stern's ecl86 amp

[stevewrigley]

Hi I have just aquired a Stern's integrated amp. the tube line up is GZ34, 2 ef86, and 4 ecl86. Can anybody help with additional details especially schematics. The cathode resister on one chanel has had a wire wound variable resister soldered in series, and this valve glows more than the others. I'm going to replace all the cathode resisters am I right in thinking all the output power of the pentode section goes through this resister? I've attached a picture of the amp, but as this is my first post anything might happen
regards

Steve

[ErikdeBest]

Hi Steve

Welcome to the forum! First advice: there are lethal voltages inside the chassis. Take much care, read and understand the specific thread about dealing with HV.

The cathode resistor does set the bias for the output tubes. The presence of the variable resistor implies that the value of resistance is probably lower than that of the other cathode resistors. A lower value implies in a higher current through the tube, and this is probably why it lights up more. BUT, maybe it is a leaky coupling cap.

In this kind of amps its 'mandatory' to change all capacitors and check the values of the resistors for drift (well, checking demands a schematic, which I don't have). I think that a search for restauration or vintage in the tube sector will show up some threads where this is discussed in more detail

Erik

[Eli Duttman]

The ECL86/6GW8 (data sheet here) is an interesting type. The small signal triode is the same as those in a 12AX7 and the power pentode is more capable than that in the 6BM8/ECL82. Sadly, AFAIK, the 6GW8/ECL86 is NOS only and not very plentiful.

A schematic would definitely help the cause.

A possibility is a variant on "El Cheapo" using 6AQ5s. The "El Cheapo" project is here. You might have to switch to SS B+ rectification to boost the B+ rail voltage, which is not a bad thing. The B- rail could be derived from the 5 VAC winding via a voltage multiplier.

[ErikdeBest]

Good point Eli Duttman

Steve, you are lucky that the chassis have 6 holes. That allows the use of a double triode + 2 pentodes (for each channel)

Erik

[stevewrigley]

thanks for the help, Ive done a schematic of what I can see unfortunately most of the capacitors are out of view. From what I can see most of the capacitors look pretty new. Any comments or observations on the schematic would be most welcome.

[Eli Duttman]

Steve,

That looks like a Mullard circuit to me. It has LOTS of gain, which mates with a low gain phono section, but not the 2 VRMS max. of a CDP.

Give the designer credit for using the ultralinear connection.

Since there is no phono section, conversion into an "El Cheapo" is (IMO) indicated more strongly.

Sourcing the 6AQ5/EL90 shouldn't be a problem in the UK. Mullard or Brimar CV4024s would be NICE in the splitter/driver role.

[planet10]

Originally posted by Eli Duttman
Sadly, AFAIK, the 6GW8/ECL86 is NOS only and not very plentiful.

You could have fooled me... i have lots (well somewhere ween 1 & 2 dozen)...

dave

[stevewrigley]

Hi, I've seen one almost identicle to mine advertised as a sterns mullard 10-10 which makes sense if you count the ecl86 as two. Iwant to try and keep this amp as original as possible, so I will be changeing all the capacitors and dodgy resisters. One last thought can anyone explain the resister, capacitor array on the output to the speaker post?

Thanks again for your input.

Steve

[planet10]

Originally posted by stevewrigley
One last thought can anyone explain the resister, capacitor array on the output to the speaker post?

You often see this to compensate for anomolies in the OPTs frequency response -- althou i've not seen one this complicated before.

dave

[RichG4PCE]

Steve,
Did you get all the info on the Stern's Amp? I can copy them to you if needed.
Are you still interested or have you moved on, I wonder.
I happen to be the proud owner of one too. I have the original docs, plus a mod that was published replacing the EF86 input with ECC83 triode.
Rich