Help with Vintage Bandmater amp

db3266

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We are trying to determine what the output load is of a 1962 6G7A Bandmaster.

Pictures of what my mate thinks the output transformers are below. (I think he has pictured input and output transformers?)



The code on the above is 022848 606 8 49

Then there is this



And I'm not sure what this is? Or even if it is relevant, but I have the pic so may as well post it.



From reading around, the 6G7A should be a 4 ohm load, but there is some discussion that Fender used whatever was available and hence it might actually be an 8ohm output?

The original transformer in his amp was replaced with an exact like for like replacement. He has been running the amp into 2 16ohm speakers wired in parrallel without too much bother, but the speakers are very old cheap items and we are trying to determine what new speakers to use (given his budget, most likely to be G12M's, but this is still to be decided).
 

rabidhamster

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The output load depends now on the replacement output transformer put in. If it sounds good at 8 ohms and has plenty of headroom it should be fine at 4 ohms. Fenders are pretty great at taing that little amount of mismatch without even changing tone too much.
 

db3266

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I can't read layouts either.
I cannot guarantee the OT was exact like for like replaced, hence the photos, but I think I have pictured the input tranny and the choke? Still don't know what the black one is?
 

db3266

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So a bot of Googling found this

022848 is the CBS parts code
606 is Schumacher code
8 49 is the date, 49th week of 1968.

So considering the amp is a 62 vintage, the OT may or may not be 4 ohms?

Is there a way to test, or do we need to take it to a tech?
 

db3266

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Can anyone else offer any insight?
Are these 4 or 8ohm amps?
 

rabidhamster

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Sort of. You can take the amp to a tech to find out what the turns ratio on the output transformer is. But that's not going to gain you anything. Well you'd know the ratio of the transformer, so you could have an idea of what impedance the output tubes see given a 4 or 8 ohm speaker load.

It does look like the Output transformer is probably original, but it's hard to say for sure from the photos. The schematic and layout don't contain any relevant info for that unfortunately

The circuit itself can take 4 or 8 ohms, and would probably be ok at 2 and 16 ohms. Fenders are well designed. I wouldn't try it at 2 ohms though all the same

The transformer itself is most likely wired to connect to a 4 ohm speaker load, but that doesn't mean fender used a properly matching transformer to begin with.

The 5881 tubes ideally want to see 6.6k ohms, but in the bandmaster circuit they might be seeing something else. That's part of why bandmasters don't sound the same as a bassman

The bandmaster most commonly got a 4 ohm speaker tap, but again once you plug into the extension cabinet that changes to 8 anyway.

Don't over think it, use 4 ohms or 8ohms depending on what sound you prefer and what speakers you have at hand.

It's a complicated subject to explain simply, but a tube amp isn't just inherently a "static" 4 or 8 ohms. Even given an ideal reflected load to the power tubes of 6.6k for a 4 ohm speaker tap, the amp would function just fine fun at 8 ohms. In your bandmaster the mismatch is already worse than that stock
 

Soul Tramp

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Is there a way to test it?

Yes, but it isn't something you'd want to do yourself. An amp tech could do the test. It requires removing the OT from the B+ supply and then running a series of voltage test with an external power supply.
 

db3266

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Sort of. You can take the amp to a tech to find out what the turns ratio on the output transformer is. But that's not going to gain you anything. Well you'd know the ratio of the transformer, so you could have an idea of what impedance the output tubes see given a 4 or 8 ohm speaker load.

It does look like the Output transformer is probably original, but it's hard to say for sure from the photos. The schematic and layout don't contain any relevant info for that unfortunately

The circuit itself can take 4 or 8 ohms, and would probably be ok at 2 and 16 ohms. Fenders are well designed. I wouldn't try it at 2 ohms though all the same

The transformer itself is most likely wired to connect to a 4 ohm speaker load, but that doesn't mean fender used a properly matching transformer to begin with.

The 5881 tubes ideally want to see 6.6k ohms, but in the bandmaster circuit they might be seeing something else. That's part of why bandmasters don't sound the same as a bassman

The bandmaster most commonly got a 4 ohm speaker tap, but again once you plug into the extension cabinet that changes to 8 anyway.

Don't over think it, use 4 ohms or 8ohms depending on what sound you prefer and what speakers you have at hand.

It's a complicated subject to explain simply, but a tube amp isn't just inherently a "static" 4 or 8 ohms. Even given an ideal reflected load to the power tubes of 6.6k for a 4 ohm speaker tap, the amp would function just fine fun at 8 ohms. In your bandmaster the mismatch is already worse than that stock
Thanks. It's been running 8ohms. We want to get two 8ohm speakers in parallel so it sees 4ohms.
 

rabidhamster

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I wont be surprised if you think it sounds better!

G12ms sound like a solid choice to me - that setup will cover a lot of ground
 


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