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[guavatone]

Any News Imo?  And the Voltages Are................


V1plate = 95-97 VDC
V1k = 1.55 VDC
V1g2 = 94 VDC

V2p = 145 VDC
V2k = 3.47 VDC
V2g = 145V

My gut says a cold solder joint in the low gain range resistors.  A lot of resistance would give you high gain (full is 58.68K) so that's why it wouldn't distort, since much resistance is much gain.  When you get to the lower range, the faulty resistor or cold solder joint causing resistance is revealed.  It could be anywhere in that path.

However, you are only getting 70V which is a bit low for V1's plate.  It could very well be that or the kathode to ground path.  I would check all resistances for resistors providing power to that input tube.

I hope you find the trouble soon and post some pics of your finished preamp.

 

[imo]

i think i figured out the problem on the one channel. The trim pot in the feedback loop was hooked up across 2 of the ground points, and the wiper wasn't going to the 160r. It sounds much better now.
I am still messing with it. The preamp sounds a bit compressed still. Moreso on the one channel that hasn't had the resistors changed. I was trying to use the 12bh7 curves to figure a good ballpark, but the plate voltage is pretty low to use them..I find it sounds best with between 5-7 volts on the cathode.

I've got 296 going into circuit..
Both my channels are around 68v on plate of the EF86.
both channels have around 100v into G2.
confusing..I've checked resistance. Plates are 100k on both, cathode measures 1k to ground. The 330k is around that(in circuit).
BOth my 1m's(grid to ground on V1) are measuring around 1.2k in circuit. That can't be right, can it? I'm wondering if there is something goofy because of the way i jumped the relay.. I basically bridged it so the input to the relay goes to the input to V1.
COuld you measure your grid resistors when you get a chance?
Ian

 

[guavatone]

BOth my 1m's(grid to ground on V1) are measuring around 1.2k in circuit. That can't be right, can it? I'm wondering if there is something goofy because of the way i jumped the relay.. I basically bridged it so the input to the relay goes to the input to V1.
COuld you measure your grid resistors when you get a chance?
Ian

I have 1.2K as well in circuit.  The tube adds resistance too.  I don't think the 12bh7 would change the voltage at V1's anode, but I may be wrong.

 

[imo]

you don't possibly have a component layout. It would make troubleshooting a lot easier with the board filled.
Thanks on the grid measurement. I don't think there is anything bad in the plate or cathode circuit as both boards are measuring the same and the circuit seems to work. I'm thinking either something around R8, R9, and or the interaction between the two tubes is throwing the voltages around a bit.
typically i see the grid 2 voltage a little bit lower as in your voltages
Ian

 

[guavatone]

here is the silk:

http://www.guavatone.com/free/0-PCB/o86silk2.jpg

 

[imo]

Charlie,
Thanks a lot. That definitely makes it easier when chasing down a problem..
I actually think the circuit is working the way it should. I am getting almost exactly the plate current on V1 that the REDD schematic shows. My 70v is from 208, giving me 1.38mA, the schematic shows 1.35..
I think the compression sound is kind of particular to the circuit. We a/b'd against a millenium and noticed a good deal less openess in the highs, but i was using channel 1, which has the unrebiased 12BH7. I'm gonna try putting in an ECC88, as per the schematic and see what i think. It is pretty easy to go back, just the one change if i like the other tube better.
Thanks again for the help.
Ian

 

[guavatone]

did you try to Change V2's Rk(R13)?  Raise it a bit to see if that helps.  maybe 300. another thing to try is changing R14 to 9K1.

 

[imo]

i think i want to build it 'stock' first out of respect to the circuit, and then maybe tweak it a bit if needed.
It is interesting that plate voltages are as low as they are, especially with the 290 going in. I wonder why they designed it that way? Seems like they could have gotten a bit more headroom otherwise.
I would imagine to clean it up, at least in my circuit, it would be raising the cathode R of V1 a bit so that the tube has more swing down to ground. V2 is pretty evenly biased, around 140 on plate

 

[Holger]

I've finished my first channel today. I've took the ECC88/EF86 6,3V DC direction.
Currently I use the Edcor XSM15K/600 for output and WSM10K/150 for input.
Since I don't have a microphone at home I used a 4dBU line signal to test the device.
Everything sounded good, but even at that high test signal level I had to use some additional gain to get unity level (my pad switch at 0 dB).
Then I found out that the input tranny should be wired reverse (connecting the secondaries at primaries).
I did that, the mic pre acts now as a mic pre level wise (I had to pad the signal at max), but the sound was very thin and noisy.
Can anyone give me a hint here?

 

[guavatone]

I would make sure your grounding scheme is correct.  Eg. only ground to chassis at the HT Power on main board.

Make sure you have close to 290VDC at D8.

Check for cold solder joints

Check total resistance of gain switch with full gain.

Check Values and solder joints on pad PCB.

 

[Holger]

Stupid me: I simply wired the pad PCB in the wrong way, everything works as expected now.
Does anybody know what the suggested Cinemag transformer combo costs?

 

[penguin]

is tis mic pre suitable for summing amp ?
how much noise i should expect at 45 db of gain to achive unity gain?

thanks

 

[Holger]

It is suitable in the way that a Mercedes S-class is suitable as a delivery car for pizzas 

 

[penguin]

sounds yummy  8)

 

[Holger]

Does anyone else miss some low end in his build?
My -3 dB point is at some 330Hz which is not so good.
I'm currently using the Edcor combo, still waiting for my Sowters.
Is it possible that my tubes are bad? I currently have only two sets, both channels show the same behavior.
I hope this will be resolved when the Sowters arrive.

 

[withnail]

Does anybody know what the suggested Cinemag transformer combo costs?

Mic input transformer
Item #: CMMI-7C
Unit price: $66.08

Output transformer
Item #: CM-9661A-H
Unit price: $135.03


BTW, I am about to embark on this build, using the v1.5 docs.  Just wondering if there've been any more errata uncovered or other changes...?  ???

 

[guavatone]

You will be fine with recommended Cinemags.

 

[Holger]

Preview, more pics and details next week:

 

[withnail]

Preview, more pics and details next week

What a TEASE! 

 

[drummerjoe31761]

Hi All.
I'm looking for the BOM, It doesn't seem to be in the build docs I have. Also I need the Values on the board. Sorry if this is a stupid request but I've looked everywhere and cant find them
thanks Joe