Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Mike_J on August 30, 2014, 10:58:28 am
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Attached is a pictures of a portion of one of the two chassis I had made for the project. They are both the same. Made from aluminum with welded corners and you can see the hole in the attached pictures for the Hoffman AC plug with the fuse insert in it. Fits perfectly.
My chassis man has agreed to program all the holes for the chassis including keyways, double d holes, etc. That would make for a much more professional build.
Thanks
Mike
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The first thing I did was drill the holes for the tube sockets. Would be much better to have my steel man drill the holes because then the holes for the screws on the preamp tubes would be straight. I can never seem to get them right although they will work just fine.
The way I drill the holes for the chassis is I blow up my layout and use it as a template for the large holes. I use a center punch and mark the center of the hole through the paper and into the chassis. I then use a 1/8" bit for a starter hole followed by step bits to complete the holes. Then I use one of each socket and a center punch to mark holes for the screws. Then I drill the screw holes.
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The next thing I did was make my Bias Board. I am placing my Bias Board on the inside front of the chassis because the wire runs are the shortest from that position. I tap and thread the chassis for a 6-32 screw and used a 1/4" standoff. I bought two one inch screws but they should have been 1-1/4" screws. Cut the head off the screws. Ran them through the screw hole I made in the chassis and put Locktite on the end of the screw and turned it in almost all the way to the end. Then I screwed the standoff onto the screw from the inside of the chassis and put Locktite on about 1/4" at the base of the screw and screwed the standoff against the chassis. Then I mixed up some JB Weld and welded the standoff into place. I waited overnight and sanded the screw ends on the outside flush with the chassis.
This morning I made and populated the Bias Board. I just took the Bias Board section out of the layout I had blown up to actual size and used it as a template for my Bias Board. Thought one of the holes was too close to the nut so I re-drilled it. The board looks like it was made from a hen scratched layout but it will work fine. If I did this for a living I would definitely find a man with a CNC machine and some experience designing and making boards to make my boards for me. It is not my favorite thing to do.
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My next step is installing my faceplate and back plate. You will notice they both have + signs scattered throughout them. This is the place to center the holes for the various components. I use the center punch and drill bits with center taps on them to drill almost all the holes. Sometimes a large hole is need and I use the step bits as I did to drill the holes for the tube sockets.
The faceplate and back plate were both designed on Inkscape which is a free vector graphic program I use. I email them to my faceplate man and he prints them on vinyl and laminates them. Costs $7.50 a side. Add the $40 for the chassis and I have $55 in it which is not too bad for a custom build. If I was going to build amps I would probably have faceplates screen printed but they have a 250 unit minimum order which is way too much money for a one off project.
The faceplate and back plates are put on as follows: 1. Get a cheap spray bottle and fill it with water and about three drops of dishwashing detergent. 2. Spray the water on the chassis. (it allows you to move the faceplate or back plate around until you get it in the right place) 3. Squeegee out the water and bubbles. 4. Set the chassis aside overnight. (allows the glue to dry)
I have found these faceplates and back plates to be very durable as long as you don't set your iron on it.
Thanks
Mike
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Here is a copy of the installed faceplate on the chassis. An explanation is needed to explain the problem I am facing with the back plate.
As probably every one on the forum knows when Marshall made their first amp they copied the 5f6a circuit used by Fender to make their 59 Bassman. This was about the time Fender got started with the blackfaced AB763 circuit which in my opinion is quite a bit cleaner largely because the tone stack comes off the plate versus the cathode follower of the Bassman and Plexi.
When Marshall made their amps they turned them over so the tubes faced up. (A good idea especially if you have four power tubes and seven preamp tubes like I have.) Marshall did not change the layout of the boards though so the input jacks are on the wrong side. None of my amps have their input jacks on the wrong side and I was not about to start here so I corrected Marshall's error of some 50 years ago and put the inputs on the left side of the faceplate where they belong. Great effort went into making sure the faceplate reflected this change. Alas, not so much on the back plate, it is backwards. The power side is on the preamp side and vice versa.
Well it is back to the drawing board. Shouldn't take nearly as long to correct. I would have had new back plates made anyway because the description above and below the Hoffman power connection with built in fuse is not properly centered and needs to be.
Thanks
Mike
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on the back plate, it is backwards. The power side is on the preamp side and vice versa.
I did that 1 time not long ago on the back plate, it happens. :laugh:
The chassis is the big 1 you don't want to mess up. Not just the $$ but the time punching/drilling it out.
