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Author Topic: carvin tube 100 tried 6L6's  (Read 5375 times)

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davidfugler

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carvin tube 100 tried 6L6's
« on: Time Format »

I tried some 6L6 tubes in a Carvin tube 100, and the lowest bias I could get was 48 milliamps . I'm wondering if there's a resister I can change, to get that lower? I read a lot of really good reviews on the Tung-sol 7581A, so I'm trying those. Also, I'm wondering how short the life of the tubes would be if I try leaving it as is? Thanks
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rnolan

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Re: carvin tube 100 tried 6L6's
« Reply #1 on: Time Format »

Hey David, I have a Carvin TS100, I bought it when my B200s was playing up.  It has 6L6 tubes, they can also take EL34s (I'd try the Mullards, they sound nice in my 1972 Marshall 50w), there's a switch for each channel inside it, you can also run one channel with 6L6 and one with EL34 (but not in bridged mode), although not something I'd bother with, although you could change one channel to EL34s and then compare.  The TS100 is a very clean tube amp e.g. you can run studio monitors off it.  I like it, and have no need for the presence knobs at all so I leave them off.  You said on the other post that you find it a bit top heavy, it's a very flat clean amp so maybe try a bit less tops and more bass on your patch  :dunno: .  With your MP2, just turn the room eq knob counter clockwise, the further you go the more bass you'll get.  The bias topic is interesting, my understanding of bias in a tube push pull output circuit is it's about limiting cross over distortion at the point where one tube takes over from the other as in one tube does the +ve side of the signal and the other the -ve (think a sine wave).  The amp manual (all 4 pages of it) explains how to set the bias, it's not about getting it lower or higher (unless you are trying to get output distortion or some such), it's about getting it right so the 2 tubes produce the signal correctly and cross over seamlessly.  BTW Carvin recommend 50mA not 48mA or lower (which will introduce cross over distortion).  The Tung-sols should be nice as a 6L6 equivalent.  As (if) you are not trying to get output tube distortion and pushing them hard (i.e. you can get 100w out of a pair of EL34s or 6L6s (50w each) but they are working hard), the tubes should last quite a long time.  I have GT 6L6s (the ones it came with) and they are still fine after quite a few years.The TS100 has 4 12AX7 tubes, 2 per channel.  The first is a buffer tube (and a lower gain 12AU7 would probably be better  :dunno: ), the second is the phase splitter (one triode does +ve side, the other the -ve side) which just splits the signal so the +ve goes to one power tube and the -ve to the other for the push pull output.  12AX7s are not great phase splitters, it's not their DNA, probably better to go with a 12AT7, lower gain but higher current, you don't want gain from a phase splitter but more current will push the output tubes harder.  Then this is suited to say an older Marshall (pre master volume control models) when/if you want to add output tube distortion and not so much for a really clean amp like the TS100.
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MarshallJMP

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Re: carvin tube 100 tried 6L6's
« Reply #2 on: Time Format »

You can even go to 60mA, don't forget this is for 2 tubes so if you have matched tubes each one will draw 25 or 30 mA
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davidfugler

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Re: carvin tube 100 tried 6L6's
« Reply #3 on: Time Format »

this is with a bias probe, under one tube, but these tubes are rated at 35 watts each. I wonder if that gives me a little more to play with. Also, I'm thinking the bias pot is 20K. maybe 25 or 30K for the pot?
« Last Edit: Time Format by davidfugler »
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MarshallJMP

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Re: carvin tube 100 tried 6L6's
« Reply #4 on: Time Format »

If that tube is rated for 35w 48mA will still be safe for the tube (around 68% of max power) but it will stress your power supply a bit more.
Is you bias pot maxed out? Can you measure the negative grid voltage? Changing the pot won't help if maxed out.
Could also be a leaking decoupling cap, do you have the same reading for all the tubes?
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davidfugler

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Re: carvin tube 100 tried 6L6's
« Reply #5 on: Time Format »

looking down from the top of the amp, the pots are fully clockwise to get the lowest amp reading. I'm just now learning this stuff as I go, and I'm probably a little slow anyway. As to the 12ax7 tubes, since the tube 100 only has one of those per channel, are those divided in half so that one side is the buffer, and the other side is the pi? Should I change those over to 12au7 or 12 at7? Right now I put the sovtek 12ax7mb tubes that came in it, back in, to help darken up the sound, since all the other tubes I tried in it, brightened up the sound.  :???:
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rnolan

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Re: carvin tube 100 tried 6L6's
« Reply #6 on: Time Format »

