Author Topic: Amp vol issue and one valve running REALLY hot/bright!!!!??? (Read 9014 times)

Twinfan · « **on:** December 31, 2006, 12:04:44 AM »

I was checking my 60w Klipp head out earlier today and removed/reinstalled the two power valves. When I powered it back up, the output was very crackly and faded in and out a bit. The right hand power valve glowed REALLY bright orange and the other looked normal. I swapped the valves for another pair and got exactly the same thing - the right hand valve seemed to overheat.

What's the likely problem? Damaged valve socket (left) causing all the voltage to go to the right hand valve? Please tell me it's not the output transformer as it's still got the original Partridges in there...

Is it going to be expensive to fix??????

Fourth Feline · « **Reply #1 on:** December 31, 2006, 01:06:04 AM »

Hi Twinfan,

Sorry to hear about the amp. The most likely culprit(s) are the bias resistors having eventually burned out over time on the base of one of the valves. This is quite natural with an older amp. Indeed many 'classic' amps get their unique sound from the fact that many resistors and capacitors have degraded with age.

If any of the resistors (large carbon or wire wound) were to fail due to old age, this would have the effect of sending the bias across the output pair right over to one side, leaving one valve hard on and the other running cool.

Bias resistors age over time, albeit a lot slower than several sets of output valves. They are however cheap to replace.

Switch off the amp and allow a little time for the large electrolytic power supply capacitors to discharge. Remove the valves, and look on the underside of the valve bases, ( I assume the KLIPP is old school hand wired etc). If not, just look for the larger resistors leading off from the base pins on the PCB.

On one valve, you should find a burnt, open circuit, or even abnormaly high resistance resistor(s) compared to the output valve bias components on it's mirror image on the other valve.

If you are fortunate, the undamaged set of components will give you the colour codes/ resistor value(s) and types you need to buy from 'Maplin' to repair the other side.

This advice is not meant to replace the attentions of a 'hands on' tech, but when I was repairing them (many years ago) the above would be the first thing I would look for.

The consolation is that if your output transformers had 'gone' - you would probably have had experienced the acrid smell of burning transformer winding insulation and blasted output fuses.

Regards,

FF.

Twinfan · « **Reply #2 on:** December 31, 2006, 09:13:20 AM »

Fourth Feline - you are a star my friend :D

I'll drop the amp off at my local place on Tuesday and have them check it out as I'm not going to chance frying myself! There was no burning smell or smoke, so your sugestion sounds like it could well be the problem. Phew! The Klipp is hand wired as you say, so it should be a simple, and hopefully cheap, fix.

Thanks for setting my mind at rest!

Fourth Feline · « **Reply #3 on:** December 31, 2006, 01:48:45 PM »

Hi Twinfan,

Thanks for your reply, I just hope that my assumptions prove to be correct in this case.

Please let me/us know what the outcome was ! :D

Regards,

FF.

Ratrod · « **Reply #4 on:** December 31, 2006, 05:55:36 PM »

If it's not what 4th Feline said, it could also be the output transformer. Hopefully it isn't cause that's an expensive repair job.

Twinfan · « **Reply #5 on:** January 01, 2007, 02:25:52 PM »

What makes you say that Ratrod? How would it make one valve run WAAAAAAAY hotter than the other?

It'll be going to my local shop tomorrow and they should turn it around in about 10 days.

Ratrod · « **Reply #6 on:** January 02, 2007, 12:04:15 PM »

My amp had something similar. One of the power tubes was glowing like a lava lamp. It was caused by a short circuit in the output transformer.

I don't know how or what but this is what the repair man said.

Twinfan · « **Reply #7 on:** January 02, 2007, 12:56:27 PM »

Oh no - I hope it's not that!

<Crosses fingers and hopes for the best>

Fourth Feline · « **Reply #8 on:** January 02, 2007, 10:49:26 PM »

Hello again Twinfan,

I unearthed the circuit diagram for my 50w Laney LH50R -II and had a look at the configuration of the output transformer. Unfortunately I can see that under certain circumstances the output transformer could be faulty enough to drive one valve harder, yet not blow an output fuse.

As a general rule, the transformer on a 50-60W (2 output valve) amp has a three wire primary winding. The 400volts comes off the mains transformer into the centre of the primary and then the anode voltage for each valve is fed off each end of the same long winding. If the coils of the primary between the centre tap and one end tap only shorted together part way along the line, the resulting feed voltage to the valve at that end of the transformer primary would change dramatically in relation to the other valve feed. In other words, the 'centre' of the transformer's primary winding would be effectively moved off to one side etc.

