t bone Posted April 14 Posted April 14 I have seen two Fender Bassman 100 heads that have differing speaker out load requirements written on the label at the back. One says "Total load 8 ohms" and the other head says "Total load 4 ohms". Both have two 1/4" out jacks. Some forums posts say the jacks are parallel and others say series. I don't think I have ever seen two out jacks in series before. The parallel wiring system seems industry standard. What is going on here? Quote
t bone Posted April 14 Author Posted April 14 (edited) Model CFA 7100 from the Silverface 1972 to 77 Edited April 14 by t bone add pics Quote
ossyrocks Posted April 14 Posted April 14 (edited) Hi, and welcome. The Bassman 100 is designed to work with a 4 ohm speaker load. I have never ever seen one designed to work with an 8 ohm load, so that pic above is highly unusual. Anyway, generally Bassman 100's (and 50's) would usually like to see a 4 ohm load. The speaker outputs are wired in parallel, but you must connect a speaker to the "SPEAKER" output first, before connecting a second speaker to the "EXT. SPKR" output. So, connecting an 8 ohm speaker to the first output, and a second 8 ohm speaker to the EXT output, would present a total load of 4 ohms to the amp, which is ideal. Fenders can usually withstand a mismatch either way though, so you could easily run a single 8 ohm cab and the amp would be ok. Equally you could run two 4 ohm cabs, which would present a 2 ohm load to the amp, and it would be ok, but it might run a bit hotter. I have a modified Bassman 50 which has a changed output transformer designed to see an 8 ohm load. I run it into a 4 ohm speaker without problems. Rob Edited April 14 by ossyrocks Quote
Bill Fitzmaurice Posted April 14 Posted April 14 Vintage Fenders aren't fussy. We used to run them with anywhere from 8 to 1 ohm loads with never a complaint. 1 Quote
ossyrocks Posted April 14 Posted April 14 47 minutes ago, Bill Fitzmaurice said: Vintage Fenders aren't fussy. We used to run them with anywhere from 8 to 1 ohm loads with never a complaint. Well, that's something I would love try sometime, a Bassman 100, through FOUR Fender 2x15 cabs !! 1 Quote
t bone Posted April 14 Author Posted April 14 (edited) Thanks for the input (haha) Rob. The pic IS highly unusual and it is stopping me from purchasing the head. This amp has had a lot of parts replaced so possibly has a different output transformer like your Bassman 50 but it doesn't explain the Model number, CFA 7100 on the label and the Total load 8 ohms on the same label. It doesn't look like the 8 has been painted on either, having said that, it does look a little bit big. Edited April 14 by t bone Quote
t bone Posted April 14 Author Posted April 14 7 hours ago, Bill Fitzmaurice said: Vintage Fenders aren't fussy. We used to run them with anywhere from 8 to 1 ohm loads with never a complaint. Bill, I have 2 2X15 cabs, each speaker 8 ohms and would like to run this but if the amp is MIN 8 ohms (Total load 8 ohms) I would rather not load it up. Don't have that much money to splash around getting it repaired. Why couldn't Fender just label it the way they did it on the Bassman 135? Having the one statement, Total load 8 ohms, under the two output jacks, does my head in! I know that for the amp's sake you should not put in less ohms than the stated MIN (only a higher amount), but it is hard to work out what that MIN is when I have also read that the two output jacks could be wired in series or parallel OR even have a disconnect feature when a lead is placed into one or the other output jack OR a combination of. Quote
t bone Posted April 14 Author Posted April 14 10 hours ago, ossyrocks said: Hi, and welcome. The Bassman 100 is designed to work with a 4 ohm speaker load. I have never ever seen one designed to work with an 8 ohm load, so that pic above is highly unusual. Anyway, generally Bassman 100's (and 50's) would usually like to see a 4 ohm load. The speaker outputs are wired in parallel, but you must connect a speaker to the "SPEAKER" output first, before connecting a second speaker to the "EXT. SPKR" output. So, connecting an 8 ohm speaker to the first output, and a second 8 ohm speaker to the EXT output, would present a total load of 4 ohms to the amp, which is ideal. Fenders can usually withstand a mismatch either way though, so you could easily run a single 8 ohm cab and the amp would be ok. Equally you could run two 4 ohm cabs, which would present a 2 ohm load to the amp, and it would be ok, but it might run a bit hotter. I have a modified Bassman 50 which has a changed output transformer designed to see an 8 ohm load. I run it into a 4 ohm speaker without problems. Rob Rob. Put aside the mismatch side of things for the moment. "Total load 8 ohms" directly under the two output jacks suggests (if the jacks are in parallel) a MIN of 8 ohms. This then suggests a 16 ohm speaker into each jack. Hmmm Quote
