Blown 6CA7...HELP!

SB Monster · Post by **SB Monster** » Mon Oct 10, 2005 7:20 am

Having scored a quad of GT-6CA7-GE's on ebay and installed them and got my 100 Metro Super bass re-biased, one of them gave up and lit up like a Christmas tree after about 8 hours continuous running. This was on the morning of a second day in the studio and it happenned as soon as the amp was turned on cold. ( day 1 there were no apparent problems )The HT fuse blew first of all. Once I replaced it one of the valves lit up like it was arcing inside. Luckilly I had a spare set of EL34's with me so all was not lost and work could go on. Should my amp tech have changed the resistor in the bias circiut?

FYI, the 6Ca7's were glassier and more transparent sounding than the EL34's with a definitely tighter and more detailed sound and gave the whole rig a stronger feel but not as "Marshall sounding" as I would like.

I'm going to try a set of GT-KT66's, again do I need to replace the resistor in the bias circuit? BTW should I punch my amp tech?

Billy Batz · Post by **Billy Batz** » Mon Oct 10, 2005 9:46 am

Yes you shuold. For KT66s I think you would need a 27k bias pot range resistor. Im not 1000% sure about that. Actually I think the fixed resistor in the old KT66 JTMs with no bias pot was 56k.

SB Monster · Post by **SB Monster** » Mon Oct 10, 2005 9:50 am

Thank's Dan, should this have also been the case for the 6CA7's, do you think that's why one of them fried itself?

Paul

Flames1950 · Post by **Flames1950** » Mon Oct 10, 2005 3:39 pm

Yes you shuold.

I thought you were talking about punching the tech.......

Typically you'll need a bigger resistor in line with your bias pot to get those KT66's biased.....my 50-watter had to go from a 47K or 56K up to a 100K to get them in line.

SB Monster · Post by **SB Monster** » Mon Oct 10, 2005 4:47 pm

Thanks Flames, no I'm not gonna punch the guy but I will give him a hard time

That quad cost me $150!

C'mon you guys, anyone else put a set of KT66's in a 100w super bass? What values of resistor did you need? I can't afford to fry any more tubes.

Paul

Guest · Post by **Guest** » Tue Oct 11, 2005 5:06 am

I did. I ran a quad of GT KT66HP's in my Super Bass and currently have a quad of GE 6CA7's in it. My amp doc did all the biasing but I do know he had to change the resistor because with the stock resistor used for E34's the KT66's ran at a whopping 90 mA! I wish I knew what value resistor he used but it's basically going down in value to get the range you need. I ALWAYS have my tubes at 30 mA, no matter what type, so my amp doc uses whatever resistor gets him in that range. If it sounds simple it's because it is. NEVER take a bias resistor for granted when changing output tube types!

Mike

bluefuzzguitar · Post by **bluefuzzguitar** » Tue Oct 11, 2005 5:08 am

Sorry that was me. Forgot to log in.

Mike

SB Monster · Post by **SB Monster** » Tue Oct 11, 2005 6:07 am

Anonymous wrote:I did. I ran a quad of GT KT66HP's in my Super Bass and currently have a quad of GE 6CA7's in it. My amp doc did all the biasing but I do know he had to change the resistor because with the stock resistor used for E34's the KT66's ran at a whopping 90 mA! I wish I knew what value resistor he used but it's basically going down in value to get the range you need. I ALWAYS have my tubes at 30 mA, no matter what type, so my amp doc uses whatever resistor gets him in that range. If it sounds simple it's because it is. NEVER take a bias resistor for granted when changing output tube types!

Mike

Cheers Mike, How did the kt66's compare to the 6ca7's tonally? What sound/style are you looking for, i thought the 6ca7's were great with loads of headroom but sounded a bit too fenderlike for me. Basically I thought they sounded quite "hard". A bit like loading up the 4 x 12 with JBL's. I'm a strat player, using mainly a sunface/cornish soft sustain in front of the amp. The bell like highs were there alright but nearly too apparent, but then it's hard to know because they must have been running too hot if one of them expired after only 8 hours playing.

Paul

MacGaden · Post by **MacGaden** » Tue Oct 11, 2005 7:11 am

Check this post by Mike/Bluefuzz: http://www.metroamp.com/forum/viewtopic.php?t=1749

It pretty much says it all.

SB Monster · Post by **SB Monster** » Tue Oct 11, 2005 11:24 am

MacGaden wrote:Check this post by Mike/Bluefuzz: http://www.metroamp.com/forum/viewtopic.php?t=1749

It pretty much says it all.

I Know - great post, read it several times before. Totally on the ball. It's technical experience with regards to bias range capacitors used with different tube types that I'm after though.

