Princeton Reverb BF bass tone control/reverb hum

[Roundtone]

Greetings to my wise and knowledgable community

So, I just finished a ground up build of a BF Princeton Reverb AA1164 (Jensen P10Q) and have a few ?s for you guys.

PHOTOS (http://picasaweb.google.com/roundtone/PrincetonReverb?feat=directlink)

First, the bass tone control seems to be shunted (very little change to tone - is this normal?) but appears to be wired correctly. I haven't ever diagnosed a tone circuit for this sort of symptom. Pot is a new Alpha.

Second, I was getting some pretty heavy hum then realized I was missing the two 100 ohm resistors off the legs of the heater feed. Putting those on helped a lot, but I am still getting more hum than I expected, especially when the reverb control is turned up past 2.

Third, reverb control goes to much to fast (any thing over 2 is too much for my taste) - what is a good way to give more subtle control without hosing up the tone?

(I like the tone the best with volume at 10, bass at 10, treble at about 6, reverb on 1 and just a hint of slow trem, tele on the bridge PUP. . .)

Thanks in advance

[TweeDLX]

Using a different tank (2 spring vs 3 spring) can help reduce the intensity of the 'verb.

Mike

[phsyconoodler]

Lead dress can cause hum problems.Go over your wiring again.
Reverb hum can be cables,jacks or even where the filtering is located.Make sure the reverb power node goes to the right place to get filtering.
You can tone down the reverb by using a 12AY7 in the reverb return position.Or a 5751,or sometimes even a 12AU7.You can also soften the bias on the return tube with different cathode resistors.

[Roundtone]

Thanks for the comments -

Wish I had another tank to try -- good call on trying the other tubes, I will let you know what happens.

[Roundtone]

Any ideas to make the bass tone control more pronounced?

[phsyconoodler]

there's not enough bass happening in that little amp.If it was a 6L6 amp the bass would be more present.It simply cannot produce more.

[Roundtone]

Here are the gut shots finally. . . They are at the end

Photos (http://picasaweb.google.com/roundtone/PrincetonReverb?feat=directlink)

[SatelliteAmps]

That looks amazing. What is the treble cap on it? Might be an issue for the bass control.

[Roundtone]

250pF silver mica if my memory serves. I will double check when I get home.

clip1 (http://picasaweb.google.com/lh/photo/Jow6t50mwf2ARCkJz5LCaw?feat=directlink)

clip2 (http://picasaweb.google.com/lh/photo/7sfZdjAfItJybsMIRyqheQ?feat=directlink) Found it in a picture ...

I also just read a note about changing a resistor value to "tame" the reverb a little. Have you heard of this?

[SatelliteAmps]

The bass control should be giving you some amount of bass response (regardless of whether it is a 6V6 or 6L6 based amp). I am leaning towards either a bad cap or a wrong value. If the 250pf is good, I would check the .047 and the .1 that are feeding the sides of the bass pot. Also check the little resistor (6.8k) on the back of the pot. And lastly, the pot is a 250k Audio pot, right?

[Roundtone]

Yes, It is a 250K audio taper pot. I will check the other components tonight and post the results.

Thanks again

[Roundtone]

All components checked out OK. During testing I had a P10R connected rather than the P10Q I mounted in the cabinet. I found that the P10R is more responsive to the bass tone control and I could hear changes better. I am assuming that this is due to the fact that the P10R is a more sensitive speaker. I prefer the P10Q characteristics in a general sense, so I am going to leave it in.

The amp is beginning to be like an oil painting to me that I can't finish. I really like it as is, but thought I better complete my due diligence to make sure it represents that circuit (AA1164) to the best of my ability.

I researched Hoffman's bias adjustment addition as well. Do you think the addition of the bias pot is a general improvement of the circuit or just an accessory?

[Roundtone]

You can also soften the bias on the return tube with different cathode resistors.

Do you have resistor value or bias recommendation for the return tube?

