This is a schematic of my new tube preamp ..... it"s very high gain and sounds great ... it is simular to most designs but also a bit different .....
I also have a PCB design for this preamp that I will post after I get the build instructions finnished ....
Cheers
Nice.
Post pictures and MP3 when available.
Congratulations :tu:
Nice work,, but I DO urr when I see so many circuits like this that just leave the tone output hanging from the end like that.
The loss is bad enough even when picked up by another stage but left like that you waste a lot of energy.
Remember these ARE passive hi Z tone circuits so they can't drive anything, cept another Very Hi-Z input.
Phil.
Even with the tone stack at the end of the circuit the signal is still very very hot ......
Here is the difference in output.
Green plot is Normal ,, using HiZ buffer.
Yellow trace is running the tone stack into a 10k load,, which is extreme but Could happen and the concept is best avoided especially if it's in a separate unit which might work with some gear but not with others.
In this example,, you are down 30 dB @100 hZ,, ouch!! :o
A buffer guarantees the same performance no matter what you plug into.
you may also come realize that you don't need to use so many gain stages,,,which helps keep the dreaded noise down also.
It's up to you? 8|
Phil.
Im running my Preamp into a LM3886 with an input impedance of 47k , I have a solid state buffer but I"m not using it because I tried it with the buffer and it really didn"t sound any different accept it was louder with the buffer but It is plenty loud enough allready , so loud that it clips the lm3886 with the Volume barely on .....
The circuit will be buffered anyways because I usually have my EQ and a couple pedals running through the FX loop ......
Thanx
After treble put 470K in series and 47K volume pot, or 1MEG/100K volume, this will tame off the signal and low the impedance.
Success !!! Ive been stumpted by this really bad hum problem so after a week of thinking on it I realized that I forgot to ground the circuit that runs the channel switching circuit ...
The channel switching circuit uses 2 5v relays to switch the 2 channels and the led indicator lites for each channel and runs off of a 6vac winding of one of the transformers that powers the tubes , it uses a footswitch that connects to the amp via a patch cord .....
After grounding the 6v winding the hum totally dissapears , sounds great and it loud as hell ....
I actually built this amp because the other amp I built died , but I come to realize tonight that it didn"t die at all , it was the 15 band EQ I bought that I used in the FX loop that died ....
go figure , you build an amp and it last longer than the store bought EQ that you bought to use with it ...... :)
Quote
go figure , you build an amp and it last longer than the store bought (insert electronics guizmo here) that you bought to use with it ...... Smiley
And what's
so surprising about that? :) :D ;D :lmao:
Congratulations :tu:
Here are a couple Pics of them amp .....
Why "High" -> "Low" -> "Mid" and not "High" -> "Mid" - "Low" ? Looks great though, I like the (I'm assuming) aluminum front plate.
hi , I did it that way because it was just a lot easier to wire up with the pots in that position ..... yes the front plate is aluminium , I got 3 of them at the local metal supermarket for about $12 ......
The rest of the chassis is from a old EQ I found in a junk pile .....
I"m going to redo the magic marker labeling at some point because it takes away from the amps awesomeness .... ;D
I also used some aluminum plate between the Power Transformers and the audio curcuits to shield some of the EM from the power transformers ....
You can"t see them in the picture but there are 3 seperate power transformers , I have a 235v transformer for the Plate supply that has a 6.3v tap that is unused and a 5v tap used for the Relay channel switching , it also has a seperate 12.6v 1A transformer for the Heaters and a seperate 100Va 2 x 18v transformer to run the LM3886 , there is also a 100mA 10H Choke .......
It was a lot of work trying to find away to fit all that stuff inside that Chassis ......
I"m now working on a 6V6 Power amp that I can use with the Line out on this amp to get that pentode tone ..... It"s allready built for the most part , just have to wire it up and find a chassis for it and hope it doesn"t catch fire ......
Cheers
Hi Minion,
Mate,, great effort :tu: Just looked at the pics. Wow a lot of work in that one so glad you got it all working.
Forgot the ground ay? Yep been there a few times. :-[
The ground is always the wire you forget to add :grr
One little thing,,, if I may be allowed?
Do I see Wires passing through holes with no grommets and around sharp metal edges?
can be an issue down the track. :trouble
Cheers, Phil.
Quote from: phatt on October 19, 2011, 09:07:25 AM
Hi Minion,
Mate,, great effort :tu: Just looked at the pics. Wow a lot of work in that one so glad you got it all working.
Forgot the ground ay? Yep been there a few times. :-[
The ground is always the wire you forget to add :grr
One little thing,,, if I may be allowed?
Do I see Wires passing through holes with no grommets and around sharp metal edges?
can be an issue down the track. :trouble
Cheers, Phil.
Thanx for the tip about the holes , there is enough slack on the wires so they hopefully it won"t be rubbing , I also don"t put it on top of my speaker cab because I don"t want to abuse it .... I will put some silicone glue around the holes to stop any rubbing .....
The amp has a sort of old school metal tone but It is hard to get the Clean channel to stay clean and be loud so I"m gona replace the two 12ax7 in the Clean channel with some 12au7 or 12at7 to get a cleaner tone ...... Using the Sonic Maximizer and EQ in the FX loop helps shape the tone quite a bit and makes it a bit more versitile .....
