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 Post subject: Question On 1962 Binson Echorec's Tube Circuit--FIXED
PostPosted: Nov Fri 16, 2018 11:15 pm 
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Joined: Aug Sun 23, 2015 6:01 pm
Posts: 950
Location: South Jersey East of Philly
A musician friend of mine walked into the shop the other day with a 1962 Binson Echorec tube delay unit, for guitars, mikes, etc. This was my first 'rodeo' with one of these. If you are familiar with Dark Side of the Moon and other Pink Floyd song...you have heard one of these delay units on the guitars. It uses a steel rotating disc and multiple head to create the echo...no tape involved.
The unit has the delay, but it is very subdued, and there should be a lot more gain on the 'wet' (delayed) signal. Anyway, I am getting a voltage reading way off on the cathode of the #4B tube (ECC83/12AX7). It is a cathode follower circuit according to the write-ups on this unit. I get 38V on the cathode pin #8.....the schematic shows 150V. There is 175V coming into the plate, pin #6..and I am getting 13V on the grid, pin #7....with no signal applied. I'm not sure about how a cathode follower circuit should look, voltage wise, in this application.
If anyone can tell what cathode voltage would be more correct...150V or 35V..that would help me get past this stage or look closer for a problem. New tubes are in the unit. Any help appreciated...
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Last edited by Tbirdkid on Nov Sun 18, 2018 2:16 am, edited 1 time in total.

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 Post subject: Re: Question On 1962 Binson Echorec's Tube Circuit
PostPosted: Nov Sat 17, 2018 1:30 am 
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Joined: Jul Mon 08, 2013 2:48 pm
Posts: 349
The 150V shown on the schematic is a typo and should probably read 50V (not critical) , if it was 150V with 175V on the plate (160V according to the schematic) that would only leave 10 volts across the tube, which is too low.

I checked the circuit and according to your measurements and the 12AX7 plate characteristics it works as it should.
Your grid voltage (pin #7) measurement is in error because your meter loaded it down after the 1M grid resistor. Grid voltage should be measured at the junction of the 1K6 and 47K cathode resistors, AHEAD of the 1M grid resistor.

The operating point of the tube in the circuit as shown (and measured) is:

Vp (plate voltage): 137V (175-38)
Ip: (plate current) 0.78 mA (38/ 47000+1600)
Ug: -1.34V (38-(47000 X 0.00078))

Which is correct for a 12AX7/ECC83.

It doesn't seem that your problem is in this part of the circuit. Also, you must remember that a cathode follower circuit has no voltage gain, which is allways inferior to unity. In this circuit it is used only as an impedance converter for driving lower impedance loads and/or long cables.

Many old Binson Echorec's are now suffering from lowish and muffled delayed (wet) signal level and the problem is usually not due to defective electronic circuits but magnetized/mis-adjusted/ worn heads and/or wear of the magnetic rotating drum. Alignment is critical and getting the original specs back nearly impossible without replacing all the worn-out parts.

Maybe you should check this:

https://www.effectrode.com/knowledge-ba ... djustment/


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 Post subject: Re: Question On 1962 Binson Echorec's Tube Circuit
PostPosted: Nov Sat 17, 2018 2:15 am 
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Joined: Aug Sun 23, 2015 6:01 pm
Posts: 950
Location: South Jersey East of Philly
Thanks for the help and tips Tubologic! At least I can eliminate that section thanks to the 'typo' you caught. I now have a fairly decent 'wet' signal to work with, although not near what it should be...it is distorted. I will have to investigate that. Just not enough gain in the 'wet' signal...distorted or not.


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 Post subject: Re: Question On 1962 Binson Echorec's Tube Circuit
PostPosted: Nov Sun 18, 2018 2:15 am 
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Joined: Aug Sun 23, 2015 6:01 pm
Posts: 950
Location: South Jersey East of Philly
:D Found the main problem...The wet signal bare buss wire lead from the .022 cap on blocking cap from the plate of #3 12AX7 was being shorted out against the tone pot shell.....a case of trying to make too short a wire work instead of allowing some slack. They pulled the soldering tab on the tone control too close to the pot's case, shorting out the wet signal.
Just goes to show that it's not always the components fault...think simple when you can. That was a tough one to spot...only because I was checking for the wet signal along it's path, that I eventually got to spot where I lost the signal. Also, The tone pot is bad at one end and we need a new playback head...plus tuning up the heads so they balance pretty good is left to do. 8)


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