A 6V6 in the Hand's Worth a 6L6 in the Bush

[jeff]

I have a 6V6 amp I built a while ago using a PT out of an old TV. The PT is physically bigger than an average 6V6 PT, It's actually the same dimentions as a 6L6 PT... BUT... the TV ran a lot of tubes so I'm not sure if the bigger size is due to the bigger draw on the 6.3V or if it's capable of running 6L6s.

When I build the amp all I had was a 6L6 OT so if I could test it for extra current draw without blowing it it's ready to go. I just don't want to fry the PT if it can't take the heat. Is there any test or way to tell if it's OK? Does anyone know and can make a guess of how much juice an old TV/picture tube draws?

I don't want to burn up a perfectally good 6V6 amp but it'd be a waste to have the extra weight and not use it as a 6L6 if it's do-able.

I dunno, a 6V6 in the hand's worth a 6L6 in the bush.
Any suggestions?

[HotBluePlates]

You can't burn up a PT by having a too-big OT. Worst case, wrong loading will limit clean output power.

[jeff]

No but right now I'm using 6V6 tubes in it. The OT is a 4K:4 6L6 OT that I'm loading with an 8 to use it as a 8K:8 for 6V6s. I'm wondering if I should take the chance of burning my PT by running 6L6s. I know the OT will take it but not sure if the PT can handle the extra current. The PT looks big enough but I don't know if it can handle the extra juice to run a 6L6 amp VS a 6V6 which I'm running now. I don't know how much of that size was because it heated a billion tubes when it was in the TV

Is there any way to tell aside from taking the chance and burning it up if it can handle it? Should I just play it safe and just have a really heavy 6V6 amp and always wonder if it could be something more?

[eleventeen]

Nobody can do much for you other than guess. But as one who took apart many hundreds of old tube TVs in his wayward youth, I *can* say that I *never* saw a TV use a 5Y3. Not even one. 5U4 only...sometimes two of them, but for the most part, one. So if you are using a 5Y3 or 5AR4/GZ34 in your amp (2 amp heaters vs 3 amp 5U4 heater) then that's a biggish factor in your favor. Secondly, transformer-operated (vs series-filament) TV sets usually had a heck of a lot more tubes than say a Super reverb, and many of them might have been octals vs 9-pin, which implies more filament current, in general. If you go back and look at TV set ads from the late 50's, many of them boast "17 tubes!!". Lots of those tubes were 6SN7s, for example, using 600 ma heater, vs 12AX7 which is 300 ma run from 6.3 volts. OK, more modern, smaller tubes? Many of those pentodes were 7-pin 6CB6 = 300 ma, same as a 12AX7, so that's a wash. And you're using lots fewer of them. No damper diode tube which is another fat 2-3 amp heater. I believe a late 50's TV required maybe an 8-10 amp heater winding. Super reverb = 2 x 6L6 @ .9 amps & 6 x 12A_7 @ .3 amps. Little under 4 amps.

So as far as total heater current, I believe you would be way, way under spec for your PT.

That leaves output tube current. What's the conceivable delta? Grossly speaking, maybe 25 more ma than 6V6 at 500 volts? But then you've deleted all the plate currents for perhaps ten tubes and at 2 ma each, very light, that's 20 ma you're not using. No fat vertical oscillator, no big switching horiz oscillator, and those oscs weren't any kind of 2 ma tube circuits.

Frankly, I don't think you have a thing to worry about.

[jeff]

OK I see what you're saying. If the TV originally ran a PP 6V6 amp for sound and the ten extra tubes for the picture, by not using those extra tubes I could use that extra current to up it to 6L6s. But I don't know how much the TV was originaly pulling. The TV only had a SE 6AQ5 for sound, so I don't know if the PP6V6 is pushing it as is.

Is there any way to put in the 6L6s and monotor it to see if there are any tell tale signs that something is going wrong before something goes wrong and it burns up?

[eleventeen]

"If the TV originally ran a PP 6V6 amp for sound and the ten extra tubes for the picture, by not using those extra tubes I could use that extra current to up it to 6L6s."

That's my general drift, but in my experience, TVs didn't use PP anything for audio. In essence, you're probably using ten fewer tubes than the TV set used. You're losing let's say 3 (and maybe 5) amps at 6.3 volts or 19-31.5 watts and 1 amp at 5 volts = 5 watts. So to me, you're pulling 25 watts less power from the tranny on the heaters. Of those tubes and the affiliated circuits, you are deleting some pretty high current items; the vert oscillator, the horizontal osc, and the damper diode---even though those items have reduced duty cycles. Let's call everything else "small signal", 2-3-4 ma per tube. 10 tubes = 20 watts, the heaters = 25 watts, the oscs & damper I dunno, 15 watts? (probably more) but 60 watts less power use. So you bump up to 6L6 and now you need 20-25 watts out of the 60 you gave up. You seem perfectly willing to go forth with PP 6V6 and reticent about using PP 6L6. The difference is just not that much, nothing to cause flames to shoot out or anything like that. If you're that worried about it, insert a 100 ma fuse in your B+ line.

