Volume ramp up on wurlitzer 530 amplifier?

I'm wondering if someone would be able to give me some input as to what's causing this. I recently had my wurlitzer 530 amplifier rebuilt, and it sounds amazing. The only thing I noticed that it never used to do is a strange "warm-up" or "fade-in" effect at the beginning of each song. It doesn't matter how long it's been playing, at the start of each record it does this. Here's how it goes. After the needle hits the record, I can count off about 10 seconds between the time the song starts and I hear the music very soft and low, and when the music gets up to the point of where I actually set it.

When the amp was rebuilt, all tubes were checked and passed with flying colors. Could a tube be acting up even though it passed testing? It almost has that "tube warm-up" sound or ramp-in.

It's my understanding though that on these wurlitzer 530 amps, as long as multiple pins are fired on the pin bank, the amp stays fully energized (not in standby) and there should be no warming up of tubes, right?

Last but not least, I have the tube removed for the auto volume control. This wouldn't cause it, would it? I haven't really tinkered with it as I received the amp back from the shop tonight.

Any advice or ideas is appreacited!

Let me take a wild guess here, you removed the 12AX7.

So, you now have much more gain and probably some distortion on loud records. However you have messed up the gain structure of that amp design.

The fade up is normal on an amp with AGC. The idea is to prevent overshoot and blasting. This is accomplished by applying a pre-bias (called squelch) to the time constant of the AGC section. This is just a voltage roughly equivalent to a very "loud" record.

When the circuit is operating normally a loud record will start slightly low and and a low cut record will take a little longer to fade up. Both should still be audible from the start.

By removing the 12AX7 the AGC circuit is no longer able to respond to the record level and you must set the volume control to a much lower setting than was intended. However the squelch is till applied, so you start off with much lower level.

Put the 12AX7 back and ignore any further "advise" from whoever told you to remove it.

FWIW: Wurlitzer AGC circuits are rather slow to response on either attack or release due to the use of what I consider a too large time constant capacitor here. On this amp that is C30 (1 mfd) and C31 (2 mfd). On my machines I lower C31 to 1 mfd.

Rob/NYC

Rob,

Thank you for the very informative post!

If I understand you correctly, are you saying that with the 12ax7 tube removed, the automatic volume control pot is still in use? Would adjusting the pot have any bearing on how long it takes for the volume to ramp up? Or is it the size of the capacitor that dictates this?

I'm only asking because it seems to me (or my ears anyways) that on my previou amplifier (a 532) I owned, that removing this tube made an improvement on sound. It seems to me that the music had better detail and was brighter. I do recall this amplifier also having the volume ramp up as well, however it was a lot less time to do so (maybe 2-3 seconds).

Before I dig into this today, would you suggest adjusting the pot and leaving the tube out? I don't mind a 1-3 second ramp up, but the 10 second thing is annoying and I miss the best part of a song often times.

Or should the tube be put back in and the pot adjusted? I'm just worried that the "softer" or "weaker" sound will come back.

What effect would changing the 1 capacitor out have on everything?

Thank you again for the response.

If I understand you correctly, are you saying that with the 12ax7 tube removed, the automatic volume control pot is still in use? Would adjusting the pot have any bearing on how long it takes for the volume to ramp up? Or is it the size of the capacitor that dictates this?

The capacitors C30 & C31 are the time constant. The pot adjusts the bias on the reactance section of the 12AU7 and given how it is connected, it would just be sued to make the gain control reasonably linear.

I'm only asking because it seems to me (or my ears anyways) that on my previou amplifier (a 532) I owned, that removing this tube made an improvement on sound. It seems to me that the music had better detail and was brighter. I do recall this amplifier also having the volume ramp up as well, however it was a lot less time to do so (maybe 2-3 seconds).

It took less time because the volume pot was set higher as was intended. With no AGC you set the pot much lower and the ramp-up appears longer.

Or should the tube be put back in and the pot adjusted? I'm just worried that the "softer" or "weaker" sound will come back.

What effect would changing the 1 capacitor out have on everything?

In my ever-so-humble opinion AGC is essential here. The circuit was designed with an excess of gain so as to properly saturate the AGC and allow it to bring up a low-cut track. Without the AGC active there is too much gain and distortion results from a 'loud" record.

Lowering the value of C31, C30 will cause the AGC to release faster (ramp up gain) and attack slightly faster. Personally, I think they used too much filtering and this made the circuit slow to react. On my 2300's and 2400's I redesigned the circuit to use one filter cap and different resistor values and a diode.

Rob,

I'm not an electronics guru so you'll have to "dumb" it down for me

So you're telling me I should do the following to improve my amp????

