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6V6GT troubles

8/23/2000 7:44 PM
Blues Lyne
6V6GT troubles
I have BFSR that has been hacked into a head (I didn't do it!). I put a 5y3 and 2 6v6's in it. The plate voltage is 408v and the tubes are biased at 23 and 26. I have 3 sets of used 6v6's I got to experiment with and these sound absolutely wonderful and totally different than the other two sets. I think it's my favorite tone I've ever had.  
So far this is all great except that the tube that has the higher bias reading will every once in a while start glowing red. This seems to be random and it will work fine for several nights and then I'll turn it on a few day's later and it will go cherry. I think this follows the tube so I'm thinking a bad tube. But what's wierd is if the tube starts glowing (and this is usually within minutes of turning the amp on) I shut the amp off and turn it back on and the tube will get bright red again. Then I shut the amp of and plug in my bias probe and leave it in while I play to monitor the bias. It never goes cherry with the bias probe plugged in. The bias will fluctuate quite a bit as I play (is this normal?) but the tube doesn't glow. So now I'm wondering what the bias probe could be doing to stop the tube going cherry. Any ideas? I've read about guys beefing up the screen resitors when using 6V6's but was told that shouldn't be necessary with this set up.  
I'd just get a new set of tubes to try but I haven't found out for sure what these are. They are labeled Hoffman, but have a GE code and a '52 date code. All the GE's I've seen are clear glass with silver tops and these are grey/black on the sides with a clear top.  
The other 6V6's just don't do it for me. With these in and a Les Paul Jr. you instantly want to play Allman Bros. licks. It sustains even at low voumes, you can't get a harsh sound out of it. It's warm and syrupy. Even with a fuzz pedal set to what would normally be buzzy and extreme, the sound is smooth and almost cleaner sounding through these tubes. Kind of that old Santna sound.  
Ok so I've rambled way too long about these tubes. Any thoughts or ideas are much appreciated. Also if you have any ideas about the tube ID that would be great also. My fear is that this incredible tone is the sound of some tubes about to die. Then I have to go back to what I used to have.  
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8/23/2000 8:07 PM
First the Bias probe will change if you play.. you set your Bias with the amp without a signal or at idle...a couple of things come to mind...try cleaning and retentioning the sockets,and check the pins on the power tubes to see if they are clean... another thing might cause the problem a bias cap going bad,coupling caps in the PI,doubt thats it cause you said it stays with the tube.. They are the last 2 big caps on the board...another reason for your tone is lower voltages on the preamp tubes... I would try to set the tubes at 20ma see any of this helps could even cathode bias the amp.. hope this helps  
8/23/2000 10:33 PM

And to add to that.. I think you DO need screen resistors... minimum of 1K to 1k5 at two watts.  
Who ever told you that probably has not really played with 6V6GTs in a Super Reverb power supply rail.  
There is only a 90 ohm resistance in the choke to drop the screen voltage down a little and if you have the stock 470 ohm screen resistors on the socket, that is the entire screen load.  
A 6V6GT does not pull as much current through the screen as a 6L6 does and because of that, there will be much less voltage drop across the choke and the 470 ohm screen resistors anyhow.  
Probably too much screen voltage!!  
You also could have an induced oscillation above your hearing range.  
Use 2k7 grid stoppers on each tube socket and try having a minimum of 1K to 1k5 ohm two watt resistors soldered on the socket for the screens.  
Bias the power tubes to about 9.5 watts each.  
The go back and reduce the value of the last resistor in the power supply dropping scheme to get the 12AX7's plate voltages back up to about 175vdc-200vdc range when using the 5Y3.  
I like Richie's idea of cathode biased 6V6s too.  
8/24/2000 12:26 AM
Blues Lyne

Thanks Bruce and Richie. That will give me some ideas to work with.  
8/24/2000 12:45 AM
Randy Jamz

Firstly, compare the Hoffman 6V6 to a Sovtek. I think they are the same. These crappy Russian 6V6s have been around for 900 years with all knids of names silk screened on them. I've seen the marked GE, Slyvania, Westinghouse, etc. They are actually made by the Russian outfit called 'Reflector' and marketed through 'Electronintorg'. These are bad, bad tubes.  
Secondly, you are a walking lesson on the pitfalls of the neurotic silliness of biasing by plate current only (you aren't really measuring plate current with a Bias Probe, but that's another story). Save your money. Walk to work and eat meat only twice a month. Then buy a good used 20 Meg oscilloscope, a dummy load, and a signal generator.  
A)Increase the screen grid resistor and/or remove the power supply choke and replace with a resistor at least 2 or 3 times the DC resistance of the choke. A 5 watt unit should do nicely. I've had good luck with a 1K value most of the time. Just make sure the screen voltage is a handful less than the plate voltage.  
B)Bias up the amplifier without the Bias Probe. Everyone will whine how 'wrong' it is to bias with a 'scope, but these people usually don't own a good 'scope or don't know what they are looking for.  
C)Compare control grid and screen grid voltages when the amplifier is biased properly. The tube with the lower screen or control grid voltages is drawing current and should be used with another 6V6 or discarded.  
That's my 2 cents. Thanks for listening.  
8/24/2000 10:28 AM
Dave H.

"Then buy a good used 20 Meg oscilloscope, a dummy load, and a signal generator."
Iíll admit it even if it does mean Iím neurotic and silly. I only use the plate current and voltage method of biasing but I have all the above equipment. How should I use it to set the bias?  
8/24/2000 11:13 AM
Ken Gilbert

"B)Bias up the amplifier without the Bias Probe. Everyone will whine how 'wrong' it is to bias with a 'scope, but these people usually don't own a good 'scope or don't know what they are looking for."
ok, smartass, why don't you tell us knuckleheads what to look for?  
biasing with a scope is idiotic.  
OTOH, biasing with a DA is feasable and recommended. not everybody's got one of those (unless you go get that software i linked out for ya a few days ago).  
sorry, but i actually like to use NUMBERS when i bias amps. they comfort me, because then i know what the hell is going on.  
BTW: i own a 150MHz scope. i know what to look for. still i bias by plate current and plate dissipation.

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