May 2nd, 2012, 02:53 PM
Advice needed here. I've got this '76 SF Deluxe Reverb. I've just replaced the rectifier tube and checked the plate voltage. 420. I was looking to set the bias on a pair of sweet, early 1960 RCA 6V6's, at about 25mA. I thought they were a matched pair, but there 4mA in difference. I've got one set at 22.3mA and one at 26.3. Is this as good as it gets?
appreciate the help
May 2nd, 2012, 03:27 PM
4ma ain't bad. The last new production matched set of 6v6's I bought came in at about 2ma apart. Did you get your vibrato channel issue straightened out?
I've got an NOS pair of GE's in my pr and they are about 4ma apart. They sound fine to me.
May 2nd, 2012, 03:36 PM
Without modifying the circuit, you have a problem. That 26.3ma of current draw in that one tube at 420VDC is around 78.9% percent of maximum plate dissipation.....that is a bit hotter than is advisable. Most of us woudl be looking for 70% or less. The tube that is pulling 22.3ma is running 66.9%. Ime, that is where I like to run tubes in a fixed-bias circuit...somewhere between 55-70%.
I would advise you to set the bias voltage a bit more negative to cool things down in that one tube. OF course, the other tube will run at a lower plate dissipation factor, too. YOu just have to live with that. FWIW, as long as the mismatch doesn't cause the amp to hum---and that 4ma of difference probably willnot cause a hum----some folks like to run slight mismatches for tonal complexity. AS you decrease the current draw, your plate voltage will increase slightly. So, if you took that 'hot' tube down to 22.3ma of draw and the plate voltaeg went up to 425---jsut an example....your plate dissipation in that hot tube would be at about 67.7% of max plate dissipation.
.0223 x 425/14(max plate dissipation in watts for that tube unless I am wrong) = 66.69%
IF the other tube were to remain 4ma less than the hot tube, that tube would be drawing 18.3ma for a plate dissipation factor of 55.6% or so.
I have seen 6V6's work with no hum with a 9ma mismatch. IF it doesn't hum and you like the sonics, play away...as long as you getthat 'hot' tube cooled down a bit.
This is where a bias balancing circuit preceeded by a bias voltage adjustment would be ideal. ONe could also unbalance matched tubes if one wanted that sonic difference. Some folks leave the bias balancing circuit that was used in the larger SF FEnders and add the bias voltage adjustment. to do thisin your DR, one would have to mod the circuit to build the balancing circuit and then add a trim pot for the bias votlage adjustment in between that balancign pot and the source of the bias voltage.
May 2nd, 2012, 05:35 PM
Thanks Wally. I'll cool that hotter tube down a bit and see how it sounds. Right now it sounds pretty awesome, but I don't want to burn this tube up too quick.
The vibrato channel is doing better. I think the main problem was me gettin to know this amp better and using my equipment, cords, peddles etc...with it. I've ordered some more reliable, vintage Sylvania Baldwin 12ax7's I'll use in the V2 and V4 spots and bout 2, RCA 12at7's, For the V3, reverb send and the V6 phase inverter. I also picked up a nice RCA 12ax7 and then with my other various 12ax7s, will switch out till I get it dialed in nice. I'll try to get that vibrato channel as quiet as I can, but with ANY pedal inline, so far, the sound magnifies by about 3 or 4 times. I'm still entertaining the thinkin that it's on this side of the amp too.................But it's the journey right? Havin a great time figurin it out. I love this amp
May 2nd, 2012, 06:03 PM
I would be surprised if that 'hot' tube were not going just a bit to the redplate side of things. Have you watched the tubes in the dark whieleyou play?
And.....when it comes to redplating....for soeone who enjoys amps on the warm side of things, an amp will sound better than it ever has right before redplating tubes fail. Hopefully, when they fail they don't take something else with them, right? I suggest to folks that if they hear a change in the sonics of an amp while they are playing...even if and maybe even moreso if the sonci change is an improvement.....that they take a look at those tubes to make sure that they are still operating safely as regards current draw.
May 2nd, 2012, 06:11 PM
ONe other thing.....have you switched positions of those tubes and then checked the current draw?? IT is likely that the high draw will follow the tube, but I am always curious about these types of things when they present themselves to me.
May 2nd, 2012, 06:17 PM
Good thought. I'll try switching the positions as I'm dialing the bias down a bit. I report back with my findings
May 2nd, 2012, 06:22 PM
hardie, I would suggest taking a measurement tas is o set a baseline and then switch the tubes with that bias voltage setting. IF the 'heat' stays in the socket, you need not do any biasing until you find out why that socket is 'hot'.
Chances are that when you switch teh tubes, though, the heat will follow the tube..and you can then proceed with bias adjustment to get that tube down in a safer range of current draw.
May 2nd, 2012, 06:55 PM
OK. Heat followed the tubes. After gettin them really good and warm, there @ 3.5ma difference. 25.0 and 21.5. Now when I adjusted the bias in this earlier, I had it out of the cabinet. When I tested the plate voltage I noticed that my Bias trem pot was accessible from this side. I saw a video from eurotubes that showed him stickin a screwdriver in from the top, (tube side) hole to adjust the bias and I think, originally, the trem pot was accessible from the top side, It very well may be now, but I'm not having any luck putting a screwdriver in this hole. It's not catchin anything and I don't want to damage it. That being said, I'll pull it from the cabinet later and dial in down a bit. It's almost dark now and I'll check for red plating too @ 25ma. My meter is a made in china job, not the best. My plate voltage may be off a bit. I think 440 is the standard for a SFDR with 25 ma bias
May 2nd, 2012, 07:27 PM
The schematics that I have for SFDR's call for 415VDC on the plates. With modern wall voltages, that is probably not going to be possible with the 5U4 rectifier.
At 440VDC with 25ma of current draw, a DR's power tubes will be sitting there at 78.6% of max plate dissipation.....hotter than we like to see.
The schematics do not give us current draw info but rather they call for a certain bias voltage....-35VDC, IIRC> I do not know what -35vdc bias voltage at a modern plate voltage will yield as far as current draw and plate dissipation. I simply measure, compute and adjust.
What rectiifer tube are you using in that amp? IT should be a 5U4.
I am thinking that you have a 5U4..because a GZ34 mgiht take the voltage even higher, right?? There are rectifier tubes that you could use to lower that B+... or....you could use JJ6v6's and keep on keeping on..because they will tolerate that high voltage and current draw. They are somewhere closer to a 6L6. Many folks figure them at 18 watts of max plate dissipation rather than the 12-14 of 'real' 6V6's.
May 2nd, 2012, 07:42 PM
Those mullard 12at7's are a nice nos tube that I have never had an issue with and they are fairly cheap. I got mine off the bay but I see them everywhere, I've had a bad run with rca's lately. So many folks sell them as nos and say tube tests strong but they all seem to go microphonic or something in a very short period of time. Are you buying your tubes from a vendor or on ebay?
May 2nd, 2012, 08:53 PM
Tubes from eBay. Those mullards you speak of, are they the re-issues? I'm hoping these sylvania (Baldwin) 12AX7's, that I've orderd, work quietly in the vibrato channel
Wally I hear you. I will tone that hotter tube down to 22.5ma and see how it sounds.
May 2nd, 2012, 09:11 PM
Real NOS Mullard 12AT7's seem to be a unique tube, none of their contemporaries and no new production tubes are as easy to drive. All others start to draw grid current well below G=0v. If you can get them, better do it, because there is no sub
May 2nd, 2012, 09:23 PM
There so F-ing expensive though. I don't mind spendin the money so much if it's going to make a real differnce.
Rectifier tube is a 5U4 wally
May 2nd, 2012, 09:30 PM
22.4ma on the hot tube and 18.9 ma on the other sounds good. Less unwanted feedback it seems too
May 2nd, 2012, 10:49 PM
No i'm talking about NOS Mullards not reissues, I bought 2 of them for my deluxe., they were 25 each, hell the reissue's mullards are almost 20 so to me it was a no brainer.
May 2nd, 2012, 11:00 PM
OK, I'm shoppin for the 12at7's. any preferance on the 12ax7's?
May 2nd, 2012, 11:08 PM
I'd consider 4ma within the definition of 'matched". Beyond 6ma is supposed to be unmatched, but frankly they'd still bias up ok. If you're off more than 10ma you'll struggle a bit but you can still compromise. One school says bias the hottest one as normal and let the weaker one hang. Another school says bias one a bit hot, and the other a bit cold. Either way will work fine. Just be sure you don't do as Wally pointed out and have the stronger tube biased hotter than 70%.
May 3rd, 2012, 06:06 AM
No to set on any one's toes. But I see no reason to make biasing such a numbers game.
Firstly, if you're using biasing probes then they're adding the screen current into measurements. Meaning, the reading is hotter than the tube actually is.
Secondly, a tube isn't going to red plate at 75% max disipation or even 85%.
Thirdly, why not set you bias to where the amp sounds best? I find it best to play the amp while adjusting your bias. Figure out what your plate disipation is and then decide if you can turn up from there. Then turn down the current going through the tube until the amp sounds less good. You might end up with something like 76% plate disipation but its not gonna take that much life off your tubes.
But 70% plate disipation is a great fail safe way to bias tubes. Just stating my preference.