1. install the tube back in

2. C-30 should remain a 1MFD cap 200V

3. C-31 should be replaced with a 1MFD cap the same as whatever I put into C-30, correct?

Last but not least, what's the procedure for setting the potentiometer inside the amp for the optimum performance?

Yes, I do suggest putting the 12AX7 back.

As for the caps, lowering the value of C31 will allow it to respond more quickly to both attacks and release. using 200 is ok, but there is really nothing much above 30vdc in this area so 100 volt is fine too. Wurlitzer probably used 200v as they got a good price on them. With old tube equipment back then higher voltages were common. It is also very important that the caps have low-no ESR as the circuit is very high impedance here and any leakage will load the circuits and reduce the efficacy of the AGC action.

As for that pot, I've never really paid a lot of attention to it. just start a record playing and adjust it somewhere in the middle of its range. It really is just there to see that the AGC changes smoothly and not abruptly along the range of control voltages. They used a standard triode as the control tube instead of the more suitable 'remote cutoff pentode" type so the linear range is a little more critical. Theoretically that is true., I've never had any problem with it.

Rob/NYC

Hello,
I had this exact same problem with my amp. (Yes,... the 12AX7 is installed). I will have to check what resister it is !!!
This is your squelch.... There is a resister in these amps that, ....because of time, drifts up in value (ohms). Because the drifting high in value really kills the operation of the squelch.
Once I swapped it, with the correct value, the fading in the volume returned to normal. (There was no LAG).
Give me a day or 2 and I can track down which resister it was. It was a 532 amp, (very similar).
Hope this helps...
Tony

I'll add that over time, some electolytic capacitors have a tendency to go UP in value.
This is probably due to the deterioration and drying out of the electrolyte and/or paper chemical breakdown.
This can easily verified by testing with a known good capacitance value checker.
I test even new ones, before installing, to insure proper operation of any given circuit.

Matt, and All,
I have found many "lytics", that have gone "up" in value, when old !--Most "prone" to this are the larger constructed ones, for some reason ? Also, low value, high AC "oil filled", types, are very prone to this (5mfd@350 vac--find them at 7-9,mfd, "all the time"). Ron Rich

Also, low value, high AC "oil filled", types, are very prone to this (5mfd@350 vac--find them at 7-9,mfd, "all the time").

You can add NP lytic's to that. Two years ago I started to notice miscounts in a Seeburg drop-in stepper in the modded K (TSU-#?). I had used a 5 mfd@300 v made in the late 80's. the stepper was washed/rebuilt by me so no mechanical problems. It had been in service for about a year and the problem was getting noticeable. I tested the cap and it read 6.7mfd. Replaced it with the five Imfd mylars that I regularly use (measured 5.1mfd) and stepper was correct from then-on.

In most cases an increase like this won't be a problem as long as it is not accompanied by high ESR/leakage. In phase sensitive circuits such as this it can lead to phase slipping. This is why I've never understood the insistence some people have in using oil filled or 'lytic caps in Seeburg steppers.


FWIW: The caps in the Wurlitzer AGC are paper-wax. I replace all time constants with mylar.

Rob

Rob,
That's exactly the cap I was thinking of, but I neglected to write "NP". I found the Chinese made caps from 10 -15 years ago, very prone to this--the later ones seem better--- Ron Rich

I have come across old NP's going up in value - an 8uf crossover cap reading 12 or 17uf.
Also a "recent" power supply filter cap of 100uf @ 250v actually reading 475uf!
Smal 'lytics can also cause havoc - .1uf, .47u, 2.2, 1u, 10u, they all can go nuts.
It depends on the manufacturer, age, even just looking at them causes problems!
Ya gotta test them, both for value, AND ESR.

I tell ya, the insulating material (paper, whatever) can deteriorate, the foils can have holes in it, all these things contribute to issues.
And yeah, poly/mylar, etc. types are more stable and reliable as replacements in most cases.

But you can't ignore older resistors either - they change value over time - lord knows how many of them I've had to change through the years.
And don't get me started on those old dogbone resistors!
woof woof! GRRRRRR!

Ok,
Provided the amp has been re-capped, and has a good set of tubes. Check the 2 resistors on the 5 pin socket for the "mute" switch on the amplifier. The 2 resistors should be connected to pin 2 ( a 390K and a 47K ) These form a voltage divider from the B+, and should have 30-40 volts on pin 2. You can check this while it is playing a record.
If the amp has been "gone through" re-capped and good tubes. One or both of these resistors have "drifted up" in value.
This is the area you want to look in.
Hope this helps...
Tony