried it like this: https://www.jogis-roehrenbude.de/Verstaerker/Gleichrichter.jpg
but as i said, here i got only 1/2 of the voltage..

 

[zeh]

Hi might be a bit late here... First R4 cannot exceed 300K or you will start to draw grid current which will cause all kinds of problems. Second if your not going to use feedback a 47pF cap across the R4 (now 300K) would limit the bandwidth and control the oscillation.
Single ended amps will get real noisy without good supply. I would also not link the Screen and the 12AX7 HV together. I like to take the B+ and drop the screen with a 2K2 5W resistor to the screen and bypass that to ground with a 10uF cap. You can do the same thing to the 12AX7 HV. If you are going to use a transformer and create the HV without a tube rectifier (not something I suggest).... it still needs like rectifier 47uF ~~ 5-10H choke~~47uF->B+ for good operation. An EZ80/6V4 rectifier would work great here super sounding and can carry that circuit well.

Hi Guys,
@Wavelength iam struggling with the R4 value, you say it cannot exceed 300K, but in the datasheet it says the maximum value for Rg1 is 1Meg, also i have seen several schematics over the internet where a 1Meg resistor is used here in this place, also R9 is the same value but you didnt talk about this one. Or is it about the voltage divider you are talking about to loose more signal here? i tried meanwhile in my simulation to place a fender tonestack between the 2 preamp tubes and this looks reeealy good now in terms of overall gain structure, nothing gets overdriven and so on, i can basically get lots of clean gain out of it (in the simulation). that leads me to some more questions. 1) as far as i understand: a tube has, depending on its DC operating point, harmonic distortions, so i can use a more linear portion of the operating range, or i could choose a more 'cold' operating point with lower grid voltage, since here the plate curves (grid voltages) get more narrow in the diagram of the datasheet, this means here is more THD, right? the other thing is, that if i 'overdrive' a tube, it will start to clip, there is 2 ways of this can happen, 1. the input voltage swing exceeds the negative bias and therefore makes the grid positive and this will lead to tube cutoff (enterering the left 'forbidden' side of the plate curves diagram), the other way is if the grid gets sooo negative that no current will flow through the tube anymore, this will 'soft clip' the signal. first i dont understand about the positive grid voltage, i thought that should never be used? so if for example i put a booster pedal in front of the amp, this will effectively increase the input voltage to a level where the grid reaches also the positive region? does a amp (guitar amp) actually have to incoorperate this 'clipping' or 'overdriving' to sound 'nice' ? i mean, the tube itself should have plenty of harmonic distortion if used in a not such a linear range, this will make the sound of the guitar warm and sound distorted slightly as well? how much harmonic distortion is pleasent? if i play for example my fender hotrod amp in the studio, i set the gain to a level so it will be very clean but if i hit the strings hard enough, it will distort a bit. i never use a high gain setting actually so i will not need this in my 'home amp'.. would be great if somebody could tell me in terms of voltage, what should happen in this (or other) guitar amp. lets say my guitar puts out about 500mV (i plugged in an AC voltmeter to my guitar and hit the strings, i get max 1.2V or so if i use both humbuckers and play loud, so i guess 500mV is about the avarage. ok, so, this will be amplified by the 12AX7, outputting several volts now.. the EL84 needs about 5V or so input voltage to get driven correclty and not too much, so actually this 12AX7 stage is enough. ok, after putting it through the tone stack after the first stage, i get more or less the same 500mV signal again since a lot of voltage has dropped inside the tonestack. this i feed to the next stage, this will bring it up to 5V or so again, this will feed the EL84, looks perfect to me in the simulation. question is, how will this sound? will i need something like a 'cold clipper' to get more 'drive'? will i even need more drive? how will an amp like this sound where the signal is visually still a sine wave that comes out of the speaker, but there is harmonic distortion happening naturally from the tube. will this be super clean? will i need to clip it in some way to make it more guitaramp like?
so many questions, thank you so much guys
kind regards chris

 

[zeh]

Thaths what i have now in my simulation
View attachment 120733

 

[zeh]

i know this PSU section and screen grid thing might not be perfect yet, but since i dont experience any noise or hum, actually the thing is very quiet, i will improve here later maybe, just need to focus on the important things for me to understand first... i have also seen other amp designs using the HV for screen and preamps so i think its 'ok' for now