Bassman transformer switch

[doveman]

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.

The standard is only one tap is wired to both jacks in parallel. The main has the shorting safety feature. The other tap is not used.

My preference: it's a 1x12 combo. So with this option, I would replace tubes - change the speaker jack - rebias. If I ever do use multiple speakers all my cabs are easily set to run off one plug. So, I'd have the most flexibility and proper tap. I would probably rarely change but this would be cool if it worked the way I think it does.

[martin manning]

OK, Allen's TO35MT (supplied with that kit) is 4K ohm primary with 4 and 8 ohm secondary taps, and you've got two holes for jacks in the chassis already. I'd just connect each tap to a switching jack (Switchcraft 12A), and connect the switching contacts with a jumper for the no-load-plugged-in short. Then, with an 8-ohm speaker use the 8-ohm jack for 6L6 and the 4-ohm jack for 6V6. It already has rear-panel bias measurement jacks, so changing tube type would be pretty quick.

[doveman]

OK, Allen's TO35MT (supplied with that kit) is 4K ohm primary with 4 and 8 ohm secondary taps, and you've got two holes for jacks in the chassis already. I'd just connect each tap to a switching jack (Switchcraft 12A), and connect the switching contacts with a jumper for the no-load-plugged-in short. Then, with an 8-ohm speaker use the 8-ohm jack for 6L6 and the 4-ohm jack for 6V6. It already has rear-panel bias measurement jacks, so changing tube type would be pretty quick.

Thanks that's what I was thinking. Put a label on the back rather than MAIN and EXTENSION. I'm going to take a look at my Dr Z RT66. It has 4, 8 & 16 ohm selections on the back. If it's done the same way that might work. The amp has a negative feedback loop I think. If so where would that connect? Right now this is just an option - but maybe a good one.

[martin manning]

I'm going to take a look at my Dr Z RT66. It has 4, 8 & 16 ohm selections on the back. If it's done the same way that might work. The amp has a negative feedback loop I think. If so where would that connect? Right now this is just an option - but maybe a good one.

With three taps you would need some jacks with more switching contacts to have a short that is opened by plugging into any one of the three. Or, you could skip the short and put a resistor from one tap to ground to put a high (but finite) load on the OT with no speaker plugged in.

The NFB should be connected to one of the taps and the feedback resistor should be selected appropriately. Maybe there is a different value recommended depending on which tube type you choose? What do the kit instructions say about that?

[doveman]

I'm going to take a look at my Dr Z RT66. It has 4, 8 & 16 ohm selections on the back. If it's done the same way that might work. The amp has a negative feedback loop I think. If so where would that connect? Right now this is just an option - but maybe a good one.

The NFB should be connected to one of the taps and the feedback resistor should be selected appropriately. Maybe there is a different value recommended depending on which tube type you choose? What do the kit instructions say about that?

The instructions don't really mention the option of using the other tap. It's drawn to match the 6L6s. The amp can tolerate an 8 ohm speaker at 16 ohm tap with 6V6s. Or David suggests if I prefer to use only 6V6s, I can wire up the other tap. The Accomplice (not Junior) has room for a switch and uses both taps. Basically the same amp just a bit bigger chassis with that switch and another control for tone on reverb.

No biggie - but I stumbled on your sketch and found it interesting. May consider it.

Still unsure how connecting both of the extra jumpers together works. I'd like to be more comfortable with how that works when only one (either) jack is used. I'm not clear on what would happen if someone hooked up two speaker jacks at the same time.

[martin manning]

Still unsure how connecting both of the extra jumpers together works. I'd like to be more comfortable with how that works when only one (either) jack is used.

Very simple. The jumper shorts the turns between the 4 and 8-ohm taps so current can flow in the secondary, dissipating the energy delivered to the OT when no load is connected. If there is a down-side, this current will be large. However, this fail-safe is only supposed to protect the OT long enough for the player to realize that something is wrong. If the amp is played with the OT secondary open-circuit (i.e. no shorting jack), the OT could be irreparably damaged in a split-second. The jumper wire could be replaced with a resistor of 5 or 10 ohms to reduce the current, and this would be taken out of the circuit if either jack is in use, just like with the jumper.

I'm not clear on what would happen if someone hooked up two speaker jacks at the same time.

If you plug a second speaker of the same impedance into a typical main-extension jack system, you get 1/2 of the expected load, or if the speaker impedances are both doubled, you match the expected load (two 8's connected to a 4-ohm tap, e.g.). With the proposed system you would have one speaker connected to each tap, so there are a number of possibilities depending on which tube type is in the amp and the speaker impedances used.

