vintageHIWATTconvention.com

howdy bros. hiwatt bob here. so the other night i was showing off my DR504 to some dinner guests--i was sort of in a rush so i didn't let the tubes warm up as much as usual. i dimed it just to scare them with volume--and it fizzled out quickly, almost like the sound you get when you power down and keep playing.

i figured i'd blown an output tube--but when i sat down tonight to try to figure it out, i tried a couple different sets of output tubes and no sound. i subbed in different preamp tubes in each position and still no sound.

the pilot light is on, all the tubes glow. i have a bias rite--and it showed the plate voltage to be around 530V on both tubes). the cathode current (mA) reading was 0 however.

took a peak inside and didn't notice any burnt parts/scorched earth. both fuses looked fine. at the time it happened i didn't notice any burnt smell, etc.

any ideas?

hopefully it's not something serious like a dead partridge.

any help is much obliged.

Since you've already tried different sets of output tubes----- and all have normal plate voltage but no current draw----#1) see if the Screen Grid resistors have failed--if screen grid resistors measure ok and screen grid voltage is good (close to plate voltage) on the tube socket pins (pin 4)--- then, #2) suspect failed OT

Dumb question -- you did check the HT fuse, right?

--mark

mhuss wrote:Dumb question -- you did check the HT fuse, right?

...as in, with a continuity tester/multimeter - not simply via visual inspection?

I'd also pull the impedance selector and check for oxidation. Sometimes, just re-seating those can clear things up.

Dave

i have a multimeter--what do i set it to in order to test the fuse? impedence?

i'll try the impedence selector plug.

how would i go about testing the screen grid resistors? i'd feel comfortable enough safety-wise to do it, just don't know which points to put the multimeter to, etc.

thanks boys!

Hiwatt Bob wrote:i have a multimeter--what do i set it to in order to test the fuse? impedence?

i'll try the impedence selector plug.

how would i go about testing the screen grid resistors? i'd feel comfortable enough safety-wise to do it, just don't know which points to put the multimeter to, etc.

thanks boys!

Check the fuse for continuity/resistance (ohms) - should read zero ohms if it's good (continuity), megohms if it's blown (open circuit).

Check the resistance (ohms) across the screen grid resistors. If they're original equipment they're probably 100 ohm. If they've been replaced they might be 1K ohms. If you get super-high readings, you've got an open (blown) resistor.

hey mikhail--

anyway you could point out which points i need to check--are they the big white ones on the output tube sockets? also, if i'm just measuring resistance, i don't need to have the amp fired up--correct?

Hi Bob,
correct, the big white ones and the amp does not need to be powered up for this.
I hope it is a simple fix!

thanks Doc Callowatt--

i'll try this stuff tonight and let you guys know what i find.

Hiwatt Bob wrote: i'll try this stuff tonight and let you guys know what i find.

I'll keep my eyes open for a purple flash on the southeast horizon .

Dave

hopefully there will be no purple flash--just a sonic boom.

ok.

tried the speaker ohm selector plug--that doesn't seem to be it.

measured the impedence on both fuses. both read very low (or high i guess)--like maybe 5 ohms (i think). they measured the same as new ones i have.

measured the screen grid (they are 1K's) and they both measured like 930-ish.

so you guys think i've got big problems?? is there anything else i can do/try? anyway to find out if it's a blown output transformer (sure hope it isn't this).

It seems that you eliminated the most common failure points.

I once had an output tube blow on one of my Hiwatts and it also took out a screen resistor, at least one electrolytic capacitor (I replaced them all), and a fuse. I suppose there's a chance you blew an electrolytic cap?

I thought that an output transformer would smell after it fails, but maybe not after a thermal shock incident.

It is best to play at low volume after start-up and roll up the volume to prevent thermal shock to the speakers and the amp's components.

We'll all keep our fingers crossed with hopes of the OT being well.

Measure some voltages (carefully!) with the amp on, not standby. Connect the black lead to the chassis, and use the red lead to measure (use the 600v DC setting or similar).
1) Voltage at the pins of the two 200uF caps nearest the fuses.
2) Voltage at the each of the pins of the output tube sockets (be very careful here, a slip of the probe and it's blue flash time!*)

--mark


* You need DR201 or DR405 voltages to get the true purple flash.

ok.

i got 30V on each of the electrolytics.

i got the following (same measurements for each output tube, see pic) measurements on the pins

(1) 60V
(2) -40V
(3) Overload -- i was able to at least hear some audible crackling when i touched to these pins.
every other pin read 0.