Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Archives of favorite topics => Topic started by: RobBozic on August 14, 2008, 11:50:34 pm
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NOTE: the drawings for this thread can be found here>>> http://www.el34world.com/Forum/index.php?topic=4437.0 (http://www.el34world.com/Forum/index.php?topic=4437.0)
I remember reading somewehere on this forum that you could disconnect one of the inputs to a output tube in a push pull amp, thereby reducing the power of the amp. I think that the Carr Viceroy has the same arrangement. Has anyone confirmed this?
Rob
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I've read something like this would work, but have not tried it myself. It was described as sort of getting a psuedo
single ended sound from a push/pull amp. One of the forum guys a few yrs ago did it and stated it worked.
With respect, Tubenit
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Ironically i just found a toneqeust report interviewing Mr Carr and he said the following regarding the power drop from 33 watts to 7 watts in his cathode biased 6L6 amp, here's some of the interview;
"The Vincent is a cathode biased, Class A amp. In most cathode biased amps you find the two (or more) output tube cathodes being tied together and run to a common cathode bias resistor.This resistor is usually bypassed by a cap. In this configuration, the bypass cap gives the amp a little more power and a faster feel. but it does not really change the output. In preamp tube circuits there is a giant difference in gain between a bypassed and non-bypassed cathode - times three or more gain difference. When the cathode bias resistor is shared in the output section, you don't see this difference with the bypass resistor. The Vincent uses independent bias resistors for each output tube, so, when the two resistors have capacitors bypassing them (in parallel with the resistor) you get full power at 33 watts. When the caps are switched out of the circuit, the output drops to 7 watts. The other nice thing besides the different f'eel from the Rambler's shared cathode resistor and the Vincent's double resistor style is with the Vincent, the need fbr rnatched tubes is not so important.
They tend to settle in near the same no matter the match. I have tried 6L6 tubes that are on the two extremes of our matching scheme - say 8ma draw and 35ma draw - and in
the Vincent they settle to within 3 ma of each other! We will use matched tubes in the amp, but it is not necessary."
So I just found the answer to my own question an hour after I posted my post
Rob
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Check out the larmar type master volume, instead of using a dual ganged pot use 2 pots, this will allow you to completly mute one output tube.
John
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When the caps are switched out of the circuit, the output drops to 7 watts.
No way! Do you think he meant to say turn off one tube AND switch the cap out??
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I agree with LooseChange ....... I don't think separate resistors with caps being switched out will change it from 33 to 7 watts. Something else is going on. Like LC said, maybe a tube is being completely shut off?
I have used a spst "smooth" switch on a Dano Centurian which removed the 47uf cathode cap on the 6V6 power tubes
shared resistor. It had very little difference in volume. It removed a little grit and bass and that was pretty much it.
With respect, Tubenit
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I reverse engineered a Carr Vincent to see how they did the 33W to 7W, and it is as described above. Here is the thread with the schematics etc.
http://www.el34world.com/Forum/index.php?topic=2060.msg15633#msg15633
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When the caps are switched out of the circuit, the output drops to 7 watts.
No way! Do you think he meant to say turn off one tube AND switch the cap out??
Still don't believe it after I looked at the thread. I have lifted Cathode bypass caps many times in the power amp stage with no tremendous loss in gain. In fact the only way to really hear the difference is if the amp is pushed hard.
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Next time you are in the music store check it out - works suprising well. The reviews of the vincent are well based, it is a nice amp.
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I am sure the amp works as advertised. I am still doubting the exhibited electronics behind it. Can we reverse engineer it again. Got some pictures??
Everybody wants to cut the wattage of their amps down. It's the latest fad. If this really works (as spec'ed) it would be all over the web. BTW, I would try it but just don't have the time.
Hey Tubenit... Got the time to try this? You are always on the cutting edge.
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LC I don't have the time either to investigate your concerns. I have already done a lot of work on this and am happy to share what I have. I will leave it as is.
With all due respect, if you doubt it, try it. If not...
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I converted a HRD by paralleling the output tubes, converting to cathode bias and using half of the OT primary. It's great for small gigs. Did not measure the power though.
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"I have lifted Cathode bypass caps many times in the power amp stage with no tremendous loss in gain."
If I am looking at the schematic correctly ........ it looks to me like the cathode caps are NOT being switched in and out. They look like they're in series with the cathode resistor not parallel. It looks like the cathode resistor is being bypassed by the spst. If I am reading that correct ....... maybe that would work as described?
