Blackface Princeton Reverb clone, el84 reverb driver mod

[varaso]

Hi, I've built a Princeton Reverb with Vibrolux PT and OT, and I would like to experiment with an el84 tube as the reverb driver.

I'm not into surf music but I like the drip effect and I want to enhance it. Tried some tanks and it was way too wet. I messed around the 500p input cap, boosted it to 15n and it did help and also made the reverb sound darker.

So I wanted to experiment with a 6v6 as found in a Reverb unit but el84 uses the same socket as the 12at7 so the mod can be reversible.

The Reverb Transformer is a Hammond 1750A with a 22.8k ohm primary and rated at 3.5W, it's not the ideal transformer but I think it can work an if I like the result I could buy a proper 10W Transformer, as found in the Reverb unit or in a Vibro king, if necessary.

The real question is how to wire an el84 in place of the parallel wired 12at7? The el84 needs another high voltage section that's not available. Could I wire it in a section that's already being used by the output tubes?

Can I just connect the red wire from the Reverb transformer to the 1st section (mentioned as Y in the attached schematic) and then feed pin 9 of the el84 via a 470 ohm resistor from section X ? Then leave the cathode bypass cap as it is (25uF 25V) and just aligator clip a cathode bias resistor until I find the right value the el84 biases correctly ? For a final adjustment, just alter the input cap(that's currently 15n, and it's the 500p in the schematic) to get a more bright or dark tone out of it because there's no room for another pot and also alter the 1M resistor to the preferred value for the correct "dwell".

Please enlighten me with your knowledge!!

Thanks in advance!

[sluckey]

I would just wire it like the original 6G15 reverb unit, ie, use the same power supply node to feed the plate and screen. IOW, connect the transformer blue wire to pin 7 and connect the red wire to pin 9. Now connect pin 9 to the desired B+ node. Look at this schematic for an example...

http://sluckeyamps.com/smoky/smoky.pdf

[Drumslinger]

MAN!!! Makes me wanna build a reverb unit!

[varaso]

http://sluckeyamps.com/smoky/smoky.pdf

Well, that's really helpful, thanks!!

Any recommendation on cathode bias resistor value and bypass cap? the b+ voltage is going to be between 390-420 V

[varaso]

MAN!!! Makes me wanna build a reverb unit!

Yeah man!! then build one.. it must be awesome!

[varaso]

I just finished it and it sounds very nice, it got potential but needs some tweeking..

any ideas how can I darken the reverb tone at the recovery stage? I tried some caps at the 1n-15n range from grid to ground, as many suggest but it didn't help at all, it made it sound like a horn

with the standard 2k2 cathode bias resistor the cathode current is about 9.5mA. With a 650 ohm resistor the tube biases correctly and the current goes up to 27mA . But I like the sound with the 2k2 so I think I'm going to keep it that way, it won't harm the tube, will it?

[jjman]

Assume net plate volts of ~350 and 27ma that's ~9.5watts. I guess you got the 10watt tranny or still 3watt?

With cold enough bias it will cold-clip if driven hard. I saw that happening on my '71DR so I heated it up somewhat. The later Twins did this. I don't know if the 2k2 makes it cold enough for cold-clipping. A scope would be needed to confirm.

[varaso]

I guess you got the 10watt tranny or still 3watt?

I got the 3.5W tranny, the plate to cathode voltage is 360V and with 9.5mA current the power happen to be 3.5W.. I'll try it in high volume after and if it cold clips I'll bias it a bit hotter.. saddly I don't have a scope!

[varaso]

any ideas where to create a simple high cut filter in the recovery section ?

[martin manning]

I guess you got the 10watt tranny or still 3watt?

I got the 3.5W tranny, the plate to cathode voltage is 360V and with 9.5mA current the power happen to be 3.5W.. I'll try it in high volume after and if it cold clips I'll bias it a bit hotter.. saddly I don't have a scope!

What you have calculated there is the quiescent plate + screen dissipation, not audio power. That depends upon the signal applied. You are still ok there since the B+ and load are unchanged. However, with idle current at 9.5 mA the driver transformer is running about twice the level it would see using a 12AT7, and it may not stand that. IMO if you want to get the benefit of the EL84, get a transformer similar to the Fender 6G15 unit that will provide the right load impedance (7.5-8k, 6G15 driver or Champ replacement OT with 8R secondary), and operate it at the correct voltage (around 300V). It doesn't make sense to run that tube into such a high impedance.

[didit]

Hello -

Fender made Vibro-King, with an EL84 driver. And Hammond makes a replacement reverb driver transformer for it https://www.hammfg.com/files/parts/pdf/1750B.pdf. It's spec'ed for 10W, which is overkill. Quite a while ago built a 6L6/K77 "clean SE" Champ-BRUT using one, with fine results.

Best .. Ian

[varaso]

However, with idle current at 9.5 mA the driver transformer is running about twice the level it would see using a 12AT7, and it may not stand that.

It doesn't make sense to run that tube into such a high impedance.

Could you please roughly explain me these? about the transformer and the impedance of the tube? Does the impedance of the tube change with its' bias?

The difference is in the sound of the reverb, when biased properly there is so much wet signal with a huge tale, lasts forever.. however with the bias as set right now, the drip effect is way more pronounced and it tames the decay a bit.. and that's what I'm after sonically..

I would like it a bit shorter in decay and not so bright, just a little darker.

Quite a while ago built a 6L6/K77 "clean SE" Champ-BRUT using one, with fine results.

Best .. Ian

Thanks Ian, I think I'll give it a try, if it fits somewhere in the chassis!

[sluckey]

I highly recommend a dwell control even if you have to mount it on the back.

[varaso]

I highly recommend a dwell control even if you have to mount it on the back.

well I guess it'll help both ways.. lower reverb decay and make it a little less bright

[varaso]

I just noticed that with the reverb pot on 0 there's still some wet signal passing through :\