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Discussion in 'Amps and Cabs [BG]' started by Lowbrow, May 17, 2021.
Ha! I figured that was probably it, glad to help, case closed!
When I read the description, the first thing that came to mind was a really simple problem I've seen on a couple of other amps with effects loops - crud in the switching jack for the effects return. Plugging your bass into the FX send is just silly. Plugging your bass into the FX return should result in a poor signal (unless the bass is going into a floor pre-amp first). The first experiment I'd have suggested would have been to run a known-good 1/4" patch cord between the effects send and receive.
Did that, too, you betcha. Def did not think running a cord into the send would do anything worthwhile (tho I did that as well for argument's sake).
The output tubes are self biasing in this amp. If you are handy with a multi-meter, you can open the amp and measure across the cathode resisters to see how much current each tube is pulling.
Each EL84 has a 100 ohm and 220 ohm resistor in series between the cathode and ground. This means each tube is biased separately. With many cathode biased amps (like the Vox AC30), the cathodes are all tied together, and then a single resistor sets the bias for all tubes. Because each tube sets it's own bias, you could pull the output tubes in pairs to see if you have dead tubes.
The amp has two circuits of two of output tubes. The two circuits of tubes are connected to opposite sides of the output transformer (OT). The current passing through each side of the transformer needs to balance or the amp will produce a hum that I describe as sort of a dull roar. So if one out of three tubes is dead or weak, the amp will produce the dull roar.
The schematic labels the output tubes V4,V5,V6,V7. V4/V5 are paired (AA) and connected to one side of the OT, and V6/V7 are paired (BB) and connected to the other side of the OT. There are three possible physical arrangement that I have seen: 1. Left/Right, AA/BB (4,5,6,7); 2. Alternating, A,B,A,B (4,6,5,7); and 3. Inner/Outer, A,B,B,A (4,6,7,5).
This image shows the tubes are physically in order from left to right as you look at the back of the amp. So the arrangement of the tubes is consistent with 1 (Left/Right).
You can pull one tube from the left pair, and one tube from the right pair. (remove and replace the tubes with the amp off, and keep in mind the output tubes get very, very hot. If the amp continues passing signal with two tubes pulled, reinstall the first pair of tubes and pull the second pair.
OK. It's really important to eliminate all the easy and safe stuff before going into repairs with a higher risk to yourself and to the amp. I've seen you around TB but I don't recall whether you do your own electronics. A tube amp on a bench can literally kill you if you do exactly the wrong thing. Given that you're complaining more of weak output than distortion, the power tubes would be the logical next things to check, as others have suggested, and you shouldn't need to expose the tricky bits on the chassis to change/replace them. If the circuit is as described above, you're not going to learn much by swapping tubes - if this is because one of them is dead, then any combination will give you a signal phase with 1 working tube and the other phase with 2, so it will sound the same. Wasnex is probably right in his statement of which tubes are paired with which others, but you don't actually even have to know that. If you LET IT COOL DOWN BETWEEN EXPERIMENTS and cycle through pulling each power tube, testing the amp briefly with a signal, and replacing it, in most removals, the output will be weaker (absent even) and possibly more distorted than what you're hearing now, but if there is a tube whose removal changes nothing, it's the dead one.
Usually the signal flow goes through the tubes in order. Ashdown labeled them backwards for the preamp. In other words the signal goes through the preamp tubes in the following order, V3,V2,V1.
The original tubes in V3 and V2 should have been JJ ECC83S. 12AX7, 7025, ECC803 are all direct replacements. 5751 is also a sub but has slightly lower gain.
The original tube in V1 should have been a JJ ECC82. 12AU7 and ECC802 are direct replacements.
This is a pretty robust little amp. Hopefully you just have a bad small signal tube.
So now, we are getting down to guessing rather than real diagnostics.
Some obvious things to check are:
1. The symptoms are present when using the high input (bypasses the low gain circuitry at the input)
2. The supply voltages are correct
3. With 100mV signal, the signal is correct at terminal 3 of the volume control with the volume turned all the way up.
4. With 100mV signal, the signal is correct (and almost the same level as terminal 3) at terminal 2 of the volume control with the volume control turned all the way up.
5. The signals at the plates of V1 (an odd way to do phase splitting) are in fact correct.
All of these tests will help identify where the problem is NOT. This assumes that you have a scope, signal generator and know how to use them SAFELY.
Try reaching out to Ashdown via a large social media site. I had a problem with one of mine and Mr. Gooday got me in touch with the leading tech with the company to diagnose the issues I was having within a matter of hours.
If you move a bad tube from one tube slot to another, it will usually change the way the amp behaves when you adjust the controls. This is why I gave the detailed information on the signal path.
I actually suggest rolling a good ECC83 and a good ECC82 into the amp. Each small signal tube has two sections called triodes. If one of the triodes is bad, it will definitely be noticeable if you replace it with a tube that has two good triodes. So rolling a good ECC83S through V3 and V2 is a good call. Also rolling a good ECC82 into V1 is a good call.