I don't think (from what I've experienced) that using a spst to engage or disengage a cathode cap will make that much of a difference ......... HOWEVER ........ that's not what the schematic is indicating.
With respect, Tubenit
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The way I'm seeing it is that the 100R resistors (R6 & R7) should be much larger (100k, or 10k at least), & are used as "anti-pop" bleeders for the caps, so that the switch is silent when thrown. (I believe this is referred to in the linked thread)
The 500R/15watt resistors are the actual bias resistors, & the 100k's (or even 10k"s) have little to no effect on the bias when they are paralleled with the 500R's.
I, too, find it hard to believe that you can go from 30w to 7w by just switching out the cap(s), but I will try it on one of my 6L6 cathode biased amps & report back as soon as I can.
Carr sez that the dual bias resistors/caps are the secret mojo.......
If it is indeed true, then it will be a great additional option for larger wattage cathode biased amps.
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OK....just wired it up to a cathode biased amp running 2x 5881's. It's exactly as the drawing shows, except that I didn't use a switch....I have the neg of the caps connected to an alligator clip wire.
There is no difference either way....with the caps connected or not, the volume/power is the same.
There must be something else going on in the Carr to drop the power to 7w(?)
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Try removing the short jumper wire between the 47uf and .1 caps.
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OK....just wired it up to a cathode biased amp running 2x 5881's. It's exactly as the drawing shows, except that I didn't use a switch....I have the neg of the caps connected to an alligator clip wire.
There is no difference either way....with the caps connected or not, the volume/power is the same.
There must be something else going on in the Carr to drop the power to 7w(?)
THANK YOU GEEZ!!! Makes you want to crack that Carr open again... Who's got that amp???
My guess is they are turning off one tube and lifting the bypass caps. (Looking at the actual amp circuit and sleuthing the switching may have caused a bit of confusion.)
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Whoops.....uh-oh.....hold it!!!!
UPDATE!!!!
The schematic must be wrong (something is shorting something else out) or maybe (quite possibly) I wired it up wrong, BUT:
I tried just doing a straight connect/disconnect of the individual bypass caps using clip lead wires (without the 0.1uf & 100k bleeders) and IT WORKS!
AND, the cool thing is that you can disconnect each cap separately (instead of both at the same time) to get (I'm guessing) ~20 watts output that has some "single-ended" characteristics.
** Both caps connected = full power (~30w) normal operation
** one cap disconnected/one cap connected = approx. 2/3 power....maybe 20w(?)
It even gets a slight SE hum with just 1 cap connected & seems to be very touch responsive & "single-ended" -like.....(unbalanced output, I'm guessing)
** Both caps disconnected = maybe 5w to 7w....it's not as loud as the "Little Wing" 6BM8 amp.
So, it really does what it's supposed to.....I'm going to put a 3-way switch in to have all 3 powers available.
Geezer
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I'll eat crow... No problem. Never figured it!
Thanks again Geez!!!
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I think about it like this. If the cathodes are tied together the current of both tubes is going through that one resistor. Each power tube is out of phase then when one tube is drawing more current the other is drawing less. So the equal and opposite IR drop across the cathode resistor keeps the bias voltage +/- the same. But with seperate, unbypassed cathode resistors for each power tube you do get the negative feedback drop. As more current is drawn through the resistor the bias voltage increases, decreasing gain. So adding the caps will keep the bias voltage constant(more gain than no caps).
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So did you use the 100R resistors and the .1 bypass?Or did you just switch out the bypass caps?
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I'm following this thread with great interest.
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So far I have just "manually" switched the caps using clip lead wires.
When I wire in the switch, I will use the 100k resistors (or 10k, whatever value stops the cap charge "pop"), but will not use the 0.1uf caps....they seem to be what caused the problem with my 1st attempt. I don't think they are needed.
BTW, the tone seems to stay very much the same at all levels of attenuation (!)
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Good work gentlemen!
I'm going to implement this into my homebrew AB763, I've got a spare hole where the extension speaker jack was.
Rob
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I jumped in too.
I mangled my 18Watt. Works great.
Got even an easier method using a SPST. Same scenario with separate cathode resistors and caps.. BUT:
If you disconnect both grounded ends of the cathode bypass caps and connect them together, but not grounded, you get full volume! Try that!!
