Fender Vibro King Repair (Question on Checking EL84 bias)

[Platefire]

Got a Fender Custom Shop Vibro King, Type CSR4 in for a loose GZ34 Rectifier socket possibly to retention socket if possible because the recitifier is floppy loose but amp is still working. I looked at the tubes and seen an EL84 in with the other pre amp tubes and got curious and started looking for a schematic. All schematics including Dougs have a 6V6 in the position where the EL84 is at in this one plus SS rectification. What model do I have? Can't find a schematic on it or much information at all with this configuration??? Anybody familiar with these? The amp sounds great though! I like it! Platefire

[HotBluePlates]

The EL84 version was the original incarnation of the Vibro King. It's a reverb driver, and the 6V6 was used in its place in later models.

Both tubes should behave very similarly in the circuit, although the EL84 will probably have a smaller cathode bias resistor.

[Platefire]

Well I found what I think is the original Vibro King schematic dated 1993(attached). It has the EL84 Reverb driver but it has diode rectifiers. This amp I got has a GZ34 tube rectifier. I beginning to think this tube rectifier was a modification??

Platefire

[DummyLoad]

schema shows PT has a CT on the HV sec. - perhaps it was a custom shop option?

--DL

[Platefire]

Yep! the Silver and black Quality Control Inspection sticker inside teh amp has EB. E=1994 & B=Feburary. So this was early production and the GZ34 was a mod because the tube chart inside the amp don't show any rectifier tube. Platefire

[sluckey]

All schematics including Dougs have a 6V6 in the position where the EL84 is at in this one

Look at page 7...

http://www.el34world.com/charts/Schematics/files/fender/vibro_king_manual.pdf

[Platefire]

OOPs! I stand corrected! Dougs schematics did have it. Same schematic that I posted off the net.
On this schematic I saw the diode rectifiers and discounted it without looking further but it appears
the GZ34 rectifier is the exception in my case. Guess it could have been a custom shop option but the tube chart in the cab not showing the rectifier makes me think it may have been a mod? Looking at the bottom of the chassis the type socket that the rectifier is in looks like a different typ also than the adjacent 6L6 sockets. Thanks for pointing that out to me Sluckey. Platefire

[Platefire]

Regarding the floopy rectifier tube, checked the socket out and I would guess it has been re-tensioned before because some of the pin grippers were a little disfigured. The five pins that are actually gripping the GZ34 pins are tight enough to make a secure connection on the pins themselves, it's just the gripper themselves are loose inside the socket pin holes letting the tube move around. I use to have a Gibson Les Paul amp like that but it worked fine. I think the only solution to this floopy tube syndrome on this amp is a new socket. Platefire

[Willabe]

Guess it could have been a custom shop option but the tube chart in the cab not showing the rectifier makes me think it may have been a mod? Looking at the bottom of the chassis the type socket that the rectifier is in looks like a different typ also than the adjacent 6L6 sockets.

Maybe, but they would need a different PT with a 5v heater wind for the rect. tube.


                Brad     

[Platefire]

Well I'll be going in to change the rectifier socket and we'll see what they did for the 5 volt. The PT is huge on this thing. I kinda dread pulling the chassis, it's going to be hard to handle. The amp itself is at least as heavy as a twin or heavier. I have a hard time managing it.

He also said it had an intermident reverb problem where the reverb goes off and he would bang on the top of the amp and it would come back on again. Could be a lot of things--cold solder joint, faulty cables, faulty connectors, driver/recovery tubes or sockets to name a few.

This is my first experiance with a reverb with dwell/mix/tone---how have I lived my life without this??? Love the control!!! About anybody could dial in your favorite preference with this. Also the "Fat" switch really brings in some good thick tone and response even at low volumes. Liking everything about this amp so far except the weight. Plate

[HotBluePlates]

Funny thing about the Fat switch is that it's just adding a bypass cap to one stage.

[Platefire]

Wow! I would have thought it was something more elaborate than that! but it does just what it says--fattens up the sound and increases the gain where you can play at a lesser volume and still get good response. Without a MV, that helps because this amp is Loud. Platefire

[HotBluePlates]

And also shows you just what some local negative feedback due to an unbypassed cathode resistor can do.

[clyde]

Had one back in the day ('94), heavy, loud, no headroom, didn't have it long.  YMMV.

[Platefire]

Yeah, I was wondering about headroom. It just makes sense if it is that responsive a low volume, if you crank it would really be breaking up big time. I really haven't looked at the schematic close, guess I need too. When I told the customer I kind of liked it, he started talking about selling it to me. Made me kind of suspicious! Platefire

[clyde]

Platefire,
I read somewhere on the Interweb that the way to control the VK's gain is with the treble control.  Mine was long-since gone so I couldn't mine that gem unfortunately, however it may be your turn to shine.  Best of luck.

