Mod'ing a tube circuit (from early 60's stereo turntable)

[Chuck H]

The only thing i've noticed is the heaters do flare up for a second or two before settling back down.

Some old tubes "flash" on initial power up. It's normal, even though it seems for all the world like the tube must be leaky to do this or the filament supply might be spiking. You can always put your meter on the filament supply and check to see if it's actually starting high and settling down.

[TubeNoob]

Parts arrived!

Going thru them all and i started to breadboard the tape saturation circuit. It sounds like its starved for power at this point (it's not) but the designer's site says to use an opamp buffer stage before and after, so i'm about to plug that in.

Have to give a shout out to these guys - their products are amazing, cheap, and they open source all the schematics and encourage people to DIY their own:
DIY Recording Equipment Store ? DIYRE
They came up with a great idea to offer a lunchbox rack unit that allows you to plug in 3 different 'colours' of your choosing. Killer format.
The lead guy (Peterson) has been cool enough to write me back himself with answers. Top notch!

[TubeNoob]

Tape Saturation (called the "15IPS Colour" on the DIYRE site) seems to now be working pretty good on the breadboard.

-Added opamp buffer on the input (NJM2068)
-Added 10uf electrolytic cap in series with the input
-Put a pot instead of the resistor that leads to the diode array to control the amount of saturation

Just did a limited test with program material bussed out of the DAW - sounding pretty punchy and pleasantly crunchy on the kick and the low-end in general.

Schematics for this are under the "Docs" section here:
15IPS Tape Saturation Colour Kit ? DIYRE

[TubeNoob]

The AMZ Muffer (clean boost / distortion) effect is breadboarded and almost 'there'.

Next up is soldering it and the Tape Saturation effects to circuit boards (x2 so they are stereo devices), then coming up with a good active mixer design with panning. Moving along...

[TubeNoob]

I have a transformer on the way (primary 230v, with two 18v secondaries which I'll tie together to make a dual supply).

Thinking to hook it up to the power source that previously went to the drive motor.


Here's the rub: I want to be able to power JUST the FX or both the FX and Amp. So my question is what's the best spot to put a kill switch on the Amp side?

Or, i can tap the AC input before the old power switch for the FX and do it that way.

Any suggestions?

[TubeNoob]

Hey Chuck - what was the story on changing the value of C11 (from .01 to .022) in your schematic for the tone-stack lift?
Does that let more upper mids through?

http://music-electronics-forum.com/a...idstuff1.0.png

[TubeNoob]

Ok, got my transformer in the mail. (230 primary, 2 x 18v positive secondaries). Also picked up a grounded power cable.

The AC comes direct to the power switch and from there we have black and red cables coming back to the xformer.

The strange thing is that the black goes directly to what looks like a ground lug on the top of the xformer. Doesn't this seem odd?

I'm probably going to tap the AC before the switch for the new xformer (for the FX) but how to best wire the new grounded cable in seems a little foggy at the moment...

[TubeNoob]

@ escherton:

If we're putting a switch on the mid control, why not have a 25k mid pot and a SPST lift control? That aught to give 'flexibility' to the tone stack

Would this 25k pot go in place of the 'normal' 15k resistor (perhaps in series with a small value resistor)?

[eschertron]

@ escherton:

Would this 25k pot go in place of the 'normal' 15k resistor (perhaps in series with a small value resistor)?

Yes.

Take a look at a 3-knob Fender tone stack. the mid control simply replaces the 15k resistor, with the wiper attached to whichever leg allows CW rotation to put full resistance into the circuit. My personal preference is to add that small series resistor you mention - say about 4.7k - to act as a default setting. Then the mid pot then can behave as a sort of 'boost' or 'drive' control, increasing the signal overall (but with boosted mids) to the next stage. There's many ways to mod a tone stack, so experiment and note your results. What works in a pleasing way for this amp may be a bit different than what has worked for others in their amps. But in general, the more resistance in that circuit element - mid resistor - up to infinity, the less attenuation there is in the circuit as a whole. Which means more 'gain' into the next stage, allowing for more grind. Not exactly the same thing as bypassing the tone stack entirely, but close.

[eschertron]

The strange thing is that the black goes directly to what looks like a ground lug on the top of the xformer. Doesn't this seem odd?

It does seem odd. The primary aught to be HV isolated from any 'frame' parts. Have you checked continuity/resistance?

I don't know about EU specs, but old-school US house wiring used black for neutral and red for hot in the fabric-wrapped cables I've come across. The wiring in the house where I'm currently living has aged to the point where the red wires were dingy and dusty enough that I thought they were a dirty white color. Surprised me when I first replaced a 2-conductor outlet with a grounded 3-conductor outlet. I learned my lesson, though, with no loss of life. (put blown edison-base fuse smiley thing here)

[TubeNoob]

Got you on the Mid-control; that's going in the soup!

I have only observed things visually; I have yet to pull the whole PT section out. But i am sure that the black side of the AC is going to a large 'ground - looking' lug on the top of the xformer. That's all i'm saying. It's weird.

I will try to get some photos together and also do some VM tests.

[TubeNoob]

Here's a quick photo of what I'm talking about...

[TimmyP1955]

The balance control is likely a log pot on one channel and a reverse log pot on the other channel, so this will need to be replaced. Unless: If you cascade the channels, the balance pot would be a distortion control with attenuation to keep the output level "kind of" the same at any distortion amount. You would also have pre-distortion and post-distortion EQ.

[TimmyP1955]

The balance control is likely a log pot on one channel and a reverse log pot on the other channel, so this will need to be replaced. Unless: If you cascade the channels, the balance pot would be a distortion control with attenuation to keep the output level "kind of" the same at any distortion amount. You would also have pre-distortion and post-distortion EQ.

[TubeNoob]

Hi TimmyP1955 - i am so tempted to keep adding mods but i think i need to draw the line; this thing is getting crazy!! :-)

Going to try to pull the PT section out today and have a closer look. Still bewildered by this black wire going to that lug on the top of the PT. Was hoping to start off with grounding the unit with the new power cable before going further.