OT? vibrochamp..newb help please?

[Ben]

Hello Everyone!

I'm a relative newb to amps. My only experience is buildig a mojo tweed deluxe kit (whic I want to marry and have children with). A friend has showed me how to safely drain caps.

I will get right to the main question and then explain the problem:
Is there a simple way to test an OT for failure without buying additonal parts?
as in... is there a way with my DMM to test it?

Heres whats going on. I bought a 73 vibrochamp. Cool little amp! played well for a couple days. I cranked it often. Then a couple days ago i had it on 10 and stepped on a dod buzzbox. The buzzbox is a horrible noise making effect. It sounds like your amp is breaking when you step on it. Well... I stepped on it and it sounded like my amp was breaking as usual...but then the amp DID break. haha. It just cut out dead silent, no hiss hum no nothing. can a pedal kill an amp? sure seemed that way but maybe its a coincidence? can the combo of an amp on ten and a super noisey pedal kill an amp?

anyway, changed tubes, all of em. No joy. Changed speaker. No joy. Open chassis and poked around the live amp with a chopstick. No Joy. No loose connections, nothing obviously exploded. No burnt smell, tho the amp did come from a smoking home and smells like an ashtray oto the max so its hard to tell. Voltages read good at caps. speaker connection checked well. guitar and chords etc all swapped out.

so Im thinking blown OT?so again...easy way to test it?
how do I choose a replacement OT?
The number codes o the back are :022905 and next line reads 606-3-01

heres some pics of the transformer and where it connects inside the amp.



as you can see, someone has replaced one of the caps with a different value. i might change that back...or not...dunno. I also wanna slow down the trem, its way too fast, and to do that i need to replace those two ceramic disc caps (.01's) you see in the very far left corner of the photo. If I have to replace the OT I wll need to make a connection to the board. I'm wondering if I can do all this without removing the board from the chassis? The grounds are connected from board to chassis by a bare stiff lead so taking the board out would be a major pain it looks like. Looks like I'd have t rewire the whole thing basically. Can I just heat up the joints and drop the components in with out removing the board?


Thanks a ton for the help guys and for your patience with me. i trying to learn

Cheers
-Ben

[Chuck H]

Is there a simple way to test an OT for failure without buying additonal parts? as in... is there a way with my DMM to test it?

Yes. Sorta. This test should tell you if the OT is "blown". Set your meter for resistance and take a read between the power tube plate pin and the first B+ node. Also, remove one of the speaker leads from the speaker and read between the loose one and the connected one. If either reads infinity the OT is certainly blown.

can a pedal kill an amp? sure seemed that way but maybe its a coincidence? can the combo of an amp on ten and a super noisey pedal kill an amp?

Yes. Sorta. I would guess that the pedal just made the amp that much more unstable and it was enough to tip the scales. You'd think that once an amp is putting out all it can that adding more to the input wouldn't change stress conditions on the output. And my logical mind believes that. But this sort of thing happens often enough that it's not just a coincidence.

how do I choose a replacement OT?

There are alot of "Champ" replacement OT's available. But if you plan to crank the amp full time (which is how they should be played) you may want to go with something beefier. Like a Hammond "off the shelf" model. Not their guitar amp replacement OT's. I'll look at the specs and find a good one.

If I have to replace the OT I wll need to make a connection to the board. I'm wondering if I can do all this without removing the board from the chassis? The grounds are connected from board to chassis by a bare stiff lead so taking the board out would be a major pain it looks like. Looks like I'd have t rewire the whole thing basically. Can I just heat up the joints and drop the components in with out removing the board?

Yes. That's typically how it's done. Repairs and mods on those amps are easy because you DON"T need to remove the board. Just heat the joint and pull/push whatever you want from/into the eyelet. But it's a good idea to use a little dental mirror to look at the underside of the work and see that no unintentional connections have been made. I've seen long slugs of solder sag from the bottom of an eyelet and almost touch things that shouldn't be connected. Also, use a hot iron and spend as little time as possible on the joints. I've had those black eyelet boards become conductive after extensive mods. If that happens you will have to replace the board.

I have no idea why they ever used those dreaden things. I'm especially clueless as to why some builders still do. I've had problems with at least half of the black boards I've come across. They're black because of CARBON. Apply enough voltage and heat and it becomes a big resistor... Stupid idea, black boards.


Chuck

[Ben]

hey thanks Chuck!