[Platefire]

Well I've been downsizing my amps over the years to where I generally play over a 1-10 or 1-12 speaker. It Don't make sense to pick up a 71 pound loud amp without MV even if it does sound outstanding. It's simply more than I can manage comfortably/safely. I really struggled with it getting it in/out of the car and into the house. So I will repair/return. Now that I got that settled, I can move on to other things  Platefire

[Platefire]

Here are some shots of the Vibro King Circuit & Amp. Other than changing out the rectifier socket, the customer said the reverb was cutting out. I never could get it to cut out with me! When I pulled the chassis I did notice that one of the reverb RCA jacks hot leg was touching the outside of the 22UF metal bypass cap on the recovery 12AX7 cathode. Wonder if that could have been shorting it out? See the Left Side pix with the orange cap and adjacent jack. Picture shows some clearance becuse I moved the cap over when I had seen they were touching. Platefire

[Willabe]

I don't think they have enough shielded cable in that amp.

     
             Brad     

[HotBluePlates]

When I pulled the chassis I did notice that one of the reverb RCA jacks hot leg was touching the outside of the 22UF metal bypass cap on the recovery 12AX7 cathode. Wonder if that could have been shorting it out?

Only if the RCA jack was touching actual metal on that cap. The metal case is connected to the - lead on that cap.

But the whole thing is wrapped in plastic, which would insulate it if the jack was touching the plastic wrap.

[Platefire]

Thanks HBP, I'll rule that possibility out. Platefire

BTW-I did a continuity check on that and let the MM hot touch the jack and edge of cap and touched the ground/black probe to different places on the cap---no continuity no way no how! 

[Platefire]

Got the new rectifier socket in last night relativly easy. The fuse and power switches have quick connectors instead of solder connections, so it made it easy to disconnect, move out of the way for installation and reconnect after installation.

On the intermidant reverb issue, I finally got it to cut out last night when I was testing the new tube socket operation. Got it to cut out a couple of times and seems to return when I messed with the RCA send/return cables. Sounds to simple but I will pursue that first. Platefire 

[Willabe]

Got it to cut out a couple of times and seems to return when I messed with the RCA send/return cables. Sounds to simple but I will pursue that first.

That's a common problem for tube verb with a tank. Check the jacks, plugs and cable. If you can swap the cables with a known good pair first. See what happens and go from there. You'll find/fix it.


            Brad     

[Platefire]

A big problem with this amp when it first came out in 1993 was the overheating EL84 tube driver that they latter changed to a 6V6. So mods where done to the EL84 version to cool it down so it would quit blowing tubes. This Vibro King has had that mod and I've been comparing the actual circuit to the schematic to see and understand what they did:

1-Relocated the power supply to the reverb transformer that supplies EL84 plate from power Node Z to power node Y. NODE Z was also feeding the 6L6 screens! This is a drop from 470 to 380V. Also found an added 25K dropping resistor and a 22uf/450 Electrlytic(to ground B+ leveling) to pin 8 screen off of Node Y.

2-Found a 1 Ohm resistor in series and just prior to cathode resistor/bypass cap to ground. The cathode resistor had been changed from 680 Ohm to 250 Ohm.

The EL84 is still very hot(will burn you fingers fast) but not red plateing that I've seen. Should have the cathode resistor gone higher instead of lower to cool the bias? I am assuming the 1 Ohm resistor was added to check bias with a MM? The reverb seems to be working well except for the intermittent cutting out. Platefire

[Willabe]

Should have the cathode resistor gone higher instead of lower to cool the bias?

Yep. But maybe because they lowered the B+v to the verb OT 90dcv they needed more current through the EL84. So they lowered the KR. 


            Brad    

[Platefire]

So I guess I could use the 1 ohm resistor added and check the actual bias to see what it is. Plate

[Willabe]

Yes.


           Brad     

[Platefire]

I'm a little confused about the readings I'm getting. Tried my Fluke 115, set it on Mv/DC and it gave me "OL" overload. Tried changing the range, still "OL". Switched to DC Amps, got 0.00 reading. Switched to my cheap Wal Mart MM. On Mv got 0.00 reading and when I switched to Amps, I go an 80.0 reading set to the 200 range???? Plate voltage is 380 and screen is 308. Platefire

Edit: I'm just not use to measuring a 1 Ohm resistor embeded/moded into a circuit. I'm use to the jacks on the back of the amp. So this resistor is between the EL84 cathode and the cathode resistor/bypass cap. I'm measuring right at the solder connection where the 1 Ohm/cathode resistor/bypass cap are joined with my MM red/hot probe and touching the black/ground to chassis----that is right isn't it??
And if my memory serves me right this measurement is done using Mv Right?

[Platefire]

Bump

Regarding previous post question by "Edit"--please look at this and tell me what I might be doing wrong. I should be getting a bias reading Mv=Ma from the 1 ohm resistor and I can't get a reading.
I just can't imagine why I can't get a Mv reading from this?

