Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Locrian99 on January 04, 2025, 07:16:27 pm
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A friend of mine recently picked up this amp off marketplace. Got it home it was working fine. He tried to do the speaker into the headphone jack thing. However he did not power the amp down when he did it, he just put it in standby. As he recalls after he switxhed it out of standby the fuse blew.
And it blew another as soon as it was powered on. He brought it to me. I checked the diodes they are all measuring fine. Its 40 years old so i just went ahead and ordered the caps as they havent been changed. Waiting on those.
I tried to power the amp on with no power tubes, it was not it standby when i did this. It made a loud hum from the speaker before it again blew the fuse. This has me concerned that it most likely is the OT? There was no smell or pop when it initially happened.
If it is the OT im not positive the best way to test, just remove the primary and secondary and check for a short? Dont want to install the new caps if its the ot…
Thanks
Well the capacitors are holding a little bit of a charge still hours after it was powered on and the fuse insta blew nearly. So I don’t think its a cap issue.
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Two possibilities, assuming you weren’t there and don’t know exactly what he did:
- He plugged a speaker into the headphone jack but left another cable plugged into the speaker jack which was not connected to anything else
- He plugged a speaker into the headphone jack but the 40 speaker jack doesn’t ground itself well anymore when unplugged (worn out, oxidation, or otherwise making poor contact)
Both of those cases would lead to the output transformer having no load and at risk of destruction when signal was applied.
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How to test OT . I do this few times with success ;
Testing Output Transformer and finding Impedance
Amp at OFF, disconnect from wall outlet
Remove power tubes
1- Signal generator set to 1000 hertz AC 0.5 volts ( I read some people with no audio generator use the 3.15 VAC from the heater. I never try )
2- Send this signal to OT secondary , check with accurate meter to have 0.5 VAC on OT tap. Or close
3- Read AC voltage at OT primary
4- For exemple if you read 14.35 VAC you divide by 0.5 ( or what you read at 1- )
14.35 / 0.5 = 28.7
5- Square the result ; 28.7 X 28.7 = 829.67
6- Multiply by secondary output impedance where you are connected , say 8 ohms
829.69 X 8 = 6631
7 - This reading 6631 is the primary input impedance
If you use two 6L6 in push pull , look on Tube Book for Plate Load resistance . You’ll see you that you need 6000 to 3800 ohms for two 6L6 in PP class AB
The reading we got 6631 is on spec
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If transformer is no good your reading will be very far from what we should get .
By experience these test do not work on Super Reverb Transformer or other with 2 ohms Output Transformer impedance
You can do this test on unknow OT to see how / where you can use it
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Well doing some basic tests on the xformer. Got an open primary, and a secondary wirh continuity to ground. So I guess thats that. Now to find a suitable replacement.
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Well doing some basic tests on the xformer. Got an open primary, and a secondary wirh continuity to ground. So I guess thats that. Now to find a suitable replacement.
How did you test?
Which tranny, PT or OT?
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Well doing some basic tests on the xformer. Got an open primary, and a secondary wirh continuity to ground. So I guess thats that. Now to find a suitable replacement.
How did you test?
Which tranny, PT or OT?
So on the ot. I used my dmm. Tested between the center tap (b+ might be the right term) and ine of the primaries 153 ohms, tried between center and otther no connection. Between the two primaries no connection. Secondary that goes to the power tube through the resistor (sorry i can never rememenr which is the right term) has continuity between the secondary tab and the chassis, ground end of a filter cap, and the case if the transformer.
White wire top left no connection. Black wire bottom left continuity with ground
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Well doing some basic tests on the xformer. Got an open primary, and a secondary wirh continuity to ground. So I guess thats that. Now to find a suitable replacement.
That is not a reliable tests.
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Well doing some basic tests on the xformer. Got an open primary, and a secondary wirh continuity to ground. So I guess thats that. Now to find a suitable replacement.
That is not a reliable tests.
I guess I don’t understand why testing these has to be so complicated. Not trying to be argumentative I’m sure you know a lot more than me. But it seems like either it should have continuity with ground or it shouldn’t. If it shouldn’t and it does isn’t that a problem? Just like the the primary being open. Its either got a connection or not?
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So on the ot. I used my dmm. Tested between the center tap (b+ might be the right term) and ine of the primaries 153 ohms, tried between center and otther no connection. Between the two primaries no connection.
That's enough to declare the OT bad. No need to do any more testing.
Secondary that goes to the power tube through the resistor (sorry i can never rememenr which is the right term) has continuity between the secondary tab and the chassis, ground end of a filter cap, and the case if the transformer.
That's normal because the common lead on the secondary is connected to ground. Therefore, all secondary leads will show continuity to chassis.
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I used my dmm.
White wire top left no connection.
Is the meter set for OHMS or CONTINUITY?
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I used my dmm.
White wire top left no connection.
Meter was set to ohms. The other primary was reading 153 ohms.
Reading never changed from the OL setting its one you dont have to set the range on.
Is the meter set for OHMS or CONTINUITY?