picking up radio interference

[smackoj]

I have a Valco Supreme reproduction amp that uses a 6SQ7 single triode preamp tube. All of a sudden it has started picking up AM radio signals and I have isolated the problem to the 6SQ7 tube. I have several tubes and I can find one that is quieter but the better sounding tubes all pick up radio. should I try to put a small value cap from one of the tube socket pins to ground or try putting it from the input jack to ground?

thx amigos   

[kagliostro]

Assuming the schematic of your amp is this

http://www.el34world.com/charts/Schematics/files/valco/Valco_SUPREME_MODEL_510-1_B.pdf

If the 47k resistor is connected directly to the input jack, you can try to use a shielded cable and/or move the resistor from the input jack to to the pin of the input tube, other thing to be try is to increase the value of the resistor to 68k

Of course you can't do it only for one input, you must do it for all

---


The last thing that I know you can try is to use a ferrite bead as in some Mesa Boogie amps



as to obtain something like this



you can also use a ferrite bead in junction with in-series  resistor

For the Mesa Boogie solution see the attached schematic

K

[Colas LeGrippa]

Don't use any series resistor like in the  Mesa Boogie

[kagliostro]

I forgot to say that also a line filter is to be tried

@ Colas

Mesa uses only a ferrite bead and no resistor

but I didn't see a reason for which a ferrite bead + an in-series  resistor can't be applied  ??

K

Edit: My be the presence of the resistor and ferrite bead at the same time push the frequency cut too high as to stop a low radio frequency, it stops only high radio frequencies

[smackoj]

yes; it is the model 510 1B Valco. Can I go thru a box of hobby beads and find one with iron in it and use that?  or do I have to order a bead from some supplier?

thx, smack em jack em

[sluckey]

http://en.wikipedia.org/wiki/Ferrite_bead

http://www.valvewizard.co.uk/gridstopper.html

[HotBluePlates]

As I think Sluckey is pointing out, a grid stopper would fix the problem.

If it were my amp, I'd add a 68-100kΩ grid stopper right at the 6SQ7 grid, between it and the junction of the 47kΩ resistors at the input jacks.

Yeah, I know it will reduce treble somewhat. But it will also reduce RF by a lot more, which seems like what you need.

Alternative: Is the tube loose in its socket? The metal shell is probably grounded via pin 1 (otherwise unused, many 6SQ7's and other metal-bodied tubes connect the shell to this pin). If the tube is loose in the socket, the shell may not be acting as a shield the way it otherwise would.

... I didn't see a reason for which a ferrite bead + an in-series  resistor can't be applied  ??

You can use both if you want. Like having a belt and suspenders.

My be the presence of the resistor and ferrite bead at the same time push the frequency cut too high as to stop a low radio frequency, it stops only high radio frequencies

Both do the same thing. Because the ferrite bead is the same as a small inductance, there would be no impedance presented at d.c., and so no reduction of d.c. voltage. A resistor, in general, could cause a d.c. voltage drop. However, in the kind of tube circuits we're talking about, this is not a problem and there's no d.c. voltage division (because of the very high input impedance of the tube grid).

A ferrite bead would be technically superior, but you could use a grid stopper resistor and get just as good results. I've never bought a ferrite bead, so I have no idea if there is a price difference that would influence the decision.

[kagliostro]

@ Sluckey & HotBluePlates

Many thanks, friends, I've always something new to learn from you

Like having a belt and suspenders

really, this is not a joke, these last few months I'm using both together at the same time

 

K

p.s.: An usable ferrite bead, with only one hole, but usable, you can easily find in a dismounted PC or PC cable

[HotBluePlates]

You are a man who wants to be sure the job will get done... 

[kagliostro]

I became a little 'too chubby and my pants fall 

K

[smackoj]

thanks for the good info.....actually, I should have said I'm having trouble with the glass bottle ones i.e. 6SQ7GT....I tried several metal case which do get grounded to the socket jacket.....no RF at all....but, those glass ones have gone nuts! one of them howls, and the rest sound like a 1950's tube table radio that got lost in a "new dimension of sight and sound" cuz the program keeps talking about something called a "Superbowl" ?? So, I'll try the grid stopper suggestion and we'll see.

Just a FYI to those who might be using similar bottles in the preamp section: The glass bottle ones sound much sweeter for music IMO but they are darn close to a 'fools errand' to find one's that don't squak. The metal case ones are more of a JAN style plug that doesn't offer much musically but could survive a nuke blast and keep tickin'....saying all that you probably wonder why am I bothering with the pesky peckerwoods?  Well, I rewired the preamp a couple months ago to use a 12AX7 because the amp was too brown sounding. But that didn't get 'er done for me so I swapped again for the 6SL7/6SN7 plugs and those are by far THE BEST. But, I wanted to put it back stock and maybe sell it, keep it for giggles or whatever, so I put the SQ7 back in and sorta woke the 'sleeping giant' again. One bright star showed up through this journey though. The metal case 6SQ7 tubes in this circuit sound really good for med-high gain bottleneck slide guitar! Anyone like Johnny Winter 'sides me? This baby will take you there with a fairly light un-doped paper coned ceramic speaker....Good stuff for real.

adios friends    

[sluckey]

I'll try the grid stopper suggestion and we'll see.

