6L6GC tube swap in Fender Super Reverb Clone

[IggyRoyale]

Hi all,

Elsewhere on these forum pages I wrote about a volume problem I was trying to resolve in my Super Reverb clone. The issue was that the amp was pumping out less volume than what I expected for a 50W amp. Alas, when I removed the bugs in the circuit, I discovered that I have unleashed a beast of an amp with a serious ear damaging potential (they start to bleed at about 5 on the master volume control )

My issue now is that the amp is too loud to be practical to use (given that I play at my church and other small venues). A magazine article I recently read suggested that it would be possible to change 6L6 power tubes to 6V6 effectively reducing the power output while preserving the original tone. Would this require any substantial circuit changes (other than just re-biasing)? What are my options?

Cheers

[Satamax]

May be drop down B+ with zenner trick.

[Steve Conner]

Well, you have a master volume, so turn it down! If you now don't have enough dirt even with the channel volume turned up full, use a booster pedal.

6V6s may need bigger screen resistors than you currently have, say 1.5k as opposed to 470 ohms, to stop them burning out prematurely. They also need a careful eye kept on them as they are being pushed to the limit in a 50w amp.

[MWJB]

Don't use NOS 6V6s, use the JJ6V6S, these will take 500v on the plates. Bias to 10W to be on the safe side, along with Steve's screen grid resistor suggestion. You may need to change your bias circuit load resistor (grounded from bias pot). Depending on plate voltage you might want to see -45v to -50vdc at pin 5 of the power tube sockets BEFORE you install the tubes, to get the bias in the ball park.

Also try a 5R4 rectifier, a 12AX7 in the PI...more dirt? Pull the preamp tube that you are not using (Normal Channel?).

[Old Tele man]

...CAREFULLY "temporarily" wire-in a pot on the screen voltage so that you can "dial" it down...adjust it until you get what volume/sounds you want, then replace the pot with a fixed resistance voltage divider (well decoupled!)

...a switch could be added to bring Vs back up to full level if you think it necessary, but remember "grid bias" will HAVE to be readjusted too.

[MWJB]

1K to 1.5K @ 5W will be fine, even (470ohms 1W or better) will be OK if your screen voltage is only a little over 450v.

1K will work fine with 6V6 or 6L6.

[IggyRoyale]

Well, you have a master volume, so turn it down! If you now don't have enough dirt even with the channel volume turned up full, use a booster pedal.

6V6s may need bigger screen resistors than you currently have, say 1.5k as opposed to 470 ohms, to stop them burning out prematurely. They also need a careful eye kept on them as they are being pushed to the limit in a 50w amp.

Steve, yeah that would be the simplest and most logical thing to do if the master volume in my amp was the "true" master volume. What it actually is, is an extra gain stage pre PI that both channels feed into with a pot wired from the grid input to the ground. Turning it down does work but it also results in a bit thinner, tinny tone .... you do give me an idea though. Has anyone here done a post power stage volume control circuit? Any idea how I would go about this?

[IggyRoyale]

Don't use NOS 6V6s, use the JJ6V6S, these will take 500v on the plates. Bias to 10W to be on the safe side, along with Steve's screen grid resistor suggestion. You may need to change your bias circuit load resistor (grounded from bias pot). Depending on plate voltage you might want to see -45v to -50vdc at pin 5 of the power tube sockets BEFORE you install the tubes, to get the bias in the ball park.

Also try a 5R4 rectifier, a 12AX7 in the PI...more dirt? Pull the preamp tube that you are not using (Normal Channel?).

Good suggestion MWJB. The plate voltage is reading at 475V which is a bit higher than usual. In one of your replies later in this thread you suggested 1k screen resistor. Does this value still hold given the above voltage?

[MWJB]

475v +/- 10v sounds normal to me, some vintage SRs run much more than this. final voltage depends on brand of tube & how they are biased, as you will most likely need less negative voltage at pin 5 for 6V6, you will probably find that B+ drops a little anyway when biased up?

If you have a GZ34/5AR4 fitted and are looking for less volume, a 5U4/5R4 would be a good idea.

1K will be fine for screen grids.

[Steve Conner]

The traditional place for a master volume is at the input to the PI. Some homebrewers put one between the PI and the power tube grids: if you want to search the archives for information on this, I think our jargon term for it is "post-PI MV".

A post-power-stage volume control would be an attenuator like the Marshall Powerbrake or THD Hotplate.

The tinny tone may partly be due to the ear's perception of sound, it has a tendency to make quiet sounds appear thin, and loud ones appear fat. Remember the "loudness" button on those old Japanese stereo amps from the 70s? It applied a bass and treble boost to make the sound better at low volumes. Maybe someone should make a master volume with a loudness switch. I believe the Hotplate already has something similar.

[IggyRoyale]

MWJB, thanks for that.

I've got a solid state rectifier (don't know what diode type it is ....).
With regards to the screen grid resistor, I just checked and it works out that I've got 1.5K 3W ...

[Bruce / Mission Amps]

MWJB, thanks for that.

I've got a solid state rectifier (don't know what diode type it is ....).
With regards to the screen grid resistor, I just checked and it works out that I've got 1.5K 3W ...

Not much of a clone if it uses a solid state rectifier.
If it has a SS plug, get rid of it.
Plug a NOS 5V4GTA rectifier in it, use two JJ 6V6s and disconnect one or two of the speakers.

[IggyRoyale]

All,

Thanks for your help on this tweak. It's been great getting input from so many of you. I feel like I learned quite a bit.

It would appear that given that I've got 1.5K screen resistors already in place, I should be able to just plug in a set of JJ 6V6S tubes and re-bias. The bias set resistor might require some attention if the negative voltage is too high.

I'll give the power rectifier a bit more thought as my power supply is a bit different to the standard super reverb circuit (due to the PT I had available at the time). Too complicated to explain now ....

Bruce, I think I found a post PI Master Volume schematic that you drew up at some stage. It would appear that it is a fairly straightforward mod. I think I'll give it a try.

I'll report back on this thread once I had a chance to apply the changes.

Till then ....