H&K Tubeman Circuit Questions (added schematics!)

Hey guys!

I've been around this forum for a week or too now, having got interested in a diy - guitar amplifier project!

I have not much experience with electric circuit design, so I'd like to ask you seasoned pros for some guidance on the following problem:

I've acquired this '92 Hughes & Kettner Tubeman recently, an amazing recording tube preamp. Unfortunately I used a 19V AC power supply instead of the suggested 12V AC.... It worked for a few hours very decently, but now it's dead. I'm an idiot I know, but now I want to fix it very desperately.

So I got out my trusty old multimeter and checked some diodes and stuff, and I noticed that two diodes (D1 and D2) were not working properly anymore, they were conductant in both directions.

Now my questions are

a) What could these diodes, in connection with the rectifier B1 just below, be for? I can't make any sensse of this.

b) Could it be that using a 19V supply fried them (and/or any of the ICs? I hope not! :-( )

Thanks in advance!!

Attachment:
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El-Ahrairah said:


So I got out my trusty old multimeter and checked some diodes and stuff, and I noticed that two diodes (D1 and D2) were not working properly anymore, they were conductant in both directions.

Now my questions are

a) What could these diodes, in connection with the rectifier B1 just below, be for? I can't make any sensse of this.



Hi

Have you tried to measure D1 and D2 out of circuit?

If not, the conduction in the two directions is caused by conduction through the bridge B1.

This diodes and bridge B1 are for clipping distortion sound, or any other sonic effect ...
 
Thanks! I have measured D1 and D2 in the circuit indeed, out of it they seem fine...

Do you think I, as a person with little knowledge about electronics, do actually have a chance to find the problem? It appears to me that the "input circuit" (rectifier, 12, 15, 300V) is O.K., and that all the outputs (mixer, guitar amp, power amp) deliver no sound makes me think the problem is somewhere in between these stages :D

How would a professional tackle such a problem? I've been trying to get in touch with real life pros but they're all way to busy to help me repair an old pedal, or even give hints on how to do such a thing! ;)
 
I've done all sorts of troubleshooting:
replaced tubes
socketed IC and swapped them in and out of working circuits
traced the signal path with signal probe

but! no dice, the problem is that the cricuit is quite compact and hard to trace b/c of the daughter riser card. In short, no obvious problems power is where it is supposed to be and signal makes it through the jacks but no serious gain on the other end...

Totally scratching my head! This is a fantastic pedal and i've tried about twice a year to get HK to sell me a schematic but they always blow me off! I've been trying for about four years and nothing. Terrible customer service, IMO. At least they could say we no longer support legacy products. I've tried the german, english, french and american "service centers".

Anyway, please keep my in mind if you ever come a cross one!

Thanks in advance!

Tom
 
Guys,
Still working on this?
Putting 19V AC into the 12V AC socket would:

1) 12.6 V DC to tube heater would go to about 20V - check that the tube still "lights" and you haven't burned out the tube heater.

2) +12 and -12 V DC rails would have tried to go to about +20 and -20V respectively BUT should have been clamped by zener diodes D16 and D17. These zener diodes would also have acted to hold the +15 and -15V rails at about the right voltage. You would have been getting excess dissipation in those zeners and they may have overheated and eventually blown.

If they did blow the +12 and -12 would have risen to those +20 and -20 V levels.

The NE5532 (IC1) will cope with that (+/-22V max) BUT TL071 (IC2) won't (+/- 18V max) and is almost certainly "cactus".

3) +15 and -15 V DC rails would have gone to +25 and -25 V respectively. The TL084 (IC3) will not cope with that (+/- 18V max) and is almost certainly "cactus".

4) The initial high voltage supply at C44 would have gone way over the 450V DC rating of that cap and so it may be "blown up".

If it were mine I would replace D16 and D17 Zeners and check R59, R60, R61 and R62 for any evidence of heat stress. that should restore the +/- 12V and +/-15V supplies.

Then replace IC2 and IC3.

Then remove C44 and check carefully to see if it has blown out its "vent " hole, if so replace it too..

