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Old 13th November 2006, 11:19 AM   #1
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Default Help with guitar amp sound

I've built a twin channel guitar amp for my son and need help with the voicing of the channels. It has two nearly identical channels, only the power amp section is common to both channels with each channel having its own pre-amp, eq, phasesplitter and master volume. Amp is very quiet, no hum or instability. Its built on an old organ amp chassis using the output transformer that came with the organ. I'm using russian 6L6's biased at about 56Ma at the moment and once it sounds ok I'll use the kt66's that came with the organ. Preamp valves (12ax7) also from the organ are mullard preamp and toshiba phase splitter. Circuit shown is the for the channel that I'm working on. The complaint from my son is that it sounds flabby--- not tight. I'm unsure what causes a flabby sound (if there is such a thing) and need some guidance on which components to change or maybe I need to add NFB. I briefly tried the kt66's and my son said it sounded different but still flabby. circuit and components are faily standard stuff so it should sound ok. Its played through a no name 4x12 cab loaded with no name speakers. Any help is appreciated.
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Old 13th November 2006, 02:34 PM   #2
lineup is offline lineup  Sweden
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Hello friend.

I would use some very good speakers
at listening testing
before I would be able to say for sure
that the amplifier is causing the major sound.

If those speakers are good, for example sings well with another good amplifier,
than I would start see
if any change in this amplifier is necessary.


And why trust in another persons ears?
I suppose you have at least one ear intact


Regards
lineup - not knowing much about tube amps
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Old 13th November 2006, 03:15 PM   #3
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Well, the big value bypass caps on the pre tubes could be a big contributor to being flubby... Try some 1uF caps in place of the 22uF and 47uF that you have.

And most improtantly add a feed back loop into the tail of the phase inverter, (add a presence control while your at it.) This will help more than anything. (you might be able to increase the value of the bypass caps back to what you had...)

I'd also suggest an additonal RC or LC filter stage for the screens on the power tubes to drop the voltage on the screens just a tick.

One other thing that I have found to be useful on farty amps is to use a low value filter cap on the preamp, (8-10uF) or so.
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Old 13th November 2006, 03:31 PM   #4
5u4 is offline 5u4  United States
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Maybe change the no name speakers like lineup suggests to known good speakers, Celestion or EV & like Renegade says, there's no feedback.

For a clean sound, you usually want to bring up the bass just before the point where it starts sounding flabby, drop the mids & adjust highs to suit. This setting usually doesn't work well with distortion though. For distortion (which I assume what the flabby complaint is about) is to drop the bass to get rid of the flabbyness, bring up the mids & adjust the highs to suit.

If you still need more bass, have the sound man add it through the PA. If your playing bars with no sound man, putting the speaker on the floor will help the bottom end.

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Old 13th November 2006, 11:47 PM   #5
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I'm going to agree with the others that the speakers can have a huge impact. Also, some feed back is called for. Look at tweed and blackface Fenders for an idea how to implement it. Don't overlook at the crazy way they had of applying feedback to the bottom of the phase splitter. It looks odd, but it works well.

I'm not sure I see why you need two phase splitters, but I might be missing something. Anyway, I would get one channel working, then worry about adding the other.

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Old 14th November 2006, 12:17 AM   #6
Tweeker is offline Tweeker  United States
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Adding to chorus of no feedback and beam tubes may mean flabby bass.

Big bypass caps should help bass, until they hurt it once the stage is overdriven leading to blocking distortion. And stages do get overdriven a fair amount in guitar amps. Smaller bypass caps recover faster. If blocking gets out of hand, the amp farts out.
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Old 14th November 2006, 10:46 AM   #7
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Thanks for the replies.
The main problem is with the distortion channel so I'll start by trying different values of preamp bypass caps first, this will be easiest thing to do. Adding nfb is not going to be easy as I've got 2 phase splitters so I'll need to add an extra channel switching relay to switch the nfb feed from the output tranny or alternatively have nfb for the distortion channel only. Circuit is taken from an orange ad140 hence the 2 phase splitters. He's still in high school and his after school job does'nt provide the funds for better speakers so they will have to wait for now.
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Old 14th November 2006, 06:22 PM   #8
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Well... one reason that the actual Orange amp that you copied is able to remain tightness of the low end is due to having a crazy high B+ voltage of 545vDC at the plates of the power tubes... You only have 400... pretty big difference... you've got to do a bit more to keep that power amp tight. 400v is pretty tame for a 6L6 / KT66 power amp.

And I really have no idea what possesed Orange (Trace Elliot) to use two phase inverters other than that was the easy way to have two Post Phase inverter Master volumes... Which IMHO is not a fantasitc way to do things... For the same reasons that I find Post PI MV Matchless amps to be big fart boxes... OK for blues but not crunchy tight rock... Which is where the classic Orange OD amps were known for.

Was there a particular reason that you used that topology?
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Old 14th November 2006, 07:05 PM   #9
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I agree with the other posters, why do you want to use two different phase splitters?.

Also, having a stereo volume control after the phase splitter is really bizarre, and bound to really affect the sound (probably in a bad way!) - as the two sides of the pot won't track very well. It also really stops you adding feedback around the output/phase splitter as well.
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Old 14th November 2006, 08:46 PM   #10
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Quote:
Originally posted by Nigel Goodwin
I agree with the other posters, why do you want to use two different phase splitters?.

Also, having a stereo volume control after the phase splitter is really bizarre, and bound to really affect the sound (probably in a bad way!) - as the two sides of the pot won't track very well. It also really stops you adding feedback around the output/phase splitter as well.

The Stereo pot deal after the Phase inverter pretty much lets the preamp pound the phase inverter into overdriven state and produces more distortion with fewer preamp stages. It is suprisingly effective as a master volume control. (Definitely conterintuitive to what the HiFi guys here are after) But I've yet to hear a completely wonderful sounding amp built this way. That particular Orange that was copied here is definitely not "The Orange" that Orange is known for... Just looking at the schematic and having built a version of about every major guitar amp topology out there. I can tell it's most likely to sound like *** espcially at those low B+ levels.

Now... If it were my amp. I'd go to a single phase inverter... Lose the Post PI Master volume completely. Use the extra tube from the abandoned phase inverter... as an additional preamp stage and Cathode Follower for the Dirty channed (A-la 2204 Marshall with some common circuit mods...) Use a pre-PI master for that channel as well... No master on the other channel, add a -feedback loop, change my screen resistors to 470 or 1k. Drop the Screen Voltage by at least 20-30v less than the Plates. And of course work out the Switching to accomodate the altered preamp, (Looks like there would be enough relays to work something out.)
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