The sketch above has been updated with a schematic, including the optional resistor.

[FUCHSAUDIO]

1 100 ohm 10-W or 20-W WW resistor (as used in Traynors) works well too.

[doveman]

Thanks for going to the trouble of updating the drawing too. The resistor information is helpful too. I love this place.

[doveman]

I know this is not art but to help me understand what is going on here ...

If a speaker is connected to the 8 ohm jack ...

• * 8 ohm tap is in use
  * NFB provided from the OT via 8 ohm tap
  * 4 ohm safety ground is disconnected via the insertion of plug

[IMG:640:319]http://i104.photobucket.com/albums/m168/dovemanpix/Dual_Imp_Outs_w_Short-8TAP_zps1086a354.png[/img]

If a speaker is connected to the 4 ohm jack ...

• * 4 ohm tap is in use
  * NFB is still provided from the OT via 8 ohm tap
  * 8 ohm safety ground is disconnected via the insertion of plug

[IMG:640:319]http://i104.photobucket.com/albums/m168/dovemanpix/Dual_Imp_Outs_w_Short-4TAP_zps4cc5e94d.png[/img]

If a speaker is connected to both the 4 ohm jack and 8 ohm jack ...

• * 4 ohm tap is in use and 8 ohm tap is in use
  * NFB is still provided from the OT via 8 ohm tap
  * Both safety grounds are disconnected via the insertion of both plugs

[IMG:640:319]http://i104.photobucket.com/albums/m168/dovemanpix/Dual_Imp_Outs_w_Short-BOTHTAPS_zpsda3385a7.png[/img]

Additional Questions:

• *Is this interpretation correct?
  *Assuming both of these speakers are single 8 ohm speakers, would this work? Not damage anything? What would the load be to the OT ... 4 ohm?
  *What is the advantage of the resistor? What does it accomplish?

Thanks for your help in this ... apologize for trashing your drawing but it helps me understand.

[martin manning]

No problem. You have it right regarding the connections when only one jack is in use. I wouldn't call the shorting thing a safety ground, though. A better term would be "safety short." The NFB could be connected to either tap with the appropriate value for the FB resistor, but some compromise between 6L6 and 6V6 operation will be necessary.

Why add the resistor? There are relatively few turns of wire from the 4-ohm to the 8-ohm tap, so if those turns are shorted and an attempt is made to play through the amp the secondary current will be large. Adding the resistor will limit that current and prevent the secondary wire from getting too hot. Again, this condition is not supposed to exist for more than a few seconds so the heat generated in the wire will be rapidly dissipated into the cold surroundings.

If you plug two 8-ohm speakers in there are two scenarios:

For 6L6 where the 8-ohm jack is correct, you have a matching load in parallel with another that is 2x the matching load, resulting in 2/3 of the matching load. This would probably be ok to play.

For 6V6 where the 4-ohm jack is the correct match, you have a matching load in parallel with one that is 1/2 the matching load, resulting in 1/3 of the matching load. I wouldn't do this on purpose.

The option Andy proposed, where a resistor is permanently connected from one of the taps to ground, has the advantage that it protects the amp in the case where a cable is plugged into the amp but not the speaker. However, if there is an NFB loop, the loop circuit provides that protection to some degree, and unless the value of this resistor is pretty big (say 5x the value of the PI's tail resistor) it will change the PI's operating point.

[doveman]

Thanks. I wasn't planning on using both but was wondering if somebody did this would it be immediately tragic consequences.

My general operation example would be:

• * Running with 6V6s (connected to the yellow) with a 1x12 combo
  * Decide I need a bit more power for a specific gig
  * With power off - change combo speaker to other (green) jack
  * Change tubes to 6L6
  * Power back up & rebias for 6L6 tubes
  * Have a blast at jam

That's basically what I had in mind. I would place labels in place of MAIN and EXTENSION to direct others. I think it would work.

[martin manning]

...And with 6L6's you also have the option of plugging a 4-ohm cab into the 4-ohm jack.

[doveman]

...And with 6L6's you also have the option of plugging a 4-ohm cab into the 4-ohm jack.

Exactly ... and that's when I might need another speaker. So I would label for that option too. Pretty cool.

[doveman]

One more question: I have 82 ohm 10W and 75 ohm 25W in the box here. I imagine the value is not that critical. Could I use these as drawn here?

[FUCHSAUDIO]

Either should be ok. It's just to assure the amp never sees "no load" at all.