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 Post subject: 6AU6 vs 6AK5
PostPosted: Oct Mon 31, 2011 5:58 am 
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I have a small Philco audio amp that uses a 6AU6 to drive PP 6AQ5s

The plate voltage on the 6AQ5s is around 250 v with screen at 240v
The plate on the 6AU6 is only about 140v and 40v on the screen.
Image

I was looking at specs of some other tubes and I saw that a 6AK6 power-pentode indicated that a 6AU6 can be used as a sub, .....even though the 6AU6 is a sharp cutoff pentode.

So.. that got me thinking if I could do the reverse and have a 6AK6 sub for a 6AU6.... so I tried it.
Well.... it worked but sounded very bad.. screwy sounding.. not right.

So while I was looking in the tube manual I also saw that a 6ak5 is a lot MORE like the 6AU6 but with lower plate voltage ratings.
So I tried using the 6AK5 in place of the 6AU6 and it really sounds wonderful.
Even draws less filament current.
I cannot tell from the tube specs why this isn't a good idea to do.
... so is it ok to do?
.. and any experience with this?
BTW:
How about C5, the .1uf cap in the feedback loop?
Can I use a 50v rated cap there?... or should it be rated higher for any audio spikes that might happen. Right now the one i put in is 50vdc

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 Post subject: Re: 6AU6 vs 6AK5
PostPosted: Oct Mon 31, 2011 7:05 am 
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Peter, the specs for the AK5 are within reason, and if it works for you, go for it.
As for the feedback cap, it only sees a few volts max.
100 ohms to ground at one end, and secondary of the OT at the other end.

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 Post subject: Re: 6AU6 vs 6AK5
PostPosted: Oct Mon 31, 2011 7:16 am 
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Pbpix wrote:
I have a small Philco audio amp that uses a 6AU6 to drive PP 6AQ5s

The plate on the 6AU6 is only about 140v and 40v on the screen.

... 6AK6 sub for a 6AU6.... so I tried it. ... sounded very bad ...

... tried using the 6AK5 in place of the 6AU6 and it really sounds wonderful.

BTW: How about C5, the .1uf cap in the feedback loop? Right now the one i put in is 50vdc


From what I've seen, power tubes usually have higher negative cutoff voltage. For a test, put the 6AK6 back in and measure the plate voltage. I'll bet it will be a lot lower than the 140V design point. So the crappy sound is probably due to clipping.

Do the same for the 6AK5, and the original 6AU6. Take note of the plate voltage of all three. The original 6AU6 might be tired, and it may show if the plate voltage is too far from the desired 140V. See how a new 6AU6 sounds. But if the 6AK5 works just fine, and you have a bunch of them, I don't see any harm.

Cutoff values: 6AU6 -5.5V; 6AK5 -8.5V; 6AK6 more negative than 18V. 6BH6 -7.7V. See how a 6BH6 sounds. Transconductance values of the 6AU6, 6AK5, and 6BH6 are also pretty close; ~5000. The 6AK6 is only ~2500.

As for C5, it usually sees low voltages. What's the output power of that amp ? If its a 10W amp, that's 9V rms, 13V peak, into a 8 ohm load. I don't know how much headroom that thing has. Crank it up, into an 8 ohm dummy load, and measure the voltage at the feedback tap. Its gotta max out somewhere, so those should be the highest excursions possible.

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 Post subject: Re: 6AU6 vs 6AK5
PostPosted: Oct Mon 31, 2011 9:11 am 
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Pbpix wrote:
Image



So.. that got me thinking if I could do the reverse and have a 6AK6 sub for a 6AU6.... so I tried it.
Well.... it worked but sounded very bad.. screwy sounding.. not right.


Screwy sound for a screwy design. The control grid of the lower 6AQ5 is tied to ground. Purdy unconventional.


