6V6 SE--bias, plate voltage, screen voltage, screen current, etc

[Willabe]

And here's some on line copies of some tube manuals;

RCA - Hallman Labs

https://hallmanlabs.com/catalogs/sylvania/

https://hallmanlabs.com/catalogs/general-electric/

https://hallmanlabs.com/wp-content/uploads/2020/01/ETR-15G_General_Electric_Essential_Characteristics.pdf

[Willabe]

Also RCA manuals say look at 6AQ5 tube for chart. Their related.

[pullshocks]

Also RCA manuals say look at 6AQ5 tube for chart. Their related.

Thanks a lot for all those links, and to Tubeswell, I did miss seeing the link in post 41.


You are right, RCA manual does reference the 6AQ5 page, but the graphs on that page are pretty minimal,  no substitute for the full meal deal data sheets

[Merlin]

And *roughly* copied my line for 8K impedence and 310v plate voltage.  So if I understand this, it would be OK for screen voltage down as far as 250v but not below?

It would be fine for a screen voltage down to 200V but below that you would lose power. You see Champ-alikes with screen voltages up to 300V with a 8k load, so I wouldn't sweat it too much

[pdf64]

I wonder why it came about that 6L6x info is so extensive (numerous different charts, typical operating conditions), whereas 6V6x info is pretty 'bare bones'?

Online loadline plotters / calculators are handy for 6V6, if one's anode characteristics chart extrapolation skills are somewhat lacking :)

https://www.vtadiy.com/loadline-calculators/loadline-calculator/

http://bmamps.com/ivds.html

[Willabe]

I wonder why it came about that 6L6x info is so extensive (numerous different charts, typical operating conditions), whereas 6V6x info is pretty 'bare bones'?

6L6x bigger/more power, better, later developed tube.

Trying to sell the newer design 6L6 series, and leave the older design 6V6 behind.

[pullshocks]

..... if one's anode characteristics chart extrapolation skills are somewhat lacking :)

Thanks, I really appreciate it.


I've tried, but I'm still "somewhat lacking."  The on line calculators are helping, but  I'm confused about a couple aspects: 


BNAmps--
1)  calls for Plate to cathode voltage --It seems like this field should be plate voltage.   You start with a selected plate voltage, but the EDIT plate to cathode voltage depends on cathode voltage, which depends on cathode resistor value, so how do you know in advance what the plate to cathode will be?


2) The notes at the top state "a) The bias control only applies to the push-pull option" but I found that changing the value in the grod bias field does significatly affect the end result, with -20v being the most favorable for 6V6 SE transformer load in the cases I tried.


VTADIY--
What info goes in the field "Out.headroom (+/-V)%"

https://www.vtadiy.com/loadline-calculators/loadline-calculator/
http://bmamps.com/ivds.html


Thanks again.

[shooter]

but the cathode voltage depends on cathode voltage, which depends on cathode resistor value, so how do you know in advance what the plate to cathode will be?

and now you "see" the difference between design, and reality.


in "design" mode, you start with the OT primary impedance and the "target" plate VDC.  Do the math and you have "Theoretical" tube current.  now with the cathode R, the tube current, more math, you're "in the ballpark", turn it over to the lab tech, have it breadboarded up, then measure and adjust

[Merlin]

how do you know in advance what the plate to cathode will be?

You lick your finger and stick it in the air. 6V6 bias under 'typical' conditions is in the range of 15 to 20V, so you just subtract that from the B+ and call that your plate-to-cathode voltage. It'll be close enough.

[pullshocks]

[, turn it over to the lab tech, have it breadboarded up, then measure and adjust
 


Yes things are in the hands of the ...uh... lab tech.  As best as he can tell with his Extech DMM, it is coming out within 20% of predicted values.  He is working up a budget request for  an oscilloscope and signal generator

[pdf64]

...
VTADIY--
What info goes in the field "Out.headroom (+/-V)%"
 ...

Just leave it blank.

[pullshocks]

how do you know in advance what the plate to cathode will be?

You lick your finger and stick it in the air. 6V6 bias under 'typical' conditions is in the range of 15 to 20V, so you just subtract that from the B+ and call that your plate-to-cathode voltage. It'll be close enough.

Thank you Merlin.  That is what I surmised.  Also, it only accepts plate and screen volts in 5 volt increments
But trying different combinations of inputs has helped me visualize  the interplay. 

[pullshocks]

...
VTADIY--
What info goes in the field "Out.headroom (+/-V)%"
 ...

Just leave it blank.

I tried entering some numbers.  Once you enter a number, it displays a power output.  With plate voltage of 330, Entering the number 290 in the out headroom box gives a power output of  5.25W and THD of about 12, which lines up with the RCA Tube Manual typpical operating conditions.

[Williamblake]

My 2 cents taken from building single ended amps: If the voltages and and currents are close to the datasheet everything it is fine, no big differences to the sound when having slightly different values there. But in one build i had only the screen supply of a SE output on a separate power-transformer winding and this gave a unique sound. I would not want this sound in an output tube but for a reverb driver it is very cool, it has very little sag, i would describe it, like walking shoes that go through any obstacle. I know SE output stages "don't sag" but it really made a difference to me. I do keep track of all my voltages while designing with soldering iron but little do i know about EE, so add your grain of salt please.

[Willabe]

..... But in one build i had only the screen supply of a SE output on a separate power-transformer winding and this gave a unique sound.

Ok, but fill in the blanks please.

