Eros voltage reading problem

[Paul Joppa]

So, what are the grid and cathode voltages on the ECC88? And the screen voltage on the EF86's? This is a direct coupled two-stage amplifier, and all the voltages interact.

One possibility, for example, is the bypass capacitor on the triode cathode, which if reversed (it happens!) will drive the tube into saturation. The voltages will give a good hint.

Incidentally, 1.8 volts is high for an HLMP-6000 LED, which is normally around 1.55 volts. 1.55v is a good bias for an EF86 that meets the original specs, but many modern tubes sold as EF86s need an actual bias that is lower; I believe I've seen as low as 0.8 volts. That's why there is a servo in the stock design. Direct coupling is a pain!

 

[Atom Shop]

So, what are the grid and cathode voltages on the ECC88? And the screen voltage on the EF86's? This is a direct coupled two-stage amplifier, and all the voltages interact.

One possibility, for example, is the bypass capacitor on the triode cathode, which if reversed (it happens!) will drive the tube into saturation. The voltages will give a good hint.

Incidentally, 1.8 volts is high for an HLMP-6000 LED, which is normally around 1.55 volts. 1.55v is a good bias for an EF86 that meets the original specs, but many modern tubes sold as EF86s need an actual bias that is lower; I believe I've seen as low as 0.8 volts. That's why there is a servo in the stock design. Direct coupling is a pain!

I made a voltage chart, Paul.

 

[Paul Joppa]

Thanks, Atom. You are right, this is strange and something is not working right. It's still hard to tell what, because the voltages and currents don't add up.

So ... some possibilities:

* I assume since you haven't said anything to the contrary that the LEDs on the C4S boards are all lit with similar brightness - a little brighter on the A side than the B side.

* If the R1 resistor on the C4S boards is off, that would change the current. Check the resistances, R1 (A side) and R1 (B side) which should be 113 and 301 ohms respectively.

* If the resistors are off, that would cause the currents to be off. Check for 27K from C3 to ground, and from C8 to ground - these are paralleled with the 100uF / 160v cap, so it will take time for the resistance reading to settle. Make sure that cap is correctly oriented (yeah I know - you've probably already done that several times by now. But it's still a significant suspect.) Then check the 47K resistors, C1 to ground and C6 to ground.

* Long shot - you posted several times "EC88" which is a single triode - not useable in Eros. I have assumed you meant ECC88 which is also called 6DJ8. The circuit is actually designed around the 6922 or 7308, though the 6DJ8/ECC88 is similar enough that it ought to work.

OK, it's nearly midnight and that's all I have right now.

 

[Atom Shop]

Thanks, Atom. You are right, this is strange and something is not working right. It's still hard to tell what, because the voltages and currents don't add up.

So ... some possibilities:

* I assume since you haven't said anything to the contrary that the LEDs on the C4S boards are all lit with similar brightness - a little brighter on the A side than the B side. [ ------------------ By my subjective observation, Paul, they all look very much alike.]

* If the R1 resistor on the C4S boards is off, that would change the current. Check the resistances, R1 (A side) and R1 (B side) which should be 113 and 301 ohms respectively. [ ------------------------ I measured every resistor on the C4S boards and all were within fractions of their indicated value.]

* If the resistors are off, that would cause the currents to be off. Check for 27K from C3 to ground, and from C8 to ground - these are paralleled with the 100uF / 160v cap, so it will take time for the resistance reading to settle. Make sure that cap is correctly oriented (yeah I know - you've probably already done that several times by now. But it's still a significant suspect.) Then check the 47K resistors, C1 to ground and C6 to ground. [ --------------------------  All OK there. ]

* Long shot - you posted several times "EC88" which is a single triode - not useable in Eros. I have assumed you meant ECC88 [ ---------------- Yup! Definitely an ECC88. ---------------------- ] which is also called 6DJ8. The circuit is actually designed around the 6922 or 7308, though the 6DJ8/ECC88 is similar enough that it ought to work.

OK, it's nearly midnight and that's all I have right now.

Well, Paul, thank you very much, that was lot of good late night advice. I ultimately found my problem! The resistor check had me stumble over it.

I'd said previously that following the manual's instructions (kudos to its authors) should result in a first-attempt success at a functioning Eros. I still strongly abide by that.

In MY case, however, I didn't quite follow some important details.  :  The MPS4250's are to be installed so that "the flats of the bodies [are] facing in toward each other." Those words and the associated picture make it quite obvious how these li'l guys should sit on the card. But, n-n-o-o-o-o-o.... I had to do it differently. I mounted both A-Q1's so that they're marching off the card in the same direction as the B-Q1's. Wrong! What a bonehead!!!  :-[

Correctly reinstalling the two transistors brought everything up to spec. Almost. (Both channels sound very good!) The cathodes' led bias and servo bias both sit at 1.56. But my B+ only came up to 158, still short of the designated 170. I'm wondering if the transistors took a beating by being installed backward.

I'm so close! 

Atom 

 

[Grainger49]

Here is one Dan didn't include.  I need often myself:

 

[Paul Joppa]

158v at the 6922 (or equiv) plate is a bit low, but the critical voltage is the grid to cathode voltage. That should be nominally the same 1.55v; if it falls below 1.3 volts or so then the triode grid may be sucking enough current to kill the bass.

Among the half dozen or so tubes that I measured some years ago, the 6DJ8 varieties were more prone to this than the 6922 types. I think it has to do with the metallurgy of the grid and cathode parts. Anyhow, that's why I built the circuit around the 6922.

 

[Atom Shop]

158v at the 6922 (or equiv) plate is a bit low......

Any thoughts, Paul, as to what might be the reason?

Atom

 

[Grainger49]

I think he is saying it is tube dependent.  Try a Soviet Military Surplus 6N23P (6Н23П, USSR Surplus, the -EV sounds better to me).  They are inexpensive versus the high price spread and sound much better than the run of the mill 6922.

 

[Paul Joppa]

158v at the 6922 (or equiv) plate is a bit low......

Any thoughts, Paul, as to what might be the reason?

Atom

If everything else is good, then a tube with more transconductance than average will require less plate to cathode voltage to obtain the same current.

Seduction operates the tube at the same operating point, nominally 3.8mA and 70v plate to cathode at 1.55v bias. Some tubes would exhibit low plate voltages, 55 or 60 v instead of the design value of 70v; this was with LED bias on the tube so you KNOW the bias is 1.55v. Eventually as the tube ages it will lose some transconductance and the plate voltage will rise.

That caveat above, "if everything else is good," means that the bias voltage as I described earlier must be adequate to prevent grid current.

 

[Atom Shop]

Thanks for everything, Paul. The Eros is on my bench getting a good inverse RIAA work-out. The 'scope display confirms what I'm hearing (and vice-versa). True Hi-Fi!!!

Atom