Ghosting on high notes (recapped AB165)

[Paul-McShartney]

Usually the bias is reported as a negative value referenced to ground. I don't see how the amp could work if it wasn't negative, but I wanted to double check.

idk it might be my voltmeter? the filter section shows as negative voltage while the bias and say, grid stoppers show positive voltage. I'm using an alligator clip from chassis to ground on my meter which could be why? Either way, I feel like the amp is sounding just as good as it did and low noise so I'm not sure any damage was done? I can't figure out why the resistance changes on the pot but doesn't function in the circuit though...

 

[Paul-McShartney]

*facepalm*

I got a new voltmeter a few days ago and the inputs are reversed from the other one I had. I haven't been paying attention to the polarity of the voltages so I didn't think anything of it. but yeah, the polarities of the bias, grid stoppers, everything should be correct. Sorry for the confusion

by the way, I'm getting 120 VAC on the negative side of the bias cap and the rectifying bias diode right next to it. this can't be right?

EDIT:
just remeasured and im not getting 120VAC, I got that measurement when I had the probes plugged oppositely, so im assuming that was a bad reading. Pretty sure something catastrophic would have occurred if there was ac there?

 

[peteb]

I am glad to hear that your amp sounds fine.

it sounds like you have learned a lot.

So did I dodge a bullet here or is there something I'm missing?

you are not finished until you measure the bias current on each tube. i would use the OT resistance method.

Measured 0.2-0.1 ohms (this is after subracting the probe resistance). Primary reads 47 ohms, and 96 ohms .

one primary is 47 ohms and the other is 96 ohms?

if it is, I would be concerned that they are not closer.

OUTPUT TUBES
425V at p3 of output tubes (brown and blue of OT)
424V at screens (p4 and p9)
45V at grid stoppers (p1 and p5)

this is a good sign. It looks like both tubes are drawing equal amounts of idle current.


I would check the bias.

 

[YellowBoots]

I can't figure out why the resistance changes on the pot but doesn't function in the circuit though...

You know, as I read the schematic, I would call that pot more of a bias pot than a hum canceller. Hum is going to cancel even if the bias settings between the tubes are different. If you can vary the negative bias voltage on one of the tubes with that pot, then it is working.

Resistors all along the bias pot signal path test fine and look unharmed.

How about those 10K resistors on either side of the bias pot?

just remeasured and im not getting 120VAC, I got that measurement when I had the probes plugged oppositely, so im assuming that was a bad reading. Pretty sure something catastrophic would have occurred if there was ac there?

Yeah, that wouldn't have been good. But still... when measuring AC, it doesn't matter which way the probes are connected, so I'm a little confused by your results, but not worried.

 

[Paul-McShartney]

I am glad to hear that your amp sounds fine.

it sounds like you have learned a lot.



you are not finished until you measure the bias current on each tube. i would use the OT resistance method.



one primary is 47 ohms and the other is 96 ohms?

if it is, I would be concerned that they are not closer.



this is a good sign. It looks like both tubes are drawing equal amounts of idle current.


I would check the bias.

Thanks, definitely been a fun process for the most part!

So using the OT resistance method....

OT resistance from center tap (standby switch) to plates:
Blue = 49.7 ohms
Brown = 47.2 ohms

Tube 1 P3 voltage= 426V
ranges from 422-426V
Tube 2 P3 voltage = 426V
ranges from 424-426V


OT center tap voltage (standby switch) = 428V

Voltage drop = 1-2 volts


Plate current 1 ----- 1.5./49.7= 0.03018 = 30.18 mA
Plate current 2 ----- 1.5/47.2= 0.03178 = 31.78 mA

I really can't tell if my multimeter is imprecise but the voltages fluctuated a bit before landing mostly on the number I eventually assigned it. Don't know if this is normal behavior in an amp or if either my meter is bad or the amp is. However, at its greater value (2V drop), the amp sees around 40mA which would still make it within the cool range. in the middle, at 1.5V, it is around 30mA which makes it a lot cooler.

I'll check later tonight again but maybe I need a better probe here as these slight inaccuracies yield a huge difference?

Multimeter is a AstroAI AM33D if you guys are wondering...

Also don't know how I got such drastic differences in resistances when I disconnected the OT but measuring the resistance from standby switch to plates, they seem a lot closer

 

[Paul-McShartney]

You know, as I read the schematic, I would call that pot more of a bias pot than a hum canceller. Hum is going to cancel even if the bias settings between the tubes are different. If you can vary the negative bias voltage on one of the tubes with that pot, then it is working.



How about those 10K resistors on either side of the bias pot?



