Rewiring record player amp- Mind Blown!!!!

[jeff]

Hi guys need a lil help here

 I got my hands on an amp. It's got two 6V6s and a 6X5 tube rect.
plate-211V
screen-205V
cathode-10V
cath resistor 220 ohms

so I did the math and each tube is running~4.6 watts. seems cold for a 12w bottle, right?
 I was gonna tweak cath resistor but noticed a 6X5 has max 70mA. Thats blowing my mind. The PT is 192-0-192 so the most I can expect for B+ is around 211V (192 X 1.1 which is what I'm getting) If the max current draw for the 6X5 is 70mA, and even if we ignore preamp tubes and screen current, that's 35mA per tube. And at 211V, 35mA is 7.3W per 6V6 which is 60%, right? ??? And that's ignoring everything else!!! It seems either the 6X5 is not appropreate or the PT is too low. Can't figure how a 192-0-192 PT, a 6X5 and two 6V6s can work


I'm so confused, any thoughts or ideas? What am I missing? Is 4.6watts way cold for a PP cathode biased 6V6 amp? Is there another 6V rect I could use? Should I ditch the 6X5 for SS diodes and get 268.8V(192 X 1.4) and hope the OT can take it?

Also the 6X5 lists the input cap to be 4mf, this amp uses a 20mf first cap. short hazzard due to inrush??   

please advise, I'm goin' bonkers

Thanks
Jeff

[John]

What make is the amp? It's possible the PT isn't putting out what it should be. Or that a bad filter cap is drawing the voltage down. I assume you've already measured voltage with the rect. tube pulled?

[sluckey]

That all sounds very typical for a '60s small record player amp.

[John]

That all sounds very typical for a '60s small record player amp.

Did the record players have a lower power amp than the reel to reels?

[jeff]

Not sure of the make but the original caps were rated 250V so I guessing PT is puttin out what it should.

How should I bias it?
  Does 23mA @ 201V seem cold for 6V6s? 

[pompeiisneaks]

The Tung Sol datasheet I find for a 6V6 says about 70mA is max rating at 250 volts, 70% of that is 49 mA (for both tubes).  If I recall from other datasheets, if the voltage goes down, the current also needs to go down, (the max for this datasheet says 250 is 79mA and for 285V is 90 mA.  So with lower voltage you may want to shave off a touch more to be safe, maybe 40 to 45mA for the pair.

http://www.r-type.org/pdfs/6v6g.pdf

Edit: This GE datasheet seems to say the same: http://www.r-type.org/pdfs/6v6gt.pdf
~Phil

[sluckey]

Tell us more. Was this a small record player? Or part of a big console? Is the amp p/p mono or single ended stereo?

A few watts per channel or a simple few watt mono amp was very typical of small record players that closed like a suitcase and had a handle on them.

The tube line up, PT voltages, and cap voltage rating are all consistent with a small record player amp. Those tubes were cheap and plentiful back then.

[jeff]

big wooden box maybe 16X16, flip top, mono, 3X4 ohm speakers.
Am I right in assuming if you were to design an amp using 12W 6V6s @ 211V youd want to bias them @ around 50mA(100mA for 2) to get the most out of em?
If so can you understand my confusion that the rect can only deliver 70mA
I must be missing somethin.

[shooter]

here's my before coffee math
190plate guesstament(B+ -cathode)
50mA * 190 = 9.6WPdis

or your 201*.05 =10WPdis @ 70% = 35mA
now I'm getting coffee

[jeff]

OK I guess I`m confusing myself.
Here's what I know.
220 ohm cathode resistor @ 10.25V
  (10.25/220=0.0466)

for two tubes so
  (0.0466/2=0.0233) per tube @ 201V(211V-10V)

So 23.3mA X 201V = 4.68Watts per tube

4.68W is 39% of the 6V6s 12W rating

Is that right? And does that seem low?
My instinct is to bias hotter but I'm limited by the 6X5? I'm confused???

Aren't cathode biased amps generally biased @ 90~95% max diss?

[sluckey]

The only thing that's confusing you is thinking that just because a tube CAN put out 12 watts the circuit should be designed to put out 12 watts. That's the guitar amp tube abuse influence. Consumer electronic circuits are usually designed with much more conservative goals in mind. There is nothing wrong with running a pair of 6V6s at 4 watts per tube (8 watts total). That's plenty for a portable record player and the tubes will last a loooong time.

I bet your OT is little too. I doubt those speakers could handle 12 watts either. This thing was designed so a 12 year old girl could play it in her bedroom or easily tote it to her friend's house.

You can't simply bias hotter to get 12 watts from that amp. You also need bigger iron, higher B+, bigger rectifier tube, bigger speakers. Just use the amp as it's designed, but rework the preamp to be more guitar friendly. You'll probably end up with a nice little 8 watt amp.

[jeff]

OK thanks
I think you're right. Just threw me when I was checkin the bias because it wasn't what I was expecting to see. I think I get it. Just because your car CAN do 200 mph don't mean you gotta. Just seems quiet for a pair of 6V6 but thats cool. Right now it has no volume control. I've just been using guitars volume and it's pretty good practice level. Might just leave it that way - no controls but the guitar's.

