Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Toxophilite on March 23, 2015, 08:06:23 pm
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Hello
I've been digging in my past posts to find this answer because i'm sure it was there
I've just had no luck so thus this post
I have a one channel AB763
If I disconnect the 270K resister to ground just prior to the phase inverter(this simulates the load of the missing other channle i believe)
Will I get it a bit more gain?
And is this a bad thing if I prefer cleans or will it give my amp more clean headroom
Excuse my ignorance at what is probably an obvious question, I've never been a one to get my amps to distort so the idea of gain has always been confusing
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Just lift one end and tell us what you think.
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Yup, did that about an hour ago :icon_biggrin:
More gain /volume, a significant amount more
Yet I can't help but think that putting more gain in front of the power amp just means more clipping/distortion? likely earlier
No more clean headroom
Hard to tell in my apartment. I should've miked it up and looked at it on the computer
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Yup, did that about an hour ago :icon_biggrin:
More gain /volume, a significant amount more
Yet I can't help but think that putting more gain in front of the power amp just means more clipping/distortion? likely earlier
No more clean headroom
Hard to tell in my apartment. I should've miked it up and looked at it on the computer
hint: make it switchable... instant boost switch. add that to a rotary switch and lift to the tone-stack for even more gain.
--pete
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hmm
That's an idea
To me the only useful switch, is a footswitch. rarely is there a space in a song for me to reach and adjust my amp!
Would this hooked up to a 1/4 out with a footswitch attached be a big noise addition??
Of course it would be a shielded cable to the foot switch
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on the tone stack, probably.
relay switching may prove useful here...
--pete
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Lifting that resistor will not give you more clean headroom. It will boost the signal and add a little 'hair' to the sound. If you are wanting more clean you probably won't like the results. However, you should be able to compensate simply by turning the volume down.
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THat's what I was thinking, It's like I'm adding a couple of digits to my volume control rather than increasing the headroom
I was curious and it's fun to experiment a little
Thanks
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For the interest of those building with AO-43 chassis
I tried my AB763 deluxe lite/AMP 1 of my stereo build (ala S luckey, thanks again Steve!) with 6L6s tonight
(actually sovtek 5881/6L6WGC cause I had them kicking around)
I biased the amp to about 33 ma with -44 volts on the bias circuit kind of like a vibrolux
though there's about 425 VDC on the 6L6 plates because of the SS rectumfire
I played for 3 hours with my very loud drummer and bass player (he was using and SVT through an 8 x 10 cab!) at the rehearsal studio playing loud rock/pop where I switch between clean and distorted(pedals) a lot
I had just enough clean headroom when I wanted to finger pick or play clean stuff and the amp seemed to handle it quite nicely
I was running the amp at about 5 on the volume treble at about 3-4 mid about 2-3 and bass around 3-4
The output transformer was quiet warm at the end but that could have been as much from it's proximity to the 2 output tubes which were nice and hot, The output transformer was a little bit warm but not as, it's farther away from the output tubes and a substantially larger piece of iron. (
I might make a little aluminium shield between the Power tubes and the OPT and check it again next rehearsal)
I also have a slightly larger one I might mount and a spare one the same as the AO-43
I checked it throughout this eves rehearsal, there where no weird smells, or signs of tunes going south, or magic smoke
I also switched the speaker so it now mounts a 40-50 watt Utah 12" that looks exactly like a C12N (in fact I thought it was originally and I've had a lot of Jensens go through my hands) on a proper nice stiff 1/2" baltic birch baffle
It has a nice sound with the one 12 and the Utah seems to be a pretty efficient speaker which helps for clean headroom
So I'm very pleased with that
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As far as my previous post goes
Do you think I'm playing with fire??
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Hard to say. At least you got a backup! :wink:
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True!
:icon_biggrin:
I'm going to build a little aluminum shield between the OPT and the power tubes (which I think is a good idea regardless as i did put them pretty close together), and then check how hot the OPT is a couple times during my next rehearsal. I figure if I screen it from the power tubes heat it'll be happier and also give me a better idea of how warm it's getting by itself
That seems like a good indicator
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Here's my amp with the 6L6s in it and a blast shield installed
It's made out of 1mm aluminium and should help keep the heat away from the OPT and the choke some giving me a better idea of the OPT heat after a 3hr rehearsal.or at least that's the plan
I'll probably install one on AMP 2 (2 channel deluxe lite) cause the 6V6s get really hot too
So from reading here
http://www.radioremembered.org/outimp.htm (http://www.radioremembered.org/outimp.htm)
and here
http://www.regiscoyne.com/tech/unknownOT/ (http://www.regiscoyne.com/tech/unknownOT/)
It says i can use the turns winding of the OPT and the ELR of the output tubes in pushpull to determine how it will work with different speaker impedances
6V6s are around 8000 ohms ELR
while 6L6GCs are 5600 ohms ELR
The hammond OPT on the AO-43 is 30/1 turns ratio (determined by running 10 vdc AC into the secondary and getting 300vdc on the primaries)
Squaring the turns ration gives me the impedance ratio
30 x 30 + 900
soo
6V6's
8000/900 = 8.88 ohms speaker as confirmed by the A0-43 schematic
6L6GC's
5600/900 = 6.2 ohms speaker
Pretty well halfway between 8 and 4 ohms so maybe not a bad fit for using an 8 ohm speaker 6L6Gcs if you want it to run a bit cool
straight up 6L6s have an ELR of about 3800
so
3800/ 900 = 4.2 ohm speaker
This is not to say the 6L6GCs won't turn the OT into a slag heap given enough time
Just that the impedance match is half decent for a 8 or 4 ohm speaker
Is that right?
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Aiiie
I wander blindly in the darkness of ignorance!! :dontknow:
I was wondering if someone could confirm my reckonings on my previous post
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I was wondering if someone could confirm my reckonings on my previous post
I reckon so.
That's a pretty beefy OT for EL84s or 6V6. You're probably safe to run 6L6s into a 4Ω cab. Meanwhile, watch eBay for another $40 AO-43 for spare parts. :wink:
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Haha! :laugh:
Happily I have a spare (exactly the same ) OT from a different hammond chassis
And a couple of there pushpull large EL84 chassis
I do like those AO-43 chassis
They're a good shape for a guitar amp build!
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Well maybe I don't have to worry about the 6L6s getting the OT too hot
I popped some 6V6s in the main amp and ran them both for an hour or two (not so loud though, apartment levels)
and they got easily as hot if not a little hotter than they did using 6L6s
It could be because they're leaning back now and retaining more heat
but whew! . The PTs got warmer too