[Syrah]

Hey Guys,

I suddenly noticed the two right EL34s of the four glowing way more than usual.  Using the bias meter showed that they were getting off the charts voltage.  I used the trim control to try to lower it, but it didn't work, so I quickly turned off the amp.  The other EL34s on the left are fine.

Anyone want to take a guess as to what's up?

Thanks,
Fraser

[JakeJ]

I believe the bias circuit resistance has drifted out of spec.

First suggestion; call or email Gary and ask him.

Second suggestion; IF and ONLY IF YOU ARE FULLY AWARE OF THE LETHAL VOLTAGES IN A TUBE AMP AND ARE A DIY GUY then you can disconnect the amp from your system, remove the tubes, flip it over and remove the bottom panel to look for obviously burned parts such as a resistor or possibly a capacitor.  If you have the schematic you may be able to trace out the circuit and test the resistors for drift.  As Dodd Audio is a very small company I doubt they give out schematics so I think you are going to have to contact Gary one way or another.

Last suggestion; do not use the amp until it has been repaired or further damage may occur.

Hope this helps.

[Syrah]

Thanks!

I was all ready to send back the amp.  I swaped it for another pair of 120s (I have three pairs), guess what - same problem.  So I'm guessing this is a tube short somewhere.  I will play with this and see if its the 7044s or the EL34s.

[JakeJ]

Sorry to seem a bit thick but...you have three pairs of Dodd 120 monoblock amps, six total?  (I'm jealous.)  So you changed both amps, L channel & R channel, and you took the tubes from the first pair of amps and put them in the second pair of amps?  Then you observed the same behavior in one pair (the right side pair) of the four EL34 tubes in one monoblock?  If you did move the tubes from one pair of amps, did you keep track of the offending tubes so that you know for certain it is the tubes and not a problem with the amp?  The other monoblock is normal?

Is this correct?

[Syrah]

It's true.  I bought two pairs of 120s (2nd version) years ago back when there was a great AC offer, then I bought a 3rd version with a view to selling one of the v2 pairs.  That never happened.  So now I'm biamping OB7s with two pairs of 120s, with an extra pair if and when I set up my second system.  The expensive part is the air conditioning.

I just tried every combination imaginable and narrowed it down to one bad KT77.   Good thing, since I live the Caribbean I'd rather pay return postage on the KT77s than the amp.  I put in my old quads of JJs to keep me going.

I never knew that tubes could glow that much without exploding.

Thanks for your help with this.

Cheers,
Fraser

[JakeJ]

It's true.  I bought two pairs of 120s (2nd version) years ago back when there was a great AC offer, then I bought a 3rd version with a view to selling one of the v2 pairs.  That never happened.  So now I'm biamping OB7s with two pairs of 120s, with an extra pair if and when I set up my second system.  The expensive part is the air conditioning.

Sixteen EL34's in the Carribean?  Yeah, A/C is a must!

I just tried every combination imaginable and narrowed it down to one bad KT77.   Good thing, since I live the Caribbean I'd rather pay return postage on the KT77s than the amp.  I put in my old quads of JJs to keep me going.

I never knew that tubes could glow that much without exploding.

Thanks for your help with this.

Cheers,
Fraser

Cool, glad it's just a bad tube.  Shipping even one amp would probably be as expensive as two months electric bill.  Kidding.

[bluemike]

Fraser

Have you ever tried the Black Treasure 6ca7 in those amps
It's time to replace tubes and looking at options
I'm guessing you're currently using the Gold Lion KT77?

[Syrah]

Hey Bluemike.  I hope you're well.  I'm actually in TO this Christmas - brrr!   Any new audio or record stores I should check out?  I used to live right around the corner from My Kind of Music, fun place.

You're right, I've got the Gold Lion KT77s.  I had the Treasure 6SN7 equivalents in my pre.  They sounded great, but did not last long.  I might give the treasures a go in at least one pair of my amps - getting 16 of them would be a pricey venture, but I could stomach 8.

I'm working on a pair of GR Research LS9s now, which should cook with gas when I biamp them with two pairs of 120s.  Luckily I have no neighbours.

[JoshK]

Nm... too little too late... 

[JoshK]

You ever want to unload one of those extra pairs....look me up.   I might be interested, but the shipping will be brutal.

[Syrah]

You're in TO now right Josh?  I'm going on Sunday.  I actually have a third pair I could sell you - providing I don't go over my weight restriction on AC.  Send me a PM.