Low Plate Classic Layout

[ChrisM]

Old data sheet's state that the second triode (6,7,8.) is lower noise than the first (1,2,3) so that's why many use the second triode as the "first".

[David Root]

Well, that explains why a lot of the old stuff was done that way, however I have yet to see that in a tube data sheet. You have an example handy?

Why would that be so? Anything to do with the parallel heater wiring?

[ampdork]

Hey Dave,

Check this data sheet for mention of it:
http://www.drtube.com/datasheets/ecc83-mazda1963.pdf

the quote: " When two the two sections are used in cascade the section connected to pins 6,7 and 8 should be as the first stage for best hum performance."

Seen it in other datasheets too but could never recall which ones.

[talbany]

Shad beat me to it..

Cool..
I can't tell any difference been there!!.. Maybe in HiFi
Didn't Fishers wrecks use 678 for V1a but wired V2 filaments out of phase...


Tony

[ampdork]

Yeah I have done it both ways and never had any reason to think it did much.
I just like the open spot to be at the rear....makes it easier for me to slot in tubes blind.

[David Root]

Thanx guys! I usually do it that way, because that 's what I saw in later HAD amps. His earlier ones weren't like that, some used 123 first, eg ODSR#13.

Never thought about that, it's easier to get tubes back in that's for sure!

[David Root]

"Didn't Fishers wrecks use 678 for V1a but wired V2 filaments out of phase..."


I know Fender didn't worry about wiring heaters in phase, but to do it deliberately? Presumably to get a hum canceling effect? No comprende, please expand on that.

[talbany]

Dave

I can't imagine it making any difference. The idea to run them in parallel is apparently that somehow the hum from the heater AC would "cancel." Except that would mean that both sides of the tube were in the same phase relationship at the same signal level with the same number of stages after them. Not likely in our case!!
Here is what I know
Normally hum induced through the heater phasing is not an issue in power stages, because the gain from the cathode to the speaker is so low, and a power tube cathode is always grounded with respect to AC signals.

12AX7s are in effect humbucking in that they have a spiral wound filament that aims to cancel out magnetically induced hum. Most NOS and current production 2a*7 tubes have spiral wound filiments

You can also get electrostatic induced hum, from the filament voltage capacitively coupling to other tube electrodes, or the filament actually emitting electrons itself, as opposed to the filament current inductively coupling into other wiring.
This electrostatic and filament emission hum is what the humbucking pot nulls out, and what you lose when wiring 12A*7s for 12V heater supply, because the hum cancellation needs the voltage across *each* filament to be balanced with respect to ground,

Floating the whole heater system a few tens of volts positive cures the filament emission problem but won't affect the electrostatic one.

Hope This Helps
Tony

[David Root]

Thanx Tony, that makes sense. As usual, we MI amp folks don't follow the Ancients' rules but it doesn't make a lot of difference because we don't need the total noise floor that they did for Hi Fi and we don't use matched pairs of stages.

I cannot recall any hum difference using 678 first vs 123 first either.

I have found that just splitting grounds into the power section and everything else pretty much like HAD did, along with paralleling the heaters always in phase makes for a dead silent amp. The last amp I finished had a little hiss, which puzzled me until I remembered I used carbon comps in the OD section!

When it gets complex, reverb, tremelo etc I like to use regulated 12VDC, or with octal preamps especially 6SL7, in high gain stages.

[talbany]

I have found that just splitting grounds into the power section and everything else pretty much like HAD did, along with paralleling the heaters always in phase makes for a dead silent amp.

I couldn't agree more.. I fly all the AC follow Dumbles ground scheme and keep the layout as neat as possible and you will need to look at the light on the foot switch to see if your in OD..

Tony

[les_guitar]

OK On with the show..

Attached is now a low plate Classic layout modified from the 124 layout I did.. AFAIK this was 124 as it was originally manufactured in 1984 before the skyline update..

To avoid any confusion on the pot tapers I labeled them by part number.. Since the Fender (CTS) were different tapers (10%-30% and the 2-35) According to my notes the 2-35 imples 2%-taper at 10%-rotation..(You must have the standard pot at "8" on a Fender knob to egual "5-6" on this pot)


you can do your own research on the part # or discuss away!!
If you have any questions I'll try to answer them..

For those looking for the Farris, Talk To Your Daughter, Carlos Rios type tones should look into this type Low Plate Classic.. Best of Luck and once again

Hope This Helps!!

Tony

Off topic a bit but what software do you guys use to so schematic layouts?

I use a Mac btw

TIA

[talbany]

Off topic a bit but what software do you guys use to so schematic layouts?

I use Visio for layouts and AutoCAD Electrical for Schematics..You can do both in Visio

Good Luck!!

Tony

[les_guitar]

Off topic a bit but what software do you guys use to so schematic layouts?

I use Visio for layouts and AutoCAD Electrical for Schematics..You can do both in Visio

Good Luck!!

Tony

Thanks. Visio is Windows only...

Anybody use OmniGraffle?

[M Fowler]

Ask Dave (colossal) I forget the name of his mac software for schematics.

PM colossal for the answer.

Mark