AO-44 Iron

[Matt D]

A friend gave me a Hammond AO-44 reverb amp and I was wondering if anyone knows how much current I can get out of it. The amp has 2 6gw8's and an ez81. The schematic says 250-0-250 so I was thinking with a solid state rectifier I could perhaps run 2 el84's and 2 12ax7's for an 18 Watt Lite IIb build. Sound reasonable?

Thanks.

[Phil_S]

It is probably best to assume the existing tube compliment is near the limit of the transformer's capacity. The point is, you don't really know how much you can push it, so don't try. Fried iron is worthless.

Look up each tube and figure out how much current it typically requires. What you see on the tube data sheets is close enough to answer your question. Go here to look up tube data: http://tubedata.itchurch.org/index.html

I'll do this one. You confirm it by going through the exercise. Then you have an example and will know what to do next time.

One 6GW8 need 43.2mA (includes pentode plate+screen and triode plate). So, that's about 87mA on the high voltage secondary.

For the filaments, one 6GW8 needs .66A and the EZ81 needs 1A. That's 2.32A for the heaters. It is reasonable to guess there is a little more available, but probably not much.

You want to run a pair of EL84's and a pair of 12AX7's.

The EL84's are a little tricky because the plate and screen current is stated as a range. I usually figure about 90ma for the plates and 20mA for screens. That's 110mA for a pair of EL84. The pair of 12AX7's you also have to guess, maybe 3mA per tube, so that's another 6mA. This makes your total requirement a little less than 120mA. I am guessing you don't have this much available and suggests it is the wrong PT. You might scrounge a separate PT for the preamp tubes to bring the load down.

For the filaments, .76A for each EL84 and .3A for each 12AX7. That = 2.42A. This is a reasonable match for the 2.32A that was existing. You'll know if you are stretching it when you measure your filament voltage in circuit. Wire up sockets for the filaments, plug in tubes and measure it.

What do you think? Maybe someone has done what you suggest and knows if there is enough in the PT for the pair of EL84s. I can't tell you that.

[Matt D]

Thanks Phil.

Perhaps I could try the same arrangement with 6aq5's as the power tubes.

The GE datasheet shows the combined plate and screen currents for these falling within a range of 75 and 92 mA (zero signal and maximum signal plate/screen currents).

Assuming an average of 83 mA or so and adding 6 mA for the 12ax7's I have 89 mA. Cutting it close but maybe.

Any suggestions for slightly lower current power tubes?

[Phil_S]

Actually, I was hoping you'd solve the problem with 6AQ5's. It's a great idea and now it's your idea . I recognize this is strictly personal taste, but I prefer the 6AQ5 over the EL84.

[Phil_S]

I was thinking...a really fine solution for a great sounding low power amp is a pair of EL95/6DL5. These sip heater voltage at 0.2A each, and only require about 30-34mA per tube to operate. I think you are looking at about 7W in PP output. You also have enough juice left over to add a 3rd 12AX7, opening up a world of preamp possibilities.

The challenge is to find a pair at a reasonable price. 6AQ5 tend to really be dirt cheap and are more common.

[Matt D]

I already have a bunch of 6aq5s on hand so that seems to be the way to go. Right now I'm working on an 18 Watt based around and old Allied/Knight 18 Watt kit amp transformer. The ao-44 will be next up. I could build something like a Gibson GA-5 Crest. I'll look around for some Marshall style 6aq5 circuits as well. Thanks again for your help.

[Phil_S]

The GA-5 with a pair of 6AQ5's sounds perfect to me. I love those old Gibsons.

[loctal]

Matt, I thought you were going to make rocket next out of that HP signal generator.

The ao-44 could be used to make a nice 6aq5 amp with a couple of 12ax7s. You could even keep the 6ca4 rectifier. That chassis will be a tight fit. The 6v6 18Watt lite is well documented on 18watt.com.

[topbrent]

Great candidate for a Lite IIB.

I bought an AO44 Lite IIB from mmoser like that and have built a few variants since then. Right now I have it configured as a Hoffman Stout and it is a ripper!

The AO-44 PT is 240-0-240 V.
The stock hammond AO-44 tube configuration draws 2300mA / 2.3A

EZ81=1000mA
6GW8=660mAx2=1320mA
------------------------------
2320mA / 2.3A

That EZ-81 rectifier tube is a total current sucking pig at 1A / 1000mA.
Ditch the EZ-81 and just run a couple of diodes. This will let the PT run happily and as a bonus, the rectified voltage should rise nicely up to around 340 +/- which is happy land for a couple of EL-84 tubes and a couple of 12a_7 tubes.

EL84=760mA x2 = 1520mA +
12AX7=150mA x2 +300mA
---------------------------------------------
= 1820mA / 1.8A


As a thought exercise, there is theoretically enough juice in that little PT to run 2 6L6's and 2 12AX7's. 6L6's draw .9A each. This combo would draw 2.1A, so you would still be less than the existing EZ81/6Gw8x2 combo

[Phil_S]

It is the high voltage secondary that's the problem, not the heater winding. A pair of 6L6 at peak current draw can get up in to the 140mA range. I doubt this PT has that much to give.

[Matt D]

Ok, How about a Superlite TMB with 6v6's? The 6v6's don't draw that much more plate current and will be able to handle the higher voltages of the diode rectification.

[topbrent]

Ok, How about a Superlite TMB with 6v6's? The 6v6's don't draw that much more plate current and will be able to handle the higher voltages of the diode rectification.

I would think it would handle this arrangement without any hesitation.
- I would run JJ 6V6's for a nice punchy sound. It isn't a requirement though, as 340v is no big thing for a 6V6.

To save space in the tiny chassis, run the tonestack caps directly on the pots.

[Matt D]

I was planning on using a larger chassis anyway. Sounds like a plan. Now I just need to hurry up and get the one I'm presently working on done......