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PostPosted: 10 Jun 2010, 06:39 
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Joined: 22 May 2010, 08:01
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While I wait for a new 6x4 tube to come in the mail, Im going to sort out the pots and jacks and start drilling all the required holes on the chassis.

I was thinking of having Gain 1, Gain 2, Bass, Treble and Master. For Gain 1 Id have a pot at the very start of the whole preamp, for Gain 2 id have a pot inbetween the tone/eq circuit and the second preamp, then for the Master, Id have a pot at the very end of everything.

In > Gain 1 pot > first preamp (left) > tone circuit (bass and treble) > Gain 2 pot > second preamp (right) > Master pot > out.

Will that all work? Can I go without the Gain 2 pot or would it be good to have it? I plan for the whole preamp to go into a solid state bass amp.


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PostPosted: 12 Jun 2010, 20:25 
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Joined: 22 May 2010, 08:01
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The other 6x4 tube I ordered came in the mail, I plugged it in, it heats up and I now get the B+ voltage. Ive got jacks wired onto it to see if I get sound and I do, its nice and clean, not too much noise which is good. The tone circuit works really nice, except that I need to replace the 1m pot for treble to a 4m pot. Ive got a question regarding the preamp tube heaters. The tube warm up, they dont get heaps hot though and they barely glow, they each are getting 11.8v dc. Is that ok? I was expecting them to heat up a fair bit more. Does the amount of heat/glow the tubes produce effect the overall output sound? How can I overdrive the tubes to get that nice sound?


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PostPosted: 13 Jun 2010, 10:49 
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Location: Winnipeg, CANADA
Barrington wrote:
The tube warm up, they dont get heaps hot though and they barely glow, they each are getting 11.8v dc. Is that ok? I was expecting them to heat up a fair bit more. Does the amount of heat/glow the tubes produce effect the overall output sound?

The tube heater rating is typically 12.6V +/-10% which is about 11.4 to 13.8V. Personally I like to stay in the 12-13V range. 11.8V should work.

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PostPosted: 20 Jun 2010, 22:43 
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Joined: 22 May 2010, 08:01
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How can I get an overdrive sound from this preamp? At the moment it is a very clean sounding preamp (im guessing that this is because of the preamp design). Is it even possible to get an overdrive sound from this preamp?


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PostPosted: 21 Jun 2010, 16:52 
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Location: Duesseldorf, Germany
Hey again,

that's what a pre amp is designed for. It provides a clean and undistorted driver for power stages.

What kind of amplifier do you want to drive with the preamp?
Or - for what kind of input (signal source) you drive with the preamp?

It shouldn't be a big problem to get an "overdrive" sound out of the amp, but you'll need to specify
the whole project that you want to realize.

The choice is
1 - an overdrive effect circuit before the preamp
2 - the same, but behind the preamp to a power stage.

One really simple overdrive effect can be done using an NPN transistor and clipping diodes
(for soft clipping), powered by a 9V battery cell. Easy job to do...

http://johannburkard.de/resources/Johann/simple-diode-clippers-and-distortion-circuits-electra.png

IMO - I wouldn't touch the running boards, so it won't hurt em.
Better feed the boards with an already soft-distorted signal or add overdrive after the output.

Maybe you can tell us something more about the whole project...

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Cheers,
Tom.

Some of my projects: TDA2050 Chip Amp, the LM3886 Gainclone Thread and the Szekeres Headamp Thread.


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PostPosted: 22 Jun 2010, 01:08 
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Joined: 22 May 2010, 08:01
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Well Ive seen some bass amps that have a tube or two in their preamp and usually when the gain is on full, the tubes produce an overdrive sound. I cant remember if I mentioned this earlier but this preamp will be used for bass guitar and will be plugged into my solid state bass amp (possibly in the send/return).


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PostPosted: 24 Jun 2010, 05:04 
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Location: Duesseldorf, Germany
Yep, some amps go into distortion / overdrive, when they are driven hard.
But that also depends on the input signal. If the input signal (on the preamp or on the power stage)
is stronger than the input sensitivity of the amplifier. A bass guitar output hasn't such a high level.
I think that the input sensitivity of your preamp or bass amp is higher than the guitar output.
So you'll have to build a seperate distortion / overdrive device.
Such an overdrive decvice will add the modified signal to your amps.
It also adds some gain to your guitar signal.

I've posted a link with an overdrive circuit in this thread. It's very simple and very easy to do.
The circuit also doesn't cost a lot, but a bit time.

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Cheers,
Tom.

Some of my projects: TDA2050 Chip Amp, the LM3886 Gainclone Thread and the Szekeres Headamp Thread.


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PostPosted: 23 Jan 2021, 09:00 
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Joined: 23 Jan 2021, 08:35
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tombethe wrote:
Barrington wrote:
The three high voltage pins are labeled 280v, 250v and 0v

Oh my, sorry, i thought this transformer is a double 250V with one "0" for ground
and i didn't take a second look onto the power supply schematic. Sorry - my fault!

OK - everything still goes fine.

The heaters for the tubes still have to be connected like meant before..
Some more informations: http://en.wikipedia.org/wiki/12AX7


The transformer really has 2 different HV sections. You'll need the pin with the measured 280V (AC) output.
Maybe the transformer has a wrong label... You wrote you've already measured the AC-output.
The schematic shows that 280V (AC) is recommended.
So - we use the pin that gave you the 280V (AC) output. (Measured from 250 / 280 to "0").

Your HV pcb is stuffed with a "hybrid" tube rectifier circuit (i double checked it to be sure and checked the schematic again).
Hybrid configurations work with one tube for the high voltage supply and two solid state diodes to build the ground (0).
So - the transformer does NOT provide a "0" or a pin for the "ground" by itself.
The label shows a "0", but this 0 can not be used to build the PSU ground.
In this case, "0" is used for the second AC connection.
This hybrid configuration works in the same way like a "graetz bridge rectifier" or "diode bridge"
http://en.wikipedia.org/wiki/Diode_bridge

OK - connect the 280V (measured) pin and the pin "0" (from the transformers HV section)
to the "H . AC" on the HV board. That's all for the HV supply (and the right connection now].
The circuit ground "0" or "ground" is provided by the 2 solid state diodes and is on pin "E" or "GND" or next to the "+B1" pin.

If you now go to measure the HV board now, you should read about 340V (DC unloaded) and 280VDC regulated / with load.

Please be careful - HV is very dangerous - treat it with respect!

I hope i could help you a bit further now...


I’m a newbie when it comes to electronics and I’ve been wondering what’s the major difference between a rheostat and a bridge rectifier? I checked some resources such as this one https://www.derf.com/how-a-bridge-rectifier-works-step-by-step-tutorial/ but I’m still trying to get the hang of it. Any help would be much appreciated. Thanks


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