Carling progressive toggle switch

[sluckey]

Doug sells a special 'progressive' toggle switch. This 3 position toggle switch can be used as a combination Power and Standby switch. See this link...

http://hoffmanamps.com/MyStore/catalog/parts7.htm  - Thanks, I added the page link Steve :)

http://el34world.com/charts/SwitchDPST.htm

Here's another option for that switch that you may find useful. Maybe you would like to use a 'sag' resistor or otherwise just drop the B+ voltage in your amp. Use this switch with a dropping resistor mounted directly to the switch to do just that. The "OFF' position is Standby mode. The center position selects the sag resistor to provide a lower B+, and the "ON" position provides full B+.

In the attached schematic let's say the B+ current is 100mA. Using a 250Ω/10W resistor will lower the B+ by 25V. The actual power dissipated by the 250Ω resistor would be 2.5W.

[terminalgs]

very cool! 

[Fresh_Start]

Neat idea!  I bought some of those switches a long time ago before Doug carried them and still have a few.

Question:  would there be any tonal/feel difference if you added the sag resistor between the plate node and the screen grid node instead of between the rectifier and the plate?  IOW drop idle voltage and and increase sag only downstream from the plate(s). In your example, put the sag resistor between the choke and the screen grid supply. No sag from a choke...

Cheers,

Chip

[HotBluePlates]

Question:  would there be any tonal/feel difference if you added the sag resistor between the plate node and the screen grid node instead of between the rectifier and the plate?

Yes, ask the folks who have replaced a resistor between those points in their power supply with a choke (on things like a tweed Deluxe, etc.).


But you should realize the current variation of the screens/preamp is very much smaller than the current variation of the output tube plates. It would take a lot more resistance to create sag.


If you don't want to create sag by way of of supply voltage variation due to output tube plate current variation, high-value screen resistors is an alternative. Switching low-value to high value screen resistance (say 100-470Ω to 2.7k-3.3kΩ) can be achieved with an average double-throw switch (with as many poles as there are output tubes to switch).

[kagliostro]

Very COOL Steve

K

[Fresh_Start]

HBP's observations bring up an interesting possibility. What if you want switchable power amp sag without changing voltages downstream for the PI and preamp?  Keep your normal screen grid resistors, one going to each screen. Put your switchable sag resistor between the junction of these two and B+. One switch pole, creates sag in power tubes, and does not impact other voltages in the amp.

Just a thought. Sorry I can't do a drawing right now, but I just moved and everything is in boxes including my computer power cord (doah!).

Cheers,

Chip

[sluckey]

I'd like to see a drawing when you get a chance.

[EL34]

Thanks Steve


Here's the page link for that switch guys
http://hoffmanamps.com/MyStore/catalog/parts7.htm

[Fresh_Start]

I'd like to see a drawing when you get a chance.

Don't know if this is useful or not, but here's what I was thinking about.  This was the first SCH file I could lay my hands on to edit.  Need to calculate power rating of the 3.3K resistor depending on potential current draw from screen grids.  3 watts might not be sufficient.

The "sag" setting would be with the switch toggle in the middle position (i.e. Pole 2 closed and Pole 1 open).

Cheers,

Chip

[sluckey]

That has some interesting possibilities. Can you try it and give us your opinion?

[kagliostro]

Mesa Boogie do the same (only uses a pair of resistors instead of one and a DPST switch instead of a single resistor and SPST Switch)
K

[PRR]

Also works as a tractor heater fan Off/Low/High switch:

[EL34]

Nice paul and thanks for the order!


You always find cool things to do with the parts
I like the Glass epoxy board being used to patch holes in fuse panels
I use scraps to prop up deck post and other wet location stuff :)

[Ed_Chambley]

I have very recently done the switch to drop the voltage in the AC15 build.  What I have noticed with having the switch you do not increase sag, at least not evident to the ear. I use the same method in a 1987 Marshall Build and I got a very noticeable sag.  I believe it works well as a sag resistor when you have SS rectifier.


