Hi all, just before the Xmas break I acquired a Fender Bassman 50 head which is a bit iconic really. I bought it not working, and the main problem was that one of 6L6GC output pair had the location lug broken off and the valve had been inserted within the wrong rotation !?! - anyways no permanent damage and all the vitals are fine.
When I first rigged it up (having sorted the original problem), I plugged the speaker into 'Ext speaker' as one might....... really thin sound and osc, thought the OP tx might be a gonner........but it turns out that unless one connects the speaker cab to the 'Speaker' jack socket, that socket has contacts which present a short circuit across the output (!?!?!?!). This would happen if no speaker jacks were connected too.
Great relief in my case, the OP tx is fine.
It is all shown as such on the schematic, I just thought it must be a mistake - but apparently not ! I wonder why Fender in their wisdom thought a short circuit better than an open circuit with no speakers connected ?
Can the AB165 amp really run into a short circuit load better than an open circuit load (for a short period) ?
How strange ?!
When I first rigged it up (having sorted the original problem), I plugged the speaker into 'Ext speaker' as one might....... really thin sound and osc, thought the OP tx might be a gonner........but it turns out that unless one connects the speaker cab to the 'Speaker' jack socket, that socket has contacts which present a short circuit across the output (!?!?!?!). This would happen if no speaker jacks were connected too.
Great relief in my case, the OP tx is fine.
It is all shown as such on the schematic, I just thought it must be a mistake - but apparently not ! I wonder why Fender in their wisdom thought a short circuit better than an open circuit with no speakers connected ?
Can the AB165 amp really run into a short circuit load better than an open circuit load (for a short period) ?
How strange ?!
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Running into a short circuit will avoid nasty spikes of back EMF from the output transformer sparking across inside the output valves or the valve socket, causing serious damage.
Thanks - yes, open circuit (no speakers) has the hazard of spikey and excessively high voltage anode voltages.
Hobson's choice, but S/C must be preferable, and that seems the choice Fender positively made here.
Maybe the short circuit on the secondary stops the transformer from working as a transformer at all, thus somehow protecting it ? Then one is left with the 6L6GCs running into an anode load of about 50 ohms - hot they will get as Yoda would say (only if driven though).
Anyways, it seems necessary to always have a load connected to the 'Speaker' jack socket on a Bassman 50 head - but not for the usual reasons !
I wondered whether this wiring arose as some mistake, where the original intention was to wire the two sockets in series? But it isn't a mistake now and is even shown on the schematic.
Hobson's choice, but S/C must be preferable, and that seems the choice Fender positively made here.
Maybe the short circuit on the secondary stops the transformer from working as a transformer at all, thus somehow protecting it ? Then one is left with the 6L6GCs running into an anode load of about 50 ohms - hot they will get as Yoda would say (only if driven though).
Anyways, it seems necessary to always have a load connected to the 'Speaker' jack socket on a Bassman 50 head - but not for the usual reasons !
I wondered whether this wiring arose as some mistake, where the original intention was to wire the two sockets in series? But it isn't a mistake now and is even shown on the schematic.
Just checked the Showman and Twin Reverb schematics, and they both have the same arrangement !
I wonder how many Fender OPTs have been falsely assumed to be duff because of this ?! All one has to do is connect to the Ext Speaker jack socket and nothing in the Speaker jack, and the amp behaves as if the OPT has a shorted turn ! It's not permanent, thankfully.
But unless a tech knows about this quirk, could easily misdiagnose as a duff OPT when there's nothing wrong with it. I nearly did !
I wonder how many Fender OPTs have been falsely assumed to be duff because of this ?! All one has to do is connect to the Ext Speaker jack socket and nothing in the Speaker jack, and the amp behaves as if the OPT has a shorted turn ! It's not permanent, thankfully.
But unless a tech knows about this quirk, could easily misdiagnose as a duff OPT when there's nothing wrong with it. I nearly did !
It depends if the person diagnosing the problem is experienced or not. On the Twin Reverb and most other Fenders with extension speaker sockets default to short circuit when un-plugged.
No offense, but anyone who has worked on a few fender amps has seen that, it is a common fender circuit element. Shorting the output doesn't usually hurt anything, and running an open output often does. it is there BECAUSE you always need to have a load connected to the output.
You are correct that it might cause someone to mis-diagnose a faulty OT, but really, among inexperienced technicians, misdiagnosing OTs is one of the most common mistakes - shorting jack or not. MANY MANY time we get a post from someone with such and such a problem, and he asks, "It's my OT, right?"
You are correct that it might cause someone to mis-diagnose a faulty OT, but really, among inexperienced technicians, misdiagnosing OTs is one of the most common mistakes - shorting jack or not. MANY MANY time we get a post from someone with such and such a problem, and he asks, "It's my OT, right?"
Yes it's the misdiagnosis aspect that nearly caught me. It sure behaves exactly like a shorted winding on the OPT even with a speaker connected to the Ext Speaker socket - because it is in fact shorted !
I'm certain the second time one comes across this it would be remembered, and (as is said) it seems common to some other Fender amps. Just thought it worth posting, esp on a DIY forum. Couldn't find any observations on it elsewhere on the web.
I'm not an instrument tech, but know my way around valve circuits and I was nearly caught by it this being my first encounter with a vintage Fender amp head.
Sounds lovely, BTW !
I'm certain the second time one comes across this it would be remembered, and (as is said) it seems common to some other Fender amps. Just thought it worth posting, esp on a DIY forum. Couldn't find any observations on it elsewhere on the web.
I'm not an instrument tech, but know my way around valve circuits and I was nearly caught by it this being my first encounter with a vintage Fender amp head.
Sounds lovely, BTW !
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