6550 Tubes for Ampeg SVT

[agedhorse]

Something I've always wondered, does cooling down the glass envelope cool down the heater filament also? If not, what is the purpose?

No, the heater is inside the cathode.

It will slightly increase the plate dissipation capability a little bit.

 

[denton57]

I think coincidentally they are the weakest tubes in each set. I wouldn't worry about it unless they red plate at the correct bias. 224 mV is really high.

I was kind of thinking the same thing. Everything seems normal now, or it did for the few minutes I’ve been able to “work” on it. This morning I ran it for 10 min with the probes in 1 and the ground and just watched it. Tweaked it a hair to get it as near 72 as I could for both sides. Sounds like the sky isn’t falling, and maybe a new set of tubes isn’t necessary yet. I guess I’ll know more as I put some time on it. I’ll definitely use it next band practice and get 3 hours on it and check for any red plating. Have a party to play Apr 15 so if all is good for the rehearsal, I’ll try it.

 

[ThisBass]

I think coincidentally they are the weakest tubes in each set.

If any then these might be weakest in an rather faulty theoretical consideration of view.
In a rather practical meaning (and also well thought out theoretical consideration) its not.

 

[bobyoung53]

If any then these might be weakest in an rather faulty theoretical consideration of view.
In rather practical meaning (and also well thought out theoretical consideration) its not.

They're in series with the other two on that side though, I would think that the weakest tube would get too hot first.

 

[Wasnex]

They're in series with the other two on that side though, I would think that the weakest tube would get too hot first.

The tubes in each trio are in parallel. Functionally, the cathodes are tied together, the control grides are tied together, and the plates are tied together.

AFAIK, a weak tube may have low current or high current.

If the tube cathode is degraded and emits significantly less electrons than it should, the tube is probably not going to red plate.

On the other hand, if the ability of the control grid is degraded and cannot hold back or modulate electron flow, the tube is likely to red plate...especially if the cathode emissions are strong.

 

[ThisBass]

Well, the tubes in the middle section of the sextet may run to a little higher temperature than the most left one and most right one. About 5..10 degree of deviation to my guessing (give or take).
Either way I strongly suggest to bring the amp to a qualified service center.
Hard to believe tubes in question become glowing red due to some reduced cooling, I'm pretty sure these go red because of something else.

[off topic on, rant on]
sometimes I do hate nitpicking
[off topic off, rant off]

 

[ThisBass]

The tubes in each trio are in parallel. Functionally, the cathodes are tied together, the control grides are tied together, and the plates are tied together.

AFAIK, a weak tube may have low current or high current.

If the tube cathode is degraded and emits significantly less electrons than it should, the tube is probably not going to red plate.

On the other hand, if the ability of the control grid is degraded and cannot hold back or modulate electron flow, the tube is likely to red plate...especially if the cathode emissions are strong.

The most thing we can do is just reading in crystal bowl.
I tried to tell that my SVT long time ago suffered on similar issue such as @denton57 described above.
Are we really that perfect with experience around our SVTs, technical wise, that we can do error diagnoses "on the fly" to such a touchy topic?
Personally, I don't think so! At least to me, I don't.

 

[6thplanet]

No, the heater is inside the cathode.

It will slightly increase the plate dissipation capability a little bit.

So cooling off a tube to say room temperature doesn't significantly change it's operation, just keep it from going above it's max temp? There's no temp thats better for operation?

 

[bobyoung53]

The tubes in each trio are in parallel. Functionally, the cathodes are tied together, the control grides are tied together, and the plates are tied together.

AFAIK, a weak tube may have low current or high current.

If the tube cathode is degraded and emits significantly less electrons than it should, the tube is probably not going to red plate.

On the other hand, if the ability of the control grid is degraded and cannot hold back or modulate electron flow, the tube is likely to red plate...especially if the cathode emissions are strong.

OK, but still the weakest tube should red plate first.

 

[bobyoung53]

Well, the tubes in the middle section of the sextet may run to a little higher temperature than the most left one and most right one. About 5..10 degree of deviation to my guessing (give or take).
Either way I strongly suggest to bring the amp to a qualified service center.
Hard to believe tubes in question become glowing red due to some reduced cooling, I'm pretty sure these go red because of something else.

[off topic on, rant on]
sometimes I do hate nitpicking
[off topic off, rant off]

They glow from too much plate current I believe, If one glows with a normal amount of plate current, time to ditch it.

 

[bobyoung53]

So cooling off a tube to say room temperature doesn't significantly change it's operation, just keep it from going above it's max temp? There's no temp thats better for operation?

I have always wondered if they were cooled to protect the components around the tube, like in transmitters without a lot of space for example and I always thought the fan in an SVT was to keep the power transformer and the whole inside of the amp box cool not the tubes.

 

[ThisBass]

So cooling off a tube to say room temperature doesn't significantly change it's operation, just keep it from going above it's max temp? There's no temp thats better for operation?

