Component Question

[Tbone55]

I'm knew to the amp building world and would like to know if there is any difference between guitar and amp potentiometers. After researching several of the more known manufacturer's sites (Bourns, CTS) I'm no further ahead. Is their a difference and if so, what is it? The only difference I've seen is the power rating, .1W, .25W, .5W. The pots that are shown here on Doug's site have no reference to the manufacturer part number and I need several that are push-pull type that Doug doesn't list. I'm putting together a list of the components I need to build a new amp. I've almost completed it but a few parts are still alluding me. Since the amp I'm looking to build isn't available on Doug's site this newbie needs a bit of help.

Thanks Kindly.

[vampwizzard]

A good thread on this topic.. to help you think through the problem.. https://el34world.com/Forum/index.php?topic=16784.0

[Tbone55]

A good thread on this topic.. to help you think through the problem.. https://el34world.com/Forum/index.php?topic=16784.0

Thanks Kindly. I'm trying to understand the circuitry and your link was very helpful. Should have realized that the coupling capacitors are only allowing the a/c signal to pass through and that is only going to be a very small amount of voltage. I think Doug sells the audio push-pull pot I need but not the linear one. At least now I know what I should be looking for. Now I need to try and source a couple of capacitors that Doug doesn't sell although maybe I can use several different values in parallel to get what I need.

Cheers!

[sluckey]

It would be very helpful if you post a schematic and tell us exactly what parts you need.

[Tbone55]

It would be very helpful if you post a schematic and tell us exactly what parts you need.

As requested, I've attached a list of the parts to build the amp a well as the schematic. Parts that are available from Doug are noted on the parts list. Parts that aren't available from Doug I've highlighted.

Thanks Kindly.

[Willabe]

Why do you want a switch on the volume and bass pots?

[PRR]

> difference between guitar and amp potentiometers

I thought guitars used one type of knob and amps used another type knob. Guitar pot shafts splined and slotted, amps often flatted solid? I think they are electrically interchangeable. But be sure you like the knobs that fit.

[sluckey]

Check your spreadsheet. Hoffman has some of the highlighted items and he also doesn't have some of the items you say he does.

[Tbone55]

Why do you want a switch on the volume and bass pots?

1. Power / Level control – Rotary ON-OFF switch with Power Level control incorporated (VRM). Actions: Off – On (low level is standby. Turning control further effects the power level of the amp. It also acts like a presence control but no exactly the same.

2. Volume Control -  Incorporates a push/pull Tweed / ‘tude switch for Clean / Tweed // Dirty / Marshall range

3. Bass - Cut/Boost. Incorporates a push/pull Thin/ Fat switch for enhanced voicing

Thanks.

[Tbone55]

> difference between guitar and amp potentiometers

I thought guitars used one type of knob and amps used another type knob. Guitar pot shafts splined and slotted, amps often flatted solid? I think they are electrically interchangeable. But be sure you like the knobs that fit.

Guitars can have either splined or plain round pot shafts. Gibsons mostly use splined shafts while some Fenders use plain round (Telecaster). I've used both types on the guitars that I"ve built depending on they type of knobs I wanted to use. Pretty standard sizing so most knobs will work with either.

[Tbone55]

Check your spreadsheet. Hoffman has some of the highlighted items and he also doesn't have some of the items you say he does.

Thanks sluckey. Got a hold of a BOM but I think it's outdated. Changed some of the resistor wattage ratings that were only 1/2W. There were a lot whose rating went from 1/2W to 1W. That's my fault as I had put them down as only 1/2 watt when I first made the list. A lot of them call for carbon film but Doug only has carbon composition. Will the wattage and composition make a difference? Would appreciate you having a look at my list again.

Thanks.

[SoundmasterG]

I'd suggest to use metal film resistors in all spots, or if you are determined, all spots except those that have lots of voltage swing, such as plate resistors for the phase inverter. There you could use carbon comp for a little bit of harmonic distortion that those create, but in every other place in the amp they just create noise and hiss. Metal film is much better than carbon comp and/or carbon film. Use metal oxide in the power supply, and possibly wirewound for screen resistors or cathode resistors depending on what your needs are.

Greg

[Tbone55]

I'd suggest to use metal film resistors in all spots, or if you are determined, all spots except those that have lots of voltage swing, such as plate resistors for the phase inverter. There you could use carbon comp for a little bit of harmonic distortion that those create, but in every other place in the amp they just create noise and hiss. Metal film is much better than carbon comp and/or carbon film. Use metal oxide in the power supply, and possibly wirewound for screen resistors or cathode resistors depending on what your needs are.

Greg

Thanks Greg. I appreciate your suggestion. Sounds like a better idea. Anything that can cut down on noise is better.

Cheers!

[vampwizzard]

I had a similar situation where I dont like footswitches for amp effects... i.e. reverb and trem. Had to source my push pull pots from somewhere else. I do try and get Hoffman parts as BOMs allow... for that reason I wont tell you what supplier has them but search for DPDT *Your Required Rating* pots and theyll come up online pretty easily.

[Tbone55]

I too would like to get all my parts from Doug's store. Since I don't have enough experience with these parts I'm not sure when I can substitute for a different one or which parts would be best. I am curious about some of the more expensive capacitors that are available. Is there really a tone difference? I know this is really a very subjective thing and what I hear and what someone else hears can be completely different. There's so much hype about "vintage" parts it's easy to get sucked into the vortex.

Thanks Kindly.

Cheers!

[Willabe]

Do caps sound different?

http://el34world.com/Forum/index.php?topic=19534.0

And most, if not all, of your push/pull pots that your looking for, I'd just use a switch.

[Tbone55]

Do caps sound different?

http://el34world.com/Forum/index.php?topic=19534.0

And most, if not all, of your push/pull pots that your looking for, I'd just use a switch.

Thanks kindly for the info. I was reading it over and it's really deep and a little beyond my understanding but I certainly saw the benefit of the testing. I did a similar thing with my guitars to compare same value capacitors with different dielectrics from several different brands. This isn't the same comparison since the amp capacitors are dealing with both AC & DC and the guitar capacitors are only dealing with a very small A/C signal voltage. To me, the material used for the caps was what made the sound different to my ears. The differences weren't huge mind you, but you could hear where there was more pronounced bass or treble. I guess I'll just stick to the less expensive ones for now and maybe try changing them later. With respect to the push-pull knobs, the chassis I'm using is already pre-drilled for the circuit that way. Otherwise toggle switches would be a good idea.

I'm working on figuring out why so many 1W resistors are called for in the BOM for this amp. When I converted my Blues Jr the vast majority of the resistors were only 1/2W. Only those that were on the B+ power supply side required 3W rated resistors. I'm going to try and figure out some of the ratings needed using the schematic. First I have to convert the AC into DC as soon as I find the formula for doing so. Does the way the amp is biased make a difference?

Thanks Again. 

[Colas LeGrippa]

PRR got it right. A guitar pot is good as long as you like the knob that fits on it. You can use a split shaft pot and put a brass sleeve over it to use a screwed knob.


Colas

[sluckey]

I'm working on figuring out why so many 1W resistors are called for in the BOM for this amp.

Where is this BOM that specifies 1 watt resistors?

[Tbone55]

I'm working on figuring out why so many 1W resistors are called for in the BOM for this amp.

Where is this BOM that specifies 1 watt resistors?

Here's the BOM. It was part of the builders guide for the amp. I was trying to do the calculations myself but am getting hung up on some of them. I'm not familiar enough with reading the schematics to figure out the proper currents going through the circuit. There are voltages marked on the schematic in various places but using the formula doesn't work out in for each spot. I'm not sure how to calculate the circuit resistance to work out the power. If I understand correctly, you need to work out the total of the actual resistance the load sees by working back from the load. Then you can calculate the total current of the circuit and then the power requirements for each resistor. Maybe I"m just making this more difficult than it should be and there's an easier way. I'll keep reading my vacuum tube amplifier book some more but if you can give me a point in the right direction it would be great.

Thanks Again, Ser Sluckey (you watch game of thrones?)

[sluckey]

The resistors that are spec at 1 watt are all carbon film. I suspect that's what coco had on hand. Just like I might spec 1/2w metal film because that's what I keep on hand. Really has nothing to do with the power dissipated. Many of those 1 watt CFs could safely be replaced with a 1/4w CC.

