Can I expect the B+ to drop enough for 2-6V6

[nandrewjackson]

Hello all,


I've got a new build underway, a jcm800/2204 with one tube reverb and 2-6V6 push pull cathode bias.


My inspiration for this one is the Hoffman/Sluckey 6V6 plexi.


I'm using a classic tone power transformer  40-18066 , it's advertised as 660V secondary, 40 watt PT.


I've got the heater leads connected to 2-100 ohm resistors to ground.


I've got the transformer and diodes mounted, so I went ahead and measured my DC at the diodes. With a single 22UF and a 220K resistor to ground I'm getting 502VDC.


With a full load of 4-12ax7s, and 2-6V6s, can I expect the VDC to drop into "safe" territory for the 6V6s?

[nandrewjackson]

Here's the sheet for the classic tone transformer

[tubeswell]

502 seems a little high, but it will drop when its loaded.


If they are JJ6V6S, they should handle the voltages (if they're biased at 14W)


If you want to lower the B+ further, consider a tube rectifier

[Latole]

502 seems a little high, but it will drop when its loaded.


If they are JJ6V6S, they should handle the voltages (if they're biased at 14W)


If you want to lower the B+ further, consider a tube rectifier

I  read 6V6S JJ maximum voltage 500 volts

https://www.jj-electronic.com/en/6v6s

[Latole]

I often built amps from scratch.
When B+voltage are too high I use zener diodes on PT secondary high voltage center tap.
I use few 5 watts diodes like 1N5349 ( 12 volts ) or 1N5347 ( 10 volts). Cheaper and easier to install than a 50 watts 40 volts diode

https://www.tedweber.com/v-dump

https://el34world.com/Forum/index.php?topic=17595.0

[nandrewjackson]

I thought about something like this.  A half power switch. I chose the red knob twin reverb as example because I've been inside one of them and know it fairly well. 


On the red knob twin it goes from 469 VDC down to 236 VDC.


I've got a couple different zeners here I could try. 4 volt and 6 volt types.


I've got a pair of genalex gold lions, their data sheet says 450 limit.


The place I ordered the PT from was out of the classic tone 22watt PT, and the 40 watt was basically the same price,  so I ordered it.

[bmccowan]

There is something confused here. I think the 40-18066 is a 20W PT and would be fine for your project. Wrong model #?

[labb]

There is something confused here. I think the 40-18066 is a 20W PT and would be fine for your project. Wrong model #?

Right on. I have that transformer in a Deluxe Lite and I have 429 VDC at B1.

[pdf64]

Regarding the high/low power switching, as you know from the Fender, a dpdt switch is needed, as the magnitude of the bias voltage needs to be reduced in the low power mode.

[66Strat]

That power transformer was designed to be used with a tube rectifier. Using diodes for rectification will result in higher voltages. You might try inserting some resistance between the secondary windings of the power transformer and the diodes to emulate the voltage drop of a tube rectifier. I would try 750 ohms as a starting point and then go higher or lower depending upon the results and your target voltage.

[sluckey]

I thought about something like this.  A half power switch. I chose the red knob twin reverb as example because I've been inside one of them and know it fairly well. 

You cannot use that circuit with the 40-18066. Full power would produce a B+ of 930V and half power would be 466V.

[nandrewjackson]

 


900 V
Good to know




Looks like I've got this all back-asswards.


I'll move forward with it like this, and *possibly * add zeners, but apparently I can't even get the model number straight 

[nandrewjackson]

And,


THANKS everyone!


Seriously. 


 

[ac427v]

So which transformer do you have?Some of the Classictones have dual secondaries which allow switching between high voltage and medium voltage. With that feature you can do all kinds of cool things without needing zeners. You could use full wave sand diode rectification or a tube rectifier. But you still can't do the the half power circuit with a bridge rectifier like the red knob twin. More info needed.

[sluckey]

If your main concern is to get proper voltage for 2x6V6 operating with cathode bias then buy a proper PT so you don't have to worry about B+ being too high.

[DummyLoad]

http://www.classictone.net/40-18085.pdf

the PT in the link above includes two B+ secondaries (660VCT & 550VCT) that was specifically made for champ B+ levels with either valve rectifier or SS rectifiers.

use that PT's red/white winding (55VCT taps) for champ with 1N4007 and B+ 6V6 plate supply in the 350-360V range. 

--pete

[nandrewjackson]

I apologize for this thread, I posted not knowing all the details about what I was about to jump into.




I thank you all for responses  and tips, but I've got it worked out.


I do indeed have the 40-18066 classic tone  for 20W builds.


I have this project up and running, I'll post a build thread in a minute, GOD willing.


When I posted this topic, it was a stressful time. I didn't know which way was up and really didn't need to be speculating about my plate voltages being too high.


Thanks, noah.

[bmccowan]

Hey,
No worries. It's a stressful time for all. It seems to manifest itself in weird ways. I have a wife and two dogs. Thank God for those dogs. Just kidding, mostly!
Love to hear how your project works out.

[jojokeo]

That power transformer was designed to be used with a tube rectifier. Using diodes for rectification will result in higher voltages. You might try inserting some resistance between the secondary windings of the power transformer and the diodes to emulate the voltage drop of a tube rectifier. I would try 750 ohms as a starting point and then go higher or lower depending upon the results and your target voltage.

corrected drawing...

[66Strat]

That power transformer was designed to be used with a tube rectifier. Using diodes for rectification will result in higher voltages. You might try inserting some resistance between the secondary windings of the power transformer and the diodes to emulate the voltage drop of a tube rectifier. I would try 750 ohms as a starting point and then go higher or lower depending upon the results and your target voltage.

corrected drawing...

Your revisions do reflect the correct diode anode and cathode polarity markings.

I'm used to following Fender schematics where the -/+ markings reflect voltage. AC applied to the anode of the diode result in a positive DC voltage at the cathode (+).

https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_ab165_schem.pdf

I can see where my methodology is technically incorrect. I think that I will refrain from using -/+ notations on diodes in the future.

[jojokeo]

I can see where my methodology is technically incorrect. I think that I will refrain from using -/+ notations on diodes in the future.

Everybody makes mistakes including Leo Fender when alive. At least the diodes are drawn and oriented correctly. If the circuit were powered you wouldn't measure a negative voltage anywhere along that string.


It is perfectly appropriate to mark components properly with regards to polarity such as electrolytic caps but diodes are not necessary to do so since the polarity is already designated by the symbol of the drawing and adding polarity markings is redundant.