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 Post subject: Low Power & fizz
PostPosted: Fri Jan 29, 2021 9:23 pm 
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Joined: Mon Jan 25, 2021 9:06 pm
Posts: 4
Hi

So a new Tramp builder here. I successfully built my amp and after having issues with the 400VDC (Centre tap on the power transformer was not soldered onto ground lug), Input signal (grounded coax wire on the input jack) I was up an running. DC Voltages checked out correct at tube pins so installed tubes and played it. Sounded good loud and clean in tweed and Dirty in Tude. Turned it on later and the amp sounds fizzy in all modes with a noticeable drop in power. It is most noticeable playing chords - I don't seem to be able to get a clean sound. Tube change has no effect. I re did the voltage check and I now have to turn the VRM all the way to 11 to get 400V on the B+ at the first capacitor, second capacitor is 450 VDC all the time. When it is at 400V DC the rest of the voltages check out correct on the tube pins.

If i remember correctly - the first time I didn't need to turn the power up to 11 to get the full 400VDC - re power supply caps, the first cap has some heat damage from a soldering iron when removing and checking the rectifier diodes(twice before i twigged it was the centre tap - Doh!) Resistors across all tube sockets check out etc.. & i figure if the voltages all check out correctly across the tube pins then the other components are working correctly. Bringing me back to the PSU. 650VAC at the rectifier i/p 450VDC at the o/p.

With the power level set to 9 & master volume & volume set to 6 - I can hear the guitar but no where near loud enough to where it was before and it is fizzy. Impedance and bias settings are correct.

Or m I chasing a red herring and should look else where - I have seen Herby's post re a new patch cable tried that, & a different guitar. Right now my 5W gretch is waaaay louder than my tramp and a lot cleaner.

Any comments gratefully received - it has been an adventure so far.

Thanks

Roj


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 Post subject: Re: Low Power & fizz
PostPosted: Sat Jan 30, 2021 12:27 am 
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Joined: Wed Sep 12, 2018 5:17 pm
Posts: 368
Location: Shawnee, OK
Post some good, clear pictures of amp interior ("gut shots"). :D

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 Post subject: Re: Low Power & fizz
PostPosted: Sun Jan 31, 2021 2:55 pm 
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Joined: Mon Jan 25, 2021 9:06 pm
Posts: 4
Took me a while to transfer iphone pictures to PC and convert etc... here are the pics requested. Thanks in advance for taking the time to look.

Roj


Attachments:
File comment: Whole amp
Whole amp.jpg
Whole amp.jpg [ 1.3 MiB | Viewed 1242 times ]
File comment: 6V6 socket
6V6 Socket.jpg
6V6 Socket.jpg [ 1.33 MiB | Viewed 1242 times ]
File comment: Rect bridge and cap damage
Rect bridge and cap damage.jpg
Rect bridge and cap damage.jpg [ 1.8 MiB | Viewed 1242 times ]
File comment: AX7 socket
AX7 Socket.jpg
AX7 Socket.jpg [ 1.42 MiB | Viewed 1242 times ]
File comment: Impedance switch
Impedance switch.jpg
Impedance switch.jpg [ 1.83 MiB | Viewed 1242 times ]
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 Post subject: Re: Low Power & fizz
PostPosted: Mon Feb 01, 2021 1:54 pm 
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Joined: Mon Jan 25, 2021 9:06 pm
Posts: 4
Just an update should anyone find themselves in this situation. Filter caps and associated resistors on B+ end lifted & check out fine with Fluke DVM. O/P transformer coils continuity & isolation check out ok, new tubes ordered just in case.

Roj


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 Post subject: Re: Low Power & fizz
PostPosted: Tue Feb 02, 2021 2:56 pm 
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Joined: Wed Sep 12, 2018 5:17 pm
Posts: 368
Location: Shawnee, OK
Looking at some of the solder joints in power section, I'll suggest what everybody else does :D Check for cold and/or bad solder joints in that area. Should be shiny and smooth around component and jumper wires at the eyelets. And check out Joey Velours post in resources "Of Solder and Solderability". Keep your tip clean and tinned; :bugeye: Crusty and black is BAD on a soldering tip. Sorry that's what I got for now.

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 Post subject: Re: Low Power & fizz
PostPosted: Sun Feb 07, 2021 4:28 pm 
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Joined: Mon Jan 25, 2021 9:06 pm
Posts: 4
A brief update:

Absolutely no problem with the remarks on solder - I ran out and am waiting for some more (Covid, Lock down, Mail order etc..). Once some parts arrive it will be tidied up - otherwise they will just be calling to me when ever i play the amp :D. Have decided I am not a fan of soldering irons with the temp switch on the handle - I kept turning it low by mistake and wondering why the solder wasn't melting.

As it turns out both AX7s that I had were 'off'. During my investigations I went back to my original assumptions re low power. I looked up the max voltage of a 6V6 and its something like 450V DC so it would make sense that I could only reach 400 in the upper limit of the power switch. An EL 34 safe voltage is much higher so I am unsure of the bias setting used when I first checked the amp.

Anyhoo - I got a new AX7 before a 6V6 turned up - so i tried it and got a lot better result. It also turns out that i had missed the jumper between the treble & bass pot effectively bypassing the tone stack. After those two changes the amp is much brighter and louder - to where I think it should be. I think a warmer Mesa AX7 sounds really nice but I haven't payed with the amp enough yet to know if I could of got there with JJ AX7s and the bass boost. Tude mode with a Supro tremelo pedal (for extra modulated dirt) & bridge humbucker sounds awesome.

Next stop is finishing the cabinet and the Tone tubby speaker.

I hope someone finds this useful. My electronics theory of 30+ years a go has had a good dusting off :D

Cheers

Roj


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