OK-- so here's an update:
The remaining iron for the project has been ordered. All of it will be wound by Jack Eliano with the exception of the output transformers, which are James 6123-HS.
I have also been reconsidering the size of the monoblocks and I think that 12" x 24" is just too damn big, so I'm going to try and make it work on a 12" x 18" dealy.
Some questions that popped into my booze-addled mind are as follows:
If I wanted to add one of them fancy panel ammeters, where should I connect it in the circuit? I'm just thinking that since a trimpot will adjust the output current, it would be nice to make the adjustments and monitor the current through a panel meter.
Any recommendations on where to get a nice, cheap panel meter would be much appreciated.
Also, I'm thinking that this meter will be in the signal path, which may be a bad thing, but maybe not.
Thoughts on this one?
Kofi
The remaining iron for the project has been ordered. All of it will be wound by Jack Eliano with the exception of the output transformers, which are James 6123-HS.
I have also been reconsidering the size of the monoblocks and I think that 12" x 24" is just too damn big, so I'm going to try and make it work on a 12" x 18" dealy.
Some questions that popped into my booze-addled mind are as follows:
If I wanted to add one of them fancy panel ammeters, where should I connect it in the circuit? I'm just thinking that since a trimpot will adjust the output current, it would be nice to make the adjustments and monitor the current through a panel meter.
Any recommendations on where to get a nice, cheap panel meter would be much appreciated.
Also, I'm thinking that this meter will be in the signal path, which may be a bad thing, but maybe not.
Thoughts on this one?
Kofi
Kofi,
Go to Euphonia Audio for those panel meters. They have cool retro-looking black round dial meters for $9.50. I've got a pair that may go into my next "big" amp and they look cool. Should be a good match for the 300b monoblocks.
BTW I built a pair of KT88 monoblocks using tube rectification on 12" x 12" aluminum plates and everything fit easily. I would think you should be able to use a 12 x 10 chassis or for extra room a 17x10.
Go to Euphonia Audio for those panel meters. They have cool retro-looking black round dial meters for $9.50. I've got a pair that may go into my next "big" amp and they look cool. Should be a good match for the 300b monoblocks.
BTW I built a pair of KT88 monoblocks using tube rectification on 12" x 12" aluminum plates and everything fit easily. I would think you should be able to use a 12 x 10 chassis or for extra room a 17x10.
A current meter has to be inline, IE the current has to flow through it unlike a voltage meter which can measure the difference in potential between two points.
On most amps the meter would do inline somewhere between the cathode and ground. This schematic is a bit different but putting the meter either above or below the 1K/12W resistor would measure the 300b current. Putting it between where the 100R and 10R resistors connect and ground would measure total current including the input stage.
On most amps the meter would do inline somewhere between the cathode and ground. This schematic is a bit different but putting the meter either above or below the 1K/12W resistor would measure the 300b current. Putting it between where the 100R and 10R resistors connect and ground would measure total current including the input stage.
OK, got it, I think. But would the meter be able to handle the voltage? I'm guessing you mean that the meter would not be parallel to the 1K resistor, it would be the path the current had to take to get to the 1K (assuming the meter is affixed above that resistor).
Am I making any sense?
Unrelated to this, I got me some Epoxy Appliance Paint that I was planning to use on the aluminum top plate, but I'd like to know if this will cause any problems given heat dissipation within the amp.
Now am I making any sense?
Kofi
Am I making any sense?
Unrelated to this, I got me some Epoxy Appliance Paint that I was planning to use on the aluminum top plate, but I'd like to know if this will cause any problems given heat dissipation within the amp.
Now am I making any sense?
Kofi
Kofi - I haven't read the entire thread so it may have already been suggested, but why not powder coat paint the top plate? There's likely a shop nearby that could do it for about the same price as the epoxy spray, and it's baked in an oven at a much higher temperature than the working amp should ever generate.
I've had several done in a nice finish called Copper Vein - highly pebbled texture dark copper with black underlay - looks very sharp indeed.
I've had several done in a nice finish called Copper Vein - highly pebbled texture dark copper with black underlay - looks very sharp indeed.
I've put them in 400V circuits without an issue. Since they are inline the potential between one terminal of the meter and the other is virtually zero volts. Maybe someone with more experience with the DRD circuit could verify the placement?
