Pulling power tubes on a Twin Reverb

[johnnyguitar]

If I pull the two outside power tubes (6L6s) on a Twin Reverb..do I have to rebias the amp?? If so do I do it any different than with 4 tubes using my Bias King?? thanks in advance

[John Phillips]

It shouldn't be necessary. The only reason it would be is on an amp with a lot of sag in the power supply (tube rectified), where the B+ voltage may rise slightly with two tubes not drawing their current - and even then probably not - but the Twin is SS rectified and has a pretty stiff power supply, so it should make no difference.

The other thing to be aware of is that the speaker impedance is mismatched if you pull two tubes - ideally, you need to run the speakers from the tap for half their impedance - which would be 2 ohms on a Twin. You can't do that, but it may be better for the tubes to reconnect the speakers in series (giving 16 ohms) - still a mismatch, but less hard on the tubes.

You will lose a little power and get slightly earlier breakup with the mismatch, but since I assume you're doing this to reduce volume that's a good thing :).


(If it's a 135W Twin - and only this model - there's a neat trick though... the speaker sockets are actually wired in series on this amp, on two different transformer taps. If you plug into the extension speaker socket, the impedance is doubled, so if you do that with the speakers connected in series and two tubes pulled, you have the correct match. Hope that hasn't confused you! Don't try it on a 100W or 85W model though.)

[johnnyguitar]

Thanks John..the only thing confusing is the speaker impendence..right now I have 2- 8 ohms wired paralell so I get 4 ohms which is what the Silverfaced 70s Twin wants to see so if I pull 2 tubes I get 2 ohms at the speaker jack why would I want a 16 ohm speaker load...guess I missed it here...

[John Phillips]

Sorry, I didn't explain it well...

When you pull two tubes you get an amp output impedance of 8 ohms - double the normal impedance, not half. To keep things in match, you'd have to double the speaker impedance too, to 8 ohms, but you can't with the same speakers. So, another way would be to set the amp to 2 ohms, if it had such a setting, still with the 4-ohm speakers. Halving the amp impedance is like doubling the speaker impedance.

Now... since you can't change the amp's output impedance (assuming it is a 100W Twin), you have an 8-ohm amp connected to 4-ohm speakers. This is a bit hard on the tubes, although it won't do any harm to the amp. So instead, if you reconnect the speakers in series (should be easy, assuming the original push-connectors are still there), you have a 16-ohm load on an 8-ohm amp. Equally mismatched, but less bad for the tubes. It won't cause any other problems either, since the power developed is lower and the flyback voltages won't be any higher than for normal operation.

Alternatively... if you're still concerned with volume, just disconnect one speaker as well! Instant 8-ohm load.

[hasserl]

OR disconnect one of the speakers to get the 8 ohm load. If you're trying to reduce volume this is going to help more than pulling two tubes will anyway.

[johnnyguitar]

Thanks as always a learning experience for me..:)

[johnnyguitar]

I pulled the 2 outside tubes so now I'm at 8 ohms..I left the 2 speakers wired 4 ohms and added an extension cab with one 8 ohm speaker so I guess I now have a 12 ohm speaker load which is OK ???????? Geez I should have taken more Math in school :D

[RussB]

Just use the 1-12 cab, No?

Why pull tubes an add speakers...seems counter-productive?

[John Phillips]

Originally posted by johnnyguitar
I pulled the 2 outside tubes so now I'm at 8 ohms..I left the 2 speakers wired 4 ohms and added an extension cab with one 8 ohm speaker so I guess I now have a 12 ohm speaker load which is OK ????????
No, very bad! You now have a 2.66-ohm load - 4 ohms and 8 ohms in parallel (how the sockets are wired on a standard 100W Twin Reverb) gives that... 1/R-total = 1/R1 + 1/R2.

Running this from an amp that wants to see 8 ohms is extremely hard on the tubes and well outside the safe mismatch range.

If you want to cut volume, you're making things worse by running the extra cab anyway.

