A word to the wise on EH 6CA7 tubes!

[MajorNut1967]

If you are contemplating using the Electro Harmonix 6CA7 in your older/vintage Marshall, Traynor or with an amp with a negative grid supply; I would stay away from this! The Electro Harmonix 6CA7 is not and I repeat “NOT” a Power Pentode like the original 6CA7’s were, in fact they are Beam Tetrode as stated on the Electro Harmonix website. But the issue is tube venders are saying it is a direct substitute for an EL34 in all situations. Be careful, its not! I you really want to use these tubes in your amps make sure you don’t have a negative grid supply mod or a Traynor Amp. In fact even if your older/vintage Marshall amp has (pin1) & (pin 8) tied to ground, I would stay away from these tubes.

I found this out on my own. My JCM 800 build project was the test bed, I had told Marty I got a set of Electro Harmonix 6CA7’s to try in it later after the amp broke-in, well I put them in today. I couldn’t get them to bias correctly. So I bumped up he bias voltage up and got them in range, but they amp sounded like crap an the PT was hot as hell. Then friggin Wilder clued me in, after all the bullshit I went through, that it’s not a real power-pentode but a damn Beam Tetrode wannabee. Wilder found it out on Solar’s amps, but didn’t bother to tell me! LOL

 

[big dooley]

I found this out on my own.

Then friggin Wilder clued me in, after all the bullshit I went through

Wilder found it out on Solar’s amps, but didn’t bother to tell me! LOL

?

 

[Jonathan Wilder]

If you are contemplating using the Electro Harmonix 6CA7 in your older/vintage Marshall, Traynor or with an amp with a negative grid supply; I would stay away from this! The Electro Harmonix 6CA7 is not and I repeat “NOT” a Power Pentode like the original 6CA7’s were, in fact they are Beam Tetrode as stated on the Electro Harmonix website. But the issue is tube venders are saying it is a direct substitute for an EL34 in all situations. Be careful, its not! I you really want to use these tubes in your amps make sure you don’t have a negative grid supply mod or a Traynor Amp. In fact even if your older/vintage Marshall amp has (pin1) & (pin 8) tied to ground, I would stay away from these tubes.

I found this out on my own. My JCM 800 build project was the test bed, I had told Marty I got a set of Electro Harmonix 6CA7’s to try in it later after the amp broke-in, well I put them in today. I couldn’t get them to bias correctly. So I bumped up he bias voltage up and got them in range, but they amp sounded like crap an the PT was hot as hell. Then friggin Wilder clued me in, after all the bullshit I went through, that it’s not a real power-pentode but a damn Beam Tetrode wannabee. Wilder found it out on Solar’s amps, but didn’t bother to tell me! LOL

Ah finally...my chance to crap on the Major!

First off..."negative grid supply"...I thought ALL fixed biased Class AB amps have these!

To clear up what the Major is saying though...EL34s are a standard pentode. Unlike a beam tetrode, pentodes have a suppressor grid (aka G3 or Grid 3) that is supposed to be either ground referenced or biased with a fixed negative voltage. Some Traynor amps do this and it's supposed to make the EL34s operate more efficiently. The Major implemented this into his JCM800 2203 build.

However, he decided to try a set of EH 6CA7 valves. He learned the hard way that these valves are a beam tetrode in an EL34 bottle. Unlike a regular beam tetrode however, they don't internally connect the beam plates to the cathode and they connect them to pin 1 in place of the suppressor grid that a normal EL34 would have. As such, if you have an EL34 amp that negative biases the suppressor grid, EH 6CA7s do not like this since they are a beam tetrode and not a standard pentode like an EL34.

Marshall has never implemented this in their designs that I know of. I found out that the EH 6CA7s were beam tetrodes when solarburn sent me his amp just because I happened to notice this. However, on normal Marshalls this isn't an issue since pin 1 is grounded instead of being negative biased like it is in Major's build. On top of that, I had no idea that the Major was going to switch to this valve...had I had known I would've informed him as such.

So bottom line...if you own an amp such as a Traynor that negative biases the suppressor grid pin (pin 1), EH 6CA7s will not work in this type of amp unless the negative bias is removed from pin 1 and pin 1 gets tied to the cathode. EL34s will work with pin 1 tied to the cathode as they have for decades in all Marshalls.

 

[MartyStrat54]

Sorry to hear about that. I feel sort of bad.I told the Major to give them a go. I too knew that they were not wired internally like a real 6CA7, but I didn't know that the Major was going to wire his amp up the way he did.

