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So i take the amp to a jam last night after all the tweaking i did with help here and the amp was just too low volume to have any kind of headroom in the power amp. The 6V6's were just driving like hell and i had the master almost all the way up. If i turned it down to where the power amp is not adding a ton of overdrive it was way to low. It's only got a 290-290v 120ma PT so i guess i'm not gonna improve it w/o replacing that, eh? Or IS there a way to get more headroom? My take on the way it works is the signal level coming out of the PI is going to determine how much clean headroom and if it takes a higher level than that to get to the volume i need i assume i'm S out of luck? It's a marshall style circuit.....CF>typical marshall tone stack>typical marshall PI with 1M/1M/470R/10k and NFB. 50/50uf can for screens and plates. 395v on plates and biased to around 24ma. Is there any way to get more clean headroom i'd like to hear it because if not i will likely shelve the amp and let it collect dust rather then buy a new PT and possibly OT. (OT is trainwreck style classictone 30w 6.6k)
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Ultimately amp headroom is limited by available output power or rather SPL.
What is the measured output of your amp?
With 2x6V6 and given power supply your amp should be able to deliver around 18W before clipping (maybe 20W depending on PT regulation).
IME 18W like my own Fender Super Champ is not enough in a blues-rock/classic rock band (except if the amp is mic'ed and signal sent to a guitar monitor).
I need at least 30W to 40W to combat with a rock drummer and 2 speakers produce more SPL than a single one.
Enzo used to say that doubling power only means 3dB more, but those +3dB can be crucial in a band context.
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Originally posted by pdf64 View Post
Yeah, it's a 18w heyboer PT. I figured 18 tube watts should get pretty loud tho. It does just barely get loud enough but like i said, at that point it's driving way too hard. I'd like to convert it to EL34's but i'm not throwing another $150-200 into the old thing. If i were gonna do that i'd have to rebuild it too due to how degraded it is from endless experimenting. And if i were gonna spend that much i'd just bite the bullet and get a fender supersonic 22 or such. Plus my modeling combo just sounded better and much louder so i'd need to know it;s gonna kill b4 putting all that $ into it, and theres no guarantees there.Comment
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I used 12L's for many years. My fav speaker. Unfortunately that would cost me a lot of $ and work because i would have to buy a 12L AND all the materials to build a new cab then slave over it for god knows how long. I hate making cabinets. All that and no guarantees....no thanks ! Like i said above, if theres no way to get a bit more with the amp as is i'll pass and it'll go back into the garage.Originally posted by pdf64 View PostEVM12L - make more of every watt
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NFB on a switch, fixed bias if it's cathode.Change the speaker, if not EVM12 there are others with high spl for cheaper. You don't want to sink any money in it how do you expect and increases in watts/sound?Comment
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It has variable NFB pot thats a no load so i can eliminate NFB altogether or anywhere in between. Speaker is a 100w celestion and it already is fixed bias. I never said i expected to get an increase w/o investment, i ASKED if theres a way and said that otherwise i'll just shelve it.Originally posted by mozz View PostComment
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Spec. of the PT will put a limit on available power, but does not mean amp is putting that out unless you have measured it. Have you measured a clean 18W out of it?Originally posted by daz View Post
If so, what is the lowest powered amp that has given you the loudness you need in band situation?
Originally posted by Enzo
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Don't know what it's putting out and don't have a scope. Lowest amp i have ever used in a band is a peavey classic 30 with 4 el84s. Don't recall having loudness issues.Comment
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I was writing this much earlier and tboy was doing site maintenance. So some of this may be redundant...
It might be worth noting that the speaker you have in there is on the lower end of the efficiency range as Celestions go. With 97dB sensitivity it's rather less than a Vintage 30 or G12H both at 100dB. It's said that improving sensitivity by 3dB is equal to doubling your wattage. But, now, some of these specs have particulars and even your cabinets compatibility with the speaker can have an influence. But if you have a Vintage 30 or a G12H (Anniversary or Heritage) around that is compatible you might want to try it and see before canning the project. IMHO the G12H Heritage would be a good choice since it has the best bottom end of the three.
The EVM12L (mentioned above by Pete) is also 100dB SPL. But you've balked the expense and I guess it wouldn't fit in the cabinet as it is? What about another Celestion then? The two above mentioned speakers aren't cheap, but affordable/used maybe. And should fit into any amp that housed a 100W Celestion. It seems you're probably not up for a speaker change but I'm including it now because I wrote this earlier when there were no responses yet."Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
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OK, I'm probably going to get something wrong here, if so I'm sure someone will correct me. You say it is a Marshall style circuit, but it has 6V6 power tubes. When I think of a Marshall 18 watt amp I think of EL-84 tubes, which have much higher transconductance, so it takes less signal to drive them to a given current. I wonder if you are running out of headroom before the power tubes (maybe the PI) so you aren't able to get full swing from the power tubes.Comment
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I've thought of that because the drive did sound more like a distorting 12AX7 plus there was a bit more volume available but it was distorting too much was the main issue. And so maybe that was the PI, but then if the PI is distorting because the signal is hot then what can you do about that? Is it possible to make the PI output a hotter signal with the same input signal? Seems to me the component values in a marshall style PI are already optimal for output volume, no?Originally posted by glebert View PostComment
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But Marshall usually uses power tubes with higher transconductance, as EL-34 is similar to EL-84 in that regard. Basically they will get the same current as a 6V6 with half of the input voltage. Since they don't need as big a signal from the PI they are less likely to drive the PI into distortion.Originally posted by daz View Post
Is your amp fixed bias or cathode biased? Don't even know if/how that plays into it.
Last edited by glebert; 10-31-2022, 06:27 AM.Comment
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I'd like to try EL84's but i don't think the trannys are gonna be right for them. And would a pair even be enough to gain me any notable headroom? I have a set of octal to 9pin c0onverters i made years ago but i don't recall what octal tube type was what the amp used i was using them with. I think it may have been el34's rather then 6v6. I might have to pull them out and see if they are wired correctly for 6v6 to el84. I guess the bias will need a new set resistor and not sure how the OT match up would be with a 6.6k tranny with a 4 ohm speaker.Comment
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I'm kind of intrigued by the fact that your approach to this seems to be either 1.) there is some basically zero-cost and not too difficult change you can make to get more headroom or 2.) you will put the amp away and let it collect dust. If you have that little regard for the amp why don't you just sell or trade it and get something that better suits your needs? Lots of guys out there looking for amps that break up at low volume.
Also, a schematic would probably help folks be able to make informed suggestions.
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