Parallel Single Ended 5F1

[TurboGuitarMelton]

Hey guys,


I built a parallel SE 5f1 and am getting some bad static/arcing sounds at higher volumes. Happens above 9 on the volume.


Saw the left power tube arc at one if its pins during one of my tests.


Amp is all stock 5f1 except I replaced the 5y3 with a solid state rectifier and used the socket for the other 6v6.


Transformers are classictone 40-18027 PWR (100ma) my B+ is 350 and each 6v6 tube shares a 250 ohm cathode resistor. Bias is around 38ma per tube @ 330v. A little hot but ok.


My OT is a classictone 40-18031. 15w SE


Using the 8ohm tap and 5000k primary. But speaker is 4 ohm so primary is 2500k like it should be.


I replaced the first cap with a 30uf to help with filtering a bit more.


Thought it was the speaker crapping out but tried it with two different weber sig8a speakers (15w) and a jensen mod 8 that is 20w.


Any ideas? Do I need grid stoppers for the 6v6s?

[DummyLoad]

Saw the left power tube arc at one if its pins during one of my tests.

at the socket or in the tube? if it arced at the socket - replace the socket. bad solder joint?

add grid stoppers - they help, but i don't think that's what's causing the arcing. too much screen/anode current is likely culprit.

have you tried new tubes? have you tried just one tube (the one that didn't arc) with adjusted OT secondary load and Rk?

5000k and 2500k loads are awfully high - you do mean 5000Ω & 2500Ω, right? 

--pete

[shooter]

+1

each 6v6 tube shares a 250 ohm cathode resistor

too much screen/anode current is likely culprit

one reason I prefer each tube have it's own Rk, you can quickly see if one tube is sucking way more than it's share, can even see it sucking just a little more 

[TurboGuitarMelton]

Thanks for the replys!


Oops! Yes i meant 5k and 2.5k


That socket is "recycled" so Ill change that if another tube doesnt fix it. Tubes are old silvertone 6v6s so that one may be going out.


It was arcing around pin 3/4


The arcing sound is really harsh. Almost sounds like blocking distortion. Only happens if I really dig into a lower bass note at higher volumes.


Amp was a normal 5f1 before I changed the OT and PT so the board is not the issue. Probably is that socket. Ill do some tests today and report back.

[sluckey]

Amp was a normal 5f1 before I changed the OT and PT so the board is not the issue.

Did you verify the NFB loop is proper phase after changing the OT?

[TurboGuitarMelton]

Okay, so I did a few tests.


I have a NFB switch and in both positions the problem persisted so Im pretty sure the phase is correct.


I also tried another 6v6 and the issue went away mostly! It did it a little for about 3 seconds or so and then went away once the tubes were fully warmed up.


Im thinking that adding two separate cathode resistors will fix problem like you guys suggested. I will order the parts and try that.


Also, while Im at it is there any way to reduce the SE hum? Seems like the idle 60hz hum is twice as loud as a normal 5f1. Im thinking of upping the screen cap to 16uf (its 8 now). Choke too?


Would a 50ma choke be suitable for this amp? Used between the main filter cap and the screen cap

[sluckey]

I have a NFB switch and in both positions the problem persisted so Im pretty sure the phase is correct.

If one position of that switch totally disconnects the NFB loop then I agree. But if your switch selects between two different resistors or some other fancy circuit then you must disconnect the NFB to be sure.

Also, while Im at it is there any way to reduce the SE hum? Seems like the idle 60hz hum is twice as loud as a normal 5f1. Im thinking of upping the screen cap to 16uf (its 8 now). Choke too?

Would a 50ma choke be suitable for this amp? Used between the main filter cap and the screen cap

Choke will help but the most effective thing you can do is add a cap/resistor between the rectifier and the plate node. A 33µF cap and 250Ω/10W would knock the ripple way down.

[shooter]

+1

two separate cathode resistors will fix problem

This doesn't fix anything, just helps ALOT identifying where a problem is.  Also good for balancing tube current as in fun with a calculator figuring out how to make || R's = 273 ohms 

[DummyLoad]

5H 100-150mA (hammond 158Q) choke will work fine for entire amp. large fender (4H 90mA) or marshall choke (5H 120mA) choke will also work, try it if you have one. 

pi filter laid out as: 5y3 > 32uF > choke > 32uF > OT > 500-1K 2W > 22uF - 6v6 screens >  PS rails to rest of amp.

that scheme will be about as buzz free as you can get with a 5Y3 for a rectifier.   

--pete

[tubeswell]

I also tried another 6v6 and the issue went away mostly! It did it a little for about 3 seconds or so and then went away once the tubes were fully warmed up.

