freeing up heater current

[tubescent]

i have a hifi amp i'm looking to convert to guitar amp.
it's output section is 2 + el84. if i rebias it to run 2 + 6aq5 (basically 6v6) i was wondering if the difference in heater ma would allow me to run another preamp valve (or two?) for reverb and maybe tremelo.
what i mean is looking at the el84 and 6aq5 (small bottle 6v6) heater specs:
6aq5 : 450ma
el84 : 760ma

that's a difference of 300ma which should be enough to run another 12ax7.
is this right or am i dreaming? it's a roundabout way to do things but i wanna save some money on iron for this build.

[Diablo]

It's possible that the PT has excess heater current capacity. You could hook up some different resistors to the heater winding on the PT and measure current draw and voltage. Find out how much current you need to draw before the voltage drops below 6.3 volts.

[J M Fahey]

Hi .
Stick to the EL84s , a 6AQ5 is *not* a 6V6 at all.
Use the available tubes for a *good* preamp, maybe add *one* extra 12AX7 ; reverb , tremolo and loop can be solid state, as in most modern commercial tube amps

[tubeswell]

You can also use a (mains voltage) lamp off the mains - to save a few extra mA by not using a 6V one off the heater winding. (However this typically will save 150mA - or 300mA at the most, depending on the 6V lamp you would otherwise have used)

[Enzo]

JM, if you look up 6V6 in the RCA tube manual you will see a note in the opening paragraph stating "These tubes are equivalent in performance to type 6AQ5A. REfer to type 6AQ5A for average plate characteristic curves."

[tubescent]

why the *stars*?
is this a forum convention i don't understand or keyboard dyslexia/typo?

[J M Fahey]

[OFF]

Hi tubescent.
I'm writing an answer for you about the 6AQ5.
Anyway now I'll explain you about the *stars*.
It's an old Internet convention to enhance text.
Fact is that although many think that Bill Gates invented Internet and before him there existed nothing, and think Windows is the only operating system, and its conventions rule everywhere, fact is: it ain't so.
Here you can enhance visibility of a text by using bold, underlined, italics, different colors, fonts, etc. but often those "cosmetics" get lost when going from one operating system to another or even to different language page coding, but *this* never gets lost.
In fact, when Gmail tells me somebody answered some post here,replaces every enhancement with *stars*.
Example:
what did you say? becomes:
what *did* you say?
Or to answer exactly what you asked: it's not a "forum" convention, but an *Internet* convention. Consider text within stars (really: asterisks) as underlined .
Hope it's clear now.
PS: since nothing seems to have authority these days if it's not backed by some Internet page, here goes one explaining *Emphasis* applied to some text.:
Source: PHP Markdown: Concepts
It says:
>>>>>>>>>>>>>>>>>>>>>>>>>>> ( these arrows mean: "from here")
Emphasis

You can put emphasis some part of your text inside a paragraph. There is two types of emphasis: normal emphasis and strong emphasis. On most websites, emphasis is shown as italic while strong emphasis is bold.

To give emphasis to some text with Markdown, we use asterisks (*) or underscores (_). Surrounding some text with one asterisk or one underscore bar gives normal emphasis, while surrounding some text with two asterisks or two underscores results in strong emphasis:

*normal emphasis with asterisks*

_normal emphasis with underscore_

**strong emphasis with asterisks**

__strong emphasis with underscore__

This is some text *emphased* with asterisks.

And it gives this result:

normal emphasis with asterisks

normal emphasis with underscore

strong emphasis with asterisks

strong emphasis with underscore

This is some text emphased with asterisks.

