Author Topic: 145/720 Tube Amp - Dubious Power Transformer?  (Read 1097 times)

Offline dottyp123

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145/720 Tube Amp - Dubious Power Transformer?
« on: November 16, 2019, 08:38:27 PM »
My first post - I've been very grateful for all the information I've found so far, but I couldn't find this exact question answered previously. I hope someone might have a suggestion:

My 720A made sound when I bought it, but since taking possession I haven't turned it on. Looking at the 145/720 power amp it looks as though the power transformer has been overheated. There are big blobs of black foam that have oozed out. I took the tubes out and measured 700V on the secondary with no load - so it is not dead. I don't see anywhere else that looks obviously cooked - but I could be missing something.

Has anyone seen this problem before?  Is there a fix for the root cause?

I was planning to replace capacitors & see how it sounds - maybe I should get a new transformer from VV?

Any tips?

Thanks a lot!

Offline pianotuner steveo

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Re: 145/720 Tube Amp - Dubious Power Transformer?
« Reply #1 on: November 17, 2019, 02:07:43 PM »
I did have one power transformer go bad, but it opened up, not shorted. It's possible yours failed due to old age, but it's too risky to just replace that part and hope for the best. You may need a pro amp tech troubleshoot it before you start buying parts.
1960 Wurlitzer model 700 EP
1968 Gibson G101 Combo organ
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1980 Wurlitzer 200A EP
1980 Wurlitzer 270 Butterfly Grand
2000 Yamaha acoustic piano
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Offline dottyp123

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Re: 145/720 Tube Amp - Dubious Power Transformer?
« Reply #2 on: November 17, 2019, 03:48:33 PM »
Thanks! I will get the rest of the circuit checked out.

Offline DocWurly

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Re: 145/720 Tube Amp - Dubious Power Transformer?
« Reply #3 on: November 20, 2019, 06:47:38 PM »
Yes, I've experienced that particular transformer going bad more than once.  oy.

Tropical Fish of Tuckahoe, NY are experts on that amp.  Call them.  Lucky you, by the way.  That's a fantastic instrument--one of the best-designed Wurlies ever, as long as the reed screws are good on it.  (and if not, that can be fixed).  What's the serial number?  The later 720A's are actually from the 145B-era (1965).

Offline Tim Hodges

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Re: 145/720 Tube Amp - Dubious Power Transformer?
« Reply #4 on: November 21, 2019, 03:38:44 AM »
Sounds like the tar has gotten hot at somepoint during it's life and leaked from the transformer. Possibly due to a bad tube or electrolytics drawing too much voltage, doesn't mean that the transformer is toast (as you have confirmed you're still getting voltages) it's probably best to get it checked over by a tech anyway.

I've had a few problems with 200 / 200a transformers not due to tar leaking but them developing an audible hum. This is down to delamination of the steel plates where the resin used to keep the plates together within the transformer winding becomes brittle and starts to oscillate. I've found 2 out of 3 N.O.S. replacements I bought had given me this issue.
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Offline pianotuner steveo

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Re: 145/720 Tube Amp - Dubious Power Transformer?
« Reply #5 on: November 22, 2019, 05:21:16 AM »
Did the transformer smoke or at least smell like it was burning?
A little different, but,
I once had a transformer for an overhead flourescent ceiling light overheat and started smoking. Tar was leaking out too. Luckily, I knew how to remove it and threw it in a snow bank outside. It was so hot that I had to use oven mits to take down the light....
1960 Wurlitzer model 700 EP
1968 Gibson G101 Combo organ
1975 Rhodes Piano Bass
1979 Wurlitzer 206A EP
1980 Wurlitzer 200A EP
1980 Wurlitzer 270 Butterfly Grand
2000 Yamaha acoustic piano
2004 Hammond XK3
2009 73A Rhodes Mark 7
2009 Korg SV-1 73
....and a few guitars...

Offline dottyp123

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Re: 145/720 Tube Amp - Dubious Power Transformer?
« Reply #6 on: November 22, 2019, 05:33:28 PM »
Thanks a lot for the replies!
The serial number is 41302. I'd love to know the age.

I acquired the piano after the "thermal incident" so I don't know how bad it smelled! There are ~3/4" diameter blobs of hardened foamy black stuff on the top of the transformer, and underneath where the wires come out.

It was too hot at some point in time. Maybe not the fault of the transformer itself - since it does still appear to be functioning.
I will get the amp checked out & hopefully it can be rebuilt with some fresh capacitors. The tubes themselves could be bad, and the output stage seems the most likely to have issues with thermal stability.

I was kind of hoping there was a known tweak - but in general it is good news if this issue isn't very common.