Modded SCT2000/Nady TCM-1050/GT-2B Frequency Question

[mattrico]

Ok,
I just savagely desacrated the poor thing during the first COVID cockdown.

Kids DON’T try this (put a EF86 in the mic) at home, pins are NOT compatible with ECC83/12AX7.
I had to hack, cut traces/rewire alot.
But the mic sounds superb on ac. guitar and on vocals. I wish I had two.

First two pics are before/during mod.
Last four, are after the ugly deed.
M

That's awesome. You really went to town with that! Did you end up changing the capsule at all?
I noticed you removed c6 and c7, I've been reading about this and some people leave it in on purpose, so what's up with that? Dave Thomas says to remove it, but it seems some people don't, so I'm definitely curious about the benefits of doing so.

 

[Migs 31]

I noticed you removed c6 and c7, I've been reading about this

Just to be clear,
I completely changed the circuit in this mic. and recycled some of the parts and connections. By using an EF86 I had to cut traces and ground at least three pins on the Noval socket.

C6 & C7 are not problematic, I just removed them because I wanted to increase the value & put good quality Nichicon elec. caps there.

The problematic ones were C9 C10 which supposedly are there to filter, but they can also muddy the sound (according to D. Thomas and others) I didn’t need them in the circuit I was building, so I just cut them, it’s that simple.

No capsule change. My new circuit sounded just sweet with that capsule. First with a Mullard EF86 & then with the Telefunken (sounded really balanced).

M

PS: Oh ok, i just read what D.Thomas also says about C6 -C7. To remove them in order to improve Bass response & less distortion. I cannot comment on this. As I have not measured/tested

 

[mattrico]

PS: Oh ok, i just read what D.Thomas also says about C6 -C7. To remove them in order to improve Bass response & less distortion. I cannot comment on this. As I have not measured/tested

Yeah in one forum they said that removing them weakened the bass, and then D Thomas says that it improves bass response. Nice and contradictory, so I was hoping you'd be able to shed more light, but since you've redesigned the circuit it's a pointless question for me to ask!
After all the mods I've done, the only thing not working for me is the gain level, it's just too low. I compared it to the AA CM67SE and it's around db lower. So unless there's a better way to add extra gain (maybe wiring the tube in parallel?), I'm going to need to swap the transformer for a higher ratio one. Any thoughts?

 

[Migs 31]

.... unless there's a better way to add extra gain (maybe wiring the tube in parallel?), I'm going to need to swap the transformer for a higher ratio one. Any thoughts?

Ok,
have to be extremely careful with what is written on the Internet. Many people make dubious claims and some times outright lies. Better ask more experienced GDIY members like Gyraf, Ruud, Khron or Kingkorg when it comes to microphones, specially Valve/tube ones.

Again, careful with transformer assumptions. It would be wise to first measure the stock transfo “off circuit” understand what turns-ratio you have there and according to your circuit decide what transfo is better in this particular case. Plate voltage (HT) amongst other factors, will influence the final “volume” output of your (plate loaded) mic.

Make volume/EQ measurements before modding. Dip your balls in Steel or Titanium coating and connect a cheap China valve in parallel. Worst that can happen: you blow a cheap valve (but I really doubt it) then report back to us.

The EF86 based circuit sounds loud & clear, with high(er) plate voltage & stock transfo.
Apologies for these old screen shots below, I cannot find the link to AA etc.
Also worth reading this:

https://groupdiy.com/threads/grounding-the-cathode-in-a-tube-microphone.87674/page-3

M

 

[Khron]

Yeah in one forum they said that removing them weakened the bass, and then D Thomas says that it improves bass response. Nice and contradictory, so I was hoping you'd be able to shed more light, but since you've redesigned the circuit it's a pointless question for me to ask!

Ok,
have to be extremely careful with what is written on the Internet. Many people make dubious claims and some times outright lies. Better ask more experienced GDIY members like Gyraf, Ruud, Khron or Kingkorg when it comes to microphones, specially Valve/tube ones.

First of all, i'm assuming this (in the first post in the thread linked below) is the schematic with the correct / same component designators?

https://groupdiy.com/threads/apex-460-noise.53875/post-687821

If so, Dave's observation / warning is correct. The C6 / C7 bypass capacitor(s) in parallel with a common-cathode gain stage's cathode resistor do indeed increase gain (more output signal level, but less headroom until clipping), but also have a high-pass / low-cut filter effect, depending on the values of said capacitor(s). The DC voltage at the cathode is usually in the single-digits, and the AC voltage superimposed on it is virtually the same as the signal present at the tube's grid. As such, one can easily use low-voltage (16-25v is generous) high-value capacitors to push the cut-off frequency as low as one desires (just about).

https://www.valvewizard.co.uk/Common_Gain_Stage.pdf

Self-biasing (via the cathode resistor) starts at page 12; details about cathode capacitor sizing start at page 25.

The cut-off frequency can be easily determined with any online RC filter calculator, and should / will be in the single-digit Hz range. Whether that's even audible, and/or counts as "reduced bass", i'll leave to the [ahem] "golden ears" to determine. My math says otherwise...

 

[mattrico]

Ok,
have to be extremely careful with what is written on the Internet. Many people make dubious claims and some times outright lies. Better ask more experienced GDIY members like Gyraf, Ruud, Khron or Kingkorg when it comes to microphones, specially Valve/tube ones.

Again, careful with transformer assumptions. It would be wise to first measure the stock transfo “off circuit” understand what turns-ratio you have there and according to your circuit decide what transfo is better in this particular case. Plate voltage (HT) amongst other factors, will influence the final “volume” output of your (plate loaded) mic.

