5E3 Power Tube Voltages vs Convention

[AHeck]

Howdy,

For your conideratio, My voltages to figure bias in my 5E3.  Am I concerned about the voltages more or the IR Drop at the cathode resister?  I'm figuring on replacing the stock 250R with a 330R to drop dissipation from >108 to around 82%.  Is my math right? i'm new at tubes, so any advice, hints, or theories are golden.  Thanks for reading, thanks for your time.

[shooter]

Current is my # 1 concern, AS LONG as plate V is reasonable.

my math show each tube ~~~ 13W.  IN PP configuration it's possible to swing a lot more than measured plate volts, hence the kinda sorta standard of 70% max plate for bias.
so your idea of replaced Rk is good, you could also add an RC  filter after the standby switch, before your current "A" tap.

the last 6V6 I did sounded good with tubes biased ~~ 8.5W each

[PRR]

It's odd to see 45V drop in each side of the output transformer.

[sluckey]

I agree. I have 340 at the center tap and 336 on each plate in my 5E3.

[dude]

About the same with my 5E3, 351 @ CT, 347v at both plates. OT issues?

[AHeck]

i'm using a schematic that has a pair of 470R between the 4K7R and my plate leads from the board.  I am not sure why, as I am not seeing them on any other print.  Assuming they were necessary, I actually bulked them up to make the plate voltage more reasonable.  Straight wire now, not powered up.  Will 374 (give or take) be a deal breaker for 6V6GTYs? If so, an RC filter upstream of the first filter cap could  suck some voltage, though I'm a little hesitant to run those numbers. I should have checked the drop at the first resister after the A tap to find out what kind of current I'm dealing with, right?  Also, from my understanding, and counter to solid state, there is a "cap" on the amount of filtering capacitance I want to cram in there, but that may only apply to hi-fi.  I'm most assuredly out of my depths here and would welcome some wisdom.

[shooter]

i'm using a schematic that has a pair of 470R between the 4K7R and my plate leads from the board.

are you sure those don't go to G2 (screen grid) 
can you show what schematic you have that shows them?

377vdc will probably end them pretty quick, especially if you're dissipating 13+W per tube.  New type JJ's can handle it

[sluckey]

I don't trust those "red" voltage reading on your schematic. Node B shows 344.3v, yet you show approx 370V on pin 4 of each 6V6. That's impossible unless you have a wiring error.

Recheck those voltages and make sure the screens (pin 4) are connected to node B, either directly as per the schematic you posted, or via 470Ω resistors as per the schematic you say you are using.

[texwest]

I think you got the voltages of the plates and screens mixed up on your schematic.

I have about 376 on my plates and use a 250r.  It puts about 40 ma thru my 6v6gt tubes.  I have a stash of NOS coin base GE 6v6gt's and they work fine without redplating.  New tubes might not handle this so it might be a good idea to  raise the cathode resistor.  If  I built a 5e3 for somebody I doubt I would let it out the door running them that hot and so I would use a higher value on the cathode.

[AHeck]

Wow, I dont trust those red voltages either. The good news is that my wiring abilities are fine, but the bad news is I apparently cannot scribe for to save my life.  So, the voltages at pins 3 & 4 need to be swapped.  I wholeheartedly apologize. 

[shooter]

another dyslexic, welcome 2 my world 

so does it make guitar sounds?
 

[sluckey]

The bad bad news is your tubes are running even hotter than you thought.

[AHeck]

It makes amazing guitar sounds.  Really stunning tone.  Not necessarily when amplifying humbuckers, but, it sounds great. 
And yes, those tubes are running god awful hot, but they are not plating, there's a nice blue haze and just the faintest points of orange at the bottom of each tube.  I would expect more catastrophic symptoms out of these numbers.

[AHeck]

My reading between pins 4 and 6 (and yes, I double checked this) on the 5Y3 is 728VAC.  thats kinda way over the 710 the PT is supposed to put out, right?

[sluckey]

My reading between pins 4 and 6 (and yes, I double checked this) on the 5Y3 is 728VAC.  thats kinda way over the 710 the PT is supposed to put out, right?

