3 valves with worn writing, possible E88CC?
 

I picked up a box of about 100 old valves a while back on Gumtree.
The box contained a pair of boxed Mullard EL36 and decided to build a push pull amp with them. These are indeed TV valves but have been used in PA systems built by Philips. It's up and running but with a bit more 50Hz hum than I'd like...

The problem was the PI, I'd used some 6N5P form a batch of 5 that were going cheap on ebay, I suspect the triodes are not well matched.

As the heaters are on pins 4/5 I tried an old Mullard ECC85 and the hum was gone :)

So, this box of old valves also contained 3 novals with gold pins but the white print was mostly gone, the top of each has the classic Philips/Mullard crimped cross. Are these E88CC?
One has what looks like R7J etched very feint though.
Measured 3Ω across pins 4/5.

Photos attached.

Doug

Re: 3 valves with worn writing, possible E88CC?
 

The Mullard/Philips factory code for the E88CC begins with "7L" in the first line. In both your pictures the first symbol is unreadable (at least for me). The second symbol/letter however could well be an "L".

I think that your "R7J" is what's in the second line. The "R" in that line indicates that the valves are manufactured by Mullard in Mitcham.

The fact that your valves have gold pins, the fact that they resemble known Mullard E88CC's, and that there seems to be an "L" as the second symbol/letter in the first line, makes me think, like you, they are indeed E88CC's.

Re: 3 valves with worn writing, possible E88CC?
 

Thanks Robert, I've never come across one of these before. I'll drop one into my simple preamp test circuit and measure the amplification for each triode of a 0.2Vpp 1KHz sine then hopefully try the best matching one in the PI position tomorrow :)

Doug.

The whole box only cost £1 and included a grey glass GEC KT66 and 2 x Russian analogues of 6L6C rebranded as KT66.

Re: 3 valves with worn writing, possible E88CC?
 

If most of them measure OK, than it sure was a good bargain.

Succes.

Re: 3 valves with worn writing, possible E88CC?
 

Here's the simple test setup I used.

Anode load resistor was 75KΩ 1/2W metal film
Cathode bypass resistor was 1.5KΩ 1/2W metal film
Grid stopper resistor was 35KΩ 1/2W metal film
Anode coupling cap 100nF 630v
Cathode bypass cap 22µF 25v

For 3 anode voltage, 50,75 and 100 Vdc, I measured the voltage drops across the anode and cathode resistors. I eyeballed the 1KHz 200mVpp sine input at the grid and Vpp output after the anode coupling cap on my CRO.

Results:
Triode 1
V Anode Output Vpp R1 Vdrop R2 Vdrop
50 5.6 1.77(1.18mA) 88(1.17mA)
75 5.6 2.51(1.67mA) 125(1.67mA)
100 5.7 3.28(2.18mA) 162(2.16mA)

Triode 2
V Anode Output Vpp R1 Vdrop R2 Vdrop
50 5.7 1.78(1.18mA) 89(1.17mA)
75 5.8 2.52(1.67mA) 126(1.67mA)
100 5.8 3.27(2.18mA) 163(2.16mA)


The triodes are perfectly matched.

Question for you all, can we calculate/estimate the gm / µ based on the results?
Would it be preferable to measure for a set of grid voltages as per the datasheet?

I dropped the tested valve in the PI socket and no hum :)

Doug

Re: 3 valves with worn writing, possible E88CC?
 

They are most likely what you suspected, E88CC, but in your experiment you're biasing it well below (anode current wise) its normal operatiing point. So you can take the gm and Mu etc from the data-sheet.
john

Re: 3 valves with worn writing, possible E88CC?
 

Thanks for the info, I found this info about testing a E88CC:

"Anode current (Ia) is tested with the official Telefunken/Philips/Mullard test setup: Va = 100V , Vg= +9 with 680 Ohm cathode resistor. Measurement after 10 minutes stabilization. Factory specification NEW E88CC: Ia = 15 ± 0,8mA."

I'll give it a try and try a plot of Vg too.

Re: 3 valves with worn writing, possible E88CC?
 

You might find it easier to use Vg= 0 and a cathode resistor of 200 ohms: that should give you -2V Vg at 10mA, and put you spot on the E88CC curve.

Re: 3 valves with worn writing, possible E88CC?
 

OK, Grid tied to gnd, 200Ω 1/2W resistor from cathode to gnd and a variable voltage (Mosfet based circuit) to supply anode. DDM to read HT at anode and 2nd DMM in series to read current. I also checked the Vdrop across cathode resister and they matched the direct reading.

Here are the numbers.

Triode 1 Triode 2
Anode V mA Anode V mA
50.17 5.28 50.08 5.17
75.2 7.86 75.4 7.79
100.3 10.53 100.0 10.47
125.1 13.3 125.4 13.28
160.3 16.17 160.1 16.36

Re: 3 valves with worn writing, possible E88CC?
 

Remarkable precision for a valve!

J

Re: 3 valves with worn writing, possible E88CC?
 

Seems the E88CC valves from the good manufacturers, cost an arm and a leg.

https://www.ebay.co.uk/itm/295215825...Bk9SR-rtzNXwYA

Re: 3 valves with worn writing, possible E88CC?
 

They're sometimes priced at an arm and a leg. But does anyone ever buy them ?

I note that Langrex have NOS Dutch-production Mullard E88CCs for £60 each plus VAT.

Cheers,

GJ

Re: 3 valves with worn writing, possible E88CC?
 

I tested all the 4 x NOS 6N5P valves I bought a while back for £10 inc p&p and found some good ones, 3 had the triodes within 2.5% of each other.

Sounded good in the amp (2 x 6P13S line scan valves) similar to EL36 but different pinout and a bit beefier (not to be confused with the 6P31S which is an EL36 drop-in) Needs a bias adjust for each output valve, set them to 60mA at idle.

Preamp is the 6N2P, 6N5P in the PI, 2mV hum measured across the speaker out. Quiet enough for a guitar amp.

https://frank.pocnet.net/sheets/113/6/6P13S.pdf

Doug

Re: 3 valves with worn writing, possible E88CC?
 

Added an extra RC stage to the HT supply and got the hum down to barely audible and at the limit of my low cost test gear.

Job done and the 4 USSR valves cost under £10 :)

Doug