As to the appliance epoxy paint. I've used the exact stuff (got mine at Menard's) to paint black Hammond Power Transformers to more closely match the champagne color of some James OPTs. The PTX gets pretty hot and the paint hasn't discolored or caused any problems at all after almost 2 years.
It has the added advantage of drying almost instantly and going on very smoothly.
That said, painting aluminum is a bit more difficult than painting steel. A primer made for aluminum (zinc chromate comes to mind) might be a good idea.
As to the appliance epoxy paint. I've used the exact stuff (got mine at Menard's) to paint black Hammond Power Transformers to more closely match the champagne color of some James OPTs. The PTX gets pretty hot and the paint hasn't discolored or caused any problems at all after almost 2 years.
It has the added advantage of drying almost instantly and going on very smoothly.
That said, painting aluminum is a bit more difficult than painting steel. A primer made for aluminum (zinc chromate comes to mind) might be a good idea.
Kofi,
A few notes on putting in a meter.
I assume you want to show the bias current in the 300B tube.
In assume you also do not want to show the AC (audio) currents.
You already have a 300B current sense element in your circuit which is that 1K resistor.
A quick look at the circuit shows that you have 245 - 177 = 68 volts across that resistor (at idle) indicating that you have an idle current of 68mA. (Carefull - in reading this off I ignored any voltage drop in the grid choke, but it doesn't affect the calcs below).
For simplicity I would then choose a meter with a 100uA movement and let the 68 Volts cause a 68uA current in the meter (to show 68 on the meter).
From the top of the 1K wire a 1Meg resistor to the +ve of the meter and connect the -ve of the meter to the bottom of the 1K. (1 Meg gives 1 uA per volt)
Now to filter out the AC and just read the DC you need a cap across the meter. Unfortunately the meter by itself has a very low impedance and so a very large cap is required.
So we pull a "stunt".
- instead of that 1Meg resistor use 2 x 500K in series (not common value so use 470K + 470K + 56K or similar).
Wire your cap from the mid point of these resistors to the -ve of the meter. We are now talking about at least 500K impedance that the cap goes across and so for say 1Hz response a cap of 330pF will do the trick.
As far as shunting the 1K goes we have 500K and 330pF in series. It will have negligable effect on the amplifier (actually it will cause a 0.2% increase in output above about 1kHz - the frequecy at which the impeance of the 330pF is equal to about 500 K ohms). You won't notice. If we were going to be total "smarty backsides" we could even manipulate the R and C values in the metering circuit to help optimise the high frequency response of the amp - BUT we won't.
You may have to recalculate according to the meter you end up using but you get the drift.
Main message is that you want to be using a 50uA or 100uA movement meter such that the meter current is negligable with respect to the current that you are measuring. I suggested the 100uA so that you don't need to overwrite the scale - just change "u" to "m". If yu used 50uA movement meter you'd then need 2 Meg of series resistor to give 0.5uA per volt and you'd have to change the meter scale printing.
Hope this is useful.
Cheers,
Ian
A few notes on putting in a meter.
I assume you want to show the bias current in the 300B tube.
In assume you also do not want to show the AC (audio) currents.
You already have a 300B current sense element in your circuit which is that 1K resistor.
A quick look at the circuit shows that you have 245 - 177 = 68 volts across that resistor (at idle) indicating that you have an idle current of 68mA. (Carefull - in reading this off I ignored any voltage drop in the grid choke, but it doesn't affect the calcs below).
For simplicity I would then choose a meter with a 100uA movement and let the 68 Volts cause a 68uA current in the meter (to show 68 on the meter).
From the top of the 1K wire a 1Meg resistor to the +ve of the meter and connect the -ve of the meter to the bottom of the 1K. (1 Meg gives 1 uA per volt)
Now to filter out the AC and just read the DC you need a cap across the meter. Unfortunately the meter by itself has a very low impedance and so a very large cap is required.
So we pull a "stunt".
- instead of that 1Meg resistor use 2 x 500K in series (not common value so use 470K + 470K + 56K or similar).
Wire your cap from the mid point of these resistors to the -ve of the meter. We are now talking about at least 500K impedance that the cap goes across and so for say 1Hz response a cap of 330pF will do the trick.
