Voltage measurements on 2 6V6 parallel CHAMP...

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7/19/1999 1:35 PM

fj

Voltage measurements on 2 6V6 parallel CHAMP... I've installed a HAMMOND 125E OT in my homebrew "SUPER-CHAMP"...running 2 6V6's in parallel...1 12AX7 and a MULLARD GZ-32 (5V4) rectifierVOL pot = 1 meg treble pot = 250 K...BASS/MID pot = 1 meg...measurements I get are as follows ;...power tubes ...pin # 3 (X2) =350V...pin # 4 = 329V...pin # 8 = 22.5V...IS THE 22.5 TIMES by 2 to get 45ma?...amp sounds VERY HOT...but this is a "good thing" when using RAT for LEAD-WORK...am I correct (in-the-ball-park) with these figures?...thanx in advance Frank

 

7/19/1999 2:36 PM

Doc

If you're reading 22.5v across a 250 ohm cathode resistor, which is common to both 6V6 cathodes (pin 8 of each tube connected to the single resistor), then yes, that would indicate a total 90ma current for both tubes, or approximately half of that, 45ma, running through each 6V6 if they are nearly matched.         Did you follow the plans for the parallel 6V6 amp over on the Angela Instruments site?

 

7/19/1999 9:36 PM
fj	It's sort of a collaboration on the ANGELA theme...I've added some minor changes...

 

7/19/1999 11:52 PM

MBSetzer

Hi FJ & All:         Looks like you are doing the right thing, is this in a Fender Champ original chassis? IIRC, the 125E should have an 8ohm load at secondary terminals 1 & 6 when the parallel 6V6's are connected with plates & ground between the brown & blue primaries. The red B+ for push-pull use would be taped back & insulated for this application.         Then if you remove one tube for half-power, you would need to move the speaker to terminals 1 & 5 for an 8ohm load. Or use a 16ohm load on 1 & 6, since if you double the impedance on one side of the OPT, you need to double the other side.         Hope this helps,     Best Regards,     MBSetzer

 

7/20/1999 0:19 AM
tboy	quote: "..if you remove one tube for half-power.." Not a good idea for a single-ended amp with cathode bias. The cathode resistor would need to be increased to maintain proper bias (but I'm sure you knew that .         --tboy

 

7/20/1999 1:02 AM

MBSetzer

Woops -         Thanks for the reminder tboy!         It's hard to think of all the implications sometimes. I have gotten accustomed to using separate cathode resistors on parallel tubes for just this purpose!         On a cathode biased parallel single-end amplifier, if you have individual cathode resistors for each tube, THEN the only adjustment necessary when you remove one tube is to make sure to also double the speaker ohm load or change it to another appropriate tap to maintain proper impedance matching.         In most circuits having a shared cathode resistor (series or parallel tubes), you would have to double the value of that shared resistor to find the correct value of individual resistors for each cathode which would result in the same bias level.         Glad you noticed this before somebody put too much stress on a good 6V6, we know they're not making those like they used to . . .         Thanks & Best Regards,     Mike

 

7/19/1999 3:23 PM

Michael Cameron

I wonder how long that Hammond 125E will hold up with 90ma of current running thru it?         You might want to try a slightly larger cathode resistor on your power tubes to bring the current down a touch, for the safety of your power tubes and tranny. 90ma times 325volts gives you a combined plate dissipation of 29 volts, a little over the max of 28 for two tubes. Drop that figure down just a hair and you probably won't notice much difference in tone and your tubes and OPT will thank you IMO.         Michael

 

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