Starting a new build - need help on a few things...

[Tonegeek]

I am starting another Dumble inspired build. It will be a 50 watter.

I need a bit of help sourcing certain parts. Heres the list:
screwdriver adj pot for bias (value to be determined - see below
suggestions on better quality trim pots than the Mouser BI brand I was using
where to get 1 ohm 1% 2W resistor (to measure for adj. bias)
export model 12.6v CT tranny enclosed (or just a domestic one with the bells)
keps nuts

Faceplate blanks:
I want to move beyond the Kinko faceplate labels to something more robust (although mine looks great, even after a year). I have access to a 60 watt laser etching machine. I am not sure this machine can also cut the part out but if it cannot, then I will need to source some blank aluminum for faceplates. If they were cut to size and anodized, that would be perfect. Otherwise I will probably anodize the plates and then etch. If anyone has done this before, i would like to hear about it. Sure would save some hassle and money if I could cut the shape and etch at the same time. I will experiment with plastic and wood also but since i use Alpha pots I need to keep the thickness down as the Alpha shafts are rather short.

Bias supply:
Anybody got a working one adjuster bias supply that will cover most tube types (mostly EL-34, 6L6 and 6550's) in a 50 watter? I want the option to try different tubes. Only caveat is it must work off 345volts (Marshall 50 watt tranny has no bias tap) I can design one, but no need to reinvent the wheel if someone has already been there. I am feeding 2 -200k resistors as seen in most Dumble schematics. I would like to use the smallest value pot that would cover the range needed. My current setup is using 2 adjusters and I am simplifying that to one.

I think I have the rest of the stuff I need for this build covered. Seems to me the face/back plates will be the biggest challenge. I really want it to look and sound good.

thanks,
Whit

[ayan]

I am starting another Dumble inspired build. It will be a 50 watter.

I need a bit of help sourcing certain parts. Heres the list:
screwdriver adj pot for bias (value to be determined - see below
suggestions on better quality trim pots than the Mouser BI brand I was using
where to get 1 ohm 1% 2W resistor (to measure for adj. bias)
export model 12.6v CT tranny enclosed (or just a domestic one with the bells)
keps nuts

Faceplate blanks:
I want to move beyond the Kinko faceplate labels to something more robust (although mine looks great, even after a year). I have access to a 60 watt laser etching machine. I am not sure this machine can also cut the part out but if it cannot, then I will need to source some blank aluminum for faceplates. If they were cut to size and anodized, that would be perfect. Otherwise I will probably anodize the plates and then etch. If anyone has done this before, i would like to hear about it. Sure would save some hassle and money if I could cut the shape and etch at the same time. I will experiment with plastic and wood also but since i use Alpha pots I need to keep the thickness down as the Alpha shafts are rather short.

Bias supply:
Anybody got a working one adjuster bias supply that will cover most tube types (mostly EL-34, 6L6 and 6550's) in a 50 watter? I want the option to try different tubes. Only caveat is it must work off 345volts (Marshall 50 watt tranny has no bias tap) I can design one, but no need to reinvent the wheel if someone has already been there. I am feeding 2 -200k resistors as seen in most Dumble schematics. I would like to use the smallest value pot that would cover the range needed. My current setup is using 2 adjusters and I am simplifying that to one.

I think I have the rest of the stuff I need for this build covered. Seems to me the face/back plates will be the biggest challenge. I really want it to look and sound good.

thanks,
Whit

1. CE/Antique sells a slotted 10K bias pot. You can always scale just about any bias supply such that 10K gives you the range you need.

2. See the attachment for how to derive a bias supply from the B+ winding. There are numerous examples of this.

3. I have mostly used Piher trimmers from Mouser. They are OK, not great... Vishay also has some that are decent quality.

4. Why a 1% resistor to emasure the bias? With a single control for two tubes, you will always be compromising between the two. I use 5% on all mine... We're measuring onthe order of say 30-40mV (across the resistor), so the current may be off by... 2mA at most. That's about as good your tubes will be matched, IMHO.

5. You can buy keps nuts from a number of on-line suppliers. The killer with hardware is always shipping... you may be needing $5 worth of nuts and bolts and pay $8.99 in shipping. Sorry, I don't have a decent answer for this.