What I have found is increasing the screen resistors in tube rectifier can get sag.  Actually I have a setup on a Super Reverb that gives me 3 different screen resistances for different volumes.  If I am playing at lower volume I found a 4.7k to work nicely.  That would be just where the amp begins to get its tone.  It is actually cool.  Then I can switch to 3.3k for club volumes and back to stock when wide open opportunity presents itself.


Since doing it on the AC15 and I did get a smoother breakup and more noticeable harmonics, I kept experimenting and found 270ohm is about as far as I wanted to take it.  It changed the amp and stays between 305 to 310 according to wall voltage change.  I tried increasing the screen resistors and not much noticeable difference.  I believe this to be because it is only 15 watts and it is easy to get it drawing current.


I found both of these modifications very valuable and each has its place except for on the Marshall.  Both worked very well and made dramatic changes in sag and tone.  I am able to go from that killer Marshall crunch to a very creamy sag laden bluesy tone.  It actually makes it respond better to single coils.


Doug mentioned he uses 3.9k screen resistors which by itself is very nice and seems to brown it up quite a bit.  Add a sag resistor to drop about 40 to 45 volts and it is a different amp all together. Impossible to describe, but lazy comes to mind.  Slow to respond which is unexpected from a Marshall.

[dude]

What I have found is increasing the screen resistors in tube rectifier can get sag.  Actually I have a setup on a Super Reverb that gives me 3 different screen resistances for different volumes.  If I am playing at lower volume I found a 4.7k to work nicely.  That would be just where the amp begins to get its tone.  It is actually cool.  Then I can switch to 3.3k for club volumes and back to stock when wide open opportunity presents itself.

Ed, is there much tonal differences in using a sag R off the S/B vs larger screen grid R,  with SS rectifier, 18 watt style amp with 6V6, 320 B+?

Thanks, al

[dude]

Thanks, I'll have to get some of those switches. They go great with a VVR using the stand/by hole too.

I never noticed them in Doug's parts, I just order a week ago but as soon as I need something else I'll get a few.

Thanks as always,
al

PS: is the spell checker gone with the new format? (anyone)

[sluckey]

PS: is the spell checker gone with the new format? (anyone)

Spell check is a feature of your browser, not part of the forum software.

[EL34]

PS: is the spell checker gone with the new format? (anyone)

Are you using Firefox?
If so, the spell check will not work.  They say it is part of the browser, but it would work on other forums and not this one.  I made a script to get it to work.
I am not going to say anything else about this other than it is isolated to this website and firefox.  How bout them taters?

I thought we had already been through this before?
Speel checker works just fine on firefox
I am using firefox right now

See the word speel right boave
How about the word above (boave)?

See the words uunderlined in red?
Ooops, I spelled underlined wrong also

Look at my screen shot below of this very post and how firefox is checking the spelling

Clear your cache, uninstall Firefox and go get the most recent version

[PRR]

> cool things to do with the parts

Fan blows.

Off/Low/High works as expected.

Took a lot of grease to get the squeal out of this 40+ year old fan.

Boring tech notes:

The lowest-R section of a generic GM heater resistor gives a nice medium speed. (This would be Med-Hi on the GM fan, so my fan must be more powerful.) Current drain without heater core is 5.5A on Hi, 3.5A on Low.

Outlet air velocity is 1,800 fpm (compare with 700 fpm in home hot-air heating). Duct size gives about 120 CFM. Low speed is about 1,200 fpm and 80 CFM. If I can get 100 deg rise in the heater core (another long story), output is above 10,000 BTUh. With 200 square feet of steel and glass in the cab, if I can stuff the gaps, I can hold 50 degree rise inside. Which would be short-sleeve shirt in freezing weather.

Resistor bracket is a Simpson joist hanger. You can see the fan is running, blades are blurred.

[eleventeen]

I like the Simpson strong-tie bracket!

[Willabe]

Resistor bracket is a Simpson joist hanger.

Very clever re-purposed use.


                  Brad