[DummyLoad]

as a 6L6 PT...BUT...the TV

my thoughts are that the TV had sweep tubes...perhaps two; vert. and horz. the horz. sweep tube alone likely at the very least, had a similar loading to a 6L6 in class A SE. TV also had video amp, and lots of current hungry mixers/oscillators, etc...

install the 6L6 as the PT is likely the least of your worries. do be concerned with OT max current/power rating and optimize load.

--pete

[PRR]

> If the TV originally ran a PP 6V6 amp for sound

Har. Lucky to get one 6V6.

The major sucker in a TV is the horizontal output. This is a tube like a 6L6 (except top-cap to stand the extrme voltages) OVER-driven 10 times harder than any guitar amp. So it may be dissipating 19W, but switching 50+W of energy (250mA at 300V 95% of the time) to sweep the electron beam, kick-up the 20,000V, heat the HV rectifier.

The vertical sweep "is" an audio amp (stuck on 59.9Hz) and often bigger than the sound output.

And all the little-tube heaters.

There's easily 100 Watts total power on tap. Plenty for two 6L6GC/EL34 and all the dressings of a guitar amp. Problem is the voltage is lower than we like for large audio. You might even go four (4KCT) or six (2.6KCT) 6V6 for 30 or 45 Watts audio at the lower voltage. (The high cathode current is a natural for DC-heating your small-signal tubes.)

[PRR]

me >horizontal output ...like a 6L6... it may be dissipating 19W, but switching 50+W of energy (250mA at 300V 95% of the time)

I forgot it is a ramp-wave.

6BG6 is the top-cap father to 6L6GC (the GC is quite different from 6L6A/B/G).

http://www.mif.pg.gda.pl/homepages/frank/sheets/137/6/6BG6G.pdf

TV systems are complex. This one starts with 400V but then captures the kick-back "DC boost" for another 150V, 550V total supply. The tube peak current is 300mA, but the average is 85mA+10mA=95mA or 50 watts total DC input power.

Running one 6L6 you can't take more than 35 Watts (30W in plate and another 10%-20% in bias and OT loss). Since there's no 3W V-sweep, no 1.3W sound out, no hot little tuner tubes, the TV PT will loaf.

Running two 6L6 at 25W-29W Pdiss each seems very safe.

If it indeed plops 400 Volts, then this is a 5F6A Bassman. Use two 5881/6L6GC/KT66/EL34 at 400V, 4KCT load, get 40W out.

[eleventeen]

Or, you could go with SS rectifier, drop another 10 watts of consumption (for the recto heater) and bump your B+ by maybe 35-50 volts and then you'd probably be comfortably in 6L6 land.

[jeff]

Cool.

Thanks for the help. I'll give it a shot.
   Jeff

[jeff]

do be concerned with OT max current/power rating and optimize load.

I think the current rating should be OK because it is a 6L6 OT.
 
 But I'm confused I thought "optimized load" isn't a big concern. It seems like every time I ask about using the "right" load, everyone says it's no big deal???

 The OT is an A-036968 which is used for Fender Pro Reverb, Bandmaster, Tremolux. It's a 4K:4 that, as the amp is now, I'm using as a 8K:8 for 6V6s by loading it with 8 ohm speaker. If I use the 6L6s I'll use a 4 ohm load.
My voltages will probally be lower than a Pro, bandmaster, Tremolux, but is that a problem, is it any different than using a VVR turned down?

[DummyLoad]

I'm using as a 8K:8 for 6V6s by loading it with 8 ohm speaker. If I use the 6L6s I'll use a 4 ohm load.

follow your plan. the reduction of B+ and the 6L6 load requirement is going to affect bias so the load needs to change and you have that covered in what i quoted - that's all i'm trying to say in that regard.

if the PT is still of concern, measure 1/2 sec. DCR  and unloaded AC RMS then sim it with PSUD2. it'd be cool to see how close PSUD2 is in that respect.

--pete

[jeff]

OK, I see, I gotcha now, Thanks.
What's PSUD2?

[printer2]

Is there any way to put in the 6L6s and monotor it to see if there are any tell tale signs that something is going wrong before something goes wrong and it burns up?

The PS voltage drops (dissipating power in the secondary) and the transformer heats up. 

