Tubes for a Dr103

[mrbig]

Hello... I'm looking at buying new power tubes for my '72 dr103. I know NOS mullards would be the best option but alas they are hard to find. I was wondering what others have tried and or are using in the power section of their Hiwatt?
I have I believe japanese made amperex tubes in there currently and my tech tells me one of them is a little weaker than the others. Any help is greatly appreciated.
Best,
G

[OldSchoolDave]

You might want to do a search on this topic, as others have previously voiced their opinions.

The two front runners for EL34's appear to be the GT "Mullard" replica and =C='s. There have been some quality control issues with the latter, though, so opt for a burned-in set if you go that route.

Dave

[ak-47]

I am getting a set of 1999 =C='s at the moment as I have also heard that the more recent stock has been a bit so so... I have some EH tubes too which are quite good and not so hard on the pocket...

[jrsdws]

Currently running Svetlana EL34's and really liked the Groove Tube EL34M's, also.

[Golds_in_the]

The Groove Tubes Mullard Replica tubes are in mine. Sound great.

[mhuss]

I have ECG Sylvania 6CA7s in one of mine, I really like the way they sound.

--mark

[mrbig]

Well I'm trying out a set of Shuguang black treasures they are matched well and biased cool says my tech. Haven't burned them fully in but they sound pretty good and I expect they will warm up a bit with some play. Unfortunately I have a gremlin playing tricks on me. One of the resistors (I believe it's a resistor, 100ohm 5% are written on it) keeps heating up to the point of de soldering itself from the turret board. Any ideas? George Schulz is going over it so she's in good hands but any suggestions might help.

[mhuss]

That can only be caused by excessive screen current on that one tube. First I'd try swapping that tube with one in another socket and see if the problem follows the tube.

I like some of the big bottle Chinese EL34s, they sound pretty good and seem as robust as anything else being made these days.

--mark

[mrbig]

Thanks! I'll suggest it to George. I had a similar problem with the tubes I got with the head, japanese amperex(?). I'm running a bunch of distortion and overdrive pedals in my chain pretty wide open, well all set to 1:00 (7), could that factor in a overheated resistor? Chain goes... noise gate(effect loop)> compressor> overdrive> fuzz> fuzz with a sag knob> eq with boost knob.

[mhuss]

Putting a square wave through the amp at high volume will stress it a bit more, but it wouldn't single out one tube.

--mark

[mrbig]

Thanks Mark,
For clarification, the 100 ohm 5% is the resistor that is heating up enough to de solder itself from the turret post. That's a screen resistor no? I saw a thread from about a year ago addressing the same issue. I'm not sure how he resolved it because there was no follow up. You suggested he use a flame proof wire wound instead of the cement/ceramic. What would happen if the value of that resistor were changed? Would you suggest going to a less resistant one or are there similar valued resistors that dissipate heat better? What effect does this resistor have on tone? What other things do you think could cause this resistor to heat up so much?
Thanks for your time. I love the hiwatt tone and don't want to have to alter my rig too much but reliability is an issue as we all know. Happy holidays!
All the best,
Gavin

[Pinus]

i have the same problem with my own

[mrbig]

Pinus,
My tech fixed the issue. He changed the ratings on the resistors in question. He doubled their value if I'm not mistaken. No problems, she's running smooth and sounding great. I also convinced to his early dismay to put in a quad of Shunguang Black Treasures power tubes. He doesn't care for Chinese products and that would be putting it lightly. After his initial bias and short burn in he was very impressed that they ran cool and stable and sounded great despite needing many more hours to truly burn in. I have been nothing but satisfied with the amp since. If you'd like I can ask specifically what he did to help you in your situation. Good luck. Now if he can finish the 201, come on Heyboer!
GP

[mhuss]

The 100 ohm resistors on the tube sockets in a 103 are part screen resistor, part fuse. The large 470 ohm on the short turret board is the main screen resistor. I believe the idea was if one tube went south, the corresponding 100 ohm would burn open, protecting the rest of the circuit. This was the only amp DR did this in, all the others from 504 to 405 have normal screen resistor arrangements.

When I build a 103 clone, I usually skip the 470 and put in four individual full sized screen resistors like the 504, etc.I think this is a little kinder to the EL34 screens.

I agree, the better Shuguang tubes have gotten pretty good both sound wise and reliability wise.

--mark

[kats1]

That's interesting Mark. When I replaced the 100r resistor my supplier had none available in 5W, so I put a 10W in there instead. After reading your post, it seems like I'm defeating the purpose and should get my hands on a 5W version of it and replace it.

Oh and PS, I put in a new Quad of =C= from tube depot and they seem very good. Mine all biased between 38-39mA. Plate voltage at 472v