6AQ5 vs. 6BQ5/EL84

[EETStudent]

What are some differences between the two? Mainly, output power and tonal qualities

[J Martin]

They sound a lot like a 6V6 and are good for about 10 watts per pair.

[Rob Mercure]

The 6AQ5 is a beam power tube based on a 6V6 and really does sound amazingly like one - only when really cranked to the max does it sound different and that's still good. The 6BQ5 is the EIAA designation for the European designed EL-84 pentode which sounds, to my ears, much like a EL-34 - a "family" resemblance." While both these bottles are rated about the same for some reason the EL84 has continued in design and production while the 6AQ5 hasn't. Which is a shame cuz I generally like the sound of the 6V6 family better (6CM6, 12AB5, some others) and in the late '90s I made a series of amp using the Mil Spec 6094 and 6005 variants of the 6AQ5 - one PP - FB amp featured fan cooling of the 6094 and put out over 20 watts without fail (would have probably melted without the massive air flow).

If you're making amps try these - all are NOS/UOS and a great bargain for the price as the worst 6AQ5 you're likely to encounter will probably be better made than the best EL84 you're likely to encounter.

Rob

[EETStudent]

What about a quad of 6AQ5s? Similar to the output of a quad of 6BQ5s under the same conditions?

[Tubenit]

If you're considering using a 6AQ5 which is a great sounding tube........ note the limits of 250v on the plate voltage. I think your challenge will be to find a PT that will give you the lower voltage but has 200ma rating for 4 tubes plus preamp and phase invertor.

http://www.nj7p.org/Tube4.php?tube=6AQ5

With respect, Tubenit

[imaradiostar]

I bet the 6005w or similar would easily take 350 volts at the plate if you keep the current down a little bit and this allows you to use some more commonly available 275-300 volt pt's. I just bought 10 of them from Canada the other week for $20 shipped- an excellent deal considering the cost of modern 6V6's. I'm hoping to build a pair of Princeton reverb style amps with them. I also hope to use a pair in my friend's musicmaster bass amp. It's sorely hurting for mods!

jamie

[Rob Mercure]

Tubenit,

You've probably noticed that vintage Fender Deluxes/Champs ran the 6V6s at voltages in excess of design as did the Tweeds push the 5881, etc. Well the same hold true for 6AQ5s - it's not voltage but dissipation that truly govern's a tube's "comfort level." And it depends on the idividual tube to some degree. The military/industrial types I mentioned (there are more) can probably hold 350V if the dissipation is watched and the consumer varieties 300 - but this is "off the cuff" engineering. But these bottles are so very cheap these days that one could afford to "TTD" ("Test (un)Til Destruction) and see what works. In general I've found that the 6AQ5 will take whatever the 6BQ5 can as long as you note that the dissipation is less and either limit the dissipation or use forced air or other cooling.

Rob

[Chuck_Farlee]

I've built a couple of champ-style circuits using the 6AQ5A and 6005, a 5F2-A Princeton and a 5C1 champ. The PT I used put about 300v plate-to-cathode on the 6AQ5. I biased the 6AQ5 tube to 12W plate dissipation and have not had any redplating or other problems.

To me the 6AQ5 sounds like a cross between a 6V6 and an EL84. It has a nice bright top end, but not as harsh or grating as an EL84 can be in SE, Class A circuits, IMHO.

chuck

[EETStudent]

Well the PT I am looking to use with these tubes is running 2xEL34s, equaling 3A of heater current. 4x6AQ5s should only give out about 2.8A so we should be good there. The voltage ratings are 375-0-375 for my PT, and about
475vdc after rectification. Obviously, 475 or anywhere in that ballpark is too much for 6aq5 tubes, but what if the voltage I sent to the grids came from a source later on down the line? I know that my voltage source into the preamp is about 278vdc, so could I draw from that so that I dont overload my power tubes?

It was also noted that I should watch the current into these tubes. Well it will cathode biased, so I guess the cathode resistor/bypass cap will affect the current. What size values should I be looking at here?

[EETStudent]

Keep in mind this will later be my main gigging amp, so i want to acheive as much output power as possible while staying under relatively safe conditions

[imaradiostar]

sounds to me like you could use a choke input power supply instead of a capacitor input and lower your voltages quite a bit. Look at some of the Hammond chokes on the internet.

You may also want to look at Duncan's power supply designer. I've found it to be really helpful in figuring out power supplies using parts in a different way from the original design.

jamie

[EETStudent]

I do have a choke in the power supply of this amp. what do you mean choke input power supply?

[Enzo]

That means that the very FIRST thing after the rectifier is a choke. NO FILTER CAP to ground, just a choke. After the choke, THEN you can put the caps to ground as always.

If you have a cap on the rectifier side - which is what you usually see with chokes - then the rectifier sees the cap and it is a common cap input filter.

A cap input filter charges up to about 1.4 times the RMS AC being rectified. A choke input filter charges up to about .9 times the AC. A substantially lower voltage from the same PT winding.

[Tubenit]

The data sheets show 250v max ratings & while I agree that some amps exceed the max ratings on the power tubes ............. you might want to look at some amps that have been built with 6Aq5's to see what has been done.

See if you can find an amp with 6AQ5's that has 350v on the plates?

Maybe there are some out there? Can you point out some?

Gibson Scout - 300v
http://www.schematicheaven.com/gibsonamps/ga17rvt.pdf

I believe the Musicmaster Bass made by Fender came with either 6AQ5's or 6V6's. The plates have 290v
http://www.schematicheaven.com/fende...bass_schem.pdf

With respect, Tubenit