Fender Bassman 100 tube head

[t bone]

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?

[t bone]

Model CFA 7100 from the Silverface  1972 to 77

Edited Monday at 09:44 by t bone
add pics

[ossyrocks]

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 Monday at 13:03 by ossyrocks

[Bill Fitzmaurice]

Vintage Fenders aren't fussy. We used to run them with anywhere from 8 to 1 ohm loads with never a complaint.

[ossyrocks]

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 !!

[t bone]

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 Monday at 23:03 by t bone

[t bone]

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. 

[t bone]

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

[ajkula66]

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.

[t bone]

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.

[t bone]

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.

[Bill Fitzmaurice]

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 yesterday at 02:33 by Bill Fitzmaurice

[t bone]

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 yesterday at 02:48 by t bone

[t bone]

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?

[ossyrocks]

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

[t bone]

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 yesterday at 08:29 by t bone

[Beer of the Bass]

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.

[t bone]

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 18 hours ago by t bone