1982 JCM 800 2203, insufficient gain after retubing? - gut shots

[A Very Special Piccolo]

I have a 1982 JCM 800 2203 that I've barely played. A good while ago, I brought it to a tech asking to have a set of kt88s and a bias resistor installed. I don’t know what the bias had been set to before I took the amp in, but it used to be el34-equipped and have a very full sounding distortion, and afterwards the distortion sound is very brittle and not full sounding, even hen the bias is pushed to its maximum. I put the el34s back in, and it sounds way fuller and smoother, but I think that I still can’t get enough gain out of it, unless the bias is set to max, and use both a boost and OD pedal in front – and even then it still isn’t as much as I’d like. I also can't find where the requested bias resistor is.

 

I took a couple gut shots of the amp, and I wonder if anyone can see what was or wasn't done to this amp to produce this outcome:

 

fullsizes:

 

http://i.imgur.com/6IEM3vp.jpg

http://i.imgur.com/pKU8LYq.jpg

 

 

 

Maybe tomorrow I can also record clips of its sound with el34s and kt88s with the bias pontimeter at max.

[WRGKMC]

Bias set to Max??? You obviously don't have a clue on what you're doing. Bias is not a gain control. Bias is set to the specific ratings of the tubes you purchase. You just don't crank it up. The damage that will occur will be stealth and occur slowly as you cook the tubes and the final result is often disastrous.

 

When installing KT88's the bias feed resistors is dropped to 100k if you want to run them close to their design ratings when using fixed bias. KT88's (and 6550's) can't deal with the high grid resistance from the 220k resistor typically found in an EL34 bias circuit and the tubes could be destroyed.

 

The other important thing to remember is that a properly biased KT88 will need significantly more bias current than an EL34, due to the higher (42 watts) dissipation rating. EL34's typically are specked at 25 watts. This means your output transformer power transformer and bias circuit need to be up to the task. You can, if you wish, run the KT88's colder at EL34 dissipation ratings, but they most likely won't sound their best. KT88's will also draw slightly more heater current (1.6A vs 1.5A), but this shouldn't be a problem for most amps.

 

Chances are the KT88's sounded like crap because they were running cold because they didn't get enough bias current from the transformer/circuit. The gain changes are the result of switching to a tube designed to run cleaner with more headroom. That's something you should have expected before you had the work done.

 

My question is did you switch back to 220K ohms before you put the EL34's back in. If you didn't you're probably damaging those tubes every time you light the amp up and it wont be long before they short out with a major circuit failure like blowing transformers. You are playing with fire here sir and the results of your tinkering will be your own fault.

 

If you were looking for different gains, you should have tried different types of EL34's. Groove tubes for example sell tubes with different current ratings that can produce different saturations depending on how they are biased. I do suggest you get the head properly biased again and stop fiddling around blindly.

 

If you want to learn how to set bias correctly you need to get a special tool that plugs in between the tube and socket that lets you measure the tubes bias current and set that bias to the specific tube rating, not by how it sounds. If you want different sounds, you do it by using different tubes and again setting the bias properly for those tubes. Do your homework before you mess with stuff like this or let a pro who already made the mistakes you're heading for making keep you out of hot water. Curiosity does kill the cat when you jack with bias and damage tubes running them out of specs.

 

 

http://www.ebay.com/itm/like/181569003485?lpid=82&chn=ps

 

http://www.ebay.com/itm/like/291163227325?lpid=82&chn=ps

 

http://www.ebay.com/itm/like/301054314656?lpid=82&chn=ps

 

 

 

 

[A Very Special Piccolo]

Thanks for the information. I haven't been playing the amp, and only turned it on to test the sound after adjusting the bios pot, which I did because the amp really doesn't sound good as it came back from the tech who put in the kt88s.

 

Because the amp sounds pretty awful with the kt88s in, but sounds good with the el34's in (with a lot of boost going on), my guess would be that the tech didn't change all the things which you've mentioned.

 

And I had asked for a bios resistor to be installed so that I could measure the bios with a DMM and without needing a bios measurement kit, but I'm unsure where that resistor is.

[WRGKMC]

The pots are the resistors but you need to realize there's a difference between running something safely and running something at optimal levels. That amp wont properly power KT88s because the power transformer and all its accompanying circuitry really doesn't have the juice to supply the tubes what they need. They were originally developed for high end Hi Fi, and bass amps that need to have low distortion and plenty of headroom. Rock guitar amps are far from being high fidelity and having low distortion. To make them work properly you'd have to gut the entire power amp section to increase the current and plate voltage needed to power them properly, not just change the bias resistors to make them safe.

 

Even then, if you're looking for increased gain saturation its not going to get you that. Its going to increase the watts and clean power so you wind up going just the opposite of what you're looking for. You would have to rely on your preamp saturation for drive instead of getting it from the power tubes, and because the power tubes are designed to produce extended frequency response, any breakup they do produce isn't going to sound pretty.

