voicing for push-pull amp without tone controls - diyAudio
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Old 9th February 2013, 11:25 PM   #1
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Default voicing for push-pull amp without tone controls

Hi all,
I'm working on designing a push-pull amp (12AX7's, 6v6's and/or 6L6's) with no tone stack. I see I have various low pass filters in the chain already: V1 cathode bias bypass caps: and coupling caps.

Seeing as how most amps are quite bright without the tone knob rolling off the highs, I'd like to ask what the range of practical values is for these caps.

I looked up the original Bassman 5C1 with tone control, and it's still using the usual .02 coupling caps- single ended though, a different story.

Thanks for any ideas.
Ted
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Old 9th February 2013, 11:33 PM   #2
tinitus is offline tinitus  Europe
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precense control ?
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Old 9th February 2013, 11:49 PM   #3
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The cap is just part of the tone shaping. The resistors are the other half.

Fred's Vacuum Tube Amplifier Blog: Basic Frequency Response Shaping
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Old 10th February 2013, 10:34 PM   #4
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If it is push pull then you can add a cross line tone control after the phase splitter. See VOX AC30 for example.
Cheers,
Ian
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Old 10th February 2013, 10:45 PM   #5
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by tapehead ted View Post
Seeing as how most amps are quite bright without the tone knob rolling off the highs......
if its just a tweeter cut/roll off, wouldn't it be possible to integrate with a volume control ?
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Old 12th February 2013, 01:52 AM   #6
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No negative feedback, no tweeter, just one or two 12" speakers.
Thanks for the link to Fred's- I'm wary of the caps roll things off- you see I have this phase allergy- but my experiments with a step attenuator have me believing that resistors can give me a nice dark solid sound. Right now I have this Alpha (CC) 1 meg pot that's awfully bright and kinda weak and hazy, but breaks up really nice and smooth and early. I'm going to try a PEC 500k pot and see if that doesn't provide the best of both worlds- the step attenuator is nice and dark and easy on the ears at 111k, 220k, but doesn't break up too gracefully, and really nice at 320k and 420k.

Thanks again.
Ted
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Old 12th February 2013, 08:05 AM   #7
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by tapehead ted View Post
No negative feedback, no tweeter, just one or two 12" speakers.
was not thinking of a tweeter.... should probably have said rolling off highs

I can read the thread title, but I still don't know exactly what you are asking
but maybe that's just me
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Old 12th February 2013, 07:55 PM   #8
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I'm just looking for other ways to voice the amp than tone controls per se. I'd like to learn how to use the existing caps, resistors, and other existing components of the amp to achieve a consistent comfortable tonal balance, rather than what strikes me as additional corrective circuitry, albeit adjustable circuitry. That's how, from my peculiar perspective, the tone controls strike me. I'm always puzzling over the tone controls, wondering "so what setting is "flat"?" and bypassing them if possible.

I'm struck by the fact that when I switch from my step attenuator (cumulative resistor values) to my current volume pot, all the sudden I need to un-bypass the EQ and roll off the top, and it still sounds thin. I suspect that one reason most amps are so bright that they need the highs rolled off is simply because of all the pots. Certainly without any pots in line, my amp here has a much more comfortable tonal character.

Hope that clarifies my intentions here. I haven't yet got into the link to Fred's, that promises to teach me a few things.

On a similar note, I was interested to see in bypassing my guitars tone and volume controls that the tone didn't really seem to need an adjustment.
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Old 12th February 2013, 09:39 PM   #9
tinitus is offline tinitus  Europe
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cathode bypass cap makes a difference
bigger paralel lytic goes lower, and is more 'fullrange'
smaller cap 'supports' less of lows
a resistor in series I think attenuates the effect some
and a LED can do something there too
but I really dont know this stuff
but I think the term 'voicing' is rubbish
but maybe thats just me
changing tube brand etc might be along what you call 'voicing'

I haven't read valvewizard's new book about guitar amps, but I'm sure its worth every penny

btw, what you say about your attenuator pots sounds more like there could be something wrong somehwere
or you connect therm wrong, they are wrong value, or some other weird stuff going on
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Old 13th February 2013, 08:19 PM   #10
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Well from what I am reading a 1 mig pot is brighter, so next I'm trying 500k. Nothing wrong with the way the pot's wired.

I'm getting this equation: corner frequency =1/(2pi*R*C). However I notice that the plate resistance of a 12AX7 is around 50k from GE specs (EH is 54.1k, JJ 62.5k), so I'm thinking I ought to be adding that to the resistor value, since they're in series.
According to this formula, without taking the plate resistance into account, a .022 uf coupling cap and a resistor value of 100k form a filter with a corner frequency of 80 hz. I'm thinking I would be using a 56k resistor and a .022 uf coupling cap to get that same corner frequency, taking that 50k plate resistance into account.

Is that right?

The next thing I'm researching is what kind of impedance the next 12ax7 wants to see from the preceding circuit, or for that matter the eventual 6v6 or 6L6 output tube wants to see.

I also have the idea that a larger resistor following the coupling cap will knock the signal voltage down further than a smaller resistor. Is that how that works?

Thanks again,
ted
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