Amp tinkerers and people who aren't afraid of 700 volts or so in here....

Moderators: Slowy, Capt. Black

User avatar
BY RectifiedAmps
#850160
Make sure the polarity of the bias capacitor is correct (positive to ground). Another possibility is that you have a parasitic oscillation happening- that’d overload the output tubes and load down the B+. Not likely given your tidy build, but always worth considering.

Also, when it happens, quickly measure the bias voltage on each grid and see if it’s just one tube or both.
User avatar
BY Cdog
#850173
RectifiedAmps wrote:Make sure the polarity of the bias capacitor is correct (positive to ground). Another possibility is that you have a parasitic oscillation happening- that’d overload the output tubes and load down the B+. Not likely given your tidy build, but always worth considering.

Also, when it happens, quickly measure the bias voltage on each grid and see if it’s just one tube or both.


Thanks for the tips! :)
User avatar
BY Bg
#850318
spent 2 hours in the outside workshop... have melted.... 28 degrees and working outside, all you building/gardening outside for a living people are mental...

Going for a drive to the wino shop to cool down
User avatar
BY Bg
#850322
Mike Clayton wrote:Mad dogs & englishmen...


well, it wasn't midday ;)

at midday I was sat having a pint of cider outside of the Lumsden, in the shade :P
User avatar
BY Timoes
#850355
So this beasty turned up on my bench. In for a check over and check out. Looks like a well made thing. Like the power valve swap out option.

I think I could maybe wrangle a deal with the dude. I have an E series Burst Strat that he desperately wants to own. I think a straight swap is the order of the day for me. But looking at prices for both, maybe I would be better to hang on to the Strat. Its a minta and the prices for good 80s strats are climbing a bit lately. I have a few amps allready that will easily cover the Orange sound.

Waddaya Reckon ?
Attachments
20200120_180823.jpg
Curious Orange
20200120_180823.jpg (4.92 MiB) Viewed 468 times
User avatar
BY willow13
#850358
Bg wrote:all you building/gardening outside for a living people are mental...



I tell myself that every summer ... then in winter when its raining I think "you idiot" :rofl:

I hoping the wife decides to go to full time work this year so I can just take an inside job that pays less :?
User avatar
BY Cdog
#850395
Cdog wrote:Amp tech question.

AB763 fixed Bias circuit, build is complete all Voltages look normal for 2x 5881s. During final tests quite by chance noticed an intermittent Bias runaway from idle at ~28mA to a over 200mA. Bias Voltage plummeted and B+ sagged from 430VDC to a little over 220VDC. Was able to kill the power and inspect right away, thankfully. No issues observed. On re-energising it problem had disappeared... but about 5 minutes later it happens again.

First thought is replace the bias electrolytic capacitor, which I've ordered.
Could it be faulty Power Tubes? Any other ideas?

Image


It was a faulty bias cap in the end. The fault was intermittent, so I'm thankful I caught it with a meter, it was only by chance. Naughty little bugger could have cooked my new Tung Sols!

After that, prolly gonna stay on the bench for a bit more testing before I box it up... :thumbup:
Sounds absolutely dreamy ♥♥♥
User avatar
BY Mike Clayton
#850663
The first parts for my '64 Vibroverb combo project have just arrived! This amp uses the classic Fender AB763 circuit like the majority of the Blackface amps but with 460V on the 6L6 plates and a 15" speaker. Very excited and pleased with the excellent quality and accurate detail right down to the brass ground plate. This is my most expensive build to date with the parts cost already over $1700.00 - speaker and cabinet yet to be added. Any SRV fans out there? And open to suggestions or offers for a suitable 15" speaker.
Chassis, plates, boards.jpg
Chassis, plates, boards.jpg (289.08 KiB) Viewed 310 times
Last edited by Mike Clayton on Fri Jan 24, 2020 2:19 pm, edited 1 time in total.
User avatar
BY H671
#850666
I have a 15" JBL K130 available. It has been re-coned before I bought it and has a paper dust cap instead of the metal one. Apparently the paper dust cap takes away some of the ice pickiness. I was using it in my Fender Excelsior, but that has now been sold.
Last edited by H671 on Fri Jan 24, 2020 2:30 pm, edited 1 time in total.
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