Same as a VVR.


http://www.duncanamps.com/psud2/index.html

[DummyLoad]

OK, I see, I gotcha now, Thanks.
What's PSUD2?

duncan's Power Supply Unit(?) Designer ver.2

http://www.duncanamps.com/psud2/index.html

web site doesn't mention that it also works under WinXP, Win7, and Win7(64).

--pete

[jeff]

I was digging around in my basement and I found the side panel of the TV with the tube chart.

It had a 5U4 rect! Good to know as far as 5V current supply goes.

Here's the list of tubes:
1B3    6AX4    6BQ?
12AT7 12AU7 12BH7
6AQ5   6AV6  6AL5
6AV6  6CS6  12BY7
6CB6(X3) 6AF4 or 6t4
6bq7a 6X8

I think I'm gonna try it soon.
The OTs AC is ~280V unloaded and I have a 4K:4 OT
With a SS rect I should be good right?

I'm curious as to what I can pull for 6.3V with all those tubes.

[Willabe]

Look up their heater current draw in a tube book and add them up.



               Brad    

[HotBluePlates]

And your "6BQ?" is either another 6BQ7 (small-signal tube) or 6BQ5 (aka EL84) as the audio output.

[eleventeen]

You can of course google any tube "6CG7 spec sheet" and get the heater current. You have 2x 12A_7 and 1x 12BH7. Those are .3 amps. As previously mentioned, you have 3 qty 6CB6 (very, very common tube in older TVs) which are the same In terms of htr current) as 12A_7.

So you have 6 qty 12A_7 equivalent, .3 amps each = Super Reverb or any two-channel Fender.

As I previously guessed, you have 10-12 more 6.3 tubes than a Super Reverb. The 1B3 runs off the flyback. The 6BQ-6_(probably) is your fat horiz output and as PRR said, is just like a 6L6, only blasted, overdriven.

So you don't even have think about this. You have the capacity to run 10-12 more 6.3 tubes than your needs.

Your HV on the PT, however...this could be another story. You're probably going to have to go with a voltage doubler to get a B+ in the mid 400's, in fact, I'll go ahead and say you almost certainly WILL have to go w/a doubler. And no way can you use a tube rectifier and suffer the loss across the tube.

Refer to Bogen schematics for how you snatch the bias voltage whenusing a doubler. It is a little tricky. You HAVE to isolate the feed for the backwards diode with a cap.

[eleventeen]

Here's your doubler supply. Sorry it is a tad fuzzy. You'll need more (negative) volts than the -25 vdc this makes, so the 56K / 5% in between the .22 ufd and the "backwards" diode needs to be reduced. Maybe 18K-27K as a guess. Across the output of the bias section, you'll throw a 50K or 100K pot and pull the bias from the wiper. The key to success is that .22 ufd cap...that has to be there and it has to be a pretty fat cap and it should have a 600 volt rating if you're trying for a ~~450 volt top B+. And use a higher voltage rated filter cap on the bias, better than 50 volts



got it from: http://makearadio.com/schematics/images/bogen-chb35a-6.jpg

[jeff]

 Thanks for the schematic. Do I need a doubler? My transformer is 280V-0-280V. Was the doubler in the event I didn't have a CT transformer?
I was thinking with a SS rect my 280-0-280 would give me ~390V DC, no? Too low?

[eleventeen]

No load, yeah. That would be a tad low but *possibly* OK. You sure wouldn't be getting full 6L6 power, if that's your intent. With a load on it you might have 350-370 volts and I think we'd be talking undesirably low volts for 6L6 plates at that point. Other than the .22 (or similar) cap, the parts count is the same. The real issue is that you'd be using one end and the center tap of the HV winding to power the doubler section, leaving the opposite end of the HV winding unconnected. Meaning, full B+ current is being pulled from only half the HV winding. I will leave it to others to chime in on the possible consequences of this.

[jeff]

I'm afraid I'd be up in 600V territory with the doubler.

[darryl]

The real issue is that you'd be using one end and the center tap of the HV winding to power the doubler section, leaving the opposite end of the HV winding unconnected. Meaning, full B+ current is being pulled from only half the HV winding.

If jeff has a 280-0-280 volt secondary, he can use all of the secondary winding with a full-wave rectifier, and ground the centre tap. With silicon diodes, the B+ would be about 390VDC, as he has calculated. A voltage doubler or bridge rectifier is not necessary.

[jeff]

OK I figured it out and I have about 6A on the 6.3V supply.

