5F6-A ugly distortion - Direct coupling question

[jmaf]

Hi, I spent the day today with a problem on an amp based on the 5F6-A circuit. In fact, not based on it, it's a verbatim copy except it's using a 12AX7 for V1.

There's an ugly distortion, sounds like clipping, when I hit the low notes a bit harder on a humbucker. With a single coil all is fine. So I guess it's due to higher signal level input and the fact that it's using a 12AX7 instead of 12AY7 on V1.

I tried substituting valve 1 for a 12AU7 and the distortion is still there, it's not a nice smooth clipping, it seems to appear at once like when the tube is either near cutoff or at very low plate voltage - lowering the gain by 5x with the 12AU7 didn't help.

I substituted all tubes for brand new ones, changed them around, etc, nothing worked. It's not the power section, I followed the signal and the distortion is there at the output of the tone stack.

All pots, tone stack caps checked, none leaking and causing offsets with DC. Checked DC voltage ofsetting grids - all ok, except one as follows:

Finally, after hours checking every voltage and every resistor value to see if any had skewed, I checked the voltage between cathode and grid on the direct coupled grid. Voltage was -710 mV between grid and cathode. All other tubes were -1.4V which as far as I know is a good working region. Signal arriving at this grid is 1.6 VAC RMS. If the direct coupled BIAS is -710 mV and my signal is 1600 mV, this grid is going positive.

So the questions are:

1) Is that what's causing the ugly distortion in your opinion? Changing those resistors is a hassle, it's a point to point wiring build and I'd rather hear from you before taking that sector apart.

2) Do you have experience with the 5F6-A with a 12AX7 on V1? Have you changed values to accommodate this tube?

3) Any other ideas as to where my distortion is coming from?

Here's what I've tried so far:

1) Injected a 100 mV sine wave on input, first output is 3.6 VAC peak, undistorted. Is gain of 30 ok here?

2) Checked all voltages, isolated the problem to the preamp section.

3) All 12AX7 plates are betwen 180 and 200 V DC

4) Cathode resistors on preamp are all 820R, I realise this was for the 2 sections paralleled, should I change to 1K5 ?

I wanted to lump all questions into one place at once. Thanks very much in advance and pardon the long post

[jmaf]

I can haz no help?

[txstrat]

4) Cathode resistors on preamp are all 820R, I realise this was for the 2 sections paralleled, should I change to 1K5 ?

Definitely. I'd even consider to go up to 2.7K when using a 12AX7. That way you'll have even less gain and therefore get closer to a 12AY7.

[MWJB]

Bassmans all over the world work OK with 820ohm cathode resistors at V1 & V2a...you are free to change this value as you see fit, but you seem to be describing a malfunction, rather than something that is a question of taste. Ergo, without wanting to sound harsh, your amp is NOT a verbatim copy...the sooner we grasp that notion the sooner we will fix the issue.

Q's 1) layout/grounding/ - post pictures 2) Yes, 99% of folks run a 12AX7 in V1, stock values, BUT a 12AY or 12AU7 might work better with humbuckers, shouldn't have any clipping with a 12AU7 in there 3) See 1..

What you have tried..
1) 3VAC RMS or over should be OK at V1A plate, voltage at V1A could be up to 55x input voltage. How do you have the controls set, see the 59 RI schem at Fender.com

2)Checked them, so please list them (dc)

3) That's not normal V1 can be anywhere from 220-260vdc with a 12AX7, a little less at V2A.

4) you can if you want, but I'd fix the issue before revoicing the amp.

[Amp Kat]

^^^^^^^^ Agree and you need to find what section is causing the problem as in pre or post. It sounds like the amp is overbiased meaning crossover distortion occurring at higher volumes. Could be the signal not hitting the grids of the PI and Output section hard enough which would be in the preamp. I always take a wire and feed the PI grids with the signal coming right off the 1st gain stage post coupling cap. If the problem exist there the chances of it being preamp related or almost slim and none.

[pdf64]

'Ugly distortion' can often be some type of positive feedback / parasitic oscillation. Even if the schematic is identical (cathode resistor?), the devil is in the detail, in this case how the schematic has been implemented. Possible causes could be
- grounding (as noted above), poor grounding can result in horrible positive feedback that only occurs at high signal levels, as the high current cause coupling from the power amp back to the pre amp.
- lead dress
- power supply, maybe a bad B+ decoupling cap with too much ESR, creating positive feedback between stages.

[txstrat]

Definitely. I'd even consider to go up to 2.7K when using a 12AX7. That way you'll have even less gain and therefore get closer to a 12AY7.