If you pull output tubes, ideally you want to do it in a way that keeps the current balanced in the output transformer. But for a short test on this particular amp, you could pull the tubes one at a time. The amp should hum because of the resultant current imbalance in the output transformer. I believe a severe current imbalance in the output transformer can cause build up of excess heat, so the amp should not be allowed to operate for an extended time with one of the output tubes missing or nonoperational.
About the only thing a layman can do is fiddle with the controls and jacks, and roll in good tubes. If none of these basic actions correct the problem, the amp should be sent to a technician.
A word of caution:
Don't assume you can simply pull and replace the output tubes in any amp. Most tube bass amps have fixed bias, which should be adjusted to the characteristics of the tubes. If you put a new tube in with different characteristics than the tubes in the amp, the new tube will not bias properly and could cause damage.
Also with an amp like the Vox AC30 where all of the output tubes are biased with a single resistor: If you pull one tube, the current in the other tubes will increase. Since the AC30 is normally biased at 90% dissipation or higher, running a partial set of tubes will easily push the remaining tubes in the amp over their dissipation rating.
I think it's safe to say all of this is over the OPs head, but these are definitely something a tech would check.
In my experience, with Ashdowns the input labeling should be interpreted as High and Low attenuation instead of High and Low gain. In other words, it's the reverse of what we normally see on amps.
From the LB30 product page, and I assume these are sensitivity ratings:
High Instrument Input 450mV
Low Instrument Input 150mV
From the Valve Series manual
IFRONT PANEL FACILITIES
INPUTS - There are two instrument inputs marked LOW and HIGH. The LOW input is high sensitivity and also high impedance to suit the output from PASSIVE (Low Output) basses. The HIGH input is low sensitivity and lower impedance to suit the output from ACTIVE (High Output) basses. Plugging an active (High Output) bass into the LOW input will overload the input, creating a fatter, warmer and more distorted sound. Experiment by plugging your bass into either input to achieve the desired sound
Not confusing at all .
As your guidance gets more technical and detailed with each successive post, am I to now believe that swapping 3 $15 preamp tubes is no longer probably the easiest and most likely to fix it strategy? Cause the result here is not giving me the power to do anything myself other than dropping it at an amp tech.
IMHO trying good preamp tubes is a good strategy. I would try this before sending the amp to a tech. You don't need three preamp tubes, one of each type should be fine.
Buying a good multi meter will likely cost more than the 3 preamp tubes, and take time to learn. So technically, yes it will be cheaper and faster to buy the preamp tubes and just plug them in. The cost goes up if instead it is the power tubes that need replacing. In which case, it still isn't that expensive to fix, a matched quad of EL84 can be had for $50 +tax/shipping. The cost then goes up more if it happens to be something else. This is the problem with the "shotgun" approach, you're basically just throwing money at it.
BUT, I would be surprised if the sound/reduction in volume you describe is anything more than a bad tube.
If we assume that they've got a scope and a signal generator, there's a basic check to be done without touching a screwdriver. I certainly get taking a serious pro-tech approach and saying that one shouldn't even start troubleshooting without a full arsenal of tools and skills, but to me, there are three distinct levels of troubleshooting: Stuff you can do with no special tools or knowledge, stuff you can do with limited tools (I've never owned a scope, just borrowed one when I've needed one), and stuff you'd do on a properly equipped bench. Not everyone can/should/wants to wrench on their amps, but anything a musician can figure out on their own is less bench time in the shop. Some of this is just systems thinking.
There are many things that *might* be wrong with your amp. A few of them - dirty jacks, bad tubes - are things you can take care of yourself. *If* the problem is a bad tube - and it might not be - it *may* be possible to identify by swapping tubes around and listening, depending on the circuit and the nature of the failure. I would not assume that to be the case.
Sigh. Back in the 20th century, the electronics shop in my area actually had a functioning old tube tester out on the floor for people to use.
Often, when musicians start working on their amps, without the proper knowledge, skills and tools, the total cost of repair often increases beyond what it would have cost before messing with it.
Sure you can swap tubes, but when those guesses are wrong (they often are), you still have to pay for repairs plus the cost of the tubes that weren’t needed.
This is the kind of amp that’s straight forward for a QUALIFIED tech to service properly.
I don't recommend that the OP goes beyond swapping tubes. Yes small signal tubes can last for decades, but they can also fail suddenly without notice. IMHO it a good idea to keep one spare for each small signal tube type in the amp. The amp has two ECC83S and one ECC82, so the OP should have two spare tubes. Therefore, there should be no cost to rolling good tubes through the amp.
Apparently I am not the only one who thinks keeping a few spare tubes and rolling them through an amp is a good idea.
From the Mesa Bass Strategy Manual
And I don’t mind this sort of guidance one little bit. Thank you!
Yes, it can be a good solution but I find that in amps like the LB, tubes aren't generally problematic unless the amp is beat around pretty hard. Not saying that it's not a tube, but most well run qualified service shops will evaluate an amp and if it's just a tube they will be quite reasonable with the cost.
This amp has been babied and exclusively home-used.
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