Brad :icon_biggrin:
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on the back plate, it is backwards. The power side is on the preamp side and vice versa.
I did that 1 time not long ago on the back plate, it happens. :laugh:
The chassis is the big 1 you don't want to mess up. Not just the $$ but the time punching/drilling it out.
Brad :icon_biggrin:
Brad
Glad I caught it before I started drilling any holes. I reprogrammed the back plate and have sent it to my faceplate/back plate guy. He made it sound like he would make it a priority so hopefully I will get it Tuesday or Wednesday. I have plenty of other things to do in the meantime.
Thanks
Mike
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Here is an update of the chassis as it stands today. All of the holes have been drilled. I didn't mention it before but all holes should be marked with a center tap and pilot point drill bits should be used so you don't drift off center.
You may notice that I mark out where the transformers and choke will be so I know where to drill the holes. Any holes under a transformer or choke must be countersunk flush with the chassis.
Thanks
Mike
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Here is the current status of the boards for the build.
Thanks
Mike
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Here is the back plate for the amp populated as it will be when the amp is completed.
Thanks
Mike
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Here is the faceplate for the amp populated as it will be when the amp is completed. I am going to have a new faceplate made because the switch to turn off the effects and the bright switch need to be swapped. Makes the wiring flow better and makes more sense for the switch to turn off the effect to be at the top. I will try to make clearer pictures in the future.
Thanks
Mike
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curiosity: what is the meaning/purpose of the "rate" control in the OD group?
--pete
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Very impressive build, Mike! I will continue watching your progress anticipating great success with this project. Bravo!
With respect, Tubenit
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Wow! That's a huge leap from the simple dual Marshall that I proposed. I can't imagine anything I did on my layout applying to this amp, other than the idea of having a 1987 and 2204 preamp in the same box. Very ambitious project. I'll be following your progress also.
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curiosity: what is the meaning/purpose of the "rate" control in the OD group?
--pete
Dummyload
As I recall the rate controls purpose is to adjust the overdrive volume level so it is not a lot louder than the clean level.
Thanks
Mike
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Very impressive build, Mike! I will continue watching your progress anticipating great success with this project. Bravo!
With respect, Tubenit
Tubenit
It is not much different than what you suggested early on with your D'Mars with OD & FX. I very much appreciate the help you have given me with this project.
Thanks
Mike
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Wow! That's a huge leap from the simple dual Marshall that I proposed. I can't imagine anything I did on my layout applying to this amp, other than the idea of having a 1987 and 2204 preamp in the same box. Very ambitious project. I'll be following your progress also.
Sluckey
My dream amps were a 100 watt Plexi and a 100 watt 800. Your idea of putting them in the same amp really inspired me to build this. Anything that was added could be switched out so the Plexi and 800 topologies could be left intact essentially like you designed it with the addition of separate tone stacks for each preamp.
When I am in the middle of something like this I can experience the "can't see the forest through the trees" syndrome. Thus the constant questions which you answered to clear up things like wiring paths which are critical to the success of an amp. I can't tell you how much I appreciate your help in that regard.
Like I mentioned I am going to have the faceplate changed to rearrange the switches in the FX circuit that will just make the wiring flow a little better and I think make a little more sense to someone that is using it. Secondly, I am having a new power transformer made that will have the same specifications except the heater current is being boosted to 9.3A. The seventh preamp tube moved the current on the heater to 8.1 amps. While 100 mA probably wouldn't have hurt anything I want to make sure this amp is built to last so I am beefing the PT up a little. Lastly, I will move the B+ chassis ground point to a place where I know it will work.
Thanks
Mike
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The Sluckey inspired Plexi/800 with OD & FX is getting close to completion. I am waiting for the upgraded power transformer that is supposed to arrive next week. The replacement PT will give the amp more heater current. I haven't completed the heater wires yet either. Since I run the heater wires over the tube sockets I like to double and triple check the wires going to the sockets before I hook up the heater wires. They will be completed before the new PT arrives.
I took some pictures of the amp and will post them below in sections.
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Here is a copy of the preamp board and the relay power board.
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Here is the B+ and screen supply board.
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Here is a picture of the bottom of the chassis. I needed to use two 50/50 @ 500 volt cap cans to make sure there is enough space for the components on the preamp board.
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Here is a picture of the preamp and tone stack pots as well as the FX and OD controls.