Somehow I thought this was a one channel amp (don't know why) and therefore thought it would be a good idea to start with changing the cathode resistor to at least 1.5k.
Please accept my apologies.

[pontiacpete]

I don't know how much of a help I can be, but I had one here once and the 12ax7 was really microphonic no matter what ax7 I put in, so I put in a 12ay7 and it calmed a lot.
the obvious question is Have you swapped out the power tubes and bias?

[jmaf]

Wow, thanks for all the input guys. I went to the shop and just returned, I didn't expect as many replies. Thanks so much for your time.

OK, I got the amp fixed and it sound great, but I don't like how I fixed it - at all. It seems as if I crippled it to get it working. First, here's what I did:

1) The direct coupled pair of triodes:I matched the two 100K resistors, first triode plate, second triode, down to as precisely as I could using my LCR bridge, found two resistors reading 105K - used them. Sound improved, but still had the sudden distortion when hitting the low E a bit harder. It sounded fuzzy, all low end notes up until the D string sounded horrible still.

2) I followed the signal from V1 on, it seemed to get real muddy on the 2nd gain stage, the one that feeds the directly coupled grid. And here's where I don't like what I did: I lifted the V1 cathode from 820R to 10K. Yes, 10K. I used a potentiometer to find the spot where the low E no longer distorted. I hate my solution, but the amp sounds amazing now and I can deliver it like this, but I don't like what I did. I had a blues player try it without telling him of the earlier troubles, he was all praise. But I think I may have masked something wrong farther ahead...

txstrat - Yes it's a one channel amp, you are absolutely right it's a one channel amp here - put it this way, it's a copy of the 5F6-A bright channel.

Amp Kat - I've isolated the problem to the preamp. But with the signal follower the sound is better than the amp, so the power section may be contributing something, though I have not found what it is.

pdf64 - My problem seems to be inverse. The amp sounds amazing cranked up. It's from 1 to about 4 that it sounds terrible with a humbucker...

pontiacpete - Any input is appreciated, thanks for your opinion. I changed all tubes around, even for 12AU7's - no luck. Using all new JJ 12AX7's now.

MWJB - Thanks so much for your input. I'm back from the shop and I have no pictures with me here, but I tested all grids and pots for positive DC, all pots read 3 decimal places zero. Grids read between - 1.21 and -1.14VDC on the inverter, V1 grid around -4 VDC with the 10 K cathode resistor.

More info:
- Tubes: All new JJ 12AX7 but like I said I tried 12AU7's and 12AT7's and the ugly distortion still appeared, just lower volume.

- Changed reservoir caps, installed an temporary external totem pole to check with brand new sprague atoms - no luck. Same distortion.

- Added a trim pot to the direct coupled cathode and plate and manually tested several positions in real time, none sounded good, distortion still there when i played notes on the low E string. Only thing that worked was matching the direct coupled pair resistors to 100k exactly.

- Unsoldered the negative feedback, not much of a change. It seems my problem is in between V1 and the tone stack.

- Changed all resistors from V1 up to the PI, all of them. Carefully matched them to spec. Nothing changed, 1 hour and a bunch of carbon comp resistors thrown away for nothing.

+B = 475 VDC
Preamp = 284 VDC
Plate V1 = ~190 VDC

I would not like the solution to have been "just cripple the V1 gain with a big as ff resistor". Because I know this kind of distortion is NOT overdrive, it's ugly distortion as with a positive grid or a tube working out of its proper region.

Any ideas folks? Sorry for the long post and thanks for your time.

[jmaf]

MWJB - I will provide more info tomorrow, I didn't write down all the readings I took, it was a full day working on this problem today. The grounding seems ok, I have a central buss of 2.5mm copper wire, everything, reservoir caps ground, output cathodes, preamp grounds, all go to this buss.

[jmaf]

One more detail, this may be significant.

(Using a resistive load for a speaker.)

I unsoldered the 22n coupling cap on V1. So the signal stopped there, did not proceed onto the rest of the amp.

I played the guitar and could hear it....but there was no speaker - and I had unsoldered the 22n coupling cap! How the heck???

Well, it turns out the output transformer was singing.... How did this signal reach there? There is no signal connection from V1 onwards....

Can this mean something? The power supply is getting modulated somehow? But I changed reservoir caps....how can this be? This may be a relevant detail, and I had never seen this before, but my output transformer can sing like a speaker if I unsolder the 22n coupling cap off V1. If I solder it back - no problems, output transformer is quiet. What is happening here?