@ How bout the fact the 1st cap is 20mfd? Should I be worried the 6X5 lists 4mfd? Heard bad news about 6X5s shorting. Would you leave it or add a 4mfd then resistor between 6X5 and 1st cap? 

Thank you
Much respect
 Jeff

[sluckey]

Don't sweat the filter cap.

[PRR]

Yeah. Design goal was maybe 7 Watts. (There may have been a 14W model at a higher price.) You shop around for a pair of tubes to do 7 Watts.... some oddballs, but you keep coming back to 6V6 working well below what they "could" do. (Especially convenient if your factory stocks 6V6 for a 4W SE job and that 14W PP job, and gets the quantity discount.)

I need a new dog-porch. I do my math. The main beam wants to be say 1.123" by 2.789". I can find a mill to cut that exact size beam. Or I can get a $3 2x4 (1.5"x3.5") at Home Depot. It's "too strong" and "too much wood", but maybe I can find the space and it sure will be cheaper as a Standard Stud instead of a special order.

And wot's the diff, 7 Watts or 14 Watts? 3dB which is not a big deal to the ear.

And the "12W" bottles cruising at ~~5W Pdiss sure won't die young. (Or if they do, it is bad welds or flake oxide, not from cooking.) In fact we routinely build houses (and dog-porches) 2X to 3X heavier than "needed" to ensure long service. (Since beams and studs are harder to replace than tubes.)

[PRR]

> PT is 192-0-192

Think outside the glass rectifier. 192V into solid-state diodes gets close to 265V. This is the split-difference between the 250V 10K 10 Watt and 285V 8K 14 Watt conditions. It would likely run near 12 Watts output. This won't be a "wow!" difference from 7 Watts; with the right drummer, it may just cross the line from not-enough to almost-equal.

[PRR]

me> There may have been a 14W model at a higher price.

My car was sold in three horsepowers on the same base engine. The base was 135HP. Mine has the hot cam for 149HP. Under another badge with hotter cams it was 185HP. The engine difference is just a few parts. The selling price difference, including different radio, nauga/leather, radio, etc, was nearly 2:1.

This block may also exist as a "utility engine" for generators and such, un-hotted for 90HP steady duty. (GM sells several of their econo-car engines re-rated for the generator market.)

[HotBluePlates]

... Design goal was maybe 7 Watts. (There may have been a 14W model at a higher price.) You shop around for a pair of tubes to do 7 Watts.... some oddballs, but you keep coming back to 6V6 working well below what they "could" do. (Especially convenient if your factory stocks 6V6 for a 4W SE job and that 14W PP job, and gets the quantity discount.) ...

Yep. They coulda used a 6K6 for a 7w amp, as that's an 8.5w tube. But if the company make 8-10 models which use a 6V6, and only need the 6K6 for a model or two then it's probably cheaper just to use 6V6 everywhere.

How should I bias it?  Does 23mA @ 201V seem cold for 6V6s? 

You shouldn't "bias" it. The circuit has a cathode resistor for the output tubes, so plug the tubes in the sockets. Biasing done.

190plate guesstament(B+ -cathode)
50mA * 190 = 9.6WPdis

or your 201*.05 =10WPdis @ 70% = 35mA

220 ohm cathode resistor @ 10.25V  (10.25/220=0.0466)
(0.0466/2=0.0233) per tube @ 201V(211V-10V)
So 23.3mA X 201V = 4.68Watts per tube

4.68W is 39% of the 6V6s 12W rating Is that right? And does that seem low?

At a B+ of ~210vdc, this is almost certainly a class A amplifier. So "70% rule" is out the window.

+1 to what everyone else said regarding not needing to operate the output tubes to their limits. Which reinforces something I've said before about idling at a specific % of plate dissipation being false rules (except that you usually don't want to exceed 100% dissipation at idle).

Further, consider that we might want distortion in a guitar amp, but distortion in a record player is undesirable. If you're idling at ~4.7w with 10.25v of bias, then you should know that idling hotter will require a smaller bias voltage. 8v or 5v or whatever.

But you should also know that tubes begin drawing grid current when their bias voltage is roughly in the range of 0-to-1 volt. Grid current equals onset of severe distortion, so if you biased your output tubes to 5v you might only be able to get 4v peak of signal voltage to drive the output tubes. But idling cooler, around 10v, allows a bigger driving signal before onset of distortion (as much as 9v peak or so).

The idle current is probably also sized to match the bias, expected output power, and OT primary impedance. Said a different way, 23mA at idle is perfect if you only need the output tube current to rise to 43mA (a 20mA increase) and fall to 3mA (a 20mA decrease). What if the output tubes could do that 40mA peak-to-peak swing with less than 18v peak-to-peak (9v peak) input signal? And what if the 6V6 needs ~50v left plate-to-cathode during the peak current swing?

