Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Feloniuspunk on April 28, 2010, 03:48:50 pm
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That's the short version of the question. Here's the long version. I have a 63 BF Showman. A friend told me that I could essentially run it at half power by pulling the two outer power tubes and running it with just the two inner power tubes. Is this true? Is it that easy? Are there any other concerns I should be aware of if I do this? Will I have to run a different speaker configuration to adjust the impedance or can I just keep using the 8 ohm speaker cab I've been using?
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Harm no.
Reduced power yes.
Speaker impedance becomes 16 ohms.
However, I'm not sure how the picture you posted related to your question.
Dave
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The picture I posted shows how to electrically eliminate the two outer power tubes by lifting their ground and running it through a switch that goes to ground. You can leave the tubes in the amp physically but flip the switch and the amp then behaves as if it only has two power tubes instead of four. On an old BF Fender amp that has a three conductor power cord the old grounding switch on the back panel is useless and can be rewired for this purpose.
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Yes, I know what the switch is for. You asked about dogs and you posted a picture of a tick. Related, but not the same.
Dave
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Actually the speaker impedance does not change, it's the transformers output impedance that changes. In a Twin, if you pull 2 tubes the output imedance becomes 8ohms, whereas when you had 4 tubes it was 4 ohms. Now, you can take out one speaker making 8ohm out to 8 ohm speaker. I have been told by a reputable source that it is also safe to wire 2 8ohm speakers in series creating a 16 ohm load on the 8ohm output. Do not, however run the speaker impedance lower than the output impedance of the amp.
Also, how many speakers are in your 8ohm cab? And how are they wired?
Chris
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Ah, how do you measure output impedance? Do you do that with a stethoscope?
Whatever tomatoe potatoe big toe whatever.
Dave
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Yes, I know what the switch is for. You asked about dogs and you posted a picture of a tick. Related, but not the same.
I thought his pic was very appropriate. Killing all current flow thru a pair of tubes is darn near exactly the same as pulling the tubes out of the socket.
Actually the speaker impedance does not change, it's the transformers output impedance that changes.
Not quite. Output Transformers don't have impedance. They simply match the tube plate impedance to the speaker load impedance.
In a Twin, if you pull 2 tubes the output imedance becomes 8ohms, whereas when you had 4 tubes it was 4 ohms.
The output impedance changed because the tube plate impedance changed. When you pull two tubes you are actually changing the plate impedance. The plate impedance on the primary side of the OT doubles. The OT still has the same impedance ratio, so since the plate impedance doubles, the secondary impedance will double also. Now you need to double the speaker impedance to match the doubling of the plate impedance. Disconnecting one speaker in the twin is an easy way to do this.
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OK Sluckey you win. :wink:
But I stand by my tick metaphor because I liked the way it sounded.
Dave
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But I stand by my tick metaphor because I liked the way it sounded.
Me too. :grin:
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It is worth mentioning that pulling two tubes often does not have the effect that many people expect it to. As far as perceived volume levels were concerned I never thought it was worth the effort. However, running two tubes instead of four gives you two hot spares (pun intended) in case your amp suffers an output tube failure that does not destroy other components in its wake and it also makes retubing only half as expensive. Both scenarios are advantageous.
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I get the impedance thing, thanks. 4 tubes = 4 ohms; 2 tubes = 8 ohms. So does using 2 power tubes as opposed to 4 effectively turn an 85 watt Showman into a 45 watt Bandmaster? I'm not talking about apparent sound pressure levels or perceived volume but rather the actual output power rating.
I've also been told that the 6L6's can be replaced with 6V6's. I know that 6L6's and 6V6's have different biasing requirements and I would attend to that. You use 25 watts for 6L6's and 14 watts for 6V6's in the calculation to set plate current.
So would four 6V6's make it a 35 to 40 watt amp and two 6V6's make it a 15 to 20 watt amp? I know there would be a tonal difference between the two types of tubes as well, but I'm more interested if running it at lower power with 6V6's would allow higher stage volumes and therefore more power tube saturation and sustain?
One more thing, Dan Torres has a mod kit that switches the power tubes from pentode to triode mode. Anyone have any experience with this mod? Is it worth the effort? Is the difference mainly in output power or tone or both? Which method provides more natural sustain, using only two 6V6 tubes or changing the mode from pentode to triode?
Thanks!
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Bandmaster?
Yes, pretty much. The power transformer won't sag, and the output transformer won't saturate, and theres no bottle rectifier, so you won't get that effect (whatever it is), but for the most part it will be equivalent to a Bandmaster.