ajkula66 Posted April 14 Posted April 14 4 minutes ago, t bone said: "Total load 8 ohms" directly under the two output jacks suggests (if the jacks are in parallel) a MIN of 8 ohms. This then suggests a 16 ohm speaker into each jack. Hmmm If the seller will allow you to open the amp, there's a couple of things that can be done: a) Presuming that the transformer is the original Fender one, there should be a code on it that you could Google and it will tell you whether it's meant to be used with a 4 or 8 Ohms total load. I've seen (otherwise untouched) blackface Fenders that have left the factory with a "wrong" transformer. b) If you have a tech who knows tube amps they can measure the specs of the transformer on the spot, and also tell you how the output jacks are wired, removing all and any confusion in the process. Good luck. Quote
t bone Posted April 14 Author Posted April 14 13 minutes ago, ajkula66 said: If the seller will allow you to open the amp, there's a couple of things that can be done: a) Presuming that the transformer is the original Fender one, there should be a code on it that you could Google and it will tell you whether it's meant to be used with a 4 or 8 Ohms total load. I've seen (otherwise untouched) blackface Fenders that have left the factory with a "wrong" transformer. b) If you have a tech who knows tube amps they can measure the specs of the transformer on the spot, and also tell you how the output jacks are wired, removing all and any confusion in the process. Good luck. So, the seller has sent me some pics but I don't think they are what you want to see or helpful. Hmmm. There have been many parts replaced over the years. Quote
t bone Posted April 14 Author Posted April 14 23 minutes ago, ajkula66 said: If the seller will allow you to open the amp, there's a couple of things that can be done: a) Presuming that the transformer is the original Fender one, there should be a code on it that you could Google and it will tell you whether it's meant to be used with a 4 or 8 Ohms total load. I've seen (otherwise untouched) blackface Fenders that have left the factory with a "wrong" transformer. b) If you have a tech who knows tube amps they can measure the specs of the transformer on the spot, and also tell you how the output jacks are wired, removing all and any confusion in the process. Good luck. So, they guy is going to get me the code off the Trans and investigate how the output jacks are wired. Cheers. Quote
Bill Fitzmaurice Posted April 15 Posted April 15 (edited) 3 hours ago, t bone said: Bill, I have 2 2X15 cabs, each speaker 8 ohms and would like to run this but if the amp is MIN 8 ohms (Total load 8 ohms) I would rather not load it up. Valve amps are the opposite of SS. They have a maximum load impedance, not a minimum. They can even withstand a dead short, while an open circuit can damage them. For this reason one of the output jacks on Fenders was a closed circuit switching jack that shorted the output when there was nothing plugged into the main speaker jack. It also meant no signal passed if you plugged into the extension jack instead of the main jack, which pretty much every Fender owner discovered at some point, to their great consternation. Edited April 15 by Bill Fitzmaurice 2 Quote
t bone Posted April 15 Author Posted April 15 (edited) 15 minutes ago, Bill Fitzmaurice said: Valve amps are the opposite of SS. They have a maximum load impedance, not a minimum. They can even withstand a dead short, while an open circuit can damage them. For this reason one of the output jacks on Fenders was a closed circuit switching jack that shorted the output when there was nothing plugged into the main speaker jack. It also meant no signal passed if you plugged into the extension jack instead of the main jack, which pretty much every Fender owner discovered at some point, to their great consternation. Oh, Bill that's awesome info. I can see clearly now... too much to think about... my head... You have an ability to put things into plain English that can be easily understood... very rare. I have only ever owned SS amps. Edited April 15 by t bone Quote
t bone Posted April 15 Author Posted April 15 So, shop manager went home. Will know something tomorrow. We know the previous owner has had some work done on the amp. I am of the opinion that the previous owner desired the tap setting on the Output transformer be changed. My question is, is it a simple case of having the tap setting changed back? Quote
ossyrocks Posted April 15 Posted April 15 53 minutes ago, t bone said: So, shop manager went home. Will know something tomorrow. We know the previous owner has had some work done on the amp. I am of the opinion that the previous owner desired the tap setting on the Output transformer be changed. My question is, is it a simple case of having the tap setting changed back? If the output transformer is the original, then there are no options for changing the settings, they didn't have multi-tap output transformers. Your mains transformer is certainly changed, the one pictured above is a Hammond, and it would have originally been a Schumacher. No big deal, but make sure the price reflects the changes. Just think about this, Fender installed speakers in their amps which correctly matched the output impedance stated on the back panels, and the actual spec of the output transformers. They also gave every amp an EXT SPKR output, which when used, immediately had the amp seeing half of the stated impedance. It worked fine, and they didn't worry. Rob 1 Quote
