ImPasse Preamplifier

[DaveM]

Well, I did a bit of measuring last night and found I made a mistake in my wiring of the heater circuit. I haven't corrected it yet, but hopefully I will get to it tonight. I reversed the pin numbers when I turned the board over and instead of wiring the heater to pin 4+5, I went to 5+6. Doh!! I hope I didn't blow anything up in the process. There was no smoke or unpleasant smells, so I still have hope.

 

[DaveM]

Well, my found mistake did nothing but take me in circles. At the end of my chasing, I am back where I began.

So here are my measurement. I measured from ground to pin on everything with the tubes in place and everything powered up. The neon bulbs lit for a while and then went out while powering up.

6SN7
Grid 2 - 1.2 mV
Plate 2 - 153.5 V
Cathode 2 - 3.83 V
Grid 1 - 1.2 mV
Plate 1 - 150.1 V
Cathode 1 - 3.85 V

ECC88
Anode 2 - 176.4 V
Grid 2 - 152.9 V
Cathode 2 - 153.3 V
Anode 1 - 177.9 V
Grid 1 - 149.5 V
Cathode 1 - 150.4 V
Screen 1.1 mV

 

[SY]

Voltages look pretty good on the 6SN7, but not the ECC88; there's a voltage divider network which steps down the DC from the 6SN7 plate. Did you put the right value resistors in the voltage divider?

 

[DaveM]

I will poke around with the meter tonight and verify my part values. I assume you are looking at the values for R8 and R10 as they are between the 6SN7 plate and ECC88 grid. I will check those and all the others tonight against the parts list.

 

[DaveM]

I did my prodding and everything appears to be the right value. I ran through all the resistors on the board and found no errors. I also verified the caps. Any other suggestions?

By the way, what sort of voltages should I be expecting to see on the ECC88?

 

[SY]

I did my prodding and everything appears to be the right value. I ran through all the resistors on the board and found no errors. I also verified the caps. Any other suggestions?

By the way, what sort of voltages should I be expecting to see on the ECC88?

R8 and R9 should divide the voltage down to 110-120V or so at their junction. That's the voltage that should be seen at the ECC88 grid. You should see a few volts higher on the cathode.

If your meter has a low impedance, it can upset some of these readings (e.g., the ECC88 grid voltage)- use one with a 10M input impedance, minimum.

 

[DaveM]

I am using a Fluke digital scope borrowed from work. According to the manual, the input impedance is only 1M ohm. I repeated the measurements with a Craftsman el-cheapo meter with unknown input impedance and got the same readings.

With no tubes in I am getting:
side 1
198-193 V across R8
193-185 V across R10

Side 2
211-205 V across R8
205-198 V across R10

With tubes I get
side 1
150-150 V across R8
150-150 V across R10

Side 2
154-154 V across R8
154-154 V across R10

 

[SY]

R10? Do you mean R9?

What are you driving with the preamp?

 

[DaveM]

Doh!! I read 9 then look at the board and measured 10. A bit of a dyslexic moment.

With no tubes in I am getting:
side 1
198-193 V across R8
.9 mV-185 V across R9

Side 2
214-208 V across R8
.7 mV - 201 V across R9

With tubes I get
side 1
152-151 V across R8
1.2 mV - 151 V across R9

Side 2
154-154 V across R8
1.2 mV - 154 V across R9

 

[DaveM]

I have a pair of F4's I want to drive with it. Presently I have a BOSOZ. At full volume it is a moderate listening volume. I'm sure this will give me all the volume I could ask for.

 

[SY]

I guess I was asking, what's the load impedance and is it balanced?

In any case, whatever your wiring error is, you've somehow bypassed the R8/R9 divider network- the grid of the ECC88 is nearly the same voltage as the plate of the 6SN7, and clearly that's wrong, since a voltage divider is between them. Is this build point to point or on a circuit board?

 

[DaveM]

I used one of Jackinnj's boards. I don't know the impedance, but they are balanced.

 

[SY]

OK, try this: with the power off, measure the resistance between the 6SN7 plate and the ECC88 grid.

Any chance you could post a pic? I'll take a look at Jack's board (I have one built up on my lab bench) and see if it's possible for a dyslexic to stick a resistor into the wrong hole.

 

[DaveM]

I am getting 1.6K Doesn't make any sense to me. I am now just checking the board and schematic against each other for continuity in the right places and part values. I'll let you know if I find anything really embarrassing.

 

[jackinnj]

and see if it's possible for a dyslexic to stick a resistor into the wrong hole.

you're opening yourself to all sorts of scatological humor with that one...

 

[DaveM]

I am certainly confused. I am looking at the schematic and very clearly I should be able to beep out one pin of R8 to R9 and R10. I can't do that. Jack, is there an incorrect trace somewhere?

 

[BDP]

Looking at the PCB, R10 is connected between R8 and R9. The non grounded end of R9 needs to be disconnected at the pad and connected to R8. Look it over and see If I have this correct.

I'am glad you found this before I get started on mine. So thanks.

BDP

 

[SY]

you're opening yourself to all sorts of scatological humor with that one...

That's why I was giggling when I typed that.

 

[DaveM]

Well, I checked all the parts are in the right holes and I lifted the non ground leg of R9 and jumpered it to the junction of R8 & 10. I can now map out the schematic correctly. Unfortunately I still have about 150 V at the junction of R8,9,&10 with the same at the grid pin.

Arrgh!

 

[SY]

When I'm back home later today, I'll look at Jack's board carefully to see if any obvious errors appear. Voltage dividers DO voltage divide. Are you sure that your R8 is 680k, not 680R?