Seeing as I have the heat and my B+ is going to be on the low side for 6L6s maybe I should use (4)EL84?
Would this be a good idea to use the 4K:4 6L6 OT with (4)EL84s considering the 18 watter uses a 8K OT and parallel I'd use half that-4K? With the right rect tube and my 280-0-280 PT I might get the voltage lower to where (4)EL84s would be happiest.
The other option is I have is a 5K OT

If you had these parts what would you make to make best use of them:
280-0-280 PT
4K OT
5K OT
(2)6L6s
(4)EL84s
any rect tube
(fixed or cathode bias)

Thank you for your opinions
  Jeff

[eleventeen]

If you like EL84's go for it....in some ways that is an elegant solution to the lowish B+ deal. I myself do not especially like them.

[jeff]

Sorry for switching gears at the last second but that might make some sence. 

I assume as long as my PT and OTs can take it I could just take an 18 watter add two more EL84s and use a 4K instead of an 8K. Is there anything else I'd have to change?(not including need for a bigger rect tube)

Thanks to all for helping talk me through it and how to use what I got.
Anyone else want to chime in all advice is welcome

 Thanks
   Jeff

[DummyLoad]

you mention that your PT measures 280-0-290 no-load.  4xEL84 may not work well. at idle in class AB1 4XEL84 are going to suck about 160mA. remember, this is at idle.  a single 5U4GB would also work but sag; a single GZ34 will also sag but will deliver some value higher B+, but not significantly higher. SSR simulates with best results. your PT is probably rated at around 265V @ 150mA to make 280V no-load. the horz. sweep tube (6BQ6) is rated @ 11W. was this PT was pulled from a smaller B&W TV?
if you post the resistance measurements of primary and secondary with your mains frequency and line voltage, closer to actual capabilities of your PT can be calculated.

2X6L6GB will make around 22W with 360V B+ and a 5K load. 
2XEL34 will make around 35W with 350V B+ and a 3.8KΩ load.

read the datasheets?? ;)
http://tubedata.milbert.com/sheets/093/6/6L6GC.pdf
http://tubedata.milbert.com/sheets/128/e/EL34.pdf

like eleventeen, personally i don't like EL84. to my ears they sound like sparkler when over-driven hard. they can be tamed by not bypassing the output stage (NFB added) and/or with use of a conjunctive filter across the OT primary.

--pete

[jeff]

if you post the resistance measurements of primary and secondary with your mains frequency and line voltage, closer to actual capabilities of your PT can be calculated.

measure 48ohms on secondary and 1.9ohms on primary.
voltage is 115~120 depending on time and day and not sure of mains frequncy but I live in MA, USA if that helps.

So maybe it's better to go with the SS rect and 6L6s to keep voltage higher and current lower?
I was figuring if a 6L6 is a 30W tube I could use double the amount of 12W tubes.
 Is the problem that in order to use the EL84s I'd need to lower the voltage(with a rect) and therefore I'd need to increase the current draw?

Not really sure how to read the datasheets. I did a exapmle circuit/design exercize for a SE amp but I'm not really sure how to use them if the screen current is not the 250V on the graph or for a PP amp. It was cool, but other then figuring a SE amp at the screen voltage on the chart I don't really know how to use them. If you wouldn't mind could you give me a quick walkthrough on how you got those numbers and/or point me in the direction to learn more about using datasheets?

Thanks for the help
  Jeff

[DummyLoad]

never mind the screen voltage, just don't exceed the max power rating of the screen. take the example given in the datasheet and compensate for the increased screen current by increasing bias voltage (make g1 more negative). the down-site is you'll need more V swing at g1 to make the same power and with the less conduction angle of the output tube the worse that small signal distortion gets due to crossover distortion.

in the example in the GE 6L6GC datasheet for a class AB1, (pg. 2 bottom of page) two examples given with the only difference being the load - 3.8k load makes 18W with -22.5V at g1, and 26.5W with 6.6K load. notice that the idle current is same but Ra-a load different, B+ is same, bias is same (-22.5V) and the screen currents are same. so by reasoning a 5K load somewhat in the middle of the two examples, so split the difference for the output power - about 22W w/ 5k load. now crank up the B+ and g2 to say 450V (super reverb) drive a 4K load - you'll get about 40W.

Morgan Jones - Valve amplifiers - 3rd. ed.
Richard Kuehnel - Guitar Amplifier Power Amps.  <--- very technical and modern reference.
Principals of Power - K. O'conner
Radiotron Designers Handbook  P. Smith - 3rd. ed.   http://www.tubebooks.org/technical_books_online.htm i think it's in doug's tube amp library as well, however, there's more stuff at that link though.

pentode press ----> http://www.ampbooks.com/


I live in MA, USA if that helps.

all of north america is 110-120V, 60Hz. attached is .png from PSUD2 sim of your PT with datasheet parameters. 5K load. also for comparison i ran a sim with circuitlab. both seem to correlate.

http://www.circuitlab.com

--pete