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Simple 5687 pre from diy paradise
Simple 5687 pre from diy paradise
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Old 21st September 2018, 05:42 AM   #11
PRR is offline PRR  United States
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> Volume potentiometer not connected

I was about to say: What is the grid voltage?

The noted numbers do not plot as 5687 (assuming grid is ground-referenced).

If the grid is floating, well, almost anything can happen. Usual case IS the grid floats a few volts positive of circuit ground, until plate voltage falls enough to quiet the electron storm.
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Old 21st September 2018, 05:57 AM   #12
vt4c is offline vt4c
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Quote:
Originally Posted by kp93300 View Post
Volume potentiometer not connected
KP,
Please try to connect a volume pot and re-check the measurement again. A floating grid might be a concern.
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Old 21st September 2018, 08:11 AM   #13
kp93300 is offline kp93300  Malaysia
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Quote:
Originally Posted by vt4c View Post
KP,
Please try to connect a volume pot and re-check the measurement again. A floating grid might be a concern.
VT4C and PRR ,
thank you for the good advice.
This is another diyer"s toy and I am hoping to learn something also.

regards

Last edited by kp93300; 21st September 2018 at 08:13 AM. Reason: amend contents
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Old 21st September 2018, 08:47 AM   #14
Bangla H is offline Bangla H  Germany
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kp.
here is my little testboard, with options, for my test.
R3/4 - grid stopper not used
R5/6 - in place + C3/4- other/better experts knowing better if it makes sense
R1/2 - 15k/2W
R7/10 - current 530 Ohm 430+100- in series for replacing other values
cathode bypass cap not used, but tested with/without

R8/9 - not used here, also not used output cap



I have tested this to drive M. Rothacher MoFo- unity gain Amp.
Around 300V on plate, behind 15k i not match the middle point ~ 120V.


But in difference on cathode stays ~ 4V in that resistor values.


Don't know if it helps for your needs.
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Old 21st September 2018, 10:41 AM   #15
Burnedfingers is offline Burnedfingers  United States
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Quote:
Originally Posted by kp93300 View Post
Thanks. Will implement these changes and report back. Will the tube work with a 80v dc at the anode? This pre will drive a Dynaco Vt 70. Source is cd player?
The CD player should have enough level (most do) to drive the Dynaco without a line stage. You can mount several volume controls in a box and make up the short cable needed to go from the RCA outs on the CD player to feed the volume control pot and then output from the volume control pot to the Dynaco input RCA jacks.
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Old 22nd September 2018, 12:34 AM   #16
kp93300 is offline kp93300  Malaysia
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The problem is the lack of reference for the grid. I tied a 20K resistor from grid to ground and the anode Voltage is 138 V dc.

This follows the schematic provided at diyparadise.
i am happy .
will test with speakers soon.
thanks to all
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Old 22nd September 2018, 12:42 AM   #17
kp93300 is offline kp93300  Malaysia
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I intend to get the anode voltage at 1/2 of B +, ie 125V.
I will increase the anode resistor from 10K to 11.5K . Is this correct??
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Old 22nd September 2018, 12:44 AM   #18
kp93300 is offline kp93300  Malaysia
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Quote:
Originally Posted by Bangla H View Post
kp.
here is my little testboard, with options, for my test.
R3/4 - grid stopper not used
R5/6 - in place + C3/4- other/better experts knowing better if it makes sense
R1/2 - 15k/2W
R7/10 - current 530 Ohm 430+100- in series for replacing other values
cathode bypass cap not used, but tested with/without

R8/9 - not used here, also not used output cap



I have tested this to drive M. Rothacher MoFo- unity gain Amp.
Around 300V on plate, behind 15k i not match the middle point ~ 120V.


But in difference on cathode stays ~ 4V in that resistor values.


Don't know if it helps for your needs.
thank you for your suggestion.Very informative for tube newbie.
MOFO is next in line for build and does this pre drive it well ?
Do you like the sound ?
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Old 22nd September 2018, 12:45 AM   #19
TonyTecson is offline TonyTecson  Philippines
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Simple 5687 pre from diy paradise
play with the cathode resistor instead, you can use a variable ppot to do that, then once you get to 1/2 b+, replace that with a fixed resistor to the nearest standard value...

going from 10k to 11.5k does nothing for you, but the cathode resistor gets you where you want your plate to be...

my quick and dirty calculation, with a b+ of 250 volts and 10k or 11.5k load resistor, a 500 ohm cathode resistor should give you a 120 volt plate...
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Old 22nd September 2018, 09:12 AM   #20
Bangla H is offline Bangla H  Germany
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Arrow 5687 for MoFo go

Quote:
MOFO is next in line for build and does this pre drive it well ?
Do you like the sound ?
For my, 2cent, opinion MoFo likes more gain producing tube, but the 5687 has solid (non
affectations) tonality with dark backround.
There are more factors to influence the sound- a way to try, the much better, experienced experts knowing better details.
The cousin 6N6P-(I/EV) in mu follower circuit also sounds good to me.
My strongest is pentode 6P9- this is a little power animal there.


Don't take a critical look to long twisted wire- it is for testing.
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