Krank Krankenstein Low Output

[m1989jmp]

Hi all,

got this amp in with very low output on both channels. I know preamp works fine because I ran it into other power amp and it worked as expected. Running other preamps into Krank's poweramp resulted in a very quiet and muddy sound.

Went to check bias, 570VDC (!) on the plates with around 60mA through each pair of JJ 6L6GC tubes. Is high plate voltage indicative of some other issue? Could powertubes be shot and still bias fine? I can definitely hear some funky noise that gets louder the longer the amp works, not unlike filament rattling.

I'll post some schematics when I dig them up, but I recall a plate voltage of around 500VDC written on the powersupply schematic.

Thanks for the answers.

[Helmholtz]

What are screen voltages at the 6L6 pins?

Please measure DCRs of the OT primary halves. Not necessary to disconnect OT wires if all power supply caps are completely discharged.

Do you have a scope?

[m1989jmp]

What are screen voltages at the 6L6 pins?

Please measure DCRs of the OT primary halves. Not necessary to disconnect OT wires if all power supply caps are completely discharged.

Do you have a scope?

I've measured plate voltage again today, it seems I've left impedance selector @8 ohm for a 16 ohm load (very briefly, less than a minute) yesterday. Now with selector @16 ohms and 16 ohm dummy load I get 515VDC plate voltage. Screen voltage is at 520VDC, which I'm sure is not fine. Could be that my other meter was acting up too yesterday.

OT primaries measure 60 and 70 DCR.

I have a scope and sig gen.

[Helmholtz]

I've measured plate voltage again today, it seems I've left impedance selector @8 ohm for a 16 ohm load (very briefly, less than a minute) yesterday. Now with selector @16 ohms and 16 ohm dummy load I get 515VDC plate voltage.

Impedance mismatch won't change DC voltages. Please compare your meters connected to the same voltage. Please check mains voltage when measuring amp voltages.

Screen voltage is at 520VDC, which I'm sure is not fine.

Measured at all power tubes (pin 4)?
While a screen voltage as high as 520V seems krank=sick, it is not uncommon (though not really desirable) that screen voltage is a few volts higher than plate voltage at idle, especially with guitar amps having choke screen filtering.

OT primaries measure 60 and 70 DCR.

Could be O.K.

Please try to find and post schematic. It's better to verify DC voltages before any signal tracing.

[J M Fahey]

You may have a shorted OT

Search for (RG´s???) ringing neon transformer tester.

[mac dillard]

I think Fahey is on to it...I would for sure check the OT. The tester he speaks of use to be on R.G. Keen's site. Don't think it is there anymore. Do a search for "The Super-secret Transformer Tester" that should bring it up.

[nosaj]

I think Fahey is on to it...I would for sure check the OT. The tester he speaks of use to be on R.G. Keen's site. Don't think it is there anymore. Do a search for "The Super-secret Transformer Tester" that should bring it up.

https://www.premierguitar.com/articl...ormer-tester-1
nosaj

[Jazz P Bass]

Quote": I have a scope and sig gen"

Drive the amp to full signal power. (100Mv input/ 1k)

Measure the output tube grid voltage (Vac)

A 100 watt amp should see 90 Vac at the grids (at least).

If that signal is present at the grids but the output at the speaker (dummy load) is low, then suspect the OT.

[m1989jmp]

I finally got around checking it. It seems literally all pots and jacks were dirty and with intermittent contact, must've been in a very humid environment.

After a good cleaning it's alive again with full power. I yet have to address high frequency oscillation when distortion channel volume and gain are both set past 7-8. The amp is completely stock so I'm thinking either the preamp tubes have too much gain or the amp is just designed this way. Doesn't happen with distortion channel GAIN and master volume set high, while channel volume is set around mid point.