[Willabe]

So this resistor is between the EL84 cathode and the cathode resistor/bypass cap. I'm measuring right at the solder connection where the 1 Ohm/cathode resistor/bypass cap are joined with my MM red/hot probe and touching the black/ground to chassis----that is right isn't it??

No and that's the problem.

The 1R is standing on top of the cathode 250R/K by pass cap. So your not measuring across 1R your measuring across the 1R and the KR which is 250R. So 251R and not 1R, when you measure from the tubes K to chassis. If the 1R was between the chassis and the ground ends of the KR/K bypass cap then you could measure to chassis.

You have to measure across the 1R.

And yes use mv (M is for meg.) = ma.


              Brad    

[Platefire]

Willabe. Thank you for straigtening me out on that! but the news is not good! I got a reading of 38.9Mv with the hot probe on the cathode side of the 1R and black on the other end of 1R.
According to the Weber bias calculator Class A EL84 at 380 plate voltage should be 28.4. Platefire

[Willabe]

According to the Weber bias calculator Class A EL84 at 380 plate voltage should be 28.4.

Is the EL84 red plating? It might be sitting on the edge so you might have to turn out the lights to see any small hot spots? You might have to try increasing it's KR?

(Mv is mega volts, mv is milli volts.)


            Brad    

[Platefire]

Har!   feeling kind of stupid letting this issue measuring bias with 1R whoop me! But well I guess it's better to plead your ignorance than to remain there. So now I know how to scab a 1R in there in any old amp to get a bias reading.

I've been watching the EL84 pretty close for red plating and havn't seen any. However just the heat coming off the tube I can tell it's running pretty hot. I might have a look tonight with the lights out and see what I can see. Don't know why I'm worrying with this because this wasn't even one of the customers issues to fix---I guess my own curiousity! How about inserting another resistor with aligator clips in parallel to the 250/5W increase the resistance to lower the bias to see what may be required?

On the reverb cut out, I installed another set of rca cables and so far it hasn't cut out again. I fire it up two or three times a day to try to catch it in the act  Platefire

[Willabe]

Your not stupid Plate. We all at times can't see things.

How about inserting another resistor with aligator clips in parallel to the 250/5W increase the resistance to lower the bias to see what may be required?

Not parallel, has to be in series. Parallel will decrease the resistance. Because now the voltage has 2 paths to go through, which will decrease the current through the R/R's. The less current that flows through an R the less voltage it will drop.

On the reverb cut out, I installed another set of rca cables and so far it hasn't cut out again. I fire it up two or three times a day to try to catch it in the act  

Sneak up on it from behind.       


                Brad       

[Platefire]

Let the amp run awhile last night and checked it in the dark. No redplating on the EL84. Platefire

[Willabe]

That's good.


                Brad     

[sluckey]

That's too hot for me. I would want that EL84 to run at about 10-12W idle dissipation. I'd make the cathode resistor larger.

[Platefire]

How Large? Originally in 1993 it was 680, currently 250. It appears the only 5 watts I have in stock is 560, 1K and 1.5K.

Unrelated, does anybody know why they put a diode to ground on the 6L6 plates on this amp?? Never seen that before!

Thanks, Platefire

[sluckey]

Try the 560 for starters. Calculate the new plate dissipation then increase/decrease the resistor until you are happy.

The diode is a protection device.

[Platefire]

OK, it's been a while since I calculated plate dissapation. I'm using the Weber Bias caculator, to check existing to get my method down. Tell me what I'm doing right or wrong.

using cathode bias calulator entered in---1 tube, 10V accross KR, 250 Actual Resistance of KR, 297 Actual Plate voltage measure between pin 3 and 7=297

Results
Plate dissipation=11
Approx plate current=38 milliamps

[sluckey]

Looks good to me. I assume you are using the 560Ω resistor? How does it sound?

[Platefire]

Sluckey--sorry I had entered the wrong plate voltage. I used 297(bad), should have been 380 before subtracting cathode voltage(10) for a total of 370 plate voltage. Haven't changed the cathode resistor yet, just trying renew my understanding of plate dissapation calculations. So I was calculating using the existing 250 resistor to refresh myself and when I try another resistor I will better understand what the change in results is. So I have re-calculated it below using my existing cathode resistor. This is class A. Please check my calculations!

Cathode current=KV/KR ohms. 10V/250=.04

Screen Current=V drop of Screen R/screen R value. 80V/25000 ohms=.0032

Plate Current=KV/KR value-screen current. 10V/250 Ohms=.04-.0032=.0368

Plate Dissapation=plate V-KV x plate current. 380V-10V=370V x .0368=13.6 Watts

It this is correct and an EL84 is not to exceed 12 watts--need to raise the cathod resistor some???

According to Weber Calculator, Dissapation is 14 and plate current is 38

[sluckey]

yes

[Platefire]

Thanks!