Blackface Bassman blowing fuses on startup

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Re: Blackface Bassman blowing fuses on startup

Postby Yoda on Tue Apr 26, 2016 10:52 pm

keithb7 wrote:To the originator: Were you able to figure this out? Any updates?
I am curious and like to learn from examples this this. Thanks, Keith.


No resolution to the problem yet, I haven't had time to go over the amp again and see if the ground switch is defective or not. As to the filter caps, one guy posted a drawing of blonde Twin filter caps but I cannot rely on that as that is a completely different amp.
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Re: Blackface Bassman blowing fuses on startup

Postby keithb7 on Wed Apr 27, 2016 3:05 am

Ya that was me who posted the drawing of the Blonde Twin amp. I custom wired it up to have a safer power supply when the amp was in stand-by mode.
The Blonde Twins were not originally wired like your BF Bassman was when new. I changed it though, so now it is wired just like your Bassman. The first two caps are wired in series on your Bassman. Have a look at this Bassman AA165 schematic:

Image


Here's a larger version: http://elektrotanya.com/PREVIEWS/63463243/23432455/fender/fender_bassman_aa165_sch.pdf_1.png

Notice at bottom right corner near the standby switch. You can see two 70uF/350V caps wires in series. See where the voltage reads +425V? The voltage travels into the first cap, positive end. Out
through the negative end of the cap, and right into the next cap right below it. See how the voltage goes into the positive end of the second cap? Then flows to ground through the negative end?
Look at your second cap from the right, shown in your photo. See the dirty aged red wire connected to the negative end of your cap? Take note of it, then look at this layout of your
AA165 amp below.

Image

Look at the wire coming off the left side of your standby switch. See how it connects to the wire coming from your bias supply? See how it shows the wire as labeled "Red"? Can you see
the voltage listed there too? It reads +425V. That is the red wire going to your second cap in your cap photo. Looking at the schematic above again, you'll see that wire
should be going to the positive end of the cap. You have it wired to the negative end of your cap. I'm no pro tech, but I'd start there and get those first two caps turned around
and orientated like the schematic shows. I too am learning as I go here so this is why I am interested. I have no problem with someone correcting me if I'm out to lunch.
I too want to learn. - Keith
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Re: Blackface Bassman blowing fuses on startup

Postby pdf64 on Wed Apr 27, 2016 10:08 am

keithb7 wrote:Am I dreaming or are those first two 100 uF caps installed backwards?

I suspect you are correct!
It definitely needs verifying.
If so, the caps should be regarded as no longer fit for service.

Whatever, use a light bulb limiter for any future power up, until all is proved good.
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Re: Blackface Bassman blowing fuses on startup

Postby Yoda on Thu Apr 28, 2016 6:22 am

UPDATE:

I reversed the positions of the two 100uF/350v caps, as they were indeed installed backwards, with the positive to ground. I had used an erroneous filter cap board diagram from Mojotone that had incorrectly colored wires and led me to install the capacitors incorrectly. I've been able to turn the amp on and play it without any problems. The only thing left is to order a cap pan/dog house cover. I also have some 1M resistors I'd like to install for biasing.

Thank you everybody for the help with troubleshooting. I'm glad it was a simple fix and that no damage occurred to the amp.
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Re: Blackface Bassman blowing fuses on startup

Postby pdf64 on Thu Apr 28, 2016 9:26 am

That's good then :thumbsup:
Note that the amp presents a lethal electrocution risk if operated without the doghouse cover in place :shock:
I suggest that it is best left on the test bench until that is sorted.

As mentioned, the caps have almost certainly been exposed to a massive reverse polarity fault current; that would have damaged them :cry:
Although they may still seem to work, I suspect that their operational characteristics will no longer be in spec, and operational life much reduced.
I would not leave them in the amp, as they are critical to the amp's performance, reliability and longevity :!:

Bias resistors (if actually cathode current sensing resistors, in the cathode 0V return of the power tubes) are usually 1 ohm; a resistor of 1M is 1,000,000 ohms :wink:
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Re: Blackface Bassman blowing fuses on startup

Postby drewleslie on Thu Apr 28, 2016 11:50 am

If you are going to install the one ohm bias resistors, make sure that you get very high quality ones. Several amps brought to me to fix noise issues have been traced to these ... one or more failing or drifting. For me, a device like weber's bias-rite is a better solution. drew
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Re: Blackface Bassman blowing fuses on startup

Postby Yoda on Thu Apr 28, 2016 12:25 pm

Sorry for the typo, I meant to say 1 ohm resistors not 1M, 1 ohm is what I have on hand for biasing. They are 2W metal oxide, 1% tolerance, best I could find locally so they should suffice.

Also, I will be replacing the two 100uF/350v capacitors, they've served their purpose in troubleshooting so now that I've turned them around and made the amp operational again I know what the problem was and I can replace with new caps without any issues.
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Re: Blackface Bassman blowing fuses on startup

Postby keithb7 on Thu Apr 28, 2016 1:53 pm

I'm glad you were able to sort the problem out. I love to hear of these old Fender amps being taken care of and set up to run for years to come.
Another tip, in the future I would recommend you take a look at F&T Caps for your B+ . Made in Germany. Great caps that I run in all my vintage amps.
Considerably less cost than Spague & Atom. I think there is some hyped up marketing pricing going on with the Spragues.