[MWJB]

Being a 1 channel amp, what have you done with the unused V1 triode?

At 475v B+, 190vdc on the V1 plates is unusually low, assuming that you are using 4.7K & 10K dropping resistors in the B+ rail.

I was asking for plate & cathode dc voltages...the grid voltages are irrelevent in themselves as no one has ever measured & recorded them. 99% of faults will show up in the dc plate & cathode (& CF grid) voltages

[Steve Conner]

Couldn't this be a matter of taste rather than a malfunction? Some people hate the sound of unmodified vintage amps. They call it farting, nasty distortion on low notes, whatever. Usually they're "modern" players running Gibson-style guitars with high-output humbuckers, which the Fender amps were never designed to cope with.

Meanwhile old grizzled bluesmen (probably playing Strats and Teles) think the same tone is wonderful.

I've got an old Selmer Corvette that used 5k cathode resistors on its 12AX7 preamp tube. I really liked the effect of the oversized resistors and ended up leaving it like that. I modded the amp radically, but that was probably the one thing that I left the same. I think if I changed them to 1.5k, the poor amp would sound like a whoopee cushion when hit with a neck humbucker.

But then if it does weird things, like the OPT singing when there shouldn't be any signal, that points away from taste and towards parasitic oscillations.

[MWJB]

Hi Steve,

I guess, yes to some degree it might be, but the OP seems to be describing something that isn't typical. For some reason his preamp voltages appear to be ~35v down, this might be just enough to underbias the preamp tubes & when hit with a large signal they suffer. It's not unusual to use lower mu tubes in V1 with humbuckers, but the amp should certainly be clean with a 12AU7 in V1, if not with a 12AY7 in V1.

It now emerges that this is a 1 channel amp, if the OP has wired the "spare" triode in series, then a much larger cathode resistor (10K unbypassed) is not uncommon for the 2nd triode (1st triode needs to be corrected to 1.5K). If he hasn't wired in the spare triode at all, then voltage at V1 should be even higher. If V1 triodes have 820ohm at each triode, rather than shared, then that could explain the low voltage and the ugly distortion.

The Selmer Corvette is a very low voltage amp, with no attenuation between the 2 halves of the 12AX7/ECC83...it probably needs those 5K to get any clean at all (Kalamazoo Model 2 also used 4.7K cathode resistors, but had the more typical vol/tone placement between the 12AX7 triodes). The 5F6A with a 12AY7 in v1 should be on par with a Super Reverb clean-wise.

Plus the biggest factor in this, is that the amp in question is not a proven, road tested amp. The schematic might be, there are tens of thousands of 5F6A that are, but this is one amp that we have never seen, built to a schematic (unseen) which is just a bit of paper that does not supply enough information to do the job. There's a lot of scope to screw up layout...Marshall did it with the 1962 RI, as you know from the Rattling PRRI thread, Fender seem to have done it with that amp too...it's too easy for a builder unfamiliar with the foibles of that particular amp to make an error. I know how easy it is, because I've done it too in the past (all values & voltages check out...the amp just never works properly, sub all the parts you like, if you just put them back in the wrong place, then it never will)...as have many of the people that make suggestions as to how to cure such problems.

I'm not precious about changing values in any amp, to suit the player, it accounts for most of the work I do...I just don't do it to mask a malfunction, make sure that everything works and you are at square one, before modding.

[jmaf]

Couldn't this be a matter of taste rather than a malfunction? Some people hate the sound of unmodified vintage amps. They call it farting, nasty distortion on low notes, whatever. Usually they're "modern" players running Gibson-style guitars with high-output humbuckers, which the Fender amps were never designed to cope with.

Meanwhile old grizzled bluesmen (probably playing Strats and Teles) think the same tone is wonderful.

I've got an old Selmer Corvette that used 5k cathode resistors on its 12AX7 preamp tube. I really liked the effect of the oversized resistors and ended up leaving it like that. I modded the amp radically, but that was probably the one thing that I left the same. I think if I changed them to 1.5k, the poor amp would sound like a whoopee cushion when hit with a neck humbucker.

But then if it does weird things, like the OPT singing when there shouldn't be any signal, that points away from taste and towards parasitic oscillations.

Yes, unfortunately I believe this amp has something else wrong with it....and hopefully I will find it later today. I'm taking note of all the feedback here to go back to the shop and test. Thanks for your input as well, it may well be excess drive but I've had some other 5f6-a's (well, who hasn't, it's the most copied ever) and this distortion is not like overdrive, it's like tubes working out of the linear region into non harmonic distortion, as if it clipped all of a sudden, not tube typical.