If you use the B15 control circuit all the pots should be audio/log taper. That will give you approximately flat at the mid point of rotation. Thus boost as you go more clockwise, cut as you rotate back.
Paul
Paul
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DIY Tube Preamp question
- Thread starter Bass4lyf
- Start date
Ok sweet.
I recieved everything in the mail yesterday. Just want to double check some things about connecting the transformer to the rectifier and see if im on the right track. Here are some pics.
Ok, So you have a wire going from 6.3v on the transformer to the terminal labelled 6.3v on the board. Then a wire from the 0 (next to the 6.3v) on the transformer to the terminal that has no label on the board next to the 6.3v terminal. Thats for the heaters correct?
Now the two terminals on the board labelled H. and AC (high ac?), Im guessing thats where the 280v goes into right? Im not sure which one connects up to the 280v or the 0v tap on the transformer. Also to the right of that, theres two more terminals labelled L. and AC (low ac?), what is that for? I think it leads to the DC1, DC2 and DC3 terminals (far right of board kinda infront of the heatsinks) which I think is for an output delay board which is optional and Im not worrying about having one. So does that mean I can ignore the L. and AC terminals?
EDIT: I just figured out that the DC1 supplies the 12v heating current to the two 12ax7's (connected in parallel), then DC2 goes to one 12au7 and the DC3 goes to the other 12au7. This means I should be connecting the 15v from the transformer to the L. AC terminals.
Also, with the transformer, how does the label on it that has all the different voltages written on it relate to the prongs above and below it? Im guessing the top row of prongs is the secondary and the bottom row of prongs is the primary (connected to the wall). If there is no copper wire connected to the prong, does that mean it is not connected? Hope im on the right track, dont want to blow anything or myself up. I appreciate the help.
I recieved everything in the mail yesterday. Just want to double check some things about connecting the transformer to the rectifier and see if im on the right track. Here are some pics.
Ok, So you have a wire going from 6.3v on the transformer to the terminal labelled 6.3v on the board. Then a wire from the 0 (next to the 6.3v) on the transformer to the terminal that has no label on the board next to the 6.3v terminal. Thats for the heaters correct?
Now the two terminals on the board labelled H. and AC (high ac?), Im guessing thats where the 280v goes into right? Im not sure which one connects up to the 280v or the 0v tap on the transformer. Also to the right of that, theres two more terminals labelled L. and AC (low ac?), what is that for? I think it leads to the DC1, DC2 and DC3 terminals (far right of board kinda infront of the heatsinks) which I think is for an output delay board which is optional and Im not worrying about having one. So does that mean I can ignore the L. and AC terminals?
EDIT: I just figured out that the DC1 supplies the 12v heating current to the two 12ax7's (connected in parallel), then DC2 goes to one 12au7 and the DC3 goes to the other 12au7. This means I should be connecting the 15v from the transformer to the L. AC terminals.
Also, with the transformer, how does the label on it that has all the different voltages written on it relate to the prongs above and below it? Im guessing the top row of prongs is the secondary and the bottom row of prongs is the primary (connected to the wall). If there is no copper wire connected to the prong, does that mean it is not connected? Hope im on the right track, dont want to blow anything or myself up. I appreciate the help.
never mind, its almost completely wired up, just one thing though, ive measured the voltage for the heaters for the two 12au7's and two 12ax7's and they are each putting out 25vdc which is way too much for the tubes. There is three heater outputs, one for the two 12ax7's in parallel and the other two for the two 12au7's. What am I supposed to do?
From the transformer there is a 6.3V feed for the rectifier tube. Two DC supplies for the pre-amp tubes both fed from the 15V winding. Both these supplies are locked at 12V DC by the 7812 regulators. These regulators need to have a minimum current flowing through them to start to work. Try putting a 1KΩ resistor across one of the 12V supplies. That'll pass 12mA or so and measure the voltage again. If the supply now reads 12V check the other. If both read 12V it should be safe to install the tubes and measure a final time. The heat sinks will get hot so don't be too surprised at that.
Paul
P.S. Do these tests quickly as the 1000µF 16V caps will be well over their rated voltage.
Paul
P.S. Do these tests quickly as the 1000µF 16V caps will be well over their rated voltage.
ok, the tubes are hooked up, thers about 12v going to each of the four on the preamp and about 6v going to the rectifier tube. The two 12au7's and two 12ax7's are glowing nicely, the two filaments in each are glowing. Except the rectifier tube (6x4) is not glowing at all. I have the 6.3vac straight from the transformer connected to pin 3 and 4. What am I doing wrong?
Check if you have any high voltage. Sometimes small rectifier tubes have a less visible heater. If you have HT the tube is working. Be very careful here - you are dealing with potentially lethal voltages. Pin 7 is the cathode and should be around 350-400V DC.
