5F1 issue...

[SleepLess]

Hi everyone!
Yeah, I know... I have successfully built amps that took ten times more time to build and that were ten times harder. Shame on me, I'm stumped on a 5F1...

So here's the drill:
- Mojo 5F1 kit with Mojo export PT (230V required here) and Mojo NOS771 OT (special because the B+ wire is actually blue instead of the regular red, I was careful on that, the specs sheet states it).
- Mojo made a mistake and sent me 3 8uF caps instead of 1 16uF and 2 8uF. Oh well... I'm back to 5E1 specs 
- The amp lits up and emits a hum without tubes and with the 5Y3 and 12AX7 in.
- I have a 462V B+ with the rectifier and 12AX7 in (tested on the board and at pin 8 of 5Y3 and pin 3 of 6V6. I couldn't test it with the 6V6 in...
- It still lits up with all tubes in but about 20 seconds after ignition there is a MAJOR hiss coming out of the amp and that's when I turn off the amp before it blows up... Really loud hiss, I think I saw some blue arcing at one point in the 6V6 jsut before I unplugged the amp
- All tubes lit up.

I really don't know where to look at. I have checked my wiring and it all looks good. I have doubts on the volume switch wiring and on the fuse holder as well, except that I know that the PT 230V wire has to go to the side lug of the fuse holder. Maybe it has to do with the power cord wires going to the fuse/volume switch (where does the white neutral go, what about the black live wire?).
Don't hesitate to ask questions if you need more info...

Here are a few pics... Thanks already!

[worth]

I wouldn't ground the 2X100 ohm virtual center tap to the main filter ground , which may cause hum , but not hiss. Is your negative feedback connected, and are the resistor values correct ? Black AC wire to fuse tip.. side terminal on fuse to hot side of power switch... black PT wire to cold side of power switch... white PT wire connects to white neutral power cord wire.

[SleepLess]

Damn... Totally not what I did on the wiring, here's what I got:

- 230V PT wire to side lug of the fuse holder
- Black AC cord wire to central lug of the fuse holder
- White AC cord wire to upper lug of the volume switch (when I mean upper I mean closest to the three pot lugs and furthest away from the board)
- Black PT wire to lower lug of the volume switch
- White PT wire not connected to anything, cut and shrink tubed...
- Red/Yellow PT wire to ground along with the green AC cord ground.

... So just to be clear on this: what are the cold and hot tabs on the volume switch?
Thanks!

[sluckey]

Your power cord is wired OK. Recheck wiring, especially underboard jumpers. Measure voltages on all tube pins and post results.

[SleepLess]

OK, so here's what I got, 12AX7 and 5Y3S inserted, amp switched ON:
V1 (12AX7)
1: 230V DC
2: /
3: 1.9V DC
4: 3.32V AC
5: 3.31V AC
6: 238V DC
7: /
8: 1.8V DC
9: 3.42V AC

V2 (no tube installed)
1: /
2: 3.31V AC
3: 448V DC
4: 423V DC
5: /
6: /
7: 3.42V AC
8: /

V3 (5Y3)
1: /
2: 443V DC
3: /
4: 335V AC
5: /
6: 331V AC
7: /
8: 447V DC

Hope that helps...
Thanks!

[SleepLess]

Do you see anything that is out of specs in my tube chart or is it fine?
Thanks!

[sluckey]

I'm guessing you meadured zero volts on the pins with no number? If so, voltages look OK. Put all tubes in and turn it on. When the major hiss starts pull V1. Does the hiss stop?

Kinda hard to tell from your pics, so do you have a ground wire on the volume pot? I ask because I just noticed the original Fender 5F1 layout forgot to show a ground connection on the pot.

[SleepLess]

Hi!
Thanks sluckey. Yeah I do have a ground wire off the volume pot. But I'm concerned about the input jack 2 ground and speaker jack ground. The speaker jack is a RCA jack. The OT is grounded on its own metal case so it doesn't have a blac wire. But then the RCA jack just has the 4 ohm wire and the signal wire that comes off the board... No black wire is soldered at the RCA jack ground metal ring... I wonder if it's normal and if I should add one...

I'm think that my issue is due to bad grounding. I'm gonna compare several layouts and change a few things and I will also do what you said sluckey, about pulling V1 when the hiss starts... If you have any idea in the meantime...
Thanks!

