JM Signature recording preamp

[Mat Cortex]

Hi,

I've build a recording/practicing amp as my first build. I didn't desing anything, i just mash-up existing design. So it's a 2R JM Sig preamp with a WFC (white cathode follower). A Big thanks to Dave O. for the PS design.

it'works so far but i've a reverb problem: it's noizy (hum/buzz) and pass a certain level it squeal.

i've test this preamp with headphone and it has plenty of output. When the output is set very high, the whole preamp start to squeal as if the WFC was overloaded or something like that.

Voltage seems ok. I swap V2 V3 and V5 with no difference.
Maybe the tube are too close each other and v5 too close to the OT ?
As a newbie my first error was to buy tube socket that don't allow tube shielding. It may have solve my problem if i could shield them. I swap the reverb tank with another amp and still hum buzz squeal. and both reverb tanks do sounded great through my vintage Gibson falcon amp.
The reverb transformer is correctly wired as the EDCOR datasheet recommendations.

any idea what can cause the reverb problem?

Please excuse my writting, english is not my first language

Best regards
Mat

[10thTx]

Try this ...............

Plug a cord into the FX send jack, then plug a cord into another amps FX return jack.

Does that sound OK? And not have hum or squeal? IF so, then maybe the problem is the Edcor 10k/600 trannie into the XLR jack.

The reason I say that is I built a PreAmp for my son-in-law using that Edcor 10k/600 & for some reason it worked with my power amp but did not work with my son-in-law's PA system. He had unwanted noise and hum.

The solution was to use a direct box instead of the Edcor output trannie.

With respect, 10thtx

[Mat Cortex]

thanks a lot 10thtx
i'll try it out.
I do not have yet a recording interface or a PA. My main goal was to use the preamp with headphone for late night practicing. I'll try to use the fx send to another amp and/or the line level output of the XSM10k/600 to a SolidState amp.

I was wondering if the reverb transformer could be accidently reverse winded. Mine is wires according to the EDCOR datasheet. Do i risk anything by trying to swap both output wires ?

Regards
Mat

EDIT: reverse winded and not reverse wired

[JazzGuitarGimp]

Notice the note on the WCF schematic that says: "11K Ohms if R1 = 600 Ohms". Did you install the 600R resistor? It belongs on the output side of the trandformer, between pins 2 & 3 of the XLR connector (where it says "to other circuitry".

[Mat Cortex]

Notice the note on the WCF schematic that says: "11K Ohms if R1 = 600 Ohms". Did you install the 600R resistor?

No I didn't !!!
I just forgot about this note because R1 wasn't shown on the shematic.
I'vet build the output as the guy who made the visio-alembic f2b preamp. ( see attached). The preamp is on my bench, so i'm up anything to make it work correctly. Worth a try ?

Someone already mentionned to put a ground lift on the output, i've not done this yet but it's on my "to do list". Not usefull for headphone use.

You've just made me notice that i may install C25 eletrolytic (my schem.) the wrong way !

Thanks a lot.
Mat

[JazzGuitarGimp]

In order to properly load the CF, there should be the equivalent of a 600R load on the final output. So, if you're going into a power amp with say, a 100K input impedance, than the 600R in parallel is in order. However, if you're driving a 35R headphone load, then a 565R resistor in series with the headphone should be used.

That part of your circuit (the WCF) came from the LA2A compressor of the late 50's / early 60's. IIRC, back then, the input and output connections on pro audio gear was a screw terminal strip on the back panel of the unit - and, again, IIRC, it was understood that if you were driving a 600R balanced output into a high impedance balanced input, that you would add a 600R resistor on the terminal strip along with your cabling. So R1 may have been an optional kind of thing. Back then, being an "audio engineer" usually also meant being an electrical engineer. So this kind of stuff (adding load resistors to the back of the unit) was standard operating procedure, and pretty much taken for granted.

All that being said, I did some quick research on the net for schematics of the LA2A, and on at least one of them, I found R1 to be a 100K gain pot. So...... who knows where the R1 reference came from on the Jensen transformer drawing you shared?

Also, there is a off-page net connection on your schematic that says "To feedback circuit". In the LA2A, this signal is fed back into the audio patch a few stages before the WCF, which in-effect will decrease the overall gain of the entire circuit. Your self-oscillation could just be a result of having more gain than you need. I also came across a documented LA2A clone build where the builder subbed-in 12AY7's for the 12AX7's to "tame the gain" a bit - he also claimed that 12AY7's sounded better to him than 12AX7's. Tempered with subjectivity, of course, YMMV, to each his own, etc ad nauseam....

[Mat Cortex]

wow, how did you know that ?