Paul
Paul
Thats also another thing, I have the high voltage hooked up and connected to the B+ on the preamp but I cant get any readings, I measure from the B+ out on the rectifier aswell as the B+ circuit on the preamp and I get like 1 volt or something.
I have a feeling I was given the wrong transformer also. The seller has one that has high volatge of 280-250 which was what I intended to buy but even though the one that I have is labelled 280-250v, When I measure the 280v pin, it reads 310v, and when I measure the 250v pin, it reads 280v. And the other transformer that they sell is labelled as a 310v-280v. So I guess I currently have 310v hooked up, Il rewire it so its hooked up to the pin labelled 250v which reads 280v and see how that goes.
I may be really stupid and missing something, or they just put the wrong label to the wrong transformer.
I have a feeling I was given the wrong transformer also. The seller has one that has high volatge of 280-250 which was what I intended to buy but even though the one that I have is labelled 280-250v, When I measure the 280v pin, it reads 310v, and when I measure the 250v pin, it reads 280v. And the other transformer that they sell is labelled as a 310v-280v. So I guess I currently have 310v hooked up, Il rewire it so its hooked up to the pin labelled 250v which reads 280v and see how that goes.
I may be really stupid and missing something, or they just put the wrong label to the wrong transformer.
ok, I changed the voltage from the power supply so that it is now 280v. Turns out I wired the 12au7 tubes wrong, so I think I fixed that. For the DC3 out for the 12ax7's, I have two wires coming off the positive, each wire going to pin 4 on the two 12ax7's, i then have a wire connecting pin 4 and 5 together on each 12ax7. I then have two wires running back to negative on the DC3 out. Each negative wire coming from pin 9 of each 12ax7. Is that the right way to do it for parallel 6.3v wiring? The tubes light up nicely exept that I still read 12ish volts, even though the two 12ax7's are running off one 12.6v output. Also the heatsink is getting really really hot, like you can only touch it for a few seconds, is that normal?
Now the 12au7's. These are not meant to be in parallel and are meant to have 12.6v to each tube. So I have two DC 12.6v outputs for each 12au7. On each, I have a wire from positive of one DC output to pin 4, then a wire from pin5 to negative on the DC output. The problem is, they are not glowing barely at all, they get slightly warm aswell as its heatsink.
Now for the rectifier tube. 6.3v does go into the filament, and I measured pin 7 at 337v (red from multimeter to pin7 and black from multimeter to ground on power supply board, think that may be wrong) but the tube isnt glowing and Im not reading any high voltage at the B+ out. No idea whats going on.
Now the 12au7's. These are not meant to be in parallel and are meant to have 12.6v to each tube. So I have two DC 12.6v outputs for each 12au7. On each, I have a wire from positive of one DC output to pin 4, then a wire from pin5 to negative on the DC output. The problem is, they are not glowing barely at all, they get slightly warm aswell as its heatsink.
Now for the rectifier tube. 6.3v does go into the filament, and I measured pin 7 at 337v (red from multimeter to pin7 and black from multimeter to ground on power supply board, think that may be wrong) but the tube isnt glowing and Im not reading any high voltage at the B+ out. No idea whats going on.
I would have expected that the heater wiring to be printed on the board and brought out to a connector on the edge. I don't think you should be wiring to the tube base itself. Not having the documentation I can't tell if what you are doing is right or wrong. While the heat sinks should get hot they should not get too hot to touch.
Paul
Paul
I would think that the 12V supplies would be going to pins 4 and 5 with nothing connected to pin 9. That way each tube is needing 12.6V at 150mA. It could be that they are paralleling each tubes heaters and series connecting them to get back to the 12.6V but at 300mA. IMO that seems a little illogical.
Paul
Paul
Can a multimeter measure amps? Seeing that wiring the 12ax7's in parallel off one dc out seems to work at the moment and that wiring the 12au7's in series off an individual dc out doesnt seem to work, could I just wire the 12au7's in parallel aswell and power that off one dc out? This will leave me one dc out spare which I could use to power a 12v light or something for power inidication when switched on.
Some meters can measure somewhat less than 1-Amp, but if you want to get a better Amperes reading (read: higher) you can either solve the problem mathematically or use a Clamp.
Typical meter pigtails aren't going to carry much more that 0.5 Amps or so for any length of time.
Typical meter pigtails aren't going to carry much more that 0.5 Amps or so for any length of time.
Each of the 12V regulators can only supply a certain amount of current. If you try to take more they'll shut down from over-current. I would expect the ones used are 500mA units. Trace out the heater traces on the board. Find out how they are wired so you can see what you are doing wrong.
Paul
Ok I think I have got them wired properly, they are both heating equally (barely glowing but I think thats normal/not an issue?) and the heatsinks are not over heating. I went off the diagram attatched.