 

I have a music man amp that I had just the opposite problem. It runs on a higher 800V plate voltage. The only EL34 tubes that could take that higher voltage were JJ tubes. Luckily EH reissued the 6CA7 tubes the head originally ran on and I re-tubed it with those. This greatly increased the headroom and warmth. Those heads are a hybrid that has a clean power amp and used a SS preamp to get saturation.

 

You should also realize, most power tubes aren't supposed to produce a large amount of saturation. It was guitarists abusing amp who made those saturated tones popular and pleasing to the ears. Today you intentionally use higher gain feeding lower fidelity components to get the sounds you want but with that comes danger. The circuits haven't changed much because the tubes haven't changed and you have to keep the voltages and currents within specs so you don't blow things up.

 

Tubes are funny animals when it comes to this. With solid state, the transistors blow out like a flash bulb when they overheat. Tubes die a slower death and you may not know they were improperly set. I have a 1967 Blackface Bassman I bought back then and It went through a period of blowing grid resistors. I was young and inexperienced. There was no such thing as the net where you could Google up the causes. I was running a high speaker impedance and the power tubes took about 3 months to blow out. I'm lucky I didn't blow both transformers out. Its a testament to the quality of the build. Eventually I took electronics in school and got my degree and now know how circuits work. Its all about balance and counter balance. You have components that have parameters built into them by the manufacturers. If you don't leave

 

a safety margin when abusing them and continually push them over their maximum limits, something's going to give that isn't supposed to. Many amps see a short lifespan when they are souped up to push those limits. Without proper fine tuning and regular maintenance, you should expect failure to occur when you least expect it. You shouldn't help it along by tinkering with something you know nothing about. Even techs have to rely on the specs a component manufacturer provides. Even if he's really experienced and knows the real limits of the components, you'll always find him add a safety margin. Otherwise he'll loose his shirt on repair warrantees. Sure he can push something up to maximum limits but you wont find him giving you a warrantee on it. You'll be running it at your own risk.

 

Again, I'm not telling you what to do. I'm just passing on some first hand knowledge as a trained technician. Best advice I can give is pull the power tubes out. Put a meter across the bias pots and sett them for exactly 220K ohms if you're running the EL34's or 100 ohms if you're running the KT88's. This would produce an even sine wave assume you are running a perfectly matched pair of tubes. Of course matched tubes are ball park close at best so the only way to know exactly how well the tubes are conducting is to use a current meter to set them. Chances are this is what your amp tech did if he's any good at his trade.

 

You may be able to balance them measuring the voltage across the bias pot, but that's really not much better then setting the resistance. Its just resistors can react differently when higher voltages are dropped across them so the actual conductivity of the tube is the only true way of knowing. You must never swap the two tubes around either. If one is weaker than the other you can throw the bias balance off by swapping them.

 

Once the bias is set the pots should have a drop of loctite added to the edge so the vibrations from the head vibrating on the circuit doesn't cause the pot values to change. The only time you break that setting is replacing tubes and having the head rebiased.

 

Good luck.

[Brewski]

right on both tech posts. I worry more about the integrity of the person who reset your amp. when you have an older great sounding amp you have special components that are sought after by tone chasers and amp techs know this. I am afraid he may have taken some of the vintage caps "to help avoid breakdown for you" and replaced with more modern "crappier sounding" caps and sold them or used them in his gear.

 

Sorry but when I find a gem no one is touching it again without my complete supervision.

[WRGKMC]

right on both tech posts. I worry more about the integrity of the person who reset your amp. when you have an older great sounding amp you have special components that are sought after by tone chasers and amp techs know this. I am afraid he may have taken some of the vintage caps "to help avoid breakdown for you" and replaced with more modern "crappier sounding" caps and sold them or used them in his gear. Sorry but when I find a gem no one is touching it again without my complete supervision.

 

I been a tech for 45 years. The last thing most techs would bother doing is replacing parts that don't need to be changed, If they are a trained in electronics they would know anything dealing with caps as giving some kind of magic tone is 100% voodoo BS.

 

Here's an analogy. You're an auto mechanic. Someone comes in and wants the best spark plugs put in. Which would you do. Go take some used plugs out of a mustang, stick them in the escort, then have to go back and put spark plugs in the mustabg or buy the exact same plugs new and install them? Used plugs from that mustang isn't going to make that escort run like a mustang. If anything its going to run like crap with used plugs. The answer is of course you'd order the better brand of plugs and stick them in the customers car. Why would anyone think its any different in electronics???

 

If anything caps get old just like batteries do and they have to be replaced before they break down and short out. On a realistic level you should change caps out every 10 years max. Power caps only filter out AC ripple and have nothing to do with amp tone besides removing hum. Coupling and tone stack caps are the only ones that actually affect tone and most of those cost $.05 at any electronics parts outlet and there is no shortage whatever of finding exact matches.

 

What you do have an endless supply of are ignorant fools being duped into thinking there's anything whatsoever unique about the parts that's going to give their amp some magic mojo. Its complete BS but there's a fool born every day and there are plenty of individuals who have no problem robbing you of your money.