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 Post subject: Re: 6AU6 vs 6AK5
PostPosted: Oct Mon 31, 2011 1:54 pm 
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Peter

Using a 6AK5 in place of 6AU6 will work but there is a slight basing difference, 7BD vs 7BK. There are a lot of TV IF amplifier tubes that should also work. Most of time 6CB6, 6AG5, 6BC5 etc are considered useless but have the same basing.

6AK6 was made as an audio output tube and requires more grid bias.

That is an unusual 6AQ5 audio circuit, grounded grid amplifier. Signal input is cathode.

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 Post subject: Re: 6AU6 vs 6AK5
PostPosted: Oct Mon 31, 2011 2:58 pm 
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Cheap. Saves the cost of a phase inverter tube and components. It is almost as rinky dink as the Philco screen grid phase inverter circuit.

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 Post subject: Re: 6AU6 vs 6AK5
PostPosted: Oct Mon 31, 2011 6:53 pm 
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The primary advantage of the circuit is inherently low distortion. It is basically a differential amplifier so both 6AQ5s run class A. Both tubes amplify both sides of the input signal so there is no potential for crossover distortion. Audio coupling is through the 6AQ5 cathodes. It requires a clean signal from the 6AU6 amplifier so any change will affect the output sound quality. Since both 6AQ5s run class A, it draws more idle current than other circuit designs.

It also is practical for driving electrostatic speakers since either 6AQ5 plate has full undistorted signal.

Philco used variations of the design for many years even after dropping the electrostatic speaker. .


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 Post subject: Re: 6AU6 vs 6AK5
PostPosted: Oct Mon 31, 2011 8:55 pm 
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What is the value of R11? Try a 100 K pot in place of R7 and connect a 0.22uFd from screen to
ground, and see if you can tweak the sound of the 6AK5.

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 Post subject: Re: 6AU6 vs 6AK5
PostPosted: Nov Tue 01, 2011 12:23 am 
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threeneurons wrote:
From what I've seen, power tubes usually have higher negative cutoff voltage. For a test, put the 6AK6 back in and measure the plate voltage. I'll bet it will be a lot lower than the 140V design point. So the crappy sound is probably due to clipping.


No doubt, the 6AK6 is a power pentode and will draw far more current than the 6AU6, plate voltage is going to be very low unless the bias is adjusted... Still the 6AK6 isn't meant to be small signal amplifier, volume will likely be less...

As far as the 6AU6 subbing for a 6AK6 as a audio output, I can't see that working well enough to even be considered...

Tom


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 Post subject: Re: 6AU6 vs 6AK5
PostPosted: Nov Tue 01, 2011 3:39 am 
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Hi Peter, Make sure the upper 6AQ5 is strong. The performance of that tube will greatly affect the whole amp. If it's weak, the lower tube will not get driven properly either. (The amplitude of each phase will be really unbalanced)This is because the upper cathode feeds the lower cathode with audio, in a grounded grid configuration. (low impedance) In a "normal" phase inverter circuit IE: long tailed pair or Williamson circuit, audio drive will still be present if one output tube is weak, missing or dead. Of course it will sound different, but not as bad as your circuit. Makes me wonder if the circuit you display, was intended to make the buyer replace tubes "all the time" to maintain "ok" audio????? Really cheap and silly design. If I were you Pete, I would remove the 6AU6 and rewire the circuit for a 6J6, as a phase inverter. It's also a 7 pin tube (dual triode with a common cathode) and can be wired in a long tailed pair configuration to give good gain.

Good Luck


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 Post subject: Re: 6AU6 vs 6AK5
PostPosted: Nov Tue 01, 2011 3:51 am 
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Peter may need the high gain from a 6AU6. A 6J6 would make an audio amp and phase inverter but may not have enough gain for a tuner or tape input?

Grounded grid amp is different. Sometimes used in amateur transmitters but haven't seen it in an audio amp. Wonder how well balanced? Good project for a scope.

