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BC30 Output Tubes


appa69

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Been getting the runaround from Gibson on this. Just want to know the safe ranges for output tubes. 5881 and 6L6GC should be okay, but what about KT66 and 350B? 7581, EL37, all the other 6L6s? Been wanting to try out some different tubes, but can't get a straight answer anywhere. Really want to try out new issue Gold Lion KT66, but read they draw quite a bit more heater current(Heater current was in the 1.5 amps per tube range as opposed to 900 ma commonly found in a 6L6). Any help would be great. Thanks.

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If your amp is designed for a 6L6/5881/7581 tube, you should ask the maker if the PT will handle the additional heater current! Or at least be prepared and willing to pay the potential price of experimentation (a new PT).

 

6l6GC heaters (Ih): 0.9amp

http://tdsl.duncanamps.com/show.php?des=6L6GC

5881 : 0.9a

http://tdsl.duncanamps.com/show.php?des=5881

7581 : 0.9a

http://tdsl.duncanamps.com/show.php?des=7581

 

KT66 : 1.27a

http://tdsl.duncanamps.com/show.php?des=KT66

350-B : 1.6a

http://tdsl.duncanamps.com/link.php?target=4125A89C

EL37 : 1.4a

http://tdsl.duncanamps.com/show.php?des=EL37

 

 

Gil...

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That's why I contacted them for the tranny specs. The best their guy could give me was "if you're going to experiment, wait until your warranty is up". Really? WTF? Why did I contact Gibson and think I could actually get an answer? I've asked to be forwarded to the tech or enginering dept, so we'll see if anyone there can help me.

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i don't know all of what works or doesn't, but here's what will, without any doubt whatsoever, work.

http://www.nostalgiaair.org/tubes/5881.htm

 

those tubes are tubes that will perform the same in a circuit designed for a 5881.

 

go here and you can search for other tubes.

http://www.nostalgiaair.org/tubes/

 

note that if you search "12ax7" it doesn't say you can substitute a 12ay7 for a 12ax7... guitar amps aren't the only amps out there, and 12ay7's don't work the same as 12ax7's. guitar amps are more forgiving, so if the tube you want to use isn't listed, it may still work, but everything you find here WILL unless it has an asterisk.

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Also, don't know how accurate they are, but I noticed on the schemo that the output tube heater winding is marked @5AMP and fused for 6.3A. If this is even close to correct, I have nothing to worry about, but it does seem STOUT. I can't seem to find any other values that seem odd/suspect on the schemo. What are the chances of this being correct?

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I can't say on the KT66's directly, but I do know that the 5881/6L6WGB, 6l6GC, 7581A, and EL34's will work. Right after I bought my BC 30 in 2006, I spoke with Pyotr Belov on the phone about the amp. While we were on the phone Pyotr plugged in a pair of 5881's, a pair of 6L6GC's, and a pair of EL34's and checked the necessary specs as he went (plate volts, static dissipation, B+, etc. - somewhere, I have the actual data, but I'd have to dig for it). All tubes were within the exceptable norms. I've only tried the 7581A's and the Tung Sol reissue 5881's in mine, though. I've been running the 5881's for almost 2-years now, and they've been great...no problems at all.

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hi, everybody, I'm new here.

I just bought a BC30 this last week. I've been building amps for several years now, and once I played this baby I just had to have it. I couldn't build it for the price most stores sell it for, plus it not only sounds great, it looks sharp, too.

Anyway, after looking at the schematic (and promising my wife I would not take the amp apart!) I decided to try some different tubes both in the preamp and output. I found that a 12AT7 in the reverb kept the reverb from getting too fizzy. I tried a 12AT7 in the PI and did not like it, though I found that a 5751 works fine. Decided to go back to the 12AX7. I tried JJ and Svetlana 6L6's in the output and discovered that I actually preferred the stock Sovtek 6L6WXT's -- go figure... Probably the most significant change was putting in a 5U4 rectifier. That seemed to nail exactly what I was after with the response from this amp.