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Here is a picture of the impedance selector switch and the speaker jacks.
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That is really an amazing build. Thanks for sharing the progress and pictures.
With respect, Tubenit
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Here is a picture of the PPIMV pot and the bias adjustment board.
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That is really an amazing build. Thanks for sharing the progress and pictures.
With respect, Tubenit
Thank you Tubenit for all the work you have shared in the past. Your D'Mars amp with FX, OD and PPIMV is very similar to this amp. Without your input I don't know that I would have made the connection to do this project. Also, your kind words and inspiration are very much appreciated.
Thanks
Mike
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I have completed the cabinet and all the wiring with the exception of the PT. Unfortunately, I am still awaiting the replacement power transformer to complete the amp. I will post further when the PT is installed.
The head is sitting on my 212 cabinet loaded with Celestion G12M65 Creambacks.
Thanks
Mike
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Here is a picture from the back of the amp.
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Did you make that turret board?
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Did you make that turret board?
EL34
Yes, I made the turret board by using the tool you sold quite a few years ago for spacing and your turrets. It is not my favorite job. I could also have printed the template you have in your custom board section of your store and used that.
If I were to make multiple boards there is no doubt I would use your custom board building service. If I had made one error on the board and had to start over again I would have had more money in the board than you would charge and there are a lot of things I would prefer doing over making a board.
Thanks
Mike
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I was just curious because it looked like my board material and my lugs :icon_biggrin:
Nice job doing it from scratch
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Really very nice Mike :thumbsup:
Franco
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PT finally arrived will install it today. Then I will go over it one more time and then fire it up.
If I can figure out how to turn my laptops microphone on I will post some sound clips, assuming it decides to make sound. Otherwise, there is a snowbird who attends my church that should be arriving for the winter any day now that has some recording equipment. I am sure he would help me lay down a few tracks
If it does not make sound or makes sound that is less than pleasing to the ear I will post for help.
Thanks
Mike.
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I was just curious because it looked like my board material and my lugs :icon_biggrin:
Nice job doing it from scratch
Thank you for the compliment. Yes it is your board material and lugs. Good quality at a very reasonable price.
Thanks
Mike
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Really very nice Mike :thumbsup:
Franco
Franco
Thank you for the compliment.
Mike
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Really nice. Gotta love all them thare toobs. :worthy1:
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Put tubes in it and got sound. Voltages are off throughout the amp though.
I have attached a copy of the schematic for the amp. It shows the voltages that the amp should be showing in red and the actual measurements in blue. Almost all of the actual measurements are low.
Any help on what to do next would be greatly appreciated.
Thanks
Mike
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I analyzed the percentage difference between my voltages and the voltages recommended by Marshall. Noticed a mistake in the red numbers for valve 3 in the previous post and made correction to same.
The plate voltages for the preamp and tone stacks for both the Plexi and 800 (valves one through four) are all about 20% low (+/- 3%) so it would seem like it could be fixed by changing the value of the power resistors. The voltages for the PI tube were 38% and 39% lower depending on the triode. This also seems like it could be fixed by changing the value of the power resistor. I am somewhat confused as to the reason for the variance between the PI tube and valves one through four. Does anyone have any idea why they would not be the same? Also, is there a mathematical formula to calculate what resistor values I need in order to get the voltages I want without having to resort to the trial and error method?
Thanks
Mike
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Can we see a schematic for the power supply? With voltages for each node.
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Can we see a schematic for the power supply? With voltages for each node.
Sluckey
Attached is the schematic for the power supply with voltages for each node. I haven't touched the PI or Plexi or 800 TS and PA voltages yet. Still working on getting the OD voltage correct.
Thanks
Mike
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Try this. Temporarily power the OD section from node H. If that gets your plate voltage where you want it then make this change. Disconnect the 470Ω/3W from node I and reconnect it to node H. Reconnect the OD power to the relocated node J.
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Try this. Temporarily power the OD section from node H. If that gets your plate voltage where you want it then make this change. Disconnect the 470Ω/3W from node I and reconnect it to node H. Reconnect the OD power to the relocated node J.
Now I know why you get paid the big bucks. I have 220 volts per triode which is 20 volts more than I need. A lot easier to get rid of voltage when you have it than to try to get more when no more is there to get.
Only problem I am seeing is switching the nodes has thrown the voltages off to the effects circuit so I will have to adjust the power resistors feeding the effects circuit again. Will I have to readjust everything again when I adjust the PI and the preamps? Everything I have seen so far says yes I will. Doing it this way I will be going around and around in circles for days until everything is just right. Has to be a better way.
