S.E.X monoblocks - filter caps

[Psimon]

Hi all,

I need to replace the filter caps in my S.E.X monoblocks (one of the earliest kits in use with the foreplay 2).

I sourced some 22uf 500V filter caps to match the ones in there and noticed that the existing are film caps with no polarity marked on. The ones I have bought are electrolytic at the same value (except their tolerance is -10/+30% while the films are +/-10%). Can I use the electrolytic as a replacement? I'm concerned about the polarity as well, as I don't want to blow the caps up. I've attached a photo of the inside wiring and existing cap.

Any advice would be much appreciated. (I'm no techie and didn't build these myself)

Best,
Simon

 

[Paul Birkeland]

What happened to warrant a cap replacement?  Those film caps should last about a lifetime or two.

 

[2wo]

The caps shown look kinda small for film caps of that value and voltage rating. If they were film, they probably wouldn't need to be replaced. I don't remember the original values but I'm sure someone does and can recommend a current replacement...John

 

[Paul Joppa]

There are four versions of the Original S.E.X., none of which had film caps in the power supply. So this is not a stock build. That 10-henry choke isn't original either, nor does the original have any iron under the chassis. So it's a little hard to tell what you have.

That said, the original used two caps of 10 microfarads/450 volts. Assuming these are the power supply caps, your 22uF/500v electrolytics should work well. The ends soldered to the ground wire would be the negative, and the other ends soldered to the choke would be the positive ends.

Because it is an unknown modification of some version of the original, it's hard to know for sure what you have. If you can post a photo of the whole top and the whole bottom, we can probably figure that out.

 

[Psimon]

Thanks for the replies guys.

To answer a few questions:

• I replaced the caps because there was a hum in the right channel. It sounded like a filter cap issue based on the pitch (I compared online with youtube video tutorials)
• The existing film caps are about 2.2inch long
• Yes I understood when I got these amps that they were different from the standard builds. I've attached 3 photos that I hope are useful. The 2 sockets on the top of the amp take 6DN7 tubes, recently replaced, but swapping them from one monoblock to the other makes no difference to the hum. I also eliminated any issue with the Foreplay by swapping output cables etc.

Cheers!

 

[Psimon]

More photos

 

[Paul Birkeland]

That certainly embodies the "E" in SEX amp.  There's battery bias, two power supply filter chokes, Magnequest TFA-204s, and a power transformer I haven't seen before. If one amp is humming and one is not, you'd want to measure the DC voltages on pins 1-6 on each socket in each amp and look for differences. 


How do those 9V batteries measure?

 

[Psimon]

Okay, I went ahead and replaced the filter caps and they made no difference to the hum. They did however reduce volume output noticeably so I reinstalled the original film caps. Hum issue is still there.

I measured the voltage between the pins and chassis on each amp. The amp with the hum has very slightly lower voltages on some of them. As follows

Good amp:
Pin  1st tube (by battery)  2nd tube     
1:  21mV                        21.mV
2:  306V                          306V
3:  12.89V                      12.88V
4:  Almost no voltage
5:  293V                          116.5V 
6:  123.5V                      4.57V   

Amp with hum:
Pin  1st tube (by battery)  2nd tube
1:  27mV                        24.mV
2:  297V                          297V
3:  13.3V                        13.3V
4:  Almost no voltage
5:  284V                          116.3V 
6:  123.6V                      4.58V   

And the batteries I measured the voltage without the Amp being powered up. On the amp that is emitting hum the reading is about 171mv.
On the other good amp it is about 70mv.

 

[Paul Birkeland]

Those are 9V batteries right?  They should read 9V.  If they are totally dead, they will also make your amp totally dead.

 

[Deke609]

Those are 9V batteries right?  ...  If they are totally dead, they will also make your amp totally dead.

Off topic: someone should be collecting these PB-isms.  There are some really hilarious gems peppered throughout the BH forum. 

 

[Paul Birkeland]

I should explain that when you see a 9V battery in tube gear, often you'll see the positive end of the battery tied to ground and the negative end tied to the grid of a tube to provide -9V of bias.  There isn't really any current drawn to discharge the battery, so it just sits there and does its thing for years and years and years, but the battery will eventually wear out and the 9V voltage will start to drop.  If your 9V batteries are near 0V, then whatever part of the circuit they are supposed to be biasing will draw way more current than it should, and the sound quality of the amp will also be terrible as the input impedance will drop substantially as you get closer and closer to the bias voltage being 0V.

 

[Psimon]

Thanks! I was really hoping it would be the batteries. I'll swap them for new and hope that resolves things. Can I use any 9V rechargeable? I have some Lithium 400mAh - the originals are Ni-MH150mAh.

 

[Paul Birkeland]

I would stick with the same chemistry that you pull out, unless they are zinc-carbon (heavy-duty), then I would replace them with something else.

 

[Paul Joppa]

There are several ways that battery bias is done, some draw no current and some provide a trickle charge during operation. In the former case, you want a battery with a low self-discharge such as an alkaline, and in the latter case you need a rechargeable immune to memory effect and tolerant of trickle charging, such as sealed lead-acid. It's a bit of a jungle out there...

 

[Paul Birkeland]

It's a bit of a jungle out there...

Yes, it looks like one end of that battery may be connected to one of the plate pins!

 

[Natural Sound]

That certainly embodies the "E" in SEX amp. 

Don't you mean "X"? 8)

 

[Paul Birkeland]

True!

 

[Psimon]

Okay, I finally was able to replace the batteries, same Ni-MH type, but the hum is still present in one channel. Also put some music through it and the sound seems richer. Any suggestions on what I can try next?

I also measured the voltages between pins and chassis again and there are some changes:

Good amp:
Pin  1st tube (by battery)  2nd tube   
1:  32.7mV                      37mV
2:  303V                          304V
3:  12.79V                      12.8V
4:  Almost no voltage      145.7V
5:  135.9V                      273V 
6:  4.86V                        149.7V   

Amp with hum:
Pin  1st tube (by battery)  2nd tube
1:  131mV                      131mV
2:  296V                          297V
3:  13.3V                        13.3V
4:  Almost no voltage        146.1V
5:  136.2V                      267V 
6:  4.7V                          150V   

 

[Paul Joppa]

It appears to me that the 9v battery replaces a resistor and capacitor on the upper triode of the mu-follower driver. If that is the case, these voltages look good and the circuit is operating properly. Some of the solder joints to the ground buss look marginal; I remember having a heck if a time getting those hot enough back in the day! Your hum might be just a failing solder joint to the buss. Poking things with a chopstick while listening might help.

Still, the power supply is always a candidate for hum. It's harder to see it; there are two chokes and three capacitors but the initial 100uF seems to be bypassed by one of the 22uF films - and then I get lost. There's a muddle of components near the film caps which I can't figure out. The perf board over the power transformer appears to be a reverse-recovery spike filter, which solves a potential buzz problem but has little to do with hum.

From the power transformer label, this amp seems to be wired for 100v power, as used in Japan., with a replacement power transformer.

By the way, NIMH batteries lose their charge fairly fast - 10% loss per month, I think.  It's not carrying any current in this design, but you'll still want to charge them up every few months.

 

[Psimon]

Paul,

Yes I'm in Japan so 100V from the power transformer. I have confirmed as you say "the 9v battery replaces a resistor and capacitor on the upper triode of the mu-follower driver" - correct. And the perf boards are indeed reverse spike filters.

I checked the joints and re-soldered anything that looked marginal, poked around with a chopstick but no difference was made.

I also used a frequency meter app and the hum is 100Hz. I'm sure it's getting louder as well.

Any other suggestions?

Thanks,
Simon