As far as shunting the 1K goes we have 500K and 330pF in series. It will have negligable effect on the amplifier (actually it will cause a 0.2% increase in output above about 1kHz - the frequecy at which the impeance of the 330pF is equal to about 500 K ohms). You won't notice. If we were going to be total "smarty backsides" we could even manipulate the R and C values in the metering circuit to help optimise the high frequency response of the amp - BUT we won't.
You may have to recalculate according to the meter you end up using but you get the drift.
Main message is that you want to be using a 50uA or 100uA movement meter such that the meter current is negligable with respect to the current that you are measuring. I suggested the 100uA so that you don't need to overwrite the scale - just change "u" to "m". If yu used 50uA movement meter you'd then need 2 Meg of series resistor to give 0.5uA per volt and you'd have to change the meter scale printing.
Hope this is useful.
Cheers,
Ian
Excellent explanation, thanks! I think I understand what to do here, but unfortunately the meters I ordered are already in mA, so I think I need to use it inline without the divider network.
It sounds like I can place it above or below the cathode resistor and make it work OK, so that's the plan for now.
As for the paint, I'll call some local powder-coat joints and see what they can do. I will admit, however, that black epoxy appliance paint looks awfully nice on my test plate-- and that's without any primer!
So, thinking ahead for a moment, I'm guessing that there's an unknown likelihood of success for any Kofi-based tube amp project. Given this scenario, I feel like I don't want to risk an initial power up with the TJ meshplate 300bs for fear of their ultimate destruction and the subsequent destruction of my marriage.
I'm guessing that one of you guys have an old, beat-up 300b or two that I could buy from you on the cheap that I could use for the initial power up.
Anyone care to make a few bucks off an old, used, vacuous envelope that no one wants? (Insert funny comparison to Mrs. Annan here)
Kofi
It sounds like I can place it above or below the cathode resistor and make it work OK, so that's the plan for now.
As for the paint, I'll call some local powder-coat joints and see what they can do. I will admit, however, that black epoxy appliance paint looks awfully nice on my test plate-- and that's without any primer!
So, thinking ahead for a moment, I'm guessing that there's an unknown likelihood of success for any Kofi-based tube amp project. Given this scenario, I feel like I don't want to risk an initial power up with the TJ meshplate 300bs for fear of their ultimate destruction and the subsequent destruction of my marriage.
I'm guessing that one of you guys have an old, beat-up 300b or two that I could buy from you on the cheap that I could use for the initial power up.
Anyone care to make a few bucks off an old, used, vacuous envelope that no one wants? (Insert funny comparison to Mrs. Annan here)
Kofi
Just thought I'd repost my last response. I think it got lost after the latest post...
Excellent explanation, thanks! I think I understand what to do here, but unfortunately the meters I ordered are already in mA, so I think I need to use it inline without the divider network.
It sounds like I can place it above or below the cathode resistor and make it work OK, so that's the plan for now.
As for the paint, I'll call some local powder-coat joints and see what they can do. I will admit, however, that black epoxy appliance paint looks awfully nice on my test plate-- and that's without any primer!
So, thinking ahead for a moment, I'm guessing that there's an unknown likelihood of success for any Kofi-based tube amp project. Given this scenario, I feel like I don't want to risk an initial power up with the TJ meshplate 300bs for fear of their ultimate destruction and the subsequent destruction of my marriage.
I'm guessing that one of you guys have an old, beat-up 300b or two that I could buy from you on the cheap that I could use for the initial power up.
Anyone care to make a few bucks off an old, used, vacuous envelope that no one wants? (Insert funny comparison to Mrs. Annan here)
Kofi
Excellent explanation, thanks! I think I understand what to do here, but unfortunately the meters I ordered are already in mA, so I think I need to use it inline without the divider network.
It sounds like I can place it above or below the cathode resistor and make it work OK, so that's the plan for now.
As for the paint, I'll call some local powder-coat joints and see what they can do. I will admit, however, that black epoxy appliance paint looks awfully nice on my test plate-- and that's without any primer!
So, thinking ahead for a moment, I'm guessing that there's an unknown likelihood of success for any Kofi-based tube amp project. Given this scenario, I feel like I don't want to risk an initial power up with the TJ meshplate 300bs for fear of their ultimate destruction and the subsequent destruction of my marriage.
I'm guessing that one of you guys have an old, beat-up 300b or two that I could buy from you on the cheap that I could use for the initial power up.
