12ax7 mic. preamp overdriving itself

[Toxophilite]

The other half of the Akai conversion I'm doing(now that I have the Ef86 pre mostly sorted(thanks) is based on a 12ax7It also uses a shure M67 input transformer.

The problem I'm seeing is that it will overdrive itself before it overdrives the inputs of my UAD apollo 11So whilst I can record a clean signal, it is not very loud. For a regular not too loud vocal this happens between 4-5 on VR1 At this point I am only halfway to distorting on my Apollo 11 unit. It would be nice if I could get a louder clean signal at the Apollo 11 unit, so I can meter there and not the U meter on the preIf I turn up the pre louder it just starts distortion and I get a distorted flattened signal reminiscent of Skeletor

Coincidentally or not, this distortion happens when the VU meter of the preamp bounces into the red. However this VU meter is actually powered by the output transformer and power amp.

Originally the schematic had a connection at the preamp output which grounded the input of the power amp when a patch chord was plugged into the preamp out., however this also turns off the VU meter which frankly is no fun!, also i jumpered this on mine and it made no difference ot the distortion issue, except to turn off the fun VU meter.

Any ideas what's overdriving here? Is this another output impedance mismatch ?

v1 plate is 93V
cathode is .923V

v2 plate is 126V
cathode is 1.19V

[Toxophilite]

help!

[shooter]

got a scope?
is the signal feeding the 1st stage "too big" ?
is the 1st stage distorting before it gets to the pentode?  (sound "clean" at the pre out jack?)


working audio signal paths require more than a multimeter and a speaker out, ya need to see the whole paths, what each stage is doing to that signal, hence a scope

[Toxophilite]

I will dig out the scope and reacquaint myself with it's usage. I will also look for a signal generator app..which means I need to drag my board, and many cables up here. Where I set up shop to work on these is very short on space but the room... with room, is COLD!


Right now I'm not  even using the power amp. Just the signal from the preamp out and that's where I'm getting the distorting. It's like the signal from V2a ends up being too strong for V2b.


 This is an Akai/Roberts conversion and the power amp is largely redundant, except for driving the VU meter

[shooter]

It's like the signal from V2a ends up being too strong for V2b.

if that's the case, throw in a lower gain tube "for testing"  I use AU7's  They usually need some tweaks to re-bias from the AX7, but for testing it should work fine, believe AU7's are "better liked" in audio, where AX7 is more better for guitar

[pdf64]

The other half of the Akai conversion I'm doing(now that I have the Ef86 pre mostly sorted(thanks) is based on a 12ax7It also uses a shure M67 input transformer.

The problem I'm seeing is that it will overdrive itself before it overdrives the inputs of my UAD apollo 11So whilst I can record a clean signal, it is not very loud. For a regular not too loud vocal this happens between 4-5 on VR1 At this point I am only halfway to distorting on my Apollo 11 unit. It would be nice if I could get a louder clean signal at the Apollo 11 unit, so I can meter there and not the U meter on the preIf I turn up the pre louder it just starts distortion and I get a distorted flattened signal reminiscent of Skeletor

Coincidentally or not, this distortion happens when the VU meter of the preamp bounces into the red. However this VU meter is actually powered by the output transformer and power amp.

Originally the schematic had a connection at the preamp output which grounded the input of the power amp when a patch chord was plugged into the preamp out., however this also turns off the VU meter which frankly is no fun!, also i jumpered this on mine and it made no difference ot the distortion issue, except to turn off the fun VU meter.

Any ideas what's overdriving here? Is this another output impedance mismatch ?

v1 plate is 93V
cathode is .923V

v2 plate is 126V
cathode is 1.19V

I’m having trouble making sense of the above. Please provide a flowchart showing your signal chain.
I think the J5 output in the schematic seems to be somewhat conceptually wonky, as it looks like it’ll be the NFB loop error signal.
That’s perhaps why the power amp input is supposed to be muted if J5 is used, as it breaks the NFB loop.
Why not use the J4 signal?

[Toxophilite]

I'm still figuring out the scope. I have an A440 going into the preamp and I'm measuring right after the 500k pot and the signal looks like a nice round sign. At the preamp output it's a much smaller cranked looking angular thing that doesn't get any bigger when you turn up the pre

[Toxophilite]

The other half of the Akai conversion I'm doing(now that I have the Ef86 pre mostly sorted(thanks) is based on a 12ax7It also uses a shure M67 input transformer.

The problem I'm seeing is that it will overdrive itself before it overdrives the inputs of my UAD apollo 11So whilst I can record a clean signal, it is not very loud. For a regular not too loud vocal this happens between 4-5 on VR1 At this point I am only halfway to distorting on my Apollo 11 unit. It would be nice if I could get a louder clean signal at the Apollo 11 unit, so I can meter there and not the U meter on the preIf I turn up the pre louder it just starts distortion and I get a distorted flattened signal reminiscent of Skeletor

Coincidentally or not, this distortion happens when the VU meter of the preamp bounces into the red. However this VU meter is actually powered by the output transformer and power amp.

