It is currently Tue Mar 17, 2020 3:29 am

All times are UTC - 7 hours



Post new topic Reply to topic  [ 6 posts ] 
Author Message
Post subject: BIASING an HOT ROD deville 212 (200X with PCB)
Posted: Mon Dec 26, 2011 12:57 pm
Offline
Hobbyist
Hobbyist

Joined: Mon Dec 26, 2011 12:28 pm
Posts: 2
Hi folks.
I hope someone can help me, I'm trying to make a correct bias on this amp, I've found a very usefull 1 ohm resistor placed betweeen the cathodes of the power tubes and ground (This is a standard way of biasing, and I'm happy this amp brings this on the pcb).

On second thought, the should be 2 resistors, one for each tube... never mind...

Usually when I found Schematis they're not so detailed, in this one said about this resistor (readings : 60mV), but as the cathodes are in a PCB and wired together, this resistor can only make a "semi" bias, cause it measures the two cathode currents together (Kirschoff nodes laws applied here).

Fender specifies 60mV ( in a 1 ohm resistor this is 60mA, divided by 2, then 30mA, multiplied by the B+ line (498V) = 14,94W plate dissipation, I'm using 6L6GC by JJ in a lot of forums I've found that 23w is the max power dissipation for a similar tube. Has anyone tried to bias this kind of amp before?, I've found that giving 48mA (24mA theoretically to each tube) has a 498*0,24 = 11,29W, lower that Fender indication, tomorrow i will put a sinusoidal signal and check the crossover to chech, but anyways.... sounds OK!! But really confused about this way of biasing.


Top
Profile
Fender Play Winter Sale 2020
Post subject: Re: BIASING an HOT ROD deville 212 (200X with PCB)
Posted: Mon Dec 26, 2011 1:51 pm
Offline
Rock Icon
Rock Icon
User avatar

Joined: Tue Jun 29, 2010 1:20 pm
Posts: 9640
Location: Indiana
Hi rafa1977,

Seems that you have a good grasp of it, that's the way that it is done. The test point is Cathode current, the Plate current will be lower by about 5 mA per tube.

If you want to be accurate, it's the Plate current used for calculations, not Cathode, and also, use the Plate voltage measured at pin 3 of the output tube as the B+ will be a little higher than the Plate.

This test point is typical of many Fender amps, some do have two points, one for each tube, and most have only one test point. The spec on the schematic isn't a hard and fast rule for setting. Typically set the bias between 50% and 70% of the tubes Plate rating.

A 6L6GC is a 30 watt tube. A 6L6GB or 5881 is 23 watt tube.

_________________
---> "The amp should be SWITCHED OFF AND UNPLUGGED before you do this!" <---

Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.


Top
Profile
Post subject: Re: BIASING an HOT ROD deville 212 (200X with PCB)
Posted: Mon Dec 26, 2011 5:02 pm
Offline
Rock Star
Rock Star

Joined: Mon Apr 05, 2010 2:37 pm
Posts: 8708
Location: Natural Bridge, Virginia
rafa1977,

Watch this video:

http://www.eurotubes.com/euro-Fender-HRDV.htm

and read this:

http://www.justinholton.com/hotrod/bias.html

_________________
Bill

Image


Top
Profile
Post subject: Re: BIASING an HOT ROD deville 212 (200X with PCB)
Posted: Tue Dec 27, 2011 9:49 am
Offline
Hobbyist
Hobbyist

Joined: Mon Dec 26, 2011 12:28 pm
Posts: 2
Thanks, the links has been very usefull.

Biased according to the links and your suggestions.

Also performed a crossover test with a signal generator and an oscilloscope... with no dummy load (as It should be done) the results are very good (damn difficult to get the oscilloscope to trigger properly, it's an analog one). Very reasonable transition and very reasonable waveform too.

The reason for biasing was that the previous owner "almost" blows the power tubes, the tubes started to make some "glitches" and go from normal color to a almost blue... in miliseconds!! Checking stage by stage finally it was motorboating due to a bad tube in the phase inverter section of the power amp.

Once located the malfunctioning tube (wrong model I must add, the owner thought a very high gain tube here will give more dist... and I seriously doubt that, the phase inverters should be working well in their lineal stage, obviously some true might come of this or the swap for an appropiate 12ax7A should't have worked or so is my guess)

Later, cleaned some contacts here and there, checked for "cold solderings", checked the screen resistors, and Ok, power on and ... no more motorboating!! biasing the old tubes was the next step, they were too high biased,(I have a new set just in case), I know that these tubes won't last long, but well, let's take advantage of their time left, biased at 34mA each, (68mV readings) they should.

BTW, the pin 3 readings are 495v/ 496v, i wasn't reading the B+ directly from the power supply.

So, 496*0.034 = 16,8 W (more or less a few miliamps, 70% of 30w is 21w) I didn't want this amp to run hot, the tubes have been abused "a bit".


Thanks again!
PD: English is not my native language, I'm spanish (from souther Spain), so expect some mistakes on grammar and spelling.


Top
Profile
Post subject: Re: BIASING an HOT ROD deville 212 (200X with PCB)
Posted: Tue Dec 27, 2011 10:22 am
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
You may also find that power output tubes of the same model and even same brand (& year made)--- pass varying amounts of current. Since, those 6L6GC were abused by the previous owner you may not get much more than 15 idle watts/tube out of them. And they may sound fine, at that setting.

Whereas, an NOS RCA 6L6GC black plate or 1990's made Svetlana SV-6L6GC may sound horrible at 15 watts idle output.


Top
Profile
Post subject: Re: BIASING an HOT ROD deville 212 (200X with PCB)
Posted: Tue Dec 27, 2011 11:39 am
Offline
Rock Star
Rock Star

Joined: Mon Apr 05, 2010 2:37 pm
Posts: 8708
Location: Natural Bridge, Virginia
rafa1977 wrote:
Thanks, the links has been very usefull.

Biased according to the links and your suggestions.


Good job. :D

_________________
Bill

Image


Top
Profile
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 6 posts ] 

All times are UTC - 7 hours

Fender Play Winter Sale 2020

Who is online

Users browsing this forum: No registered users and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum

Search for:
Jump to: