Back to work on the Fender Pro Reverb Ultra-Linear

[gui_tarzan]

So I'm going to keep this with the stock 6L6GCs and just let the guy know not to crank it and overpower the 40w OT, which I can't see him doing with an acoustic anyway, but I have a couple of questions.

The OT has a 4 ohm tap which I'm using as the primary tap for the pair of 8 ohm speakers in the cabinet but he asked if I could use the 8 ohm tap for the extension cab, in case he wants to try other speakers internally that add up to 8 ohms or an external cab with 8 ohms. My thought is to use a switching jack for the ext. cab jack that will disconnect the 4 ohm tap. Is this configuration wise?

Second, I've read in multiple threads that removing some of the pre-amp cathode caps will lower the output, is this true? Will it break up sooner or be cleaner? I understand it drops the power a tad, is this worth pursuing to tame this beast a little?

[potatofarmer]

Does the ext. cab. out already have a switching jack? They did some wacky stuff at the end of the Silverface era.

Removing cathode bypass caps in the preamp will make the amp less gain-y but won't change the ultimate output.

[Stevem]

Yes you can also use the 8 ohm tap while the 4 ohm tap is powering the cabinets own speakers.
Yes, you can remove a cathode bypass cap or two and the amp will still have enough drive signal left to get you full wattage, so he will still have to watch the volume thing, but with a acoustic guitar plugged in you would have to 60 feet away from the amp to not get feedback with 40 watts of power flat out, and that being said I do not see how a extension cabinet is going to help him in the feedback department unless it's of a closed back design and set out in front of him!

[gui_tarzan]

I don't think it's a matter of feedback avoidance, Steve, he mentioned that the had an external cab, possibly for trying different speaker combinations. Which of course he could do in the cabinet itself anyway so that's probably a moot issue.

PF, someone replaced the original external cab jack with a standard, non-shunted jack. So it's really up in the air which way I go with it. He just wants to have options.

[gui_tarzan]

I think this one is finally done. At least as good as it can be given what I had to work with. Just to update everyone, I got this Pro Reverb UL a while back and had to figure out why it didn't make any sound. It had sat in a closet for a dozen years or so, and it looked like someone had tried to get it running at one point but failed. I don't know if it was the owner or someone else but it was a mess.

The original special extension output jack was replaced by someone with a standard jack, which I think led to the destruction of the OT. All I had on hand was a 45w Heyboer OT and this is a 70w ultra-linear rig with a pair of 6L6s. The owner wanted it to work again and really wanted to tame it so I set off on a course to do that.

With the help of several of you I was able to figure out how to lower the screen voltage to decrease the power output enough to make the new OT usable. I ended up putting a 1K cement resistor between the original connection and the tubes, and put a 470 ohm on pins 4 which dropped the voltage enough to call it good. It has a good current draw, not too much or too little so I think I found a good balance. Between that and some creative bias work it's functioning now, sounds ok and should last him a long time.

So I wanted to update you all like I said and thank everyone for their help on this beast.

[martin manning]

Yes you can also use the 8 ohm tap while the 4 ohm tap is powering the cabinets own speakers.

If he does this he will be running at half of the intended load impedance. Given the high plate voltage this is likely to cause red-plating, and will run a lot of current through the OT.

[Stevem]

I guess possibly!
I can not see the owner being able to use more than 20 some odd watts of this amps power with a acoustic plugged in so if he sets the bias to no more than 18ma each, I do not think he would push the output stage into red plating, do you ?

[gui_tarzan]

The voltages I measured were -55v on each pin 5, and 26ma on each pin 8 through a 1 ohm resistor. I let it run for probably an hour and watched closely, no red plating.

[martin manning]

My concern is with playing the amp with two speakers plugged in. I would advise the owner against that, regardless of where the idle bias is set.

[gui_tarzan]

Are you talking about it putting too much of a load on it with 4 ohms?

[martin manning]

Yes, connecting speakers to both the 4- and 8-ohm taps, which Steve suggested would be okay. I don't think that's what you were asking about above, though

[gui_tarzan]

Oh, after talking to the customer we decided it wasn't worth trying to put in a complicated 4/8 system and are just going with the 4 ohm output. Both speakers are 8 ohms so in parallel they match.