I created a box today that has a 25K and a 100K pot in it to make the adjustments. I added two 1K / 2W to each pot to beef them up. Seems like I need to make enough pots for every node so I can adjust all of the power resistors at one time. Doing one at a time gets you a little closer each time but it will take a long time to get them all correct.
Thanks
Mike
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If you are gonna fine tune all voltages to a gnats ass then you're gonna need a bucketful of resistors and a lot of time. Or, you could switch to parallel nodes so you can change a node voltage with only swapping one resistor and you won't affect the other nodes. Take a look at this power supply from a Matchless Lightning...
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If you are gonna fine tune all voltages to a gnats ass then you're gonna need a bucketful of resistors and a lot of time. Or, you could switch to parallel nodes so you can change a node voltage with only swapping one resistor and you won't affect the other nodes. Take a look at this power supply from a Matchless Lightning...
Sluckey
Thank you for your help. I think I am going to go with having a potentiometer for each node after the screen supply node and adjust and adjust and adjust until I get the voltages reasonably close. Then I will measure each pot and get a dropping resistor of the same value if I don't already have one on hand. When I change tubes it will have an impact on voltages so getting exact is probably impractical. Just need to get close enough for it to sound good.
Do you think the Matchless system would work with the branches needed to feed the 800 and Plexi circuitry? I can't see how I could get that to work. Hopefully you are seeing something that I can't see like you did with fixing the overdrive voltage above.
Thanks
Mike
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HOW DOES THE AMP SOUND? That's a lot more important than matching plate voltages to some internet reference. BTW, where did you get your "Marshall recommended" voltage references? I've never seen voltages on the Marshall schematics I have. And what about the stuff that ain't Marshall? Ie, OD and FX Loop? What's your basis for saying those voltages are not correct? Most of this stuff will work just fine with a wide range of voltages. Just trying to get you to look at this from a practical standpoint. Many people will tweak an amp until it sounds pleasing to them (may not sound pleasing to me). At that point they would quit changing things and take a baseline voltage reading set for the entire amp. That becomes their basis for what's correct.
The Matchless parallel B+ nodes will work fine in most any amp, including yours. There is one drawback to using parallel nodes. You lose the progressively greater filtering that a series B+ rail gives as you reach the end of the rail. This can be mostly overcome by using larger caps and larger dropping resistors. But the obvious advantage is the ability to set the voltage for any node without affecting the voltages of the other nodes.
As for the C, D, E, F, and G nodes all being too low... look at the common denominator. Node C feeds all those other nodes. Increase node C and all the other nodes will increase too. IOW, decrease that 20K dropping resistor between node B and node C to a 10K to increase all voltages. If that gets you where you want to be, fine. If not, adjust the value up/down until you have what you want.
You have a one of a kind amp. It's OK to have a one of a kind set of voltage readings too. So, once again, HOW DOES THE AMP SOUND? :wink:
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HOW DOES THE AMP SOUND? That's a lot more important than matching plate voltages to some internet reference. BTW, where did you get your "Marshall recommended" voltage references? I've never seen voltages on the Marshall schematics I have. And what about the stuff that ain't Marshall? Ie, OD and FX Loop? What's your basis for saying those voltages are not correct? Most of this stuff will work just fine with a wide range of voltages. Just trying to get you to look at this from a practical standpoint. Many people will tweak an amp until it sounds pleasing to them (may not sound pleasing to me). At that point they would quit changing things and take a baseline voltage reading set for the entire amp. That becomes their basis for what's correct.
The Matchless parallel B+ nodes will work fine in most any amp, including yours. There is one drawback to using parallel nodes. You lose the progressively greater filtering that a series B+ rail gives as you reach the end of the rail. This can be mostly overcome by using larger caps and larger dropping resistors. But the obvious advantage is the ability to set the voltage for any node without affecting the voltages of the other nodes.
As for the C, D, E, F, and G nodes all being too low... look at the common denominator. Node C feeds all those other nodes. Increase node C and all the other nodes will increase too. IOW, decrease that 20K dropping resistor between node B and node C to a 10K to increase all voltages. If that gets you where you want to be, fine. If not, adjust the value up/down until you have what you want.