[TweeDLX]

All components checked out OK. During testing I had a P10R connected rather than the P10Q I mounted in the cabinet. I found that the P10R is more responsive to the bass tone control and I could hear changes better. I am assuming that this is due to the fact that the P10R is a more sensitive speaker. I prefer the P10Q characteristics in a general sense, so I am going to leave it in.

The amp is beginning to be like an oil painting to me that I can't finish. I really like it as is, but thought I better complete my due diligence to make sure it represents that circuit (AA1164) to the best of my ability.

I researched Hoffman's bias adjustment addition as well. Do you think the addition of the bias pot is a general improvement of the circuit or just an accessory?
As an oil painter, I can appreciate your dilemma! I heartily reccomend the bias pot, and adding the one ohm resistors and test jacks to the chassis makes biasing incredibly easy. I did the bias pot arrangement shown on Hoffman's site. Works well for me. Your amp looks beautiful!

Mike

[Roundtone]

As an oil painter, I can appreciate your dilemma! I heartily reccomend the bias pot, and adding the one ohm resistors and test jacks to the chassis makes biasing incredibly easy. I did the bias pot arrangement shown on Hoffman's site. Works well for me. Your amp looks beautiful!

Mike

Thanks Mike - for the advice and the encouragement.

[Roundtone]

That looks amazing. What is the treble cap on it? Might be an issue for the bass control.

Thanks again Adam for your encouragement as well.

[kurtlives]

What taper are you using for the reverb? If your using a linear, I would switch to log. With a log taper all the reverb intensity dosent get "jammed" up at the beginning.

I would also isolate your RCA jacks. I use these nice high quality gold ones sold at Antique Electronic Supply. With insulated jacks you can avoid a lot of noise and a nasty ground loop. Ground the jacks back with your preamp grounds.

[Roundtone]

Again from memory, it is a 100K linear. I think I have enough room to pull the RCAs out of the chassis for a test. I will post my findings.

Thanks

[Roundtone]

Update - I pulled the reverb and footswitch RCAs out of the chassis and isolated them and routed a separate ground. Bad news - no improvement, Good news - found an unsoldered ground connection on the reverb send ground ring :) Put her back together and THEN there was an improvement. Think I am going to leave the 100K-L in there to match the schematic, now that things have quieted down a little.

[SatelliteAmps]

Loose or unsoldered stuff can be a nightmare to track down. Glad to hear it is working better. Any improvement in the bass control?

[Roundtone]

Adam, I could really use an original 64 to sit beside my amp for comparison! I can hear it change but just not the sort of change I am used to hearing in other circuits. What do you think about adding the bias pot? I got one thumbs up and so far no nays.

[Roundtone]

As an oil painter, I can appreciate your dilemma! I heartily reccomend the bias pot

So... I pulled the trigger on the bias pot and it is very cool indeed. I used a 10KL bias pot and the rescued resistor off of the bias cap. Thanks for the advice. Very simple non-invasive mod to aid in biasing the circuit. I honestly could tell any difference in the tone at the same bias prior to the change.

Now I have another issue. I am getting some nasty distortion during full output on attack of the strings. It is a popping noise on the attack and will continue on a loudly a power chord for a second or two and then give way to normal power tube distortion (that I dig).

Any more advice?

[SatelliteAmps]

Bias pot is a good idea. Makes life so much easier.

Does it change if you change the bias of the amp? Could be biased way off (double check to make sure).

If that isn't it, it could be an oscillation inside the amp somewhere (which can be a pain to located, but it can be done, even without a scope). The next step will be to try and locate it in the circuit. Does any of the controls make a difference in whether you hear it or not?

[Roundtone]

Better wrap this thread -- and thanks for all the help. Turns out to be a bad old stock preamp tube, which is sad cause I paid a little too much for it. Anyhoo after tube swap and bias this amp is singing like an angel. Really complex harmonic content and strong trem. (I actually love the sound of the trem with the power tubes biased cold) The bass tone issue was rooted in competing hum. Lead dress (me luv um chopstick) lowered my noise floor and there was my bass.