Then check the 300V DC rail, if it is not there replace C45 as well and then if still not there replace all of D10, D13, D14, D15. Note that if the high voltage supply is good it will go quite a bit over 300V until the tube warms up and then settle back to about 300V.

Hope this helps.

Cheers,
Ian
 
update

My Transformer is blown on the 250V side, this seems to be common with these pedals. I pulled it and it was open on the high voltage side. The p/n is E 958 049 and it runs MSRP $28.00 from Yorkville sound in USA. the phone number is 716-297-2920, ask for Jim Welch. I found a suitable replacement for it for $3.99 but I will have to double the voltage to make it work but the ratio is right Philmore TR006 and the size is right with flying leads. I will install the Philmore to test the circuit and see how if it can take the strain. I am putting new caps in on the 300v side. Will report back tomorrow with results.

I have posted the schematic in JPG here for eternity;)

http://picasaweb.google.com/dunham.thomas/TubemanSchematic#5275624657196258226

I armed with new ecaps, new ic's and a new transformer on the way and an ok transformer in hand.

Thanks again!
 
update II

Success!

The cheap transformer fed ~ 210 V to the 300 V rail so not ideal but plenty good enough for testing. I turns out that the transfomer is just a little too tall to shut the case on it. Pitty because I think it sounds a little better this way than it used to. However, the tranformer got pretty hot 62C and I likely need one of the hammond transformer with the correct rating to get both the temp and the voltage down... maybe next time! Also, the difference in tone might be related to the different IC I'm using, I'll know more once get the power supply back to stock.

I installed my caps (22uf 450V instead of the 15uf 450V which I have on order) and the Tubeman fired right up. The original caps were very leaky and most likely caused the transformer to fail. There is a strong possibility that those caps get pretty hot, they look like it anyway. The caps cost me $1.32 each from Mouser and are radial instead of axial but they are the right size. All existing axials are much bigger in length than the F&T. I should just ordered them from Yorkville when I had them on the phone, live and learn!

You will know if your caps are leaky if your Tubeman "pops" once in a while. I would recommend highly that the 15uf 450v be replaced ASAP if you they have not already been replaced.
 
Hi Guys,

I need some help, I replaced my 200v temp transformer with the proper one from H&K and the pedal sounds like new but not as good as the lower b voltage tranny. With big valve amps I've heard of guys bringing down the B+ with big zeners, is this something I can pursue with this pedal? I am looking for a cheap fix here having spent $40 bucks on new caps, IC's and the tranny.

The pedal sounds more sterile now and going through my collection of vintage and new tubes, I found the circuit to be vastly more sensitive to tube quality issues and emf on the bench. Some tubes I tested justed squeled at the highest gain and I could stop them but putting my hand over the tube as to sop radiation. I can equate this sensitivity to higher gain in the tube @300v vs. 200v and this a good thing but the tone actually suffers a bit on top of the added noise.

To be precise, the attack is improved (typical tube rounding) but the the color of the distortion is more sterile and a lot harder on my tubes. Any suggestions would be much appreciated!
 
Tom,

Suggested Modification list.
1) to reduce the B+
No need to add zener regulation, just find R58 the 56K 1 Watt resistor, increase its value, try 100K for a start but you can go higher if you want. Increase it until you get the B+ voltage/sound you want.

2) On the squeel - this is most likely parasitic oscillation of the 12AX7 triode section(s). On the schematic see board connections 9 and 13 which go to the tube pins marked "G" for Grid. These two "G" pins will be pins 2 and 7 on the 12AX7. You need to add "Grid Stopper" resistors on the feeds to these pins. Use 2K2 resistors with the body of the resistor right against the tube pins. Then connect the wires which used to go to pins 2 and 7 to the other side of these added 2K2 resistors. That should stop the squeel. If not try 4K7 resistors instead of the 2K2.

Cheers,
Ian
 
Gingertube, et al.

Thanks for the advice, 200k brought it down to 200v and it sounds great. Once the B+ was down the squealing was no longer an issue. Thank you very much for all your help, I couldn't done it without your assistance to the original poster and of course the schematic!

Tom