But any Super Bass players who've tried other output tubes, I'd be interested to hear how you got on.

Paul

SB Monster · Post by **SB Monster** » Tue Oct 11, 2005 11:41 am

bluefuzzguitar wrote:Sorry that was me. Forgot to log in.

Mike

BTW Mike. DB II Welcome has largely helped me on my way on this Superbass Quest - Just browsing and came accross your "Whats the fuzz" thread!

Paul

bluefuzzguitar · Post by **bluefuzzguitar** » Tue Oct 11, 2005 5:40 pm

Paul,

In response to your last couple of posts:

The sound/style I'm looking for is that late 60's/early 70's bluesy classic/southern rock sound. I'm talking Paul Kossoff, Duane Allman, early Clapton. I find both the KT66 and 6CA7 will do the job. You see, IMO you need a hybrid tube that will give you Fender-like cleans coupled with British mid grind. Bear in mind that these players used Mullard EL34's which sound nothing like contemporary EL34's. The Mullards were cleaner with a flatter response. The GT KT66 and any good 6CA7 will get you in that territory.

The difference? I found the KT66 to have dirtier mids and less glassy high end. Judging from your response so far this may well be the tube for you. I currently have 6CA7's in my Super Bass and love them as much as the KT66's. I consider them two different animals equally capable of producing my desired plexi sound. The KT66 sounds tighter but at the same time dirtier, more British. The 6CA7 definitely has a more American 6L6-like flavor to it, so your description of them being Fender-like sounds accurate to me. I don't find the 6CA7's to be overly hard or anything, that probably comes from the tubes having been overly biased. Like I said, for some reason I seem to like the tubes biased fairly clean, some might actually say cold (30 mA). It worked when I had Svetlana EL34's, it worked when I had KT66's and it works with the 6CA7's now.

Yeah, DBII got me into playing a Marshall Super Bass as well. And fuzz faces, of course. He got me on the right track but I ended up with a different take on the plexi sound which is what it's all about. I can say one thing, tho, it's all Super Bass for me! I tried several Super Leads. Great sound but waaaaaaaaaaaaaayyyyyyyyy too bright for me. There's something about the warmth, depth and complexity of a Super Bass that can't be beat!

Mike

P.S.: Wish I could help you with the tech data but I don't know the first thing, I have an amp doc for that.

Billy Batz · Post by **Billy Batz** » Tue Oct 11, 2005 8:19 pm

I agree with BFG on all counts there. Im into the same sound in Marshalls.

SB Monster · Post by **SB Monster** » Wed Oct 12, 2005 11:19 am

Thanks again Mike, I think you could be right about the 6ca7's but they're hard to come by so it's gonna be the kt66's. I'll post a comparison. What was interesing from that POV was that the 2nd day in studio was back to jj el34l's after the 6ca7's gave up. More takes of the same tracks with the different tubes and the difference was there to see in all the honesty that only a studio situation can reveal. I have to say that I preferred the 34's. More warm fat fizz and spit, more "marshall" although a little too blurred in some solos.

I was really interested on your take on fuzzfaces. I'm using an AM nkt 275 sunface. It's germanium as you'll know and I think you could be on to something as far as the si variety is concerned. I do find that the AM "closes up" in tone and loses that raspy spit and snarl that I want from a fuzz, the more you drive it. For sure all fuzz units are very sensitive to guitar vol but even using the internal trimmer, I've never been able to find a sweet spot with it. By coincidence I've found a guy in Belfast selling this si one on the "bay".

http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... %3AIT&rd=1

Never came accross one before but the recipe seems right. High gain bc109. I'm going to see him tonight and check it out.

You are very right on two counts though, SuperBass IS the way to go for those Kossoff, Allman, early EC, Bramhall tones and/or if you're a strat player

and there is NO tone like your own.

BTW if anyone want's my take on strat single coils let me know, I've spent a small fortune trying just about all of them.

Paul

bluefuzzguitar · Post by **bluefuzzguitar** » Thu Oct 13, 2005 6:58 am

Hey Super Bass Monster!

Sounds like the KT66's will be just right for you. You clearly dig the spit and snarl of EL34's vs. the cleanliness of the 6CA7's but when you dig in on the geetar you find the EL34's compressing too much. The KT66's do have that EL34-like dirty edge to them plus they will hold up when you push them too. Expect tight low end and beautiful yet nasty mids. Lemme know how it works out for you, Paul!

And yes, I'd love to hear your take on strat single coils. I currently own two Strats, both of them 1980 Tokai Springy Sounds. One is loaded with Van Zandt Vintage Plus pups and one has VZ Rock pups. I'm very happy with the VZ's but am open to other suggestions!

Mike