AC30 Schematic -- I Can't Believe my Eyes... 160ma ?!?!

[bob p]

So I'm looking at an old AC30 schematic. Here it is:



If you take a look at the power transformer, it's spec'd as 580 VCT (280-0-280) at 160mA. That's right, it says 160mA. Hard to believe, for sure.

After looking at the 6BQ5 data sheet I was thinking that I'd need to source a PT that delivers about 250mA. Wow. That's a huge discrepancy.

So I started looking at published specs for reproduction AC30 transformers. The Hammond 290NX specs the current at 262mA and the 290MX specs the current at 230mA:

AC30 Vintage: http://www.hammondmfg.com/pdf/EDB290NX.pdf
AC30 Reissue: http://www.hammondmfg.com/pdf/EDB290MX.pdf

What's up with that value on the schematic?

[Enzo]

As RG has explained before, the wire in the transformer can get as hot as it wants, as long as the insulation doesn;t fail. Perhaps exceeding the ratings of the power transformer was part of the model's particular sound.

Vox could get away with whatever they got away with. Hammond would have a hard time justifying selling an under-spec'd part as a replacement. They would have to recommend using their product beyond its ratings. Even if it would survive.


Just some thoughts...

[bob p]

I had wondered if they might have really used iron that was under spec'd for the job, either on purpose or by accident. that's part of the big mystery, isn't it?

Looking at the AC15 type of builds, conventional wisdom says that if you're going to build the 2xEL84 amp then you're going to have to choose the 270EX that is rated at 144mA, and if you're going to go with 4xEL84 then you should choose the 270HX at 230mA. There's just no way that a 160mA tranny makes sense for the AC30, unless getting that wrong is part of the magic. funny that *nobody* is doing that today. Not Hammond. Not Korg. ClassicTone is using 230mA iron. And that Mercury iron that has no specs is friggin' huge.

This makes me wonder if some dummy just copied the wrong info on to the schematic. It would certainly make sense if the explanation was that some dummy copied the AC15 transformer specs onto the AC30 schematic. But then to further complicate the issue, I find this post where Bruce says that a 175mA tranny is good enough for the job.

AMPAGE Archive: Re: Hammond 270 FX /AC30 clone

Everything is contradictory. Harumph.

[Ted]

60s AC30s often eat their power transformers. It's a fact of life. Possibly explained by the fact that they were underspec'ed to begin with.

Modern insulation materials can be run hotter and modern wire enamelling techniques are much more consistent than they were in 1960. So a modern transformer (underspec'ed or not) with the same rating will be more reliable than one made 50 years ago, and will stand up to more abuse.

[Bruce / Mission Amps]

Many tweed era 6L6 amps used a power tranny that was rated at only 160ma... but think about that.
160ma at 340vac = 54 watts
280vac at 160ma = around 45 watts
Sounds OK to me even if they do get hotter then a $2 pistol in the back alley of Chicago.

[Dave H]

You were supposed to play clean in the ‘60s

It has to be 560V CT x 160mA = 90VA. The output section is running at 12.5V across the 50 ohm cathode resistor at 30W output which is 330V x 250mA = 82VA (if the supply is 330V) but that would be for a continuous sine wave. It’s going to be less than that on average for a peaky guitar signal unless you’re square waving it. AC15 schematic says 300-0-300V @120mA

[Ted]

You were supposed to play clean in the ‘60s

It has to be 560V CT x 160mA = 90VA. The output section is running at 12.5V across the 50 ohm cathode resistor at 30W output which is 330V x 250mA = 82VA (if the supply is 330V) but that would be for a continuous sine wave. It’s going to be less than that on average for a peaky guitar signal unless you’re square waving it. AC15 schematic says 300-0-300V @120mA

The cathodes sit at about 10.5V at idle. That's 210mA.. Its no wonder they get so hot....

[bob p]

> AC15 schematic says 300-0-300V @120mA

Indeed it does. You must be looking at one of those hard to read Jennings schematics. I've been looking at the V-1-5 schematic because it's easy to read. It doesn't have the 120mA spec on it. Knowing the 120mA spec blows my theory about mis-copying the wrong value to the AC30 schematic. So I guess the 160mA rating on the above Jennings AC30 schematic is not an error.

Thanks.

[bob p]

+1 on the original amps running hot. Joe P recently mentioned that the replacement PT in his trap head has been gigging for a couple of years without getting very hot. I'm thinking that it's got to have better current specs than the original, and might actually lie in the camp of the PT with high current specs.

OK, we've established a few things:
(A) the original PT were under spec'd at 160mA
(B) today everyone's using a higher current spec of 230-260mA
(C) today you can get away with (A), but the PT will get HOT.

But the real question is whether there is any putative advantage to doing (C) on purpose vs. using a "proper" PT as in (B).

I'm still trying to get a good grip on what possible behavioral benefits could come from trading off reliability and intentionally using an underspec transformer.

As an example: I have a spare 175mA 550 VCT PT that's taking up space here, begging to find a home in a head, but I can't see a good reason to intentionally build using an underspec transformer without hearing a decent hypothetical reason for why doing that would be better than just picking up another PT with a "proper" current rating. The cost of buying another PT isn't really a factor in the equation, and reliability will become a factor in the equation, but only if nothing else matters.

I'm hoping someone can talk about the Secret Black Magic of the under-sized transformer recipe, if there is any.

[Jazz P Bass]

I'm still trying to get a good grip on what possible behavioral benefits could come from trading off reliability and intentionally using an underspec transformer

The manufacturer got a boat load of them cheap?

[bob p]

Everyone's a comedian.

[olddawg]

A power transformer is just that, a voltage/current source. Does it really matter if the original is a bit under specifications and runs hot or the replacement is the size of a refrigerator and made by NASA if it provides the necessary and accurate voltage and current? The amp shouldn't care?

[pdf64]

The smaller PT would likely have worse regulation, eg be more saggy, so may impact on how the amp responds, and also act to limit the amp's max continuous output.
Pete

[Steve Conner]

Well, a smaller transformer would sag more under load. But the AC30 is biased very hot, almost in Class A, so the current draw doesn't vary that much with the audio signal, hence sag is less of an issue.

The original AC30 was also famous for eating tubes. Maybe you could extend tube life and get away with the 175mA PT if you modified it to run at a lower idle current.

PT current ratings vary. Some specify the RMS current rating of the winding, others specify the average DC current you can pull from a capacitor-input rectifier. In tube amps the difference between the two ratings isn't that great, but if you intend to run right on the ragged edge, it's good to know.

[bob p]

160ma at 340vac --> 54 watts
160ma at 280vac --> 45 watts

It's tempting to just grab onto those numbers and think that the VA rating is sufficient, but then I have to remind myself that those are input watts not output watts, and that there are a lot of thermal loss watts that have to be accounted for along the way. Like tubes, resistors, etc. ...