Hiwatt DR103 FX loop

[SoundCity85]

Hello,

I recently finished building a DR103 preamp into a Sound City 6x EL34 amplifier. It sounds huge, but there is still need for an FX loop. Especially for effects like reverb/echo/delay, it's the sound of the preamp that needs to run through the FX instead of the signal of the guitar. I attached the schematic. Where is the best position to install the loop, and what kind of loop would be preferable? I have one free triode available (V2B), but still room on the chassis for additional tubes. I'm not really familiar with FX loops and limited knowledge, but if I understood well a signal coming from a cathode follower would be a good spot for FX loop?

Best regards

Dominic

[tubenit]

https://store.metropoulos.net/collections/amp-mods/products/zero-loss-fx-loop-board-only

This may be of interest to you? 

That schematic looks like some variation of a "bootstrapped" cathode follower?  I am not sure I've ever seen that in a schematic before? And it looks like it may have some type of local negative feedback and/or a voltage divider. Unusual set up. Hopefully someone with more knowledge can explain it.

http://www.valvewizard.co.uk/dccf.html

I have no idea how to advise you on trying to use the CF with the unused triode as an active FX loop?   I probably would try the Metro FX loop after the master volume pot prior to the next triode's grid (V3-7)

With respect, Tubenit

[2deaf]

Here are a couple of ideas to kick around:

[2deaf]

That schematic looks like some variation of a "bootstrapped" cathode follower?  I am not sure I've ever seen that in a schematic before? And it looks like it may have some type of local negative feedback and/or a voltage divider. Unusual set up.

V3, pins 1, 2, 3 is just setting the DC voltage for the LTP grids.  The DC voltage at pin 6 is too high for direct coupling to the LTP grids because it would make the voltage drop across the tail resistor too large thereby decreasing the available output voltage swing of the LTP. 

[tubeswell]

Here's some more ideas (from Merlin Blencowe's 1st pre-amps book)

[PRR]

You want to figure a level-diagram so you don't smoke the inputs of your loop effects.

You want to define impedances low enough that it isn't fussy what effects you use.

[SoundCity85]

Thank you all for the feedback. I have some things to look into now. It seems on first sight that the FX loop directly after the treble pot is the easiest way to go.

[SoundCity85]

Here are a couple of ideas to kick around:

Hi 2deaf,

Thank you. Where did you obtain these schematics from?

Best regards Dominic

[2deaf]

Hi 2deaf,

Thank you. Where did you obtain these schematics from?

I created them myself.  Here is one with a bypass switch.

[SoundCity85]

Hi 2deaf,

Thank you. Where did you obtain these schematics from?

I created them myself.  Here is one with a bypass switch.

That looks great. How would you perceive the 10k pot? As an internal or external component?

[Latole]

v
That looks great. How would you perceive the 10k pot? As an internal or external component?

I must be external.

[SoundCity85]

Ok. So we have "in" and "out", a level control pot of 10k and a double on-on switch in the back to create a true bypass. I'll go for it

[2deaf]

That looks great. How would you perceive the 10k pot? As an internal or external component?

I would mount it so that it can be accessed externally just like the other control pots.

[Williamblake]

And so do i. And don't just say this just to get an even number of triodes.

[SoundCity85]

thank you for the advice. this forum is very supportive

a question popped in my mind when looking at the schematic. what would happen if you jumper send and return? will the V2 triode increase overall gain, or does it only recover what is lost in this loop?

[pdf64]

thank you for the advice. this forum is very supportive

a question popped in my mind when looking at the schematic. what would happen if you jumper send and return? will the V2 triode increase overall gain, or does it only recover what is lost in this loop?

That would be completely dependent on the fx send control setting.
I think it would be an improvement to insert a small resistor between the bottom of that pot and 0V. So as to avoid the signal being accidentally muted by an inadvertent knock of the send control.

[SoundCity85]

What would you consider small? 1k or something like that?

[2deaf]

a question popped in my mind when looking at the schematic. what would happen if you jumper send and return? will the V2 triode increase overall gain, or does it only recover what is lost in this loop?

