PRR Vari-Mu question

Hi,

I've got my PRR Vari-Mu running now, but it's not yet working properly. I'm still waiting for the 12AU7s to arrive, so I put in 12AX7 for the moment and only one channel is fitted with transformers right now. Opamp and tube voltage are a little too low (only ca. 14V op and 11,6 tube under load).

The real problem is, though, that it's not passing audio. The transformers are installed correctly, and there is a faint tube buzz (at -90db), but I can only hear the audio I put in for a short time (at -30db peak) when I pull the plug (distorted). Is there any obvious mistake?

DC voltages are:

Tube
9 5,9
8 11,8
7 0
6 104,7
5 11,6
4 0
3 11,8
2 0
1 104,7


5532:

1 -12,5
2 -9,8
3 -0,2
4 -14
5 11,8
6 11,9
7 12,4
8 13,2

TL074:

1 1,6
2 1,6
3 1,6
4 13,2
5 1,6
6 1,6
7 1,6
8 1,6
9 1,6
10 1,6
11 -13,9
12 1,6
13 1,6
14 -11,9


Thanks and happy holidays!
Gregor

Any idea? It should at least pass sound with 12AX7s mounted, shouldn't it? I've used this schematics:

http://www.conditionedresponse.com/DIY/C5/index.html

yeah! they don't have identical specs but they work.....a little noisiers that 12AU7....

Yea, I thought they should... thanks! I can't understand why there is only audio after I pull the plug though...

I've fitted both channels with transformers now, same problem on both... Is there anything else I could try? What should I look for?

Thanks!
Gregor

Well, at least it's definitely not related to the tube type. I put in a PCC85 (with reduced heater voltage): same thing.

So, now I've got the 12AU7, everything installed, still no sound on either channel. It's only there very faintly and distorted when I pull the plug, then it slowly fades away.


DC voltages are as follows:

tube 1

9 5,9
8 12,1
7 0
6 106
5 11,9
4 0
3 12,1
2 0
1 106,3

tube 2

9 5,9
8 12,1
7 -4,(1-7)
6 106,6
5 11,9
4 0
3 12,2
2 -4,(1-7)
1 106,7


5532

1 12,7
2 12,1
3 12,1
4 -14,9
5 12,1
6 12,1
7 12,8
8 13,5

TL074

1 1,6
2 1,6
3 1,6
4 13,5
5 1,6
6 1,6
7 1,6
8 3,7-8,7
9 1,6
10 1,6
11 -3,1 to 14,9
12 1,6
13 1,6
14 -12,4

There are a few values that go up and down (the two on the TL074 go up and down a lot).

The +/- 16V lines on the power supply read -14,8V and +13,4V DC...


Any help would be greatly appreciated.

Thanks,
Gregor

You hear sound for a second when you "pull the plug?"
Meaning, a 1/4" jack? / instrument jack?

I had this problem on a guitar effect.
Turns out, the pins on one side of the jack shorted when the plug was inserted, and connected when the plug was pulled - which created a similar effect - audio for just a split second as I pulled the plug.

If you have the same weird jack I bought, look closely at the contacts as you insert the jack, you'll see they "lift up" and disconnect upon insert.

I know it's unlikely that this is your problem.
But it sounds so similar to what happened to me, I figured I'd mention it. I did troubleshooting for hours before discovering the culprit jack...

Hi kato,

no, I did mean when I pull the power plug. Right after the click the incoming audio can be heard at low volume and sounding very distorted, it then fades. Other than that the unit is dead quiet, no hiss or hum although it's not in a case yet. There are no shorts I'm aware of, I've send a sine wave in and measured what got out of the transformer, it's all there. There's also tube buzz in the noise floor which goes away when I take out the opamps and/or tubes...

The only real deviation from the schematics are the transformers. I didn't have 600-600 line transformers, so I'm using an OEP 1:6 transformers wired backwards at the moment on the inputs and edcor XS1100 xformers as plate transformers. They're a mismatch, but the thing should at least pass audio anyway, shouldn't it?

Thanks for your suggestions anyway.

Gregor

Doesn't anyone have any idea? Could someone please confirm if the values are OK?

I send in some audio, a sinewave at 0db, and measured 1,8V AC after the input transformer, but then it dissapers. It doesn't even get to the plate transformer. Something seems to be very wrong with the tube stage, but I build it exactly like the PCB shows it, and I couldn't make out any deviations from the latest schematics either. It's the same for both channels and isn't related to the type of tube used (all dual triodes of course).

Could someone please point me to where and what to measure?

Thanks in advance!
Gregor

Just a thought, as you seem to use 12V for tube heaters,
the pin 9 should not be connected, only pin 4 and 5
(as shown on the PCB layout from your link)
Or if you use 12V center tapped, the center tap should go to pin 9
For reference (pins shown from bottom view) :


From a 12AU7 datasheet

Hi keefaz,

thanks for answering! Pin 9 is not connected to anything, in accordance to the schematics and the PCB.

I know the voltage values are a little off, but that shouldn't prevent the build to pass any audio at some audible level, should it? I could post pictures of my build if that's of any help, but it's not very clear looking since I used veroboard and made every connection by wire...

I don't know why audio doesn't pass, but the audio circuit has to be closed though
I mean, looking at the shematic, I would not expect to hear audio directly at TX out without the ICs connected
I don't know how your circuit looks like right now, if it is completed and connected like the shematic or not so I can't tell

There isn't any audio (AC) present at the input of TXout (coming from the tube). That's why I think the problem has to be in the tube part...

I hope you solved your problem but from what I see you've got something really wrong with the tube :
You should have those values :

TL 074

4 ~= +16V ( + PSU value ) you don't have the right value but it could be OK if there wasn't that thing :
11 ~= -16V ( - PSU value ) :shock: :shock: :shock:

TUBE

9 ~= 6V OK
8 ~= 0V :!: :shock: :!:
7 ~= 0V OK
6 ~= 40 V :!: :shock: :!:
5 ~= 12 or 0 V OK
4 ~= 0 or 12 V OK
3 ~= 0V :!: :shock: :!:
2 ~= 0V OK
1 ~= 40V :!: :shock: :!:

Did you really used a 100 ohm pot for the thumb nulling ?

Don't bother if you only have + and - 14 V on the opamps, mine runs OK with + and - 12 V. I use it for both heaters and opamps...
At the moment I use OEP tranfos with ratio 6.35 : 1 on the input and 1:1 on the output and it's working...

I know this an old thread and probably resolved now...but, it is odd that you have +12volts on the cathodes...escpecially since anodes are at 104volt ( no currentdraw through anoderesistors?)...look for shorts around heaters/cathodes...with the catodes resting so much higher than the grids, the tube is basicly shut off
j