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carvin tube 100 tried 6L6's

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davidfugler:
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.  :???:

rnolan:
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.

MarshallJMP:
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 µ of 20 which is way to low for this amp. Also heater current is double that of a 12ax7.

davidfugler:
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.

davidfugler:
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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