Looking at the metal part of the transformer from the top: it is 2.25 inches long, about 1 inch wide, and about 2 inches deep.
The coil has a diameter of about 2 inches, but it is not really a circle. It is more of an ellipse. The 2 inches is the widest part. The coil is 1 1/8 inches "deep."
How does it's size or the voltage across the caps influence the size and ratings of the substitute?
The coil has a diameter of about 2 inches, but it is not really a circle. It is more of an ellipse. The 2 inches is the widest part. The coil is 1 1/8 inches "deep."
How does it's size or the voltage across the caps influence the size and ratings of the substitute?
The transformer size gives an idea of the power or VA rating. The AC secondary voltage can be computed from the DC voltage on the caps. See the 1.414 factor discussed here:
http://sound.westhost.com/power-supplies.htm
http://sound.westhost.com/power-supplies.htm
Well I read over that page. That calculation makes it seem like I need about 50 volts on the secondary. I read over that part about capacitors effecting the size of the transformer that part lost me a little bit...
I'm thinking this part will help some
"Transformer
* The voltage is determined by the power that you need from the amplifier. Calculate power from the formula ...
P = Va² / R Where P is power in Watts, Va is RMS speaker voltage, and R is speaker nominal impedance
* The supply voltage (allowing only for basic losses) is calculated as follows
VRMS = Va * 1.1 Where VRMS is the transformer secondary voltage (for each supply rail)"
Well we know that the power in watts of the transformer in the amp that I have is 30. I don't know what they mean by RMS speaker voltage(how to find it). I'm guess that R is 8 ohms.
I'm thinking this part will help some
"Transformer
* The voltage is determined by the power that you need from the amplifier. Calculate power from the formula ...
P = Va² / R Where P is power in Watts, Va is RMS speaker voltage, and R is speaker nominal impedance
* The supply voltage (allowing only for basic losses) is calculated as follows
VRMS = Va * 1.1 Where VRMS is the transformer secondary voltage (for each supply rail)"
Well we know that the power in watts of the transformer in the amp that I have is 30. I don't know what they mean by RMS speaker voltage(how to find it). I'm guess that R is 8 ohms.
I'm doing that formula wrong. I don't know how to find the RMS speaker voltage, and I don't know if the P is total output watts or some other kind.
The 1.414 formula is pretty simple. I put in 35 and got 25 back. Earlier I thought I had to add the +35 rail and the -35 rail together, and that is why I said that I thought it was 50.
Pete I like your teach-a-man-to-fish method. I'm learning a lot but this transformer thing is frying my brain! What steps would you take in calculating a substitute? Thanks for all your help.
The 1.414 formula is pretty simple. I put in 35 and got 25 back. Earlier I thought I had to add the +35 rail and the -35 rail together, and that is why I said that I thought it was 50.
Pete I like your teach-a-man-to-fish method. I'm learning a lot but this transformer thing is frying my brain! What steps would you take in calculating a substitute? Thanks for all your help.
You've been really trying to use more than one method to determine the transformer spec.
You measured essentially the no load voltage, which is all you need to determine the secondary voltage which comes out to roughly 25-0-25 or 50V center tapped. 48V CT is a very common value, this or 50 will work fine, 48V CT is actually closer to what you measured. Look for one about the same size with a decent power rating, say 50-75W so that you emulate the power supply droop. You could go a bit larger, and add series resistance if it is too powerful.
The peak output power assuming that the caps hold up for a bit of time is, roughly:
Short term RMS Power = ((.707*(33 -1))^2)/8 = about 64 Watts
If we assume 30W continuous RMS then, with a continuous test signal you should see the supply lines droop to:
Vsup = 1.414 * sqrt(30 * 8) = about 22V
These would be the same if the power supply was regulated, probably not what you want in a guitar amp.
Pete B.
You measured essentially the no load voltage, which is all you need to determine the secondary voltage which comes out to roughly 25-0-25 or 50V center tapped. 48V CT is a very common value, this or 50 will work fine, 48V CT is actually closer to what you measured. Look for one about the same size with a decent power rating, say 50-75W so that you emulate the power supply droop. You could go a bit larger, and add series resistance if it is too powerful.
The peak output power assuming that the caps hold up for a bit of time is, roughly:
Short term RMS Power = ((.707*(33 -1))^2)/8 = about 64 Watts
If we assume 30W continuous RMS then, with a continuous test signal you should see the supply lines droop to:
Vsup = 1.414 * sqrt(30 * 8) = about 22V
These would be the same if the power supply was regulated, probably not what you want in a guitar amp.
Pete B.
if you know an amp tech that has lots of "parts beasts" stacked up in his shop. you should be able to get a reverb tank for a very low price. about 75-80% of practice amps use the same reverb tank, and another 10% of the ones that aren't the same can be modified to work (usually just by moving the anchor points of the support springs to change from a vertical to a horizontal mounted tank)
Well I decided to think about it for a few days before I invested in a 20 to 30 watt amp, and I'm glad I waited. I have a 15 watt practice amp, I should invest in a much larger amp. Pete, you were saying that I should consider the G50 as a alternative. It is very similar to the G100. The G100 does not have a note about what wattage the resistors are or what the caps voltage maximum is, but the G50 schematic does so I think I can use those values. I think making the 100 watt one would be a better economic choice.
