1 6L6 (of 6) at 2x bias Super Twin Reverb

[PRNDL]

I have a Super Twin Reverb with one of the 6L6 output tube socket bias at 50 mA while the rest bias at 25-30 mA. I swapped the tubes and it's the socket.

I measured and compared the grid and anode voltages and they are similar (-61V; 510 and 513V). The bias currents for the 6L6 tubes were 29, 24, 18. 52, 34 and 24. I have a Bias Rite with 2 sockets.

Adjusting the matching pot changed both bias currents, but one was still twice the other.

This is a fixed bias amp with an ultralinear transformer and a ton of interesting features: reverb, EQ, tube distortion.

It sounds great, but doesn't have the Fender clean tone I'm familiar with. It's a bit darker, but that may be the UL transformer.

I'm not sure what to look for. The pins look fine and are tight. Besides, you wouldn't get double the current. The two resistors looked and measured fine (1.5K and 470) on all the sockets.

Any ideas??

I'm thinking the amp is fine.

[Amp Kat]

Did you swap the tubes around to verify maybe a bad tube conducting current ? 50 ma's is way to much

[PRNDL]

Yes, and it stayed with the socket and not the tube, which is why I checked the resistors and touched up the solder joints.

[Amp Kat]

Ok I see after rereading where you said you swapped the tubes. Must be something on the socket or a bad connection on the socket. If two are not doing it means the coupling cap is ok. If the bias is what the others are the only thing it leaves is a faulty socket or open cathode. Try putting the tube in 3/4 of the way and ohm out the cathode to ground. You could have some expanded pins on the tube socket not making good contact.

[PRNDL]

I checked the connections and they are fine, then I re-soldered everything.

It turns out the high current does move with the tube. I'm pretty certain there are several bad tubes, which is making it very hard to figure out since I only can test two at a time and there are three in parallel.

At $15-20 apiece, it's pretty expensive to replace all of them ($120 for six), but they all seem to be the original ones that came with the amp (Fender labels).

At first I had a bad impression of the amp, but it does seem to be pretty cool -- a new set of output tubes might really improve the tone. With a bit of fiddling you could get two clean tones (one with the EQ), plus it has a reverb switch.

Is the general idea to test tubes sequentially and find the weak one(s)?

I suppose it would be easier if I had some spare 6L6's to put in.

[Amp Kat]

Yes put your bias rite to work for you. The tubes drawing the most current are probably the weak ones especially the one drawing 52 ma's. If I'm not mistaken those fender labeled are Sovtek WXT which are the tubes I hate the most and have had the most problems with. Get some JJ's or Winged C tested and matched. You do know you don't have to use all 6 to make it run and cutting back to four will only give you a -3dB cut so think about it and yes you can run two.

[vintagekiki]

PRNDL "The bias currents for the 6L6 tubes were 29, 24, 18. 52, 34 and 24. I have a Bias Rite with 2 sockets."

Don't use tubes with currents 18/52mA; Use tubes with currents 29/24mA as tubes V6&V7 and 24/32mA as V10&V11. This is approximately matched tubes for your amp. Leave amp on for at least 10 minutes. Master Volume or Output Level control (100kOhm) set to "zero". With screwdriver adjust Output Tubes Matching or Output Balance control (400 Ohm) for the least audible noise (usually half way between the control extremes).

The best for you and for your amplifier is new (it's birthday for amplifier) matched sixth tubes. Remember amp power descending with square tube currents ( P=RxI2 ); If tube emission is 70% amp power is 50%. Old tubes is unsafe tubes.

Btw bias current adjust to 25mA with changes 2.7kOhm/ 2 watt. In this case output balance control (400 Ohm) set at "neutral" position. With extended parallel or serial ressistor you increase or decrease tube current. If you use matched sixth tubes adjust current for one tube (it's enough) in socket.


About my knowledge English: Web translator; sorry

[Enzo]

You can sit down and just test tubes. Pick a socket, and plug one tube into it. use your bias meter to characterise the tube. Now remove that tube and install the next tube in the same place, and test. COntinue one at a time. That way you can get to know each tube. The amp won't amplify well with just one tube, but we are not checking amplification, distortion, tone, or whatever. All we are testing at this point is the ability of each tube to bias up at a reasonable level so we can weed out bad ones.

I agree that a fresh set of good tubes is the real answer.

[Rick Erickson]

[At first I had a bad impression of the amp,]

Your instincts are strong... trust them. Then again, Ted Nugent used to play them so how bad can they be?

[Steve Conner]

Oh, also if you're concerned about the cost of retubing, it might be worth looking for those Sovtek 6Pi3S-E tubes. They're the same tubes that get rebadged as 5881 and 5881WXT, and I've seen them go by the crateload on Ebay.

http://cgi.ebay.co.uk/6N3C-E-5881-6L...item5828662991
(just an example, these are from a UK seller and a bit overpriced)

[Bruce / Mission Amps]

Oh, also if you're concerned about the cost of retubing, it might be worth looking for those Sovtek 6Pi3S-E tubes. They're the same tubes that get rebadged as 5881 and 5881WXT, and I've seen them go by the crateload on Ebay.

http://cgi.ebay.co.uk/6N3C-E-5881-6L...item5828662991
(just an example, these are from a UK seller and a bit overpriced)

I have too... do they sound any good.... are they any good at all?

[Amp Kat]

They don't sound real good but they are tough as nails and last a long time but tone is not one of their forte's.

[Steve Conner]

I have a pair of the "5881WXT" version, and yes, I guess they won't exactly set the world on fire with tone. But they're cheap and tough, I remember Ken Gilbert trying to destroy some with 600V plate voltage, and they lived.

I heard that Groove Tubes and Fender both used them as "5881s" or "6L6GCs" for a while. Don't know if they still do. You can recognize them a mile off, they have an almost 6550-sized plate with two oblong holes in each side.

[PRNDL]

We put in three matched sets of Groove Tubes, which came out at about 22 mA bias. This amp has a nice dark sound. It's not the bell-like Fender tone, probably because of the ultralinear transformer. With some tweaking of the EQ, you could switch between some nice tones.