21v - 50v = 160v peak plate voltage swing.
43mA (peak plate current) - 23mA (idle plate current) = 20mA peak plate current swing.
160v / 20mA = 8000Ω (look familiar for 6V6 output transformers?)

Output Power = (peak plate current * peak plate voltage swing)/2 = (0.043A * 160v)/2 = 3.44 watts

I know I can't dime a Champ-sized amp in my apartment without pissing off neighbors. I'm sure I wouldn't want my kid's portable record player to be louder than 3-4w of clean output power (maybe not even that much, considering my son is driving me crazy with some of his t.v. shows...).

[HotBluePlates]

... There is nothing wrong with running a pair of 6V6s at 4 watts per tube (8 watts total). That's plenty for a portable record player and the tubes will last a loooong time.

I bet your OT is little too. I doubt those speakers could handle 12 watts either. This thing was designed so a 12 year old girl could play it in her bedroom or easily tote it to her friend's house.

You can't simply bias hotter to get 12 watts from that amp. You also need bigger iron, higher B+, bigger rectifier tube, bigger speakers. Just use the amp as it's designed, but rework the preamp to be more guitar friendly. You'll probably end up with a nice little 8 watt amp.

Well, I'm guessing you'll end up with a nice little 3-4w amp. But there is much wisdom in Sluckey's suggestion to let it be what it is, while only tweaking the preamp to be guitar-friendly.

Given the 6V6's, you could try to replace the rectifier, filter caps (higher voltage rating for 192 * 1.414 = 271v dc), possibly bigger OT (core may not pass 12-15w without severe bass-loss and/or core saturation), replacement speaker. And the PT is probably only big enough to do what the existing amp requires (though Fender cheated this rule in the 60's Champ & Princeton amps, likely for quantity discounting).

That's a lot of expense, when you could perhaps only add a jack, a few resistors, maybe a few caps and a pot or two to convert what you have into a guitar amp.

[Ritchie200]

I'd be willing to bet it was originally a 6K6 as every drug store tube tester on the planet would have been listing a 6V6 as a substitution.

Jim

[PRR]

> coulda used a 6K6

My readings suggest that when 6V6 came it it was aggressively pushed into almost all 6K6 applications. Gain is a bit better. Heater power is a bit lower. But I suspect that Ken-Rad was selling 6V6 cheaper than 6K6. Which makes it a no-brainer for any slim-profit product anywhere near a 6K6/6V6 size.

[jeff]

I need a new dog-porch. I do my math. The main beam wants to be say 1.123" by 2.789". I can find a mill to cut that exact size beam. Or I can get a $3 2x4 (1.5"x3.5") at Home Depot. It's "too strong" and "too much wood", but maybe I can find the space and it sure will be cheaper as a Standard Stud instead of a special order.

OK think I get that analogy and get why the amp is what it is.
I guess I may have gotten carried away thinking of it like this:
I buy your house and move in. I look at your dog porch and think "Wow! The frame on this thing is way over built! I don't have a dog.....but I do have a monster truck. All I'd have to do is replace the floor boards and I could park my monster truck on that thing!" Or in your car analogy, I bought a used 135HP model, with a few parts and some elbow grease could I supe it up to 185HP?

But it seems like the investment is not worth the return. I think maybe the best thing to do is like you guys say, keep it as is and just rewire the pre section.

Thanks for the help.
jeff

[jeff]

Could I use an 8 ohm speaker in place of the three 4ohm speakers(wired series for 12 ohms)?

[PRR]

> All I'd have to do is replace the floor boards and I could park my monster truck on that thing!"

My dogs are not THAT big!

Anyway, look at the whole structure. That beam could be sitting on a rock which could stand your 4 tons. Or the rock could be frost-split and will settle with 1 ton. Or 'round here the beam may be on a small rock on peat or damp clay, will start to sink-in with 500 pounds. Safe for 120 pounds of dozing dog but no more.

An amp's "foundation" is the PT. Also rectifier which may be easily upgraded. Still this is looking like 8W-12W of foundation, even if it has "6V6 beams" capable of 14W-25W. And likely your "floor board", the OT, is also a bit shy of big power. Perhaps 5W of deep clean bass or 15W of very funky bass (which may be OK for guitar).

12 ohm secondary? Yeah, put 8 ohms on it. The cathode-bias amp won't be hurt (especially as the 6V6 aren't near stress). Impedance match and parasitic OT loss mean some less power but not a big deal.

[jeff]

OK thanks. Makes sense. It is what it is.
One more thing, the cathode resistor, as is, is unbypassed. Would it be a good/bad idea to use a bypass cap?
Thanks again
 

[PRR]

> Would it be a good/bad idea to use a bypass cap?

Buy a cap, tack it in, play HARD, take it out, play HARD, make your own mind.

It does nothing for small output. The action in OVERdrive may or may not be different, or good.

Also try 22-47uFd versus 10uFd and 470uFd. The recovery time after a heavy pluck may be a musical difference.

As these are low-volt caps you can easily re-use in something else, the cost to experiment is insignificant.

[jeff]

Wow! Got the amp going and it sounds great. Thanks to all for the help.