Yeah, that wouldn't have been good. But still... when measuring AC, it doesn't matter which way the probes are connected, so I'm a little confused by your results, but not worried.

Hmm for sure, yeah I'm jsut confused by the fact that before, a little movement in the pot would yield significant hum while now, moving it one way will increase hiss (which I'm assuming increases plate current).

Those resistors are good too!

Also was very confused by how I got AC there, really didn't make any sense but I've yet to see that again.

Also the hum is back again within the chassis so there's that, though still no more pop when flipping standby on

 

[Lowerleftcoast]

OT resistance from center tap (standby switch) to plates:
Blue = 49.7 ohms
Brown = 47.2 ohms

I am glad to see these numbers. The first meter attempts left me rather concerned.

FYI the actual voltage drop across this resistance will be metered at the same place these resistances were taken. (It meters only a few volts but you know there are over 400 volts there.) It is much more accurate than subtracting 426 from 428. The meter will jump a little when measuring the actual voltage drop.
To be most accurate the OT should be at operating temperature. Like all coils, the resistance of the OT coil changes with temperature.

This is working with high voltage. No jewelry. Shoes on. Rubber mat. One hand in back pocket. All the safety stuff. This can be done hands free with alligator clips.

 

[Paul-McShartney]

I am glad to see these numbers. The first meter attempts left me rather concerned.

FYI the actual voltage drop across this resistance will be metered at the same place these resistances were taken. (It meters only a few volts but you know there are over 400 volts there.) It is much more accurate than subtracting 426 from 428. The meter will jump a little when measuring the actual voltage drop.
To be most accurate the OT should be at operating temperature. Like all coils, the resistance of the OT coil changes with temperature.

This is working with high voltage. No jewelry. Shoes on. Rubber mat. One hand in back pocket. All the safety stuff. This can be done hands free with alligator clips.

for sure, I'll remeasure it the same way I did the resistances and leave the amp on for a bit just to know exactly what plate currents are at.

Anyways, besides that is there anything else I should check on before I confirm nothing got damaged during the mis-wiring? I've checked OT resistances, plate currents, the resistors around bias pot, the bias pot itself, the bias cap, 470 ohm resistor, the diodes and everything seems good to go

 

[peteb]

Plate current 1 ----- 1.5./49.7= 0.03018 = 30.18 mA
Plate current 2 ----- 1.5/47.2= 0.03178 = 31.78 mA

right on!

 

[Lowerleftcoast]

Anyways, besides that is there anything else I should check on before I confirm nothing got damaged during the mis-wiring?

Everything seems to be checking out OK. I am thinking you got away with the mishap. I would stay nearby for the next several uses so you can shut it down should it misbehave.

So far so good.

I don't think you have mentioned what type 6L6 you have. Depending on which 6L6... they can be 19W tubes to 30W. I am interested in some accurate plate dissipation numbers because I am still thinking about the ghost even though that local NFB seems to have cured said ghost.

...and there is still hiss and crackle near V2 right?

 

[Paul-McShartney]

Everything seems to be checking out OK. I am thinking you got away with the mishap. I would stay nearby for the next several uses so you can shut it down should it misbehave.

So far so good.

I don't think you have mentioned what type 6L6 you have. Depending on which 6L6... they can be 19W tubes to 30W. I am interested in some accurate plate dissipation numbers because I am still thinking about the ghost even though that local NFB seems to have cured said ghost.

...and there is still hiss and crackle near V2 right?

For sure, glad it looks okay to you too. I'm just having trouble with the fact nothing seems wrong after the miss-wire hahah. I figured +425 on that middle lug would surely destroy something but I guess I caught it in time to do any real damage.

okay so yeah I remeasured as follows (good to know this is much more accurate by the way, though I was pretty close with my estimate):

Plate current 1 ----- 1.62 / 49.7 ohms = 30.72 mA

Plate current 2 ----- 1.45V / 47.2 ohms = 30.93mA


Im running 6l6GC which should be 30 watts. also turning the bias pot doesn't really do much on the outer tube but has a lot of control for the inner most tube, which makes sense since it is bias balance circuit. so I think all is good there. It is really cold for these tubes and I can't go any higher so I think I might add a trim pot after the 15k resistor on the pot to adjust overall bias too. Wanna see what my preference is, though I do really like the way it sounds cold surprisingly, the crossover distortion is actually quite nice to me.

Also yes, unfortunately the crackle is still there after my deep cleaning of the board, changing the plate resistor, and changing the two 470k's on the local NFB (the latter did seem to help a bit, one of those resistors was seriously acting up).

