Metropoulos Forum

Hi there. New member, first post. Kinda excited that I just might get some answers to some of my questions. I'm a Marshall/general tube amp nut, recent E/E grad., wanting to put the practical alongside the theoretical now. All I've ever wanted was to understand and tweak tube amps. I've been at this for years, but just recently finished the education behind the experience.

On both my '69 and '71 1987 heads I have a pair of caps - appear to be electrolytics, but no obvious polarity, just metal cans - which have deteriorated - oil leaks - they are at the end of the board, just after the rectifier, and seem to be in that part of the circuit, 0.25uF/600V.

Several questions:
- What function do these serve? They seem to be too low in capacitance to be useful in the rectifier section;
- They "appear" to be electrolytics (axial metal cans), but 1) very low cap value and 2) they don't appear to be polarized, so they - kinda don't fit my understanding of electrolytics;
- It's been suggested by others that they can simply be removed from the circuit without replacement;
- If I were to replace them, what type of cap would I use? I can't find any electrolytics at that spec.
- The schematic I have seems to show these caps as 22uF, if I'm looking at the correct section. Lots of schematics have wrong specs, though.

Thanks for any insights/suggestions. I'm hoping this forum can accelerate my learning curve on amp repairs and understanding the various component functions.

Hello Tony

Welcome to the forum.
Those capacitors are referred to as snubber caps (bit like sort of cap you see across a high voltage switch to avoid pos and crackles.
It has been suggested that these caps were introduced to smooth ot switching noise from the diodes of the rectifer circuit but it is equally possible that they were there to buffer against high transients when flicking the standby to the on position.
They are not polarised. Usually the Plessey metal can 0.25uF 1000v or Dubilier 0.22uF 1000v. Both can be removed safely but there is a wiring error that needs to be corrected first (many PTs died because of it!).
Make sure that the white bias feed wire (to the 220k resistor hooked up to the bias diode) is fed from the hot side of the standby switch, ie when you have the mains switched on but the amp on standby the bias circuit will need to be hot. That way the bias circuit to the output valves is effective before you hit the rest of the amp with HT voltage.
It may have been corrected already but a photograph of your amps rectifier and switch area will clear that question.

Thanks for the reply and the explanation. Snubber circuit makes sense. I"ll take them out and check out the noise/arcing and then make the decision to replace, or not.
Thanks also for the tip on the bias wiring. Here's a pic showing the bias feed from the switches:
Regards,
Tony

TonyRS wrote:Thanks for the reply and the explanation. Snubber circuit makes sense. I"ll take them out and check out the noise/arcing and then make the decision to replace, or not.
Thanks also for the tip on the bias wiring. Here's a pic showing the bias feed from the switches:

SnubberCaps.JPG

Regards,
Tony

Both of yours have been modded already (the white wire lenthened under that bit of black insulation tape on the 71. I would replace the wire with one piece personally and take care to loop the wire through the board next to the turret and wrap it around properly (at the moment it is just tacked on).
I can see you have the more traditional snubber cap (red Hunts 0.05uF) next to your polarity switch on the 71 and your bias feed resistor looks to have been replaced (can't read the value on the red/brown resistor) on the 69 - that wire should be pushed up through the turret and hooked over the top slightly before the bent end of the resistor is fed in (to stop it dropping out).

Thanks Neil. Got it. It'll get straightened up like new.

Tony

The snubbers may or may not be beneficial depending on the state/quality of the rectifier. With modern fast recovery types they are pretty redundant though, but 50 watters with bridge's may be a different story. I've yet to get my grubby hands on one old enough to see.

demonufo wrote:The snubbers may or may not be beneficial depending on the state/quality of the rectifier. With modern fast recovery types they are pretty redundant though, but 50 watters with bridge's may be a different story. I've yet to get my grubby hands on one old enough to see.

Sorry I only have one and that is in an amp, so you cannot have it (if I find any markings on it I will let you know)!

We do know that the rectifier this era is a bit under specced as only two diodes are actually used (750 PIV rating but only 1A as far as I know). For clones I have preferred 1000PIV and 3A rating. Not sure of the rating of the blocks that George was supplying?