JCM800 6550 issue

[newtube]

Hello,
I have a 1981 Marshall JCM 800 2203 100watt amp that I was going  to check the bias on, but noticed that pins 1 and 8 of the output tubes are tied together and grounded...except for 1 tube, the pins are tied together but it is not grounded. The tubes are RCA 6550's. Would there be any reason for this because it doesn't make sense to me? The amp is also very loud as it should be.

[SILVERGUN]

6550 tubes do not utilize pin1 ...BUT....EL34s do
Some manufacturers will go ahead and wire pins 1+8 together to ground to accomodate EL34s which DO utilize pin 1 for an additional grid (suppressor grid)

The tube that doesn't have the ground connection SHOULD... and it is OK to leave 1+8 tied together even though you are not using EL34s because there is no internal connection for pin 1 inside your 6550s

That year 2203 was equipped with EL34 tubes for amps sold in the UK and 6550 tubes for amps exported to the United States

If that is an original (unmodified) 2203 from 1981 it is very sought after and very valuable....be careful

[HotBluePlates]

... pins 1 and 8 of the output tubes are tied together and grounded...except for 1 tube, the pins are tied together but it is not grounded. ...

Then that tube doesn't have a connection from its cathode to ground, and can't pass current. Verify by measuring d.c. voltage across the screen resistor for that tube/socket. If you get anything more than 0vdc across that resistor (from one leg to the other), then current is flowing and you overlooked the gorund connection somehow.

6550 tubes do not utilize pin1 ...

Some versions of the 6550 have pin 1 tied to the metal shell of the base. Therefore, they should have pin 1 grounded same as the EL34 (for a different reason).

Safest plan for any common guitar-amp octal tube type is to tie pin 1 to pin 8 and ground them.

[SILVERGUN]

Some versions of the 6550 have pin 1 tied to the metal shell of the base.

See that newtube...we've all got things to learn here

And just in case you don't understand what HBP is saying...the tube that doesn't have the ground connection from pins 1+8 can not work like that.
If there is no current flowing through the tube from a ground path then it is not "working", meaning that it is not putting out any output. Pin #8 is the cathode connection and you HAVE TO have a path to ground for it, or else that tube is just a very dim light bulb.

[newtube]

Thanks everybody for the replies! My gut feeling was right...I knew that the tube should be grounded, just wanted to be sure that this wasn't someones idea of a good thing. This is an original 1981 vertical input JCM800 2203, unmolested, the previous owner had vintage RCA 6550's in it and Westinghouse yellow stars ECC83's in it. Could the disconnected tube have damaged anything?maybe the other tubes etc..I mean the amp works great, it's incredibly loud and sounds awesome so it doesn't seem like it. The amp appears to have the original Drake transformers in it, dated 6/81.
The overall story with this amp is pretty cool, I have another Marshall head that needed a speaker cabinet, but because I am cheap I did not want to buy a new cab. So I spent about 5 months looking for a cab on ebay and CL, until one day I saw an ad on CL saying "JCM800 Marshall amp and speaker, amp needs work $500". No pics or anything just a phone number. I didn't call because it seemed like it was a BS ad, until a bit later in the day I decided "what the hell". The guy said to come look at it now, which I did. When I got there, it was in his living room...and wow it was a Lead Series 100w 2203 AND a 1960av cabinet with greenback celestions, basket weave grille and all! He was the original owner, said he got out of music and was putting together a Corvette and needed the money. He told me he hadn't even plugged it in since 2005, but the amp doesn't work for sure. I asked him if was firm on the price, he said he could slide 50 or 100 bucks, so I gave him $460. Took it home, the amp powered up but no sound...pulled the chassis, found a piece of a broken bolt with a nut on it rolling around inside. Looked for the rest of the bolt or the hole it belonged in, and found a ground wire should have been under that bolt! Thinking "great if thats the only thing wrong with it", powered it up and it now made some sound but it was distorted, weak and a bad humming coming from it, killed the power, took it apart again, this time I found another broken bolt. I thought that was the one I just took out of it and tossed it on my bench, when it landed it landed next to the other broken bolt. I said "really, two?". Sure enough another ground wire should have been under that broken bolt. This was back in September 2012. Plugged it in and holy crap! this amp was loud when played and very quiet while idling! What a score I thought!  I gave it to my 16 year old son for his band. He plays a lot of the classic stuff like Led Zep, Cream etc..It has been played regularly since then. This past Saturday his band did an awesome show at a local theater and it performed wonderfully. I decided it was now time to open it up and check it, especially given it's age and the use it has been getting recently and found the ungrounded tube! Luckily I never touched the metal base of that tube while it was powered up!
What is the consensus about recapping it? I know it needs it. It has DALY filter caps in it, probably the originals, replace with F&T"s a good choice?

[newtube]

One other thing, the bias mv is set at .033 when checked across the 1ohm resistor and pin 3 voltage is 445. Is this a good setting or is it too cold? Like to know your opinions. Thanks, NT

[HotBluePlates]

... Could the disconnected tube have damaged anything? ...

Having that tube disconnected doesn't cause any harm to anything. The amp might not play as loud, as clean with only 3 tubes.

What is the consensus about recapping it? I know it needs it. It has DALY filter caps in it, probably the originals, replace with F&T"s a good choice?

A recap is good preventive maintenance, unless you're already having problems with hum. Yes, Daly would be the original caps for that era. F&T does make fine caps.

One other thing, the bias mv is set at .033 when checked across the 1ohm resistor and pin 3 voltage is 445. Is this a good setting or is it too cold?

That's fairly cold for 6550's, however...

When you bias hotter (more idle current), you do so by reducing the amount of negative voltage on pin 5. That allows more current to pass through the tube with no input. But all normal guitar-amp output tubes run out of steam when the incoming signal voltage momentarily equals the bias voltage. If your bias voltage is -45v, then that's an a.c. voltage that develops a 45v peak on the positive-going side of the waveform. The tube may show varying amounts of distortion before this point, but will definitely distort with any signal this big or bigger.

So while you bias hotter, you're also reducing the amount of maximum input signal, causing the tube to distort sooner, and possibly making a little less output power (depending on other factors).

But 6550's were fitted in a lot of American-distributed Marshalls, especially after master volumes were fitted to the amps. You could argue that with a 100w amp and 6550's, that the goal really isn't output tube distortion without insane SPLs. So biasing a bit cool will tend to keep the output stage clean, and the tubes will probably last longer.

REGARDLESS OF ALL THE ABOVE... if it sounds good, then it IS good.

[PRR]

> bias hotter, you're also reducing the amount of maximum input signal

Not really. Up to a point, the MAX power has very little to do with the bias point.

Yes, the gain increases some when biased hot, but the difference is small, perhaps less than the slop in the volume-knob.