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For the EL34, pin one can be tied to ground, or the cathode. All the other bit octals are tied to cathode internally, so I usually just add a jumper on the tube socket between pins 1 and 8. Easy. Chances are, that may already be done. An ohmmeter will tell for sure. Just pull the power tubes and insert the meter probes in the tube socket's pin receivers. No big deal. Be sure to check for voltages first, then switch your meter to measure ohms.

 

Gil...

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I can't say on the KT66's directly' date=' but I do know that the 5881/6L6WGB, 6l6GC, 7581A, and EL34's will work. Right after I bought my BC 30 in 2006, I spoke with Pyotr Belov on the phone about the amp. While we were on the phone Pyotr plugged in a pair of 5881's, a pair of 6L6GC's, and a pair of EL34's and checked the necessary specs as he went (plate volts, static dissipation, B+, etc. - somewhere, I have the actual data, but I'd have to dig for it). All tubes were within the exceptable norms. I've only tried the 7581A's and the Tung Sol reissue 5881's in mine, though. I've been running the 5881's for almost 2-years now, and they've been great...no problems at all. [/quote'] Blues I remember that conversation you had with Pyotr .But I thought he said that the 34's where at !00% so they where way too hot and would not last very long at all ,So his recommendation was for 6L6's
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Did some checking of my own today, and indeed the Heater winding is fused at 6.3a. Also checked the bases and pin 1 and pin 8 are definately tied together, but both show 239ohm to ground :- seems odd... looking at the tube specs, I don't see why this amp could not handle EL34s. I'm going to look for a So-Cal schemo, I have a feeling I'll recognize alot of what I find in there, too.

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At 239r shared, I'd say that amp will be biased a tad cold for EL34. Which means it'll work, but it may have crossover distortion, in which case that'd be easy enough to tweak if you were dead set on running 'em all the time.

 

Also consider the JJ KT77 as an alternative. It's reported to have the sweet clean tones of the 6L6, and the grind of the EL34, like the best of both! It's billed as a direct swap for EL34's, spec-wise; so like the EL34, it'll also run cold till you rebias for 'em.

 

Gil...

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239ohms is the cathode bias res. Should Work if pin 1 disconnected from 8 and pin 1 ran straight to ground. Not much different between the BC30 and the So Cal other than SS rectification and fixed Bias. And a "presence" control #-o .

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From what I keep seeing on the schemo, it seems to me that ELs would be almost perfectly suited for this amp in Class A(P/P triode). Am I seeing this wrong? Based on Bloozcat's old post with the info from Mr. Belov, EL34 Had Plate 367V , 57mA per tube(140 total). In Triode this would yeild about 16 watts with a shared res of 225ohm, and well within specs for both the tube and the amp. Class AB is a different story... still crunching the numbers.

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Could we check the part number on the SoCal Transformer to see if its the same as the one in the BC30? That would let us know if it can handle the extra current draw.

 

According to the schematics they are the same PT. I also opened mine up and verified that the output tube heater winding is fused at 6.3a. That's more than enough to handle any tube in the expanded 6L6 family. Anything from 5881 through 350B should be fine.

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Should Work if pin 1 disconnected from 8 and pin 1 ran straight to ground. .

 

If this was a fixed bias amp, you would need to do that, but for a cathode biased amp it's not necessary at all. Save yourself the wire and the headache. It'll already work fine with an el34, so leave pin 1 connected to pin 8, just it as is. It's actually a little less stressful on the suppressor grid being lifted from ground potential. Pyotr knew what he was doing when he did it that way.

 

If you're going with the KT66, just plug 'em in. Nothing to tweak. Not even the bias. Personally, I'd want to check the heaters to make sure they're not being loaded down below 5.9v (pull 'em if they are), but other than that, they should do fine.

 

Gil...

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Has anyone noticed the 60V tap on the supply transformer. This amp could easily be converted to fixed bias a la 5F6/6G6/AA165 Bassman.

But

With the o/p pair on its separate heater circuit it's unlikely to overload it (do check). You can put 6L6GC etc into an amp built for 5881 but not the other way around. With this and the self-bias / cathode-bias tube swaps are easier. Pyotr got this right too.

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