728 is only 2.5% more than 710. I consider that pretty insignificant.

[dude]

Looking at the schematic with the voltages in red in your first post, you have 344v at node B, that 344v should go right to the screen grids, threw a 470 ohm R, like Sluckey mentioned. You must have wired those screens wrong. I assume you corrected the problem..? But with a little less then 344v on the screens and 332v (seems low, should be like 360v) at the plates and with a 250 ohm biasing R, over 13 watts dissipation seems high but do the math. If you raise that 250R, 20/30 ohms, you should be in the ball part of 11 to 12 watts dissipation, what is the voltage across the 250 biasing resistor? And what is the voltage at node B?

[AHeck]

Looking at the schematic with the voltages in red in your first post, you have 344v at node B, that 344v should go right to the screen grids, threw a 470 ohm R, like Sluckey mentioned. You must have wired those screens wrong. I assume you corrected the problem..? But with a little less then 344v on the screens and 332v (seems low, should be like 360v) at the plates and with a 250 ohm biasing R, over 13 watts dissipation seems high but do the math. If you raise that 250R, 20/30 ohms, you should be in the ball part of 11 to 12 watts dissipation, what is the voltage across the 250 biasing resistor? And what is the voltage at node B?

yes, I got my voltages mixed up, but the amp was wired correctly.  I'm gonna put in a 300R at R23 and try to drop that dissipation. Resister just arrived and I'll do it straight after work.  Right now the IR drop over R23 is 21v +/-. Same as listed on the schematic as is node B.

[PRR]

> only 2.5% more

A modern 2x4 is nominal 1.5" thick, with wide variation from planing, moisture, crook, etc.

A 2x4 that is 2.5% fat is like gluing a matchbook cover on a perfect 2x4. No carpenter works to that precision. (A cabinet maker does, but never uses framing lumber, or not as-sold.)

[dude]

Do this:
1. Measure the voltage drop on the cathode resistor to ground. Write the value down.

2. Divide this voltage by the value of the cathode resistor. This gives you the amount of current being drawn by both power tubes in milliamps. Write this value down.

3. Measure the voltage on the plates of the power tubes to ground. Write this down.

4. Now, subtract the voltage from the cathode resistor in step 1 from the voltage measured on the plates. Write this value down. Take this value, and multiply it by the current (milliamps) from step 2. This will give you the dissipated power (in watts) of both power tubes. Write this figure down.

5. Take the figure from step 4 and divide by 2. Write this figure down. This is the power dissipation (in watts) of each tube. For 6V6s, if it is over 12 watts, then you need to install a higher value cathode resistor. If it's 10.5 watts or less, you need to install a lower value cathode resistor.

[AHeck]

Pretty sure I plugged all voltages in correctly this time.  Are these reasonable changes to the circuit after increasing the cathode resister's value by 50 ohms?  If not , does it indicate any mistakes I might have made?  I don't know if this is relevant, but would the higher voltages than normal be something expected when using a 5Y3GT rectifier as opposed to a straight 5Y3?  I wouldn't know, and I'm sure I could find out, but maybe y'all could clue me in .  Thanks again for those of you hanging in there with me.  Everyone's knowledge is very appreciated.

[dude]

Your 6V6's are cooking at 14 watts each. You're B+ is a little high for a 5E3, probably the PT your using. You can lower the B+ voltage with a 15volt, 5 watt Zener off CT on PT to ground but this isn't going to solve your problem...?
Maybe someone else can step in... issue with his 6V6's...?


Do you have a 300 ohm cathode Resistor in the amp giving these voltages and the voltage drop across that R is 23.5 volts?

[shooter]

probably the PT your using

my guess too
I personally start with a PS PI filter prior to my "A" tap, each tube getting a 550 ohm and adjust for 8-9W Max plate/tube, but that's just me 

[sluckey]

6V6 is rated for 14 watts. Many self biased tubes are run at 100%. I would not worry.

But, if you want to cool it down just put a 470Ω or 500Ω on the cathode. Or you can reduce the B+ with two 250Ω/10W, one in each plate lead of the rectifier.