Historically, it's usual to annotate power requirements only for resistors that need a higher wattage rating. Looking at a schematic, you'll see some resistors that don't specify a power requirement. Most times those resistors would be 1/2w rating. Many times you'll even find a note on the schematic saying something like this... ALL RESISTORS 1/2 WATT UNLESS OTHERWISE SPECIFIED.

So quit knocking yourself out and just buy a set of resistors. If you like 1W CFs then get'm. If you like 1/2W MF, then get those.

If I was gonna keep a full set of one or the other I'd stock up on 1/2W MF. 1W CFs may be built well but I've replaced several 1/2W CFs over the years that the leads had just separated from the resistive film inside the body. Look perfectly good but measure open, or even worse, work when you put it in a certain position, only to fail when you put the chassis back together.

[Tbone55]

Thanks Kindly Sluckey.  :BangHead:I'm really tired of banging my head against the wall. When I compared my Blues Jr build schematic I started to see where the same value of capacitors at the same locations were only 1/2W but they were metal film. I can get metal film in 1W, 2W,3W, and 5W but the 1W are a lot more expensive than the 1/2W. I don't mind buying the metal film capacitors where the higher wattages are needed. Going with metal film 1W capacitors really bumps the price up compared to the 1/2W. Where the wattage requirements are 3W or 5W is there a advantage to using metal film in place of metal oxide or wirewound? The little bit I read seems to indicate metal film are not as noisy. Also, the schematic indicates that there's a max B+ voltage of 400v. There are a lot of the capacitors on the BOM with 630v rating. Is this just being cautious or would 400v rating do just fine?

Thanks Again, Ser.

[Tbone55]

Well, I've completed going through my BOM list and I think I've got it completed. I've replaced most of the 1W CC's with 1/2W Metal Film as was suggested by Sluckey. I should be able to buy a lot of the parts I need from Doug. Just to play it safe I stayed with some 1W in the power section where no power rating was shown on the schematic. There were several others that also didn't have a power rating on the schematic but when I looked at their location on the layout the power rating was shown (2W) so I'll go with that. I have one question. Do the resistors have to be flame proof? I noticed when I was looking up specs that some were and some weren't. Is this a must have?

Thanks All.

[vampwizzard]

Ive never purposefully gotten "flame proof" resistors.. ive just designed the gear to not cause flames during normal operation lol.

But nah.. you should be good without a special "flame proof" resistor

[pdf64]

It is a very good idea to ensure that resistors which may overheat significantly when another parts enters a likely failure mode have a flame proof / retardant coating.
In the Tramp, that might cover R10, 15, 16, 17.

See Aiken http://www.aikenamps.com/index.php/technical-q-a

[Willabe]

It's a good thing to use FP R's for the power supply dropping R's and the power tube screen R's.

[Tbone55]

It is a very good idea to ensure that resistors which may overheat significantly when another parts enters a likely failure mode have a flame proof / retardant coating.
In the Tramp, that might cover R10, 15, 16, 17.

See Aiken http://www.aikenamps.com/index.php/technical-q-a

Thanks kindly for the link. That's a great bit of technical information and although above my level a very worthwhile read. 

[MakerDP]

correct me if I'm wrong guys but I'm pretty sure flame proof resistors are where they are because of regulatory requirements.

[Tbone55]

It's a good thing to use FP R's for the power supply dropping R's and the power tube screen R's.

Thanks. Sound advice.

[Tbone55]

correct me if I'm wrong guys but I'm pretty sure flame proof resistors are where they are because of regulatory requirements.

Would be interesting to find that information. Thanks.

[bmccowan]

Sluckey's experience is interesting:

1W CFs may be built well but I've replaced several 1/2W CFs over the years that the leads had just separated from the resistive film inside the body. Look perfectly good but measure open, or even worse, work when you put it in a certain position, only to fail when you put the chassis back together.

I have used 1 watt CF as my standard resistor for several years - I have chosen those over 1/2 watt because I'm older, have arthritis, and the larger size is easier to work with and easier to read the bands. Now I see that I lucked out on another reason. I do recall some failed new 1/2 W resistors years ago, but figured I had overheated them with the iron. Now trying some Dale MFs (makes me think of the wallet scene from Pulp Fiction) for the first time. No color bands! (oddly some vendors say the exact same Dale resistor is 1/2W and some say they are 1W, and some say it differs if its a military application?)