Hey David, I just realised you have an old Tube 100 not a TS100 which is the current model.  :facepalm: Sorry, my bad, I should have looked at the schematic you posted earlier.  Your amp just has a Pi (12AX7) and no buffer tube (1 triode feeds +ve power tube, other feeds -ve power tube (ie push pull)).  The newer TS100 has a buffer tube and a Pi per channel.  I also looked up the manual (https://www.manualslib.com/manual/1006804/Carvin-Tube-100.html?page=16#manual), it makes no mention of using 6L6 power tubes only EL34s.  While these tubes do a similar job I think they require slightly different bias circuits  :dunno: The EL34 is a pentode, while the 6L6 is a beam power tube. (hence the newer TS100 has a switch to change the circuit for each type). Check this thread (https://www.thegearpage.net/board/index.php?threads/el34-and-6l6-interchangable.103648/).  It seems to me that putting EL34s back in your amp may fix it  :dunno: as that's what it was designed for, although it may have been modded to have 6L6s  :dunno: (my guess (and it's just a guess) is it wasn't modded (ie bias resistor) and therein may be the problem).For the Pi tube, a 12AU7 will have less gain (and you have no buffer tube) so probably not a great idea.  A 12AT7 also has less gain than a 12AX7 (not as much less as a 12AU7 IIRC), the advantage of the 12AT7 is the additional current (if the power supply can cope) as it will push the output tubes a bit harder.  BTW for Pi tubes you want matched triodes so each output tubes input is balanced (and matched pairs for output tubes).  Another choice for a Pi is a 6CG7 (https://www.tubedepot.com/products/electro-harmonix-6cg7-eh-gold-pin-preamp-vacuum-tube), these are very popular in HiFi tube circuits.
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MarshallJMP

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Re: carvin tube 100 tried 6L6's
« Reply #7 on: Time Format »

Well I compared both the T100 and TS100 schematics and as far the bias goes they are almost the same except for the TS has that switch but that only lowers the bias voltage if set to EL34. It seems on both amps the max negative bias voltage is -57V (TS set on 6L6) so that should be more then enough for running 6L6 types of tubes.
So I don't understand why the bias current is so high. That's why I asked to measure the bias voltage.

As for the PI tube stick with the 12ax7/ecc83 tubes else you going to loose some gain.

The 6CG7 is not suited for this amp, first this is 6.3V heater tube so you have to do some modification to the heater circuit and second this tube only has a ยต of 20 which is way to low for this amp. Also heater current is double that of a 12ax7.
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davidfugler

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Re: carvin tube 100 tried 6L6's
« Reply #8 on: Time Format »

haven't had a chance to take it apart, but, strangely enough, these tubes sound much better than either of the EL34s, which are all treble. do you think I'll toast the amp if I use them anyway? I doubt that I can return them.So far I've spent 100 on a set of el34s, and another 130 on these 7581As, and 400 on an amp that sounds way too top heavy with the el34.
« Last Edit: Time Format by davidfugler »
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davidfugler

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Re: carvin tube 100 tried 6L6's
« Reply #9 on: Time Format »

I don't play out anymore, haven't in 25 years, so I only play at lower volumes, is there a resister I can change to make this work?
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rnolan

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Re: carvin tube 100 tried 6L6's
« Reply #10 on: Time Format »

Hey David, have you got the presence control turn off ?  Turning it up will add 5khz - 6khz and make it sound top heavy, particularly at low volume.  For your ears to perceive the bass, mids and treb as "flat" (equal loudness) you need to play at 98db which is quite loud.  At lower volumes your ears will perceive the mids (particularly 1khz to 5khz) to be much louder than the bass (fletcher munson curves (loudness etc) https://en.wikipedia.org/wiki/Equal-loudness_contour ).If you have the presence off (and I would) and you are playing at low volume, you'll need to boost the bass for it to sound "normal" (like the loudness button on a stereo).From what MJMP has said either type of tubes should work so I don't think you'll toast the amp.  However, he wants you to measure the negative grid voltage (see his previous posts)
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MarshallJMP

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Re: carvin tube 100 tried 6L6's
« Reply #11 on: Time Format »

Should work, just strange you can't lower the anode current.
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davidfugler

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Re: carvin tube 100 tried 6L6's
« Reply #12 on: Time Format »

I've been trying to figure out how to measure negative grid voltage. I read it somewhere more than once but can't find it again. something like pin 3 to ground?
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MarshallJMP

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Re: carvin tube 100 tried 6L6's
« Reply #13 on: Time Format »

Pin 5 to ground
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davidfugler

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Re: carvin tube 100 tried 6L6's
« Reply #14 on: Time Format »

-43.5volts, all 4 tubes. pin 3 to ground, 447 and 448 all 4. haven't checked bias on all 4 yet.
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