Noodles
314 posts
Sep 08, 2012
10:02 AM
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I recently read an article by Gerald Weber and as an amateur harp-amp nerd, I learned a couple of things. Here’s the topics he writes about, (if you're interested).
Low plate voltage on the preamp tubes
Larger than normal coupling caps
Simple two-stage design. Less is more
Paraphase style Phase Inverter
A Cathode-biased, Class A push-pull output stage
High idle plate current in the output stage
Tube rectifier
How to Stop Feedback and Fatten the Tone
OK, enough already! Show me the article
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didjcripey
378 posts
Sep 09, 2012
8:36 PM
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Very interesting article; sounds easy, though I haven't had enough experience with amps to do it myself.
When they talk about the effect of speaker polarity on feedback, does that mean I can just swap the speaker wires round? If it was that simple, wouldn't that be the way they were set up?
What do you reckon my chances taking that article and my amp to an amp tech and asking them to do it? (there is no one in my area that knows anything about amp mods for harp)
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Lucky Lester
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Tuckster
1138 posts
Sep 09, 2012
9:56 PM
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I saw this long ago and wondered about it. Some of it is certainly true,but some of it is questionable. In particular Class A Push Pull. To me that's an oxymoron.
Also reversing speaker polarity is a 50-50 chance.It might already be out of phase,depending on how many gain stages there are.Still,no harm trying. One way will be better than the other,I suppose.
We need 5F6H's expertise on this one.
Last Edited by on Sep 09, 2012 10:03 PM
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SuperBee
557 posts
Sep 09, 2012
10:14 PM
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I've done the first two things, and seen them recommended many times.
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5F6H
1350 posts
Sep 10, 2012
2:11 AM
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@ Tuckster - the speaker can never be "out of phase", if there is just one speaker, or one amp/cab. We're just talking here about whether the cone fires forwards or backwards when given a positve signal (polarity & phase are often confused, bt they have slightly different meanings). For the puroposes of class, Gerald has a practical view that if both power tubes conduct for 360 within normal use, then it's class A (this doesn't always correlate with others' definitions, but class, like power W RMS are more relevant to a diagnostic state that to real world use - but you're right, push-pull Class A amps are so rare as to be of virtually no significance).
@ didjcripey - Speaker polarity on it's own has a negligible effect on feedback. It mainly has a subtle effect on tone. As Tuckster says, no harm trying, depending on what brand of speakers were fitted to some amps, some left the factory one way...others left the factory the other way. You would only detect it in a comparative test. Your chances of getting a good result from going to your local amp tech with these suggestions are typically not good...too many variables, not least which amp you are taking to them. I know a couple of guys who tried just that...both had their sugggestions dismissed out of hand & the respective companies took their own route...& failed! Some techs just don't like being told what to do & how to do it.
It's worth putting the Texas Crude harp amp in context, it uses some great innovations, Gerald's a very clever guy, but if I was going to buy a Kendrick I'd get the bassman. ;-) The Texas Crude was built to an "old school" recipe (more in the mold of an early 50's amp), thick grindy tone, but ultimately less power & volume from 2x6L6 than a fixed bias amp.
Only a few of the 6L6 amps made today, aimed at harp players today use cathode bias with high idle currents, most are fixed bias with moderate bias. Only a few use paraphase PIs, most use regular "long-tail".
I'm not saying these things are "wrong", far from it...if it gets you the tone you like at a useable volume then great, just putting these things in context. Like many features you see on harp amps, they may only pertain to that brand/model & are typically not universal accross all "harp amps".
Points #1 & #2 - Widely used, other than tube & speaker subs, it was really Gerald who set the ball rolling in specifically designing a tube amp for harp use...even if you have a Twin Reverb with dropped preamp voltages, it's a fair bet that the idea stems from Gerald's article.
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www.myspace.com/markburness
http://www.facebook.com/markburness
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didjcripey
379 posts
Sep 10, 2012
5:23 AM
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Thanks for the valuable info guys; my chances of getting good advice locally are close to nil.
I am fairly happy with the tone I get now, but am always looking for ways to tweak it.
I tried the polarity swap on the speakers; very slight difference: hard to tell.
Sometimes something sounds better because it is different to what you are used to, it is such a subjective thing.
I have a friend who tools around with amps, I might see if he'd do the plate voltage and coupling caps for me.... or maybe I'll stick with: 'if it ain't broke don't fix it', and work on my microphone technique and playing instead.
Cheers
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Lucky Lester
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atty1chgo
467 posts
Sep 10, 2012
6:05 AM
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Informative article on tube amps.
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Tuckster
1140 posts
Sep 10, 2012
9:44 AM
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5F6H= Point taken on calling it polarity and not phase. Heaven knows there enough misinformation on the 'Net. I don't wish to contribute any more.
Yes,to me Class A means the tube conducts 360.Is that the same as single ended? In a transistor amp you can parallel output transistors to increase current and thus power. Can you do that with tube Class A? (Showing my ignorance.)
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5F6H
1351 posts
Sep 10, 2012
10:29 AM
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@ Tuckster "Yes,to me Class A means the tube conducts 360.Is that the same as single ended?" Well, because single ended amps have no partner on the other end of the OT primary winding, to pull when they push, they are all class A in their clean range...but folks just love the sounds of S-E amps pushed into cut-off, where signal is strong enough to push the bias, under signal, cold enough so it no longer conducts for 360...in these conditions S-E amps sound very similar to moderately biased P-P amps! Some techs also consider grid conduction in the whole class A debate, presence of global NFB etc...I prefer to keep things simple myself!;-)
SF Champs tend to actually be biased to mid point (ideal) Class A, more by accident than by design, but they typically have to idle VERY hot to do so (20W+ on a 14W tube)...a lot of other tubes won't take this.
