Help With Tube Preamp

[jsefer]

Hello All, I have just finished a project, I transplanted the Akai M8 mono blocks into a 3u enclosure, and modified them to take in 4 mic-inputs. Attached is the schematic. 

Referring to the schematic I made a few adjustments.:

1. I removed the power section [everything enclosed in red] so you can disregard that.
2. I changed the 500k R10 resistor to a 10k to match my input trasx
3. C9 and C11 were replaced w PIO .1uf caps
4. C10 and C2 were replaced w 100uf/25v
5. R13 was changed to 100k to stabilize voltage.
6. I split the sections so they get power independently from the power-supply and are not in series.

My problem is this: I have 2 monoblocks, one of them works perfectly. The other only the EF86 stage works with out faults. The 12ax7 stage, something strange is happening.

As soon as I power it up, the Gain wich is VR1 500k on the 12ax7 stage, does not seem to have effect when turned, and the output volume has little effect. Also, there is only sound for about 5 seconds then it fades away.

I already checked all my wiring, and compared it the working unit and they seem to be exact. Can anyone help me try and give me some places to try and check to see if I can get this to work.

I appreciate anyones time that can help me try and get this working.

 

[jsefer]

Here is a sneak peek of the front panel.

 

[lassoharp]

I changed the 500k R10 resistor to a 10k to match my input trasx

  If by match you mean match the rated Z on the secondary - 150:10K loaded with 10K will reflect back 150r - a heavy load -which will make some mics unhappy.  You generally want to load much higher.  The original 500K or 100K would be a better choice.

I split the sections so they get power independently from the power-supply and are not in series.

Will need to show exact changes on schematic.  There are any number of things that could be happening with a rearranged power supply that could be causing problems.  Too much for people to guess here.


I would suggest starting by redrawing your rearranged PS schematic.

 

[jsefer]

I changed the 500k R10 resistor to a 10k to match my input trasx

  If by match you mean match the rated Z on the secondary - 150:10K loaded with 10K will reflect back 150r - a heavy load -which will make some mics unhappy.  You generally want to load much higher.  The original 500K or 100K would be a better choice.

I split the sections so they get power independently from the power-supply and are not in series.

Will need to show exact changes on schematic.  There are any number of things that could be happening with a rearranged power supply that could be causing problems.  Too much for people to guess here.


I would suggest starting by redrawing your rearranged PS schematic.

What I cant be explained is that I have two of these units and one works perfectly and one only the EF86 section works. I have uploaded the schematic of exactly how i have it wired, except showing the 500k load resistor. I was recomended 10k by dave at cinemag, should I definately increase it to 100k??

The input tranx is a CMLI 15/15b
http://cinemag.biz/line_input/CMLI-15-15B.pdf


Even when I unplug the ef86 section from power the condition still exists... Im stumped..... could it be a component? A bad resistor or cap?

The one that works sounds damn great...

 

[lassoharp]

If Cinemag recommended loading their iron with 10K I'm not going to question that, though I would normally guess 100K.  Your post said you changed the resistor terminating the Shure transformer  "R10, 500K"  to 10K.


Ok, you have one channel with working AX7 stage, one channel with non working AX7 stage.  Have you taken a set of voltage measurements (plate and cathode) for each channel yet?

 

[jsefer]

My apologies, I forgot to mention I am using a cinemag transformer, cinemag advised I load it with atleast 10k, so I started with that, might changed it depending on the sound I get/want.


Voltages of the good unit
12ax7 heaters [AC]
6.5v
Plate
166.5
Cathode
1.491v[@ R11] / 1.75 @ R14

Voltages of the bad unit
12ax7 heaters [AC]
6.5v
Plate
172.4
Cathode
1.501 [@ R11] / 2.35 @ R14

One thing i just noticed, at R15 wich is pin 6 of the second stage, on the bad unit, when I turn off the unit the voltage goes from 128v to about 240+ then lowers, and as soon as I turn it back on it shoots to 340v then steadily lowers back to 128v

On the good unit the same pin its stable at 191v when I turn the unit off, the voltage drops. Turn it back on voltage shoots to 205, and never passes that then steadily drops to 191 operating...

 

[pucho812]

might be a dumb question but you never know "are you sure you have good tubes?"

 

[jsefer]

might be a dumb question but you never know "are you sure you have good tubes?"

I have 2 ax7 tubes tried both in the good unit and both work..... they are cheap chinese, dont want to stick in the good ones just yet...

