Krank 100w head w/ only 14v output

[Rockin' Andy]

Hi all. I've got a krank rev1 on the bench w/ very low output (only 14v across an 8ohm (250w) resistor when the scope shows output wave almost ready to clip). I have traded out all tubes, to no avail. I am injecting the signal into the return, so the preamp is rulled out. I have checked the voltages on the tubes, and they seem more or less correct. The OT impedances seem more or less normal - around .7 ohms on the secondary, around 130 across the outer legs of the primary. However, the outer legs to the center tap of the primary are a little off from each other - 60ohms on one side and 73 on the other. Impedance from primarys and secondaries to ground is near infinite. I've checked output voltages coming straight off the secondary of the OT(thus bypassing the impedance selecter switch) and all is well. Nevertheless, the output just below clipping is only 14v (and 14X14v / 8ohm = 24watts (and it's a hunerd watt head!). I'm almost ready to send this POS to krank - any ideas? (Sadly, I do not have a schematic - patrick at krank SAID he'd send one, but so far nothing - all in all, I've asked about 10 times and have yet to get anything, I'm starting to suspect that I'm getting the run around🙁
Anyway, I thought I'd see if anyone has any ideas. Any help at all would be mega-greatly appreciated!!!
many thanx,
drew kramer
vintage grooves (mostly solid-state) amplifier repair shop🙂

 

[Rockin' Andy]

ps

ok, two things I forgot to mention: 1) the amp was purchased at the musician's friend scratch and blem store (it was a return), so it might never have worked. 2) the head looks like it was dropped - the PT brackets are bent - however, I pressed my sharpie everywhere it could be pressed for about 20 minutes and visually inspected the solder joints, so I don't think it's just a bad connection. What gets me is that the sine wave I'm inserting is not distorted in any way when I scope it at the output. It just clips at 14vac instead of 24 or whatever it's supposed to be.

 

[Ty_Bower]

The OT impedances seem more or less normal - around .7 ohms on the secondary, around 130 across the outer legs of the primary. However, the outer legs to the center tap of the primary are a little off from each other - 60ohms on one side and 73 on the other.

That's perfectly normal. To keep the AC impedance equal on each side, the winder needs to ensure there are the same number of turns on either side of the center tap.

With some output transformers, the "top" tube is wound on first and the "bottom" tube is wound after. As the windings lay down, each successive layer is longer than the one before. (Think about an LP record - a trip around the record near the spindle is shorter than a trip around near the outermost edge.) Since the windings are longer, the DC resistance is higher - but the AC impedance is the same, since they have the same number of turns.

I can't comment much on the rest... Good luck!

 

[Ty_Bower]

Oh... you never mentioned what kind of tubes are in it?

 

[magicsamin]

Have you checked the screen resistors? Shorted output tubes will often take them out, resulting in a much lower output.

 

[Rockin' Andy]

Thanx, Ty - that makes sense. the tubes are 5881s and 12ax7s.
Magicsamon, I haven't actually pulled anything out of circuit, but I did check voltages on either side of all the resistors on the power tube board (I think there were 8), and found more or less the same voltage on either side of all of them, so I assumed none of them were open.

 

[Ty_Bower]

A pair of 5881 in push/pull is good for about 26 watts. See here:
http://tdsl.duncanamps.com/show.php?des=5881

Are there eight power tubes in there? I'm wondering how they thought they'd get 100 watts out of the thing. Perhaps it is an enthusiastic manufacturer rating...

 

[Ty_Bower]

It seems in the world of guitar amps, you can get a "100 watt" head out of four 5881 or four 6L6 tubes. I wonder if your Krank has a schematic similar to this:

http://www.ampwares.com/ffg/schem/twin_6g8_schem.gif

 

[Rockin' Andy]

I'm sorry, I was too ambiguous - when I said that there were "eight of them," I meant resistors. There are four power tubes - which should equal around 25v. Yes, I would guess that be pretty close on the schematic - four power tubes in push pull - from what I can see of the wiring, the power section seems fairly normal except for the output impedance switch. It just doesn't make sense to me, though - the power tubes are clipping, and the sine wave looks normal just before clipping. So, I know the preamp section is passing a signal to the output section that is both accurate and of sufficient voltage to drive the output section into clipping. So, the problem has to be (ok, "seems like it should be") with something AFTER the power tubes. Yet, the OT passes basic tests, I've bypassed the speaker jacks just in case one was letting output leak to ground, and I can't think of anywhere else I could be losing signal. So, I'm left to conclude that I'm not losing signal and something is causing all four power tubes to lose equal amounts of signal such that my output waveform stays nice and uniform. PT, maybe? Could a physical stress cause just enough damage so that my voltages were off just enough to damper power a little. Or, could a physical stress cause just enough damage to an OT that it would pass a signal along accurately but at half volume? It seems like a reach, but I can't think of anything else *sigh*. Oh well, maybe I'll sleep on it🙂

 

[Ty_Bower]

Does the OT have multiple taps on the secondary? If you're inadvertently connected to the 4 ohm tap when you thought you were connected to the 8 ohm tap, the voltages are going to be half of what you expect.

Is there a negative feedback line connected somewhere?

 

[tubewade]

Ty may be on to something. Try your tests with your load connected to each different output tap. Could be that someone inadvertently connected them wrong to begin with. Which ever tap gives the most power into 8 ohms is the one I'd use.