[SleepLess]

Here's the link to my PT (sorry I don't know how to copy paste a snapshot of the specs sheet...)
http://www.mojotone.com/amp-parts/amp-transformers-fender-style/Tweed-Blackface-Princeton-Champ-Power-Transformer-EX

And here's the OT, it clearly says that the red wire should go to the 6V6 pin 3 and not to the board...
http://www.mojotone.com/amp-parts/amp-transformers-fender-style/Tweed-Champ-Style-NOS-Output-Transformer

Sluckey I can't do your V1 pull test. The amp just emits such a loud hiss and the 6V6 seems to turn blue, I don't have enough time to unplug V1 and see if it stops or not... I think I'm going to take the board out and recheck everything... I don't understand, everything seems to be where it should be now...

[SleepLess]

Right... So I have unsoldered almost everything, taken the board out. My underboard wiring was good. Now that I'm at it I have found a 22uF 500V cap in my stash. DO you guys think I could use it in place of the 16uF or do I stick with the 8uF? 22uF is only 6uF away from the stock 16uF, 8 uF is 8uF away... What would be the sonic differences between the 22uF and 8uF caps?

Still stumped on this one... I'm going to use a ground buss bar this time instead of multiple wires...

Here's also what I did:
- Tried another 5Y3
- Tried another 6V6
- Soldered a wire at the RCA jack ground to ground
- Reversed AC cord black and white wires

All that was useless, I still get that piercing hiss when the 6V6 receives signal (nothing at first, but about ten seconds after turning the amp ON the hiss appears)

Thanks!

[SleepLess]

OK... Amp totally rebuilt with new ground buss bar and the 22uF cap instead of the 8uF (if you say it's bad on this amp, I'll install the 8uF back...). It's even neater like this, it looks great and clean!
But what good is it of having a neatly built amp if I have exactly the same problem... The hiss comes up once the 6V6 receives the signal. Sluckey I have done your test and pulled V1 out once the hiss arrived. The hiss stopped... I assumed then that I had a bad 12AX7 but after trying in two other 12AX7s I still have the same problem... Don't bother about the long white and black AC cord wires, I left them long enough to swap them in case I made a mistake on that. Once I will have all of this sorted I will cut them to proper length...
Help please!  

Oh and I have 2A fuse in the amp, that's what the amp came with, I don't know if that's the right value for 230V...

New pics of the build in case it helps:

[Willabe]

Try twisting the white/black AC power cord wires together just like the red/red and yellow/yellow for the rec. tube and put them over to the far left end against the chassis.

[sluckey]

Sluckey I have done your test and pulled V1 out once the hiss arrived. The hiss stopped...

That's a big divide and conquer clue. It means the problem is probably in the 12AX7 circuit. You've already eliminated the tube as a suspect.

1. Now turn the volume control to zero. Does the hiss stop?
2. Disconnect the NFB loop directly at the speaker jack. Does the hiss stop?

Take some resistance readings between chassis ground and these tube pins (one meter lead on chassis, the other directly on the tube socket.) POWER MUST BE OFF!!!

V1-2 should be 34K with nothing plugged in.
V1-3 should be 1500Ω.
V1-7 should be zero with VOL pot set max CCW and should be 1MEG with VOL pot max CW.
V1-8 should be 1500Ω with NFB loop disconnected or 1400Ω with NFB connected.

Now check the resistance of the OT secondary, one meter lead on chassis and the other on the tip of the speaker jack. Should be about zero ohms.

Answer the two questions and post the five resistance readings. BTW, the 22µF cap is fine. With filter caps I usually up-size when I can't find the exact size.

[SleepLess]

Sluckey I have done your test and pulled V1 out once the hiss arrived. The hiss stopped...

That's a big divide and conquer clue. It means the problem is probably in the 12AX7 circuit. You've already eliminated the tube as a suspect.

1. Now turn the volume control to zero. Does the hiss stop?

I can't turn it to 0 or it's like turning it OFF. When I turn that volume knob to switch the amp ON the knob actually lands on 3. On other tweed amps I have a 0-12 margin, but here it's OFF between 0 and 2, turned ON at 3 and goes up in volume (I suppose) up to 12.

2. Disconnect the NFB loop directly at the speaker jack. Does the hiss stop?

I suppose the NFB loop is the wire that comes from the board and goes to the speaker jack? If so, do you mean you want me to turn the amp on, wat for the noise and then unsolder that wire with the amp still ON?

Take some resistance readings between chassis ground and these tube pins (one meter lead on chassis, the other directly on the tube socket.) POWER MUST BE OFF!!!

V1-2 should be 34K with nothing plugged in.
V1-3 should be 1500Ω.
V1-7 should be zero with VOL pot set max CCW and should be 1MEG with VOL pot max CW.
V1-8 should be 1500Ω with NFB loop disconnected or 1400Ω with NFB connected.

