Farty bass on '66 Fender Pro

[Phazor]

Look at the power tube sockets and make sure there is no shorting. An old Socket can burn thru and arch between screen and plate pins. I would also try a new 5AR4 rect tube. Those Rectifier tubes can get soggy sounding. I keep a weber solid state GZ34 around to check for that problem. They have sag and lower voltage like a GZ34. The other thing that can cause the problem is a bad screen resistor so check both of the 470 ohms.

[stoo]

Don't know the amp but is there Neg. feedback? Sure sounds like it's not working!
stew

[rfgordon]

I got the go-ahead from the owner to rebuild the power supply, so the parts are on order. When I brought it up, he then told me the last cap job was about ten years ago. Wish he'd a told me that when I started...

[rfgordon]

I recapped the PS. The amp does sound better and punchier, so the new filters werean improvemnt--but they didn't solve the fizzy distortion problem. I may try changing out the dropping resistors in the PS.

Tomorrow (my wife scored two free tickets to the Rod Stewart show for tonight) I'm gonna replace the grid & screen resistors on the 6L6s.

I tried a different recto tube, no change. (Well, except that the JJ 5AR4 lasted about two seconds before dying a most spectacular death!! One fuse and some awesome, glass-encased fireworks later...I'll know never to buy those again!) My standby "made in West Germany" 5AR4 worked fine in the Pro, but the same fizzy sound was there.

I pulled the preamp tubes for the Vibe channel and played thru the Normal channel to free up current, but the problem was still there. Then again, it could be the PT--this amp is so rusty, you'd think it was recovered from a sunken ship! Could also be the choke, I suppose. I've got one of those I can put in, but I don't have a PT lying around.

That only leaves about a half dozen resistors that the channels share in the mixer, PI and feedback. I suppose I could replace those--they're all 1/2 watt CFs of unknown quality.

Tis a very mysterious problem.

[dehughes]

He wanted it converted to cathode bias, which I did...

What value of cap/resistor did you use? I'm doing something similar with a SF Pro Reverb...

[rfgordon]

dehuges,
I useds a 330 ohm resistor and a 33mF cap. On my usual builds I have lower voltages on the 6L6 plates, but this Pro has the 444VDC, so I went with a pretty big resistor. sounds great, apart from this mysterious noise! I have found that there is great variation among current production 6L6s as to how much current they can handle in cathode bias before they red-plate, so you may have to experiment. If you use a lower value, say 250 or 270 ohm, some tubes may tolerate it better than others. Note: this resistor is shared by the two tubes.

[dehughes]

Right on. Thanks. I was considering a 300 ohm 25w Dale resistor, but if the tubes are looking at 500v on the plates...then this might be too low a value for the resistor.

Thanks!

[rfgordon]

An update:
I've ruled out the PT as the problem. I ran the amp off the B+ from another amp, and the noise was still there. Also checked/replaced the feedback resistor and another coupling cap.

Next I reckon I'll change the resistors on the output tube sockets.

[kralgard]

A guy brought his well-worn 66 Fender Pro to me for some work. It has really nasty farty distortion on the bass notes.

How about reducing bass response on pre amp? I guess it's more or less typical for pre amps with large cathode bypass caps that they give farty bass when overdriven. See e.g. http://www.guitarnuts.com/amps/g40v/index.php: "Another thing this cathode circuit will do is severely attenuate low frequencies...when you overdrive at low frequencies the result is not very musical, the tendency is for low frequencies to sound "farty" or buzz when overdriven."

Of course if it is clear that the problem is not in pre amp this hint is worthless

[jelle]

Well other fender amps have 25uf cathode bypass caps and sometimes even .047 V1b output caps and they do not have this response that this amp has.

But the ab763 fenders have some thing called 'stiction' going on at higher freqs. It happens when the power tubes are working hard. I'm sure you are familiar with this. I'm not sure on the technical details but it involves lowered B+ and bias current hat is also lowered resulting in a very cold biased amp when the low notes are played at high volumes, especially in amps with a rectifier tube. But smaller caps between the pi and powertubes should solve this......and you have tried this.

Another thing before I start to suspect lead dress in the amp: are you 100% sure that all the ground connections are OK?

If so, please post a pic of the guts of the amp. I have had a dumble clone that also did this and it drove me NUTS! I tore out the entire preamp, redraw it, rebuild it and the problem was gone. I hope this is not going to be the case with his one!

Good luck,
Jelle

[Bob-I]

Well other fender amps have 25uf cathode bypass caps and sometimes even .047 V1b output caps and they do not have this response that this amp has.

Another issue may be that the caps have drifted in value over the years. I've measured 25uF caps at over 80uF. Even new one's can measure as much as 40uF. Can't hurt to measure them.

[rfgordon]

This problem is not a bass response issue. I've been down all those roads. Yesterday I replaced the grid and screen resistors on the output tube sockets. No change.

One of the things I'm learning thru this is that, while the web is full of amp-builders, it is woefully short of amp-fixers, or at least ones who will share their knowledge.

[jelle]

I'm tryin to help....

Jelle

[jelle]

Have you tried the ampage?

Jelle

[Chris333]

+1 Ampage for this kind of stuff.

http://music-electronics-forum.com