Vibro King Reverb Transformer

[mikeboone]

I am having trouble finding an exact replacement for the reverb driver, Fender p/n 024038. I have seen this very same trans referenced as the OT for a Champ, 10W Single Ended. I cant find any spec's on it though

All I can find in Fender reverb drivers is p/n 022921. This trans is 25K ohm : 8 ohm.

What I cant find is what Z trans I need for an EL84 driver. All the amps referenced with the 022921 are driven by 12AX7's.

I checked the existing trans and I get 240 ohms on the pri and 0 ohms on the sec. The output of the rev tank is also open.

Any info would be helpful.

[mikeboone]

Additional Info

See attached service manual and schem. I have the unit that has the EL84 reverb driver.

[52 Bill]

Why are you looking to replace the transformer?

[Alex R]

Do I remember correctly that Fender did a recall on these amps to fix a problem caused by too much voltage on the EL84 plate? I know they changed it to a 6V6 later.

[mikeboone]

Thanks Alex... I actually have the schematics for the 6V6 rev. driver. I can reverse engineer the circuit and replace the 9pin socket with an octal. The transformers for the 6V6 are more readily available. Mercury wants me to send them the bad tranny that I have so they can design an Axiom series product for the V/K.

Any thoughts before I start reconfiguring this circuit??? At a glance it looks pretty straight forward.

[Alex R]

- not really. The 6V6 replacement is a wise move. Generally it's a fine-sounding amp. I have built the vk preamp circuit in other amps. Once I put it in a blackface bassman alongside a classic black face preamp as a project for a customer, and I found the VK-type preamp superior - you could really hear differences between pickups, and it seemed much more subtle and sensitive. I feel that the cathode-follower first stage is a key to the vk sharp tone and touch-sensitivity, though without really having grasped the science of that.

Bill asks why you're replacing the transformer - are you sure it's bad? I doubt whether there's much significant difference from the 6V6 version, the valves are expecting similar impedances. otoh if Mercury are calling...

[mikeboone]

I somehow missed Bill's question... I mentioned above the DC resistance values I am getting on the existing trans. (Pri=240ohms Sec=0.5ohms) Are those DC R values typical? I am guessing so since Bill asked, but I want to be certain.



An added note: My rationale for assuming that the trans is bad is that pin 7 on the socket is arc'd and completely detached. I re-connected it with a mini-grabber jumper and the circuit is inoperable. Reverb tank is open on output and 2.2 ohms on input.

As for the 6V6 mod, I think it'll be an interesting project.

I will keep the VK preamp suggestion close. I am working on a design for a super simple 40W amp. Preamp with V & T only, LTPI, 2x6L6 output... Simple & Loud. After I get this amp straighten out I will pay particular attention to the detail that you noticed and consider using it for my preamp stage. A story for another thread in another forum!

Thanks guys...

[Enzo]

If the pan output is open, then of course you get no reverb.

[mikeboone]

Enzo... Any suggestion on the DC R values I found on the rev drive trans? I was a bit concerened with the state of trans due to pin7 being obliterated by current.

Thanks...

[52 Bill]

The resistance values that you mentioned are not out of the ordinary, but that does not mean that it's ok. Fender used the same transformer for either version of the amp, 6V6 or EL84. Have you tried ordering one from Fender?

[mikeboone]

Thanks Bill... I was able to verify that the EL84 was bad. I tried it in a friends Blues Jr, it's caput.

I put another EL84 and a rev. tank from from my son's princeton chrorus practice amp. The circuit does work. I ordered another tank. So that issue is solved.

Thanks everyone for the input!

Mike

[Alex R]

...it will probably soon burn up that EL84 too, so if you're taking the thing apart anyway I would put in an octal socket and a 6V6. If not, try the E84L type, which are reputedly a bit tougher.

[mikeboone]

Thanks Alex... That exercise was to confirm Bills theory that the transformer may not be bad. I still intend to mod the circuit to 6V6 as you recommended

Thanks again...

Mike