Fender Super Bassman Automatic Bias issues

[nevetslab]

While there's no scale to gauge these by, the pitch on the 20-pin header is 2mm ctr-ctr, both across the 10 pins and the two rows. THAT'S why this is so hard to find. I get a bit outraged hearing the policy of some service centers, stating they will NOT sell the part, but will ONLY provide it in the process of they're installation and service fee. I still have the spare Auto-bias board that I had ordered, which didn't solve the problem on the amp in this thread I posted in July 2015. Somebody has to offer the IDC connectors and the raw ribbon cable to make these. I don't recall how wide and long I made my search...probably just the main resources...Mouser, Digi-Key, Allied, Newark. With 2mm ctr-ctr pitch, that would make the ribbon 1mm pitch. Looking at the 2nd to last photo, I see the other end of the short ribbon connects underneath the power amp PCB. I'm assuming that header is the same as what's on the bias board (both shrouded males)...though I can't really tell. My notes didn't detail the cable. I did try to pull the defective one apart, but that didn't work on the re-assembly. It was a PITA to install as I recall.

[johnk_10]

yeah, and that tiny 3 conductor flat ribbon that plugs into the auto bias board was nearly impossible to reconnect. they didn't leave enough length to plug it in easily.
IMO, whoever designed this amp had no business using components that are hard to obtain and service. who'da thunk a 300 watt tube bass head would have more in common with a computer than a bass amplifier.

[Enzo]

I might wager the pitch is 0.100 INCH rather than metric. That is the standard spacing between pin on an IC.

[nevetslab]

yeah, and that tiny 3 conductor flat ribbon that plugs into the auto bias board was nearly impossible to reconnect. they didn't leave enough length to plug it in easily.
IMO, whoever designed this amp had no business using components that are hard to obtain and service. who'da thunk a 300 watt tube bass head would have more in common with a computer than a bass amplifier.

DITTO on that! That computer board came from their Super Sonic 100, though that amp only has 4 tubes (6L6GC's). I do recall dealing in that cramped space with hands the size of a small child being required, and it received some verbal abuse from me in the process!

The bias computer will work ok if you have a new matched sextet to start with. It don't do nothing but cause grief if you're trying to cobble a working set of tubes together from pulls, instead of spending $250-$300 for a fresh matched sextet of tubes. AND, you can't use KT-88 tubes in the chassis, unless you really want broken glass during transit on the road! I wrote up a thread on this subject and the abuse their bias computer inflicts on ya last month...7/03/18. What everybody needs in their amp....a bloody computer!

[johnk_10]

what does the tiny 3 conductor mylar ribbon that connects to the auto bias display do? i'm not sure that i have it seated in the tiny connector that goes into the auto bias through the rear panel?
i have the schematic to the amp but it doesn't show it. it's nearly impossible to seat it in the receptacle since it's so short.

[nevetslab]

I might wager the pitch is 0.100 INCH rather than metric. That is the standard spacing between pin on an IC.

Actually it IS 2mm pitch, and that format IS avaialble....just not very plentiful. I double-checked it on the spare automatic bias card I have on hand. Definitely NOT 0.1" pitch. The other connector on that board IS 0.1" pitch (16-pin 2 x 8)

[Enzo]

Fair enough.

[johnk_10]

well, after massaging the original connector and reflowing some solder on it, it's working perfectly now. thank GOD.

[nevetslab]

I only have the Super Sonic 100 schematic. I don't have any of the service doc's on the Super Bassman. J3 is the Autobias 3-pin connector. There is a connection to a pair of 2 momentary switches S1 & S2, which applies +3.7V to the IC controller Pins PA12 & PA15. If I had to guess, I'd say S1 & S2 are the UP and DOWN buttons on the Autobias control panel.

[nevetslab]

well, after massaging the original connector and reflowing some solder on it, it's working perfectly now. thank GOD.

NICE WORK, John!

