'66 Fender Princeton Reverb tone problem

[bohema79]

Hi Guys!
I have a 1966 Fender Princeton Reverb.
The amp was stock when I got it.
It sounded ok but compared to my friends stock 1966 Princeton Reverb it is lacking sustain, harmonics, sag and has more hights. Not as wide "3D" sounding.
- both played through the same speaker!!!

Ive replaced the bypass caps and bias cap with Sprague Atoms 25uf/50V & 50uf/100V and some bypass 1.5K resistors that have driffted way of.
- not much of a change.

All the vintage tubes in the amp test 85% and 100% and sound great when used in the other amp.

The bias is set at 22mA and the plate voltage is 439V

Can some one please tell me what can make these amps sound so apart and how can I make my amp have more harmonics, sag, sustain and less treble.

Both of amps are the AA764 circuit and use a GZ34 and a Jensen 10Q speaker.

The fillter caps can is replaced and tested fine.

Thanks!

[eleventeen]

Offhand, I would say you probably have a leaky coupling cap somewhere that isn't letting much signal through (or is crushing it) Also possible is a shorted or near-shorted tone cap. Unlikely, but you sound like you've at least checked the easy stuff. It is further possible that you have a bad solder joint that worked for 46 years but suddenly pooped out in year 47.

Does one amp crackle a lot when you turn the tone knobs? This is indicative of DC on the pot(s) and thus implies a leaky cap.

Best would be to 'scope the signal, amp-to-amp, with the same tone generator on the amplifier input.

Lacking that (a scope) I would be checking and recording plate and cathode voltages along the signal path; pulling out the reverb driver, the tremolo tube, and comparing. If you have a really low plate voltage reading somewhere, that tube is not going to be able to swing a good sized signal. This, too, could be caused by a leaky cap, because such a cap could mis-bias the next preamp stage by adding (unwanted) DC volts to a later stage. You might check ohms across some of the coupling caps...and this would have some validity but is not optimum.

[HotBluePlates]

I have a 1966 Fender Princeton Reverb.
The amp was stock when I got it.
It sounded ok but compared to my friends stock 1966 Princeton Reverb it is lacking sustain, harmonics, sag and has more hights. Not as wide "3D" sounding.
- both played through the same speaker!!!

There may be nothing wrong with the amp.

I've owned a number of blackface and silverface Princeton Reverbs. Some just sound different than others. Unless you have a good means of going through and measuring every component in both amps to figure out what's different, you may never get them to sound alike.

Even if you do figure out what's different, you may never get them to sound alike.

Case in point: I once knew a shop owner that had a pair of Vox AC-50's. One sounded like a typical AC-50, loud n clean. The other had a distorted grind that would make any 60's Marshall 50w amp envious. The shop owner wanted me to figure out what was different about the grindy one, and make the clean one sound like it (cause he intended to keep the grindy one, and had a customer wanting to buy it, so he needed the other amp to sound the same way).

I measured every part in both amps, used the same exact tubes in each (by swapping the same tubes between amps), plugged into the same speakers. I couldn't find anything that was substantially different in either amp. In the end, I simply had to modify the clean AC-50 to respond the way the other did (in other words, make the clean amp non-stock in order to get it to break up while the dirty amp still stayed stock).

[Willabe]

The bias is set at 22mA and the plate voltage is 439V

Isn't that on the cold side? I have mine set at ~35mA.

What is the amp's bias set at?


            Brad     

[Platefire]

I agree, going warmer on the bias can make a world of difference--just watch the tubes for red plating--the old tubes my not can handle it. Platefire

[bohema79]

Offhand, I would say you probably have a leaky coupling cap somewhere that isn't letting much signal through (or is crushing it) Also possible is a shorted or near-shorted tone cap. Unlikely, but you sound like you've at least checked the easy stuff. It is further possible that you have a bad solder joint that worked for 46 years but suddenly pooped out in year 47.

Does one amp crackle a lot when you turn the tone knobs? This is indicative of DC on the pot(s) and thus implies a leaky cap.

Best would be to 'scope the signal, amp-to-amp, with the same tone generator on the amplifier input.

Lacking that (a scope) I would be checking and recording plate and cathode voltages along the signal path; pulling out the reverb driver, the tremolo tube, and comparing. If you have a really low plate voltage reading somewhere, that tube is not going to be able to swing a good sized signal. This, too, could be caused by a leaky cap, because such a cap could mis-bias the next preamp stage by adding (unwanted) DC volts to a later stage. You might check ohms across some of the coupling caps...and this would have some validity but is not optimum.

Hi,
Ive noticed today that even with the treble control at "0" the amp has a diecent amount of presence/treble in the signal... ?!

[eleventeen]

Most Fender amps, if the tone controls are turned all the way down, put out pretty near nothing.

[Willabe]

Ive noticed today that even with the treble control at "0" the amp has a diecent amount of presence/treble in the signal... ?!

Are you sure nobody put a small treble bypass cap across the treble pot? That would do what you are describing.

Turn the bass pot down to zero also with the treble pot. Now what do you hear?

What about the bias? Your bias is pretty cold and will sound on the thin side.


             Brad        

[bohema79]

Ive noticed today that even with the treble control at "0" the amp has a diecent amount of presence/treble in the signal... ?!

Are you sure nobody put a small treble bypass cap across the treble pot? That would do what you are describing.

Turn the bass pot down to zero also with the treble pot. Now what do you hear?

What about the bias? Your bias is pretty cold and will sound on the thin side.