 

There just aren't as many educated technicians telling kids to watch out for these low lives selling snake oil. Or they just don't want to believe because electronics is a complete mystery to them so they choose to believe the voodoo BS they read all over the place.

 

You want a good sounding amp, it begins with a good design, then you use better then the bargain basement parts (or used junk) so you purchase components that have low tolerances. Caps are the worst because they can be off by as much as 20%. That's not usually a big deal in power supplies because the value simply needs to meet of exceed capacitance to do the job properly. 5% tolerance resistors is much better then 10 or 20%

 

Once you purchase all the parts you test all the components and pick the ones with the tightest tolerances. This was a common thing you'd do as an electronic tech, especially working on stereo gear when you want both sides to match in tone. You go through maybe 10 resistors of the same value and pick the two that match the closest. You'd do the same for caps, coils, transistors and tubes etc.

 

Of course that takes time and runs up the bill. If you run an assembly line then its just the law of averages weather you get something that's closer to specs. Your better gear is made with tighter tolerance parts and they usually last longer with the parts running closer to how the engineer designed them.

 

Once the units built you can then tweak the tone by intentionally straying from the initial schematic. There may be many reasons for this. There are many things you can see on a scope or meter you don't hear with your ears. You don't get that either on an assembly line and if any testing does occur its usually done by uneducated workers who couldn't tell you the difference between an ohm or an amp.

 

The test equipment a tech uses lets him visually look into the circuit and see what's going on inside that's otherwise invisable. You may move a wire to taper off some stray hum, increase the frequency roll off to match a particular speaker or any of a hundred items to get the best performance.

 

Its no different then tuning up a car to run its best. Its just most end users haven't got a clue. Its not like they own several amps nor do they use other peoples amps much. On the other hand a person may own dozens of cars in their lifetime and ride in hundreds more so they have a much broader comparison to base their opinions on them.

 

There's surely nothing unique in that Marshall that you cant buy exact matches from a hundred different vendors, and in many cases the newer caps being made today are not only a much higher quality but have lower tolerances. If anything,the tone stack caps will be the key items that can be tweaked to get the best tone and that's often a matter of taste. Since most amp EQ's are designed to go well beyond what an end user may need, getting those frequencies right isn't that hard to do. You can vary the voicing based on the values you choose to use, And with a good scope and signal generator you can easily target exactly what is needed.

 

 

The only thing that might happen on old vintage amps is the caps drift out of specs so far it does give the amp a unique tone that owner may like. You could also match that by using ohms law and grouping a pair of caps, but you do have to remember those amps were never designed to last more then 10 years and keeping those old components in there makes for a weak spot that can fail at any time.

 

 

I see caps especially as hardening of the arteries. When those caps dry up and fail to pass electricity properly its like what happens in your veins when your arteries clog up. Your amp wind up having a stroke or heart attack when its pushed too hard.

 

The hardest part is getting people to realize this and get them to pay for the parts and labor to replace them all. Most could care less and prefer to buy a new one when it does blow out. Most amp techs could care less because when it does blow, he can earn a bigger paycheck with a larger repair.

 

Robbing an amp of used parts? Worst thing you can do because that eats up all your profits as a technician. It creates too many call backs that you have to repair on your own dime.

 

You can go over to a parts cab and pull out a $.20 cap, clip the leads of the old one and solder in the new in about 10 minutes tops. Remove that cap from the other amp, stick in in the one you're working on, then have to go back and fix the one you robbed the part out of takes 3X the work and effort. No tech in his right mind is going to be bothered with that unless he truly is an idiot.

 

There are some cases where you may have a bone yard of old amps. If a customer comes in with something truly unique that's no longer being made and they want it restored, then yea you might dig through the bone yard to find that part or call other dealers you know to find that part. If it cant be found you tell that customer to go pound sand because all that time it takes finding that part cuts into your profitability and chances are if they are too cheap to buy a new amp, they're too cheap to pay you for all that effort finding that rare part.

 

Sorry for going through explaining all this but it does rub me the wrong way when people think techs are crooks. If anything they bust their asses to help people and the heard work they actually put into giving what their costumers want and never get the recognition they should as being honest repair guys just trying to earn a living.

[realtree71]

Lets not forget that kt88 though more powerful than the el34, it also has less distortion. High output low distortion are its main charactoristics

[WRGKMC]

True, and add to that, the tubes only produces that extra wattage if the amp circuit designed to support its maximum potential. Without heftier power and output transformers and all the support circuitry optimized to supply those higher voltage/current, the tube only has the potential to produce a higher wattage. Even with the bias adjusted up, the best it can do in an EL34 circuit is run cold. This may give more clean headroom but actual wattage isn't going to increase by anything significant.

 

You may read that it does increase the RMS wattage, but you have to remember, RMS is based on clean, undistorted sound. If the EL34's can be turned up half way before distortion and read 40w and the KT's can be turned up 3/4 and read 50w clean, the distorted sound of the EL34 may still read 50W at 3/4. Its just the waveform is clipped.