See here: https://www.tubesandmore.com/products/capacitors?filters=Brand%3DF%26T

I believe you can also get a new dog house cover from these guys. I have bought many many parts from them to keep my small army of vintage amps alive. I recommend.
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Re: Blackface Bassman blowing fuses on startup

Postby Yoda on Sun May 01, 2016 1:31 am

So I installed the 1 ohm resistors for biasing and installed a brand-new pair of Tung Sol 5881s and biased them at 39.5mA. I left the amp on for a while for the tubes to settle in (connected to a 2x12 cab, volume on 0) and out of nowhere there's a very loud shrieking sound coming through the speakers, I turned the standby switch off (to "standby") and the noise went away, and then I turned the amp completely off. Any idea what may cause this?
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Re: Blackface Bassman blowing fuses on startup

Postby keithb7 on Sun May 01, 2016 4:02 am

Were you into the output transformer connections at all? Reverse the 2 to the speaker by accident?
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Re: Blackface Bassman blowing fuses on startup

Postby Yoda on Sun May 01, 2016 5:16 am

keithb7 wrote:Were you into the output transformer connections at all? Reverse the 2 to the speaker by accident?


Not at all, I've never had any reason to mess with any of the transformer wires.
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Re: Blackface Bassman blowing fuses on startup

Postby pdf64 on Sun May 01, 2016 8:41 am

Had your new power tubes been tested at high voltage by the vendor?
Had they been burned in by the vendor?
The 39.5mA you measured would be the cathode current; what was the HT or plate voltage at that current draw?
Note that with saggy power supplies in such amps, voltage will fall as the current supplied rises.
Bear in mind that those tubes have a max rated plate dissipation of 23 watts http://www.tungsol.com/tungsol/specs/5881-tung-sol.pdf
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Re: Blackface Bassman blowing fuses on startup

Postby Yoda on Sun May 01, 2016 7:27 pm

I measured the plate voltage at 460v prior to installing the 5881s, and biased them to 39.5mA using the simple bias calculator from tedweber.com.
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Re: Blackface Bassman blowing fuses on startup

Postby Dean Jr. on Fri May 06, 2016 5:22 pm

Yoda, that plate voltage reading prior to installation of the tubes that you biased at 39.5ma is not valid for purposes of understanding the biasing, ime. the plate voltage is best read at the bias point with the tubes that are being biased. that plate voltage swings with current draw. When the current draw increases, the plate voltage decreases and vice-a-versa. When the current draw swing is not very large, the plate voltage swing might be neglible, but I like to know what it is at any certain bias point.
FWiw, IF the plate voltage remained at 460Pv with the 5881's at 39.5ma, those tubes are dissipating somewhere close to 79% of max plate dissipation. that is not overly hot, but some would question why it needs to be that high. I believe in making the decision based on sonics, and ime the difference between say 69% and 79% with regard to sonics is not worth the extra wear and tear on the tubes at the higher plate dissipation. YMMV>.... But....what if the plate voltage is NOT 460vdc? IF you took that plate voltage with tubes that were running even hotter, that voltage might be highernow with the differetn tubes at a lower bias point???
I worked on a '76 vibroverb last week in which someone had installed a gZ34 rectifier as opposed to the 5U4 that should be there. Plate voltage was 451VDC. Installing a 5U4 dropped the voltage down to 431VDC...still too high...and the amp still sounded terribly thin and harsh. I then checked the biasing....20% of max plate dissipation....very cold---harsh, thin, non-musical. I quickly adjusted the bias to something just over 50%. The plate voltage dropped volts to around 410....right with the BF VR schematic---the SF VR's seem to do this...thankfully. The amp began to sound like a musical instrument. (;^) AFter all of the work was done, I did a final biasing to about 58%, which is a good point for this Western Swing player.....good sonics with long tube life. the plate voltage hung right there at 410VDC....no big swing because the biasing was in the zone.
IF the tubes in that amp of yours were drawing an really high current draw, than it might be ill-advised to accept that the 460VDc is what your plates are seeing right now. I am not saying that the amp is not set-up properly. I am simply saying that without measuring that plate voltage at the bias point you have right now, you do not know what the plate dissipation is....and that is what you want to know. EX: IF the biasing was very hot with the old tubes when you measured 460VDC and IF that plate voltage went UP when you installed the 5881's and biased them to 39.5ma of current draw, you are running hotter than 79% of max plate diss. What if the Pv went up to 470vdc at that 39.5ma of draw???
then, those 5881's are working at a couple of points higher....almost 82% of MPD.
Long story short, I like to know what is going on, and there is only one way to know that. You have to take your measurements for the equations with the tubes and the biasing that are being considered. I think that there is a really good chance that your amp is operating in a safe manner, but one wonders....and without knowing the Pv at 39.5ma of draw, one doesn't know exactly what is going on.
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Re: Blackface Bassman blowing fuses on startup

Postby Yoda on Fri May 06, 2016 6:25 pm

Thanks for the info, I'll poke around inside the amp again when I have time. I do remember checking the plate voltage prior to all this work and seeing 410v so that may be where it's at now.
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