Anyone care to make a few bucks off an old, used, vacuous envelope that no one wants? (Insert funny comparison to Mrs. Annan here)
Kofi
OK-- so no bite on the solicitation of the used 300b. No big deal, I'll just be careful with what I've got. And what I've got is now almost complete!
I'm just waiting on the reactor (DRD choke) from Jack, which should arrive Monday, and the parts list will be complete. I'm taking care of some casework today and I'll try and post pictures of the build as it happens so you can spot the screw-ups.
Yeah!
Kofi!
I'm just waiting on the reactor (DRD choke) from Jack, which should arrive Monday, and the parts list will be complete. I'm taking care of some casework today and I'll try and post pictures of the build as it happens so you can spot the screw-ups.
Yeah!
Kofi!
I particularly like Watco antique oil finish, natural color.
It soaks into the grain, and hardens inside the wood. There is a small percentage of varnish that protects the wood. But to the touch and sight, you wouldn't think the wood is anything but stained. Feels dry, not glossy or shiny.
Readily available, and at a decent price.
It soaks into the grain, and hardens inside the wood. There is a small percentage of varnish that protects the wood. But to the touch and sight, you wouldn't think the wood is anything but stained. Feels dry, not glossy or shiny.
Readily available, and at a decent price.
The only reason to consider Poly is if your worried about those unsightly rings from careless glasses. Oil is the way to go. Either tung or Watco, They look great and are so easy to apply.
What I do is take a plastic bag, like what you get at the supermarket to put produce in. Put your hand in like a glove and grab a small hunk of old sock, or other lint free cloth. Put a small amount of oil on it and wipe down the wood. Don’t go nuts, just enough to wet the wood but leave no excess. Sort of like a wrung out bar rag over the counter. I find that for the first coat or two after I wipe the second base the first is ready for another light lick and I call this "one coat" .I am making this sound a lot harder then it is, Just don’t goop it on.
So, were I was going with this, is when your done. Grab the rag and pull the bag off your hand over the rag and seal it with a twist tie. Not only is your hand clean but you have a pre primed rag ready to go for the next coat. This may sound like a little thing but flip the bag over your hand add a couple of drops of oil and wipe. two minutes three tops. Not counting the time you spend admiring your work
Do this morning, dinner and nite for two or three days. That is six or nine coats. It will be gorgeous…John
What I do is take a plastic bag, like what you get at the supermarket to put produce in. Put your hand in like a glove and grab a small hunk of old sock, or other lint free cloth. Put a small amount of oil on it and wipe down the wood. Don’t go nuts, just enough to wet the wood but leave no excess. Sort of like a wrung out bar rag over the counter. I find that for the first coat or two after I wipe the second base the first is ready for another light lick and I call this "one coat" .I am making this sound a lot harder then it is, Just don’t goop it on.
So, were I was going with this, is when your done. Grab the rag and pull the bag off your hand over the rag and seal it with a twist tie. Not only is your hand clean but you have a pre primed rag ready to go for the next coat. This may sound like a little thing but flip the bag over your hand add a couple of drops of oil and wipe. two minutes three tops. Not counting the time you spend admiring your work
Do this morning, dinner and nite for two or three days. That is six or nine coats. It will be gorgeous…John
Thanks!
Just got some tung oil, so I'll add a coat to a test board tonight. I'm guessing that there's no need to sand between coats, is this right?
Also, it sounds like the first coat would be wipe on, wait about 10 minutes, then wipe on another one. After that, it's just one wipe per coat, right?
Kofi
Just got some tung oil, so I'll add a coat to a test board tonight. I'm guessing that there's no need to sand between coats, is this right?
Also, it sounds like the first coat would be wipe on, wait about 10 minutes, then wipe on another one. After that, it's just one wipe per coat, right?
Kofi
You could sand after the first coat hardens after a day or so, but it isn't necessary. Once you get into multiple coats, you don't want to keep sanding. This isn't specifically varnishing, where sanding between coats is a good thing.
The instructions, if I recall, are to add the second wipe within the hour. After that, if you want to go with the multiple coats, you might want to extend this to 3 or 4 hours.
The instructions, if I recall, are to add the second wipe within the hour. After that, if you want to go with the multiple coats, you might want to extend this to 3 or 4 hours.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.