I used two 250R resistors and two 20uf caps.
What did you use?
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Very interesting!Wonder how that works?No ground source for the bypass caps should be no different than lifting them.
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I used 2x 500R/10w, since my amp has 2x 6L6/NOS Tung Sol 5881's, and I used 8uf/150v caps....the 47uf made it a bit mushy.
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You guys are gonna love this... My 18 watt has a few controls that made this easy to experiment.
Totally variable from full on Push-Pull to low 5 watt single ended... Relates right back to the first post.
See the attachment. Geez, you'll like this one!
OR put a 1M pot between the two cathode caps and get variable Push-Pull power. Didn't draw that.
I'm really waxin' it up tonight!! ;D
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.sch & gif of the switch I implemented
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Hey Loose, how does it sound in SE mode? Does it have some fizz to it when you drive it? WE NEED SOME SOUND CLIPS!!!
Thanks,
Jim
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You guys are gonna love this...
Totally variable from full on Push-Pull to low 5 watt single ended...
I'm really waxin' it up tonight!! ;D
You're right about us loving it! :D I had to look at your drawing a couple of times before I saw the 1 meg pot in parallel with the 470K resistor to get 330K.
I jumped in too.
I mangled my 18Watt. Works great.
Got even an easier method using a SPST. Same scenario with separate cathode resistors and caps.. BUT:
If you disconnect both grounded ends of the cathode bypass caps and connect them together, but not grounded, you get full volume! Try that!!
I used two 250R resistors and two 20uf caps.
What did you use?
LooseChange - is your drawing missing one of the 20uf caps?
LC & Geezer - thanks so much for sharing the results of your experiments!
Chip
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The SE sounds good. It still has a full tone as compared to the P-P. Interesting note: Before this mod, turning that pot didn't do much unless you went to both extremes of the pot. This is a HUGE improvement.
I had to look at your drawing a couple of times before I saw the 1 meg pot in parallel with the 470K resistor to get 330K.
If you want the standard 220k values in there use the 1M pot in parallel with a 280k.
LooseChange - is your drawing missing one of the 20uf caps?
The drawing is for the P-P to SE idea, it only requires one cap.
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I've been playing with the "modded" amp all nite....this is one the most useful features I've ever put on an amp!
Three (3) definate, distinct power levels that do not significantly affect the "tone" of the amp.
I also tried LC's idea of a pot between the negatives of the bypass caps for "infinately adjustable" power....it works, too. I found a 5k linear pot worked best, but most of the useable sweep is at the last 25% or so of the dial....maybe a reverse audio pot would work better(?) UPDATE: After experimentation, I believe a 2k pot is best....see posts below.
Here's how to do the variable pot>>> (UPDATED to show switched pot)
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So tell me please how this works?The 5k pot seems to be a better way than the DPDT switch,unless the pot fails.The ground reference for the bypass caps goes backwards?
No comprende???
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So tell me please how this works?
I honestly don't know, I just know that it does work!
When the cap negatives are tied/connected together (NOT grounded, just connected to each other) the amp has full power as normal. If any amount of resistance is placed between the negative ends, then attenuation begins.
I experimented with different values (100Ω, 220Ω, 470Ω, 1kΩ) and each value gave an incremental step of attenuation.
At 2kΩ, there is only a slight noticeable increase in volume over the total disconnection of the cap negatives. Therefore, I think a 2kΩ pot with a spst switch would be perfect (Mouser actually sells one).
I envision it working like this:
*With the pot completely CCW & the switch "off", the circuit would be "open" with maximum attenuation (lowest volume).
*Turning the pot slightly CW as to turn the switch "on" would "close" the circuit & bring the pot's max 2kΩ resistance into play, giving a very slight increase in volume.
*As the pot is turned further CW, resistance is decreased, thus increasing the volume.
(I have updated the drawings above to reflect this change)
I have found the PPIMV on this amp is no longer needed....I just leave it all the way up. That way the power tubes are receiveing the max signal from the PI & the tone is unaffected.
unless the pot fails.The ground reference for the bypass caps goes backwards?
No...if the pot fails "open", then the volume will be at max attenuation. If the pot fails "short", then the amp will be at normal full volume w/ no attenuation.
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Looks like this trick is very promissing... but does anybody really understand what it does ?? it's quite confusing to me.
