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Old 10th Apr 2016, 7:39 pm   #1
FERNSEH
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Default Small PCB with two valveholders.

Recently found this printed circuit board. Absolutely no idea what equipment it was originally used in, perhaps an audio application. It could be a push-pull audio output stage?
Judging by the date codes on the capacitors it was made in the mid sixties.

DFWB.
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Old 10th Apr 2016, 9:44 pm   #2
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Default Re: Small PCB with two valveholders.

Hi David,

I've made an attempt to reverse engineer your PCB with the limited clues available. The circuit around one valve holder looks like a two stage R/C coupled amplifier possibly employing the ECC82 or something similar.

The low pass filter to the next stage looks a bit unusual.

See diagram below.

Note to self: "Must get a life."
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Old 10th Apr 2016, 10:17 pm   #3
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Default Re: Small PCB with two valveholders.

Further examination of the print side of the PCB reveals the word SEPARATOR.
So it's possible the board was removed from a television receiver.
The valves employed are double-triodes but not the usual ECC81/2/3 series, unless the heater supply was parallel 12.6 volts which is unlikely.
It's most likely the valve used here was the Mazda 6/30L2, a which has a similar pin connections as the ECC82 except the heater does not have a centre tap.

DFWB.

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Old 10th Apr 2016, 10:22 pm   #4
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Default Re: Small PCB with two valveholders.

Quote:
Originally Posted by ukcol View Post
I've made an attempt to reverse engineer your PCB with the limited clues available. The circuit around one valve holder looks like a two stage R/C coupled amplifier possibly employing the ECC82 or something similar.
Hi Col, I've just done the same thing and come up with the same answer, one of the valves is a two stage amplifier. Strange to have such a high value capacitor in the grid circuit of the second valve.

DFWB.
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Old 11th Apr 2016, 7:46 am   #5
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Default Re: Small PCB with two valveholders.

Sorry, there was an error in my drawing. I connected the output to the wrong side of the coupling capacitor.
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Old 11th Apr 2016, 11:55 am   #6
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Default Re: Small PCB with two valveholders.

If pin 9 is isolated then it probably won't be a 6/30L2, as that had an intersection screen on pin 9 which should be grounded (or connected to a cathode).

My guess is not an ECC82, as the cathode degeneration on the second stage would not reduce gain but simply increase anode impedance. ECC81 is my guess.
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Old 11th Apr 2016, 2:07 pm   #7
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Default Re: Small PCB with two valveholders.

If the valves were ECC81s it is unusual to find that the heaters were supplied from a 12.6 volt source rather than parallel connection of the heaters for 6.3volts.

DFWB.
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Old 11th Apr 2016, 3:23 pm   #8
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Default Re: Small PCB with two valveholders.

Both at 12.6V would need 300mA, just right for a series 'P' chain.
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Old 11th Apr 2016, 8:15 pm   #9
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Default Re: Small PCB with two valveholders.

I will have a use for the PCB. I'll be needing a preamplifier for the low output magnetic pickup fitted in the SP25 record deck I'm considering for the RGD radiogram. A two stage amplifier using the ECC83 should be realisable from this PCB.

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Old 12th Apr 2016, 10:48 am   #10
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Default Re: Small PCB with two valveholders.

A two-stage amplifier using ECC83 would have an awful lot of gain! Voltage gain about x3000?
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Old 12th Apr 2016, 7:26 pm   #11
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Default Re: Small PCB with two valveholders.

X3000: That assumes that each triode amplifier stage has a gain of X54.

I intend to modify the PCB to accept the circuit shown in the attachment.
The gain of the amplifier will be reduced by the negative feedback loop which also incorporates the RIAA compensation. V1A and V101A stages will be omitted. An additional double triode valve will function as the output cathode follower. V1B and V102B in the circuit diagram.

DFWB.
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Old 12th Apr 2016, 7:47 pm   #12
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Default Re: Small PCB with two valveholders.

is it to be a mono RIAA booster?
Just that if so theres a few circuits about that might fit the bill, usually its an ECC83 using both halves for amplification and EQ into half an ECC83 as a CF. You could use a 6AV6 for that (EBC90?) as its effectively one triode of an ecc83 with a double diode that you can simply strap to earth, although its on a B7G base so could be tricky. I guess you could use half an ECC83 or maybe parallel the first stage to reduce noise?

Probably more circuit permutations around than there are days in a year.
Nice idea to use the board for that all the same.

Andy.
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Old 12th Apr 2016, 8:02 pm   #13
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Default Re: Small PCB with two valveholders.

Hi Andy,
It'll be a stereo phono booster, the left and right channels can then be added together to feed the amplifier in the RGD 1046G. A switching arrangement will allow stereo operation if required at some time in future.

DFWB.
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Old 12th Apr 2016, 8:44 pm   #14
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Default Re: Small PCB with two valveholders.

OK Gotcha!
Then apologies for my previous post and grasping the stick at the wrong end.
If you like I can share a design that I know works well and doesn't use a lot of parts. Passive EQ though.
There's also the EF86 stage that Leak used, not the greatest of fi but i reckon could be fine in a vintage radiogram. The second EF86 in those preamps was basically used for the baxandall tone control with a bit of added gain, i reckon you could use a low-ish gain triode stage to bump the signal up if needed. But you'll need a coupple of extra valve sockets if you need that plus a cf output.
Nice idea to build a valve RIAA step up, It kind of seems logical if you have enough spare current to steal a couple of mA HT and enough for the heaters.

Andy.
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Old 12th Apr 2016, 8:55 pm   #15
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Default Re: Small PCB with two valveholders.

Quote:
Originally Posted by FERNSEH
X3000: That assumes that each triode amplifier stage has a gain of X54.
I estimated about 60 for the first stage and 45 for the second stage. Rough calculation, done in my head so don't hold me to the details! Enough loop gain to add RIAA, as you plan to do.

6AV6 is EBC91. EBC90 is 6AT6; the triode is half a 5751.
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