My EL34s in my Klemt S100 are drawing too much current

[El_ThirtyfFour]

Hi,

I have a Klemt S100 100W amp with 4 EL34s, 3 ECC808s and one ECC82. When I turn it on it is great and I biased the tubes to 45ma each. But, after a couple of minutes it heats up and if I check the tubes they are pulling over 200ma, the sound also goes bad, crackly and then fades as the tubes get over-heated. Just replaced the power tubes, so this doesnt seem to be the problem. Any ideas very much appreciated.

p.s. The klemt S100 was made in germany during the early -mid 60s, it is similar to the S40.

[cbass]

Are all the tubes pulling to much current?
Not familiar with this amp at all.is it fixed bias?
Possibly leaky coupling caps or bad electro's in the bias circuit.
Do you have another amp to try the tubes in ?

[El_ThirtyfFour]

Hi,

it has adjustable bias, there are small pots. When it is stone cold, it allows proper biasing to 45ma , but, after about 10 minutes, if I check again, the ma will climb to> 200 ma and the output will become distorted then fade away.

There are some caps as well as a goofy german power supply/transformer that has about 6 different voltages.

I dont have another el34 amp, but, I have 10 EL34s and the behavior is the same with all of them. I have a tube bias meter also so I have a fair degree of confidence in the symptom but am fuzzy on the cure.

[ampgeek]

Welcome EL!

Do you know if the amp has had any service and/or repairs since it was made in the mid 60's?

As cbass mentions, the symptoms could be caused by many things if it hasn't been maintained over the years.

Electolytic caps drying out and failing to function comes to mind first.

Track down a schematic to share and mention anything that you know about maintenance over the years and you will probably be inundated with suggestions on where to start.

Open it up and snap a few pics for us to peek at if you are comfortable. You will have to do that at some point if you intend on fixing it yourself.

There is a very helpful crew here. Just get us going in the right direction!

Cheers,
Dave O.

[El_ThirtyfFour]

Thanks, there are no known schematics of this unit. Here are some pictures though, and it seems like the circuitry is very simple. The top has 4 EL34s, I do have a matched set but since they are getting toasted I am using some old ones. THe bottom of the board has this metal tube which prodcues the virbrato. I notice a large Sprague ATOM dry capacitor TVA 2740, perhaps this is the culprit?

There are schemtics of a similar amp at: http://www.peel.dk/Dynacord/pdf/Echolet ... ersion.pdf

but this amp has 3 x ECC808 while mine has 2, mine has 4 el34s and mine is 100W whereas this is 30.

[overtone]

Looks all original.
These Klemt Echolette amps show up often on Ebay in Germany. There is a small following of fans and they were used by many big name touring bands back in the sixties, but maybe that was the only rental gear available. I have never played through one, so I don't know what I'm talking about.

Throwing out some thoughts:

This link http://www.backfromthesixties.co.uk/kle ... owstar_s40 has some repair guidance on the S40 model and has an excellent schematic by Chris Devine which is rather easier to read than the teutonic schematic in the above post. These amps have a vibrato which is switched off by placing -ve from the bias circuit on one of the g1 ECC82 grids.

Some questions that may help:
Are you using a current limiter?
Is this one with a grounding polarity switch?
Is this one with a VDR /MOV across the PT secondary?
Can you safely get any voltages? Plate, g2 stopper supply and at at g2 itself?
What kind of bias probe is this and do you leave it in the whole time on your tests?
Is DC being passed by the coupling caps to the output stage?
What load have you got hooked up to the OT?

tony

[overtone]

one other thing, something tells me it gets mighty hot inside that amp.
A component in the bias circuit could be opening up as the heat builds up.
I would be looking at that last bias trim/balance pot among other candidates.

[Phil_S]

I'm thinking that tremolo tube is a LDR (light dependent resistor) and an incandescent bulb of fairly low voltage and low intensity. The tube is probably long just to allow connections at points that are unusually far apart. Indeed, I finally got the schematic you pointed to to load and that one shows at "ORP 62" a unit with a photocell and a 48V 200mA lamp.

I don't see the electrolytic caps, but old ones will need replacement no matter what. They weren't intended for 50 years of service life. First, though, figure out the problem. Sorry, I'm not able to contribute to the discussion more.

[Structo]

Do you feel comfortable working around high voltages in this amp?