John
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What about disassembling the pot and scraping off a small area of the carbon trace?That would act like a 'switch' when fully CCW.
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Looks like this trick is very promissing... but does anybody really understand what it does ?? it's quite confusing to me.
Jeff detailed the basic action. The excerpt from ToneQuest is talking about how a single tube using an unbypassed cathode resistor has negative feedback due to the current flow through the resistor. The voltage developed across the resistor is directly proportional to the amount of current flowing, and that current is wobbling up and down during a signal. That means your bias voltage is also wobbling up and down, with the end effect of counteracting the current the tube is passing due to an input signal. Hence it is "negative feedback" but at one given tube stage.
Normally, we counter this effect in a preamp tube by bypassing the cathode resistor, which bypasses the alternating current around the resistor and keeps the voltage across the resistor steady. It restores gain lost due to negative feedback.
In a push-pull output stage, you could use a shared cathode resistor, and not need a bypass cap. If you assume somewhat matched tubes, then during a signal swing one tube is conducting more current while the other tube is conducting less current by more or less the same amount. The total current passing through the resistor remains unchanged, and so the voltage across the resistor remains unchanged. Therefore, there is no negative feedback effect, even without a bypass cap.
All I have just written is a restatement of exactly what Jeff already said.
So tell me please how this works?
Well, if we don't use a common cathode resistor, then there will be negative feedback at the cathode due to the current flow during a signal. We would have to bypass each resistor to eliminate the negative feedback and have full power. If we did not bypass the resistor, the negative feedback due to the resistor reduces the current swing through the tube due to signal, but that also reduces the voltage swing at the plate of the output tube, because the voltage swing is caused by a current swing through the primary impedance of the output transformer.
In the end, less current swing times less voltage swing equals less power output.
What about the bypass caps? If we bypass, we get full power. But what's up with the funky effect of connecting the negative ends together??
Look again at Geezer's drawing.
(http://www.el34world.com/Forum/index.php?action=dlattach;topic=4401.0;attach=3928;image)
If the negative ends of both bypass caps are grounded, are they not effectively connected together? Yeah, they are connected to true ground too; however, if you disconnect the negatives from ground, but keep them connected to each other, they have a "virtual ground" at their junction and have the same end effect. In fact, 2 caps connected in this manner are essentially in series, and the net value is half of the value of each individual cap. The pair of (maybe 22uF) caps could be replaced with a single 10uF cap. This idea (a pair of bypass caps being replaced by a single cap of half-value connected between the 2 non-grounded points to bypass) has been used before by Quad amps and was explained by Morgan Jones in Valve Amplifiers.
If you wanted to replace the pair of bypass caps, you'd want a non-polarized cap, but you'd need ~10uF which might be bulky in a film cap, and a non-polarized electrolytic is basically 2 back-to-back electrolytic caps in one package (and much more expensive). If you did replace with a single cap, I'm not sure how effective a single pot would be from one leg of the cap to a single output tube cathode, but it could be tried out for the sake of experimentation.
Anyway, a single pot between the 2 caps as Geezer shows destroys the bypassing effect due to a virtual ground as the resistance is increased. In the end, this is atypical, fairly unusual, but not voodoo or make-believe. Pretty cool rediscovery/re-application if you ask me.
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what i figure is happening is the bypass caps are actually acting as coupling caps to scrape dcv off the signal at the cathode of one power tube and use it to limit bias at the other power tube. the 2k linear pot and the cathode resistor of the other tube form a voltage divider - when the pot is out of the circuit, bias voltage is the same at each cathode. you put the pot in the circuit and you impose a less negative bias voltage due to the ratio of the 2k pot and the cathode resistor - less output. it works due to the fact that the tubes are out of phase class ab operation (as previously stated) - one tube is idling while the other conducting. with no signal both tubes are at idle, so i figure the caps are also there as bypass caps to allow you to do your bias calcs for each tube individually.
HAH! HPB entered an essay while i was typing this ... i'll leave mine anyhow; i think we are saying the same thing.
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thank you both, i think im starting to get it... it's probably gonna take me the rest of the day to digest HBP post ;D
John
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I have posted the drawings in the schematics section to have quicker access for those wanting to try this attenuation system.
Geezer
http://www.el34world.com/Forum/index.php?topic=4437.0 (http://www.el34world.com/Forum/index.php?topic=4437.0)
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Does it work well with Class AB or just Class A?