I'm sorry Major. I hope you didn't burn up your OPT because of it.

Sort of like the Chinese KT88's. They are the same as their 6550, just a different bottle and PN.

 

[racko7566]

Will the EH 6ca7'S harm a dsl 100, or are they ok to use. Mine sounded great with them but the tubes died in like two months.

 

[Jonathan Wilder]

Will the EH 6ca7'S harm a dsl 100, or are they ok to use. Mine sounded great with them but the tubes died in like two months.

No...this is why I wish Major hadn't have said anything. Creates an un-needed scare.

Marshall amps have never applied negative bias to the suppressor grid pin on the amps they build for EL34s. They've always connected pin 1 to the cathode (pin 8) on their EL34 amps and as such the 6CA7s will work in any Marshall-made amp that's set up to run EL34s with a cathode referenced suppressor grid pin.

Some of us who build Marshall clones will negative reference the suppressor grid pin as Major did on his and the EH 6CA7s will not work in these amps without converting them to a cathode referenced suppressor grid pin. But not very many of us do this (Traynor is the only other amp maker I can think of that does this and I will soon be doing some experimenting of my own with it) so these amps are the exception to the rule.

 

[racko7566]

Thanks Jon.

 

[MajorNut1967]

Sorry to hear about that. I feel sort of bad.I told the Major to give them a go. I too knew that they were not wired internally like a real 6CA7, but I didn't know that the Major was going to wire his amp up the way he did.

I'm sorry Major. I hope you didn't burn up your OPT because of it.

Sort of like the Chinese KT88's. They are the same as their 6550, just a different bottle and PN.

Marty its not your fault at all, I should have researched it more on my own, no worries mate! I tend to always use a negative grid supply on all my EL34 amps mainly because of the crappy CP tubes available. But I think that EH should make the consumer aware that these are not true Power Pentodes.

 

[MajorNut1967]

No...this is why I wish Major hadn't have said anything. Creates an un-needed scare.

Marshall amps have never applied negative bias to the suppressor grid pin on the amps they build for EL34s. They've always connected pin 1 to the cathode (pin 8) on their EL34 amps and as such the 6CA7s will work in any Marshall-made amp that's set up to run EL34s with a cathode referenced suppressor grid pin.

Some of us who build Marshall clones will negative reference the suppressor grid pin as Major did on his and the EH 6CA7s will not work in these amps without converting them to a cathode referenced suppressor grid pin. But not very many of us do this (Traynor is the only other amp maker I can think of that does this and I will soon be doing some experimenting of my own with it) so these amps are the exception to the rule.

Hey wankey boy, I explained it clearly about my amp and the Traynor in the beginning. And also Racko said his tubes died in two months! So if you want to go out on a Limb and recommend these to Marshall owners, you go right ahead but I see two reasons to use an alternative right here.

Oh update I just got an email from a mate of mine in New Zealand, he tried these in his 1971 Model 1987 50 watt. The tubes last a whole two hours before they super nova'd and took out his OT. He said that he had change the range res. in the bias circ. just to get it set correctly. He said they went poof and they are full of this white snowflake crap inside now. And that it is wired stock with pin 1 grounded as well as pin 8. So this wasn't a scare tatic but a word to the wise

 

[thrawn86]

Bummer deal Maj. So does everything check out now? (i.e. the OT)

 

[MajorNut1967]

Bummer deal Maj. So does everything check out now? (i.e. the OT)

Thanx Bro! all is well, nothing fried crispy chips. OT is fine the PT is what really got hot! but no major casualties yet, we'll see.

 

[rjohns1]

yeah, I tried these tubes a few years ago. lasted a whole month and pop.Luckily they didn't take anything with them.

 

[MajorNut1967]

yeah, I tried these tubes a few years ago. lasted a whole month and pop.Luckily they didn't take anything with them.

Thanks for the Info Mate. We are kind of seeing a bit of a pattern here.

 

[racko7566]

What really sucks is that they sounded better and felt better than any powertubes I've tryed so far. Be it I haven't tried any expensive tubes yet but ive tried eh el34s jj el34s and svetlana c's.

My tubes didnt fry, but one thing ive learned here so far is to watch for and prevent (POOFS). My bias started drifting alot on one side, I switched them around and the drift moved over so I new it was a tube, Which I learned here. Thanks to you guys. So with my constant bias checking I feel confident enough to give them another shot. If this set goes then I'm done with them.

 

[Jonathan Wilder]

As I've mentioned before, the screen grids in the power valves are really stressed in guitar amps. This is a known but hidden fact in the valve community as people would much rather blame the plates for not being able to handle the voltage.

