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I recently had an idea for my diy 5E3X2. I put a switch in that lifts the cathodes of 2 power tubes on ONE SIDE of the OT changing it from Class A push-pull to Class A single-ended. I realized this would work when I observed that the current would stay put for the 2 tubes on one side being that the amp is in Class A. Just wondering what people's thoughts are on this or if there is something about doing this that is not allowing maximum performance in single ended mode. Maybe impedance mismatch? I don't know. I've played it and it sounds good. There is a definite perceived loss of some low end.
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Hold it.Originally posted by lowell View Post
The 5E3X2 is NOT a class A amp.
Another point is that the output transformer in the 5E3X2 was actually designed for two 6L6, class AB, push pull tubes idling at around +15 watts each and an output of around +35 watts max.
And as such, the smallish output tranny is not really built to handle the idle current of a pair of class A 6V6s (which again, they are not in Class A) and the subsequent current imbalance through the OT while one side is running all by itself in high idle current... as would be found in a true SE class A PA or the 5E3X2.
That, and the speaker is creating an impedance mismatch now, is probably why the tone is suffering
Point three, something is not right there if lifting two of the power tubes does not increase the B+ rail and thus, increase the idle current on the remaining two tubes.
Is this a homebrew version of my 5E3X2 or a Weber kit?
Email for a little trick I do to these to simulate a little class A sound with reduced power. -
Hi Bruce,
Class A is cathode biased right? That's how I have my homebrew wired. I also am using 4 6V6 output tubes. I built this amp from scratch but the transformers are Weber. So using 2 6v6 on one side is causing higher B+ and impedance imbalances? Can you explain?Comment
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'Class A is cathode biased right?' No
Class A means that at max output before clipping, the output device/s don't go into cut off, they are always conducting over the full waveform.
Does the voltage across your cathode resistor go up in response to a signal? If so it is in class AB.
You are losing low end because your OT is saturating, as it's designed for push pull (balanced currents), not single ended (totally not balanced).
Try reconnecting the cathodes, but put the output of one side of the phase splitter on to a volume pot, so that you can turn down the input to one side 6V6s. That way you'll keep the OT dc current balanced, but the even harmonics won't cancel out (which is probably liking about your modded set up).
Peter.Last edited by pdf64; 10-26-2008, 12:49 AM.My band:- http://www.youtube.com/user/RedwingBandComment
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Lowell, a lot of Class A amps use cathode bias, but that is not what makes them class A. You can also use fixed bias to run a class A amp. Cathode bias is just a simple way to bias amps, especially lower power ones.Education is what you're left with after you have forgotten what you have learned.Comment
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Ok. I guess I think ALL cathode bias amps are class A because most commercial if not all commercial amps that are cathode bias ARE class A right? Correct me if I'm wrong, but A cathode bias classic Fender circuit is going to be Class A right? I'm guessing that an amp w/ a negative grid bias to shut the tubes off for part of the cycle can also be cathode biased w/ a cathode resistor. Let me know if I have this right.Comment
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No, very few are actually Class A, despite what the marketing copy (or internet hype) may say.Ok. I guess I think ALL cathode bias amps are class A because most commercial if not all commercial amps that are cathode bias ARE class A right?
Which cathode bias classic Fender circuit? Champ? Yes, it's probably class A. Deluxe? No I don't think its class A.Correct me if I'm wrong, but A cathode bias classic Fender circuit is going to be Class A right?
As stated above, cathode bias does not equate to class A; or rather, method of bias does not equate to operating class.Comment
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wow... seems I need to go back to school a bit here. I just read Aiken's page on the AC30 and he debunked the Class A myth. I see that it'd be pretty hard to get an amp over 20watts to be Class A.......... eh? What amp really IS class A that I could listen to?Comment
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Sorry I should have put a link to aiken's site.
Like hasserl says, a fender champ or most anything single ended pretty much has to be class A. For push pull, class A is very unusual, tends to be a specialist boutique thing. Peter.My band:- http://www.youtube.com/user/RedwingBandComment
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Yes , it is. On a couple amps I built that were 36 watts SE Class A, I used two SE output transformers. However, if I were to do that again, I could have probably gotten by with just one.Originally posted by lowell View Post
Now, to answer the question which amp is really class A to listen too ? One could always listen to one of those low power 5 watt amps, not that they sound too particulary good, like a stock Valve Junior, Blackheart, or Gibson GA-5. But if you could find an old VOX AC-4, that would be a good start.
Now, if you wanted to mod an existing push/pull class A/B amp for single ended, one could always lower the bias toward class A, keep both power tubes right where they are, and then just apply your drive signal to only one of them.
-gComment
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I have been wondering about class A as well for a long time until I found this site:
http://www.diyguitarist.com/GuitarAm...erTubeBias.htm
There are "bias charts" that tell WHEN a tube is actually class A at a specified voltage.
I don't know if those charts are really correct, since I haven't found another site or book, where it is explained in a similar way.
I'm wondering why, cause it's just a matter of calculation, isn't it?
MattLast edited by txstrat; 10-28-2008, 09:09 AM.Comment
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"I have been wondering about class A as well for a long time" ...not trying to be funny but why?
Doesn't it make more sense to build an amp that you like the sound of, rather than build something primarily as a technical excercise to meet determined technical perameters? Players can hear the difference between low plate voltages vs higher plate voltages, hi plate current vs moderate plate current, NFB vs lack of NFB...many can hear the difference between fixed vs cathode bias...so why not explore the factors that are clearly audible to get a result that you like, if it falls into class A then great, if not what does it matter as long as it sounds good?
If you have to actually measure things to determine whether tubes are conducting for the full 360 then you're missing the point.
"cause it's just a matter of calculation, isn't it?" No it's a matter of what the result sounds like. An SF Champ is typically biased to class A territory, plenty of folks cool off the bias but still like the way they sound.Comment
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You're right MWJB the sound is the goal.
I didn't mean the sonical point of view in class A rather than the technical. I was just wondering (out of curiosity) at what values a specific tube will run in class A, no more. I believe some people talk about (especially) class A without actually knowing how it sounds (including myself). I'd just like to know what the difference is and be able to differentiate by sound.
Did you have a look at the link I posted? Do you think the values shown are correct? I mean it would be interesting to try out different bias voltages to run the amp in different classes just to learn how the sound changes.
Actually I don't care what class an amp runs in when I bias it. If it sounds good - good, if it's class A - I don't care just out of that sort of hype.
BTW, my personal sound favorite is cathode bias, even though most of my amps have fixed bias.
MattComment
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Well, the first place I look for those values is what is listed on the data sheet.Originally posted by txstrat View Post
-gComment
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