Make volume/EQ measurements before modding. Dip your balls in Steel or Titanium coating and connect a cheap China valve in parallel. Worst that can happen: you blow a cheap valve (but I really doubt it) then report back to us.

The EF86 based circuit sounds loud & clear, with high(er) plate voltage & stock transfo.
Apologies for these old screen shots below, I cannot find the link to AA etc.
Also worth reading this:

https://groupdiy.com/threads/grounding-the-cathode-in-a-tube-microphone.87674/page-3

M

I'm impressed you have all these screen shots. I remember reading that whole thread a bunch of times years ago. It sucks that it's not there anymore, even just to be able to reference and get ideas.
So it's a lower ratio transformer I'd need, not a higher one, I always get confused with that. But then you're saying it's best to measure it and figure out what I have first.
I did a recording session with it today, for a jingle with a female singer, and it sounded really good. Once the track is out I'll post up some snippets here so people can have a listen. It was really easy to get it to sit in the mix and didn't need a lot of EQ, I was quite impressed. I only have to add a bit more gain than the CM67SE so maybe it won't be an issue in the grand scheme of things. It sounds so good I'm inclined to not touch it again

 

[Migs 31]

I always get confused with that. But then I only have to add a bit more gain than the CM67SE so maybe it won't be an issue in the grand scheme of things. It sounds so good I'm inclined to not touch it again

Yes I read that thread a lot, too.
When I find interesting/pertinent Info, i take a screen shot & archive it. Nothing lasts forever, as we know

Dave on AA seems to say that a 6.5:1 transfo would be better!? He also uses a 12AY7 (but less gain than a 12AX7).

Thread carefully, one part/bit at the time. If you f**k up something, reverse, easy to undo. My advice.

M

 

[mattrico]

Thread carefully, one part/bit at the time. If you f**k up something, reverse, easy to undo. My advice.

M

That's what I thought, until I swapped the capsule on this one and got all these noises. Luckily I managed to get it working again. It definitely says a lot for cleaning and soldering properly, re-stripping wires, that sort of thing. I'm inclined to leave the transformer alone, as I've been doing all my voiceovers on it for the last few days and I'm finding that the gain level is very usable. I'll probably get in there and swap a few more caps as I know there are still gains to be had. Plus I'm also wanting to give a bit more attention to my Nude C12 with OPR OP12 (CK12 style) capsule in it, to see if I can bring that up a level or 2 in sound quality.

 

[mattrico]

So as an update, I decided that I should replace the C4 cap as well, and in the process found out that good caps in the right spec are hard to come by in Australia, so I ordered some from the Netherlands. I'm going to replace the c4 in this mic, and also do it in my Nude C12 (in which I've already swapped out the C8, removed C9 and C10, and put an EH 6072A instead of the garbage stock 12ax7).
I'm still undecided about removing C6 and C7 for both mics. Is it necessary to keep them if I decide to do the cathode follower remove? Or the "tube in parallel" thing? I've read so many pages about mods that I've had brain overload and I don't know what relates to what anymore!

 

[Migs 31]

Or the "tube in parallel" thing? I've read so many pages about mods that I've had brain overload and I don't know what relates to what anymore!

I dare you !!!
Make history: be the first Ozzy diy’er to put a 12ax7 in parallel (easy to undo ) and report back to GDIY. ( I suspect that Røde has allready done it in the Classic II). Test with a cheap China valve/tube first. Your plate resistor (is it R2?) might have to change though, IF I REMEMBER WELL ... don't quote me on that, ok.

BTW, caps from Taiwan or Japan are really good (and closer geographically) Nichicon, Panasonic etc

Mod overload is a curable disease.
I am clean (almost) for 3 yrs now.

https://groupdiy.com/threads/6072-in-parallel-dual-triode-mode.60886/

https://groupdiy.com/threads/t-bone...s-of-the-tube-in-parallel.46772/#post-1018830

M

 

[Migs 31]

Mod overload is a curable disease.
I am clean (almost) for 3 yrs now.

My fellow GDIY’ers I relapsed.
I hadn’t touched a soldering iron in three weeks straight. All it took was a couple of puffs of solder smoke and I knew I was in trouble again.

Paralleled a 12ax7 in an SCT-800. Similar to the SCT-2000, but smaller/Cardioid only, same basic topology. Had to change resistor R2 for 220K instead of the stock 100K. And cut C9-C10. So far the China 12ax7 has not exploded. Will try a 12AY7 and 6.5:1 transfo, once I come down.
M

EDIT: Note the 3K (R7) cathode resistor on this one, as opposed to 1K ( on SCT-2000 ).
It affects the Bias point for sure. I already put 1.8K (better) Try 1.2K-1K ?
Valve was running hot with 3K (R7).
Mic sounds definitely quiter to my old ears (note small ceramic cap between Anode-Grid)

 

[mattrico]

BTW, caps from Taiwan or Japan are really good (and closer geographically) Nichicon, Panasonic etc\

Funny you say that because I'm actually running Panasonic caps for C8 in the 2 that I've modded, and I ordered some Philips to put in place of C4.

And nice that you actually went ahead and did the tube mod, great job! I'm definitely curious to know how it compares to running the tube in its original configuration. If it's worth the extra mucking around to do it or not. I've got plenty of crappy chinese 12ax7s here that I can experiment with, and a 5:! transformer as well that I can drop in too. I'll just need to buy some resistors to be able to do that (or steal some from an old failed mic mod, but then I need to check all the values because I have no idea what's on there).