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Does it work well with Class AB or just Class A?
Good question. Re-reading HBP's post, it almost seems that this approach would work better with Class AB than with Class A. My theory, if you can call my mutterings a "theory", is that in Class AB the tubes take turns handling the signal and generating current. However, in Class A both tubes would be drawing the same amount of current all the time. Does that mean the plan won't work as well with a Class A push-pull amp? Don't know. LooseChange's drawing showed only one cathode resistor being bypassed and one tube being gradually "turned off" IOW swamped with negative feedback. (That sounds bad but is "good" in this situation.)
Chip
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I like this mod so much that i am going to install it on at least 2 other amps.....a 2x EL34 amp that is currently fixed bias (but soon to be cathode bias) and my Sluckey November.
Both amps sound fantastic, but I never use them because they are just too loud for most of the venues that I play.
I'm ordering a few of the 2.5kΩ pots to have the "infinately variable" version on these amps....can't wait to see if it works as well on these as it did on the 1st one!
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DaGeezer,
THANKS for your contribution to the forum and developing the schematic information on this and sharing your experiment!
I am very appreciative of the creative ideas and innovative thinking/approaches you bring here. The variable version (pot) sounds like a cool idea.
Keep coloring outside the lines!
With respect, Tubenit
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Hey guys!
This is VERY interesting thread and it got me thinking that since we have individual bias resistors could we use mixed pair of tubes (for example 6L6 and EL34)?
Also LCs drawing gave me an idea of a balance pot. I have no idea if this would work or not but i drew it anyway. All comments are welcome.
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Disconnct the bottom leg of that pot and that is exactly what I did. It works really well and after doing that Blackheart conversion I'm liking the single ended EL84.
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HI there!!!!!
After reading this topic....it took me to realize that this would be well regarded in a 100w amp....which could actually be switched to cathode bias and then through the attenuation steps when in cathode bias mode....actually I thought abouth it since I`m building a mean machine and this could be a very useful to bring it to acceptable volume levels in a small club...even though this could still spill blood....
Any ideas of how to do it?? I`ve been trying to figure out....should i use ONE RESISTOR PER CATHODE? and swithces to engage them one at a time? or could I use ONE RESISTOR for each opposite pair??? may sound dumb but 4 tubes always screws things a bit....
I should mention that this well be a 4 - 6550 with a switch to go from Pentode to U.L. and a switch to fixed/cathode bias...and i would like to install it...what you guys think??
Thanks for sharing this info
With Respect
Best Regards
Rzenc
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While still experimenting with the attenuator, I found a better way to wire the switch which (1) eliminates the "Single-ended" hum, and (2) allows you to choose any power level you wish for the "medium" power setting.
The 330Ω resistors shown on the following "improved" drawing gives ~1/2 power. By increasing or decreasing that value, you can choose slightly higher or lower power output.
For example, increasing the value to 470Ω would yield slightly less output, while decreasing the value to 220k would give slightly more power. The more the caps are isolated from ground, the less output power is generated.
I'm also posting these in the schematics thread......
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It is so cool that you guys are thinking outside the box and sharing your ideas. Many thanks! :D
Chip
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I tried this on my cathode-biased AB763/reverb.... 560 ohm resistor/25@50 cap on each 6L6, ( 2 6L6s' )... I found that this mod DID drop the output, but in a way that made it sound thinner. The tone was not the same. The effect is like switching-out pre-amp bypass caps. Possibly I'm missing something ?
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Hi...
I came up with this schem for 4 POWER TUBES
What you guys think?
With Respect
Best Regards
Rzenc
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Here's a layout of Geezer's improved 3-selection switch. Looks better if you zoom in. Craig
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I just tried the attenuation on an 18 watt lite I just finished and it indeed works.It's hard to say how much it drops the wattage down because the 18 watt is low wattage already,but it dropped it quite a bit.The tone is in debate because I couldn't crank it up yet,but the cleans seem to be alright.
I'll reprt back later today with a real report on the tone differences.
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I tried it at all volume levels and it definitely works fine.I just used a switch for the caps and it doesn't pop even with no resistors.The low watt sound is good but not outstanding.For bedroom gigs it gets an A.
It works very well with a strat and not bad with humbuckers.I have a PPIMV on this amp too and that rounds it out nicely.
