Experimenting with a ‘65 TRRI, I plan on removing two power tubes and disconnecting one speaker. Do I need to adjust the bias? If so, what should my voltage read?

You need to re-bias after removing the tubes. I would suggest to measure the bias before removing to know were you currently are, and turn the bias all the way cold before you remove the tubes. You can experiment with different bias setting to see what you like most, just make sure you don't overload the tubes.

As correctly Tqi said, this is a dangerous job, not only for the amp but for you too! You should have some technical background to do this, so I hope you have studied on the subject first. Otherwise let this job to some experienced technician.

I’m not going to lie I have zero experience around a chassis but after extensive research I feel comfortable removing a few tubes and turning a pot a few times (with my basic understanding and having a handy background) I called a tech to schedule me in for maintenance and a good clean of the chassis, that I will not do. but trying to save a few dollars, this also seems like something every amp owner should be able to do on there own when needed. My main question is, do I need to cut the bias pretty much in half once I remove two power tubes (after I check the bias with all 4 6l6’s)? Or a little less if anything?

Tqi wrote:

AndrewTesta wrote:

this also seems like something every amp owner should be able to do on there own when needed.

Getting a little off topic, but to be clear - this is not the case. Most amps will need pre-amp tubes replacing maybe once every 50 years (maybe), and power amp tubes maybe... once a decade? If you use cheap tubes and rag the amp? The only other reasons to rebias would be if a circuit component exploded, or the your electricity grid permanently changed from say ... 110V to 125V.

I’m just trying to calm down the signal of the monster we call a twin reverb that’s staying as a bedroom amp for the foreseeable future so I planned on removing two of the four power tubes and disconnecting a speaker. Am I overthinking or do I need to do further modification in the chassis to achieve this

psychonaut wrote:

AndrewTesta wrote:

Tqi wrote:

AndrewTesta wrote:

this also seems like something every amp owner should be able to do on there own when needed.

Getting a little off topic, but to be clear - this is not the case. Most amps will need pre-amp tubes replacing maybe once every 50 years (maybe), and power amp tubes maybe... once a decade? If you use cheap tubes and rag the amp? The only other reasons to rebias would be if a circuit component exploded, or the your electricity grid permanently changed from say ... 110V to 125V.

I’m just trying to calm down the signal of the monster we call a twin reverb that’s staying as a bedroom amp for the foreseeable future so I planned on removing two of the four power tubes and disconnecting a speaker. Am I overthinking or do I need to do further modification in the chassis to achieve this

Just get an attenuater if low volume is your goal.
https://www.tedweber.com/gadgets/attenuators/
You can then crank the amp as much as you want and lower the overall volume much more than by pulling tubes. This will allow you to play the amp at its sweet spot without the neighbors calling the cops.

Thought about it, I was leaning more towards the free alternative that can always be changed back to factory. Would also like to get to know my amp a little better, Guess I should just sit and think about it before diving in

edwardsand wrote:

Also, removing a speaker means you are changing the impedance load on the output transformer, say from 4 ohms to 8 ohms. Fender amps can tolerate a bit of an impedance mismatch like this, but I wouldn't make it a long term thing.

A better option would be to replace the speakers with a pair that are way less efficient or run through an external cab with a single, much less efficient speaker.

I was under the impression that lowering the bias would take care of that issue, maybe I should just get an attenuator
That being said, does anyone know an affordable attenuator that suits a twin reverb? (100 watts)

Last edited: Jun 13, 2023 11:38:09

DaveMudgett wrote:

AndrewTesta wrote:

edwardsand wrote:

Also, removing a speaker means you are changing the impedance load on the output transformer, say from 4 ohms to 8 ohms. Fender amps can tolerate a bit of an impedance mismatch like this, but I wouldn't make it a long term thing.

A better option would be to replace the speakers with a pair that are way less efficient or run through an external cab with a single, much less efficient speaker.

I was under the impression that lowering the bias would take care of that issue, maybe I should just get an attenuator

If you remove two output (6L6) tubes (on a Twin Reverb, either remove the inner pair or the outer pair - i.e., labeling the tubes 1 2 3 4 in order left to right, either remove 1 and 4 or 2 and 3), then to maintain the correct speaker load impedance (8 Ohms instead of the stock 4 Ohms for 4 tubes), you can simply disconnect one of the speakers. This assumes that your speakers are both 8-Ohm speakers, which is certainly true if they're the originals.

Alternatively, you could use two 16-Ohm speakers, which with the stock Twin parallel wiring, gives 8 Ohms.

If you simply remove the inner or outer pair of tubes and change the load to 8 Ohms and the bias is correct for the tubes now, then you should not need to change the bias voltage.