Cheers,

Gil

[Guitarman18]

As a newbie I’m not going to disagree with anything that ayan has said, however I went for these 1 ohm 1% 2 watters : Mouser ~71-CPF2-F-1/R

http://www.mouser.com/Search/ProductDet ... ZqtA%3d%3d

[ayan]

As a newbie I’m not going to disagree with anything that ayan has said, however I went for these 1 ohm 1% 2 watters : Mouser ~71-CPF2-F-1/R

http://www.mouser.com/Search/ProductDet ... ZqtA%3d%3d

You should, and I mean that! I just happen to use 2W, MO 5% resistors in my amps, that's all.

Gil

[Tonegeek]

As a newbie I’m not going to disagree with anything that ayan has said, however I went for these 1 ohm 1% 2 watters : Mouser ~71-CPF2-F-1/R

http://www.mouser.com/Search/ProductDet ... ZqtA%3d%3d

You should, and I mean that! I just happen to use 2W, MO 5% resistors in my amps, that's all.

Gil

Kinda hard to argue with Gil's math. I suppose if you were setting bias to run your tubes at the maximum output AND your resistors (1 on each tube) were 5% out of spec the wrong way (giving you less than one ohm) AND your meter was a bit off (reading low) AND you had crappy tubes that could not meet published specs, THEN that extra 2-3 mv might cause a redplate.

Since Gil got me thinking about this it gave me an idea. I have a bag of 10 1 ohm 5% 2 watt resistors (that I bought before I got some 1% ones at a local store). I can at least measure and find 2 resistors that come close to matching each other. At least this way I will still know that the current draw relative to each tube is pretty accurate. The total current draw may still be off a bit but if I choose resistors that are slightly above 1 ohm, then I will have a built in margin of safety. (I am assuming my ohm meter resolution is accurate enough for comparing these small value resistors but not sure about that.) I am hijacking my own thread but that's fine. I would like to hear more about this, (especially if my logic is screwed).

[ayan]

Since Gil got me thinking about this it gave me an idea. I have a bag of 10 1 ohm 5% 2 watt resistors (that I bought before I got some 1% ones at a local store). I can at least measure and find 2 resistors that come close to matching each other. At least this way I will still know that the current draw relative to each tube is pretty accurate.

Just one word of caution: some (cheaper) multi meter have a hard time reading low resistance values. Sometimes you may be better off measuring a resistor of a higher value, say 100 ohms first. Then add the 1 ohm in series and measure that... Compare to what you measure using the 1 ohm resistor alone. If your meter is good, you will be right on the money; otherwise, you will be significantly off.

Gil

[Tonegeek]

Since Gil got me thinking about this it gave me an idea. I have a bag of 10 1 ohm 5% 2 watt resistors (that I bought before I got some 1% ones at a local store). I can at least measure and find 2 resistors that come close to matching each other. At least this way I will still know that the current draw relative to each tube is pretty accurate.

Just one word of caution: some (cheaper) multi meter have a hard time reading low resistance values. Sometimes you may be better off measuring a resistor of a higher value, say 100 ohms first. Then add the 1 ohm in series and measure that... Compare to what you measure using the 1 ohm resistor alone. If your meter is good, you will be right on the money; otherwise, you will be significantly off.

Gil

Good point. I have a digital Fluke but it is almost 18 years old so i think I will be looking for a new one soon. I need better capacitance range anyway to test those pesky ceramics.

[glasman]

Not sure if you have ACE hardware stores in your area. But they are great sources for hardware.

that not being the case, McMaster is THE place. all sizes, lengths etc etc etc. But you will have to pay shipping.

Gary

[Funkalicousgroove]

My ESR Meter works much better than my multimeter for measuring 1 ohm resistors.

Look in your local phone book for the hardware, I have a place here in Denver called Timberline Fasteners, they have a $50 min, but I get 4x/$ what I get from McMaster before shipping.

[Tonegeek]

I forgot about ACE - they are 5 minutes from my house.

Still working on faceplate ideas and I am debating the trimmers. Should I use cermet or carbon? I am looking at Piher (10mm) since Gil suggested it and I just want the right ones for tone. I have some Bourns cermet on hand which would actually fit my layout better than the Pihers, but Mouser does not have a Bourns in 250k so I will have to redo my layout a bit anyway. The BI trimmers I used in my last build had long leads so they fit in a tight spot better than some, but now I am all about the best for tone. Also, the BI's seemed a little flimsy since they are so small.