Please note that there is absolutely no difference between using underscore and asterisks: both give exactly the same result. Choose the one you are the most confortable with.
<<<<<<<<<<<<<<<<<<< (these arrows mean : "to here")

[J M Fahey]

Hi Enzo, thanks for what you post
I *do* understand what you say, but tubescent may be misled by our conversation, please let me write a more extended reply in his benefit.
Yes, the RCA Tube manual says so.
I don't have it handy but, if you say so, your word is good enough for me, period.
Maybe I should have used a few more words in my answer, but I wanted to make it short, my mistake ; so now I'll make it more boring, .... ahem!, .... detailed.....
The point here is that our friend tubescent thought he could replace 6BQ5 with 6AQ5 with no ill effects (and saving a few mA heater current).
That was the real question, and my answer saying "don't do it" still stands.
Unfortunately he was misled by a bad analogy : 6AQ5 can replace 6BQ5 *because* "6AQ5=6V6" and "everybody knows that 6V6 is as powerful as 6BQ5".
That's the implicit reasoning, or why else would he have mentioned 6V6, a tube that is neither the original one nor the proposed substitute?
Problem is, and this is a basic point: is 6AQ5=6V6?
Yes, "up to a point" , or "sometimes", or "in less critical applications", take your pick.
Is "6AQ5=6V6" valid when substituting 2xEL84 in an amplifier? : NO !
Please let me justify this statement.
Since I don't want to compare pears to oranges, I searched until I found all three tube datasheets, from the same manufacturer, so the data is presented in comparable form.
I also wanted a "traditional/classic" maker and not , for example, Sovtek, Shuguang or JJ, with all due respect they deserve.
I found General Electric (USA) to provide such data, I hope you agree to trust such a classic source.
What did I find?
Just to begin with:
Sylvania says:

Yes, I also read a warning there: within its maximum ratings (not beyond)
And what does it mean?
-"Is 6AQ5 a "light" 6V6 ?"
-"Can 6V6 do things that 6AQ5 can not?"
-"Can 6AQ5 replace 6V6 (on light duty), but not beyond certain limits?"
I'd say, "all of the above", but let's see what GE says:


Well, I see a difference here: 6AQ6 : 250V tops; 6V6 tops 315V. A big difference.
And what does 6BQ5 support? Because that is the original tube. Let's see:

Well, enough of cold specs, let's see what can they do in an amplifier:



Interesting:
6AQ5: 10 watts with 250V
6BQ5: 17 watts with 300V (either class AB1 or B)
6V6: 10 W with 250V, 14W with 285V, probably 17W with 300 V, because we know it stands 315V per spec.
We see that 6V6 can substitute 6AQ5, as correctly stated by Enzo, but the opposite is not true, 6AQ5 can neither substitute 6BQ5 nor 6V6.
Which I stated earlier using far fewer words.
Sorry if I didn't make it clearer from the beginning.
So, in a nutshell:
Dear tubescent, please do not substitute 6AQ5 for 6BQ5 because you are going to a much weaker tube.

[olddawg]

You can also use a (mains voltage) lamp off the mains - to save a few extra mA by not using a 6V one off the heater winding. (However this typically will save 150mA - or 300mA at the most, depending on the 6V lamp you would otherwise have used)

Also, if it has a tube rectifier you can use a SS rectifier. If the rectifier tube is using the 6v filament line and not a 5v tap, you will then have that current to use. The SS rectifier will also raise your B+ a bit which may be better for a 6V6 build. Personally, I would keep the EL84s, especially if you have some good old tubes. I find that these old Hifi units use a lower B+ than most guitar amp applications. Using and SS rectifier usually brings the B+ more in line with what you want even for an EL84 amp.

[soundmasterg]

You could try a pair of 6CM6's, which are also very similar to the 6V6 but are in a 9 pin bottle. Check out the data sheet:

NJ7P Tube Database Search

Jim McShane (a respected tube dealer) has torture tested these and they will definitly take more than the 315v max just as vintage 6V6's will take more than their max. Best of all, they are CHEAP and sound good. You can get some from Antique Electronics Supply. Antique Electronic Supply .

I gave a pair to a friend for use in his hifi in place of an EL84 pair and they work fine and sound better.

Greg

[Enzo]

Quite a thesis JM. Yes, they are not the same tube repackaged. Was just making the point that they were also not totally separate.

[tubescent]

you guys have all given me good solutions, info, and food for thought.
really appreciated