Edit:

Just want to add that all resistors are original from unit, old FOX resistors, probably 50yrs old, Im leaning to think i might have a bad resistor in there.....

 

[lassoharp]

The resting voltages for the 1st sections look normal.  The 2nd section plate voltage of the bad channel seems a little low and on/off behavior odd.  Check/replace C30, It's electrolytic and old and may be leaky.  Also try quick swapping a different tube in there. 

Are you able to document voltage wise what happens when the sound fades away after 5 sec?  I'm assuming what you described was that with power on, you get sound for 5 sec and then it goes away (power still on).  And does it do this consistently?

Q: Is R20 (5K) still in circuit as shown?  Have the 6BQ5 plate and screen connections been fully removed?

edit - if you don't have a 25uf cap handy you can stick a PIO 0.1 or any film cap in there temporarily just to see if it's a cap related problem

 

[jsefer]

The resting voltages for the 1st sections look normal.  The 2nd section plate voltage of the bad channel seems a little low and on/off behavior odd.  Check/replace C30, It's electrolytic and old and may be leaky.  Also try quick swapping a different tube in there. 

Well actually according to my readings the Plate on the good unit is 10volts lower than on the good unit... what is strage to me on the bad unit, the second section cathode is 2.3 volts. thats 1volt higher than on the good section...

Are you able to document voltage wise what happens when the sound fades away after 5 sec?  I'm assuming what you described was that with power on, you get sound for 5 sec and then it goes away (power still on).  And does it do this consistently?

Q: Is R20 (5K) still in circuit as shown?  Have the 6BQ5 plate and screen connections been fully removed?

edit - if you don't have a 25uf cap handy you can stick a PIO 0.1 or any film cap in there temporarily just to see if it's a cap related problem

The caps are all brand new, as I stated the coupling caps are 25v 100uf sprauge, I actually used 4 25uf in parallel.

I would highly doubt is a cap issue but can certainty try. these are brand new caps...

All the stuff I didnt use was completely removed. I practically rebuilt the amp section...

And the behavior of it going of and on, I can replicate it, almost all the time, just have to turn off the power, wait then turn it back on and the sound is present then it fades with a crackling in the background, sort of as I i had turned off the amp and sound was still crackling through.

As for R20 I left that in there, when I had a chance to talk to one of the members on tapeop who was amongst the first to try this out, he said I wouldnt hurt to leave it in there to tame the voltages....

 

[lassoharp]

And the behavior of it going of and on, I can replicate it, almost all the time, just have to turn off the power, wait then turn it back on and the sound is present then it fades with a crackling in the background, sort of as I i had turned off the amp and sound was still crackling through.

Tube swap give same results? 

Crackling sound on it's own could be pointing to loose connection/solder joint problem.  Prodding with a chopstick will reveal those pretty quickly.

I would highly doubt is a cap issue but can certainty try. these are brand new caps...

All the stuff I didnt use was completely removed. I practically rebuilt the amp section...

Missed that - no need to replace if those caps are new.  I would comb all the connections around the bad channel ax7 and double check all the wiring.

 

[jsefer]

And the behavior of it going of and on, I can replicate it, almost all the time, just have to turn off the power, wait then turn it back on and the sound is present then it fades with a crackling in the background, sort of as I i had turned off the amp and sound was still crackling through.

Tube swap give same results? 

Crackling sound on it's own could be pointing to loose connection/solder joint problem.  Prodding with a chopstick will reveal those pretty quickly.

I would highly doubt is a cap issue but can certainty try. these are brand new caps...

All the stuff I didnt use was completely removed. I practically rebuilt the amp section...

Missed that - no need to replace if those caps are new.  I would comb all the connections around the bad channel ax7 and double check all the wiring.

The crackling happens as the sound is fading, tube swap same results... Im going to try and use some chop sticks to pry arround. the project was all asembled on a point to point board... so i dont think i missed anything... you dont think by any chance it could be a bad resistor...?? things are 50yrs old...

Im going to try and swap the resistors, I was recommended to use 100k plates by a buddy of mine for flatter response, so Ill probably try that. 1watt should be good right?

 

[lassoharp]

. you dont think by any chance it could be a bad resistor...?? things are 50yrs old...

Those HDK/FOX resistors are precision types and I would judge them to be very good quality - more so than typical carbon comps you would see from that era.  I have a small drawer full, many of which were pulled from an M8 I rebuilt for someone.  Most all measure like < than 1% tolerance, many of them are spot on.

What do the plate and cathode voltages do after the sound starts to fade with the crackling?  Stay the same?

