5F6A won't break up

[MWJB]

"MWJB:
OK I will try. I noticed that comes less light from V2 filament respect to the other two tubes (excuse for the explanation, but I think you can understand) maybe it's the tube." - That doesn't necessarily indicate a problem, a voltage check will be more conclusive.

[el_fela]

Hi! ive been following this thread and it has been very helpful to me. However, i cannot solve this problem i have.
Ive built my amp following the the ceriatone 5f6a layout, and i get a nasty distortion.
Previosly, i had a problem with my amp, a serious shortcut at v5 socket. I dont know if something is fried or what...

Anyway, it works now, but there is still the nasty distortion problem, when playing loud notes...
It may be blocking distortion, but i have some high readings at v3:

V1
1: 165
2: 0
3: 2,6
7: 0
8: 2,5

V2
1: 176
2: 0
3: 1,2
6: 325
7: 180
8: 180

V3
1: 258
2: 40
3: 70
6: 246
7: 44
8: 70

V4
pin 2: 436
pin 3: 435

V5
pin 2: 436
pin 3: 435

Bias -52VDC


I really dont know what it can be... i tried substituting a large number of components, but still, i cant solve this.

Please help me!! I spent my last two weeks on this...

Cool forum, by the way.
Peace.

[MWJB]

What is the voltage at the junction of the 470ohm resistor and the 10K (or 6.8K) PI tail resistor that are connected to V3 pin 3 & 8. Double check component values, ensure a good ground connection from the presence pot. You definitely have a 12AX7/7025 in V3, not a 5751 or similar?

Power tubes pin 2 should not have hi dc voltage on them, are you sure that you don't mean pins 3 & 4? Your power tube plate voltages are low, what PT did you use? You say you have -52vdc at pin 5, what is the actual plate current (in mA)?

Do you have some pics of the build?

[el_fela]

Hello!
Here are some pics:








What is the voltage at the junction of the 470ohm resistor and the 10K (or 6.8K) PI tail resistor that are connected to V3 pin 3 & 8

69VDC, just 1V below pins 3&8. I know this should be about 32V, but i dont know why its so high. I cheked the resistors at his junction and i have the correct values (1M, 470 ohm, 1M and 10k)

ensure a good ground connection from the presence pot.

Pot seems ok. at least it works as expected.

You definitely have a 12AX7/7025 in V3, not a 5751 or similar?
Yes, tung sol 12ax7. I tried replacing all tubes with no results.

Power tubes pin 2 should not have hi dc voltage on them, are you sure that you don't mean pins 3 & 4? Your power tube plate voltages are low, what PT did you use?

Yes, sorry, I meant pins 3 & 4.
Im using a power transformer made by Saint Vith, a guy that makes these for DIY tube amps.
When there was this shortcut at the sockets, i think the PT, the 5ar4 rectifier or the OT might have suffered some damage. The rectifier was sparkling inside, didnt look good.
I tried a 5u4g tube, and the voltage difference wasnt noticeable, so i guess the 5ar4 is not at 100%. At the PT high voltage secondaries I have 325 + 325.

You say you have -52vdc at pin 5, what is the actual plate current (in mA)?

I lost the exact measurement, but it was about 34 ma, maybe less. at -41 Vdc the plate current gets to 50 ma, but voltage goes down to 385V.

Thanks for the reply! that was fast...

Felipe

[MWJB]

1v at the junction of the PI cathode & tail resistor = 2mA/tube...sounds about right. The hi voltage here could possibly be due to a poor ground connection at the presence pot...check or remake the ground connection here.

OK the B+ you mention sounds about right with the PT you have.

Mid 30's is fine as far as plate current.

Your transformers & choke are all oriented the same way, PT & OT should have the laminates at 90degrees to each other.

I can't zoom in on the pics, you have used a very deep chassis, I would shield grid wires to the 6L6 from the PI & shorten/shield grid wires to the 12AX7s. You may have a layout issue. Can you explain the grounding scheme please?

[el_fela]

Thanks MWJB, that was really helpful! I found out that the presence pot was rated 25k, and I placed a 6.2k resistor in parallel for testing. I think the voltage at the PI should be ok now, cause i tested bypassing the pot and the voltage dropped a lot. Then my multimeter had an internal short and I couldnt continue with the tests.
Anyway, the sound is a little darker now, but the ugly distortion still occurs.
Its not that bad... i would describe it like a resonant harmonic sound that washes out. It only happens at mid to high volumes.

Could it be the OT that is not working right after the shortcut? Or maybe blocking distortion??

The grounding was set this way: all preamp wires connected at one point, except for rhe pots, for wich I used one run of wire, from the presence pot to the input. One wire from that to one of the PT screws, were i connected everithing else (PT ground, bias, power tubes and the OT ground).

Then i tried leaving alone the PT grounds and connecting everythinh to the chasis near the input jack, except for the long run of wire trough the pots and the input.

The amp was very silent when idle in both schemes, even at high volumes.

i will try again tomorrow with new chinese 6l6's, but i think i did that already with similar results.

Thanks a lot!
Felipe

[MWJB]

A 25K presence pot is a good thing, look at the presence control on a 6G12A Fender Concert, or the 59 Bassman reissue...these use a 25K pot in series with the 0.1uf cap, with cap & pot in parallel to ground with a 4.7K resistor. This presence circuit is quieter in operation than the original 5K pot circuit.