Nope, the heater contributes slightly to plate dissipation, at most, that's all.
No mojo to be found in things such cooling down to room temperature, or anything else.
Personally I don't see any sense by cooling down a tube to room temperature?
May it happend there was some HiFi mojo involved to this consideration?

 

[ThisBass]

They glow from too much plate current I believe, If one glows with a normal amount of plate current, time to ditch it.

Its too much dissipation power when they start to glow red

 

[Patrice B]

I have always wondered if they were cooled to protect the components around the tube, like in transmitters for example and I always thought the fan in an SVT was to keep the power transformer and the whole inside of the amp box cool not the tubes.

The fan in my SVT-CL pull air from the amp and there’s an opening right behind it accessing the area under the boards. So, I believe it serves two purposes: cooling the tubes to a certain extent and evacuating hot air away from the innards.

 

[bobyoung53]

Its too much dissipation power when they start to glow red

Well yes, but why is the question, I think tubes just wear out after a while and can't handle normal plate current.

 

[bobyoung53]

The fan in my SVT-CL pull air from the amp and there’s an opening right behind it accessing the area under the boards. So, I believe it serves two purposes: cooling the tubes and evacuating hot air away from the innards.

In other words, yours blows outward? On the old ones the fan blows inward right onto the power transformer.

 

[ThisBass]

Well yes, but why is the question, I think tubes just wear out after a while and can't handle normal plate current.

From where did you got this wisdom?

 

[bobyoung53]

From where did you got this wisdom?

"We should also mention that when it comes to power/output tubes another issue can be red-plating. This is sometimes due to wrong biasing of the amp, which really isn’t a tube problem. But there are times when a properly biased amp has a tube that begins to red-plate. This is a sign that that specific tube is failing and in a “run-away” state where the current can’t be controlled by the bias voltage. A tube like this should be replaced. If not, the amp will eventually blow a fuse or worse damage other parts."

How To Tell If Your Tube Is Bad - www.thetubestore.com

 

[agedhorse]

So cooling off a tube to say room temperature doesn't significantly change it's operation, just keep it from going above it's max temp? There's no temp thats better for operation?

It’s impossible to cool the inside of the tube to anywhere near room temp. Just the glass envelope.

OK, but still the weakest tube should red plate first.

Or not. Tubes that red plate are conducting more current. Why is important.

They glow from too much plate current I believe, If one glows with a normal amount of plate current, time to ditch it.

It won’t.

I have always wondered if they were cooled to protect the components around the tube, like in transmitters without a lot of space for example and I always thought the fan in an SVT was to keep the power transformer and the whole inside of the amp box cool not the tubes.

Both

Well yes, but why is the question, I think tubes just wear out after a while and can't handle normal plate current.

They do but they no longer function the way they are supposed to. This generally does not include red plating or excessive current unless the grid is damaged.

 

[coreyfyfe]

I can't vouch for the quality of the tubes, but I am sure I am not the only TB member with a very favorable impression of TC Tubes. I trust them more than most other tube vendors, which is not necessarily saying a lot.

FYI, The Winged C KT88s are a fairly recent addition to TC Tubes' offerings. Before Covid they sold primarily current production New Sensor tubes, with a few NOS/ANOS mixed in from time to time. I believe these tubes (and all the other output tubes they currently sell) popped up as their new stock dwindled, just before prices went through the roof. AFAIK TC has not raised the price since the tubes were listed.

AFAIK, tubes from the last year or two of Winged-C production were very problematic. I believe they shut down in 2012. The tubes TC is offering appear to be from earlier when Winged-C was still considered among the best available.

Note the description from the product page:
View attachment 5019238
So these do not appear to be from the last years of production.

It's possible these are seconds. If you don't know, big companies can buy tubes that have been tested/QC'ed to different quality levels. When they receive a big batch of tubes, the company does their own testing and return tubes that don't meet their standards for credit.

Not all rejects are considered bad tubes. Often functioning rejects are sold to other tube vendors, who test, grade, and sell them as A-stock. So it's possible these are not the top quality tubes sold by premiere sellers, but they still may be of the same quality sold by various tube vendors back in the day.

If you have concerns, call TC Tubes and ask for additional details, and also ask if the tubes carry any sort of warranty.

At $90 a pop, I would choose current production JJ 6550 at $60 each. Back when Winged-C was still pumping out good tubes, Winged-C and JJ were generally considered the best.

TC also offers some VA 6550A for $49. These should be Shuguang 6550A, which had a decent reputation. Unfortunately Shuguang has also stopped production.

I can also vouch for T&C, Tyler is a very helpful guy and I’ve always gotten great tubes from them. They’re always buying and selling stuff so it’s possible they got the winged Cs for a reasonable price and have them listed for what they think is a reasonable markup. I’ve sent him mix and match old glass 7027As before and had them test on their equipment and build me matched sets for my ampegs from my stock and theirs. Since they test and match everything at high voltage I’ve found their stuff very reliable.