Im not too up on the terminology like B+, i need to learn, i study but it kinda goes right over my head.....but using my photo above, i took a measurement of the resitance from where that red wire meets the upper right corner of the board to pin 3 on the power tube (the blue wire), which according to the shematic as best as i can read it is the plate. The metter didnt move, it stays at 1. measuring the speaker wire as you decribed I get no resistance. That is to say the meter jumps up a certain amount then goes to zero. I dont know what you meant by 'infinite" resistance?...do either of these readings qualify? Is my OT toast?

Thanks for the help! much appreciated!
-ben

[metalangst]

The transformer should read a few ohms across the input(red/blue) and a few across the output(black/yellow), if it were burned or open it would have a high or infinite resistance. Another DMM test is to check the A/C mA of the audio signal. This is how you can use the DMM like an ocilloscope, it will show that an audio signal is present. Set the DMM to A/C volts, 200mA range. Connect the black lead to ground. Probe for the audio signal at the input jack, pin 2, 1, 7, 6 of each pre-amp tube and pin 5 of the output tube. With the amp on and running signal into the input, you should see a reading at these points. You can use your guitar, turn the guitar volume from off to on at each point to determine if it's the guitar signal you're reading.

[Enzo]

If your amp goes silent, the transforme is not the first thing to suspect. Of course it could be bad, but so many other things are suspects first - at least i my view.

Have you lost power supply voltages, has a resistor opened? Are all the tube heaters lighting up?

Infinite resistance is the reading you get when you hold your meter probes in the air not touching anything or each other.

[MWJB]

"The transformer should read a few ohms across the input(red/blue) and a few across the output(black/yellow)," More like a couple of hundred ohms between blue & red, probably less than an ohm across yellow & black, meter might even go to default lowest setting. it doesn't really matter with the yellow & black wires, it is far more likely that a short would occur on the primary winding (between red & blue).

Before I ripped out the OT I would be checking dc voltages at pins 1, 3, 6, 8 of the 12AX7 and at pins 3, 4 & 8 of the 6V6.

[Ben]

well, I'm learning alot here, thanks!
IM not sure how to check power supply voltages, but my freind took some readings of high voltages and said they were all good. he checked em vs the schematic and we were doing this with the amp on and open. i have no doubt my friend would have discovered the problem, but he only had 2 minutes to help me before he had to leave. anyhow, the more I do on my own , the more i'll learn..long as i dont zap myself its all cool. Tubes all light up fine if thats any indicator of power? power light come son etc.

1. when testing the tranformer, is it necessary to disconnect it first?
My readings with it connected were, slight resistance from black to yellow, from red to blue the meter doesnt move on any range setting...but this red wire also connects to several other components and the bue wire connects to an empty tube socket. Does there need to be a tube in that socket to get the prorper reading?

2. I found a resistor that is not reading correctly and it is the last thing the board sees before going to the speaker jack. This strikes me as a likely candidate for the cause of my problem..but i know nothing. The resistor is supposed to be 2700 and is located i the middle of the board. a pink wire conects the resistor n the board directly to the speaker jack, where it meets the yeelow wire from the OT,. This resistor is only giving me 50 instead of 2700. Is this a more likely cause of my "silence of the champs" than the OT?
needs replacement at minimum yes? Maybe its BOTH...and somehow the resistor failing could cause failure of the OT? or vice versa?

3. so an infinite reading on a DMM would look like "1.00" ? thats what it reads when Im not touching the probes to anything.

thanks again. dang these amps are COOL!

[MWJB]

1. No, OT can be measured in place, just pull the power tube to measure primaries (red/blu) and the speaker to measure secondaries (yel/blk). Replace speaker before powering up.

2. That 2700 is the "NFB dropping" resistor, it's usual for resistors to rise in value or go open (infinite ohms) under stress. Quick test - disconnect and insulate the pink wire from the speaker jack. Do you get sound back? The amp will work OK without the NFB dropping resistor connected but 50ohms would be a very low value for NFB dropper and would seriously impair performance. Alternatively, make sure that you are not accidentally reading the value of the 47ohm "NFB load" resistor to ground?

3. Yes.

[Ben]

Okay. yellow to black I now get 0.00 where before I thought I was getting around 50. red to blue I get 1.00 meter doesnt move at all. so OT is kaput?.
Failure of OT could take down that NFB dropping resisitor? could a vice versa scenario be possible where the failure of that resistor takes out the OT? doesnt sound like it from your description.

I wll try the test you suggested with the pink wire tonight, have to leave for work in a minute here. Thanks again so muc for the help.


**If anyone could suggest a replacement OT that would be much appreciated as well. In choosing a replacement I'd like to return the amp to as close to spec as possible. i dont want the amp to be louder or sound different. I dont think I want a "beefier" transformer, but if it wont change the sound or volume, Im open to it I guess. The current transformer has four wires: yellow,black, red and blue and the codes are posted in my first post.