6V6's?
You could replace 6L6's with 6V6's. 4 6V6's would be kind of like 2 6L6's when calculating for speaker load. Some might argue that the B+ voltage is too high for 6V6's. I have notices on a lot of blackface twins that the actual B+ runs pretty low in the neighborhood of 420 volts. Your amp uses the same PT, so measure it. If it runs around 420, it should be safe enough for 6V6's. That's what Deluxe Reverbs ran at and they worked pretty good. Try it and see what you think. I'm not sure I'd try to go all the way down to 2 6V6's with a 32 ohm speaker load. That just seems like a little much.
Triode mode?
You don't need a kit to run it in triode mode. There's not much to it really. You can read all about it by doing a search on this forum. Output power is definately reduced. Tone definately changes, darker for sure, everything else is in the ear of the beholder.
2 6V6's?
Again, I won't condone that under any circumstances.
Dave
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Just curious. If you turn off two of the tubes (per your picture) leaving the plates connected, does the impedence really change?
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Thanks for the deeper insight Sluckey on the whole impedance change. I was unaaware of what was really going on in there.
Chris
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does the impedence really change?
Yes. It helps to think of the tube as a resistor.
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What if you would instead ground G1 input grids of the unwanted tubes (like a volume control turned to zero).....would the reflected imp be affected?
I did this on my SuperNova (Sluckey November w/ 4x EL84's) & don't notice a tone difference when the 2x G1 grids of the extra output tubes are grounded. Works great, but just don't know if imp is actually changed by the grounded grids(?)
G
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What if you would instead ground G1 input grids of the unwanted tubes (like a volume control turned to zero).....would the reflected imp be affected?
I don't think so. You still have the plate/cathode connection with static current flow, so the plate/cathode still looks like a resistor. Maybe some of the smart guys will jump in and clear this up for us.
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For my 2 cent opinion
Reading the question the first thing was to remember an old thread about by Geezer that put this question and experimented about it with good feedback
So I agree with Geezer and the explanation of Sluckey (the two resistors in parallel)
as I can see the impedance don't change but only two tubes give an output
the desired effect is obtained and no change in the reflected impedance is present
hope that a skilled amp wolf can confirm that to us
Kagliostro
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All this works but the tone of the amp will be very poor unless modded.
As far as I know the OT is a ratio, pull 2 tubes and you get 4ohms turning into 8 and an 8ohm into 16.
But there are other issues like filtering, voltages and the size of the Iron being huge. This will not sound like a bandmaster at all. Does anybody really remember how a BF bandmaster sounded with those small cab with Oxfords?
I would say leave the thing alone, it's a vintage piece of gear. Ask yourself what you really need and buy a kit of build something, man there are so many kits out there that can even be modified. For upgraded 1960's Fender sounds David Allen probably has the best stuff out there.
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This is getting silly... you're fine removing the 2 outside tubes using an 8 ohm speaker... a lot of people PREFER that tone.
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"Even minutiae should have a place in our collection, for things of a seemingly trifling nature, when enjoined with others of a more serious cast, may lead to valuable conclusion."
George Washington
Dave
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This is getting silly... you're fine removing the 2 outside tubes using an 8 ohm speaker... a lot of people PREFER that tone.
I agree.
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Even more silliness. Pull 2 tubes create an output impedance of 8, then wire 2 8ohm speakers in series creating a 16 ohm load. More even order harmonics (according to some).
Chris
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Ummm,
He's been talking about a Showman the whole time. Its an 8ohm load in its natural state.
Talk about sillliness :shocked:
Dave
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I can vouch for the fact that it hardly makes any difference at all in volume levels, its easier to turn the vol down a tad. I recall reading on some post over at ampage a couple of years back that you only get about a 3dB reduction from pulling two tubes. But if it floats your boat then put the boat in the moat.
Edit - there's a neat little summary here where he's talking about increases and decreases in loudness as the result of power outputs relative to 50W (5th para)
http://www.amptone.com/g112.htm
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Let me make sure I understand the situation. You're running 4 - 6V6's and using a 4 ohm tap into a 4 ohm speaker load. You pull the outside 2 - 6V6's. Now the OT wants to see an 8 ohm load. UNLESS.... what if you switch to a 2 ohm tap off the OT? Won't that fool the OT into thinking it is seeing the correct load... or, to put it another way, won't this actually BE the correct load for 2 tubes?
