I've done it again (Vibrochamp woes)

[rafe]

Well, I am pretty happy, but what I don't need is another amp, I couldn't resist a 74 Vibro champ on E-bay
I found it by accident and it had one bid on it for 200 with 14 minutes left.....I bid what I was willing to pay and waited till the final few seconds and bid. I got it for the next increment they had only bid 200....
222 including shipping, it is purported to be in fine shape SANS speaker, but I have one. I'm happy so far
more will be revealed

[Geezer]

Excellent deal! 

[rafe]

It got here today. Pretty slow shipper. It's in very good shape after a cleaning tolex mint faceplate pretty good I took a look inside and there was a fried 25-25 cap I replaced that with a 22-25  it has a 470 ohm resistor that has drifted to 300 I didn't have a 470 handy it's on pin 8 of the 6v6gt......I put in a 4ohm speaker that I had in another amp every thing works and it is very quiet but it is a little bit ill....if I crank the volume and play hard it starts to cut out. I noticed that the 6v6 is redplating when this happens ...put in another tube and the same thing. So I'll have to replace that with a new 470ohm. So slow shipping and he claimed it was fine!! You know what I'm still really happy and will pass my joy back to him with a good rating, however if I had paid more I'd really be pissed! And now a question, it calls for a 1amp 120v fuse it but it has a 1amp 240v fuse is this the correct fuse? I have a nice jensen alnico I'm having reconed that will be nice in this amp. It temporarily has a 4ohm ceramic MOD jensen                                                   

[rafe]

  I replaced the 470ohm resistor and the symptoms remain= redplating the 6v6,cutting out...It appears that I am the first one in there no mods no repairs
   .....enough for now
I'll need to go through it more carefully any body have any ideas? I'll check all the voltages and post them later or tomorrow

[bigdaddy]

Did you check the tube dissipation? It really should not be more than 12 watts. You might need a higher value because of the high voltages. What are your voltages on the power tube?

It should have a 1 amp 120v fuse in there. Does it have a 5Y3 rec tube? Maybe it was replaced with a 5AR4/GZ34.

Try adding another 200 ohm 5 watt resistor in series with the 470. I wouldn't trust a 25/25 in the amp, that should really be a 25/50.

The vibro champ is different then the champ, make sure the vibrato is off when working on the amp. When the vibrato is on the voltages fluctuate on the power tube if I remember correctly it did on mine. Once you have the amp set up if it does it with the vibrato on then you have an issue there.

I would take that .047 cap off AC.

[rafe]

I went out to the shop to get some voltage readings. I cut out the .047 on the fuse and turned it on to warm up after a few minutes before I got any readings the cap blew..... scared the crap out of me. So now I am dead in the water till I get parts. It is a 5y3 in there and a different one makes no difference.....Now I am wondering about polarity on that cap, I replaced it the same as I found it = positive to ground. But the other one was fried. it appears to have been original. I'll look at the schematic and see if it specifies polarity.........

[bigdaddy]

Now I'm confused, you said you cut it out...how can it blow? You mean the fuse blew?

No wonder if it's the fuse.....that makes sense. You could have some real issues with that amp. That's why it was $200.... ...you still got a bargain.

The best NEW speaker for that amp is a weber CVC 8". That is the closest thing to a real C8R. The reconed P8R will need some time to break in so don't give up on it if it's not right at first.

Replace fuse, make sure you installed the cap correctly but I would change it to a 25/50. Add on a 150-200 ohm 5 watt in series with the 470 and make sure the vibrato is off by grounding the wire to the switch if I am not mistaken, use a jumper. I'm pretty sure grounding it, kills it.......I have mine hooked to the volume control with a switch on the back right now.

Once you get it working you'll have a cool little screamer and an amp with a surprisingly good sounding vibrato when set up.

[rafe]

Don't fret, i'm confused too Yes the jumper turns it off! No the fuse didn't blow and the cap I took out is fine .....that bias cap was in backwards so I replaced it the same way .....It actually worked for a few minutes each time I played it, It would play fine  but if I cranked it up and drove it, it would redplate and cut-out so I'd shut it down... the one(470ohm cap) I replaced sure looked like the original and matched the others in the amp (brand wise) and I am 100% sure of the way it was installed when i got it ....However on the layout here in the library it calls for the ground to be negative....It really sounded great if I didn't push it. I'm wondering if the previous owner's played it on 3 without knowing it had a problem? Anyway I'll try again tomorrow and see if that rectifies the problem......I will put a 25/50 in as well as a higher value resistor, but not tomorrow..
Yeah, I'm real happy with the deal, but here's part of the description "This is in good condition, but its missing the speaker. when purchased it had a mod speaker and I wanted an original vibro champ speaker but I just couldn't find one. It sounded great but I didn't know that this decision would lead to the amp sitting there all sad and unplayed. This amp has a beautiful chimmy sound and can get a great sound out of it."      ...LOL it's Sad alright ....still I'm very surprised that It got no other bids and at how well and easily it cleaned up I'll be putting in an old alnico 5pm jensen........I should be able to sell off all my other small amps when this is working proper!!
 

