Schematic: Please check for mistakes

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skyboltone
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Re: Schematic: Please check for mistakes

Post by skyboltone »

drhulsey wrote:Dave, how could anybody complain :?: At least, you DID it with the best info you had at the time :? I really needed an excuse to use my new ConceptDraw program and thought adding some labels might be helpful to some- just a little sharing :wink: Thanks for all your sharing :!:
Your energy inspires and depresses me Tim. I've got Visio sitting new in the box over there with my stack of stuff for 6 months and I really need to put it in the machine. Ya know? But the thought of learning a new program just kills me. I hate it. I hate it. I hate it.

Dan
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drhulsey
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Re: Schematic: Please check for mistakes

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skyboltone wrote: Your energy inspires and depresses me Tim.
Dan, if you could see my mesh deck chair, leaned back at 45º, my studio flat panel monitor at the edge of the desk, a Contour Shuttle with programmed buttons! About the most energy I expend is drawing a LONG line :D As far as programs go, I just start using it and only go to the manual if I NEED to. Most Mac programs have a straight forward interface. Really, I'm L-A-Z-Y :oops: Re-drawing a schematic or layout just helps my atrophic brain understand the circuit better. It's nice to be able to produce a little helpful info, since I have gained SO MUCH from this forum :!: I'm not knowledgeable enough to give electronic advice, so, I'll do some leg work :wink:
Tim

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jem
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Re: Schematic: Please check for mistakes

Post by jem »

Nice work!
drhulsey wrote: Any suggestions about making it better and correcting any mistakes will make it better for all of us.
A couple of suggestions:

Jacks need designators (J1, J2, J3).
Impedance switch added and designated.
MOV added.
Are the 100 ohm for the heater center taps there?

I'd prefer to see the actual 2 2W 9.1K in series rather than a single 4W 18.2K (if that is what really is used)
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drhulsey
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Re: Schematic: Please check for mistakes

Post by drhulsey »

jem wrote: A couple of suggestions:
Jem, thanks for the suggestions. Appropriate changes have been made. The schem shows a center tap for the heater supply, therefore "virtual" center tap not shown. Either is acceptable as far as I know.
Tim

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gcenker
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Re: Schematic: Please check for mistakes

Post by gcenker »

I thought the bias was to be set at -30VDC?
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drhulsey
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Re: Schematic: Please check for mistakes

Post by drhulsey »

gcenker wrote:I thought the bias was to be set at -30VDC?
Most amps can work within a range of bias. Ken used -30mA and said "set it and forget it." Allynmey found bias at -49mA in two of Ken's amps. Some builders have found a better sound with a bias other than -30mA. The bias is tweakable :D
Tim

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gcenker
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Re: Schematic: Please check for mistakes

Post by gcenker »

drhulsey wrote:
gcenker wrote:I thought the bias was to be set at -30VDC?
Most amps can work within a range of bias. Ken used -30mA and said "set it and forget it." Allynmey found bias at -49mA in two of Ken's amps. Some builders have found a better sound with a bias other than -30mA. The bias is tweakable :D
I was just curious because I installed a 1ohm resistor across the cathodes (ala Komet style) and created dual test points.

First I measured across pin 5 of the EL34(rated for 25W) and dialed it in at -30VDC. Then I measured the plate voltage which was 382VDC. Next I measured across the bias test points and it read 49.3mV (which correlates to 49.3mA). By using the formula (mA/watts) x plate = dissipation factor, then (0.0493/25W) x 382 = 75.33%

So by dialing in -33VDC makes me think the tubes may be biased too hot.

Your thoughts?

-Greg
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Re: Schematic: Please check for mistakes

Post by paulster »

gcenker wrote:First I measured across pin 5 of the EL34(rated for 25W) and dialed it in at -30VDC. Then I measured the plate voltage which was 382VDC. Next I measured across the bias test points and it read 49.3mV (which correlates to 49.3mA). By using the formula (mA/watts) x plate = dissipation factor, then (0.0493/25W) x 382 = 75.33%

So by dialing in -33VDC makes me think the tubes may be biased too hot.
You haven't factored in that the screen current is being measured across your 1R cathode resistors as well. If you subtract the screen current (approx 5.5mA in mine) then you'll be down at 43.8mA which is 67% dissipation at your plate voltage.

They can be biased pretty hot as the plate voltage is fairly low compared to similar 2x EL34 amps.

I've never bothered to measure the negative bias voltage but I've ended up settling on 49mA across the cathode resistors, which is coincidentally what Allyn has found from some that Ken had serviced.
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gcenker
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Re: Schematic: Please check for mistakes

Post by gcenker »

Doh! I forgot to factor out roughly 10% for screen current.

I do find I like the sound of about 49mA though, seems really thick & harnmonic rich. Harmonic feedback is instantaneous.

Thanks for the information.

-Greg
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drhulsey
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Re: Schematic: Please check for mistakes

Post by drhulsey »

I corrected the value on the presence cap (C11) to 0.1µF on pdf_107
Thanks, Dave :D
Tim

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Ron Worley
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Re: Schematic: Please check for mistakes

Post by Ron Worley »

Hey Tim-

I think that I read here (UR12 perhaps??) that the third leg of the bias pot should be grounded per the schematic- in that case, shouldn't the layout drawing indicate this? I wired a jumper under the PS board on my build (which I have not powered up yet, waiting on tubes...)

The experts here will know if this change is correct...

Thanks for doing the layout, it was indispensable during my build!

Ron
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drhulsey
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Re: Schematic: Please check for mistakes

Post by drhulsey »

Ron Worley wrote:... shouldn't the layout drawing indicate this?
Change made :!: Thanks for catching it :D
Tim

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