pultec pops when turn rotary in the low freq

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skal1

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Joined
Jun 4, 2008
Messages
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Location
Birmingham,uk
pultec pops when turn rotary in the low freq i did buy mbb switches but you never know they could have sent me bbm ,how can i test for mbb switches?


skal1
 
set your multi meter to continuity test and connect both leads to two neighbouring contacts. When you turn the switch there should be a breef bleep....or whatever signal your dmm gives for continuity.
 
trying what you said to do , i have put the red probe on contact 1 and the black on contact 2 is this correct. this is a 2 pole six pos rotary sw.

skal1
 
Don't know what you mean by fakes. Clones of the original design, or "based on pultec" designs?

drpat said:
A Pultec uses mbb, and does not pop. It's been reported that some of the fake Pultecs will pop no matter which type of switch that you use.
 
Does not compute.

There is no DC present in the filter (if correctly built), thus no way to get switching clicks in a no-signal condition.

In signal-present condition, there is no way around getting switching clicks - they may be loud or soft depending on where at the curve-form you switch (or if you use MBB or BBM switches), but is unavoidable in total.

Jakob E.
 
Interesting....mine only click/pop once (after powering up). After that there is no pops/click..i think it's some thing to do with filter cap charge ? ???
 
Hi,
I know its an old thread but i seem to have exactly the same problem.
In a no signal condition i will get pops hitting -18 dB when switching between 20-30-60
I get no click from 60-100
On the high freq selection i will get clicks when switching between 5-8.

This is my first build so im definitely not an expert!
I have measured for DC on the input and output, in the passive filter, and they are both at zero?
When a cap is not selected, some of the filter caps will charge up to around 30-100mV?
Im really confused at this point, i dont really know what to look for.

Any help is much appreciated.

 
radardoug said:
There should be NO d.c. in the filter section of a Pultec if built correctly.

Hi radardoug, thanks for your response.
Everything should be build correctly, and voltages in circiut looks ok.
BUT i get 27VDC on the 12AU7 grid?

I did try two old aerovox caps as coupling caps between plate and grid 0.25 uF 400V  ::)
These tested around 300mV with a 1Meg to ground when i first tested it out.
Could this maybe be caused by a leakage on the 12AU7 grid?


 
In a standard Pultec there is an input transformer, then the eq network, then another transformer. So there is NO d.c. in the eq section. If you put one of your aerovox caps from the plate to a 1M, and get 300mV of d.c. across the 1M, the cap is leaky.
Buy a new cap.
 
radardoug said:
In a standard Pultec there is an input transformer, then the eq network, then another transformer. So there is NO d.c. in the eq section. If you put one of your aerovox caps from the plate to a 1M, and get 300mV of d.c. across the 1M, the cap is leaky.
Buy a new cap.

Right on, thats why i am so confused about this, i do not measure any DC on in/out in the passive, but from sign. gnd to switch terminals i can see the DC rise up when cap is not selected, and this discharge will then make the thump.
I just cant get my head around this strange voltage on the 12AU7 grid, yes i will definitely change the 0.25 caps, but +27VDC on the output tube grid, (measured with tubes in circiut)
The pultec sounds great, no distortion, and very low noise. the pots are not scratchy, sorry im still very new to this, but everything seems to work, but this grid voltage on the 12AU7 could that maybe  cause of the pops?

12AX7 VDC          12AU7 VDC

Pin 1 = 145              Pin 1 = 286
2 = 0.0                        2 = 27
3 = 1.2                        3 = 39
4 = 57                          4 = 57
5 =  57                        5 = 57
6 = 149                      6 = 286
7 = 0.0                        7 = 27
8 = 1.2                        8 = 40
9 = 57                          9 = 57

Cheers


 
Is your interstage transformer wired correctly? Sounds like you might have the windings connected wrong. The primary should be grounded one end, and go to the output of the eq at the other end. The secondary goes to the 12au7 grids.
 
Re-used capacitors with high dielectric absorption, maybe - holding eltrect-like charge?

..put a 2-10M Ohm resistor in parallel with the capacitors that are acting up...

Jakob E.
 
radardoug said:
Is your interstage transformer wired correctly? Sounds like you might have the windings connected wrong. The primary should be grounded one end, and go to the output of the eq at the other end. The secondary goes to the 12au7 grids.

I have used an UTC A 16 as interstage, but the SEC will go to 12ax7 grids right?
I have made an centertap with two 22k resistors like this
2ywahxi.jpg

I will then have the 270pF cap in series with a 43K resistor between the SEC windings
like on the original schematic.
I have tried another 270pF to see if that helped, but nothing.
The 56V input should be set up correct as well, i have checked the DCR and looked for shorts before i wired it up.  :eek:

Cheers


 
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