You have a one of a kind amp. It's OK to have a one of a kind set of voltage readings too. So, once again, HOW DOES THE AMP SOUND? :wink:
I played the Plexi channel after I set the power tube bias and it sounded somewhat like a Plexi but it wasn't quite bright enough. I haven't yet put a bright cap on the bright volume pot. Will figure out the value I like there after I get the voltages settled. One thing I didn't like is the amp had too much sag when I dug into the strings for my taste. I am attributing that to the low voltages on the tubes. I hope I am right.
Thank you for the Node C suggestion. It will be next on my list. I have attached the Marshall voltage chart for the Plexi and the 800 amps. It has the voltages for both the 50 watt and 100 watt amps. I you have any trouble reading it let me know and I will help you with it.
On the very good side the amp is so quiet that it is almost spooky.
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I got all the voltages almost spot on. Plexi channel sounds like a Plexi but still needs a bright cap over the bright volume control. The 800 channel has a problem. Volume is intermittent. Problem seems to be the !MA normal volume pot. When I giggle it the volume comes back but makes a vibrato sound while I am giggling it. Does anyone have any suggestions? I will change the pot and recheck the connections to it but don't know what else would cause it.
Thanks
Mike
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I will change the pot and recheck the connections to it but don't know what else would cause it.
+1, sounds like best place to start.
Brad :icon_biggrin:
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Changed the 1MA pot and the problem went away. Used the OD with the 800 channel with a SSS Strat. Thought it would have too much overdrive but it does not. Actually sounds pretty decent. Will have to test with humbuckers and see if that will give me a tone I am looking for. I was leaning towards changing the overdrive circuitry to the S.I.R. #36 circuit but I think I will give this a chance for a little while. Haven't played with the FX much yet but did notice I could reduce the volume substantially with it but didn't play with it long enough to see how it impacted tone.
Very pleased with the low or almost no noise level this amp has when the strings aren't being played. I was concerned that I would have problems with having all this circuitry in one chassis but it hasn't shown up yet.
Thanks
Mike
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Changed the 1MA pot and the problem went away.
:icon_biggrin:
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Latest on the amp is I have decided to use the CTS 1MA potentiometers with the SPST switch built into the pot (pull it out to turn on the switch). They will be used to set the bright caps for a Strat and a Les Paul. The Les Paul should require a larger cap if I am correct. Have a friend coming over with his Les Paul in a few days and I will use it to set the higher bright cap value. My experience with my Strat is a 500pF cap across the bright volume pot on the Plexi side is just about right. Haven't decided yet on the 800 side.
Pleasantly pleased with the overdrive channel using my Strat. Does a convincing Texas Flood sound. Hopefully my friend can help me get some audio on the forum.
Thanks
Mike
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Put the CTS potentiometers with SPST switch built in for the bright volume on the Plexi channel and the volume pot on the 800 channel. Set the Strat pot at 420pF and .0047uF when the pot is pulled out. The 800 channel has either no cap or 1000pF if I pull the pot out.
All was going well until today. The Plexi pot is not working properly. Have to play around with it to get it to make sound. Enough wiggling and it works okay until I want to change its value. This is the second pot I have had trouble with on this amp. Is anyone else having problems with their potentiometers? I don't think there is something I am doing improperly. Only choice if I want to keep it the same is to buy a new pot and use a 1MA in the meantime with a .0047 bright cap across it.
Secondly, I don't like the overdrive. Sounds acceptable at low volumes but squeals like a pig when the volume is turned up, especially on the neck pickup. Any suggestions would be appreciated. Considering the Slash #36 as an alternative but I am open for other suggestions.
Thanks
Mike
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Now it is making sound, most of the time, but it is not getting any brighter when I pull the switch out. It was working well the other day. Can anyone think of a reason it would quit working? All the connections look and seem sound with the chop stick test. Another thing I noticed is the normal volume control is having very little effect on volume or gain. Almost all volume is controlled by the bright volume switch. Shouldn't both volume controls have an impact on volume and gain? Any ideas as to what is wrong?
Thanks
Mike
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I think I fixed the problem. Got a second hand Mullard that must have a very weak triode because the other tube I put in V1 made both the normal and bright channels work. Getting my Hickock gm tester out to check the tube. Guess is it wasn't what the seller said it was.
The switch problem may also be fixed. I had three pF caps on the pot to tune it in. I removed one of them and it looks like one of the cap wires was precariously close to the ground tab and was probably touching it intermittently. I clipped the wire shorter and all seems to be working now.
Thanks
Mike