The loop has a maximum gain of 1.3 from treble to master volume with a typical 12AX7.

[2deaf]

What would you consider small? 1k or something like that?

I wouldn't go any larger than that.  I originally designed it with a 2.2K, but then I decided that I had better build the DR103 stage with the tone stack just to see what signal levels were possible.  I was surprised by the peak voltages that made it through that stack.  I removed the 2.2K because it was possible to exceed 0.5V_p with the Send Level at minimum. 

[SoundCity85]

Ok, perhaps bring it down to 500 then? I only have a 10k log pot laying around. Would that also do the job?

[pdf64]

Ok, perhaps bring it down to 500 then? I only have a 10k log pot laying around. Would that also do the job?

They should be fine, but as it's a log pot, you may need to turn it up past halfway.

[SoundCity85]

Thanks. In the end found a linear 10k CTS that measures fine. By the way, what is the reasoning behind the 4M7 going to ground?

[pdf64]

It mitigates ‘pops’ when the bypass switch is flipped.
Without it, in bypass mode, both of the cap’s plates will charge up to the tube’s plate voltage. When the loop is re engaged, that charge will create a momentary voltage spike in the next stage’s grid circuit.

[SoundCity85]

All done. Testing is for tomorrow. Hopefully didn't make any mistake wiring the jack sockets, that's still difficult for me to grasp. Wasn't really sure how or where to ground, so I connected to my central ground buss after V2A and connected everything on that ground wire

[SoundCity85]

Works like a charm!

[Williamblake]

Here's some more ideas (from Merlin Blencowe's 1st pre-amps book)

The one at the left on the bottom looks convenient. I wonder how the recovery stage is biased without a grid resistor?

[pdf64]

Here's some more ideas (from Merlin Blencowe's 1st pre-amps book)

The one at the left on the bottom looks convenient. I wonder how the recovery stage is biased without a grid resistor?

The dc path between grid and cathode goes via 1M feedback resistor, load resistor, 1k5 cathode resistor.

[Williamblake]

Thanks for pointing that out. So the load would be 220k if you put this before the master volume in a DR103?

[pdf64]

Thanks for pointing that out. So the load would be 220k if you put this before the master volume in a DR103?

Sorry, yes, thanks for pointing that out!

[Williamblake]

Yet another question: the send has a 100uF capacitor or did i read that wrong? Would a 70V polarized Frolyt  cap do? Dont other effect loops use 1 to 10uF? I dont mind big caps but this is an odd value, isnt it?

[sluckey]

It's a cathode bypass cap. 25 volt rating is fine.

[Williamblake]

Sorry, i should have marked it in the picture right away. 100uF is a lot for a non polarized cap?

[sluckey]

Sorry, i should have marked it in the picture right away. 100uF is a lot for a non polarized cap?

That 100µF is also a polarized electrolytic. But since it's connected to a cathode follower it will need to be a higher voltage rating.

[Williamblake]

Thanks.  As this cap is seeing one fifth of the cathode voltage i assume a 70V cap will do fine.

[pdf64]

100uF seems way higher than necessary to feed a 10k load from a roughly 8k source impedance - may be an error?
I think about 1uF would be more appropriate, with the benefit that a film type cap could be used.

The rule of thumb I remember reading in a vintage electronics mag is that with an audio amp using RC coupling between several successive stages, for minimal bass rolloff, each interstage C x R should equal at least 10, where C is uF and R is kohms. That works out to a corner freq of about 16Hz.

[tubeswell]

100uF is in Merlin’s book. He recommends 63V for that part.


10 or 22uF will be fine.

[Williamblake]

Thanks for checking on this, unfortunately i cant find my copy right now.

[wavley]

Sorry to dig this one up.

I like the version of this that's simple basically interrupting the signal path, I also see a version around with the unused triode as a makeup gain stage.  I usually use that triode as a plate mixer.  I never really use effects loops and would like to keep my knob and switch count down. 

Does this amp really even need make up gain? How has the super simple version worked for the folks that tried it.  Would I be better off ditching my plate mixer in favor using that triode's cathode to drive my send?

Thanks for your input.