\


Most likely a bad tube, or the tube in question was drawing much more current than the other tube. Have you got separate Rg2s? (and what is the total load resistance, plate and screen voltages, and bias voltage?)

[TurboGuitarMelton]

that scheme will be about as buzz free as you can get with a 5Y3 for a rectifier.   

I actually don't have a 5y3 in the circuit. I took it out and used the hole for the other 6v6. Im using diodes for rectification.

Most likely a bad tube, or the tube in question was drawing much more current than the other tube. Have you got separate Rg2s? (and what is the total load resistance, plate and screen voltages, and bias voltage?)

I think it may have been the tube too. Both 6v6s are sharing a 250 ohm resistor with a 25uf 50v cap.


If I remember correctly when I checked the bias the plate voltage was around 335v on each tube and the current through the resistor was around 78ma.


 

This doesn't fix anything, just helps ALOT identifying where a problem is.  Also good for balancing tube current as in fun with a calculator figuring out how to make || R's = 273 ohms 

So you dont think the separate cathode resistors would help?

[TurboGuitarMelton]

Also, forgot to mention that I am using a Weber WTC thermistor before the B+ to help tubes a power up since there isnt a 5y3 rectifier. Could that thermistor be causing the problem?

[shooter]

you dont think the separate cathode resistors would help

it will help;
1. it allows you to balance tubes so they each carry = I
2. It will help identifying which tube could be bad by observing I as you swap tubes, like 1 tube draws 2X and does in either socket kinda thing
3. it helps some with AC swing and the ability to "balance" gain with bypass caps
so tube 1 is hotter than tube 2, use less bypass cap on the hot one - this gets into cork-sniffing though 

I don't believe the MOV is an issue - possibly a hum source, but you can lift one end just to see 

[tubeswell]

Have you got separate Rg2s? (and what is the total load resistance, plate and screen voltages, ...?)

[TurboGuitarMelton]

Separated the cathode resistors and the problem persists... Even does it with different tubes. Could the fact that the grids, plates, and screens are in parallel be causing some crosstalk/ossilation at high volume?


I looked at an angela super single ended amp and mine is virtually the same except the two rectifiers. They do have 1.5k grid stoppers. Maybe these are necessary?





Voltages:


V3 6v6
cathode resistor 466.6
Cathode voltage 19.98


V2 6v6
Cathode resistor 465.8
Cathode voltage 17.91


Pin 3 (connected on both) 337.9
Pin 4 (connected on both) 300.3


B+ is 358.2


Before NTC its 360.4


I also bypassed the NTC no change

[TurboGuitarMelton]

At the end of the video you can hear the issue. Sound like the speaker is going to blow but I've tried 4 different speakers.

[TurboGuitarMelton]

Was doing some tests while my wife strummed the guitar and found that moving the plate wires of the 6v6 made the noise go away. Saw the wire from V2 plate to V3 plate arc near middle of the wire. Odd... How reliable is vintage style cloth wire??? Im thinking it might not be insulated enough and is causing the cross talk between the wires and chassis. Moving them with a chopstick definitely made the noise change/go away.


But that kinda doesnt make sense because I also took out one of the 6v6s and ran the amp with a 8ohm speaker (8ohm to 5k) neither socket had the same issue on its own. It only happens when they are used in conjunction.


 

[DummyLoad]

that section of wire that is arcing/has arced is junk and likely the socket terminal as well. i'd replace the section of wire & the socket + use teflon insulated wire. 

good luck with the amp. :-)

--pete

[TurboGuitarMelton]

that section of wire that is arcing/has arced is junk and likely the socket terminal as well. i'd replace the section of wire & the socket + use teflon insulated wire. 

Changed the socket and all the wire. Still is making the blocking distortion/rattle sound at 9 or above... :(


Also, the preamp voltages drop from 110v to 85v while you strum the guitar at high volume. Is that normal? Or likely the issue?

[jojokeo]

Can’t see your build but have seen this in various forms. Check to be sure that you have a grid leak resistor for the return path /bias of all tube grids. It seems grid related, possible an old coupling cap could be leaking a bit of DC? Anyway, a couple things to check

[shooter]

Cathode voltage

Was doing some tests while my wife strummed the guitar

did you monitor Vk and see if one "spikes" at your problem point?

I had 1 PSE 6V6 build that had a crazy un-solved problem where DC voltage "built up" as I got to max volume.  troubleshot extensively, re-build PA/driver section twice, finally "fixed" by limiting the volume/ AC input to driver, in other words, 10 became 8 on the dial.

consider it a challenge not a burden 

 

[Willabe]

I looked at an angela super single ended amp and mine is virtually the same except the two rectifiers. They do have 1.5k grid stoppers. Maybe these are necessary?

