I will get some photos up tomorrow.... but I definitely do remember that pins 1 and 2 are soldered together....could this be the problem?
thanks
thanks
Those pins are internally connected in the tube so it's OK for them to be soldered together. But if you have 3v there you have a problem. It should be 0v to ground unless your amp is fixed bias. If it's fixed bias the voltage would be 4x that, normally. Is there a resistor to ground on pin 3 about or larger than 100 ohms?
All of the light grey caps (there seem to be several of them) should be replaced since they known to be prone to early failure. They seem to labelled Rubycon but Suzuki is another typical brand used in Japanese gear. Regardless of brand, all of them need to go.
The small blue caps and the black bodied electrolytics (the PS caps) are not the originals and should be OK. The PS caps seem to be 40uf and two 10uf. It's possible that whoever worked on this previously changed the values of these. Little 5 watt guitar amps didn't usually have 40uf caps in the PS and, depending on what rectifier tube is used, 40uf may be too much. Besides the 7189A, what other tubes are used in this? Do you have a schematic? I couldn't find one online.
The grey "dogbone" resistors are original and, I believe, are wirewound construction. They should all be checked to see that they are in spec but, generally, wirewounds hold up pretty well.
The small blue caps and the black bodied electrolytics (the PS caps) are not the originals and should be OK. The PS caps seem to be 40uf and two 10uf. It's possible that whoever worked on this previously changed the values of these. Little 5 watt guitar amps didn't usually have 40uf caps in the PS and, depending on what rectifier tube is used, 40uf may be too much. Besides the 7189A, what other tubes are used in this? Do you have a schematic? I couldn't find one online.
The grey "dogbone" resistors are original and, I believe, are wirewound construction. They should all be checked to see that they are in spec but, generally, wirewounds hold up pretty well.
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Yes there is a 130ohm resistor in parallel with a 10uf electrolytic cap on pin three.
Yes I did replace all the electrolytic caps...the originals had the same values. I don't have an exact schematic but this one is very close:
teisco-10 diagram - same as express valve amp_zps3oiaignc.jpg Photo by jimmy6122 | Photobucket
Yes I did replace all the electrolytic caps...the originals had the same values. I don't have an exact schematic but this one is very close:
teisco-10 diagram - same as express valve amp_zps3oiaignc.jpg Photo by jimmy6122 | Photobucket
OK. You have a self-biased amp then. There should not be any DC voltage on pin #2 grids. There will be a coupling cap going to each one of those and those can leak voltage to the grids when they get old or are defective. 3v there is not good. Are those still the originals? Did you get voltage reading there on all 4 tubes at pin 2? Also, did you do any other mods, like speaker terminal changes?
The voltage on the grid is due to a leaky coupling cap - one of those grey ones. Again, replace ALL of them.
If the same rectifier is used in yours - a 6X4 - you need to use a smaller value for the first cap after the rectifier. Yeah, it may have come stock with the 40uf cap but that is 4 times higher (!) than the tube data sheet recommends. Arcing, leading to an internal short, is likely with that high a cap. Here's a link to the 6X4 data sheet: http://tubedata.milbert.com/sheets/049/6/6X4.pdf
Sometimes you can get away with going a bit over the recommended cap - maybe like 25% over - but going 300% over is asking for trouble. No point in risking damage to the tube and the transformer.
If you look at most all of the typical small guitar amps like the Fender Champ and all the others that are patterned after it you'll usually see them use a 5Y3 rectifier, which is more heavy duty than a 6X4. Even though the 5Y3 allows up to 20uf first cap they usually use either 8uf or 16uf.
I'd suggest you rewire your PS with one of the 10uf as the first cap, then leave the other 10uf on the screen of the 7189 and use the 40uf to supply the 12AX7.
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Ok I swapped the two electrolytic caps around so now the 40uf cap is on the preamp and the 10uf for the rectifier.
I tested the other pin 2's on the two 12AX7's and I'm not getting any voltage on those. So that would mean that only 1 of 0.01uf caps is to be replaced?
I haven't done any other mods to the amp...I changed the wiring for the speakers
thanks
I tested the other pin 2's on the two 12AX7's and I'm not getting any voltage on those. So that would mean that only 1 of 0.01uf caps is to be replaced?