Now check the resistance of the OT secondary, one meter lead on chassis and the other on the tip of the speaker jack. Should be about zero ohms.

Answer the two questions and post the five resistance readings. BTW, the 22µF cap is fine. With filter caps I usually up-size when I can't find the exact size.

There you go for the resistance readings:
V1-2: 33.7K
V1-3: 1470 ohms (how do you do that symbol btw?)
V1-7: 0 --> 1054 ohms
V1-8: 1450 (nothing disconnected)
OT secondary: 0 ohms.

Thanks guys!

Oh and Willabe I did what you asked and nothing changed...

[sluckey]

Disconnect the NFB wire directly at the speaker jack and THEN turn the amp on. Is the hiss gone?

I'll stick around for a while this morning if you want to tinker with it.

[sluckey]

(how do you do that symbol btw?)

Hold down the 'Alt' key and type 234 on the numeric keypad. When you release the 'Alt' key the symbol will appear.

Here's a list of other ASCII characters you can use with the 'Alt' key...

[SleepLess]

Disconnect the NFB wire directly at the speaker jack and THEN turn the amp on. Is the hiss gone?

I'll stick around for a while this morning if you want to tinker with it.

Hiss is gone with that wire disconnected!
I'll stick around too, thanks Sluckey!

[sluckey]

Hiss is gone with that wire disconnected!

OK. Now, ignore what Mojo told you about the OT primary wire. Reverse them (red to the board and blue to the tube). Reconnect the NFB wire. Hiss should still be gone. Is it???

[SleepLess]

OK done, no hiss! Damn Mojo, their OT specs sheet was wrong then!!!

[sluckey]

Does the amp sound OK with a guitar?

[SleepLess]

Does the amp sound OK with a guitar?

I'll have to wait until my 3 year-old son wakes up from his afternoon rest to tell you that...! But already a million thanks Sluckey! Damn I can't believe they send misleading specs sheets! That's insane!

[sluckey]

Sounds like we're done. Give us a sound report when you can.

[SleepLess]

I will! Thanks a lot Sluckey and thanks to Willabe and drgonzonm as well!

[SleepLess]

Oh my God, what a sound this thing has! Awesome blues/rock machine! At full tilt it's pure tweed heaven without blowing up your ears. Fantastic!

Thanks again to all of you! 

[SleepLess]

Hi!
This thing is a tone machine, really! I love it!

BUT I find it to hum quite a lot when at full tilt. It's almost silent up to 6/12, but then it increases and on 12 it really hums. I don't know if it's normal or not. I've tried it with a strat and a LP and the result is the same.

My grounding is such, I have 2 ground points only in there:
1. Ground lug at one of the PT bolts with the AC cord ground and the HT CT ground (red/yellow wire)

2. All the board grounds and volume switch ground and input jacks ground (ground buss bar)

Do you know how I could reduce that hum and if that's ever possible at all or if I have to admit that Champs do hum when on full tilt?
Thanks again!

[Willabe]

I've never built a champ type (SE) amp, but I did build a Fender stand alone reverb unit, which (pretty much, more or less) has a champ type (SE) amp in it to drive the verb tank and it's very quite too me.

I used a wired ground star schem or as KOC calls it a wired galactic ground, all jacks insulated from the chassis. Here's a link to Merlins site that has info on wiring up grounds;

http://www.freewebs.com/valvewizard2/Grounding.html

I also used a DCV stand-off (70-80 dcv) to float the 6.3acv heaters.

Also, I've seen a lot of guys here add a 2'nd stage of filtering to the B+ _plate_ supply, ie, filter cap-R- filter cap or go with filter cap-choke-filter cap, either seemed to work fine. The choke would have to be big enough to handle the whole amp and IIRC the large Fender choke Doug sells will handle it at 4H/90 mA.

SE amps do not have the noise cancellation that a PP (push/pull) OP stage amp has, so you've got to do more to make it as quite as a PP amp.


                                  
                           Brad            

[Willabe]

Here's the drawings from my reverb unit. You can see how/where I numbered what to tie together for each ground star. Star grounding is in green on layout. Ground stars #2/#3 aren't nessary, I was trying to drop a few B+ volts so I added them.

There's other ways to do this but it worked for me.      


                            Brad      

[sluckey]

Do you know how I could reduce that hum and if that's ever possible at all or if I have to admit that Champs do hum when on full tilt?

As Willabe said, "I also used a DCV stand-off (70-80 dcv) to float the 6.3acv heaters." Most SE amps (like your Champ) can benefit from referencing the heaters to a dc voltage rather than ground. Push/pull amps have a builtin filament hum cancellation just due to the common mode rejection characteristic of the push/pull action.

