Mark wrote: ↑Sun Jan 22, 2023 9:54 pm
How do the 6SL7’s sound different to the standard 12AX7’s.
I have assumed the differences wouldn’t be significant enough to go to the trouble of using 6SL7’s.
6SL7s have an amplification factor of 70 as opposed to 12AX7s at 100. This means that the amp is overall less gainy which I find make it easier to dial in the gain needed.
Also, the octals have more harmonic content. This translates into easier transition to smooth distortion.
Just my personal observations, of course.
I have done this a few times and have plans to do it again.
Excellent question. Hard to answer your exact question without having an identical amp here utilizing 12AX7s. However I’ve wondered about the nature of this tube for long enough that I decided to just start building amps with them to see what it was about so I started with some 5F1 Champ style builds utilizing the 6SL7 and sold them locally with excellent results. In fact they were sort of a test bed before going all out on a more complex amp such as the 2204.
The hi-fi guys will rave about octals having a more flat or neutral frequency response but there’s a video by D-Lab on YT demonstrating that the frequency response is almost identical so I’m not gonna go there. What I will say however is that the amplifiers I’ve built with the octal tubes exhibit a more open and three dimensional character to their tone as well as the different character to the overdrive. Bruce from BC Audio says they provide a power-tube like overdrive in the preamp and this is probably an accurate way to describe it.
The lower plate resistance of the 6SL7s will bring the actual circuit gain up closer to a 12AX7 after all is said and done.
I will be building more amps in the future with 6SL7s for sure.
Yoda wrote: ↑Mon Jan 23, 2023 12:18 am
What I will say however is that the amplifiers I’ve built with the octal tubes exhibit a more open and three dimensional character to their tone as well as the different character to the overdrive. Bruce from BC Audio says they provide a power-tube like overdrive in the preamp and this is probably an accurate way to describe it.
The lower plate resistance of the 6SL7s will bring the actual circuit gain up closer to a 12AX7 after all is said and done.
I will be building more amps in the future with 6SL7s for sure.
I think you hit it on the head when you mention 3D... It is almost the same as having a Klon buffer up front. I tried value-for-value identical circuits with better results (from others as well as myself) with a change to octals. I had a set of noval to octal adapters to make a quick change... let people play it for the 1st half of a jam session then switch, blind when possible. Everyone who tried it preferred the octals. Well, a couple couldn't hear a difference. Different sets of tubes too... didn't matter. Very nice paired with the 2203/4 circuit. And I completely agree with the "power-tube like overdrive". Even the lower gain stuff could be pushed to crunch and ring a lot easier than with the 12AX7s all at relatively the same volume level and settings.
"Education is what you're left with after you have forgotten what you have learned" - Enzo
Yoda wrote: ↑Thu Jan 12, 2023 1:21 am
Yes I fabricated the chassis myself, knobs recessed on top for a unique look. Just need to build a head shell for it.
Loaned it out to a couple different band players for some feedback (kind of like Leo Fender would do with his creations before putting them into mass-production).
Main issue right now is some noise. The amp is giggable by all accounts but not suitable for recording. I will be looking at installing a DC heater circuit for the preamp tubes but will test using a 6v lantern battery first to see if the DC will make a difference. Moving wires around no longer makes a difference in noise.
Interestingly when I ask the real players about FX loops they say they don’t care, they don’t use it or want it. But when I’m listing an amp for sale the tire-kickers ask if there’s an FX loop as if it’s a deal-breaker. Hard to please everyone, but I’m inclined to just include the loop on any and all my builds just to have one more selling point to advertise.
Nice looking build! I especially like the metalwork. Do you have a giant CNC or are you really patient with a file and metal break? I did an all octal tube Trainwreck inspired thing last year and really liked it. https://ampgarage.com/forum/viewtopic.php?f=1&t=36121
Elevating your heaters would be much easier than going DC, but I bet your noise problem is either lead dress or grounding. I’m guessing the easiest way to improve your ground scheme would be to separate the clump of preamp filter grounds and run wires to the cathode grounds of each corresponding stage, or at least farther from the noisy stuff. Original 2204's didn't have a perfect setup, but they did ground the preamp cap can near the input.
I want to experiment with the elevated heaters before going the DC heater route, thanks for reminding me about that. I’m also open to any improvements in grounding. I pretty much followed the original Mojotone British 800 wiring diagram (but not layout) and Marshall Mk.2 Lead 2204 schematic and have not modified much yet until I had a working amp but now I’m ready to really start modding it for improvements in noise floor.
Yes I have access to CNC plasma cutters as well as being a metalworker by trade.
How was the noise in your Trainwreck build utilizing octal tubes? Any real tricks or aha moments to keeping noise down or just proper execution or grounding and lead dress?
My build was very quiet. No real tricks; was using 2w metal films throughout and took a lot of care to get the ground scheme and layout as quiet as possible. To be fair, I was using some tubes other than 6sl7, but I have never noticed any noise problems with those on other builds. Just treat them like anything else.
Looks like you could do a voltage divider thing on your existing terminal strips to elevate the heater. 220k and 22k 2-3w in series from HT to ground with the heater CT in the middle and a 20 to 100uf cap across the 22k resistor is a nice place to start. I'm sure that would help, but I would start with fixing your grounding.
I added a voltage divider and capacitor to provide a heater elevation of about 50v. Made no audible difference in the noise floor so I will try separating the preamp grounds from each other before possibly trying DC heaters in the preamp.
Marshall didn't twist their heaters, elevate, or go DC. I'm guessing dc heaters are more likely to cause more problems than fix them. Grounding or lead dress are much more likely to be the cause. Noisy tube is possible, not sure if you have shielding under the amp, OT too close to the first preamp tube is possible contribution as well. Lots of possibilities, but fixing the grounding is important.