If you suspect the OT, measure the dc resistance from each plate wire to the centre tap...should be within 15% or so (don't panic if not necessarily identical), you might read from 70-150ohms depending on the OT? This is a rough test, ideally you would apply a small AC voltage (measured) at the OT secondary & measure voltage deleloped at the primary to work out the turns ratio & ensure that AC voltage from both plate wires to the centre tap was identical.

Blocking distortion - You should have 1.5K resistors feeding pin 5 of the 6L6s already? You could try larger values here, but really if you find yourself having to use 10K or more, then really you should be looking to revise the layout. Confirm that you have 470ohm screen grid resistors feeding pin 4 of the 6L6s.

I use 2 ground points - one at the input jack for buss wire from the pots, preamp cathodes & PI & preamp filter caps (all using one wire per component, NOT daisy chained on the board) the other ground point is the PT bolt, for PT CT/bias supply/main & screen filter caps (again - one wire per component to the ground point). Buy some switchcraft metal jack sockets and ground the OT secondary at the speaker jack.

[el_fela]

Ok, I added 10k grid stoppers, nothing happened.

Confirm that you have 470ohm screen grid resistors feeding pin 4 of the 6L6s.

i had, but when i was testing, i replaced the one at v5 for a 220ohm resistor, and the distortion thing improved a lot. I also tried connecting a 100pf cap from V3 pin 1 to ground, and it imrpoved the bass part of the distortion. I really dont know what it means...

One last bit of info: when the distortion occurs, the blue light inside the power tubes "glows" acording to the sound of it. I mean, you can "see" the distortion...

im gonna try that AC test with the OT. I read about it somewhere, I know what youre talking about

Thanks again

Felipe

[MWJB]

"i had, but when i was testing, i replaced the one at v5 for a 220ohm resistor, and the distortion thing improved a lot. I also tried connecting a 100pf cap from V3 pin 1 to ground, and it imrpoved the bass part of the distortion. I really dont know what it means... " Nothing useful, that I can determine, you should be able to run 470ohm/1K screen grid reistors happily enough. Voltage at the screens should be around a volt under the plates.

The tubes will glow when conducting signal, even if that signal is there unintentionally.

Just a thought, where are your filter caps mounted?

Any bigger/better pics available?

Your bass cap & middle cap are the wromg way around on the circut board, make sure that the 0.1 uf cap connects to the wiper of the bass pot & the 0.022 connects to the wiper of the mid pot.

[el_fela]

Hello again. Ive been replacing components, trying with different values without positive results.

[COLOR="rgb(65, 105, 225)"]Your bass cap & middle cap are the wromg way around on the circut board, make sure that the 0.1 uf cap connects to the wiper of the bass pot & the 0.022 connects to the wiper of the mid pot.[/COLOR]

Err... i maybe you were looking at a different shcematic this time?

[COLOR="rgb(65, 105, 225)"]Just a thought, where are your filter caps mounted?[/COLOR]

Inside the chasis, near the mains connector, the tube recto and the two 6l6s, why?
Should I place them outside?

I want to clarify that, right now, the amp sounds good in general, but when overdriven it makes this washing distortion. Its not so bad, so im beginning to think it might be normal (i never really played a 59 bassman). If that is not normal, then what could be the cause? Components too close from one another? too long/unshielded wires? intermittent ultrasonic oscilation ?

Next time I'll upload some pics, and some audio if I can ( a least "for the record")

Thanks in advance,
Felipe.

[MWJB]

The bass cap sits between the middle & treble caps, in the same order that the pots fall on the control panel (T, B, M, P). Check that the correct cap connects to the appropriate pot wiper, you wouldn't be the first to switch them around. I was not looking at any schematic or layout, I was looking at your circuit board.

The distortion you mention is definitely not normal. Yes it is most likely due to one, or all of the causes you mention...rebuild the circuit board, line up components on the board so the grid wires connect to the tube sockets with as short as possible wires, shield wires from the inputs to the first tube's grids, shield wires to the power tube grids.

Have you changed the orientation of the PT yet?

Filter caps should be fine mounted in the chassis. You seem to have plenty of room.

[el_fela]

Well, the pots are wired ok. I tried moving the OT and nothing. The PT's wires are to short now so moving it would be a terrible mess... ill try that before i build a new board.

I found that if i cut the end of the 100k resistor at v2, breaking the connection between pins 6 and 1, leaving pins 1 and 7 connected between them, the signal goes away, but the ugly intermodulation remains alone!

Again, i dont know exactly what it means, so, what do you think?
Thank you MWJB !!
Felipe.

[MWJB]

Cutting the resistor at V2 isn't the most useful thing to do, as it is mounted on the tube socket there is little margin for this component being at fault (outside of poor/sloppy soldering). It would make more sense to just pull the preamp tubes & reinstall one by one, starting with the PI, noting when the unwanted symptoms occur, or if they are related to a particular stage.

Do you get any untoward symptoms with just the rectifier & power tubes installed?

Have you replaced the output jacks & grounded the OT secondary to the chassis yet?

Honestly, I would wait until you have revised the circuit layout & transformer orientation to troubleshoot.

[el_fela]

Hello again! OT is grounded to casis, and I tried cahngin the orientation of the PT. Just to experiment, i built a 5e3 circuit and used the 5f6a chassis, OT, PT, recto and power tubes. The circuit is porrly gounded, but i just get the same horible sound. Here is a sample clip:

Download - MP3 Ringtone by - Free Music Hosting

Bye!

[MWJB]

Are you saying that you have removed the 5F6A circuit and replaced it with the even more troublesome 5E3 circuit? :-o

"The circuit is porrly gounded, but i just get the same horible sound." Well, this suggests that you know whereto start looking already.

More pictures please.