Laney VH100 OT failure?

[mrr3000gt]

Guys,

another quick question for us:

I have a Laney VH100 that appears to be having OT issues. I had it for a few years, and did this to it:

1) Made sure all sockets and connections were perfect (amp was able to have 1-4% matching between tubes and each pair of tubes)
2) rebuilt power section - grid & blocking
3) ensured all ground connections on output jacks are solid.

I had a couple of solid years of use to establish this amp was in excellent condition.

The guy hooked up a cabinet at 16 ohms and one wired for 32 ohms into two 16 ohm connections and ran it hard at full volume for about two weeks.

He is now reporting volume drop-outs. The tubes test good (and I even replaced them with a set he had). I got it biased at 18 watts each (KT77s). Its intermittancy and lack of other data leads me to believe its the OT failing (on this 15 year old amp).

The problem can happen now immidiately or in minutes/hour of playing. I did check to see if it is a paracytic osc but checking to see if particular notes cause the dropout (loud volume, check notes chromatically to see if it is a particular frequ). Short of measuring phonomenom, this can detect a problem also connected to an intermittant connection that is vibration-aggrivated. I cant get it to consistantly resurface but did witness it once (and checked jack, cords, FX loop jacks, switches).

The amp never had a history of this in the three years I had it.

I measured with an ohm meter each leg of OT is about 15.5K ohms to ground, and 32K ohms across the OT. If this was any other amp, I would think since it measured greater than 2K ohms across the secondaries, it is on its way out.

And: I did the typical 'chase-the-voltage" stuff to check it is not something dumb like a failing connection or resistor. I cant find anything that looks wrong based on ym 11 years of fixing and building amps. The only failed OTs I have seen have been low/no volume constantly or no outout. I have not had one on the verge of failure in my shop. I dont want to incure a 265 dollar cost to the customere before I am sure (so new tubes didnt make the intermittan problem go away).

Thanks!

[sluckey]

I measured with an ohm meter each leg of OT is about 15.5K ohms to ground, and 32K ohms across the OT. If this was any other amp, I would think since it measured greater than 2K ohms across the secondaries, it is on its way out.

I'm not sure what you are saying? The resistance readings that are important are primary, plate to plate, plate1 to CT, and plate2 to CT. Should be only a few hundred ohms. Then secondaries, all should be just a few ohms, or even less. There should be no reading between primary and secondary if you disconnect the primary CT.

Recheck those readings. If you are really reading 2K across the secondary, the OT is bad. I would not expect the OT to be good one minute and bad the next.

[mrr3000gt]

As I said: its intermittant. I have tested OTs in the past by eliminating everything else as I cant measure it under load conditions (as you know).
But, I did do a quick assessment with my VOM: with tubes out measure Ohms plate to ground, then Ohms plate-to-plate. I did not disconnect the center tap. I did not check the secondaries and primaries against each other: I was looking for a smoking gun and have not dove into it that deep.

I rewired his wronly wired cabinet for him, instructed him on proper loading for the amp, replaced & biased the tubes (KT77s) to a calculated 18 watts for each tube, and tested it for an hour (and rechecked everything).

I told him to contact me if he got any more problems and we would check his OT. There is just not a whole lot that can be going on, as the amplifier checks out fine everywhere else.

My question was I have never observed a OT 'on its way out' - only OTs that were fried or not functional. The last bad OT I fixed was for a 10 year old Peavey blues Junior copy that had very little output (like a mini AM radio sound). Replacing the OT and checking everything (for possible causes of the blown OT) resulted in everything checking out.

The complaint after running the amp at his 'creative loading' was volume drops on occasion (and one occurance of a burned smell that did not eminate from the tubes or board).

I have learned in 10 years or so fixing amps (and 20 years in Info Tech) that users will tell you a story of what happened that leaves them out of the cause (more often than not) and you almost everytime have to start from scratch in diagnosing problems.

[sluckey]

As I said: its intermittant.

Yes, I know. And that's the main reason I don't want to suspect the OT.

it measured greater than 2K ohms across the secondaries

Did you recheck that reading? It should be verrry low, not 2K.

[stratele52]

There a test you can do with voltage on your transformer, with you'll see if it is good or bad.

This is I do and I read that on some Gerald Weber books . You have those books ? You should

The test;
 Amp off, power tubes remove

1-Signal generator at 1000 hz output at 0.5 volts, plug it to secondary of your output transformer.

2- Take mesuremtn of ac voltage you have at the primary

3- Let say you mesure 14.35 volts ac, Divide this voltage by your 0.5 volts ( see 1 ) this give you 28.7

4- Square ; 28.7 X 28.7 = 829.69

5- Multiply by the speaker impedance you want ; we use 8 ohms ; 829.69 X 8 = 6631.52 ohms. You can also do this with 4  or 16 ohms.

6- 6631.52 ohms =This is the transfomer primary impedance for working with a speaker load of 8 ohms

7- Now check which tubes need this primary impedance in some book like RCA manual.

 We know 2  6L6 in push pull need a plate load resistance of 5000 ohms . So this transfomer will doo the job FOR WORKING WITH A 8 ohms speaker load with two 6L6

In your case a lost of power will mean IMO too high impedance at the primary. You have to find what your 4 X KT77 need , I think this will bee something close to 2000 ohms ?

I hope this help you.

[plexi50]

Would those KT77 tubes you are using be made by JJ ?
Been a lot of threads online about the JJ KT77 tubes failing right outa the box

[six el six]

I test output transformers using the "advanced" method on the GEOFEX "tube amp debugging" page. Each half of the primary should read exactly the same. Most bad OT trannies I've found were dead (no sound). One I took out of a blackface Pro Reverb still put out signal (sounded awful, low output) but sure enough, the primaries were off by a few volts. Hope this helps.

[mrr3000gt]

I saw the GeoFX test too: thats the only test I know about.

And the JJ tubes: I am not a fan of JJ tubes. But I like them better than Chinese tubes. The JJs give me the impression they should be sold at McDonalds. My cheap tubes used to be the Sovtek EL34s they used to make, now my cheap but usable tubes are Electroharmonix EL34s at 27-ish bucks a pair.
I prefer SED or Svettys over all others.

I have found a hidden gem EL34: Groove Tube EL34-Ms. They are chinese-made, have the GrooveTube sticker (no painted label), have duel XF getters, and sound better than any other EL34s I have used. I have been an anti-chinese tube proponent until these tubes (75 bucks a quad delivered). These were the tubes the guy who got the Laney fried out (the labels were blackened and cooked). We put in those JJ KT77s into it. They did sound killer, but again, I am not a JJ fan - my perception is that they are not premium tubes and these KT77s cant do the 35 watts KT77s are supposed to do. I set them at 18 and that was because the customer wanted them hot. I wanted them at a nice & safe 14-15 watts.

Thus far, he has not called meaning there is no problems. I do agree that intermittant problems (the worst bug to fix) usually points to (1) tubes or (2) connections. I have been over every square inch of that amp in the three years I owned it and have many solid hours over many months of good behavior with it.

But I will say that the VH100 has some things I dont like: such as lots of micro-components (itsy-bitsy capacitors & resistors & transistors) like you see on JCM900s. Also the chassis is not rigid: its friggin bendy (but not bad). I have seen where bendy chassis result in the PC board mounted pots and switches & jacks solder jopints crack.