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 Post subject: Re: 6AU6 vs 6AK5
PostPosted: Nov Tue 01, 2011 4:12 am 
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Philco had some interesting circuits... I have a small 6AQ5 amp of probably similar vintage that has R & L channels, plus a center bass channel that is operated push-pull... In one channel it uses a 6AU6 for audio amp and the other has a 12AU7, as audio and phase inverter... The plate of the inverter has 13v B+...

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 Post subject: Re: 6AU6 vs 6AK5
PostPosted: Nov Tue 01, 2011 4:23 am 
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Ever see an amp that takes preamp filament voltage from cathodes of pair of 6L6's? A 12AX7 filament was wired between cathode and B-. This allowed the preamp to operate with DC on the filaments without additional cirtcuitry.

The Philco grid #2 phase inverter is different. Never did check if it was well balanced.

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 Post subject: Re: 6AU6 vs 6AK5
PostPosted: Nov Tue 01, 2011 4:24 am 
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Tubenut wrote:
Hi Peter, Make sure the upper 6AQ5 is strong. The performance of that tube will greatly affect the whole amp. If it's weak, the lower tube will not get driven properly either. (The amplitude of each phase will be really unbalanced)This is because the upper cathode feeds the lower cathode with audio, in a grounded grid configuration. (low impedance) In a "normal" phase inverter circuit IE: long tailed pair or Williamson circuit, audio drive will still be present if one output tube is weak, missing or dead. Of course it will sound different, but not as bad as your circuit. Makes me wonder if the circuit you display, was intended to make the buyer replace tubes "all the time" to maintain "ok" audio????? Really cheap and silly design. If I were you Pete, I would remove the 6AU6 and rewire the circuit for a 6J6, as a phase inverter. It's also a 7 pin tube (dual triode with a common cathode) and can be wired in a long tailed pair configuration to give good gain.

Good Luck

Thank you:
I didn't realize one of the pair had more importance. The tubes that came are all Philco. and seem to be ok... so I haven't "tested" them in my tube-tester yet as i had no question.. but i'll do that later.
I also ordered six NOS 6AQ5s from FindATube.. and he sent me 6 of the military 6005W (Sylvania)... all new. So I have them. I cannot hear a difference compared to the Philcos so I haven't swapped them in permanently.
Edit:
I just checked all 4 6aq5 Philcos on my Precision 10-20 Tube-tester and the 4 philcos all measure similarly ( 10% into the "good"-green)
The JAN 6005Ws test 15% into the "good"-green
So they all seem good.




If you think I'd really like the 6J6 idea.. and if you think it would make a serious performance difference.
Could I ask you to please sketch those suggested changes for me showing any component value changes as well please?

The 270 ohm (common) cathode bias resistor finds it's path to ground by way of the full 2ndary winding of the OT where it meets up with the feed back path... which confuses me a bit to say the least... lol

Will that need changing?

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Last edited by Pbpix on Nov Tue 01, 2011 5:09 am, edited 2 times in total.

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 Post subject: Re: 6AU6 vs 6AK5
PostPosted: Nov Tue 01, 2011 4:46 am 
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The circuit will operate at about 25% power without V3. V2 must be there for V3 to function.

The circuit is not too bad as designed. I am not sure what you intend to accomplish with modifications. What exactly is your objective, distortion, power, current drain? The output transformer, speaker, components, power supply and circuit are selected to optimize the performance. Changing any one will necessitate other changes to make a difference.

A 6J6 would not be a first choice for audio amp. You may want to try the more likely 12AU7. That requires punching out the 6AU6 socket for a 9 pin and extensive circuit changes. In the end, I am not convinced you will hear any difference.


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 Post subject: Re: 6AU6 vs 6AK5
PostPosted: Nov Tue 01, 2011 4:58 am 
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Philco Don wrote:
The circuit will operate at about 25% power without V3. V2 must be there for V3 to function.

The circuit is not too bad as designed. I am not sure what you intend to accomplish with modifications. What exactly is your objective? The output transformer, speaker, components, power supply and circuit are selected to optimize the performance. Changing any one will necessitate other changes to make a difference.


