Fender Vibrosonic tube pairs

[pbman1953]

Would anyone know the Vibrosonic enough to know the tube pair match up in the amp?


Thanks

[sluckey]

Would anyone know the Vibrosonic enough to know the tube pair match up in the amp?

Do you mean how the 6L6s are paired? If so, V7 and V8 drive one side of the OT. V9 and V10 drive the other side of the OT. Looking at the back of the amp V10 is the left most tube, then V9, then V8, then V7.

If you want to run only two 6L6s you would remove the two inner tubes or remove the two outer tubes.

Or did you mean something else?
 

[pbman1953]

Yes,  this owner wants to run just a pair.  Do you know if this has a pot or only adjusted by resistor changes?

[sluckey]

Look at the schematic and you tell me.

[pbman1953]

When I search for I get a Twin reverb.  If I read it right it has a pot, marked 10k-L.T

Actuall at the bottom there's a box that shows the amps the diagram is for.

[sluckey]

See how easy that was. 

[pbman1953]

So I suppose and you can correct me that the Vibro bias is like a Bassman in that you balance the pair?

[sluckey]

That amp has bias balance. Some bassman amps have bias balance. Others don't.

[pbman1953]

My Baseman AB165 is.


Thanks for all the help!

[sluckey]

The AB165 is a slightly different bias balance circuit but it operates the same as the Vibrosonic.

[pbman1953]

The amp just came and left . Sadly , no pot anywhere.




Couple noticed items-


A past owner installed a 1" done tweeter, but that is not connected.


There's an EV- 15L- 15" bass speaker mounted instead of the JBL D-130.


Besides some non- factory mounting and trim screws the amp seems stock.


The caps have not been changed- need suggestions on that. The owner and I wrote down the values.2 silver can Mallory's  220's, 3- Paper Mallory's(50uf), a 70 uf and a Sprague Atom(forgot the value).


The amp has a little  hum but not horrible.


Tubes are not original except for 1- Phillips in the v2 position. All others are current Sovtek 7025/12ax7.  I don't know if New Sensor labels them like that otherwise they could be original. The power tubes are a pair of JJ- 6L6


Thanks

[sluckey]

The amp just came and left . Sadly , no pot anywhere.

Hmm, now I'm confused about which Vibrosonic you have. The early 5G13 and 6G13 were brown tolex and had no bias pot, but they also only had two output tubes. No reverb, but harmonic vibrato, and a presence pot.

Then there was no blackface Vibrosonic.

Next came the 100 watt Vibrosonic Reverb amps. They all had a bias balance pot and four output tubes.

You say yours only has two output tubes. So, why even ask about how the tubes are paired? Had you not seen the amp when you started this thread?

[pbman1953]

The owner pulled the inner pair.


Black Tolex


I saw the amp for the first time last night.


Here are the measurement from my Biasrite-




512V/ 18


514Volt/ 13


Any thoughts on replacing the caps?

[sluckey]

Replacing the caps may take care of the little hum you have.

Your tubes are biased very cold for 6L6s or even 5881s (9.2W and 6.7W idle power). Are you sure there is no bias pot? All three of the Vibrosonic schematics show a bias balance pot. This pic of a 1973 VS clearly shows a pot right underneath the rectifier board. The easy way to run the tubes hotter would be to increase the value of the resistor that connects to the bias diode on the rectifier board, or decrease the value of the resistor mounted on the balance pot.

If the amp truly has no bias pot then you have a very unique amp. Please post some pics if this is true.

[Ed_Chambley]

In the 80.s Vibro sonics came with EV speakers. Post the chassis serial number. Probably has a balance pot.


If the owner wants a service, and you should replace all Electrolytics. Replace plate resistors. Remove the pot and input Jack's and wire brush the brass plate. Replace power tubes. Visually inspect all joints, and especially chassis grounds. Doug's library has a service guide you can follow.


This amp changed and if it has a black faceplate it is made in 80 or 81. It also was 100 watts and 135 watt. Does it have a UL Output transformer? If it had a black faceplate and EV it is a rare amp and I have been looking for one, so I know them a little. The 1981 had a line out on the back.


