Soldano SLO 30 : How to bias check amplifier when replacing the power tubes (2023)
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- เผยแพร่เมื่อ 29 ส.ค. 2024
- For guitar amplifier technicians and electronics repair personnel...this video is educational only.
Do not attempt to measure voltages or repair an amplifier if you are not qualified to do so. Internal voltages can be lethal.
NEW TUBES : Sovtek 5881WXT, matched pair
Plate voltage ~ 430VDC (approx)...measure @ pin 3 of each power tube
Target Bias range ~ 34mA cool 60% ~ 37mA median 65% ~ 40mA hot 70%
Measure bias across R63 & R64 1-ohm resistors) with meter set to 200mV DC voltage range, 1mV~1mA
Adjust VR12 bias trimpot to set desired dissipation.
Recommend watching on 1.25X - 1.5X speed...I am from Texas so I talk slow as hell, sorry!
Paul @gremlinmachineshop
Also keep in mind with an SLO you are getting virtually all your gain & crunch from the preamp, so there's not much need to set the power section to a hot bias imho...better to run the tubes cooler and extend tube life.
5881 tubes are so good though!
Definitely xlnt choice
FYI you can technically do the math for the 5881WXT as a 30-watt equivalent, but I chose to do a conservative rating BECAUSE one of the original tubes was wasted, possibly due to the factory bias being set at 70% for a 30-watt rating. Coulda just been a faulty tube though.
I think the TAD 6L6WGC-STR (small bottle design) will fit into the headshell dimensionally if you are looking for an alternative to the Sovtek 5881WXT.
Thanks for the great video!
Yvw! The video summary has the instructions also, if you want a quick reference guide anytime
@@gremlinmachineshop Thank You appreciate it!
I have the SLO 100 and it's amazing. I'll probably make it hotter to see how it sounds.
But for now, it's sounds awesome from the factory. They usually bias the amp on the cold side to make it safe probably
I would like to bias mine very cold....counter clockwise would run them colder correct? my mesa boogies bias are not adjustable fixed bias ) and are biased very cold making it so easy to change tubes without a problem, i have been changing tubes in my mark III for 30 years without any issues as long as the tubes were good and obviously not shorted. do you know what turning the bias trim pot all the way left would be at its coldest bias and work? Thanks
I think so, but use your meter to confirm. So very easy to double check it!
Do you recall what the amp was biased at stock? Word is that SLO 100 are biased cold from factory, like 28mA. I understand the reasons why. Just wondering if the 30 were the same, and what might be your observations between stock and after you swapped tubes and rebiased.
Mine was showing something like 42mA or whatever right at 70% was (based on 30W). Those 5881wxt are usually not prone to failure so I am not 100% sure what happened to it tbh for one to fail. As I said in the video you can certainly do the math using 30W instead of 25W for the dissipation factor but I was playing it on the safer side.
@@gremlinmachineshop so I have the combo version that came out earlier this year. The power tubes are different. They look like Tung Sol 6L6. I'm waiting on response from Soldano/BAD to find out what they are exactly. I tried some tube swapping, and I didn't think to check what it was stock. I just set it at 38mA when I swapped and when I put the stock tubes back in. The origin of my question was for comparison, cause I don't think it was in the 20s stock. Then I was doing some research and found that the big ones are cold biased. The manual is useless. No bias recommendations or even tube assignments mentioned.
@@1974zah just regular 6L6? or 6L6GC...we can do some math as needed. Well let's just say regular 6L6 are 25W and 6L6GC are 30W, and you can measure the plate voltage on Pin 3 still to calculate it. I'd guess you understood that part tho.
@@gremlinmachineshop I appreciate that. I'm not quite sure what they are. It's labeled 5881, but the only tube I found that resembles it is the current production Tung Sol 6L6 STR, the black base, not the red one. I should be safe with 38mA for now . I generally bias mine at 65% . I was mainly curious if yours was biased cold before. Hopefully BAD will get back to me with details soon.
@@gremlinmachineshop I actually had a good set of those Sovtek 5881 I had forgotten about. Popped those in this afternoon and the bias is still around 35mA with my probe. Now the amp is sounding like I had imagined. The stock tubes are just kinda bland sounding. Doing some tube rolling in the preamp now.
I just bought one of these and the gain on this amp is extremely compressed and over the top even on the crunch channel and I've played a few other ones. They were definitely more tamed than mine. Do you have a clue what it could be? Thank you.
Maybe you just got a "strong" preamp tube(s). You could always run some lower gain factor tubes in the V1 or V2. Like a 12AY7 or 5751. If you're not familiar with lower gain preamp tubes Google it. There's a handful of other tubes that you can easily substitute for less gain without needing to re-bias the amplifier.
@@-DILLIGAF Ok, thank
you.
@Zebram5 In some of the forums, some people shared the same sentiment as you. They went with lower gain tubes and said it worked a charm. I do it all the time in amps. In a way, it's kind of like having different wattage light bulbs. If 100w bulb is too bright, just lower it down to a 75, 60, or 40w bulb. It's about as easy as changing a light bulb to change to a lower gain preamp tube. Pull one out and put a lower gain one in its place. You don't need to re-bias as mentioned earlier. Good luck if you decide to do it!
what do you mean by the "all the tube drama that went down"? I havent been paying attention to tube news for a few years
When Russia invaded Ukraine, everybody freaked out and hoard-purchased tubes for a bit there, kind of like toilet paper during Covid, et al. Prices went thru the roof for a few months. All the Russian-made Sovtek and EHX prices are still higher than before I think. Funny b/c the other 2 countries China and Czech still produce tubes, no problem. It was an ignorant freak-out, for sure.