This isn't exactly starting over, because I've learned a lot of info from studying the practice amp. I'm just not sure about some of things on the schematic. http://www.gibson.com/Files/schematics/G100A.pdf
In the area where it says note 1, is that an incandescent lamp and a thermistor? How can I calculate the values of these?
-Josh
This isn't exactly starting over, because I've learned a lot of info from studying the practice amp. I'm just not sure about some of things on the schematic. http://www.gibson.com/Files/schematics/G100A.pdf
In the area where it says note 1, is that an incandescent lamp and a thermistor? How can I calculate the values of these?
-Josh
I suggest that you build the preamp first and get some experience with building and debugging with inexpensive parts. Then do the power amp.
That is a photoresistor, and look at what it is connected to -diodes for distortion. It will work fine if you leave it out for testing you just won't have the distortion control.
Photoresistor:
http://en.wikipedia.org/wiki/Photoresistor
Probably a cadmium sulfide cell.
The lamp is probably a 6.3 V low current type, check the rating of the transistor and use a lamp below the current rating. The voltage divider in front of the emitter follower seems to put 5V max across that bulb, try 6.3V if it is not bright enough try a lower voltage bulb.
Pete B.
That is a photoresistor, and look at what it is connected to -diodes for distortion. It will work fine if you leave it out for testing you just won't have the distortion control.
Photoresistor:
http://en.wikipedia.org/wiki/Photoresistor
Probably a cadmium sulfide cell.
The lamp is probably a 6.3 V low current type, check the rating of the transistor and use a lamp below the current rating. The voltage divider in front of the emitter follower seems to put 5V max across that bulb, try 6.3V if it is not bright enough try a lower voltage bulb.
Pete B.
The lamp inside of the amp makes sense now. Would one of these http://www.futurlec.com/Photocells.shtml work as the photoresistor? And what value would work?
For the bridge diodes I'll follow that "linear power supply design" page you showed me, and use a 35A 400v rectifier. What values will work for the other diodes?
I also have some specific questions that I'll just list to save time and space.
-What kind of thermistor should i use for TH
-Is CB1 necessary
-Can I use any op-amp for mc1741P
-What's a sub for 2N5668
-What's a sub for 2N6254
-Why is R37 factory selected and what value can I use
T1 has the numbers "2-40" under it. What does that mean? T2 has "2-24A" written in the power section. Are these values? Using that page with the 1.414 thing, T2's secondary should be 56VCT. I don't know the primary.
I'm going to try to save money on shipping by buying all of the electrical parts off mouser and futurlec at the same time, but I think I'll take your advise and make sure the preamp section and everything works before I finish up the power section.
Thanks for all of your help. And take your time responding, I asked wayyy too many questions for one post.
For the bridge diodes I'll follow that "linear power supply design" page you showed me, and use a 35A 400v rectifier. What values will work for the other diodes?
I also have some specific questions that I'll just list to save time and space.
-What kind of thermistor should i use for TH
-Is CB1 necessary
-Can I use any op-amp for mc1741P
-What's a sub for 2N5668
-What's a sub for 2N6254
-Why is R37 factory selected and what value can I use
T1 has the numbers "2-40" under it. What does that mean? T2 has "2-24A" written in the power section. Are these values? Using that page with the 1.414 thing, T2's secondary should be 56VCT. I don't know the primary.
I'm going to try to save money on shipping by buying all of the electrical parts off mouser and futurlec at the same time, but I think I'll take your advise and make sure the preamp section and everything works before I finish up the power section.
Thanks for all of your help. And take your time responding, I asked wayyy too many questions for one post.
DeadSpeaker said:
In my opinion, that harmonic multiplier circuit is a joke and I would remove it completely. It will not sound extraordinarily good (more likely it has exactly that annoying 60-70’s “transistor buzz” that most people hate in SS) and I don’t see the point of such a complex arrangement for adjustment either (using transistor to control a lamp that controls series resistance of the diodes). First of all, replace the old-fashioned lamp with a LED and a suitable series resistor - it’s a much better solution. Second, I would just put an ordinary potentiometer in series with the LDR (photoresistor) and put a simple on/off footswitch arrangement in series with the LED. That is if you even need the whole HP circuit. Any external diode clipper pedal will do the “harmonic multiplying” trick (a.k.a. clipping) much better. The LDR must have a very low resistance in light and very high resistance in dark in order to work properly as a switch.
Do not use a thermistor and series diodes: Replace with a proper “VBE multiplier” bias servo circuit instead. These are old solutions that never worked very well and that’s why you don't see these circuits in most modern amps any more.
Let’s just say that I wouldn’t build an amp without a primary side fuse.
The OpAmp must be able to drive the load presented by the spring reverb tank. You do know what is the input impedance of the tank, right?