Yes, you should put in grid stoppers on the power tube grids.

They should be mounted as close to the grid pin as possible. With maybe only a 1/4" of lead between the grid pin and R end.

[TurboGuitarMelton]

Can’t see your build but have seen this in various forms. Check to be sure that you have a grid leak resistor for the return path /bias of all tube grids. It seems grid related, possible an old coupling cap could be leaking a bit of DC? Anyway, a couple things to check

Grid leak is a single 220k to both 6V6s and on the 12ay7 there is a 1m on the jack and if Im correct the 1m volume pot is the grid leak for the 2nd triode.


Coupling caps are all brand new. (I built the original incarnation of the amp about 2 weeks ago)


All that being said, I noticed that when I turn the volume pot the sound does in fact change frequency if I turn it up or down. To me that tells me the issue is after the volume pot. So from the 2nd triode on.


Another odd thing is that it only happens if you pluck the string in a specific way. So its frequency/harmonic dependant. I know its not the guitar/cable checked all that with another amp.


This amp is bascially the "Gibsonette" Ga-8 after studying the schematic there is a suspicious 470k grid stopper going to the 2nd 6v6. Im going to clip one of those in and see if that fixes the problem. The Angela super single ended also has those grid stoppers. I may be off the right track on this but its worth a shot.


Found another forum where another guy had the same issue and he added smaller grid stopper and it helped. Also said that changing the 220k grid leak helped. How does changing the value of that resistor effect the operation? The gibsonette has a 220k though

[TurboGuitarMelton]

FIXED!


Grid stoppers are a necessity for this circuit. 1.5k on both.
 

Yes, you should put in grid stoppers on the power tube grids.

 

[Willabe]

How close to the power tube grid pin did you install those grid stoppers?

The closer they are to the pin, the better they will work.

This amp is bascially the "Gibsonette" Ga-8 after studying the schematic there is a suspicious 470k grid stopper going to the 2nd 6v6. I'm going to clip one of those in and see if that fixes the problem.

Clipping in a grid stopper might not kill off a problem, has to be close to the tube grid pin for it to really work best.

[shooter]

way to hang in! 

I did a PSE GA-8, one of the better liked builds, beefed it up to 6L6 for "more" of what the amp already did.

for future reference;
typically if a vol/gain pot "effects" a problem I start there and go back - prior to that stage.  the pot can interact, depending on circuit, to the "next" input also, so it's not "golden"

I noticed that when I turn the volume pot the sound does in fact change frequency if I turn it up or down. To me that tells me the issue is after the volume pot

[PRR]

> Grid stoppers are a necessity for this circuit.

Grid stopper need is more often about layout (too compact or crossed-over).

[TurboGuitarMelton]

Amp sounds so good! Im really happy! Thanks for the help guys! I also added the 250 ohm 10W resistor and 30uf cap to the power supply. Made the amp SUPER quiet.
 

How close to the power tube grid pin did you install those grid stoppers?

Right on the pins (as close as it gets!)

Grid stopper need is more often about layout (too compact or crossed-over).

For sure this is the case. Everything is pretty cramed in there. This is a normal 5F1 champ chassis. Had to squeeze the diode rectifier/extra power cap and resistor on a terminal strip above the PT on the side wall of the chassis. Perfectly fits with enough space but barely.

[shooter]

Amp sounds so good! Im really happy!

 
as a non musician, I still get giddy when that happens 

find a 4X12 well stocked cab for even more fun 

[Calboy]

I finally got around to building a dual singled ended 5f1/5f2a clone and it seems I had the same issue as OP and this thread helped solve the issue. 
Basically, I had a 5f2a clone and just added another 6v6 in parallel.  I had a bad noise like OP that went away when I added 1.5k grid stoppers to pins 5&6 of each 6v6.
I used separate cathode resistors for each 6v6.
I used a 5k 8ohm 5watt OT connected to 8ohm speaker.
Compared to one 6v6, dual 6v6 seems louder with more headroom and less breakup.  Very pleased with results.
I still have to try 2x12 4ohms to see how that works and I also have a large SE 15watt OT as well. I will also experiment with nfb.
I still like a regular 5f1 but I've been meaning to try parallel 6v6 for 5-10 years and finally got around to it.

[shooter]

still have to try 2x12

roll as many speakers through that you can scrounge, I had about 8 laying around, ran all the combinations n settled on a 90's cream-back Celestin n a re-issue Eminence
amp got even fuller n louder.