Amp re-bias?

[bleujazz3]

Hi,

I own a Fuchs Overdrive Supreme amp head and cab. I recently purchased new power tubes because the old tubes had worn out.

Originally, the stock tubes were Tube Amp Doctor 6L6 WGC-STR tubes. The designation on the tubes were #7.

To make a long story short, the new Tube Amp Doctor tubes are the same make tubes that have a designation of 07D, PC 46 and making matters more confusing, TC 655.

I contacted Andy Fuchs, president of Fuchs Audio Technology, who suggested re-biasing the amp myself.

What worries me is that Andy said I had to go out and buy a digital multi-meter. I have done so. By disassembling the amp, and measuring the voltage between chassis ground and the 8-pin on the tube (yes, he said tube, not in the 8-pin socket receptacle) the amp should first be turned off, and the amp plugged into a speaker. Controls should be set to zero, and the amp switched on, allowed to warm up, then switched from standby to on. The amp can be biased by adjusting the adjustable pots on the amp.

What I need to know is, where is the 8-pin on the socket or tube? My first inclination is to turn the amp up on its side, and look at the socket. The socket, if I remember correctly, has an arch of holes in it. Let me know if the #8 pin is located at 8 o'clock on the socket. If so, I can measure the voltage correctly. Andy said that the voltage should read 36 mV.

I am guessing that Andy mistakenly suggested that the tube be measured, not the # 8-pin receptacle with the tubes NOT inserted. Is it safe to run an amp without power tubes in the amp? Also, once the test is complete, can I turn off the amp, safely insert the tubes, and allow the amp to run?

I know that after the test, the amp should be allowed to run for 30 minutes, then tested and adjusted again if necessary.

Please advise me of any precautions I should take to prevent hazards.

One other suggestion was made to make sure that the reverb jacks are connected before reassembling the amp. Andy said to make sure that the #5 screw NOT touch the circuit board. I am unsure if Andy meant that I would have to disconnect the reverb pan when I disassemble the amp. If yes, the reverb pan does need a disconnect, how can it be made easier to reconnect without stress?

Sounds like I'm making a iced tequila drink. And, this continuing ed is not all that it's cracked up to be. I'll be happy once I get this done and under my belt. :D

[Bruce O'Donnell]

Since you're using the same tubes (TAD 6L6GC STR's) and assuming TAD matches their duets/quartets, I doubt it needs a rebiasing. The hardness of the tubes doesn't change the nominal power dissipation, it's just a measure of how soon they distort. If they are not matched, then a rebiasing may be in order, but I'm not sure what Andy is telling you do. Here's how to bias an amp with a DVM.

1) Pull one of the power tubes, turn on the amp connected to a speaker with nothing connected to the input, and measure the voltage in the socket across pin 3(+) and pin 8(-). That is the plate voltage. For a given tube type for a given plate voltage, there is a nominal no-load power dissipation that represents the ideal setting for the tube. For example, 6L6 and EL34 have a nominal power dissipation of 21.6 watts at no load with a plate voltage of 500V.

2) Reinstall the power tube, power up the amp again, and measure the cathode current through the tube. For example, with a plate voltage of 500V and a 6L6 or EL34 tube power dissipation of 21.6 watts, you should have 36 mA flowing through the tube at no load. This is where a bias tool is invaluable. A bias tool is a dummy socket you insert between the socket and tube that has leads you plug directly into a DVM. Since most DVM's don't measure current all that reliably, most of the bias tools are internally shunted to voltage, such that 1V=1mA. You adjust the bias pot until you get the correct cathode current, and you are biased. Check the other tubes, and adjust as necessary if the others are out. I normally split the difference between the coldest and hottest tube around the nominal bias point.

Unless you have small aligator clip leads and can lift the tube slightly in the socket so you maintain good contact, it's not a one person operation to hold the leads to the pin and adjust the bias pot. Plus if you slip off the pin and hit another, it can be ugly. A bias tool is money well spent. I use the GT tool, but there are others.

