Awesome tube preamp project: Top Drive

Tube or solid-state, this section goes to eleven!

Re: Awesome tube preamp project: Top Drive

Postby danielzink » 05 Apr 2019, 13:55

ppluis0 wrote:Hi Dan,

You're right: 12AX7 and 12AU7 has the same pinout, so both types can be employed in this project but with sonically differences.

I forgot to say that the numbering of a tube pins is viewed from bottom and counting clockwise.

It's OK to have 0 volts at pins 2 and 7 in each tube, but 0 volts at pin 6 is wrong. Missing connection perhaps ?

At firs sight, at the pin 5 of V2 you must read 12Vdc instead of zero volts, but that tube appears to be active... :scratch:

(Coincidentally V1 looks as if be cold. There is a chance that the value of 12Vdc belongs to pin 5 of V2 ? )

Are you build the same PSU proposed in this link ? http://freestompboxes.org/download/file.php?id=31987 or are you using the voltage multiplier topology included in the original PCB ?

I re read again the entire thread, and looks as nobody complete one of the PCB's provided for this project...it's it true ??

Keep in contact,
Jose



To be clear - this is a PCB purchased from PDF Electronics back in the day.

And yes. That is the frustration.

As far as I know - no one else has populated and tested the PCB.
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Re: Awesome tube preamp project: Top Drive

Postby ppluis0 » 05 Apr 2019, 16:48

Hi Daniel,

My best guess is that you -inadvertently- has a connection crossed from PCB to tube sockets.
Try to check if the bundle of violet and green wires are soldered to the same pin number at each socket.
Also measure again the voltages at pins 5 of V2 and 6 of V1 and tell me what you see this time.
As a side note: if you have 0V at pin 4 and 12Vdc at pin 5 of each socket, at the same time you can be able to read 6Vdc at the unconnected pin 9 of each socket, since there is an internal tap in the heater inside each bottle.

Cheers,
Jose
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Re: Awesome tube preamp project: Top Drive

Postby danielzink » 13 Apr 2019, 02:44

ppluis0 wrote:Hi Daniel,

My best guess is that you -inadvertently- has a connection crossed from PCB to tube sockets.
Try to check if the bundle of violet and green wires are soldered to the same pin number at each socket. All connections checked with*no* tubes in...triple checked - all connections correct


Also measure again the voltages at pins 5 of V2 12Vdc (tubes in)

and

6 of V1 51Vdc (tubes in)

and tell me what you see this time.


As a side note: if you have:

0V at pin 4 and 12Vdc at pin 5 of each socket YES (tubes in),

at the same time you can be able to read:

6Vdc YES (tubes in) at the unconnected pin 9 of each socket, since there is an internal tap in the heater inside each bottle.

Cheers,
Jose


I'm leaning towards a fault with the PCB itself - I have one more unpopulated PCB here....anyone want to take a $20 gamble ?
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Re: Awesome tube preamp project: Top Drive

Postby ppluis0 » 14 Apr 2019, 02:37

Hi Daniel,

Perhaps exist a mistake in the PCB itself... :roll:

But, after re read the schematic of this preamp I found the cause for those tube sections are not operating: the cathode resistors present an unusual high value so the triodes are polarized at cut off

The usual values for the resistors connected at pin 3 and 8 must be an order to magnitude (10 times) lower than those published in that diagram.

Try to replace R26 with 1K; R22 and R24 with 1K6 and R27 with 1K2 resistors to put the tubes in a better operating bias.

Eventually, replace these resistors one at a time and in the same order indicated above and see if the corresponding stage pass (and amplify) the signal.

Cheers,
Jose
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Re: Awesome tube preamp project: Top Drive

Postby Manfred » 14 Apr 2019, 04:12

How much is the recommended B+ for the givenTop Drive circuit?
I took it 48 Volts from the information on this topic.
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Re: Awesome tube preamp project: Top Drive

Postby ppluis0 » 14 Apr 2019, 06:28

Manfred wrote:How much is the recommended B+ for the givenTop Drive circuit?
I took it 48 Volts from the information on this topic.