My only problem now is that Im still not getting a B+ voltage and the 6x4 tube isnt heating or glowing at all. The choke is connected. Could it be a faulty 6x4? Im going to swap the fuse for another one just incase I might have blown it and not realised it, though it doesnt look blown at all.
My only problem now is that Im still not getting a B+ voltage and the 6x4 tube isnt heating or glowing at all. The choke is connected. Could it be a faulty 6x4? Im going to swap the fuse for another one just incase I might have blown it and not realised it, though it doesnt look blown at all.
Attachments
While I wait for a new 6x4 tube to come in the mail, Im going to sort out the pots and jacks and start drilling all the required holes on the chassis.
I was thinking of having Gain 1, Gain 2, Bass, Treble and Master. For Gain 1 Id have a pot at the very start of the whole preamp, for Gain 2 id have a pot inbetween the tone/eq circuit and the second preamp, then for the Master, Id have a pot at the very end of everything.
In > Gain 1 pot > first preamp (left) > tone circuit (bass and treble) > Gain 2 pot > second preamp (right) > Master pot > out.
Will that all work? Can I go without the Gain 2 pot or would it be good to have it?
I was thinking of having Gain 1, Gain 2, Bass, Treble and Master. For Gain 1 Id have a pot at the very start of the whole preamp, for Gain 2 id have a pot inbetween the tone/eq circuit and the second preamp, then for the Master, Id have a pot at the very end of everything.
In > Gain 1 pot > first preamp (left) > tone circuit (bass and treble) > Gain 2 pot > second preamp (right) > Master pot > out.
Will that all work? Can I go without the Gain 2 pot or would it be good to have it?
It all depends on what power amp you are intending to use. If it is tube your Master would be fine, if SS then it would not be. SS amps run at an input impedance much lower than a tube amp. Paralleling a stereo SS power amp makes things even worse. You don't need three level controls. One or more of them will always be on full.
Paul
Paul
I plan to put it in the effects loop of my ss bass amp. Peavey Tour 450. So I wouldnt need a master control because id just be using the master control on the actual bass amp instead? So I will only need one gain pot at the start? Will I beable to get an overdrive type sound from this preamp? How can I overdrive the tubes? Will I need a seperate booster pedal infront of it?
The other 6x4 tube I ordered came in the mail, I plugged it in, it heats up and I now get the B+ voltage. Ive got jacks wired onto it to see if I get sound and I do, its nice and clean, not too much noise which is good. The tone circuit works really nice, except that I need to replace the 1m pot for treble to a 4m pot. Ive got a question regarding the preamp tube heaters. The tube warm up, they dont get heaps hot though and they barely glow, they each are getting 11.8v dc. Is that ok? I was expecting them to heat up a fair bit more. Does the amount of heat/glow the tubes produce effect the overall output sound? How can I overdrive the tubes to get that nice sound?
You can check your tube data sheets at Invalid Link Removed
A very good resource as you progress.
As for heaters, I have built a few amps and the "tube glow" depends on the design and brand of the tube used.
For example; new Tung-Sol 12ax7 vs new Chinese 12ax7. One lights like a bulb while the other is barely visible with the lights off.
Glad to hear your preamp is working!
IMO- output is not related to heater voltage. Tube life is. If you run a heater very low on voltage, the volume will diminish due to no output from the cathode. I would imagine that this could damage a tube after some time but could give some of that fuzz with minimal mods.
Extra gain stages will get you more distortion... Some other manufacturers accomplish the fuzz with very low plate voltages (12~30V). I forget which distortion pedal did this but I am pretty sure it was a well-known company. I have not played any of the above to vouch for them sounding good or bad.
Play around a little... Look at your favorite amp schematics and see what they are doing. I promise, in time this all starts making wicked sense and you can start trying some of your own tricks!
Just enjoy what you are doing and don't get burned out by trying too many things at once!
A very good resource as you progress.
As for heaters, I have built a few amps and the "tube glow" depends on the design and brand of the tube used.
For example; new Tung-Sol 12ax7 vs new Chinese 12ax7. One lights like a bulb while the other is barely visible with the lights off.
Glad to hear your preamp is working!
IMO- output is not related to heater voltage. Tube life is. If you run a heater very low on voltage, the volume will diminish due to no output from the cathode. I would imagine that this could damage a tube after some time but could give some of that fuzz with minimal mods.
Extra gain stages will get you more distortion... Some other manufacturers accomplish the fuzz with very low plate voltages (12~30V). I forget which distortion pedal did this but I am pretty sure it was a well-known company. I have not played any of the above to vouch for them sounding good or bad.
Play around a little... Look at your favorite amp schematics and see what they are doing. I promise, in time this all starts making wicked sense and you can start trying some of your own tricks!
Just enjoy what you are doing and don't get burned out by trying too many things at once!
