Vibro-King EL84 Modification?

[Howard Smith]

I have a Vibro-King (CSR4) that uses an EL84 in the reverb circuit and it runs very hot and gets through valves quite fast. I have read that there was a mod that Fender came up with (and Torres too) to remedy this but I am struggling to find out more.

Can a poor EL84 cause bias problems in the output valves? I tried various EL84s to try to cut down on microphonic problems and with one, the power valves started glowing like light bulbs before the fuse went! It seemed OK again when I replaced the fuse and tried another valve.

Any help gratefully received - Fender didn't bother to reply to my email!

Howard

[kawe]

Hi Howard,
did you get a reply?

Greets
kawe

[Robert M. Martinelli]

Hi,
your Vibro King, having an EL84/6BQ5 as the reverb driver, is probably one of the earlier versions; on later versions Fender moved to a 6V6GT.

( BTW, 6BQ5s are pentodes, 6V6s are beam tetrodes ).

Modifying your amp is surely possible, but you' d need to build a socket adapter ( EL84/6BQ5s are miniature 9-pin, 6V6s are octal ) and to modify the cathode bias network. The plate and screen voltages on the two versions of your amp are similar, so no change is necessary there.

The two tubes also have a different plate ( load ) resistance, so it could also be necessary to replace the reverb tank or to do some tweaking to match the tube output and the reverb input, OTOH tubes are quite forgiving and chances are it would work just as well.

For all the above, plus safety reasons, I'd caution you against doing such mod by yourself....

Now the good news :

Before deeming this mod as absolutely necessary, however, I'd say that you could try some "different" 6BQ5s/EL84s that could "fit the bill" without having to modify the amp

I'm talking about the 7189 ( mil-spec version of the 6BQ5/EL84 ) and the Russian 6P14P. The latter has the same maximum plate power dissipation a 6V6 has, 14W. AFAIK both are pin-to-pin compatible with 6BQ5s.

Hope this helps

Best regards

Bob

[kawe]

Hi Bob,
thanks for your reply!
Do you think the 7189/6P14P can handle the voltage of ~ 480 V?

Greets from Bavaria/Germany
kawe

[Robert M. Martinelli]

Well,
actually the voltage dropping through that tube is less than that, if you look at the schematic you can clearly see that stage is cathode biased by a fairly high value cathode resistor ( 680 Ohm ), so I guess a 450VDC drop is in the ballpark there.

The EL84 absolute max plate voltage is 550 VDC, this makes me think that those tubes were failing due to excessive power dissipation rather than to the high voltage, OTOH it' s also true that, AFAIK, the mains voltage in the US has been raised from 110 to 120 VAC and this could give rise to some reliability problems on "borderline case" designs like this.

Back to the issue, being the 7189 and the 6P14P mil-spec versions of the EL84/6BQ5, I expect them to be more rugged and reliable; the 7189 has 13,2 W max plate power dissipation, the 6P14P can dissipate 14 W max, they both can also withstand higher voltages.

I think that the main reliability problem with the early Vibro Kings was the rather poor quality of current production EL84s ( when compared to the "old days" ones ), this is why I suggested the use of mil-spec tubes as a "cure".

Lastly, I'd like to point out that I didn't intend to offer my advice as the "absolute truth"; I simply pointed out that, before having to tear the amp apart by modding it, it was worth to try something "less invasive"; as I told already, chances are it could work.

Cheers

Bob

[kawe]

Thx a lot for your statement, Bob. I've just ordered a 6P14P-EB (7.95 €).

Do you own a "pre 6v6" Vibro King, too?
Looking at the schematics I've found out the following, which seems a little weird to me:
The amp version with the EL84 reverb driver has a .002 uf capacitor between the dwell pot and the grid. The 6V6 type has just a 100 k resistor in there.
My amp (with the EL84) has no capacitor but also the 100 k resistor.
What do you think of that?

Best wishes
kawe

[Robert M. Martinelli]

Hi,
the little differences in the schematics ( grid stoppers, caps ) seem to be intended to fine-tune the circuit for that specific application. A reverb pan is anything but linear as a transducer, so it's usually necessary to put RC filters around....( and remember that inter-electrode capacitance has to be taken into account, so 6V6s and EL84s might need different RC values ). Anyway, the differences are not that pronounced IMHO.

Oops...I was forgetting....If you want to add some "peace of mind" to the design, try to swap the 680 Ohm 5 W cathode resistor for a 820 Ohm 5W. This will bias the EL84 ( er...6P14P ) a little bit colder, moving the tube's operating point towards the safe side. The cathode resistor is fully bypassed by a 100 uF capacitor, so no change in "tone" ( frequencies ) will occur.

Cheers

Bob

[kawe]

Thanks again, Bob, for your great support!

Have a nice weekend
kawe

[Alex R]

I installed the Torres mod for a guy once, and it worked fine. Er, no I didn't take notes... Uses a big dropping resistor I think.

[Howard Smith]

Thank you all very much for your replies. I ended up getting a pair of Groove Tube EL84s for the customer (he now has a spare as well) and the one I installed was pretty quiet and it sounded alright. I will tell him about the possible EL84 substitutes and changing the cathode resistor but he is fairly content with the GTs for the present. Thanks again and keep up the good work.

[clyde1]

Yeah, I also had one of the earlier VK's with the EL84 and thought it might be nice to install an ANOS EL84 instead of the crappy new production one. Bam,
instant red-plate. In fact the only one that didn't was the one that was in the amp from new. I'm not sure if it was the mil-spec EV EL84, but I did change the cathode resistor to a higher value too. I don't really miss that amp.