I haven't done any other mods to the amp...I changed the wiring for the speakers
thanks
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Ok I've taken the readings on the two 12AX7's aswell:
12AX7 1
Pin 1: 305vDC
Pin 2: 3.1mvDC
Pin 3: 3.2vDC
Pin 4: 6.7vAC
Pin 5: 6.7vAC
Pin 6: 210vDC
Pin 7: 207mvDC
Pin 8: 1.75vDC
Pin 9:
12AX7 2
Pin 1: 209vDC
Pin 2: 0
Pin 3: 1.63vDC
Pin 4: 6.7vAC
Pin 5: 6.7vAC
Pin 6: 257vDC
Pin 7: 285mvDC
Pin 8: 3.2vDC
Pin 9:
12AX7 1
Pin 1: 305vDC
Pin 2: 3.1mvDC
Pin 3: 3.2vDC
Pin 4: 6.7vAC
Pin 5: 6.7vAC
Pin 6: 210vDC
Pin 7: 207mvDC
Pin 8: 1.75vDC
Pin 9:
12AX7 2
Pin 1: 209vDC
Pin 2: 0
Pin 3: 1.63vDC
Pin 4: 6.7vAC
Pin 5: 6.7vAC
Pin 6: 257vDC
Pin 7: 285mvDC
Pin 8: 3.2vDC
Pin 9:
Again . . . replace ALL of those grey caps. They are notorious for their high failure rates. No matter where they are in the circuit, no matter what value, replace them ALL!
After doing so, check the values of all the resistors. Generally, +/- 15% is considered in spec.
The schematic doesn't show any voltages other than at the PS caps so it's hard to comment on voltages unless something is obviously wrong - like the voltage you had earlier on the grid (pins 1 & 2) of the 7189A. So just replace those caps (ALL of them), check the resistors, and then see what kind of voltages you're getting at each of the PS caps.
OK. Let's start with the wiring for the speakers. This could be the reason for the squeeling. If you swapped the secondary leads coming off the OPT and now have the lead that was originally the Common or (-) ground exchanged and is now (+), this could create positive feedback. If you did any OPT lead work, then try swapping the leads to the speaker terminals.
Is there only 1 output tube, 7189? Not stereo?
If the other coupling cap is not used for blocking high DC volts like the one going to the output tubes does, then there is no need to change it if all's well otherwise. I'm assuming it's going to the second 12ax7 grid? If there is plate voltage from the first 12ax7 going to the cap but not leaking through to the next grid then you don't NEED to change it now, but it would be a good idea to anyway.
As mentioned in earlier posts, this is a low budget Japanese guitar amp (Teisco Express 5), not a stereo or hi-fi amp. He also posted a link to pics of the internals and a schematic of a similar Teisco amp. There doesn't seem to be a schematic for this model available and it's unclear what the differences are between his amp and the schematic.
There were many brands / models of similar 5 watt guitar amps which were considered entry level. Most of them are very similar - one or two 12AX7s (depending on whether the amp has tremolo or reverb) and, typically a single-ended 6V6 (or sometimes 6BQ5 or 6L6) output stage. They mostly used a single 8" speaker.
The parts used in this amp are the same types used in vintage Sansui (and other Japanese) stereo amps. If you read about restoration procedures written by those who have a lot of experience in these you'll see that the universal, number one recommendation is that the grey (oil type, usually) caps should ALL be removed because of their propensity for failure. If this were an American made amp I wouldn't recommend replacing all the caps because they used different parts that seem to hold up better.
OK, I see it's SE.
Let's get the amp to stop squeeeling so he can get a tube in there for some quick B+ and cathode voltages. I think the amp is going to run too hot for anything other than a true 7189. It's probably too hot for one of those. I've read a lot of stuff that sounded like it was coming from experts that was just unnecessary and wrong. Too much alarmist advice striking fear and panic in newbee souls. I don't shotgun anything. I don't think swapping the PS caps was a good thing, either.
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As you suggested earlier, it's very likely that he switched the wires to the speaker without realizing and, if so, getting them right should correct the squealing. I agree, he should probably stick with a 7189A and, yes, with the higher line voltage that's common today even a true 7189A might be stressed.
