PT question

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tubeswell
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Location: Wellington. NZ

Re: PT question

Post by tubeswell »

The diagram at the bottom of the page from this linked Merlin Blencowe article may help to understand what to expect in the way of voltage-drop across each supply resistor in the power rail of a typical tube amp.

Each supply node (at each filter cap) caters for the current drawn by the sum of all the stages that derive their DC supply beyond each node in the supply rail.

In Merlin's picture, we see the pre-amp node has 2mA dropped across 4k7 resistance, which is 4700R x 0.002A = 9.4V. The next resistor back in the chain is seeing 4mA dropped across 4k7 = 18.8V, etc:

http://www.valvewizard.co.uk/smoothing.html

But without any tubes installed, there will be no DC drawn through the supply resistors, and all the voltages at each node in supply rail will be the same.
He who dies with the most tubes... wins
MikeR670
Posts: 85
Joined: Sun Jul 10, 2016 7:56 pm
Location: B.C. Canada

Re: PT question

Post by MikeR670 »

Yes, very fair points which I accept:)

I play in a band which has a very diverse setlist, and I have to perform both rhythm and lead guitar duties, as well as lead vocals. I have tried to group different song types into separate sets so I can alter my rig during breaks, but you can't play too many similar things in a set or it becomes dull for listeners.

First set is easy as it is mostly Eagles and Blue Rodeo type stuff. A nice clean jangly sound works most of the time there ( I play a 2013 Japanese '54 reissue strat, all stock and the best sounding strat I've ever owned).

Second set gets more tricky (for me too manoeuvre between sound types), as it often mixes Radiohead, Tom Petty and Chilli Peppers, etc. I use a 1970s original Japanese made Boss SD 1 overdrive, and have a zoom g3 pedal for loops and things like delay, compression, eq, etc. I don't use any drives or distortion from the zoom as it's digital and to me they sound ugly.

Third set often contains Floyd, Skynyrd, etc depending on the scene.

ATM I'm using my Fender SChamp with the speaker bypassed into a 2x12 cab. It's a bit loud for practice, partly why I had wanted to build something with lower output.

But after beginning this journey, I sort of fell in love with the idea of building an amp that really can produce huge wide searing and sustained but sweet drive - while cleaning up enough for rock rhythm with guitar volume control. I'm no Frusciente, but I use guitar volume a lot, things like the violin type swells in rhcp "don't forget me" live at cigale intro.

I've bought into the premise that an amp built close to instability can be much more touch sensitive and while I'm sure it would be a steep learning curve playing it, I think the tonal rewards might be phenomenal.

Hence my trainwreck attempt.....I really want to use the stuff I have and I'm ok with a long experimental ride. I know it's all been done, but I also know that the sounds we all chase are very personal and an amp will not suit or be used the same way by two different players.....
MikeR670
Posts: 85
Joined: Sun Jul 10, 2016 7:56 pm
Location: B.C. Canada

Re: PT question

Post by MikeR670 »

tubeswell wrote: Sun Apr 23, 2017 3:58 pm The diagram at the bottom of the page from this linked Merlin Blencowe article may help to understand what to expect in the way of voltage-drop across each supply resistor in the power rail of a typical tube amp.

Each supply node (at each filter cap) caters for the current drawn by the sum of all the stages that derive their DC supply beyond each node in the supply rail.

In Merlin's picture, we see the pre-amp node has 2mA dropped across 4k7 resistance, which is 4700R x 0.002A = 9.4V. The next resistor back in the chain is seeing 4mA dropped across 4k7 = 18.8V, etc:

http://www.valvewizard.co.uk/smoothing.html

But without any tubes installed, there will be no DC drawn through the supply resistors, and all the voltages at each node in supply rail will be the same.
Thanks for that info, I'll study it before re planning my initial filter and smoothing circuit :)
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Phil_S
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Joined: Tue Oct 23, 2007 10:12 pm
Location: Baltimore, MD

Re: PT question

Post by Phil_S »

You want this for practice or gigging? Either way, I've got a wild hair idea for you that will take good planning and lot's of patience and maybe a few more dollars than you intend to spend.

From the set list you describe, I'm thinking you need three different preamps. This is where most of the tone shaping happens. You can build a three channel preamp into the power amp of your choice. You can probably live with preamp selection on the amp panel, but that sends you back to the amp, which may be inconvenient and look odd to the audience that you have to dither with your amp so much. You might control the preamp with a footswitch, but this will take extra planning and you'll need to ramp up on the in's and out's of switching. This wild hair idea is going to need a BIG chassis, there will lots of knobs on the front panel, and it will use several 12AX7's. The power transformer will need extra juice to support all those tubes. Really, I don't think this sort of project is for a novice. There's a great deal that can go wrong with something with that much stuff in it.

I am sorry if I am bursting your bubble here. Have you thought about a modeler for the preamp and feeding the signal to a tube power amp? That would be a whole lot simpler. I recognize it isn't all tube and some may see that as a compromise. I'm not sure how many people in the audience actually know the difference.
MikeR670
Posts: 85
Joined: Sun Jul 10, 2016 7:56 pm
Location: B.C. Canada

Re: PT question

Post by MikeR670 »

Thanks Phil, an awesome idea that would certainly fit the range of my needs! :)

You're right that it's probably beyond my capability right now - but who knows as my learning develops...I can still remember back when I began learning the comfortably numb solos and thinking I'd never be able to play them - nowadays I'm pissed if I cock up one note during it lol.

Now that I've built the express preamp section but with output section power being so high, I guess I might as well complete it with 6550s in the power amp section and mess with it from there....trying different preamp tubes from my stock etc....there's still the option of dialing in some preamp drive and here's an interesting thing: when I jumpered input channels on the old pa the drive tones were amazing. Obviously it was not breaking up the 6550s, but it was that kind of jet (cold hard bitch) drive sound, and huge. I found I could still roll back guitar volume for clean ish, but it got very sterile quite quickly, and lost character.

Once again, I really appreciate the input and I'm learning a great deal as this rolls along :)
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