[johnnyguitar]

I'm running the ext cab 1-15 AND the 2-12s for tone..great blend..soooo now I'm better to run 4 tubes if I use this speaker config....thanks as always..this Math thing is getting my head spiinning..no wonder we burnt up so much stuff in the 60s and 70s..YIKEEEEEEEES:confused:

[John Phillips]

Yes, run all four tubes. That way you're safe down to 2 ohms, which is OK with any combination of any normally available cabs. That's why Fender made them like that - so they were 'musician proof' :). Which they pretty much are... more so than most other vintage amps, anyway.

You still can swap the combo speakers to series (16 ohm) though, and run those and the 8-ohm cab for a total load of 5.33 ohms - with two tubes pulled, that's well within the right range. This also will send more power to the 15 - at the moment, 2/3 is going to the two 12s, but if you swap the combo speakers to 16 ohms, 2/3 will go to the 8-ohm cab.

Now you're really confused.. ;)

[johnnyguitar]

Thanks again John...I may be heading your way sometime and would love to look you up along with another forum member from Scotland..perhaps we could get together..thanks again..I'm learning a bunch..

[dionysusmyth]

I only use my TRRI at home these days. I don't need all the volume and would enjoy having a couple of speaker tones to choose from. After reading this thread and the Eminence speaker thread, I'm thinking I might pull the outside power tubes and install a Cannabis Rex and a Tonker speaker with a selector switch to allow a quick choice between the two, or both.

What type of switch should I use?

Any potential problems?

[RussB]

Originally posted by dionysusmyth


...What type of switch should I use?



No need for a switch of any kind. Just make up another short cord with a 1/4" right angle plug, and 2 crimp-on spades. Then you'll have one for each speaker, simply unplug one, and plug in the other.

[dionysusmyth]

Good point. The KISS principle applies here. Thanks for the suggestion.

[paulg]

Just a few observations from my experience. I pulled two tubes , rebiased and left the speaker ohms as is. I also swapped the 6L6's for 6V6's (4), again no ohm change. Never had any problem.

[dionysusmyth]

Originally posted by paulg
Just a few observations from my experience. I pulled two tubes , rebiased and left the speaker ohms as is. I also swapped the 6L6's for 6V6's (4), again no ohm change. Never had any problem.



Paulg,

How did replacing the 6L6's with 6V6's affect the sound of your amp?

[paulg]

In a positive way, breaks up a sooner. Less headroom, but not mushy like a lot of 6v6 amps. I suspect the power supply makes it sound that way. For me the 6L6's were just too loud. I'm using JJ's right now.

[RussB]

Blackface Twins put out around 450 volts on the plates, early Silverface Twins, around 400v. Later SF ultralinear Twins have around 500v. Be wary of that before you install 6V6's.

My '69 Twin is a "400 volt" unit. and would work OK with a robust set of JJ 6V6's, or a set of JAN Phillips 6L6WGB's...

...just throwing that out there :)

[dionysusmyth]

Gentlemen,

Thanks for the feedback. I used to own a blackface Vibrolux (:( sold it when I started raising a family ). Loved the way it sounded when it started to breakup. Perhaps 6V6's would bring my TRRI closer to that sound. I just read about some new 6V6's at The Tube Store that are supposed to be fine with higher plate voltages. Back at you when I've had a chance to try them out.

[dmagalhaes]

It shouldn't be necessary. The only reason it would be is on an amp with a lot of sag in the power supply (tube rectified), where the B+ voltage may rise slightly with two tubes not drawing their current - and even then probably not - but the Twin is SS rectified and has a pretty stiff power supply, so it should make no difference.

The other thing to be aware of is that the speaker impedance is mismatched if you pull two tubes - ideally, you need to run the speakers from the tap for half their impedance - which would be 2 ohms on a Twin. You can't do that, but it may be better for the tubes to reconnect the speakers in series (giving 16 ohms) - still a mismatch, but less hard on the tubes.

You will lose a little power and get slightly earlier breakup with the mismatch, but since I assume you're doing this to reduce volume that's a good thing :).


(If it's a 135W Twin - and only this model - there's a neat trick though... the speaker sockets are actually wired in series on this amp, on two different transformer taps. If you plug into the extension speaker socket, the impedance is doubled, so if you do that with the speakers connected in series and two tubes pulled, you have the correct match. Hope that hasn't confused you! Don't try it on a 100W or 85W model though.)


Can I do this on a Twin Reverb II? It has a weird non-linear PS from what I've been reading.