t bone Posted April 15 Author Posted April 15 (edited) 56 minutes ago, ossyrocks said: If the output transformer is the original, then there are no options for changing the settings, they didn't have multi-tap output transformers. Your mains transformer is certainly changed, the one pictured above is a Hammond, and it would have originally been a Schumacher. No big deal, but make sure the price reflects the changes. Just think about this, Fender installed speakers in their amps which correctly matched the output impedance stated on the back panels, and the actual spec of the output transformers. They also gave every amp an EXT SPKR output, which when used, immediately had the amp seeing half of the stated impedance. It worked fine, and they didn't worry. Rob So, if it is the same OT then the label on the back becomes a moot point. Problem solved, yes? Having said that I read the 100 and 135 were identical except for the obvious, EQ, master vol, power and the ultra linear output section. Now, having said that, I believe the 135 had a multi-tap OT. Edited April 15 by t bone Quote
Beer of the Bass Posted April 15 Posted April 15 I'd want to get a really close look at the output transformer and the label itself. If the 8 looks to be written on or a sticker applied after the original manufacture, and the output transformer is the same as the stock 4 ohm model, the only other reason I could think of for relabelling it is if someone had been running it with one pair of power valves removed for lower power. 1 Quote
t bone Posted April 15 Author Posted April 15 (edited) 15 hours ago, Beer of the Bass said: I'd want to get a really close look at the output transformer and the label itself. If the 8 looks to be written on or a sticker applied after the original manufacture, and the output transformer is the same as the stock 4 ohm model, the only other reason I could think of for relabelling it is if someone had been running it with one pair of power valves removed for lower power. Ah. The plot thickens. So, removing power valves would increase the impedance. 4 to 8 ohms. This could explain the 8 being "painted" on the back panel. Edited April 16 by t bone Quote
bremen Posted Saturday at 14:44 Posted Saturday at 14:44 On 15/04/2025 at 10:17, Beer of the Bass said: ...the only other reason I could think of for relabelling it is if someone had been running it with one pair of power valves removed for lower power. I have the same amp with a missing valve. I've been thinking of pulling its complement and running it into 8 ohms...sensible? Quote
ossyrocks Posted Saturday at 14:58 Posted Saturday at 14:58 12 minutes ago, bremen said: I have the same amp with a missing valve. I've been thinking of pulling its complement and running it into 8 ohms...sensible? Running just a pair is fine to do. Make sure you run them on either side of the output though, ie run a pair in the middle two sockets, or the outer two sockets, and not two on the left or two on the right. Quote
Bill Fitzmaurice Posted Saturday at 15:39 Posted Saturday at 15:39 On 15/04/2025 at 05:20, t bone said: So, removing power valves would increase the impedance. 4 to 8 ohms. The load impedance seen by the power valves is the output transformer primary. All that you accomplish by removing two valves is to reduce the current that the amp can deliver, and while that does reduce the power it's not a good idea. Guitar players who came up with that notion were only fooling themselves as to its effect, which at best might result in a measly 3dB reduction in maximum output. Quote
bremen Posted Saturday at 17:35 Posted Saturday at 17:35 2 hours ago, ossyrocks said: Running just a pair is fine to do. Make sure you run them on either side of the output though, ie run a pair in the middle two sockets, or the outer two sockets, and not two on the left or two on the right. Understood. Thanks for confirmation. Quote
bremen Posted Saturday at 17:36 Posted Saturday at 17:36 1 hour ago, Bill Fitzmaurice said: The load impedance seen by the power valves is the output transformer primary. All that you accomplish by removing two valves is to reduce the current that the amp can deliver, and while that does reduce the power it's not a good idea. Guitar players who came up with that notion were only fooling themselves as to its effect, which at best might result in a measly 3dB reduction in maximum output. The load on the secondary is reflected back to the valves, though? Quote
Bill Fitzmaurice Posted Saturday at 18:36 Posted Saturday at 18:36 (edited) Yes, it is, which is why that load must be within the secondary specs. If you want to get really picky about it you match the reflected load current to the primary current, but when you try to do that via the speaker impedance and/or the number of output valves you run into the issue that speaker impedances aren't a fixed value, they vary with frequency and the enclosure design, so you can't really get it perfect. For the most part close enough is good enough so long as you don't have a gross mismatch, like using a 4 ohm tap with a 16 ohm speaker. Edited Saturday at 18:54 by Bill Fitzmaurice Quote
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.