Are you experienced at soldering and replacing components?

If so, I would replace all electrolytic capacitors paying close attention to the voltage rating on the new ones.

The bias cap will have the positive end going to a ground reference, so pay attention to polarity.

Also as mentioned make sure the bias trimmer pots operate correctly and don't lose contact.

Once that work is done check for leaky coupling caps, especially in the phase inverter section.

[El_ThirtyfFour]

Hi,
you guys are great. TO answer a few questions at once:

I have tried changing polarity (switching the AC leads).
THe bias pots are the little white wheeled things in this pic. THey seemed to work fine, but I can check them with a meter.

THis big electrolytic cap, a sprague ATOM 2740 450V 40mfd is there and looks like it was replaced in the past, though I have had this amp for 15 years (sitting in my garage, not in use), so it cant have been recent. The wire on one side looks tired.

I cant seem to find the same cap anywhere, and since this is an archaic part, is it a good idea to replace it with something more modern? If so what cap should I use. One of the reasons I am thinking that this cap might be the culprit is that it is way back near the power supply and thus might be causing this universal downstream effect.

I am very comfortable working with soldering and replacing caps.

THe bias meter I use is this one:

http://www.ebay.com/itm/300601127735?ss ... 1497.l2649

I have only 2 probes, but it does give continuous readings.

Also: My symptom has changed: now it doesnt need to heat up more than about 20 seconds and the ma reading just keeps climbing once it starts and cruises right up to >200ma.

[ampfab]

what is your bias voltage when all this happens?
what happens if you disconnect the coupling caps from the pi?

[ampgeek]

I spent a minute during lunch looking at the (similar) schematic and thinking about your situation.

As ampfab suggests, we need to see what your bias voltage is doing.

So....try this:
Pull the power tubes
Make sure the vibrato is off
Power it up (you won't need a speaker load with the tubes pulled)
Measure the voltage somewhere along the red line in the attached
Report back what you find

Note that you have two bias set pots and two bias balance pots. I can't tell from the pictures which is which so you will need to ferret that one out somehow. You want the bias set pot as shown.

You might be able to get your + meter lead clipped onto the back of the actual pot somewhere on the leg that feeds point "3". Negative lead goes somewhere on a good ground. You will be looking for a steady -40'ish VDC to -20'ish VDC reading if all is well.

Adjust the bias set pot from one end to the other and record the voltages at each end.

Repeat same for the other bias set pot.

A couple other items to note:
Be prepared to buy and install all the electrolytic caps highlighted in yellow.
I see a couple of 1 ohm resistors on the cathodes of the power tubes in the similar schematic. Do you have those? If yes, you don't need your bias probe. Measure the voltage drop across those. 1 VDC = 1 ma.

Above all...be careful! You will be gussying up close to lethal voltages poking around in a live amp. I always hold my left testicle with my left hand (through my pocket of course!) to assure that I don't provide a current path through my heart should I fuck up somewhere. Ladies should choose an equivalent anotomical feature of choice in the same region.

Good luck!
Dave O.

[Phil_S]

I'll go out on a limb and say the just about any modern cap at the same or approximately same spec will do the job nicely and better than that old Atom. I'd be cautious about Chinese and Taiwanese caps, which are just fine if genuine, but there have been bouts of counterfeiting where the caps inside the wrappers weren't what the spec label said. Make sure you buy from a reliable supplier, that's all. The new cap will also be much smaller and should be easier to place in the amp, though you do want to secure it from moving around.

I'll guess you can use a 32uF, 33uF, or 47uF. I'm not sure if you'll find a 40uF or not. Don't sweat the exact uF value. Voltage rating is very important. Don't go below the voltage rating on the existing cap.

[overtone]

I'm wondering if those old filter caps will stand the higher voltage with all the tubes pulled?

[El_ThirtyfFour]

Hi all, again thanks for the help. I will try to take some of the measurements tomorrow. I was ordering some new capacitors and had an issue with replacing this 40mfd sprague atom electrolytic cap in that the original cap has 2 leads coming out of the positive side, (and one in the negative). I was planning to replace this with a 47uf, 450V axial capacitor but how should I handle the 2 leads? Should I use 2 47uf caps tied together at the neg and routed individually to the end points of the current positive leads (one of which is gnd, or, use 1 cap and connect both wires from the positive to the single lead?