Bassman transformer switch

[aurelius]

So I just probably bought a bassman 50 with a fuchs ods mod for about $1050 shipped (also comes with 2x10 cab and speakers. Good deal? Don't know much about pricing here). One thing I didn't notice though was that impedance for the thing is 4 ohms. that really throws a wrench in my plan as i needed something with 8 ohms.

So my question, Changing out that transformer would be the step necessary to get it to 8 ohms right? (granted the new transformer supports it.) Would it be a very involved process? I never opened up an amplifier, but i'm not unfamiliar with soldering. although kinda worried. I hear these things tend to retain voltages even when unplugged. Would I have to drill new holes in the chassis? would pots, caps, tubes, etc. have to be switched?

And maybe more of a question for andy from fuchs if he still reads here: I hear this mod gets you to about 70% of the way to a production line ods. with a new transformer in there, how much closer? do you guys sell parts that you use in ods amps? would you even considered it a worth while venture? I really want a hand built ods 50 hrm, but the price tag is just....I'm working 76 hours a week to try and afford one of these amps and even then it'll probably take me about 2 months and a half.

just trying to salvage a botched operation on my part. any help will be greatly appreciated.

[martin manning]

So my question, Changing out that transformer would be the step necessary to get it to 8 ohms right? (granted the new transformer supports it.) Would it be a very involved process? I never opened up an amplifier, but i'm not unfamiliar with soldering. although kinda worried. I hear these things tend to retain voltages even when unplugged. Would I have to drill new holes in the chassis? would pots, caps, tubes, etc. have to be switched?

You can buy a drop-in replacement transformer with 4 and 8 ohm taps (no drilling, no other component changes, just soldering) for $62 from Magnetic Components (PN 18-008). and I'd wire the two existing jacks for 4 and 8. Dumble used Fender transformers so you won't get any closer than you are now to a "real" ODS. You could spend a lot more for a Mercury transformer, but I'm not convinced they are any better than other good quality parts that replicate the originals. Discharging the filter capacitors is easy, and if you have soldering skills and a decent multimeter you could make the swap.

[ToneMerc]

You can buy a drop-in replacement transformer with 4 and 8 ohm taps (no drilling, no other component changes, just soldering) for $62 from Magnetic Components (PN 18-008). and I'd wire the two existing jacks for 4 and 8. Dumble used Fender transformers so you won't get any closer than you are now to a "real" ODS. You could spend a lot more for a Mercury transformer, but I'm not convinced they are any better than other good quality parts that replicate the originals. Discharging the filter capacitors is easy, and if you have soldering skills and a decent multimeter you could make the swap.

Ditto Martin

@ OP...the Dumble 50W amps mainly had the Bassman OT with only the 4 ohm tap and surely were run with an 8 ohm speaker load.
If you are not capable of doing it yourself, just pay a tech $50 to swap the OT if thats whats you feel you need to do.

TM

[alvarezh]

Here is another alternative:

http://taweber.powweb.com/store/zmatch.htm

All the best.

[aurelius]

Thanks for all the responses. Dropping in a new transformer doesn't seem like a bad idea. I've always kinda wanted to get into amp build. Maybe this will be my start.

Also to clarify, the issue with the ohms isn't so much a cab problem. The plan is to use the amp mostly to record. I'm gonna be plugging into a vb-101 from two notes, and the vb only supports an 8 ohms impedance. That's where I hit my problem.

[martin manning]

One thing that has bothered me about using a multi-tap OT without an impedance selector is that you will typically lose the shorting feature which protects against running in a no-load situation. You can put a resistor from one of the taps to ground, but here's another option. Both jacks need to have a switch contact so you may have to replace the extension jack, but otherwise no chassis modifications are required. Plugging a speaker into either jack opens the short.

[alvarezh]

Nice little circuit Martin, thank you much.

Cheers!

[mat]

Is that so that middle and upper lugs are connected when there is no plug inserted ?

[martin manning]

The shorting jumper connects the tip terminals together when no plug is inserted. This shorts the two lower terminals and therefore the windings between the 4 and 8 ohm taps.