Believe me, I don't "shotgun" either but when experienced and qualified techs who have a lot of experience with Japanese gear consistently recommend replacing all of those grey oil type caps, I don't think passing that advice (and the reason why) to a someone to be alarmist. Better to replace them all now rather than having to go back into the amp again later. Generally, I only suggest replacing out of spec parts which is just what I said about the resistors.
Guitar amps are often run on the ragged edge and beyond. The designers regularly ignored tube data sheets, as they did in this case, which is why I suggested swapping the PS caps. The 6x4 data sheet, which I posted a link to, clearly states that the maximum cap value when used in a cap input filter is 10 uF. I'm guessing the tube manufacturers know more about how to run this tube than the designer of a cheap Japanese entry level guitar amp.
So you think it's better to use 40uF? Really?? I find the suggestion to use 300% more capacitance than the data sheet allows alarming.😱 But, hey, what could go wrong . . . let's see, the tube could arc and be destroyed but, if you're lucky
maybe it won't take out the power transformer too. I mentioned that most of these 5 watt amps used the 5Y3 rectifier, whose data sheet allows up to 20uF with cap input. Despite this, every one of these designs that I've seen, including the iconic Fender Champ, used less than this - usually either 8uF or 16uF. Part of the reason for this is that, unlike stereo and hi-fi amps, a certain amount of "sag" in the power supply is considered beneficial in order to enhance their tone and ability to compress in a pleasing way.
Another advantage of going to a 10uF as first cap is that it might drop the B+ just a bit, which would be a good thing. As you said, the voltage may still be too high. I'm curious to see what kinds of voltages he gets.
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Hope he upgraded the voltage rating on them, if they're the same as the schematic.
Yes - Good call! I was wondering about that myself earlier so I took another look at his pics. The pic isn't super clear but it looks like they are all 450v caps.
Fortunately, the 6X4 is indirectly heated so there will be at least a little bit of a slow start. I'm not sure if it's as slow as, say, a 5AR4 but anything will help. A CL80 thermistor on the AC line in would be a good idea too.
I also noticed that the schematic shows only a single heater secondary which is not center tapped and has one side grounded. Is it possible to run the rectifier and all the other tubes off a setup like that?? Maybe the schematic isn't showing a separate CT winding for the 6X4?
Wow...this is really getting interesting. Ok I will proceed to changing all the oil caps...then I'll get some readings for you. Yes the new PS caps are all 450v...the old ones were about to blow especially the 40uf one. The start up time is around 10 seconds.
The amp was squealing before any work was done on it. The original wiring was solid core and a very poor repair job was done on them, so I just replaced the whole wiring from the OT secondaries to the speaker lugs. That's one of the reasons why I suspected the power tube and the OT as being a possible cause of the squealing. I was quite certain that a JJ EL84 would not be correct for 380vDC, and since the previous power valve is unmarked I'm guessing it too may be an EL84 and not a 7189A....but there's no way of telling.
The amp was squealing before any work was done on it. The original wiring was solid core and a very poor repair job was done on them, so I just replaced the whole wiring from the OT secondaries to the speaker lugs. That's one of the reasons why I suspected the power tube and the OT as being a possible cause of the squealing. I was quite certain that a JJ EL84 would not be correct for 380vDC, and since the previous power valve is unmarked I'm guessing it too may be an EL84 and not a 7189A....but there's no way of telling.
J,
To clarify some things important to this process..
Since we're using a diagram that isn't for this amp we need to make sure we don't assume that anything suggested is going to be appropriate for your amp... that said,..
The speaker wiring polarity swap, as a possible cause of the squeeling, would only matter if your amp has a negative feedback line tied to the (+) terminal (secondary lead) somewhere. That line is not on the diagram you've posted. I suggested this possibility before going to your photo folder. SO, so we don't get caught up in chasing something that isn't there, look closely at the secondary circuit and follow any lead that goes back to the drivers. It could be attached at the speaker terminal or it might be connected at a terminal strip, but it would be pretty obvious that it is another lead that goes back to the front and will be connected to the components that go to one of the 12ax7's. That is the feedback line. If you can't find it, we can focus on other things, you can swap the leads if you want anyway, just to confirm suspicions. Hope you get the jist of this FB polarity thing, if it's relevant to your amp at all.
Since this thread is discussing a guitar amplifier the most appropriate place for it is in instruments and amps. I'll move it there now..- Status
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