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ecc40 circuit adjusting posible fault
ecc40 circuit adjusting posible fault
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Old 7th April 2019, 04:33 AM   #1
Pauld35 is offline Pauld35  New Zealand
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Default ecc40 circuit adjusting posible fault

hi all,


I have been restoring a pp amp to run a guitar through gone through the ps and replaced the dogey bits, the input goes


----> ecc40 pre -----> passive tone switch then master volume------>

------>ef86 2nd pre ------> el41 pi and power tubes.


if the guitar run clean or boosted and bypass the ecc40 works very well strong loud output,


if the guitar is input to ecc40 i get thin but ok signal at low volume, when i try to go louder on the master the output volume actually gets quieter,




I'm not sure if the gain of the ecc40 is wrong for the guitar signal, or if it is another fault or if the wiring is wrong from the factory. i have drawn the ecc40 circuit and bypassed the tone stack/switch for now to eliminate that as a fault until i test that part of the circuit.


i could video a demo if needed or would help?


cheers
Attached Images
File Type: jpg IMG_5395[1].jpg (733.3 KB, 72 views)
Attached Files
File Type: pdf ECC40.pdf (885.3 KB, 11 views)

Last edited by Pauld35; 7th April 2019 at 04:43 AM. Reason: add attachments
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Old 7th April 2019, 05:29 AM   #2
PRR is online now PRR  United States
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The input needs a grid to ground resistor.

The output NEEDS a blocking cap, or the next box will be VERY distressed. Perhaps the removed tone network did that function?

I won't watch a video but I would like to see the thing this is based on.
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Old 7th April 2019, 06:15 AM   #3
Pauld35 is offline Pauld35  New Zealand
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Quote:
Originally Posted by PRR View Post
The input needs a grid to ground resistor.

The output NEEDS a blocking cap, or the next box will be VERY distressed. Perhaps the removed tone network did that function?

I won't watch a video but I would like to see the thing this is based on.
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Old 7th April 2019, 07:04 AM   #4
Pauld35 is offline Pauld35  New Zealand
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hi, yes it has series cap between volume and ef86, the tone network still needs resto work because of very old style dial selector switches have tarnished over time and gives intermittent faults and need to check leakage on caps in tone circuit had 28% v-loss, thought that might be quite high so took it out the loop for now to carry on testing.

cant find any guitar amp schematics online that use an ecc40 as a pre to compare. was going to put a 1meg to ground on input grid but thought problem is probably more than that. tube is fine,

the schematic is factory unless i put the 47nf's and 100nf back on the wrong tabs.
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Old 7th April 2019, 07:11 AM   #5
Pauld35 is offline Pauld35  New Zealand
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here is photos for you
Attached Images
File Type: jpg IMG_5396[2].jpg (869.2 KB, 71 views)
File Type: jpg IMG_5398[1].jpg (667.0 KB, 67 views)
File Type: jpg IMG_5397[1].jpg (832.7 KB, 62 views)
File Type: jpg IMG_5399[1].jpg (595.7 KB, 62 views)
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Old 7th April 2019, 09:38 AM   #6
Parafeed813 is offline Parafeed813  Netherlands
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The cathode follower triode isn't properly biased.
Either insert a cathode bias resistor between cathode and 100k resistor and move the ground part of the grid leak there, or connect the grid directly (without grid leak) to the anode of the input triode.
The first triode is biased very cold with that 10k cathode resistor. You probably won't have a lot of voltage drop over the first anode resistor.
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Old 8th April 2019, 04:40 AM   #7
Pauld35 is offline Pauld35  New Zealand
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Quote:
Originally Posted by Parafeed813 View Post
The cathode follower triode isn't properly biased.
Either insert a cathode bias resistor between cathode and 100k resistor and move the ground part of the grid leak there, or connect the grid directly (without grid leak) to the anode of the input triode.
The first triode is biased very cold with that 10k cathode resistor. You probably won't have a lot of voltage drop over the first anode resistor.

Hi i think you are right i think im getting cross over cancellation when i try to play loud. I'm cant quite visualize what adjustments you are describing,
if you have time to draw me a sketch that would be great, or if you very busy I found a schem for a philips p.a. if you think i would be better to re-wire the ecc40 the same as in the yellow circle if the out put would be at a similar level to instrument or line level?
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File Type: jpg Philips EL641-01 Schematics 1.jpg (632.0 KB, 26 views)
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Old 8th April 2019, 04:54 AM   #8
gingertube is offline gingertube  Australia
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The Cathode Follower gets bias OK.

Its grid voltage is direct coupled from the anode of the first triode via the 2 x 220K resistors.

The network which includes the 2 x 200K is called a "Twin T" Filter and it is a notch filter.
One "T" is formed by the 2 off 220K resistors and the 100nF to ground and the other "T" is formed by the 2 off 47nF capacitors with the 100K to ground.

10K seems way too high for the cathode bias resistor on the first triode. The data sheet suggests it should be 2K2. Thats sets the first triode idle current and hence the anode voltage . That anode voltage is what is seen at the grid of the cathode follower so it effectively sets the idle current for the cathode follower too.

Cheers,
Ian
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Old 8th April 2019, 08:32 PM   #9
Pauld35 is offline Pauld35  New Zealand
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hi, thanks I hadnt seen a Twin T Filter before (or at least not the way i traced sketched it) so thought it may have been wired wrong.

I'll try adjusting cathode bias.
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Old 8th April 2019, 08:49 PM   #10
Pauld35 is offline Pauld35  New Zealand
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hi, just done some research,
Notch Filter sharply attenuates frequency-specific noise like mains hum or electrical whistle with minimal damage to the remaining audio, by cutting a "notch" out of the frequency


might have been added there to remove unwanted noise from a tape or pu motor
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