I am just surprised that the resulting breakdown in insulation did not make the transformer give off at least some melting shellac/arcing smell . This could be because it happened very quickly and made an instant contact at the 'new' point, avoiding arcing etc.

It also looks like Laney run the valve cathodes straight to earth, thereby removing the hope that a cathode bias resistor may have gone faulty. There are still the two grid bias resistors to consider (and hope faulty), but I fear the worst if your "Klipp" is wired like my LH50.

I cross my fingers and await with baited breath...

Regards,

FF.

Twinfan · « **Reply #9 on:** January 03, 2007, 08:47:53 AM »

Oh no :(

If it helps, here's a circuit diagram for a Klipp 100w:

http://www.drtube.com/schematics/laney/klipp100.gif

If the transformer HAS gone, what's a good replacement to consider?

Is there still a chance it could be a faulty valve socket? I know they're prone to issues on the Klipp amps?

Fourth Feline · « **Reply #10 on:** January 03, 2007, 09:29:17 AM »

Hi Twinfan,
It is the same basic circuit as the 50/60 types, except in a 100/120w version the power output valves work in pairs, the top pair in parallel with each other and the bottom pair the same. This in effect makes two 'super' valves.

For this reason in a Fender "The Twin" (where the bias is externally adjustable from the back panel) one can drop out the inner two valves and re-bias to get through a show on half power.

As for replacements, Laney would probably have to provide that, or at least their very helpful technical department could confirm the voltage input/ output requirements of the transformer, so the damaged transformer winding may be rewound by a local firm that can do that kind of thing.

I would confer with Laney technical dept by phone on this one, as they will advise as to cost and availablity whilst probably being best able to re-wind if not replace their own product.

Still a worthwhile price for a classic hand wound amp though, and an assurance that many years of hard use can be relied upon from hereon in.

Based on my contact with them, I feel that if any company will charge sensible prices and deal with you in a down to earth way, it would be Laney.

All the best TF.

FF.

Fourth Feline · « **Reply #11 on:** January 03, 2007, 09:29:47 AM »

Hi Twinfan,
It is the same basic circuit as the 50/60 types, except in a 100/120w version the power output valves work in pairs, the top pair in parallel with each other and the bottom pair the same. This in effect makes two 'super' valves.

For this reason in a Fender "The Twin" (where the bias is externally adjustable from the back panel) one can drop out the inner two valves and re-bias to get through a show on half power.

As for replacements, Laney would probably have to provide that, or at least their very helpful technical department could confirm the voltage input/ output requirements of the transformer, so the damaged transformer winding may be rewound by a local firm that can do that kind of thing.

I would confer with Laney technical dept by phone on this one, as they will advise as to cost and availablity whilst probably being best able to re-wind if not replace their own product.

Still a worthwhile price for a classic hand wound amp though, and an assurance that many years of hard use can be relied upon from hereon in.

Based on my contact with them, I feel that if any company will charge sensible prices and deal with you in a down to earth way, it would be Laney.

All the best TF.

FF.

Twinfan · « **Reply #12 on:** January 03, 2007, 09:40:25 AM »

Thanks FF. The amp has gone to a company in Liverpool for repair via my local music shop. They're called "GEE Electronics" and I believe they have a very good reputation for valve amp repairs.

Will a new output transformer (if required) drastically change the tone of the amp?

Fourth Feline · « **Reply #13 on:** January 03, 2007, 09:53:12 AM »

No, the output transformer serves a fairly utilitarian current transfer function and to provide the same voltages, they will have to use the same ratio/number of windings.

The crispness or squashiness of power transfer should therefore remain the about the same. The only variable can be if they use a different size 'former' on which the windings are mounted. Even then, it is not a solid state/ valve type of difference in sound, more a subtle change in power output.

FF.

p.s I have sent a PM off topic. :D

Twinfan · « **Reply #14 on:** January 03, 2007, 10:06:39 AM »

Thanks for the reply FF. I've asked the repairers to call me before doing any work so I can check what they do. I'd be gutted if they binned a repairable Partrdige transformer!!!! I'm still crossing my fingers that it's something really minor...

I'll keep an eye out for the PM (not arrived yet...)