Thanks again for the help!
-Ben

[MWJB]

Like I said, I don't think that the NFB dropper has dropped from 2700 to 50ohms...it's very unlikely - double check this.

Your OT does look like it's shot. Any tolex Champ replacement OT from somewhere like Mojotone would be fine...but given that the original OT has blown, you might want to consider a beefier version if you're going to give it the same punishment...like an Allen Amps TO8C or a Kendrick 118 OT. If going with the stock replacement, maybe reduce the current draw of the 6V6 by changing the 470ohm 1W cathode resistor for 800ohms at 5W.

Whilst you are there I'd swap that crappy RCA speaker jack socket for a switched 1/4" jack socket & plug.

[Ben]

Thanks , that is some very helpful advice.

I'll double and triple check that resistor again tonight when I get home from work. Ive done it a million times already tho casue I found it hard to beleive also. theres nothing else really connected there to get in my way and give me a false reading...its a pretty open spot on the board with just that resistor and two nice long (compared to others components) leads to measure from. I could be doing something stupid tho and not reading the meter correctly, tho it does show up as 50 or .5 or .005 consistently depending on the meters range setting.

about beefier transformers. The original fender OT's werent strong enough to withstand playing these amps on 10? wouldnt everyone have ablown OT n their champ if that were the case? just trying to understand. I didnt think I was abusing this amp by playing it on ten. I thought that was the whole point of this amp? what i dig about the amp (well a large PART of what dig about it)is the semi-cruddy breakup on ten and its relatively low volume at that setting.

I dont wanna do anything to make it louder or make the breakup crisper.
More headroom is the opposite of what I want also. dont know if any of this would result from a beefier OT or not?

right on , gonna get this thing working again and this is kinda fun diagnosiing and fixing (with your help of course!) Cheers!
-Ben

[MWJB]

It's possible that the OT failed because of another issue that hasn't come to light yet, but you might have to face the fact that he OT died from being forced to deal with well over its rated output of, what 6W RMS (6 clean W). The OT probably has to deal with 50-60mA of plate curent at idle, but was designed for less (more like 40mA-ish).

Yes, SF champs are usually pretty bullet proof, but no amp is indestructable under all circumstances. As well as playing on 10 you were using an OD pedal - more volts AC in = more work for the amp. An OT with a higher current rating may or may not sound just as good as the original, you'll just have to try it - but it doesn't make sense to fit a part that is likely to fail if you carry on using it like you have been.

[Ben]

Understood. Thanks.

Yikes!! If its an issue that hasnt come to light yet, then throwing a new OT in there would yeild another blown OT no? Hmmm...i dont wanna take this to my tech, he keeps things forever and isnt cheap, plus i juts wanna learn how to work on these things for myself finally. on the other hand I dont wanna be buying OT's and popping them like popcorn.

Is there a way to track down the cause of the OT's failure with certainty?
any steps I can take to avoid blowing a brand new OT?, anything to look for?


thanks much!

[MWJB]

"Is there a way to track down the cause of the OT's failure with certainty?". No, but if it worked OK, then you bludgeoned with a maxed out OD pedal, amp on 10, for hours on end...then that's the most likely cause if failure.

"any steps I can take to avoid blowing a brand new OT?, anything to look for?" Lots - reduce plate current to the OT by rebiasing the 6V6 with a 800ohm cathode resistor, fit a higher current handling OT, not play with an OD pedal maxed out with amp on 10, ensure that the speaker is working & in good condition, ensure speaker wire is making a good connection with the speaker, make sure that the speaker plug & jack are making a good connection, check dc voltages as advised (use a meter with clips/wire grabbers, black probe to the chassis, red probe to points noted, DO NOT TOUCH ANYTHING ELSE, power down & unplug amp to remake connections & measure next point)...and any combination of some, or all of the above - though some will change the sound of the amp a little.

[Ben]

absolutely awesome help! thanks!

I will do as you suggest on all accounts.
I'll start tonight buy checking that resistor and maybe if i am brave enough will try and get some voltage readings.
Then tomorrow maybe i will order some parts.

btw:the od pedal wasnt dimed, its output and gain was set quite low and its not really an OD pedal, tho it is certainly adding a gnarly gain, along with a suboctave and bunch of other weirdness. I had only been playing the amp for 3 minutes and the pedal was only on for 3 seconds before the amp bit it. Maybe this pedal just has some extreme output peaks or something? DoD buzzbox...maybe yutube has a demo or something.

okay man thanks again. I will report back once I have that resistor pulled and I'll just plan on taking some or all of the steps you suggested to prevent a reoccurance of this problem.