Originally the schematic had a connection at the preamp output which grounded the input of the power amp when a patch chord was plugged into the preamp out., however this also turns off the VU meter which frankly is no fun!, also i jumpered this on mine and it made no difference ot the distortion issue, except to turn off the fun VU meter.

Any ideas what's overdriving here? Is this another output impedance mismatch ?

v1 plate is 93V
cathode is .923V

v2 plate is 126V
cathode is 1.19V

I’m having trouble making sense of the above. Please provide a flowchart showing your signal chain.
I think the J5 output in the schematic seems to be somewhat conceptually wonky, as it looks like it’ll be the NFB loop error signal.
Why not use the J4 signal?

input
input transformer
 out of input transformer into grid of V2a
out of plate V2a into 500k pot
out of 500k pot into grid of V2b
out of V2b cathode to preamp output jack




THe J4 output is a speaker output and I really think it would probably roast most recording gear as it's meant to drive an 8 ohms speaker with 3-5 watts. I really should've ,left that part of the schematic out as I'm only passing signal through the preamp. I think I left it in in case the power amp was in someway responsible for the distortion

[Toxophilite]

It's like the signal from V2a ends up being too strong for V2b.

if that's the case, throw in a lower gain tube "for testing"  I use AU7's  They usually need some tweaks to re-bias from the AX7, but for testing it should work fine, believe AU7's are "better liked" in audio, where AX7 is more better for guitar

I put a 12DW7 which has the 12au7 side before the 500K pot and the 12ax7 after, same results. It's like V2b can't handle what V2a is putting out and damed if I know why

[Toxophilite]

Since I don't know enough to figure out what's going o here perhaps I should start partly anew, like this maybe

[Toxophilite]

Small angular line is the output from the cathode of V2b big angular line is taken from the plate of V2b. Nice smooth sine wave is from the grid of V2b.
If you turn up the 500K volume the nice smooth sine wave gets really big and stays nice and smooth while the angualr shape gets somewhat bigger and more angular

[pdf64]

The J5 output in the schematic will be the NFB loop error signal.
That’s perhaps why the power amp input is supposed to be muted if the J5 output is used, as it breaks the NFB loop.
My guess is that your mod (removing the mute) has caused your issue. 

[Toxophilite]

I really wish that were the case but I've tried this via the muting as some of these were wired and by simply removing the 6BQ5 . In fact those sillyscope readings are with 6BQ5 removed.

No difference in the results either sonically or visually.


These units (which might be a Roberts 770) actually didn't have the mute wired up at all, so I didn't modify them at all in that regard.

[BrianS]

C9 in good shape?  No DC on the grid of the second stage?

[Toxophilite]

I'll check it. It's actually brand new, a .047 orange drop. I forgot to update the schematic. Everything else is right though


Checked, no DC

[Toxophilite]

Well as I was not achieving much in my limited trouble shooting I tried rebuilding the pre as a NYD one bottle preamp in a temporary sprt of way to try it. Much better results in my opinion. Interestingly my 100kA pot gain pot didn't seem to be able to turn the signal all the way off but it was an old pot dug out of my supplies and some of the other components were approximates though very close.  I tried it by itself and out of curiousity I tied it back into the power amp to see if it moved the meter and that worked too. Tempted to just build the 12Ax7 part of the pre as this circuit as I'm better at building anew than modifying and trouble shooting a problem wth the circuit design.


I have one question though. There's a 3.3 uf cap on the output, that's not an electrolytic. Voltage at the plate could be up to 190+ volts and non electrolytic caps at the local electronic supply store seem to be rated at 100V, could I get away with a high voltage electrolytic in this application ? (that 's actually what I used in my test of this circuit because that's what I had

[PRR]

> no electrolytic caps at the local electronic supply store seem to be rated

Maybe nobody else in your local does cap-coupled tube amps? Shop at a specialist:
https://www.parts-express.com/speaker-components/crossover-components/crossover-capacitors/metalized-polypropylene-crossover-capacitors/capacitance/3.3%20uF/voltagerating/400

[Toxophilite]

Grrrr typo, tat was supposed to be NON electrolytics caps. They have plenty of high voltage electrolytic caps.


Interestingly this place sells lot's of tubes, as well as all the high voltage caps you see in guitar amps, including axial electrolytics which some insist upon. Maybe I'll try giving them a call and see if it's just their site.


So i can't use an electrolytic in that spot?

[pdf64]

Electrolytic cap tend to have far higher leakage current than film etc.
So they don’t tend to get used for RC coupling purposes when the R term is large, as there will be V DC across it.