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Hey everybody! I'm finishing up a fairly straight forward 5E3 and would like to implement this attenuation method. I currently have a 25uF 50v cap and 270ohm 5watt resistor in place. Do I just separate the cathodes of the 2 tubes and add a second cap and resistor of the same values for the other tube? Or do I need to change the values of those components? Thanks in advance!
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You need to double the resistor value with separate ones.Thw same value cap will be fine,but usually you halve it when using two.
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Does the attached drawing of a DPDT switch look correct here?? I have actually wired one up on a friend's amp and it worked but not sure I have drawn up how I wired it????
Please confirm or correct the drawing, thanks. If it's incorrect can you post a DPDT drawn up in this style to show what it would look like.
Would both of these work as I have drawn them???
I'm trying to draw the one on the top right to be the same as DaGeezer's on the bottom.
With respect, Tubenit
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Hi, I think you are missing the caps to ground connection on the full-1/4power switch.
The other one seems to be ok.
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VMS (& his added drawings) are indeed correct.
T, how did the mod sound on your friend's amp?
G
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VMS ........ THANKS for your help it is appreciated!
DG, the amp sounded GREAT with the mod. As I heard others say, there was little to no tone difference which I liked alot. It was on a Carolina Blues Special (Bogen conversion) that belongs to my friend (incredible guitarist) who lives up there within a few miles of you. He bought a condo and needed even lower volume to continue playing without the neighbors complaining.
While the wiring values I used were called 2/3 & 1/8 ....... it sounded more like 2/3 and 1/4 to me.
Very useful mod. I'm grateful that you shared your success with it on the forum.
With respect, Tubenit
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This idear has really got me curious: I see it implemented with a pair of EL84's- will it work with 6L6's (such as my Red-Headed Stepchild)? It looks simple enough that a ham-handed tube novice such as myself. How would that affect the adjustable bias mod? (Or is this too complicted fer my pea-brain?) ;)
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This circuit applies to a cathode biased amp. I think your stepchild is a fixed bias amp.
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Glad to help!
This mod is something I'm definitely going to try in some future build.
Maybe 6bm8 amp with even lower wattage. :)
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Hmmm, (doin' my best Arsenio Hall impersionation)
http://www.el34world.com/charts/fenderservice5.htm (http://www.el34world.com/charts/fenderservice5.htm)
I think the above link details how to change from fixed to adjustable bias. Is this the same thing? Or is my profound lack of knowlege rearing its ugly head (again)? Got my money on 'ignorance'- but I could be wrong! ;D
(Yer gunna have to think of me as that kid in the corner with the thick crayons)
;)
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I think the above link details how to change from fixed to adjustable bias.
Yes. But it doesn't change to CATHODE bias.
Fixed bias applies a negative voltage to the control grid in order to bias (control) cathode to plate current.
Adjustable bias is a form of fixed bias with a pot to fine tune the negative voltage applied to the grid, thus providing fine control to the cathode to plate current.
Cathode bias uses a resistor between cathode and ground to develop a positive voltage on the cathode that will control the cathode to plate current.
Your amp is fixed bias (even if you can adjust it) and this circuit applies to cathode bias circuits ONLY.
(Upper case used for emphasis, not shouting) :)
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OK, that makes sense :)
So, Fixed = PLATE bias
Now, the sixty-four dollah question: is there a way to do something similar to a fixed (plate) bias amp? or is there an adjustable bias configuration that will work with my iron?
I wish I'd bet real money on my ignorance... coulda hada winnah.
(Upper case used for emphasis, not shouting) :)
('s OK, I'm used to gittin' shouted at, wife does it all the time) ;D
She sez it's fer emphasis too... but I don' think so unless it's followed up by the fryin' pan, now THATS emphasis!
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So, Fixed = PLATE bias
I've never heard of PLATE bias.
Now, the sixty-four dollah question: is there a way to do something similar to a fixed (plate) bias amp?
May have to think outside the box.
or is there an adjustable bias configuration that will work with my iron?
Sure.
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So, Fixed = PLATE bias
I've never heard of PLATE bias.
Don't mind me I'm either low on caffiene, or stuck on stupid... same difference, just tryin' to hone my 4H electrical skills ;) Just making a gallant attempt to follow the bouncing electron with my thick crayolas.
