Single Rectifier without power tubes

[PaulSM]

Hey there,

I'm new on this board, lots of great stuff on here !
I could spend my nights reading just that...!

I've got a question about running my Mesa Single Rectifier without any power tubes in it. Reason is I want to use only the preamp section of the amp to do "slient" recording during night studio sessions. However, even with the master volume lowered to the bare minimum, there is still an output signal to the cab. I built a 10 ohms dummy load and measured the output voltage with the master level fully counterclockwise and it's about 1Vrms, that's enough for me to worry about running the amp unloaded with master level off.

I'd like not to waste my power tubes and heat the room for no valid reason if I can just pull these two power tubes out. The only thing stopping me from doing this is that the B+ will probably rise up like crazy without plate loading from the power tubes.

I could not find a proper schematic for the Single Rectifier, but if it is anything like the Dual Rec, the B+ is at 460V when fully loaded and according to some basic calculations of mine, it could easily rise up to 509V without any loading (assuming a 120Vrms line voltage). Filtering caps in B+ circuit are supposedly rated at 500V which is one of my worry, but I'm also in doubt about the preamp circuitry if it is run with higher plate voltages.

Anybody's got some input on that ? I could always continue to use a dummy load as I do right now, but if I can pull the power tubes out, why not?

I remember having read somewhere that if IS possible to do it, but I need more experienced input about B+ rising too much. I am aware it might also affect the tone.

Thanx a bunch!
Paul

[Chuck H]

I can't say for sure about the actual B+ voltages in your amp, but your concearn is valid. Pulling tubes for testing is done all the time, but if you'll be running the amp that way a lot it would be more of a concearn. The filter caps in most Boogies are 500V as you noted.

FWIW I don't think you'll hurt the preamp with the higher volts. Mesa tends to run preamp tubes a bit low when compared to other amps so a little more would probably be fine. Again, I don't know the actual voltages in your amp.

You could beef up the first AND second filter nodes by using two caps in series for each node. A 100k to 220k 1W resistor across the leads of each cap will equalize the voltage on the totem pole arrangement. Mesa uses Sprague Atom's. These can be purchaced at Mouser Electronics on line. If your starting with, say, a 47uf cap, a pair of 80uf 350V caps in series will give a filter of 40uf that will handle 700V.

But...

I honestly wouldn't worry about having the power tubes on for the tubes sake. With the master volume all the way down all the power tubes are really doing is keeping their cathodes warmed. Not much tube wear that way. If the issue is excessive heat adn/or power consumption, get another, smaller amp for silent practice and recording.

JM2C

Chuck

[cbarrow7625]

I built a 10 ohms dummy load and measured the output voltage with the master level fully counterclockwise and it's about 1Vrms, that's enough for me to worry about running the amp unloaded with master level off.

Why not just run the amp into the dummy load while recording? 1Vrms into a 10 Ohm load is only 0.1Watts. Even a small dummy load will work. The power tubes will be perfectly happy running into a 10 ohm dummy load. As Chuck noted, you won't be wasting your power tubes, they are just keeping the cathodes warm at that point with very few electrons being emitted. If you are concerned about the heat in the tubes just put a fan on them to help keep them cooler while recording.

[Gtr_tech]

I wouldn't bother with it. The tone (if you call that tone) will change due to the increased B+ supply in addition to the risk of overvoltage to the first caps in the string. Dummy load is the key, but it will sound buzzy(er) and thin(er) without using a mic'd cab.

[Enzo]

Gtr Tech has a point, your preamp output won;t sound the same as what wold have come out your speaker. And the presence and resonance controls in any amp are part of its power amp stage. Certainly try it to see what you think of the results, but be aware the speakers are one of the largest contributors to the sound of an amp. Play your amp chassis through a PA cab of you want to hear what the amp sounds like.

[PaulSM]

Thanx for all the input!

I've always recorded my amps using speaker cab and mic, but since I can't mic an amp anymore due to volume issues and nightly recording sessions, I use software cabinet modeling such as power amp + cab impulse responses and I get more than satisfactory results (if not better than with a simple SM57 mic + cab). It goes without saying that ANY distorted preamp sounds bad when not using some kind of speaker + power amp emulation and I would never use the preamp output straight to a recorded track.

I've been recording with the dummy load at the output for a while now, but as I've read on some other forum that it was possible to simply remove power tubes altogether if the power amp wasn't used, I thought about giving it a go.

However, it seems it might be a better idea to keep using the dummy load and leave the power tubes in the amp, although not using them at all.

Paul

[Don Moose]

Have you considered one of Ted Weber's MASS speaker-motor-based attenuators?

A safe load, some EQ and a line-level out.