This can be done very easily on your champ. First, remove the two 100Ω resistors from your indicator lamp base (These resistors reference the filaments to ground.)) Now we're gonna put those resistors on the 6V6 tube socket. Connect one resistor between pin 7 and pin 8. Connect the other resistor between pin 2 and pin 8. That's it! Now the filaments are referenced to the dc voltage on the cathode of the 6V6. Hopefully the hum will be reduced some more.

Filtering the B+ even more with a choke and/or extra stage of filtering should reduce hum too, but this may not be practical in your chassis.

And don't forget, the 5F1 was the bottom of the Fender line when it was new. It may just be what it is. I do recommend moving the 100Ω resistors though.

[SleepLess]

Do you know how I could reduce that hum and if that's ever possible at all or if I have to admit that Champs do hum when on full tilt?

As Willabe said, "I also used a DCV stand-off (70-80 dcv) to float the 6.3acv heaters." Most SE amps (like your Champ) can benefit from referencing the heaters to a dc voltage rather than ground. Push/pull amps have a builtin filament hum cancellation just due to the common mode rejection characteristic of the push/pull action.

This can be done very easily on your champ. First, remove the two 100Ω resistors from your indicator lamp base (These resistors reference the filaments to ground.)) Now we're gonna put those resistors on the 6V6 tube socket. Connect one resistor between pin 7 and pin 8. Connect the other resistor between pin 2 and pin 8. That's it! Now the filaments are referenced to the dc voltage on the cathode of the 6V6. Hopefully the hum will be reduced some more.

Filtering the B+ even more with a choke and/or extra stage of filtering should reduce hum too, but this may not be practical in your chassis.

And don't forget, the 5F1 was the bottom of the Fender line when it was new. It may just be what it is. I do recommend moving the 100Ω resistors though.

Sluckey you're awesome!!! I Will do this!!!!
Oh and Willabe you're awesome too... But if I can reduce hum by modifying my build as little as I can so much the better!!!!

Oh and just one thing Sluckey: this "trick" works for a SE amp. But does it also work for, say a 5F4? Would I have to use 2 100ohms resistors per 6L6 on those same two positions?

Thanks so much again!!!

[sluckey]

But does it also work for, say a 5F4?

5F4 is a push/pull amp and may not have as much effect as on a SE amp. But, the 5F4 is also a fixed bias amp and has the cathodes connected to ground. Putting the resistors on the tube socket would be exactly the same as putting the resistors on the indicator lamp socket.

Would I have to use 2 100ohms resistors per 6L6 on those same two positions?

You only need one pair of resistors. Using the 6V6 tube socket is just a convenient place to put the resistors since you will be connecting between the filament and cathode. You can physically mount them anywhere.

[Willabe]

Sluckey you're awesome!!!

Yep, Sluckey's top shelf! I'd listen to him first.    

But if I can reduce hum by modifying my build as little as I can so much the better!!!!

Makes good sence to me. Try what you can and see if/when it gets you to where your happy.    

If you look at my drawings you'll see a little different way to do the dcv stand-off and it works for either cathode or fixed bias and the dcv can be adjusted by changing the R's, but I wouldn't go lower than 120K/100K on the R to ground. Most guys here seem to go with 30-40dcv for the stand-off, Merlin and Kevin O'Conner go with 70-80dcv.    

This is why I also suggested to you a few posts back to use twisted pairs on the AC power cord wires and tucked up against the chassis
just in case, it all adds up. Some things more than others, but it's still good lead dress practice when building IMO, to nip these things in the bud right away as you wire the amp up. To me then you have less things to try to have to figure out what could be the problem.

Every time you find and _get rid_ of the loudest noise source _then_ the next loudest noise source comes up to be heard.    


                                  Brad          

  

[Paul_Fawcett]

If you look at my drawings you'll see a little different way to do the dcv stand-off and it works for either cathode or fixed bias and the dcv can be adjusted by changing the R's, but I wouldn't go lower than 120K/100K on the R to ground.

Er, actually, you don't want make this shunt resistance *larger* than 47k-56k, lest you exceed the commonly published ratings for maximum permitted heater-cathode resistance..... If you want a higher elevation voltage, you're more-or-less stuck with making the series resistance smaller, and burning a bit more current in the divider. I generally target right around 50V.

[Willabe]

Er, actually, you don't want make this shunt resistance *larger* than 47k-56k, lest you exceed the commonly published ratings for maximum permitted heater-cathode resistance

Hi Paul,

Depends where you tap the B+ at.

I like to tap the B+ further down stream so, 1. it's cleaner dc and 2. it's at a lower voltage already.

