Low Level Pop During Start Up

[Gerry E.]

Hi:

One of my Paramount V1.1  300B amps may have the beginning of a problem.  Approx. 10 - 15 seconds after turn-on, there is a relatively low level audible pop.  I have tried 3 different 300Bs and it happened with all of them.  It does not happen with the other amp. 

Even though it's supposed to be closer to 30 seconds, I'm wondering if this could be when the B+ is applied?  In any case, is this an indication that a part may be starting to fail?  I wouldn't swear to it but over the course of the last week or so the pop maybe getting louder.  Thanks!

Gerry       

[Paul Birkeland]

The 300Bs will fully conduct and be ready to operate within a few seconds after you turn the amp on, so I wouldn't be looking there.

I would definitely be evaluating the voltages around the driver tube though, as well as trying a different driver tube.

[Gerry E.]

The 300Bs will fully conduct and be ready to operate within a few seconds after you turn the amp on, so I wouldn't be looking there.

I would definitely be evaluating the voltages around the driver tube though, as well as trying a different driver tube.

The Good News:  It's not my Tung-Sol round-plate 6SN7.
The Bad News--: It's not my Tung-Sol round-plate 6SN7.  That means it's something else.

Here are the voltages on the 9 pin socket (Incoming voltage measured 122 VAC, replaced the 6SN7 with a WE 396A for measurements, expected values in paren):
B1  3.2   (3.1 VAC)
B2  4.1   (4.1 VDC)
B3  0      (0 VDC)
B4  192  (175 VDC)
B5  0      (0 VDC)
B6  297  (300 VDC)
B7  0      (0 VDC)
B8  6.79 (6.3 VDC)
B9  3.19 (3.1 VAC)

While B4 and B8 are a bit high, not excessively.  Anything else to check?  Thanks PB!

Gerry         

[Paul Birkeland]

You can use a clip lead to clip pin 3 of the 300B socket to ground, which will ground out anything before the 300B from being sent through to the speakers.  If this removes the pop, then you know it's something in the driver stage.  If the pop is still there, then there are other steps that can be taken.

[Thermioniclife]

About 20 years ago I built a pair of 300b mono's that popped like that and I couldn't find the problem so I used an industrial delay on make timer that held the 300b grid to ground for 30 seconds, Worked great

[Paul Joppa]

I'm out of town throough next week, so I don't have all my notes available. But can you say a little more about the non-stock 6SN7 adaptation? there may be a relevant issue.

[johnsonad]

Lee, have you tried putting a 5670 in and seeing if it happens?

[Thermioniclife]

Hey Aaron, I do not have those amps any longer but thanks for the suggestion, I have some 5670's on hand for the future. I built a a new set of the 300b mono's about 5 years ago and do not have any issues with them. Thank you.
Lee

[johnsonad]

Sorry buddy, I meant to ask Gerry. That’s a good looking amp and I appreciate the text 🤣

[Gerry E.]

Guys - Thanks for all the replies!  The first thing I did after PB's initial reply was to swap the 6SN7 for a WE 396A.  The problem remained, so that's how I knew it wasn't the TS RP 6SN7 nor the adapter (Garage1217).

Here's a bit more information about the symptoms.  Upon turn of the amp in question, there's hiss that slowly dissipates into a POP and then dead silence.  Of course once I hit play, the amp plays fine and sound great. 

Gerry           

[Paul Joppa]

I assume you adjusted the drive plate voltage to 175v when switching tubes

There is a 300K resistor from the driver plate (pin 4) to ground - check that resistance.

When developing the soft-start circuit, I had a similar pop, and that 300K resistor was added to prevent it. It keeps the 300B grid at ground until the driver cathode warms up and the soft-start begins to conduct.

[Gerry E.]

I assume you adjusted the drive plate voltage to 175v when switching tubes

Ummm... no, but I will do that.  Is 175v still the target voltage even with a 6SN7?  The reason I ask is I re-did the 9 pin voltage measurements that I already did for the WE 396A.  With the 6SN7, I stopped at pin B4 because it measured 125v (it was 192v with the 396A - s/b 175v).

"There is a 300K resistor from the driver plate (pin 4) to ground - check that resistance." 

While I didn't see any resistor connected to pin B4 the resistance from pin 4 to ground did measure approx. 300K ohms.

Thanks! 

Gerry

[Paul Joppa]

The optimum plate voltage for the driver is 175v. That's the  heart of the linear region, and it's the reason for the voltage trim pot.

I don't expect adjusting that voltage will eliminate the pop, but it's worth checking - it's hard to envision every possibility!