I don’t know about transistor substitutes but personally I would pick a modern design to substitute the power amp circuit in the first place. These old amp designs are often quite unreliable and I see few issues in that design that would lead to very early destruction if you happened to have a bad day. If you accidentally short the speaker the whole output stage is toast in an eye blink. I would seriously consider adding a current limiter to the circuit. …And a DC detector to protect your speakers. I believe Rod Elliot’s Project 27 would be a quite good option for a power amp circuit instead. Not bomb proof but certainly better than this circuit.
R37 is selected so that the output of the power amplifier has the lowest DC offset. The idea is to balance the input impedances of the differential pair transistors. Right value is probably close to 56K due to R41.
T1 is a generic inductor that is typically made by wrapping 15 – 30 turns of heavy gauge wire around the resistor R55. The exact inductance is not that important as this is basically a pretty transparent component and more related to speaker’s parameters than to those of the amp. Do note that the inductor will practically pass all the speaker current so build it accordingly.
T2: I wouldn’t care of what the schematic reads. Just figure out the proper VA rating (e.g. 3 x the expected output power of the amp) and make sure the transformer has 2 x 30V secondaries. That’s about it. And seriously, if you don’t know the primary voltage you really shouldn’t be touching the mains circuitry at all…
Do not put that neon light to the primary side. Instead use a LED and a series resistor across the secondary rails. It will also work as a “bleeder” that drains of the supply capacitor charge.
C38 and C39 must be special capacitors! If you do not know all the details of why they are “special” then omit them. Putting generic capacitor in their place is a serious safety risk! Personally, I would omit C39 completely.
DeadSpeaker said:
With regard to the recent comments here, I'm going on the assumption that you want to essentially copy this design, any changes make the task much more complex. It is good advice to be very cautious around line/high voltage - you should read up on this if necessary. And I agree remove that cap to ground.
There are very nice distortion pedals on the market, my son likes the one from Line 6 and it would be reasonable to leave it out.
The circuit is not that complex on the other hand. I'd use the lowest resistance photocell if you go with it.
Most of that power amp is a very standard design, similar to the RCA reference design and the Harman Kardon Citation 12. It works just fine. There is an interesting trick with regard to how the second set of output devices are connected to the first. I would not add protection since it is normal to drive guitar amps into overload and who knows what a protection circuit will do to the sound. I would instead use good sized output devices, as they did in the original design, and you might want to add +/- supply line fuses in case of a shorted output.
D1-2 1N4148
D3-5 Use a tab mount transistor wired as a diode, or Vbe mult
D4-5 1N4004
Outputs 2N3773
OP amp I suggest a 5534, check circuit details.
Pete B.
oh geez........... I don't want to come off as indecisive, but I don't think it would be smart to build a crappy amp. If I'm gonna put that kind of money into something, it should sound... pretty flippin' amazing, or at least sound nice with my DIY fuzz face or sansamp running through it.
Teemuk, you recommended the Project 27 (Looking at the Mk II version) I had seen this before but I had no idea how to build it, but now that I have been educated by Pete and everyone who has helped, I think I can handle it now.
Thanks for everything.
Teemuk, you recommended the Project 27 (Looking at the Mk II version) I had seen this before but I had no idea how to build it, but now that I have been educated by Pete and everyone who has helped, I think I can handle it now.
Thanks for everything.
I don't want to start a whole new thread for this quick question so I'll just ask it here.
On the ESP website, Rod Elliot says "Try to get speakers that are rated at at least 100W each - this safety margin is a requirement for guitar, since the amp will be overdriven for much of the time and this produces up to double the rated output of the amp."
He is talking about 2 12" speakers being 100watts each. I've been reading his articles and he seems to take a lot of precautions. Is this over kill or a good thing to do.
My budget is $200 US (excluding casters and the finish on the cabinet and other thing to make my amp look awesome) I think this is a realistic budget as long as I shop around. I was looking at these 50watt speakers.
http://www.tonetronix.com/pc/Jensen...-Speakers/Twelve-Inch-12-Jensen-Speakers.html
I was hoping that they will work, but I can always go with the next step up which is the same thing at 70watts. What are your guys thoughts on it?
Tech Specs: http://www.jensenvintage.com/mod12-50.htm
On the ESP website, Rod Elliot says "Try to get speakers that are rated at at least 100W each - this safety margin is a requirement for guitar, since the amp will be overdriven for much of the time and this produces up to double the rated output of the amp."
He is talking about 2 12" speakers being 100watts each. I've been reading his articles and he seems to take a lot of precautions. Is this over kill or a good thing to do.
My budget is $200 US (excluding casters and the finish on the cabinet and other thing to make my amp look awesome) I think this is a realistic budget as long as I shop around. I was looking at these 50watt speakers.
http://www.tonetronix.com/pc/Jensen...-Speakers/Twelve-Inch-12-Jensen-Speakers.html
I was hoping that they will work, but I can always go with the next step up which is the same thing at 70watts. What are your guys thoughts on it?
Tech Specs: http://www.jensenvintage.com/mod12-50.htm
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.