Most tube spec sheets I've seen for the common 9 pin types we use are all at least 90 to 100v, + or -, cathode to heater.

I've only done this twice so far but, I've had no problems with the dc stand-off at 70-80 dcv.

KOC favors this for noise reduction and writes in his books that he likes to use the 70-80 dcv range.    

In Merlins book he writes not to lower the shunt R lower than 120K, so you don't drop to much current from the B+.

Makes sence to me, but          
      
                            

                                            Brad      

[sluckey]

lest you exceed the commonly published ratings for maximum permitted heater-cathode resistance.....

"maximum permitted heater-cathode resistance"??? I've never heard of such a rating. Can you explain? Maybe even post some links where we can read about such?

[sluckey]

Regarding maximum heater-cathode resistance, One source on valve amplifiers " The valve Wizard uses the exact language.

He also states "Many data sheets do not quote this". I had to dig thru several tube manuals  to find HK max resistance. Found it in a Phillips manual. Never saw it in the RCA or Sylvania manual. I certainly would not call it a 'commonly published rating'.

Thanks. Never too old to learn something new! 

[John]

I made a voltage divider for my heater CT since I used 6SL7s (and a 5879). Used the 100K bottom resistor with 470K on top. Gives 72V at the junction with a B+ of 365'ish.

If something melts down the road, I'll be sure to post it. 

[Ed_Chambley]

Never built a 5F1, but enjoyed this thread.  I have a question. the input jacks are switchcraft 11 type and not 12a?  How does it keep from humming with an open jack?

[sluckey]

The original schematic shows type 12A. And SleepLess used type 12A also. Look at pics closely. Why do you think type 11?

[Willabe]

So what is heater-cathode resistance and what are the raitings for tubes like; 12_ _ 7?

None of my tube manuals have this raiting listed.


                           Brad        

[sluckey]

So what is heater-cathode resistance and what are the raitings for tubes like; 12_ _ 7?

From an Amperex manual listing for a 12AU7...

Max Heater to Cathode resistance... 20KΩ (self bias circuit)
Max Heater to Cathode resistance... 150KΩ (phase inverter circuit)

http://www.tubezone.net/pdf/12au7.pdf

or... from a Mullard manual for a 12AX7

Max R_h...k is 20KΩ (120KΩ when used as a phase inverter immediately preceding the output stage)

http://www.tubezone.net/pdf/12ax7ecc83.pdf

I've made it 45 years without knowing about that obscure rating. Now that I'm better educated, watch me crash and burn during the last 217 days of my career!   

[John]

Now that I'm better educated, watch me crash and burn during the last 217 days of my career!   

Be sure to post pictures! 

[Ed_Chambley]

The original schematic shows type 12A. And SleepLess used type 12A also. Look at pics closely. Why do you think type 11?

I see now, it was just one of the photos dead on and I could not see.  I looked at the photo showing the angle shot and I can see it is a 12.

[Willabe]

OK, but what does it mean and how do you figure it?

From R ground to cathode + R ground to heater?

If so why would that matter?       


                          Brad      

[sluckey]

Just a guess... Maybe increasing the cathode resistance beyond some max value makes the cathode element impedance high enough that hum from the nearby heater element becomes a noticable factor. Hi-Z circuits are more easily affected by heater hum than Lo-Z circuits, especially when dealing with small signals.

I've never thought about it before today. Most cathode are operated with a relatively small cathode resistor, typically under 10K.  The cathode followers and split load phase inverters use the highest cathode resistor values (usually 47K to 100K) I've seen in audio circuits ('course the signal level is much higher too, which probably provides some immunity to the unwanted hum). Maybe that max parameter is the reason for not seeing larger value cathode resistors.

I'm just a fixer with a decent electronics background. Maybe some of the tube theorists will chime in.

[HotBluePlates]

Yeah, I'm guessing that once that resistance is too high, heater-to-cathode leakage becomes a bigger factor. With that resistance low, the external resistance shunts any leakage. Heater-to-cathode leakage, in a bad case, causes hum.

All this is really moot, because you don't have many situations where you might have a larger resistance there. Even if you thought you might because of a voltage divider to provide a d.c. reference, you should just scale the values of the divider to have a smaller shunt resistance.

[SleepLess]

Hi!
Very interesting read guys, thanks!

I just wanted to come back to say that I installed the resistors on the 6V6 instead of on the indicator lamp as per what sluckey advised. The hum has reduced a little. I would not say the change is drastic, but there is a difference. The amp sounds great! It makes me change my mind about single ended tube amps! I may try the Princeton one of these days!