If possible, you can try taking the output signal from C11 (Right where it connects to the switching jack) - this would be the tip (+) lead, negative lead connected to ground.  Unplug any cables from the jack itself.  Best to run this signal straight into a Hi-Z power amp input or guitar amp for monitoring.

 

[abbey road d enfer]

My apologies, I forgot to mention I am using a cinemag transformer, cinemag advised I load it with atleast 10k, so I started with that, might changed it depending on the sound I get/want.


Voltages of the good unit
12ax7 heaters [AC]
6.5v
Plate
166.5
Cathode
1.491v[@ R11] / 1.75 @ R14

Voltages of the bad unit
12ax7 heaters [AC]
6.5v
Plate
172.4
Cathode
1.501 [@ R11] / 2.35 @ R14

One thing i just noticed, at R15 wich is pin 6 of the second stage, on the bad unit, when I turn off the unit the voltage goes from 128v to about 240+ then lowers, and as soon as I turn it back on it shoots to 340v then steadily lowers back to 128v

On the good unit the same pin its stable at 191v when I turn the unit off, the voltage drops. Turn it back on voltage shoots to 205, and never passes that then steadily drops to 191 operating...

Looks like there's something wrong around the second half. Quiescent current seems too high. Are you sure the pot is correct? It may be that the wiper has lost connection and the grid is floating. Good practice says "put a grid leak resistor".

 

[jsefer]

. you dont think by any chance it could be a bad resistor...?? things are 50yrs old...

Those HDK/FOX resistors are precision types and I would judge them to be very good quality - more so than typical carbon comps you would see from that era.  I have a small drawer full, many of which were pulled from an M8 I rebuilt for someone.  Most all measure like < than 1% tolerance, many of them are spot on.

I mesured a few this morning before I left to work, and some seem out of spec, reading at about 240k...

What do the plate and cathode voltages do after the sound starts to fade with the crackling?  Stay the same?

If possible, you can try taking the output signal from C11 (Right where it connects to the switching jack) - this would be the tip (+) lead, negative lead connected to ground.  Unplug any cables from the jack itself.  Best to run this signal straight into a Hi-Z power amp input or guitar amp for monitoring.

The voltages are the same. There is olnly sound the first 5 seconds after I turn the amp on, but as the voltages settle to operating voltage, it fades.

As far as the output, the output is fed high-z into my small fender practice amp. I will try this out, this would bypass the output fader correct?

 

[jsefer]

My apologies, I forgot to mention I am using a cinemag transformer, cinemag advised I load it with atleast 10k, so I started with that, might changed it depending on the sound I get/want.


Voltages of the good unit
12ax7 heaters [AC]
6.5v
Plate
166.5
Cathode
1.491v[@ R11] / 1.75 @ R14

Voltages of the bad unit
12ax7 heaters [AC]
6.5v
Plate
172.4
Cathode
1.501 [@ R11] / 2.35 @ R14

One thing i just noticed, at R15 wich is pin 6 of the second stage, on the bad unit, when I turn off the unit the voltage goes from 128v to about 240+ then lowers, and as soon as I turn it back on it shoots to 340v then steadily lowers back to 128v

On the good unit the same pin its stable at 191v when I turn the unit off, the voltage drops. Turn it back on voltage shoots to 205, and never passes that then steadily drops to 191 operating...

Looks like there's something wrong around the second half. Quiescent current seems too high. Are you sure the pot is correct? It may be that the wiper has lost connection and the grid is floating. Good practice says "put a grid leak resistor".

So putting something like a 100k resistor on the second half to grid and seeing if it passes signal.... Im going to try this as soon as I get off work... Pots are new also, but it is likely that something might have gone wrong, maybe a bad pot.

 

[pucho812]

when the sound fades do you still get your around 6v for your heaters on the tube?

 

[jsefer]

when the sound fades do you still get your around 6v for your heaters on the tube?

Yes 6~ AC heaters are a constant.

 

[pucho812]

hmmmm. o.k.  I had to ask as I had a similar problem with an amp I had on the bench and it turned out the bridge rectifier failed. but you have ac heaters so not a problem.  Do you have access to an o-scope and a tone generator? Might be time to start  to scope it out. Also double check all the solder joints and such.

 

[lassoharp]

I mesured a few this morning before I left to work, and some seem out of spec, reading at about 240k...

That is around 5% which is perfectly acceptable.  It is when they go excessively high (towards open) you should suspect a bad one.  A 250K that reads say 2M or more.

this would bypass the output fader correct?

correct.