Since i have two output in parallel ( XLR and 1/4 ), you're suggesting that i put a resistor in series with both channel(left/right) of the 1/4 jack. Maybe put a switch to choose wich output will be used ? So i can have both output: 1/4 for headphone use and XLR for recording or PA use.

I've tryied to drive only the WCF with my loopstation pedal throught the fx return. The voltage sended from the loopstation wasn't high enough to make the output stage squeal but it was loud enough to ear clearly trought the headphones. i'd say about conversation level. Dead quiet what ever the knob setting are. That's obvious but i had to try.

I've also notice that when a guitar is pugged in, noise level went down. When flipping the TMB switch, the squeal are louder or the gain pot gets less adjustability before squealing and reverb was squealing as before.

C25 was indeed install backward, but it changes nothings.

Is there a way to feedback the output throught my preamp so it can tame the gain, or is it just a better option to use lower gain tube?

EDIT: After a long thinking, The output impedance is 600 ohms and the headphone is only 35. I thought that impedance mismatch was fine with headphone application. Basicly the resistor in serie on the jack will be to lower the gap between this mismatch ?

Do you think that i should change the MV to 500K ? With the gain maxed, the MV has never gone higher than 10 o'clock. On full power, it could damage my ears.

thanks for your help, both !
Best regards
Mat

[JazzGuitarGimp]

Mat, now that I am at my desktop computer, I can actually read your build schematic - couldn't do it on the iPad. Is there a schematic of the TR JM sig floating around here for comparison? I'm curious as to how closely your build follows the donor amp. I'm thinking a 1Meg FX loop output impedance seems really high. Well, I guess it's 1Meg || 207K || 68K, but.... it still looks high to me.

[JazzGuitarGimp]

Yes, a switch for to select between 1/4" and XLR output would be useful. Have you tried connecting the XLR output to a recording or PA rig yet? If so, were you able to get a good clean tone out of it with enought volume and no oscillation?

[Mat Cortex]

firts, I taked the schem floatting around .Wich was report to works well. I change nothing on the preamp section. (see attached)

Yes, a switch for to select between 1/4" and XLR output would be useful. Have you tried connecting the XLR output to a recording or PA rig yet? If so, were you able to get a good clean tone out of it with enought volume and no oscillation?

I didn't have a PA to plug to. I've done all my test with headphones (1/4). I was able to have a good clean tone even some "clean sustain" or a very small breakup when digging hard. I'm able to have both RVB SEND and RETURN pot at 10-11 o'clock before it start to squeal. Volume/gain at 3 o'clock and MV at 9 . I can turn the MV much higher before the amp start to squeal but it's way to loud !

Could those squeals being cause by a ground loop via the star ground ?
Squeals happen at different value( little less or more) on the volume gain depending on tone stack setting. AS the reverb goes up the more trebly it get.

Thanks
Mat

[10thTx]

Can you plug your 1/4" headphone plug into the 1/4" FX send jack? And then remove V5 tube.

Any problems with oscillations or hum with that? IF not, then the Edcor trannie going into the XLR could be a problem?

With respect, 10thtx

[dorrisant]

Maybe you could reverse EITHER your two primary wires OR your two secondary wires on your reverb TX... This would reverse the phase of the reverb and maybe take the squeel out of it.

Don't ask me how I know.

Tony

[Mat Cortex]

Can you plug your 1/4" headphone plug into the 1/4" FX send jack? And then remove V5 tube.

Any problems with oscillations or hum with that? IF not, then the Edcor trannie going into the XLR could be a problem?

With respect, 10thtx

Wow, it works... signal is weak but it sounded great. No squeal at all. a little noisy on the reverb but i could dial a big lush reverb sound....

Now we can say that the problem is in the output section for sure.

My actual WCF circuit ( when populating the board) has been tested throught a hammond 125 OT and a 12" speaker with great result.

That's only left the EDCOR 10k/600 to cause the squeal .... mismatched impedance as JazzGuitarGimp mention ?

dorrisant: thanks for the help but after the 10thtx test, reverb do sound great.

Regards
Mat

[dorrisant]

dorrisant: thanks for the help but after the 10thtx test, reverb do sound great.

No worries... Glad you got an answer.

Tony

[10thTx]

That's only left the EDCOR 10k/600 to cause the squeal

Glad that was a useful experiment/test. Next step is to remove (or bypass) the Edcor trannie and XLR and install a 1/4" jack and plug into a direct box.

That's what happened with the guitar PreAmp project I did.

My son-in-law removed the Edcor trannie and simply used a direct box velcroed to the top of the amp chassis as in the previous illustration I posted in my first response.

In my thinking the Edcor trannie and the direct box are sort of the same kind of thing.

with respect, 10thtx