BTW - re-biasing does not change the output load or significantly change the output power. Typically, running the amp a bit 'cold' (higher bias voltage, which reduces the current with no input signal) increases the headroom in the amp, up until you reduce it to the point where there is so-called 'crossover distortion', which is pretty ugly-sounding. Running a bit cold also tends to extend the life of the tubes because it reduces the current you're pushing through the tube - it actually does run cooler temperature-wise. If you run the amp 'hotter' - i.e., with a lower bias voltage which causes a higher current with no input signal - the amp will tend to hit the distortion point a bit quicker, but at the expense of faster tube wear - you actually are running the tubes 'hotter'.

If you run Rob Robinette's Tube Bias Calculator (to which Tqi linked) for 6L6WGB tubes (max design plate dissipation = 26 Watts per tube), which is what I have in my '66 Twin, with a plate-to-cathode voltage of 450 Volts (which is about what my Twin runs), you'll see the typical range of bias currents range from 28.9 ma (50% of max dissipation - cool) to 34.7 ma (60% of max dissipation - average) to 40.4 ma (70% of max dissipation, which is generally considered as high as you'd ever want to run them). And it is no surprise that I run my bias current between 32 and 36 ma - I will adjust in the range to where it sounds best to me. My general philosophy is to run power tubes as cool as possible (to extend tube life) while still sounding good. Obviously - if you're running different 6L6 power tubes, calculate for those, not the 6L6WGB's that are in my Twin.

But really - if the amp sounds good to you now, but is too loud, you really should be able to reduce the volume significantly by pulling two tubes and disconnecting one speaker. Make sure you tape up the speaker wire ends of the speaker you disconnect and secure them so you don't have hot flying leads running around the cabinet to cause potential havoc. I've pulled tubes and speaker many times - it does help. The best part is that none of this requires that you open up your amp at all, and costs nothing. But definitely - unplug the amp even when changing tubes, and disconnecting and taping up the speaker wires. If pulling tubes and disconnecting a speaker isn't enough, then maybe you really do need an attenuator.

I will also second (or third) the warning about messing around inside a tube amp if you don't know what you're doing. If you don't clearly understand how to protect yourself in the presence of high voltages (typically 400-500 Volts DC), you should not be messing around inside a tube amp. Period. IMO. Wait until you learn, and until then, find a good tech.

I appreciate the feedback, I can assure everyone that I would never jump into a chassis blindly. I’ve done a lot of personal digging on the do’s-dont’s regarding working on an amplifier or around a chassis and it’s high voltages. A number of people talk about doing this so nonchalantly that it seemed like a simple solution to no longer have angry neighbors. If you’re saying I do not have to take the chassis out and simply just remove either 1-4 or 2-3 of my power tubes, along with disconnecting a single speaker I feel comfortable tackling it myself. But based on a number of these responses I feel more deterred than encouraged to do something that I thought was an easy musicians “trick” to lower some wattage on a very loud amplifier. Do I absolutely need to take the chassis out to do this? If so, might just be worth investing in an attenuator considering I would have to pay to have this done anyway by a tech
I have my multimeter, I have alligator clips, I have enough safety precautions in my head for everybody and their mother but based on the responses I’m at a loss now

Last edited: Jun 13, 2023 12:58:45

DannySnyder wrote:

Have you considered foregoing the amp and using your computer with some decent amp sims? (if you have an Apple, Garageband is pretty amazing and it's free). You can crank your headphones as loud as you please. I've worked that way for decades.

Ideally I would but I’ve wanted a FTR for as a long as I can remember, now that I have one and now that it’s my only amp I’m trying to get the most out of it as possible. One day I would like to get it mic’d into a nice interface that I can record straight into my computer. However easy/complicated it might be, I want to hear my guitar through the twin. But you’re right it really is efficient I sometimes plug directly into a little handheld tascam recorder I acquired a long time ago. Very clean and you hear exactly what the instrument is doing. Once I have a mic I can run my amp directly into the recorder but an interface going to a computer is better imo

Last edited: Jun 13, 2023 14:00:15

DaveMudgett wrote:

I do not understand why you are still thinking about resetting the bias. If it sounds good now and the plates of the 6L6 tubes are not red-plating, you don't need to reset it. Resetting the bias will not change the fact that a Twin Reverb is a loud 80-100 Watt amp.

You can pull two tubes and disconnect a speaker and see if that does it for you. If the speaker wires are connected to the speaker terminals with female spade lugs, this just takes a minute - just pull the spade lugs, wrap with tape, and pull either tubes 1 and 4 or 2 and 3. Plug amp in, turn on, play. If that does it, you're done. If the speaker wires are soldered to the speaker terminals, you need to de-solder the wires from the speaker you want to disconnect. As long as you're comfortable with a soldering iron, it's easy. You don't need to open up the amp chassis to do this. Just unplug the amp and do it. Obviously, don't try to remove the tubes when they're hot.