What was the SE power tube, what plate dcv, what was the screen dcv, K R, and how many watts output with the set up?

[shooter]

My 2 cents taken from building single ended amps:

6V6, 6K6, EL84, 6L6, EL34, 6550, KT88 xSE builds, the most "versatile" SE amp has a Power amp with a stiff SS PS, an oversized OT with 40% taps into the best speaker money can buy.


This gets you Class A PA, drive it with whatever "front end" you choose, keep the drive signal "available" at ~~~ 20% over PA tube datasheet for "max signal G1"
you can add all the flavor you want, up front, an SE PA will just pass it to the speaker unmolested by  PA builders creativity.


If you just have to OD the PA for "that sound" the 20% extra drive signal is there while keeping the PA "in-check", more than that and 8 outta 10 times the PA heads for some death metal sounds that are real hard to "design out"

[pullshocks]

Thank you Shooter and Williamblake.  Here’s my 2 cents, to go with your 2 cents.


After studying Merlin’s SE page and playing with the simulators, I decided I should change the output transformer to 8K.  It had been on a 5K tap.  It took a while to make that change, so I couldn’t do much of an A/B comparison, but I think it sounds better.  Or it could be confirmation bias rearing its ugly head again.


I also think it sounds better with the 5V4, but of course that also affects voltages in the preamp and cap drive reverb, so not a controlled experiment.  Either way, it is set up using a cathode resistor close to the BMAmps calculated value for my voltages and OT impedence.  It is not overdissipating.  It sounds good.  I get good feelings as I play.  I’ll probably try tweaking RK, to see if I can hear a difference, but I doubt I’ll be able to tell.


However, beyond  certain settings of preamp volume and master volume, I don’t like the sound any more, and I think this is what Shooter is getting at.    I can’t find a spec for “max signal G1” on the datasheet, and don’t know how/don’t have the equipment to measure drive signal anyway.  I would not be surprised if my “by ear” setting  lines up with  Shooter’s Law of Single Ended Amps


Speaking of signal level, in the BMAmps simulator, there is a data field for “Signal Input” that defaults to 66.6 volts.  For quite a while I just left that value, but then started playing with it and eventually worked up the graph below. 


Attached is my latest block diagram, showing revised power supply dropping string and updated voltages, bias resistance, and OT connection .

[shooter]

I can’t find a spec for “max signal G1”

try Peak signal G1  the 6v6 sheet I looked at had it at ~~13vac at plate 315  NOTE: peak, is only 1/2 the sine wave, so when you use a scope n sine wave you're looking at ~~26 vac p-p sine wave.


you really need to sell a kidney or something n buy a scope if you want to keep tinkering at this 

[pullshocks]

you really need to sell a kidney or something n buy a scope if you want to keep tinkering at this 

You've been very generous with your help, so I hate to ask another favor, but, OK, help  me pick (or suggest something else adequate for this work, and easy to learn to use)


Leader 8103 100MHz Oscilloscope + Ham Radio CB RF Sampler - electronics - by owner - sale - craigslist


Hantek DSO2C10 Digital Storage Oscilloscope 100MHz Bandwidth 2CH Dual Channel 1GSa/s 8M Memory Depth with High Voltage Probre: Amazon.com: Industrial & Scientific


Handheld Pocket Digital 120 MHz Color Oscilloscope Replaces Velleman HPS140MK2

[Merlin]

Not the handheld one. I'd probably lean towards the analog scope for tube amp work, since you get a 'smoother' image. But digital is great for giving you instant readouts of voltage, frequency, true RMS etc. Real gangsters have one of each.

[shooter]

what's your long game?


will you be doing repairs for others?
do you tinker in the digital realm, breadboarding, micro-controllers....?
simply to "see" what's up on your builds?


unless the scope is real cheap AND works fine, I would avoid CRT based scopes like the 1st pic, you can tell in the image "somethings up" by the uneven trace intensity, normal on left, then bright.
the 2nd one looks at 1st glance to be a good all-round "bench scope"  something you'll use on a regular basis for repairs, analysis, design.
the last one is closest to "just a good tool to have when I need it"


this one was "reviewed" by Sluckey awhile back, and the price is good for that quick look at things, quick and handy for tweaking but I wouldn't put it in the "I use a scope often" group.
https://el34world.com/Forum/index.php?topic=22702.0

[Merlin]

you can tell in the image "somethings up" by the uneven trace intensity, normal on left, then bright.

That's just a camera artefact. I don't live in America, but $200 sound like a lot for a secondhand scope like that. OK it's 100MHz, but it looks well-used and otherwise bog-standard. $100?

[pullshocks]

what's your long game?

Thats the hell of it, this is not a long game proposition.  I want to dial in this project and then get out of the tube  amp building hobby.  So buy, use it for a month or 2, sell for whatever I can get back out of it, no doubt at a loss.  After looking into this, I also need a signal generator, a bucket of probes, cables, and adapters, and apparently an isolation transformer
Starting to  think it would be more cost effective to take it to a tech, if I can find one who is willing to work with me

[shooter]

this is not a long game proposition.

Starting to  think it would be more cost effective to take it to a tech, if I can find one who is willing to work with me

we're free here, otherwise I charge $100 over cost for the no frills, add trem add $80 add verb....


post your design idea schematic
build it
debug the VDC and "gross" signal issues
match up a speaker
play for 1 month
take it to a tech for "tweaking" and dialing it in.