I have no agenda per se.
Just trying to rebuild these and hook the two together as a stereo pair.
So I've picked up a dual volume pot to install. Then I'll mechanically tie the two little chassis together and try to build a common front panel... something like that.
A learning experience is the primary goal and if they perform well ... then the pleasure of that aspect too.

But I already have a sweet little PP5902 home-brew I built last year that i love:
Image

Do you remember seeing the thread for it here on ARF?
(pages 4 and 5 show final drawings and completed project)
http://antiqueradios.com/forums/viewtop ... 3&start=80

In addition I JUST bought the super-wonderful ASL AQ1003DT PP-EL34 amp in "mint" condition that was my first real serious tip-toe experiment into the "heady" audio pleasure-land.

So I don't need this Philco to take their places.. but to learn more with and have fun ... and I'll have the others to compare it to.

So do you advise that I just keep them as designed and enjoy them as is?
I don't have enough of an "ear" to consider them good or bad yet.

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 Post subject: Re: 6AU6 vs 6AK5
PostPosted: Nov Tue 01, 2011 5:51 am 
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radiotechnician wrote:
What is the value of R11? Try a 100 K pot in place of R7 and connect a 0.22uFd from screen to
ground, and see if you can tweak the sound of the 6AK5.

Hi:
R-11 is 470k
The 6AK5 already sounds good... it's the 6ak6 that sounded bad.

So you want me to put the pot in and a 0.22uf cap to ground from the screen at pin 2 & 7 point where it meets the 10uf cap?
And this is to improve the 6AK5 or 6AK6... which?

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 Post subject: Re: 6AU6 vs 6AK5
PostPosted: Nov Tue 01, 2011 6:44 am 
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Many designs took the output cathode curent to supply the preamp filaments, I have a couple of Scotts and I think Fisher did it too. Wouldn't think of it now, for a couple of LM317's could do it just fine. but it was an elegant design way back when.


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 Post subject: Re: 6AU6 vs 6AK5
PostPosted: Nov Tue 01, 2011 7:32 am 
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Hi Peter. Here is a link explaining the phase inverter. They show a 6J6 circuit. I haven't built this version, but it looks ok. In order to match phase amplitude, (the VR that they show) you will need a dual trace scope. There are some guitar amps built with the 6J6 as a phase inverter, so it is not uncommon in audio. This would be the least invasive way to convert that chassis, without making a larger tube socket hole.

link: http://mysite.du.edu/~etuttle/electron/elect33.htm


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 Post subject: Re: 6AU6 vs 6AK5
PostPosted: Nov Tue 01, 2011 10:37 am 
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Tubenut wrote:
Hi Peter. Here is a link explaining the phase inverter. They show a 6J6 circuit. I haven't built this version, but it looks ok. In order to match phase amplitude, (the VR that they show) you will need a dual trace scope. There are some guitar amps built with the 6J6 as a phase inverter, so it is not uncommon in audio. This would be the least invasive way to convert that chassis, without making a larger tube socket hole.

link: http://mysite.du.edu/~etuttle/electron/elect33.htm

Thanks:
That does seem simple to implement...

Image

but what of the treble tone control that I have in the orig circuit??
It now uses a cap from the 6au6 plate to a pot.
Wouldn't that have to be done in duplicate somehow?
How does that get handled?
Could I possible still do the same as now but just have two separate caps feeding one single pot ...each cap coming from the two out of phase plates of the 6j6? ... Or would that just be coupling the two plates together though the caps making a mess?
Maybe that's why they chose this circuit in the 1st place?... to provide tone controls .. but simply?

What do you think?
Is it easy to do with phase-splitting too?

Edit updated:
After I shut the computer off last night I realized that the treble tone control as it exist now would probably be able be connected to pin #1. That way the adjusted-tone would affect both phases ... right?
I think it looks like that would work. Am I right?

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