Vintage tube mixer input impedance

[Levente]

Originally Posted by ms660

It's hard to say without better photo's showing what's what for sure.

Lawrence.

The PSU tap 8 connected to pin 1 on the 6x4
The other PSU ac line is connected to pin 6 on the 6x4

The PSU tap 8 is also connected with one H170 diode to pin 7 on the 6x4 socket (a junction point?) where also the 2nd diode goes out from into a massive resistor and then to the AC filter cap.

sorry for the explanation in words..i know a drawing would have been better..

[ms660]

Looks like a substitute for the valve rectifier.

Lawrence.

[Levente]

Originally Posted by ms660

Looks like a substitute for the valve rectifier.

Lawrence.

Thanks Lawrence... this mixer came with a 6x4 already plugged into the socket.. should I take the tube out and leave the diodes or take the diodes and use it with the 6x4? the two i am sure would not love each other...

[ms660]

The 6X4 might be a good one, who knows, possibly thrown in with the sale, only way to find out is to take the silicon diodes out of circuit and try, if it's no good then remove it and connect the diodes back in circuit.

Lawrence.

[Levente]

Originally Posted by ms660

The 6X4 might be a good one, who knows, possibly thrown in with the sale, only way to find out is to take the silicon diodes out of circuit and try, if it's no good then remove it and connect the diodes back in circuit.

Lawrence.

Thanks I did try and plugged into the AC 117V ( as this is again a US made gear) with the diodes in place. The power transformer started to buzz and rattle slightly which as soon as one of the 12au7 tube came on, this rattle from the power transformer faded away. This buzz/rattle lasted about 10 seconds. Is this something to worry about?

No smoke, signs of burn or anything, only this slight buzz/rattle from the power transformer for about 10 seconds or so...

[ms660]

Originally Posted by Levente

No smoke, signs of burn or anything, only this slight buzz/rattle from the power transformer for about 10 seconds or so...

I wouldn't worry about the buzz so long as everything's working ok.

Lawrence.

[Levente]

Originally Posted by ms660

I wouldn't worry about the buzz so long as everything's working ok.

Lawrence.

Cheers Lawrence... i think the mixer is not OK...removed the diodes and tried with the 6x4 ( i had a tested working one for 100%) left it on for few minutes observing if anything happening.. started to smell a bit...no smoke but quickly unplugged as I saw this huge 4kOhm resistor going from the rectifier tube the filter cap stared melting or at least tiny bubbles appeared on it...you can see those bubbles on the pic... I didn't touch it how hot it was...i did not feel like poking around there with my finger...

On the other hand, the rattle buzz from the transformer has gone as soon as I swapped the diodes back to the 6x4...

Something is not right there...

[ms660]

Could be anything across the HT downstream of the resistor.

Lawrence.

[Levente]

Originally Posted by ms660

Could be anything across the HT downstream of the resistor.

Lawrence.

Thanks Lawrence, will start and check everything one by one... do you know what am I looking at on this pic? I have never seen anything like this... it could be a small wax/drop capacitor from the 50s? It is very small drop-ish one connected the following way:

Gain/Volume pot middle wiper-> this unidentified cap-> 12ax7 pin7- mica cap to chassy.

This mixer has a 600 ohm output, not a power mixer...more like a mic pre to 600ohms and a headphone output.

Thanks !

[ms660]

Is there a resistor connected between Pin 7 and ground?

Lawrence.

[Levente]

Originally Posted by ms660

Is there a resistor connected between Pin 7 and ground?

Lawrence.

Nope... there is none. Only this brown waxy little drop on the pic is at pin7 which goes to the middle tap of the volume/gain pot, and the mica cap from pin7 goes to a ground point. nothing else is connected to pin 7.

[ms660]

Originally Posted by Levente

Nope... there is none. Only this brown waxy little drop on the pic is at pin7 which goes to the middle tap of the volume/gain pot, and the mica cap from pin7 goes to a ground point. nothing else is connected to pin 7.

The mystery component must be resistive then, try and measure its resistance.

Lawrence.

[Levente]

Originally Posted by ms660

The mystery component must be resistive then, try and measure its resistance.

Lawrence.

Ok, will do... i am awaiting my testing gear to arrive here as I am in a different location... in the meantime I have noticed something odd at the rectifier and the multisection filter cap. ( see photo too)...

so someone clipped off the first section of the filter cap, but left an orange lead there connected to the rectifier... also, that big burning 4k resistor is connected directly from the rectifier to the second stage of the filter cap(where this goes to the output stage 12au7+OT)... this is bad! so the first stage of filter cap is just hanging there with clipped off leads still connected to the rectifier..

the output tube 12au7 is burned out...came in a state like this... the d shaped getter bended downwards and the bottom of that tube is black. I havent seen anything like this... i think this messed up everything here..

[ms660]

The photo's not too clear, how many capacitors are there in the large electrolytic can, and what are the values and voltage ratings and which (if any) is the -ve (ground) tag)?

Lawrence.

[Levente]

Originally Posted by ms660

The photo's not too clear, how many capacitors are there in the large electrolytic can, and what are the values and voltage ratings and which (if any) is the -ve (ground) tag)?

Lawrence.

the cap is a 4 times 20 UFs 450V Aerovox there is one ground point as far as I can see, a pin with a whole connected to another cap's -ve ... these caps has the phenolic insulation i can not see clearly if these caps are having any connection to the chassy, but might have..

On the pic, where the big resistor is right underneath there is an orange lead going into the 1st section of the cap, and from there no connection further...its just there with some clipped off lead ( i think thats where the big resistor should have been and not directly connected to the rectifier ?)

Levente

[ms660]

So far as I can make out it looks like the 4k is connected to the cathode, a section of the can (reservoir) also looks to be connected to the cathode, the 4k looks to be connected as a first stage filter resistor, that normally wouldn't cause the resistor to get hot and bubble, I would investigate that first, maybe remove the silicon diodes and use a good 6X4.

Lawrence.

[Levente]

Originally Posted by ms660

So far as I can make out it looks like the 4k is connected to the cathode, a section of the can (reservoir) also looks to be connected to the cathode, the 4k looks to be connected as a first stage filter resistor, that normally wouldn't cause the resistor to get hot and bubble, I would investigate that first, maybe remove the silicon diodes and use a good 6X4.

Lawrence.

Yes, will check that resistor... and exactly as you described it is, however, one section of that cap is not connected further anywhere else but the cathode...

[ms660]

Originally Posted by Levente

Yes, will check that resistor... and exactly as you described it is, however, one section of that cap is not connected further anywhere else but the cathode...

That wouldn't be that unusual because the maximum ripple voltage will be across the reservoir capacitor.

Lawrence.

[Levente]

Originally Posted by ms660

That wouldn't be that unusual because the maximum ripple voltage will be across the reservoir capacitor.

Lawrence.

I see.. but then there is no use of that section of the cap... would not be better if the resistor is connected there as the first filter/voltage drop and the going to the second, third, and 4th down the line?

[ms660]

Originally Posted by Levente

I see.. but then there is no use of that section of the cap... would not be better if the resistor is connected there as the first filter/voltage drop and the going to the second, third, and 4th down the line?

The reservoir will have the most ripple voltage (hum) across it.

Lawrence.