But we don't really know your ultimate goal. Do you want a fairly loud amp that just isn't as loud and clean as a Twin Reverb, or do you want a bedroom amp. Nothing is gonna turn a Twin Reverb into a bedroom amp. Power attenuators are OK to drop the output some, but if you try to bring a Twin Reverb way down to quiet bedroom levels, even running two tubes at 40-50 Watts into a single 12" speaker, it will sound thin. I have a Fryette Power Station, which is undoubtedly the very best attenuator/re-amplifier I've ever tried. But even that will not tame my Twin to conversational level and still sound good to me. Great on a gig if I just don't need to be 80-100 Watts loud.

As Danny says, maybe a good amp sim. Or maybe an inexpensive modeler into your Twin at a low volume. Or into headphones. Personally, I don't like playing into headphones, but if the modeler/sim is good, running into any clean amp at any volume should sound OK.

Or maybe an inexpensive bedroom amp. There are tons of them out there. Unfortunately, great small tube amps like vintage Champ/Vibro-Champ/Princeton/Princeton-Reverbs are crazy expensive now. But there are other tube alternatives. I would avoid a modern PCB amp with lots of bells and whistles - the more complex, the more potential reliability problems. But there are simple tube amps out there on a PCB that will probably do the trick and be reasonably reliable. Hand-wired amps, even small ones, are pushing a $700 to a grand these days.

I know there is a bias in the guitar world against solid-state Peavey amps, but I have an 80s Peavey Backstage and a Backstage Plus that are just fine for relatively low-power clean guitar. They're a lot cheaper than a good attenuator. For more power, I also have an 80s Studio Pro 40 and a Studio Pro 50. I got all of these in the last few years for under $200 total. They're giggable in the right context, I've done it. The onboard reverb isn't great, but I just use a reverb pedal like the Topanga, or a 6G15. Turn the Post Gain up, turn the Pre-Gain only as high as you need it, and leave the Saturation Control on 0. Adjust Bass/Mid/Treble to taste. I generally turn the treble and mid down fairly low on these, bass up, on a Strat or Tele. These suckers are built tough for the road. No, they're not a Princeton or Deluxe Reverb, but they can do the job if you know how to work them. I use these a lot at home.

Also - check some Quilter stuff out. They have some relatively low-power heads that sound very good for a clean Fendery sound to me. My Quilter Tone Block 201 (a 200-Watt 4 pound head that I use mostly for pedal steel) has saved my butt a couple of times when the provided backline Twin Reverbs sounded like garbage. I just unplugged the Twin's speaker cable from the chassis and plugged it into the 4-Ohm output of the Quilter. Sounded great, actually pretty comparable to a good Twin Reverb. One of the guitar players I work with uses one of the lower-power Quilter heads. It sounds very good.

There are lots of ways to get a good sound at reasonable volumes that don't have to cost an arm and a leg, or require you to tear apart a Twin Reverb.

In a previous response to my question I was told I need to re-bias the amp after I remove the tubes I’m glad it’s as simple as you’re stating. But don’t get me wrong I love my TR and knew going into it that this thing is a beast and like you said, not at all a bedroom amp. I recently got rid of a FBJR limited edition tweed and even that was loud, but it sounded like garbage, hated the tone. So when I saw this twin at the guitar shop I had to get it, it does phenomenal at gigs/band practice. But now that I’m mainly playing at home I figured I could absolutely leave it as is and be happy or indulge in a hobby that could potentially give me good results(?) Thanks for the wisdom!

Last edited: Jun 13, 2023 19:06:27

A couple of thoughts.

Volume is measured in decibels, abbreviated dB. If you cut the power of an amp by 50%, the volume will be 3 dB lower. Three dB is roughly the threshold of perception, which is to say that if you pull two power tubes and take one speaker out of the picture, the volume drop would be just enough to notice. This may not sound sensible, but volume changes work on a logarithmic scale, which is contrary to intuition, but mathematically works for acoustic phenomena.

As a low cost experiment, you can pull either power tubes 1 & 4 or 2 & 3, disconnect one speaker insulating the spade connectors as suggested above, and your Twin will have become a 42.5 watt amp, and all for the cost of a couple of inches of electrical tape. It’s still going to be a very powerful amp.

If you are looking for a way to make it a bit more tractable at low volumes, replacing V1 (Normal Channel) or V2 (Vibrato Channelj with a 12AY7 will reduce the preamp gain, and may lower the “coming in” volume, where the amp first begins to be audible. This will not, however, reduce headroom, and may actually increase headroom. Nonetheless, this is the first thing I do to any Fender amp I own. Really, it just makes the volume control a bit less sensitive on the lower end, but my Twin does just fine, if I choose to play it at living room volumes.

If you are looking for natural overdrive, then you might consider buying a large parcel of land, because even with half the power tubes removed, a Twin is still loud. I live out in the country, and I took my Twin out on the front porch and cranked it up to about 8, which was about as loud as I wanted to go. It was still clean. Flocks of birds fell from the sky, rivers changed course, and the nuclear test sit in Nevada called in a noise complaint, but it was still clean.

The artist formerly known as: Synchro

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