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

Moderators: Slowy, Capt. Black

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BY Mike Clayton
H671 wrote:I have a JBL E130 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.

Sounds interesting. How much are you asking for it?
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BY H671
I made a typo, it's a JBL K130 15" - 103 Db SPL perfect for surf music. Looking for$150.
User avatar
BY Mike Clayton
Cdog wrote:
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?


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 ♥♥♥

Meant to say, what a great looking build :clap:
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BY Mike Clayton
'64 Vibroverb chassis is starting to take shape. Drilling done, valve bases installed, heaters wired (far canal, hate it!!!), bias board populated/installed & transformers mounted. After struggling with heater wiring over many builds I'm gonna try and get push-back 18 gauge solid core heater wire for next time. Should make life way easier.
Chassis Build Progress#1.jpg
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BY Cdog
Mike Clayton wrote:Meant to say, what a great looking build :clap:

Cheers Mike! :)

I have never built a Fender before, and eyelet boards are new to me too. It took some head scratching and lots of Google image search to figure some things out. Finding a decent layout diagram on robrobinette's site was very helpful. Here's a couple of better quality pics of it as it stands, still incomplete. Couple of finishing tweaks to do yet, haven't decided if it needs a MV. I'll do a completed build post once it's done... Maybe in another year or so :lol:

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BY Cdog
bender wrote:If you don't add a Master Vol, get a good attenuator- these sound best in the 4-6 range, which can be pretty damn loud.

Cheers, good to know. I look forward to getting to know this amp's personality. :)
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BY Cdog
Mike Clayton wrote:Corey, was this a kit or did you source all the bits individually? And what are those tag boards doing near the 6L6's?

Mike, it was a Mojotone kit, but I changed a few things. :)
To answer your question, the blue things near the power tubes are MOVs, a protection measure for the OT in case of speaker disconnection or other sudden failure. Next to those I have a couple of 1-Ohm resistors in series with the power-tube cathodes (to ground), to make for easy biasing.

There's a few other geeky things going on. Film resistors for better reliability. Switchable speaker impedance for the OT. All PT windings except bias are fused. IEC fused AC inlet. No standby switch, but a NTC thermistor on the Phase line to try reduce inrush current (not convinced it's doing much). 5AR4 rectifier instead of GZ34 to knock down the B+, it's around 430VDC running 5881's instead of 6L6GCs. :geek:
Just tried this out tonight. It's a 5e3 with robrobinette's mods, master vol, lead and normal channel. Its consideribly easier to overdrive compared to stock 5e3's. It's quite crunchy.

I need to get some more dial bulbs. I am using the old radio dial lights as an indicator.

The radio/pa would have originally done duty in a school or motel

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