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just wanted an update:
how many of you guys are still using this mod and happy with the sound?
also any problems with wear on the transformers? (someone mentioned to me it might be an issue with this mod)
thoughts much appreciated before i go ahead and drill my 5e3
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I've read something like this would work, but have not tried it myself. It was described as sort of getting a psuedo
single ended sound from a push/pull amp. One of the forum guys a few yrs ago did it and stated it worked.
With respect, Tubenit
In the cathode biased amp i tested coneccting to ground one of the tube grids and it work REALLY WELL. Even better thant split cathode resistor/cap with the caps switch.
How could it be done in a fixed bias amp?
I suposse that bypassing one of two 220k bias resistor will work, but i´m afraid about bias voltage going crazy.
any idea?
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Is there any chance of recovering the images from this thread? I'm very interested in this topic. I think I understand the concept well enough to figure it out on my own, but I'd like to see the different implementations.
Also curious about the original question. If one tube in a push pull pair has no input signal (e.g. by grounding its control grid), does this cause problems in the output transformer?
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Is there any chance of recovering the images from this thread?
Not likely. Hoffman periodically purges forum images from his server in order to make room for new images.
If one tube in a push pull pair has no input signal (e.g. by grounding its control grid), does this cause problems in the output transformer?
No problems as long as you don't disturb bias. You can directly ground the grid of a cathode biased amp, but not so with a fixed biased amp. For those you'll have to kill the input signal some other way.
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Is there any chance of recovering the images from this thread?
Not likely. Hoffman periodically purges forum images from his server in order to make room for new images.
If one tube in a push pull pair has no input signal (e.g. by grounding its control grid), does this cause problems in the output transformer?
No problems as long as you don't disturb bias. You can directly ground the grid of a cathode biased amp, but not so with a fixed biased amp. For those you'll have to kill the input signal some other way.
Just to make sure I'm understanding this correctly: the output transformer only becomes mismatched when both tubes are pulling different amounts of DC? That's why we ground the grid instead of lifting the cathode (or why we can't just pull one tube out of its socket)? This way both tubes are still drawing the same DC current, but one has the additional AC riding on it, which doesn't matter because the OT only cares about keeping the DC balanced between both sides? We would of course lose the hum cancelling effect of being push pull?
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We would of course lose the hum cancelling effect of being push pull?
No. The high B+ ripple will still cancel as long as each tube is drawing the same DC current.
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... If one tube in a push pull pair has no input signal (e.g. by grounding its control grid), does this cause problems in the output transformer?
No problems as long as you don't disturb bias. You can directly ground the grid of a cathode biased amp, but not so with a fixed biased amp. For those you'll have to kill the input signal some other way.
Envision a Volume control ---> Coupling cap ---> Bias supply + Grid
Simply "turn down the volume" on one side of the push-pull output. That side still idles, but has no AC-input.
... the output transformer only becomes mismatched when both tubes are pulling different amounts of DC? ... one has the additional AC riding on it, which doesn't matter because the OT only cares about ...?
Be careful with the word "mismatched." Mismatched to what?
At lower power-output levels, each side of the push-pull output sees 1/2 of the total "plate-to-plate impedance" of the primary. At higher output levels in Class AB, one side is cut off and the remaining side sees 1/4 of the total plate-to-plate impedance. That allows the side that is on to flow more current for the same AC-voltage-drop (which is most of how Class AB delivers higher power output that Class A).
Turning off the drive to one side of the push-pull power section but retaining the DC idle current effectively never "shuts off" that side of the transformer primary. The driven side always sees the "1/2 of the total plate-to-plate primary impedance."
Now whether that load is "optimal for max power output" or not probably doesn't matter, because the user wanted less power anyway. And the tube doesn't really care what its loading is, it is simply attempting to pull current through the primary in response to an input signal. Therefore "mismatching" is an arbitrary and/or academic notion, and best forgotten (it's common due to the way people first learn about transformers, but the concept is over-emphasized past a point where it stops being useful).
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Therefore "mismatching" is an arbitrary and/or academic notion, and best forgotten (it's common due to the way people first learn about transformers, but the concept is over-emphasized past a point where it stops being useful).
This is probably where I'm at. I was under the impression that push-pull transformers required some sort of equal-but-opposite current running through each side, and this was what made them different and more compact than single-ended transformers (something about an "air gap"...). Given that understanding, and reading through this thread, it started to sound like that requirement only applied to the DC, and not the AC.