Hi Manfred,

Daniel measured 51 Vdc at anodes of all triodes.

Assuming that all are biased at cut off, then the +B present at the voltage multiplier should be very closed to this number.

Also danielzink has plugged a couple of 12AX7 that becomes erratic when are supplied at low voltages. For such reason the design indicates 12AU7 that runs more linear with low supply.

As said in my previous post, I think that the cathode resistors are ten times greater in value and can be originating the fault.

Cheers,
Jose
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Re: Awesome tube preamp project: Top Drive

Postby Manfred » 14 Apr 2019, 09:49

Hi Jose,
you are right at all points.

About 50 Volts, this was my first thought too.
I have taken a very detailed look at curve traces on low voltages of the 12AX7 and 12AU7.
The anode current of the 12AU7 is significant bigger than the current of the 12AX7 for the same grid voltage at low anode voltages too.
that requiers different bias settings.
Cheers,
Manfred
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Re: Awesome tube preamp project: Top Drive

Postby danielzink » 14 Apr 2019, 15:30

I'm going to order a pair of 12AU7's and go from there.

I'll post back in a few days and update.

I know it "shouldn't" make that much difference - but you never know.
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Re: Awesome tube preamp project: Top Drive

Postby danielzink » 29 May 2019, 21:24

12AU7's installed - still silence.

Boo
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Re: Awesome tube preamp project: Top Drive

Postby ppluis0 » 30 May 2019, 01:31

Hi Daniel,

Quoting one of my own previous post, again suggest yo try this:

Try to replace R26 with 1K; R22 and R24 with 1K6 and R27 with 1K2 resistors to put the tubes in a better operating bias.


Tell us what happen after that modification.

Cheers,
Jose
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Re: Awesome tube preamp project: Top Drive

Postby danielzink » 02 Jun 2019, 00:42

Okay.

I've had a success !!

It seems that all of my problems are stemming from the "JR1" switch (the FET Boost switch that Chris Moir had wired into an XLR)

Image

Image

I wired it into a regular on/off switch (far left).
The other switches are for reference if they'll help rectify the issue (on/on and 3pdt toggle)

Image



So....

Image
Image

If I connect the wires as such:

Image

The FET Boost LED comes on and the FET Boost works..

if I do this:

Image

Voila ! the preamp section works !

Connect all three ?

Image


And we have preamp and FET Boost !

Now....what @#$%! switch do I need to accomplish this ? grrrr !

Thanks !!
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Re: Awesome tube preamp project: Top Drive

Postby danielzink » 02 Jun 2019, 20:01

It was a kludge...but I got it done and she's up and running..


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Re: Awesome tube preamp project: Top Drive

Postby danielzink » 02 Jun 2019, 20:04

That being said....

Here it is.

One of my oldest boards...and what I believe is only the second assembled...operational (the first being the prototype)....

Top Drive Pre-Amp

I opted to not mount the transformer on the board - I went "wall wart" style.


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Re: Awesome tube preamp project: Top Drive

Postby Manfred » 02 Jun 2019, 22:57

Please, post the last working schematic.
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Re: Awesome tube preamp project: Top Drive

Postby danielzink » 03 Jun 2019, 20:28

Manfred wrote:Please, post the last working schematic.


https://www.dropbox.com/s/zbi2rplso97xk ... tic_bw.pdf
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HENK (04 Jun 2019, 11:40), LoveRock69 (16 Jun 2019, 00:17), Manfred (03 Jun 2019, 22:14)
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Re: Awesome tube preamp project: Top Drive

Postby Manfred » 03 Jun 2019, 22:16

Thanks for sharing the files. :thumbsup
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Re: Awesome tube preamp project: Top Drive

Postby LoveRock69 » 16 Jun 2019, 00:17

danielzink wrote:
Manfred wrote:Please, post the last working schematic.


https://www.dropbox.com/s/zbi2rplso97xk ... tic_bw.pdf


Are you able to do some sound clips??!!
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