Maybe a 100Y grid iron would be better today. 

According to the schematic the original amp uses a 6J7 pentode, not a 6SQ7 triode/diode.

[HotBluePlates]

... I should have said I'm having trouble with the glass bottle ones i.e. 6SQ7GT....I tried several metal case which do get grounded to the socket jacket.....no RF at all....but, those glass ones have gone nuts! ...

Seems like you should use the metal ones, or find a shield for the glass ones. Hopefully the grid stopper will help.

According to the schematic the original amp uses a 6J7 pentode, not a 6SQ7 triode/diode.

Smacko never did confirm whether that schematic matched his amp or not. He says his amp uses a 6SQ7, but Kagliostro guessed the supplied schematic was for the amp.

[DummyLoad]

couldn't you just slug RF with a 47pF to from grid to gnd?

--pete

[kagliostro]

Right, a shield

a little piece of alluminium pipe can be easily found



just remember to connect it to ground (look to the image)

K

[HotBluePlates]

couldn't you just slug RF with a 47pF to from grid to gnd?

Yes, you could do that.

But the tube already has Miller capacitance from grid to plate, and which is amplified by mu+1. So all you need is a little series resistance to for a low-pass, due to feedback of high frequency from plate to grid.

[PRR]

> All of a sudden it has started picking up AM

What changed?

> Well, I rewired the preamp a couple months ago

What didn't you put back JUST like before?

[smackoj]

<smackoj never confirmed the amp> HBP   Sorry guys I built this amp that is called the 510-1B by David Barnes at Vintage47amps who builds a lot of them for sale on the web. But, after looking over the scheme library, I see that the 'Supro 510-1B is a different setup with the diff. preamp tube. The scheme I worked off along with some mild tweaks given to me by David at Vint47amps is this Gretsch scheme.
http://www.el34world.com/charts/Schematics/files/gretsch/gretsch_g6156.pdf

Sorry for the confusion....I'll try harder to be accurate in the future.

 

p.s.  I should add for clarity that David Barnes' main interest was, and may still be, harmonica amps. This early field coil model from Valco built with all new parts and put in a pine cab with tweed covering, is bought and used by many harp players. As I stated earlier, I have spent many hours trying to get a sound I liked from it for blues, soul, jazz type guitar work. It's been hard for me to get from 'here to there' though after trying nine or so different speakers, raising voltage to the preamp section, trying different preamp tubes, and trying different OTs (I always stayed with 6V6s in the output section). The best it ever sounded was with a 6SL7 in V1, the same in the PI and I used a big vintage inductor where the field coil would have gone in the B+ supply.

I just wanted to add that caveat for other people who may have an interest in these older 'early' amp designs.   

[DummyLoad]

The scheme I worked off along with some mild tweaks...

http://www.el34world.com/charts/Schematics/files/gretsch/gretsch_g6156.pdf

i think that schematic has a couple of errors. please see attached - hopefully someone with more expertise will set me straight...

--pete

 

[kagliostro]

I'm less experienced than you are Pete

But yes, the connection you labeled as <no connection> seems really must not exist or in the grid of the bottom 6v6 we'll have DC

and the other connection must exist or it will be no DC on pin 5 of the 6SC7 and pin 6 of the 6SQ7

Franco

[smackoj]

> All of a sudden it has started picking up AM

What changed?

> Well, I rewired the preamp a couple months ago

What didn't you put back JUST like before?  Senor PRR

I don't know PR, sorry. When I first got this design done I didn't like the sound it supplied for guitar (not harp). I've tried lots of small changes and as I said, the best it ever sounded for strait guitar work, no pedals, no groupies in the crowd, was using 6SL7GT in Vone and Vtwo. But, now I have it back to stock and that's as far as I'm going today. I will try the grid stopper R and the alum. tube over Vone looks worthy of a try too. But, if the wolfe still howls, I'm perfectly ok to put the metal 6SQ7 plug into Vone and call it a 'dedicated' design for bottleneck blues guitar. It's got the Johnny Winter sound screaming to get out and this amp has never before 'screamed' at any settings.....so Serendipity came over and blessed my labours....Like Tom Petty told some interviewer for an article on his success, "If you're not eager and willing to explore new sounds and musical ideas, then you're not living where the majic happens."  (paraphrased)

Thanks so much for your input and interest fellas.....it's helped me get my mind off this demonic flu virus. Via con Dios     

[sluckey]

Smacko never did confirm whether that schematic matched his amp or not.

I sure thought he confirmed with this statement, "yes; it is the model 510 1B Valco". Anyhow, maybe the 6J7 or 6SJ7 would sound more guitareesh.