[sluckey]

I have an amp right now that is full of 1W CF. They are very sturdy compared to 1/2W CFs.

[Willabe]

Now trying some Dale MFs (makes me think of the wallet scene from Pulp Fiction) for the first time. No color bands! (oddly some vendors say the exact same Dale resistor is 1/2W and some say they are 1W, and some say it differs if its a military application?)

I love Dale R's.

I think it's the 65 series I buy. (We moved and all my stuff is in boxes until we buy a house, renting right now.)

They probably de-rate them for mil-spec.

[Tbone55]

I've gone with the 1W CF's rather than 1/2 W. I do have 2W CF's as well that I can use but have decided to go with 3W metal oxide for the power section and at the power tube. Placed my order with Doug yesterday. Couldn't get everything I needed through Doug which is a bit of a PITA. I've been trying to understand the circuitry for this amp but I need to do a lot more reading up. I have a book by EJ Jurich, Vacuum Tube Amplifier Basics. It's a bit difficult for me to follow the schematic and understand what's going on at this point even though I have a basic understanding of what's going on. I don't want to just build something because I can follow directions from a plan.

Thanks Guys.

[MakerDP]

correct me if I'm wrong guys but I'm pretty sure flame proof resistors are where they are because of regulatory requirements.

Would be interesting to find that information. Thanks.

Well I've personally witnessed a Bassman 50 catch fire and then personally replaced the old carbon-comp screen resistors with flame-proof rated resistors so it would not happen again... just sayin...

[shooter]

don't want to just build something because I can follow directions from a plan.

a lot of folk that have been doing electronics forever 
started that way, before girls n cars n rocknroll, I wore out my radio shack 75:1 electronics thing. a couple years with the fore-mentioned distractions I wound up in the NAVY and this course, ~8,000 class hours later, I made it here 


http://www.fcctests.com/neets/neets.htm

[sluckey]

I have an amp right now that is full of 1W CF. They are very sturdy compared to 1/2W CFs.

I should mention that I would never use these 1W CFs in my builds. They don't fit into my layout scheme. Forget about trying to stuff another lead in the same turret hole with a 1W CF. I would have to change my whole board layout scheme to use them. A major reason I chose 1/2W MF is the skinny size of the leads. I can easily put four leads in one turret and that suits my board layouts quite well. Like this...

[MakerDP]

Totally agree with Slucky on that. I bought a "Joe Knows" 1/2W MF resistor kit off Amazon a few years ago. Comes in nice ziplok bags, labeled clearly, in a convenient cardboard box, came with 10 of each value. Now before I do a build I count-up what I need and if I don't have enough on-hand or it takes my quantity down to less than 4 or so (more depending on the value - like 100k) I just reorder 10 or 20 new 1/2W MFs with my parts order to replenish my supply.

It's kinda a pain to use resistors larger than you need ... more expensive, bigger leads... everything he said. 

[Tbone55]

don't want to just build something because I can follow directions from a plan.

a lot of folk that have been doing electronics forever 
started that way, before girls n cars n rocknroll, I wore out my radio shack 75:1 electronics thing. a couple years with the fore-mentioned distractions I wound up in the NAVY and this course, ~8,000 class hours later, I made it here 

WOW! That's a lot of hours! I took some electronic courses back in high school and really enjoyed them but that was a long time ago. I did build a radio shack tube radio and some other electronic devices. Wish I still had them. Glad to have folks like yourself on board here. Your knowledge and help is greatly appreciated. Us newbies wouldn't get anywhere without your help.

Cheers!


http://www.fcctests.com/neets/neets.htm

[Tbone55]

I have an amp right now that is full of 1W CF. They are very sturdy compared to 1/2W CFs.

I should mention that I would never use these 1W CFs in my builds. They don't fit into my layout scheme. Forget about trying to stuff another lead in the same turret hole with a 1W CF. I would have to change my whole board layout scheme to use them. A major reason I chose 1/2W MF is the skinny size of the leads. I can easily put four leads in one turret and that suits my board layouts quite well. Like this...

Yes, I can see why. The board I'm using has eyelets rather than turrets so I shouldn't have fitment problems. I would have preferred a turret board as it easier to use IMO. Your set up looks amazing! Now that's pro grade work space.