 

[Lowerleftcoast]

That cold bias and that crossover dist. might be the reason for the ghosts.

Just for giggles clip in a a 220k parallel to the 15k on the back of the bias balance pot. That should be the equivalent of 14k. Check the bias. Still cold? Clip in a 100k. That would equal about 13k. Check the bias. I bet around 55% to 60%MPD will be nice to your ear.

Iow, a bias pot is nice for dialing in the resistance but it can be done without a pot, just like the original AB165 circuit.

 

[Paul-McShartney]

I gave that a go and oddly enough, 100k was still not enough. The 15k resistor did drifted up a little bit and so adding 100k yielded around 14k. still I think maybe adding a 25k trim pot in series with a resistor there would be better right?

fyi, 100k in parallel with the 15k made the outermost output (which is fixed) be around 47% plate dissipation

 

[Lowerleftcoast]

The bias pot is just a handy way to adjust this voltage divider. Paralleling the 15k with another resistor is reducing the tail resistance. If a pot was installed the same thing would happen, it is just a handy way to quickly change the tail resistance.

I am in a minority of the shock brothers. I do not mind having a bias supply with fixed resistors. Then again I am not opposed to installing a pot.

 

[Paul-McShartney]

For sure, yeah I get you about achieving the same with paralleling another resistor, I just really like the idea of adjusting the tail end on the fly!

If I was to do this, would having a sweepable range from 10k-20k be optimal? I guess I could achieve this by running a 10k resistor in series with a 10k trim pot?

 

[peteb]

If I was to do this, would having a sweepable range from 10k-20k be optimal? I guess I could achieve this by running a 10k resistor in series with a 10k trim pot?

you are on the right track. Any resistance added on to the end (in series with) of the existing tail resistor will make the bias colder. You are right. You would need to first reduce the tail resistor.

 

[peteb]

It looks like the AB165 bias is adequate at 30 mA.

one thing players like about the blackface amps is the adjustable bias, allowing one to fine tune the bias.


I looked up the well known Andy Ruhl comparison of the AA864 and the AB165 Bassman amps and surprisingly, that is not what he said

AA864 Bassman vs AB165 Bassman

“Bias supply - Fender really screwed the pooch on this one. They set one tube up as standard non-adjustable fixed bias and then you adjust the other one to match the fixed one with the pot. Seems like a good idea right? Wrong. These amps almost always hum when they are adjusted so each tube draws the same amount of current. I don't know why, maybe an engineer can explain it to me. So when you adjust the pot to make the hum go away, the tubes are not idling the same and you get nasty distortion when you turn it up.”

 

[Lowerleftcoast]

Imo we are OK with just changing the tail resistance.

For your education... bias circuits create a negative DC voltage by rectifying AC from (usually) the PT. Sometimes the voltage source (PT) is quite high. Other times, like with this amp, there is a dedicated bias tap from the secondary of the PT providing a lower (closer to the voltage we will need for bias) voltage. In either case there are resistances, essentially in series, to arrive at the desirable bias voltage. These resistances many times have the rectifier (diode) in between the resistances so technically we can't call them a *voltage divider* but the result is similar. When changing these resistances we want to watch out for dropping a lot of current to ground. Too much current will make it to ground when these resistances are low. So we should be looking at the dropping resistor (AB165 it is the 470R) along with the tail resistance (AB165... this is the 15k along with the resistance created by the two 10k and the 10k pot balance arrangement). Iow sometimes we might change the 470R to get where we want to go. Sometimes changing the 470R and the tail resistance is needed and sometimes we can change just the tail.

If I was to do this, would having a sweepable range from 10k-20k be optimal? I guess I could achieve this by running a 10k resistor in series with a 10k trim pot?

Well with these tubes ~13k is still cold so I am thinking the tail might want to be less than 10k to sweep to what you might want. Maybe 5k with a 10k pot in series? Maybe a 25k pot with 47k in parallel?
Consideration might be given for the possibility of using 5881 or EL34 tubes so a wider sweep may be desirable? Decisions. hah.

 

[Paul-McShartney]

thanks for explaining how these resistances come in play for adjusting overall bias voltage, it's always easier to understand what I'm doing when I have a grasp on the fundamentals of it.

I'll definitely try a 5k in series with a 10k then.

 

[peteb]

This got me thinking about the prevalence of adjustable bias in Fender amps.

in the entire history of hand wired Fender amps, adjustable bias on the larger amps started and stopped with the blackface amps. Possibly or possibly not a reason why they are so popular.

the bassman had adjustable bias for less than one year.