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Thread: Hammond AO-29 power transformer question

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    Hammond AO-29 power transformer question

    Hi all,

    I have a power transformer from a Hammond AO-29 amp. The transformer (AO-24110) powered push pull 6V6s. I have been trying to find specs on this transformer, and Google results vary. My question is, does anyone know the current rating of the high voltage secondary? I read a thread somewhere saying that the transformer can handle 6L6s up to 35 watts. I'm thinking about building a cathode biased Fender Pro 5E5, which ran at about 25 watts. Does anyone have any data or experience to show that this transformer could handle that? Thanks!

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    Bent Member Chuck H's Avatar
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    No personal experience. The specs I found on another forum are:

    335-0-335V @90MA
    5V @2A
    6.3VCT @2.2A

    I'd say no on a pair of 6l6's. Even using a 5u4 rectifier you'll probably get around 410 plate volts. The 5e5 ran a fairly hot bias too if you want it to sound right. I would expect to use up that 90mA minimum with the whole amp just idling.

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    Supporting Member The Dude's Avatar
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    Plus, if those are indeed the specs, you'd be pushing the current limit for the filament winding since the 6L6 has about twice the filament current as the 6V6.

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    Last edited by The Dude; 01-20-2018 at 02:52 AM.
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    Quote Originally Posted by The Dude View Post
    Plus, if those are indeed the specs, you'd be pushing the current limit for the filament winding since the 6L6 has about twice the filament current as the 6V6.
    Thanks for the insight. I would like to try this transformer as a learning experience if it's not going to be DOA, so I've got a ton of follow-up questions.

    First, the filament winding's not so much an issue. The AO-24110 heater current draw is 3.3A, so probably has a 3.5-4A winding? Draw on the 5E5 will be 2.7A, 2.4a if I don't use a pilot light. So a little wiggle room there. So:

    With the not fully loaded 6.3v winding, would the HV secondary see any benefit? If the answer is yes, would it also be worth it to use a GZ34 instead of the stock 5U4? I'd save 1A on the 5v heater winding. If HV is too high with GZ, install maybe 4-5 10v zeners in seres with HV center tap to ground to bring it down?

    Then possibly, bias the 6L6Gs a little cool.


    What do you think?

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    Quote Originally Posted by Chuck H View Post
    335-0-335V @90MA
    5V @2A
    6.3VCT @2.2A.
    Thanks Chuck, but I think at least some of these are off? The AO-29 uses a 5U4GB so it's got to have a 3a 5v heater winding. And the tube complement (with 10 6.3v-heated tubes) has to be at least 3.3A for the collective current draw. The only part of the puzzle I didn't have was the ma rating of the HV secondary. Thanks!

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Diverted View Post
    Thanks Chuck, but I think at least some of these are off? The AO-29 uses a 5U4GB so it's got to have a 3a 5v heater winding. And the tube complement (with 10 6.3v-heated tubes) has to be at least 3.3A for the collective current draw. The only part of the puzzle I didn't have was the ma rating of the HV secondary. Thanks!
    I didn't look up the AO-29. I looked up the transformer number provided. Certainly the one account I found with specs could be wrong. But I will say that the only other amp I saw mentioned that used that transformer was a 2Xel84 amp and I never found any 6L6 references to that transformer. So I'll stick to what I said above and suggest that you don't use that PT for a pair of 6L6's.

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    "Back to the amp. It makes horrible sounds when I play my guitar thru it... because I suck at playing guitar." Mike6158

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    Thanks for the insight.

    The plot thickens ... found a good schematic for the amp this transformer came from. From what it looks like here there may be enough on the HV secondary?Click image for larger version. 

Name:	transformer.jpg 
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    Bent Member Chuck H's Avatar
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    That looks better. 140mA should be alright without much room to spare, but Hammonds, even the older ones, are usually pretty good about that.

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    "Back to the amp. It makes horrible sounds when I play my guitar thru it... because I suck at playing guitar." Mike6158

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    Good to know. I'll try and play it conservative Will let you know how it goes. Thanks for the help.

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