[rafe]

Well puting the cap in correctly did not solve it here are some voltages Spec voltages in parentheses
6v6 (V4)
Pin 3 304 (342) Pin 4 318 (340) Pin 5 12 (?) Pin 8 56 (21)
12ax7a (V3)
Pin 1 138 (170) pin 3  1.5 (?) Pin 6 317 (340) Pin 8 0 (175) Pin 9 0  (1.6) (8&9 tested w/vibrato off)
12ax7a (V2)
Pin 1 190 (205) Pin 3 1.4 (1.6) Pin 6 106 (200) Pin 8 0 (1.5)

The 470ohm (Bias)resistor I replaced is blistering hot and the 6v6 is over 100 ma (my bias meter goes to 100 but the needle was pegged)

[bigdaddy]

What does it say on your PT and OT transformers in the way of EIA # and part #?

I'd remove the wire connecting pin 4 from the 6V6 to pin 6 of the 12AX7 for the trem. Then start working on the amp. This could be causing all the problems.

[rafe]

PT L 010020    -606-4-11    CSA 827
OT  022905      -606-3-20

[VMS]

Well puting the cap in correctly did not solve it here are some voltages Spec voltages in parentheses
6v6 (V4)
Pin 3 304 (342) Pin 4 318 (340) Pin 5 12 (?) Pin 8 56 (21)

If you have voltage on pin 5, then the problem could be a leaky capacitor (.02)

[bigdaddy]

You have the right trannys.

VMS is probably right. I have to check one of my amps and see. But even if that cap is leaky I don't know if that would cause this issue.

[plexi50]

Remove or just lift one end of the 25uf/25V bias cap. What happens?

[rafe]

Plexi , when you say what happens, what am I looking for? a change in bias, or changes in voltages?
let me re-iterate one thing in case it is an issue here ....I didn't have a 25/25 I did have a 22/25 and used that ...I would rather wait (if possible) to put in an order for parts until I'm sure of what I need...and should I disconnect the wire connecting pin 4 from the 6V6 to pin 6 of the 12AX7 for the trem. before i do that as per Bigdaddy or wait?

[plexi50]

I assumed you had a 470 ohm bias resistor and 25uf/25V bypass cap on it. If you had then i was thinking the cap may have been bad making the 6v6 redplate. What is the actual plate voltage on the 6V6 tube pin 3. Im a little confused reading your earlier voltage post.

[punkykatt]

I say replace the .02 coupling cap coming off pin 6 of V-1, and check the value of the 220k resistor to ground. there should be "0" volts at pin 5 of the 6V6. try another 6V6 also.
Punky

[rafe]

Pin 3 has 304v .... the schematic indicates 342 so I put that number in parentheses sorry about the confusion.....it's 11% off not too bad schematic calls for +or-20%
I have a 470ohm bias resistor with a(new) 22/25 bypass cap.........

Punky, I have tried other 6v6's same deal, I will look at that 220k resistor and replace the .02 cap thanks

Pin 8 of the 6v6 is way high it's 56 and should only be 21.....I read that a 750 ohm bias resistor is not unheard of with these amps.  so I am wishing I had a 200 ohm 5 watt to tag on the 470 but I don't! However I do have a 1k 5 watt too much? Or will it tell me what's needed even if it's biased too cold with it??
will I need to increase the bypass cap?

[rafe]

Below is a post authored by J.Philips on"The gear page" about a 74 vibrochamp he wrote it in 2002. I hope it's OK to post it here it seems a bit relevant. What do you think?