             Brad        

Hi,
There is no treble bypass cap on the pot.
The amp is biased at precisly 70% of the RCA 6v6GTA's dissipation. That isn't on the cold side at all.
If you put my plate voltage in the Weber Bias calculator for 6V6 GTA you get 22.3mA bias for 70%.
If I run the vintage RCA's at more than 70% they wont last to long and I wont get any tremolo cause iits bias variable. It seems to me that your bias for the princeton reverb is an overkill. Does your tremolo work?

When I also turn the bass control to zero and the treble is to zero I hear a thin sounding amp with some presence/treble on the notes. :(:(

[Willabe]

My trem is not hooked up. I stole the triode to make a LTPI.     

My point was is the other Princeton biased the same as yours? I understand about the trem and the dissipation but if the other amp is biased hotter that might be a big part of the difference in the sound? If it's biased hotter it's not a fair comparision. Just a thought.

IIRC, Pysconoodler has posted that that he runs 6V6's in PR/DR up to 30mA to 35mA and they sound great. He said he knows that it shouldn't work but it does. Maybe he'll correct me on this?


             Brad     

[Willabe]

When I also turn the bass control to zero and the treble is to zero I hear a thin sounding amp with some presence/treble on the notes.

Some pots don't turn all the way off. That by it self I would not worry about since you don't play the amp with the treble/bass pot turned to zero.

To check the coupling caps for leaking dcv you have to lift the far end (end towards the next tubes grid, leave the cc end that's hooked up to the tubes plate connected) of the coupling cap clip your + meter lead to it the - lead to ground, meter set for dcv and turn on the amp. Should be less than 1dcv, more like a fraction of 1dcv. Go through them 1 at a time and see if any are leaking. If any are they can kill the amps tone. If any of them leak a lot they can stop the amp down from making any sound.


             Brad         

[HotBluePlates]

The bias is set at 22mA and the plate voltage is 439V

Isn't that on the cold side?

No, that's dead-typical for a blackface Princeton.

440v * 0.022A = 9.68w
9.68w / 12w = 80.7% dissipation

Yes, you might subtract screen current from cathode current (but these amps don't have 1Ω resistors stock, so he may be using the OT shunt method of measuring plate current). Yes, you might use 14w instead of 12w as your plate dissipation number. However, I know when I had my old Princeton serviced by a competent "guru" (in the days before I knew much of anything), he biased it to 22mA at idle.

When I also turn the bass control to zero and the treble is to zero I hear a thin sounding amp with some presence/treble on the notes. :(:(

Ummm... don't tun the tone controls to zero? 

Google "Duncan Tone Stack Calculator". Download it. Select the "Fender" tab (which is selected by default when you launch the program). Set the "Mid" slider to about 6.8. Set the "Treble" and "Bass" sliders to zero. Output with be reduced as shown on the graph but will not be off in any setting. The 6.8 setting of "Mid" mimics the 6.8kΩ resistor you have in your Princeton's tone stack.

I think you you may be perceiving things as problems which are not problems. Without hearing both amps & tinkering them in person, it is impossible to understand what difference you heard between them, much less what might be causing such a difference.

I'm sympathetic to your position because when I got my first tube amp (a '67 Princeton Reverb), I tinkered with almost everything in it, thinking I make it sound "better". I started with an amp that sounded like a Fender Princeton and ended up with an amp that sounded like a Fender Princeton.

If there's not an obvious problem (redplating tubes, crackling output, huge hiss, hum louder than the guitar), I'd humble recommend letting the amp be what it is. Which after all is a pretty cool old Fender...

[bohema79]

The bias is set at 22mA and the plate voltage is 439V

Isn't that on the cold side?

No, that's dead-typical for a blackface Princeton.

440v * 0.022A = 9.68w
9.68w / 12w = 80.7% dissipation

Yes, you might subtract screen current from cathode current (but these amps don't have 1Ω resistors stock, so he may be using the OT shunt method of measuring plate current). Yes, you might use 14w instead of 12w as your plate dissipation number. However, I know when I had my old Princeton serviced by a competent "guru" (in the days before I knew much of anything), he biased it to 22mA at idle.

When I also turn the bass control to zero and the treble is to zero I hear a thin sounding amp with some presence/treble on the notes. :(:(

Ummm... don't tun the tone controls to zero? 

Google "Duncan Tone Stack Calculator". Download it. Select the "Fender" tab (which is selected by default when you launch the program). Set the "Mid" slider to about 6.8. Set the "Treble" and "Bass" sliders to zero. Output with be reduced as shown on the graph but will not be off in any setting. The 6.8 setting of "Mid" mimics the 6.8kΩ resistor you have in your Princeton's tone stack.

I think you you may be perceiving things as problems which are not problems. Without hearing both amps & tinkering them in person, it is impossible to understand what difference you heard between them, much less what might be causing such a difference.

I'm sympathetic to your position because when I got my first tube amp (a '67 Princeton Reverb), I tinkered with almost everything in it, thinking I make it sound "better". I started with an amp that sounded like a Fender Princeton and ended up with an amp that sounded like a Fender Princeton.

If there's not an obvious problem (redplating tubes, crackling output, huge hiss, hum louder than the guitar), I'd humble recommend letting the amp be what it is. Which after all is a pretty cool old Fender...

I just found out that the 250pf treble ceramic cap on the board has driffted to 460pf...
So that should be the reason of the trebly sound.

[HotBluePlates]

I just found out that the 250pf treble ceramic cap on the board has driffted to 460pf...
So that should be the reason of the trebly sound.

Unlikely. That will just shift the frequency of the midrange dip.

Prove that last bit to yourself by playing with the value in the Tone Stack Calculator.