[dude]

Just curious, why not the Zener off the CT to ground, rather than the two 10 watt 250 ohm's? It's "much cheaper," a lot of techs use the Zener and a lot go for the PS...?


I though max plate dissipation for 6V6gt was 12 watts.

[sluckey]

Two resistors will cost $2. Personal preference.

I have three RCA Receiving Tube manuals. RC-19 says a 6V6 max plate dissipation is 12W. RC-20 and RC-30 both claim 14W max plate dissipation. I like the number 14.   

Here's the data from RC-20...

[dude]

Well, that's that. I guess several ways to skin a cat... 


If I were AHeck, I'll lower the the B+ and rebias, with probably the the two resistors, PS way. Because the Zener on the CT gets hot and probably not as reliable as the PS method.  Just that I have a supply of 15 volt Zener and no 10 watt R's, like Leo, he used what he had on hand.  


Why.., every time I use a emoji here the print gets small.....?

[sluckey]

I don't like to put anything in the CT of the HT winding. It's OK if the amp doesn't use the HT winding (or a tap on the HT winding) as a source of negative bias voltage. So, good for this self bias 5E3, bad idea for a Deluxe AB763.

Also, using a zener in the CT to drop voltage never yields the voltage drop you probably expect. IE, a 15V zener will not give a 15V drop in B+ when used in the CT.

I prefer resistors between the PT winding and the rectifier, mainly because I have a bunch of high wattage resistors on hand. So I do understand your preference for on hand zeners.

Can't say about your emoji issue, other than to say that there are two font tags in your message and that is what changed the print size. Emojis work fine for me.                         

[PRR]

> RC-19 says a 6V6 max plate dissipation is 12W. RC-20 and RC-30 both claim 14W max plate dissipation.

12W Design Center, where you throw radios together and do not put a meter in every one.

14W Design Maximum, where you do meter every one (or use other design techniques to control abuse).

DIY guys with meters, Design MAX is an appropriate guide.

The 6V6 specifically has always been under-rated (to drive big designs to higher-price 6L6). Like the 1960s Hemi claimed 425HP (they run closer to 475HP). Many 1970s Fenders ran well over 14W. The recent Champ reissue is reported to run at 17W. 14W measured is interesting but not alarming.

[dude]

Good info. Those 10 watt R’s to lower B+ in a fixed biased amp, come off the a/c winding before rectifier? Always use 10 watt or when would 5’s be  appropriate? Always learning.

[shooter]

470Ω or 500Ω on the cathode. Or you can reduce the B+

when choosing there are small sonic differences since you're deciding which end of the tube gets the shorter end of the deal 

personal choice, I like the cathode to get less swing in small bottles, and try hard to make 'em swing ~~~ = for the biggins

[Willabe]

I don't know if this is relevant, but would the higher voltages than normal be something expected when using a 5Y3GT rectifier as opposed to a straight 5Y3? 

What brand 5Y3?

Is it a newly/current made tube, or an NOS (new old stock) tube?

The newer 5Y3's give a little higher output dcv than the NOS tubes. (Some say the newer Russian 5Y3's are really not a 5Y3 but a Russian rectifier tube that's close in specs to a real 5Y3.)

GT stands for glass tube. NOS tubes that have an A or B after them can be a further developed  tube for a little more current/less loss.

But newly made tubes it's hard to say if it's a better tube, they seem to just add letters after the main name to make it seem like a better tube.   

[shooter]

just add letters

[texwest]

I worked on a 1956 tweed deluxe that ran the plates at 376v with a wall voltage of 124.  So you are close.

I would also recommend using an old 5Y3.  I used a TAD 5Y3 on a build and it ran a higher B+ than a NOS.  I switched it out and was happier.  You can find old or even NOS in the box 5Y3 tubes for a very good price if you shop around on Ebay.  I recently got a deal on 3 NIB NOS 5y3 tubes for about $20.  no one bid them up because they were rebranded Motorola.  There's really no reason to not use NOS with 5Y3 tubes.  I understand if someone doesn't want to afford a Mullard GZ34, but even then the Mullard will be more economical in the long run because it will last forever.