Wem Custom Copicat (tube version)

[thomasdf]

I will check these resistors and wiring, but hum is not my main concern. It's not that noisy, there's hiss of course, but nothing to worry about for now on. My concern is the super distorted echo signal.

So I hooked up my scope with the probe in x10 mode, the AC at 6BR8 pin 2 is approx 30V (so it's 300VAC / 600VAC, dunno), at a 55KHz freq, that's the lower I was able to adjust the bias with the coil winding. It's supposed to be 44Khz but I can't go down to that. I have no idea if this is causing my problem ?

 

[Tubetec]

Theres other reasons you might have poor sound , what about tape tension ? alignment visually as the tape moves across the heads
The rollers often develop a flutter when there worn out.

Did you degauss the tape path ?

 

[thomasdf]

Tape tension seems about right, there's the arm with the spring, the wheels turn super freely and the tape is turning smoothly at a pretty regular speed... head alignement, I can't tell but it seems correct too, at least visually. Here again, flutter / wow doesn't bother me, it's really that saturated echo ...
I've degaused the whole tape path, not much of a change...

If I lower my input gain, the echo seems clearer, but then the output of the copicat is super low, and the echo is MUCH louder than the dry signal and the output is drowning in hiss and noise. If I want to achieve a correct dry/wet signal output, the echo volume is almost turned all the way down.

I am a bit surprised by the two 1M resistors after the preamp tube ... They drop a whole lot of dry signal, dont they ? If I want a correct signal/noise ratio on the output of the unit, I turn my preamp almost all the way up ... maybe that's the reason of my distorted echo after all ? A previous owner has carved (yes!) level marks on the front panel of my unit, and the marks are between 1/4 and 1/3 of the input gain...

How do you use yours ? Cranked all the way ?

 

[Tubetec]

First off,
What input are you trying to drive with the echo output?
You want Hi-Z , so either instrument input of your audio interface , active DI or tube amp input stage

Typically I want just delay at the output , never a mix of wet and dry ,
theres an easy way to tap the echo output , the footswitch jack provides the wet only output ,
again has to be a HI-Z input stage or the signal will vanish into noise and distortion.

 

[thomasdf]

At first I wanted to use it in a mixing situation on a board or converters input, so I added a switch to cut the dry signal right before it mixes with the wet signal, before the output transistor. It never work properly because for some reason when the dry signal is cut the echo signal is even dirtier as the background noise / hiss makes a huge jump up in level ( ?! )
Back then I also had tried driving it with a reamp box, and output to a DI. Not much better...

So right now, I've put this line level idea aside and I'm trying to have a working unit the way it was intended in the first place, the input drived by a Fender Strat, and the output going to a guitar amplifier (Pignose 7-100 on my bench, sounding great even with the most difficult fuzzes and all). I can try DI it or feeding a tube input straight but I doubt the result will be much different.
The dry signal is a big "soft" and "not punchy" at the moment, but it doesn't sound like an impedance mismatch at all. So I suppose my distorted echo comes from an electronic problem or me trying to use the unit at the wrong signal level rather.

The more I think about it, the more it feels like reducing the value of the two 1M resistors might give me a lot more dry output, allow me to keep the gain low and maybe an easier blend between clean and wet at the later stage ?

 

[Tubetec]

Its all in the way you use it ,
back way off on the input signal to get the input controls into a more usable part of their range . line level is likely way too hot .
The switch you added sounds like its not functioning as intended , its not needed to access the echo only output in any case ,that appears at the footswitch connection.

 

[thomasdf]

I've made some progress. I had been misleaded by some weird mods performed by a previous owner. I've rebuild to output section from the ground up around the transistor, used the original Ge Transistor too, and now the echo is cleaner. The thing is the balance between wet and dry is much more usable so I don't have to crank the input that much, which means the echo is cleaner.
Now my concern seems much more familiar : noise. Now that the overall level is lower, I have quite some noise and hiss, so I'll try using shielded cables here and there, and maybe cleaning up the power supply a bit.
Also, the preamp is a bit thin, it cuts a little too much bass for my taste on the dry signal. Echo is super filtered but that's not a problem I suppose. When I crank the preamp input and roll the guitar knob back it gets better, ballsier / warmer, but that's not how I'd like to use it.

I think I can play with the values of C1 and C2 (0.05uF) to get more low end out of the preamp, right ? Maybe 0.01uF instead ?

Edit : replaced C1 with 0.01uF, it seems to help but its very subtle, I don't know if its in my head or not The big chance I did was replacing the output capacitor, swapped the 0.01 for a 0.1uF and now I have way more bass, much rounder sounding unit, it's a delight.
Still some noise I'll need to take care of, and maybe tweak the quality of the echo a little bit, but now the single tube preamp is sounding gorgeous ! Very different animal.

 

[Tubetec]

The long wires at the end of diagram are the ones that benefit from screening most , I tied the screen to 0V at one end only .
As I removed the sources of noise I was able to open up the bandwidth of the recovery amp more ,
In the end I got rid of the bias trap 47k+0.005uf and all the extra capacitance of the switch bank loading the heads and output stage ,
much brighter more present echo as a result , its easy to apply a LPF in subsequent equipment if you want low fi again

Bear in mind , if you make the coupling caps bigger in the preamps it will mean theres more bass (and hum ) to tape also .
If you want to increase the bass you need to make the cap larger not smaller !

 

[Tubetec]

I ended up adding an extra stage of RC filtering on the HT line before the 6BR8 , that cleaned up much of the hum going to tape from the bias osc stage , a little less hum from the preamps due to the better filtering also .

 

[Tubetec]

Just read your edit ,
You have added bass to the echo output and made the straight through sound even thinner
By chopping and changing values of coupling cap your also upseting the the record and playback LF EQ curves in the tape path ,
I suggest not worrying so much about tone until you have noise well under control .

Its a bit of a shame they never rectified the noise issues in these units , with a few simple tweaks Ive been able to banish any trace of hum and the tape paths fidelity is massively improved ,

bridge rectifier ,
extra RC filter in the HT supply ,
screened cable between Hi-z areas ,
removal of LP filters on the replay amp .
Series pots rhesostat connected in parrallel with each of the heads instead of switches ,

I might try incorporating 3 dual gang pots , now each head has independant volume and passive tone cut , gives an extra dimensionality to the echoes .

 

[thomasdf]

Back then I suppose everything was drowned in noise so I suppose it didnt make much of a difference ? Or maybe components have drifted and it wasn't so noisy back then.

Anyways, I came went the the stock 0.05uF C1 and C2 value, but kept 0.1uF on the output <3

Now I need to see what I can shield, reroute etc to get a better signal / noise ratio. Tubetec, what version do you have ? If that's the same as in your schematic, it's different than mine. Mine is a MK3, it's almost the same but with a Germanium transistor output buffer / stage.
As far as noise is concerned, mine has a "D#" note / sine / ghost going on, from the motor obviously. It's not super loud but it's still a little bit annoying