The reason you need to rebias is that unmatched tubes can be all over the place. Even matched tubes - I bought a matched quartet of GT branded 6L6GC RCA NOS, and in my Peavey 5150, which has a plate voltage of 495V, the tubes ran from 13 - 19 mA. So a supposed "matched" set of tubes had this much variation. This was a fix-biased amp, so the tubes ran ungodly cold and in constant crossover distortion. I replaced the fixed resistor with a 5k fixed (safety, so the amp doesn't short with the pot at zero) and 10k pot and biased the amp so the coldest tube was running at 33 mA. I checked them all afterwards, the the hottest tube was running at 39 ma. Nominal is 36mA for 6L6 at 500V, but 33 mA is very close to no crossover distortion and I'm not eating tube life that much by running the other tubes slightly hotter.

[bleujazz3]

Bruce,

Andy tells me that I can just measure the voltage from chassis ground to pin 8, and set the bias by adjusting the pot until the DVM reads 36 mV.

Andy doesn't tell me that I need to have tubes in the amp, but I didn't ask him if I needed to.

Correct me if I'm wrong, but should I put both new tubes in, pull one, or should the old tubes be used? I'm thinking they should be the new tubes, not the other way around.

The new tubes are a perfect matched pair by The Tube Store. I just don't know if the amp will need a re-bias, since the Tube Store was unsure about the original tubes.

I also know what pin is pin 8. If I disassemble the amp, have the it face front, and tip it upwards, letting it rest on its transformers, the 8-pin is the lower right pin of the key slot.

One thing before I begin, I will check with Andy once again to see if it is necessary to have the only one of the tubes in the amp. If yes, I'll be alright. If I can check the voltage in the pin (socket) of the extracted tube without fear of causing a problem, that'll be enough. I'll get back to you after I hear back from Andy.

[bleujazz3]

Since you're using the same tubes (TAD 6L6GC STR's) and assuming TAD matches their duets/quartets, I doubt it needs a rebiasing. The hardness of the tubes doesn't change the nominal power dissipation, it's just a measure of how soon they distort. If they are not matched, then a rebiasing may be in order, but I'm not sure what Andy is telling you do.

Bruce, thank you for providing that wealth of information, but the bias test that Andy was able to provide assistance was simple and straight to the point.

First, I disassembled the amp. Thinking that I might need to remember how to reassemble later, I wrote down the steps I took to disassemble the amp.

First, the reverb leads that connected to the reverb pan had to be removed. Then, the wood block on the left side of the cab that acted as the back panel support had to be removed. (One of the transformers wouldn't slide past it) The wood block was screwed into place from the side, and the block was easily removed.

Once I had the chassis out of the amp, I removed the tubes and replaced the them with a fresh set. In order to re-bias the tubes, I used clip leads looking down from the top of the chassis, and the tube sockets were marked, as were the tubes. Clamping onto pins 8 of both tubes, and clamping the black prob from the multimeter to the chassis, I switched on the amp. (The amp was no front end load, controls set at zero, connected to a speaker.) It idled for a couple minutes, and then from standby to on. The bias point from the red prob onto each of the # 8 pin's clip lead was measured, and adjusted via adjustable bias pots on the circuit board. After a few minor goings back and forth, the multimeter settled at 36 mV for each tube.

I turned the amp to standby and let it idle for 30 minutes. After the 30 minutes, I went back to re-test and re-adjust. The amp didn't need much re-adjustment, so I followed the instructions backwards to re-assemble. The only thing I need to check is if the reverb leads are not resting up against the circuit board, otherwise if I turn the amp on and the connections are pressed up against it, the amp will likely short and need a more detailed repair. Prevention is key here.

Andy was spot on by providing the assistance, and although I appreciate your efforts, Andy's amps are simple and elegant to work with. The resistors didn't need changing, or any other measurements needed. A multimeter was the only tool needed, along with two of the clip leads that I snagged from a computer grounding strap.

Needless to say, the amp should work now. I'll test the amp tomorrow controls at normal settings and make sure levels are still good. Wish me luck. I had the opportunity to graciously thank Andy for the ability of going out his way to be polite and not condescending, for a newbie like myself to amp repair.

And thank you again for your advice, although most of it was above my head. I'll let you know how the amp tests first thing Thursday morning after I have my usual dose of silliness and getting my brain to work correctly. From then, I'll be good to go. See you first thing in the morning. :)

[Bruce O'Donnell]

There you go - that's why Andy Fuchs makes such a great amp. Exceptional tone, and designed for ease of maintenance and biasing. There are very few amp manufacturers that go that extra mile. I've never serviced or played through a Fuchs - yet :D