[g1]

Not sure if it's the same auto-bias board, but some info on ribbon cable replacement for Bassman 100T here:
https://music-electronics-forum.com/...l=1#post550819

[nevetslab]

Our former Guitar Dept manager inherited a 'Broken' WHITE Fender Super Bassman and brought it to me right around when I was in the middle of the Mike Campbell Vintage Amp restoration project middle of last year. It came back to my small shop after I had to move back into my smaller shop space and has been sitting on the floor by the door, used to have other Ampeg SVT-CLs and such stacked atop it since last July. With no staff in this building, and my 72 yr old body telling me AIN'T NO WAY YOU'RE PICKING THAT UP UNASSISTED, it sat there until this past week, when I got somebody to help lift it up to the checkout bench.

I pulled it apart, removing the power amp / power supply chassis to the test bench and went thru it to see what I could find. I found just one bad part....D208 1N4006 rectifier diode that was near-short, reading 60 ohms in both directions. Replaced that, and not finding anything else, apart from a Snap-In IEC 320 AC mains connector that never remains tightly fitted (only one pair of flex-tabs to hold it into place), I finally tack-soldered in my Cathode Test cables so IF the power amp came to life, I'd be able to read the plate current on them.

As the Power Amp chassis has Preamp Output / Power Amp Input jacks on the rear panel, I was wondering if I needed to even have the Preamp connected. Never had tried that, and, no service doc's to go by. Powered up that way to see.

It powered up just fine without the Preamp attached. I had dead-patched the Power Amp input jack just to be sure. The Auto-Bias circuit worked....CORRECTLY, TO MY AMAZEMENT! The six GT 6550 matched power tubes all worked, and apart from one reading a touch high, it was a good balance. I was able to switch in and out of Standby and all continued to work. I could change the Bias from coldest to hottest settings, seeing the results on my test probes as well as on my power analyzer. Then, connected my test speaker, patched in Burst Pink Noise, and switch back out of Standby. All working fine, so YES, you CAN run this with the Preamp disconnected. Cranked it up, watching the Auto-Bias periodically tweak the tubes, and seeing the Power Analyzer's Wattage readings follow the burst waveform.

Powered down, then moved the Preamp chassis in to sit on the cleats in the cabinet, so it could be partially open, in case i needed to go digging into it. Powered back up with the Preamp connected, and it lit up just fine. Began with the dummy load, so see if there was anything wrong. No surprises, so changed to the test speaker, plugged the Burst Pink Noise into it, and carefully walked thru the two channels, one by one. Found a couple noisy pots, but they exercised out just fine. EVERYTHING WORKS IN THE PREAMP!! Bloody amazing.

I haven't yet plugged my 60's vintage Fender P-Bass in to give it a listen, but will get there in the morning.

Nothing like inheriting an amp like this for free, with Fender's Repair Dept not able to figure it out. No idea WHAT they did prior to it coming to me, but only finding a simple rectifier diode bad, I'd say Chuch (new owner of the amp) SCORED BIG TIME HERE!

[5dbworkshop]

Hi!

Coming my way are two super bassman's (A/B rigs for a well-know british band), both with potential bias issues. Having discovered this thread via google, I had to sign up just to thank everyone, especially nevetslab for the very useful information given here.

I have pre-emptively ordered premade 2mm cables and a couple of connectors from Farnell in the UK and will post back with my findings.

Amps are being dropped off on the 13th and the gig is on the 18th - yikes!

Thanks again

[nevetslab]

Well, I'll keep my eyes open for your posts, and best of luck. I've made myself a pair of 3-channel cathode-sense cables that terminate in some old molex 5-pin female connectors so i can plug my probes in and measure the cathode voltage across their cathode resistors so you can read the bias current (think they use 10 ohms, or maybe it's 1 ohm...have to check. very handy support cables. The power and output xfmrs can be removed from the cabinet so you can set these up on the bench outside of the cabinet. Very clumsy, but that's what the minds at fender gave us.

[thekirkmanator]

New member here. I fixed my auto bias with the ribbon cable change, then screwed up and burned r132 and r134 on the rear I/o pcb. Can’t read their values. Does anyone happen to know what their values are? Can’t find a schematic for super bassman 300.