You must bias the amp by the resistor on the balance pot, and lower value will increase negative grid. No big deal, it is like any fixed bias. You have to check to insure the balance pot is centered if you want to check matched tubes. Is the negative grip voltage the same on both sides with the tubes pulled? If not, I will make it so before checking tubes. This will let you know if your tubes are matched.


You must do this to insure the balance pot is working correctly from the center. Lot of idiots think it is necessary to change it to a bias adjust because it is part of blackfacing a CBS amp. The balance system is fine. At the time it was hard to get new tubes that would stay matched. Hence the balance pot.


Ed Jahns was the guy responsible for most of these. Not bad amps at all except the master volume, which cannot be changed to the dual pot for a PPIMV unless you use a 16mm pot. The chassis tilt makes the pot hit the chassis bottom. I have put dual Alpha pot in and will fit barely, but the 16mm works better, or you can a different single pot master or install a dual pot on the back ext speaker hole, but you must use shielded and I would use sealed pots.


The amp is not a twin, with the chassis number you can get the year then the correct Schematic. Recap, re tube if needed. If UL Output Trans remove the NFB wire if you plan to use it for guitar. It will make it have a more forgiving feel. I have installed a switch for NFB, but after they are played the stay off. Makes a big change.

[pbman1953]

Honestly, no pot. I'd notice that 10K because it's the same as my Sunn 2000s.




On the picture you sent the 10K must have access from the top of the amp? I looked for access holes and there were none on the top side.


I have a 10K so I may get this back to install

[pbman1953]

This is bigger job that I have time for at the moment . Have any of you ever heard of Bob Meztger? He was Leonard Cohen's guitar/slide player for many many years. Bob also worked at Fender with Leo as a tech. He loves to work on these old amps. He lives in the Boston area near me. This would be a perfect job for him and he's very reasonable.




I have pictures taken if possible. Thanks!!!

[tubeswell]

I had a 73 Vibrosonic in for repair last month.
Pre-repair gutshots attached. This one had a 'blackface' faceplate    and they moved the MV to the back panel (and somebody bastardised the eyelet board with squirty-shite-out-of-a-can, shielded cable everywhere and odd looking caps    - wasn't me)

What Steve said - the layout is the same as a silverface twin of the same era (except the OT is wound for a single 8R secondary) practically an AA270.

[pbman1953]

I'm not sure if it means anything but the amp I looked had all navy blue  caps instead of the browner ones.

[Ed_Chambley]

I had a 73 Vibrosonic in for repair last month.
Pre-repair gutshots attached. This one had a 'blackface' faceplate    and they moved the MV to the back panel (and somebody bastardised the eyelet board with squirty-shite-out-of-a-can, shielded cable everywhere and odd looking caps    - wasn't me)

What Steve said - the layout is the same as a silverface twin of the same era (except the OT is wound for a single 8R secondary) practically an AA270.

Yep this one looks pretty bad. Lot different than that EV 1981 I am looking for. I had one here we changed the tubes to the jj6v6s and switched the OT to a 3k3/4,8,16, actually a Marshall Drake copy from Heybour. You can pull 2 tubes and use 16 and it runs as a Deluxe, but cleaner due to the EV and larger iron.


Turned out nice. Twins are not bringing but about 600 to 750 for silverface. It really is good way to make very versitle amp. I just really like the sound of the EV 15, but it is one heavy amp. As long as you like the jj tubes, you can still get a very nice handwired amp under a grand if you have the ability to do the work.

[pbman1953]

I just received a $250 estimate to do the work, incl. parts and labor. Not including tubes. He's using a pair of newer JJ's.


Is this a fair estimate?

[tubeswell]

I just received a $250 estimate to do the work, incl. parts and labor. Not including tubes. He's using a pair of newer JJ's.


Is this a fair estimate?

If this is to replace electrolytic caps (including bypass, bias and filter caps) and old rectifier diodes and trem roach and LFO caps, and test for DC leakage on coupling caps and replace any leaky ones, then yes it is extremely reasonable, because that's at least $60 in parts and 5 hours work @$40/hr. (NZ prices) Tubes would be extra.

[sluckey]

I thought you were going to do the work.

[pbman1953]

I thought about doing it, but the owner would be needing it back faster than I could dedicate to it. This is his main go to amp and would need as soon as possible. I'd do it at my leisure if I had the time.