"Ah - if it's dead stock it will probably be biased wrong. They did that, except for the very first ones. After a short time, they replaced the power transformer with one from a Princeton, which increased the output power to 6W (from 4), but they didn't change the cathode bias resistor to account for the higher voltages. Later BF and all SF Champs/Vibro-Champs run far too hot because of this - probably no-one noticed because the 6V6s back then were good enough to take it... typically they run at 15W or more. But this also makes the tube clip too soon on the forward end of the waveform, which lowers the clean headroom and makes the distortion quite harsh - kind of like overdriving a mixer , it's like a clean tone underneath with a farty, buzzy distortion riding on top.

To fix it, you need to raise the value of the cathode resistor from 470 ohms to 1K, and change the bypass cap from 25uF/25V to 50u/50V (the voltage rises to about 35V, and you need to double the value to maintain the same bass roll-off since the resistor value has doubled). It's also a very good idea to fit a screen-grid resistor (move the wires from pin 4 to pin 6, and fit the resistor between 4 and 6). I'd use wirewound types for both, 5W for the cathode resistor and 2 or 3W for the screen."

[rafe]

Punky , I just read another article and the 220k and .02 are on the grid of that 6v6 and they definately help control the bias I'll check them in the morning ....to early to tell, but it could cause the high bias..

http://archive.ampage.org/threads/1/gagd/032174/Vibro_Champ_single_ended_6v6_questions-1.html

[stantheman]

I would rely on the schematic and use the polarity as in the layout/schematic. I had the same problem with a vibro champ. The cap was positive to ground but I oriented as in the layout and it worked fine. I had a bsket case that I rebuilt and my friend is ecstatic with the amp, he brings it to all the rehearsals.  I used an 8" ceramic weber that I payed 24 dollars. My friend didn't want to pay a lot out, but I must say the amp sounds killer.

[sluckey]

Your highest priority task for fixing this amp should be to get rid of that positive voltage on pin 5 of that 6V6. The number one reason for a positive voltage to be on that pin in that circuit is a leaky coupling cap, the .02µF that has been pointed out by others. It's not critical for that cap to be .02. For testing purposes it can be anything from .47µF on down. Just replace it with whatever you have handy (use at least a 400v rating!), but remember, you'll want to ultimately use a .02µF to keep that holy grail sound.

If you don't have a cap, just simply remove that .02µF. I bet pin 5 will go to zero volts and all the other voltages on the 6V6 will look more normal. You won't get any guitar sound though, but you will have fixed the amp.

If you want to absolutely prove the cap is leaky, just unsolder the end that connects to point 'X'. Turn the amp on and connect your voltmeter between chassis ground and the dangling end of that cap. A leaky cap will have some positive voltage (more voltage = more leakage) and a good cap will have no voltage (or very, very little voltage).

[bigdaddy]

I'm glad people who know more than I do chimed in here. I was about to go and test one of my amps to find out.

I am surprised that a leaky cap there would cause those issues. The only time I experienced that problem was due to the high voltage and too low a value bias resistor. That's why I said to remove the trem off the tube, even when it's off unless it's grounded out it causes fluctuations in the plate voltage.

I would definitely add a screen and grid resistor to the amp once you have the problem solved.

Try the cap and resistor change, definitely do the resistor too. Those carbon resistors drift and can crack starting from the part you can't see on the board itself. Many times for instance they look fine until you take them out and there it is hiding a burn or small crack(especially plate resistors).

[rafe]

Rather than just replacing that .02 cap I went ahead and tested it as Steve suggested, It was fine, no leakage at all...at that point all I could do was hope for a bad 220k resistor but I ended up finding that it was not grounded to the chassis it looks like it never was, the wire was just laying there....go figure, any way  all the voltages are where they should be and it sounds great ...the vibrato is a little weak , but I'll get that another time ....Thanks to all, I do as always  appreciate the feedback.....

[bigdaddy]

That's not the first time I have heard or seen that. I had a Blond bassman many years ago that did not have some resistors grounded. They were laying there against the pot or the chassis. Someone had missed those places to solder them. They were solid against against the places they should have been soldered from age I guess.

[rafe]

I have a marshall on which the low input was never grounded, it took forever for me to find it because I rarely used it and it would scream violently when i tried, so I left it alone....I believe there is a similar story on this Vchamp I think I'm the first to even look inside....I think it was written off as not fun to play and sat most of it's life 36+ years. I don't know how long the previous owner had it ,but they claim it came with a jensen MOD and they sold that right away and hoped to get an original speaker, so it sat sad and unused. The only 4ohm I have for now is a MOD and there is nothing sad about it. I drove 24 miles to a monthly open blues jam with it tonight....it was -CANCELLED- 24 miles back...Hell Yeah, I have the blues   :iroc: