General Advice AB763 - is it better to have slightly HIGH or slightly LOW B+?

[jordane]

My question is generic.  Maybe the answer is "stop worrying about this."  I am too novice to know.  TL;DR below...


I just built an AB763 using Hoffman's two channel board with two 6V6 power tubes (Deluxe Reverb style), using the hardware from a Weber 6A20 kit.  It is running and playing nicely, but while biasing I ran into some questions...

The Weber PT has two sets of high voltage secondaries at 150mA.  One is 540v (the Weber schematic suggests using 540v).  The other is 680v.  I have tried both.

To complicate matters, it also includes a 120v primary AND a 125v primary.  I have tried both.  See attached image for a diagram.

My wall voltage is 123.2vAC.


When connected to 120v primary and 540v secondary, I am getting 370vDC at B+ (biased).  When connected to 125v primary and 680v secondary, I am getting 440vDC at B+ (biased).  The original Fender schematic calls for 420vDC.

All these numbers have been measured using the Weber WZ34.  I have also tried the a standard 5AR4 (Fender brand, I believe it is a JJ), which increases B+ by about 10v DC in each case.

TL;DR:
So, the first question:
Is it preferable to run B+ slightly high or slightly low?

Second question:
Is there something I am missing that will get my amp to 420v DC, besides a brown box? 

[sluckey]

When connected to 125v primary and 680v secondary, I am getting 440vDC at B+ (biased).  The original Fender schematic calls for 420vDC.

I would definitely use the 125V primary. But you should also try the 125V primary and the 540V secondary to see what you get. I bet you'll get close to 400VDC. I'd rather run with 400VDC than 440VDC, but would not be nervous about running at 440VDC.

Adjusting the bias pot will cause the plate voltage to change. Biasing hotter will lower the B+, while biasing cooler will cause the B+ to rise. This is all due to the loading of the B+. ALWAYS RECHECK THE PLATE VOLTAGE WHENEVER YOU CHANG THE BIAS POT.

[jordane]

But you should also try the 125V primary and the 540V secondary to see what you get. I bet you'll get close to 400VDC. I'd rather run with 400VDC than 440VDC, but would not be nervous about running at 440VDC.

Thanks, Sluckey.  I've probably read more of your posts and tips than you know.

I will try the 125v/540v combination, too.  My method of biasing (pulled from RobRob) has me checking plate voltage so all good.

[labb]

I have a deluxe AB763 running 440 on the plates and the player loves it. It is a guitar amp + or - 20% on the schematic voltages. If you like it let it bump. Be happy. Don't worry. I would recommend JJ power tubes. 

[jordane]

I have a deluxe AB763 running 440 on the plates and the player loves it. It is a guitar amp + or - 20% on the schematic voltages.

I’m at ~435 on the plates. 420 to 440 is about 5%. Just wanted to make sure I wasn’t missing something obvious. Thanks for the reply!

My PRRI runs a little hot, too.   

[pdf64]

How does the heater voltage respond to which primary tap is used?
Mains voltage can vary.

[tubeswell]

If you're running JJ6V6S, they will run quite happily at B+ 450V. I'd be hesitant at running New Sensor's present range of 6V6GTs at those voltages tho'. YMMV

[labb]

My PRRI runs a little hot, too   


Fellow had me build him a PR without trem or reverb...I used a set of Deluxe transformers. The OT is a classictone 40-18090 20 watt. amp runs 450 vdc on the 6V6 plates. Thing sound pretty good. He does all the effects with pedals.

[ac427v]

Pretty much always agree with pdf64. But especially this time talking about the primary wires. I find the 120v primary wires on Weber power transformers put too much voltage on the heaters. I always use the 125v primary wires so I don't damage/wear out my tubes. 10-20 volts off spec on the secondary taps is insignificant in my view so getting the heaters right is always my priority.

[jordane]

How does the heater voltage respond to which primary tap is used?

Thanks for the replies to all, I'm understanding a lot more about power transformers, heaters, etc. with each post and question.

I attached a chart, but the bottom line is the 120V tap puts the heaters (at the bulb) at 3.42V per side, while the 125V tap is at 3.28V.

I have my guess which one I should run, the 125V / 680V.  Correct?

[pdf64]

Yes, I suggest to use the primary set for 125V, the HT secondary set for 680V.
If the rectifier heater winding will cope, consider trying a 5U4. If not, then a 5R4.

[jordane]

If the rectifier heater winding will cope, consider trying a 5U4. If not, then a 5R4.

According to the Weber PT info sheet (in the first post), the filaments (green, 6.3V) are 5A.  Is that enough information to know they’ll cope with the 3A of the 5U4 or do I need to measure with a multimeter?

[sluckey]

The 5U4, 5AR4, 5R4, etc., cannot be run from the 6.3V filament winding. They must be run from the 5V/3A winding, which has enough current capacity for the 5U4 that requires 3 amps.

[jordane]

Oh, duh. I had the preamp filaments mixed up with the rectifier filaments.

They’re wired correctly in the amp, though.

Yes. Rectifier filaments (YELLOW) note 5V and 3A, so good to run a 5U4. Thanks!

[ac427v]

My actual voltages with a Weber WO 25130 transformer using 5 12A_ preamp tubes and 2 JJ6V6S power tubes with a 5R4GYB rectifier:
Wall Voltage 124 AC Volts using the Blue and Black wires

Red Secondaries 353 AC Volts
DC to 32uf first filter capacitor 410 DC volts
Heaters 6.6 AC Volts
Rectifier heater 5.5 AC Volts

[jordane]

My actual voltages with a Weber WO 5130 transformer…

Thanks. I don’t have a 5R4 or a 5U4 but found a nos source for ~$8 per tube, so I will probably buy one of each and test the voltage drop.

[pdf64]

It’s a bit disappointing that the rectifier heater is getting 5.5V.  Though the rest of it looks as good as it’s likely to get.
But it’s worth asking what idle current the output valves are drawing with the HT at 410V?

[ac427v]

24ma and 19ma on those "matched" JJ 6V6S power tubes. I'm sure I tried other rectifiers but did not keep a record of those voltages.
Ten years ago I used an older WO 25130 on a cathode biased amp and had lower voltages all around.

Wall voltage 122vac    Secondaries 343vac    Heaters 6.2vac    Rectifier heater 5.0vac    B+ DC volts:    5AR4 448    5V4 427    5U4/GB 406    5R4GYB 387    5Y3GT 383

[dude]

Yes, I suggest to use the primary set for 125V, the HT secondary set for 680V.
If the rectifier heater winding will cope, consider trying a 5U4. If not, then a 5R4.

Can his PT handle the 3a current draw..?

[jordane]

Can his PT handle the 3a current draw..?

Yes, my heaters are 5V / 3A (yellow secondaries, first post)

[dude]

Can his PT handle the 3a current draw..?

Yes, my heaters are 5V / 3A (yellow secondaries, first post)

Sorry, didn’t open the PT spec’s

[jordane]

Got a GE 5U4GA/GB in the mail today, plugged it in.  Ran warm-up and then biased.

Heaters still at ~3.3VAC per side.  B+ now right at 421VDC biased, so basically right on the money using the 125V/680V taps.  I haven't played it yet as it is late in the evening.

Thanks for all the help! 

[Willabe]

My PRRI runs a little hot, too   

Fellow had me build him a PR without trem or reverb...I used a set of Deluxe transformers. The OT is a classictone 40-18090 20 watt. amp runs 450 vdc on the 6V6 plates. Thing sound pretty good. He does all the effects with pedals.

There's an inexpensive currently made Russian 5V4 with a 5v/2A heater. Slow warm up time too. It will drop the B+ some. It's B+ current might be a little low for 6L6's but, it'll work great with 6V6's.

Doug used to sell them, I don't know if he still does?

Edit; Maybe not currently made, skinny bottle, straight sides, Sovtek;

https://www.tubeampdoctor.com/en/5v4ga/gz31-russia