And the chanell switching curcuit works flawlessly , I made the footswitch out of a plastic project enclosure and a heavey duty push button switch and a 1/4 in female jack , it connects to the amp with a regular patch cord .......
It is by far the hardest project I have done which was componded by my total ignorance in Tube theory and my pathetic and dyslexic math skills .......
I will eventually post the PCB artwork , stuffing guide and wiring diagram I just haven"t had the time to finnish the stuffing guide .....
Cheers
Quote from: Minion on October 19, 2011, 07:08:59 PM
I will eventually post the PCB artwork , stuffing guide and wiring diagram I just haven"t had the time to finnish the stuffing guide .....
Cheers
I wait eagerly! :dbtu:
I've been having a close look at this circuit and there are more than a few curious features that deserve comment.
I have redrafted it so that it has component designators for identification.
Starting at the input, a minor point; it is more conventional to place the grid leak (R1) after the grid RF stopper (R2) than before it, as here. Unlike in an output stage where the function of a grid stopper is to prevent self-oscillation, in preamp stages the grid stopper is to prevent external Radio Frequencies entering the amplifier, and is therefore normally placed as close as possible to the input socket.
What initially grabbed my attention when glancing at this circuit is the value of R3, the first stage cathode resistor, in conjunction with the value of the anode resistor R4. The Mullard databook give typical operating conditions for valves such as the 12AX7, and for around 300 volts HT with a 220k anode load (R4) the value for the cathode resistor R3 should be 3k9. The operating current of this stage is around 0.6mA and the effect of such a low value for R3 is to reduce headroom and increase stage distortion.
The cathode bypass, C1, has been set to a fairly low value meaning that the stage has about a 4dB rise from 100Hz to 1kHz, clearly aimed at tenor guitar.
There are a number of components that seem to have no useful function, and R5 is the first we encounter. Since there is already a DC path from C2 to ground via R6 and gain control U5, which are also of much lower value, there seems to be no point including R5.
The gain control circuit, R6, C3, U5, seems to be at odds with itself. The effect of R6 & C3 is to provide some fixed treble boost, however this is minor and doesn't come into effect until about the fourth or fifth octave above Middle-C. It would be more normal to omit R6 altogether and take C3 to the wiper of the gain pot, then becoming a more conventional "top coupling" cap.
Again we seem to have a component, R7, which has no function and can be omitted.
Given that some top boost has been applied via C3 the inclusion of R8 more than throws it all away again. The object or function of R8 is obscure, but its effect is to act as an ill-defined top-cut filter in conjunction with the Miller capacitance of the following valve, U2.
An important aspect of circuit design is to try and make the circuit as insensitive as possible to the aging of components, and to replacements during repair. The main effect of R8 is to make this stage more sensitive to changes in the Miller capacitance of U2, and to different Miller capacitance of any replacement valves inserted at that position. Similarly R8 is large enough to add bias due to grid leakage and thus make the actual DC operating point of U2 less predictable. It is also worth mentioning that because the grid leakage current can flow through the wiper of the Gain control it will tend to make this scratchy in operation, through no fault of its own, and which no amount of cleaning will cure.
The value of cathode resistor R9 is spot-on for its anode load (R10) of 100k. Again the low value of the cathode bypass (C4) shapes the low frequency response of this stage towards tenor guitar.
Having obtained around x60 gain from U1 and x55 from U2 (~x3300) we now encounter R11 and 12 which form a fixed attenuator of x0.6, reducing the effective gain at this point to 3300*0.6=1980 or about x2000. The reason for doing this is not at all clear.
Again we encounter a grid series resistor, R14, which has a much lesser effect than R8, similarly seems to only make this stage less predictable and more sensitive to device variation in U3.
More predictable is the effect of C6 across the anode load of U3 (R15). This provides a heavy roll off above about 1kHz, negating the effect of C3.
I would personally reserve the title "Ultra gain" for a preamp where all the stage anode loads were constant current sources, thus making them Maximum Available Gain or MAG stages, but in stages U3 and 4 we see that the cathode bypass resistors aren't bypassed, meaning the application of local negative feedback and quite a bit less than MAG from each stage.
Similarly R17 at 470k provides less stage gain than by using a 1Meg.
Again we have a large value grid series resistor in R18 which has a similar effect to R8 discussed above. By the time we get to the entry to the tonestack at C8 we have about 120dB of available gain, but we also have a pretty savage overall roll off above about 1kHz.
And again in C8 and R21 we seem to have two components that serve no useful function. Both could be omitted (as they are in many similar amplifiers) and the anode of U4 taken straight into the tonestack since the DC path is already blocked by C9, 10 and 11.
Phil has already touched on having the unbuffered output of a High-Z tonestack subject to following loads, and with this particular Marshall configuration nothing less than about 1 megohm load will produce acceptable performance. The use of loads as low as 47k, particularly at high setting of the volume control U9 will result in savage tilting of the response downwards into the bass from about 1kHz.
By re-positioning the tonestack between U3 and U4 it could be fed either conventionally from the anode of U3, with R16 bypassed to provide high gain (or perhaps from U3 as a cathode follower), and U4 could certainly be used as a cathode follower output buffer which would make the tonestack totally indifferent to output loading.
Taken overall this is a preamp that isn't sure of what it really wants, with contradictory and ill-defined response shaping, gain mixed with attenuation, and the use of stages (particularly U3 and 4) which aren't employed to best effect.