[mhartman]

So I just probably bought a bassman 50 with a fuchs ods mod for about $1050 shipped (also comes with 2x10 cab and speakers. Good deal? Don't know much about pricing here). One thing I didn't notice though was that impedance for the thing is 4 ohms. that really throws a wrench in my plan as i needed something with 8 ohms.

So my question, Changing out that transformer would be the step necessary to get it to 8 ohms right? (granted the new transformer supports it.) Would it be a very involved process? I never opened up an amplifier, but i'm not unfamiliar with soldering. although kinda worried. I hear these things tend to retain voltages even when unplugged. Would I have to drill new holes in the chassis? would pots, caps, tubes, etc. have to be switched?

And maybe more of a question for andy from fuchs if he still reads here: I hear this mod gets you to about 70% of the way to a production line ods. with a new transformer in there, how much closer? do you guys sell parts that you use in ods amps? would you even considered it a worth while venture? I really want a hand built ods 50 hrm, but the price tag is just....I'm working 76 hours a week to try and afford one of these amps and even then it'll probably take me about 2 months and a half.

just trying to salvage a botched operation on my part. any help will be greatly appreciated.

Unless you really want to buy and change the transformer, just run the 4ohm output to your 8ohm cab. It will be fine and probably sound better that way.

[PaisleyTube]

+11 regarding mismatching (and it's THE dumble way )

[FUCHSAUDIO]

FWIW: The amp will gladly accommodate 4 or 8 ohms. A slight power loss, but no harm done to the amp. A transformer replacement is easy to do. I would recommend Heyboer (Weber sells them), or Classic Tone, not Mercury. The Weber matcher does match the impedance but will suck tone. It's an autoformer, and that's a few miles of wire that will change the way any amp responds for sure....

[alvarezh]

The Weber matcher does match the impedance but will suck tone. It's an autoformer, and that's a few miles of wire that will change the way any amp responds for sure....

Good to know Andy, thanks.

Good luck on the amp show!

All the best.

[doveman]

One thing that has bothered me about using a multi-tap OT without an impedance selector is that you will typically lose the shorting feature which protects against running in a no-load situation. You can put a resistor from one of the taps to ground, but here's another option. Both jacks need to have a switch contact so you may have to replace the extension jack, but otherwise no chassis modifications are required. Plugging a speaker into either jack opens the short.

Have you tried this? As I read it, you use either jack to engage either tap. What happens if someone unfamiliar with amp plugs something into both jacks?

This would be awesome in my Allen Accomplice Jr. It has a dual tap OT (6L6 8 ohm & 6V6 16 ohm) or (6L6 4 ohm & 6V6 8 ohm). Same as the Accomplice but no room for a switch since it's a smaller chassis. But this would allow swapping tubes and rebiasing. But you could have the proper match with the OT too. In Allen's design, he chooses the first tap because it matches 6L6 at 8 ohms and tolerates 16 ohms with the 6V6. Wondering if the 6V6 set up might get a bit more volume with the matching tap? I'm just starting my build.

[martin manning]

Have you tried this? As I read it, you use either jack to engage either tap. What happens if someone unfamiliar with amp plugs something into both jacks?

No. I haven't actually done one like this, but I don't see any down-side. If you plug two speakers in the shorting jumper isn't connected to anything, and you get a miss-match like plugging two speakers into the main and extension jacks on a Fender (a bit different depending on whether the speakers match the high or low impedance, or both).

This would be awesome in my Allen Accomplice Jr. It has a dual tap OT (6L6 8 ohm & 6V6 16 ohm) or (6L6 4 ohm & 6V6 8 ohm). Same as the Accomplice but no room for a switch since it's a smaller chassis. But this would allow swapping tubes and rebiasing. But you could have the proper match with the OT too. In Allen's design, he chooses the first tap because it matches 6L6 at 8 ohms and tolerates 16 ohms with the 6V6. Wondering if the 6V6 set up might get a bit more volume with the matching tap? I'm just starting my build.

Do I understand correctly that the recommended wiring on the Accomplice Jr. is just connecting one jack? If so, then this would be better, IMO. You would have two jacks, one for 6L6 and one for 6V6, both being 8 (or 4) ohms, and you preserve the shorting safety feature.