Some EL34s are only rated at 450V on the screens whereas others claim to be rated at 500 (Winged =C=). Well most of the newer Marshalls run them right at that limit or slightly higher. Some valves are more tolerant of this than others. It's becoming obvious that EH 6CA7s aren't very tolerant of the screen voltage in Marshalls.

The problem is that guitar amps have been running the same old style LC/RC supply in them for decades. The problem with this supply is that the plate voltage affects the screen voltage since they're on the same rail, although the choke/sag resistor and screen filter caps decouple the two, they're only a few volts apart from each other. This type of supply sets a limit as to how high you can run the plate voltage because of the fact that the screen voltage is only a few volts lower than the plate voltage. Once plate voltage is at 450-460V, ideally you can't go any higher than this because now you're at the screen voltage limit.

 

[solarburn]

I had one drift at the 3 month mark and got a replacement that I haven't used yet. I have other tubes in my DSL right now. I loved the tone of these too but I'm not liking what I'm hearing. I have a good set again but I have to really think about using them. One thing I do regularly is check bias. Like 3 times a week or when I am trouble shooting. If I use them I'm going to keep a very close eye on them. Meaning bias checks frequently.

 

[solarburn]

As I've mentioned before, the screen grids in the power valves are really stressed in guitar amps. This is a known but hidden fact in the valve community as people would much rather blame the plates for not being able to handle the voltage.

Some EL34s are only rated at 450V on the screens whereas others claim to be rated at 500 (Winged =C=). Well most of the newer Marshalls run them right at that limit or slightly higher. Some valves are more tolerant of this than others. It's becoming obvious that EH 6CA7s aren't very tolerant of the screen voltage in Marshalls.

The problem is that guitar amps have been running the same old style LC/RC supply in them for decades. The problem with this supply is that the plate voltage affects the screen voltage since they're on the same rail, although the choke/sag resistor and screen filter caps decouple the two, they're only a few volts apart from each other. This type of supply sets a limit as to how high you can run the plate voltage because of the fact that the screen voltage is only a few volts lower than the plate voltage. Once plate voltage is at 450-460V, ideally you can't go any higher than this because now you're at the screen voltage limit.

A tube site I was on recommended not using these on amps with PV's above 450V. The other site said they have a great record and are rated to handle 800V. I think we are starting to see whats what...

 

[Jonathan Wilder]

A tube site I was on recommended not using these on amps with PV's above 450V. The other site said they have a great record and are rated to handle 800V. I think we are starting to see whats what...

Yeah and it's not even the plates that's the problem. The problem is due to the ages old guitar amp power supply design that derives the screen voltage from the same positive rail in the supply that the plate does. As such, when you increase plate voltage, screen grid voltage also increases and it's always just a few volts lower than the plate. This is exactly why they recommend not to run them in amps with a plate supply of 450V or more...it's because the screen voltage ain't far behind it and the screens can't take it.

If you had them in an amp that had the screens on a different rail than the plate supply, you could run the plates close to 800 volts while running the screen supply at 400 volts and probably pull some big power out of them that way with the proper plate load. But ain't no way in hell they're gonna handle 800V when the screen supply running on the same rail would only be about 10 volts lower than that.

 

[big dooley]

so to say it in my words....
them tubes are more like a KT77, but with a misleading nametag?...

a dutch webstore, selling these things, quotes them as "developed at exact philips design 6CA7"

it's about time CP-tubes are named at what they are, with specs they actually have and not intend to have...

 

[Jonathan Wilder]

so to say it in my words....
them tubes are more like a KT77, but with a misleading nametag?...

a dutch webstore, selling these things, quotes them as "developed at exact philips design 6CA7"

it's about time CP-tubes are named at what they are, with specs they actually have and not intend to have...

I'd consider them to be more like a 6L6 that's made to run in an EL34 environment. The screen grid max rating of these things is MUCH lower than the screen rating on a KT77 (650 max for a KT77 as compared to a screen max rating of 400-450 for the EH 6CA7). I think they're hit/miss in the DSL/TSL amps because the DSL/TSL run a 460-470 volt plate supply, and due to the supply design topology (where the screens are powered from the same rail as everything else) the screens aren't far below the plate voltage. This runs the screens right at their max and taxes the screens a bit. Again some can tolerate it better than others.

I think this is what makes the Winged =C= EL34s a good match for Marshalls since they have a max screen voltage rating of 500 volts. Every other EL34 screen seems to be rated for 450ish.