Thanks,
Dave
t
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If you pull the outside two tubes, you double the speaker load connected to whatever tap you are using.
Yes, a way around it would be to use the same speakers with the next lower speaker tap.
Dave
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Great!
Thanks,
Dave
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I recently experimented with the DC30 circuit which BTW is a great sounding amp IMO. Anyway... it has a 1/2 power switch that kicks out 2 bottles and doubles the cathode resistor (cathode biased) so that the current stays roughly the same. When kicking out 2 tubes, the entire B+ rail increases 20 volts form 350V to 370V. I'm using a beefier PT than necessary so it's plausible this variation could be even greater. I really have no point to make here. I'm simply sharing a recent observation that fairly on topic.
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"it has a 1/2 power switch that kicks out 2 bottles and doubles the cathode resistor (cathode biased) so that the current stays roughly the same."
A far simpler solution for cathode biased tubes would be to seperately bias the inside and outside pairs.
Dave
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"it has a 1/2 power switch that kicks out 2 bottles and doubles the cathode resistor (cathode biased) so that the current stays roughly the same."
A far simpler solution for cathode biased tubes would be to seperately bias the inside and outside pairs.
Dave
Funny, I just came on this board looking to find out whether that was a good idea. Has anyone used separate cathode bias resistors for each pair of a cathode biased 4 tube output, like on an AC-30? I'm considering doing that to my AC-30. Although it sounds like it might not be worth the trouble to retrofit, given the modest change in output volume, I'd probably design some sort of half power option into any 4 tube amp I made in the future.
Thanks,
Chris
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Chris, that IS the solution for an AC30 or you can Power Scale or VVR it.
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IMO the DC30 goes from great sounding @30W to OK sounding @15W. It has a master volume so the 1/2 power seems pointless.
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The picture I posted shows how to electrically eliminate the two outer power tubes by lifting their ground and running it through a switch that goes to ground. You can leave the tubes in the amp physically but flip the switch and the amp then behaves as if it only has two power tubes instead of four. On an old BF Fender amp that has a three conductor power cord the old grounding switch on the back panel is useless and can be rewired for this purpose.
I found this link from Google and while the last post was 10+ years ago I wanted to add my thoughts.
Using a switch to "lift the ground" from the cathodes of the two outer power tubes, while there is still voltage on the plates, screens, grids and heaters makes me wonder what the operating mode of these valves will be. We know that the cathode is internally tied to the beam forming plates, and the GE datasheet shows the internal capacitance between the cathode and other elements. Then you have the wire from the cathode to the switch acting as an antenna, and any stray resistances in the switch, wiring or tube socket. Then you have someone poke around with a multimeter to check the voltage on the cathodes or the resistance to ground...
I just don't like this, having the cathodes floating in an unknown state, and I don't believe running 2 tubes like this as "hot spares" will extend their life. Using a switch that the user can toggle at any moment while the amp is running without being able to simultaneously switch the speaker load doesn't sound like good practice to me.
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last post was 12 years ago prior to yours "+" was a tad light
If you want to do it differently then toggle off the screens however toggling off the cathodes is not near as much voltage on a switch. Reading the thread we see that there isn't much difference in sonic sound level by turning off two tubes, but if I was going to do it I'd physically remove the two inner or two outer pairs thus alleviating your concerns .
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... Using a switch to "lift the ground" from the cathodes of the two outer power tubes, while there is still voltage on the plates, screens, grids and heaters makes me wonder what the operating mode of these valves will be. ...
"No Mode" because the tubes will not be passing current, so not-operating.
... We know that the cathode is internally tied to the beam forming plates, and the GE datasheet shows the internal capacitance between the cathode and other elements. ... I just don't like this, having the cathodes floating in an unknown state ...
It's okay for you to not-disconnect tubes in this manner if you do not like it.
... I don't believe running 2 tubes like this as "hot spares" will extend their life. ...
I don't think anyone claims "tube life is extended."
Alternately, it is implausible to say "tube life is depleted" by disconnecting the cathodes (and thereby not incurring wear to the cathodes).
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I built a cathode biased amp with 4 power tubes and a 4k primary impedance OT (Vox AC30 style).
I put two pairs of cathode resistor/cap.
It will run four 6V6’s normally, yet can run two 6V6’s if I pull inside or outside tube pairs and use an OT mismatch.
It will also run two 6L6’s if I pull one side of either sides pair—no OT mismatch.
All of these combo’s are happy wrt bias.
I should look for the wiring diagram…