You can parallel tubes in S-E, just like you can in P-P with say a twin, but at what point they drop out of class is still dependent on bias conditions for each tube, rather than number of tubes. You still get twice the power of one tube under the same conditions. Parallel S-E amps tend to be pretty rare (there was GA8 Gibson, a few boutique builders - check out the Victoria Regal & 2 Stroke - & hobbyists have made them), good high power SE OTs can get expensive (though P-P OTs reconfigured are good enough for rock'n'roll)...given the price of iron & customer expectation, most manufacturers tend to go push-pull when using more than one power tube - one 6V6 S-E can only make ~6W (dirty), 2x 6V6 in S-E will only make twice that...but 2x6V6 in P-P will make 20W+...which amp would you buy? ;-)
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www.myspace.com/markburness
http://www.facebook.com/markburness
Last Edited by on Sep 10, 2012 10:32 AM
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bloozefish
44 posts
Sep 10, 2012
10:56 AM
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@5F6H
I have a '48 Supro with 2X6V6 in parallel, and I almost mentioned it in the "best harp amp" thread. It is the most breath-sensitive amp in my modest collection (a guitarist would say touch-sensitive.) This thing just screams with the 6V6 sweetness, and can go from a whisper to a roar with just a little mic technique. It has a very different feel from my 1X6V6 SE Harpgear 1, 1X6L6 Masco, my Kali 2 clone, or my push/pull Premier 110 and Magnatone Melodier.
Not sure what it is about that parallel SE circuit, but I like it a lot!
james
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bonedog569
629 posts
Sep 10, 2012
10:57 AM
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Glad to hear from 5F here. I was def. influenced by this - "...even if you have a Twin Reverb with dropped preamp voltages, it's a fair bet that the idea stems from Gerald's article." Yes - and bigger coupling caps too.
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MJ
464 posts
Sep 10, 2012
10:59 AM
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bloozefish, I agree on the 2x6V6 SE. As a matter I named the one I built in the "Best Harp Amp" thread. It is a great harp amp setup with wonderful tone and enough power to hang with some pretty loud situations.
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5F6H
1352 posts
Sep 10, 2012
12:51 PM
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Bloozefish & MJ - I never said they weren't good, just that compared to 1x6V6 S-E & 2x6V6 P-P they were rare. :-)
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www.myspace.com/markburness
http://www.facebook.com/markburness
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MJ
466 posts
Sep 10, 2012
12:56 PM
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5F6H, I did not imply that you did. I was merely adding to the thread and seconding Bloozefishes statement which I did not see as any repudiation of anything you have said. Sorry you saw it that way. I should have made myself more clear. ;)
Last Edited by on Sep 10, 2012 12:57 PM
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5F6H
1353 posts
Sep 10, 2012
1:28 PM
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Sorry MJ (& Bloozefish), I also didn't intend to be adversarial. I love to hear about folk enjoying their amps...MJ, you say you built yours yourself?
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www.myspace.com/markburness
http://www.facebook.com/markburness
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MJ
467 posts
Sep 10, 2012
2:11 PM
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Yes I did 5F6H. It has a number of embellishments from the original 5F1 circuit. A couple of which are mentioned in the Weber article. I have found that the Cab size and shape also seem to affect the final sound.
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Last Edited by on Sep 10, 2012 2:23 PM
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5F6H
1354 posts
Sep 10, 2012
2:18 PM
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Nice work.
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www.myspace.com/markburness
http://www.facebook.com/markburness
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MJ
468 posts
Sep 10, 2012
2:42 PM
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5F6H, Thanks. I would like to add that I do find much of your information both entertaining and useful. Your posts add a lot to this site.
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bonedog569
632 posts
Sep 11, 2012
3:59 PM
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MJ - tell (or show) us more about your circuit design. Lone Wolf shared a 3 paralle 6v6 plan and a forum member there shared a 2 6v6 parallel design . There are those and the Gibsonette. Not sure how the Champ circuit is utetilzed. It is single ended.
http://lonewolfblues.com/proj3x6v6se.html
http://lonewolfblues.com/forumamps.html
(the first one on the forum amps page is by our esteemed 5F6H - aka Mark Burnesss - the second one is the parallel 6v6 design
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MJ
471 posts
Sep 12, 2012
10:53 AM
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Bonedog, The changes I have made in my amp probably would not be called a design change. Tweaks would be a better word. I installed switches for bypass caps on the preamp. Pin #1 with a 22/25 cap. Pin 8 with a 1uf cap. .1 oil in paper coupling caps replace the .22 that are in the schematic. The dropping resistor to the preamp I changed to 150k which dropped the plate voltage. I changed the 470 ohm 5w resistor that goes to pin #8 of the 6V6 to 1k and the cap that also goes there I changed to 50/50. This allows me to run 6L6 if I wish. In the 2x6V6 I have not done this, however I am using a 6L6 in one of my other Jaybird amps that have only one power tube. The 2 6V6s are wired in parallel, and I upgraded the OT to a Hammond 125DSE. So as you can see, there is nothing too radical in the line of change, but it makes a good difference in the sound to me.
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bloozefish
46 posts
Sep 12, 2012
2:32 PM
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bonedog,
here's a link to the schemo of my parallel 2X6V6 Supro:
http://api.ning.com/files/KBrEfAoS3SFvFgiHA4Jkc*0Z-bKQ3x-CmHsRAEKQb1lu*lZYRVWUTesfkcTalFNBo8KiBAty1SsTJAQNKbjRZxCmUHqjq6fL/1948supro26v6SE.jpg
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bonedog569
634 posts
Sep 12, 2012
5:24 PM
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thanks guys - I'll look at these options more when I get a little time.
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Last Edited by on Sep 12, 2012 6:19 PM
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