can anyone help diagnose a dr103

[jasperok]

Ok - SO major redplating going on in an old dr103 (its my friends amp who got me interestred in the hiwatt sound.

On first inspection i noticed one of the rectifier diodes had come off the turrett so i soldere that back up.

I didnt realy know how to trouble shoot the redplating but i followed markh 's first startup guide of the amp (which i wil soon use for my own VHR build!)

anyway

Lug 8 of power tube socket = 528v
Lug 5 of power tube socket = -35v
preamp board lug 18 = 482v

so when i turn the amp on before standby heaters all glow good and normal

with all controls ZEROED when i come out of standby i get a loud hum through the speaker
after 8 seconds I get redplating on tube to your outer right (amp orientated : power tubes at your belly and preamp tubes far away)
maybe the rest would redplate too but i dont like to risk letting it redplate its just that outer right lights up first.

So i took out to outer right and left tubes to run at 50watts and halfed output impedance to compensate

Exact same problem but except being the outer right tube is one of the middle tubes.

Mayeb to not when the amp is switched out of standby i get a very white flash out of V2 for a second then it consistently glows as normal

can anyone help?

this guy helped me discover hiwatts and i'd like to do him a good turn.

thanks

[riba2233]

Are you sure it's lug 8? Lug 8 should be 0 volts (connected to chassis), and lugs 3 and 4 should be high voltage...

[Dr.HI-TONE]

I recently had a similar situation with a 1968 DR103.

make sure the power board matches the manual and or layout. Don't get hung up on each turret exactly matching the layout. I have seen many old HIWATTs that had components moved one turret to the right or left. What is important is the circuit. Use your DMM and continuity test to verify that the circuit is correct.

Posting pics is a big help for us at this end too.

The more info you can give us, the more help we can be. Except for Prof. Huss, he fixes these things in his sleep!

If the power section is tru to spec then I would ask, What power tubes are in the amp? New production Sovtek or EH (any New Sensor brand) cannot handle the voltages found in old HIWATTs.

[riba2233]

I recently had a similar situation with a 1968 DR103.

make sure the power board matches the manual and or layout. Don't get hung up on each turret exactly matching the layout. I have seen many old HIWATTs that had components moved one turret to the right or left. What is important is the circuit. Use your DMM and continuity test to verify that the circuit is correct.

Posting pics is a big help for us at this end too.

The more info you can give us, the more help we can be. Except for Prof. Huss, he fixes these things in his sleep!

If the power section is tru to spec then I would ask, What power tubes are in the amp? New production Sovtek or EH (any New Sensor brand) cannot handle the voltages found in old HIWATTs.

Unfortunately, I've tasted that on my skin

[jasperok]

with the preamp tubes in and the power tubes out - each of the power tube sockets reads the same on 4,5,6,7 = 533v 531v -35v and 530v respectively

I just tried with the two tubes i HAVEN'T seen redplating in the center position and guess what - it plays!

2 bad tubes then? is that definitive enough or do i need to check more?

kinda worried about that white flash when in v2 when i bring her out of standby.

thanks again

[Zells]

What power tubes are you using? If two of them already red-plated, then the other may probably will soon. Most power tubes, especially modern ones, do not handle a DR103 well.

kinda worried about that white flash when in v2 when i bring her out of standby.

A bright flash in certain vintage pre-amp tubes (Mullard & Amperex) is totally normal during cold start-up when the 'Mains' switch is turned on. I haven't seen any tubes flash when the 'Standby' switch is moved up or down, so I'm not sure what happening.

[menace2u]

Just to chime in here- my VHR kit which remains true to the original DR504 circuit had the bias current at almost double the rated dissipation for the 2 EL34s. Those EH tubes did just fine until one night when a buddy came over wanting the FULL Hiwatt experience. To quote the cop for the first National Lampoon Vacation movie....."Poor little guy- probably kept up for the first mile or so."

Regardless of what you may or may not find, I do suggest that you consider doing a bias mod to the amp if your friend plans to gig with it and IF the amp is not a collector piece.

The stuff made today sounds good- but the saying could never be more true "They don't make em like they used to."

[mhuss]

Yeah, it could just be two bad (or badly mismatched) output tubes. I would suggest changing out all four from a known good amp if possible.

--mark

[jasperok]

i got another while to look at this this morngin

there seems to be something still wrong

i plugged in y bias rite to the two tubes (i am not using the outer two tubes)

from the second i turn it on they read 70 and 90 so i switched it off staight away pretty much

they both read 490v within a volt of each other

what could cause this?

[OldSchoolDave]

what could cause this?

Not sure. Maybe your bias resister has failed ?

Check the reading of this guy (connected between turrets 5 and 8 ) and report back:

Bias Resister 001.jpg (330 KiB) Viewed 2626 times

Dave

[mhuss]

Check the voltage on the 22k resistor (Red Red Orange Gold) going to pin 5 of the output tube sockets (either end). This should be in the neighborhood of -45vDC (you can check this on standby). If it a lot lower (closer to zero), you have a bias problem.

The other thing that happens on rare occasions is one of the PI output caps gets leaky. When this happens, the -45 on the 22K resistors will be OK on standby, but when when you switch it on, this voltage will drop a lot.

--mark