A word to the wise on EH 6CA7 tubes!

[big dooley]

650V max screen rating for a NOS KT77? or does that include the CP-KT77's from JJ and gold lion? (if those are real KT77's of course )

 

[Jonathan Wilder]

650V max screen rating for a NOS KT77? or does that include the CP-KT77's from JJ and gold lion? (if those are real KT77's of course )

Not sure on the Gold Lion KT77s, but JJ KT77 screens seem to be rated a bit higher at 800 volts. At least that's what their datasheets claim. I'm a bit skeptical on that though.

 

[big dooley]

yes, i've been looking into those specs too... but if their KT77's seems a bit overrated, where would the EL34's and E34L's stand with their claimed 450V?

 

[Jonathan Wilder]

yes, i've been looking into those specs too... but if their KT77's seems a bit overrated, where would the EL34's and E34L's stand with their claimed 450V?

450V on the 34 and 34L seems to be right on the money for these. I've ran these a bit higher than this (close to 500) and have never had them prematurely fail on me. But 450-500V seems to be the typical screen voltage spec range you'd see for any EL34. 450 is more common, but the Winged =C= EL34s seem to be rated at 500.

 

[Joey Voltage]

Not sure on the Gold Lion KT77s, but JJ KT77 screens seem to be rated a bit higher at 800 volts. At least that's what their datasheets claim. I'm a bit skeptical on that though.

Per our conversation earlier today, even the MO KT77's were not reccomended to have anywhere near 800V on the screens, I think even the old datasheets quote a maximum of 350Vdc for the screeens when used in a typical fixed bias class AB push pull amplifier. They make mention that when used in ultra linear designs, that a slightly higher screen supply can be tolerated IIRC. To my knowledge, the only valve curently being produce that can handle higher screen voltages in a run of the mill PA is the KT-88, and even they have limitations.

I blame some of the misuse on the manufacturers frankly. These days they don't seem to give a flying fuck, and don't go into detail of very important limiting elements, like the old datasheets used to. Not that I think anybody would pay attention anyway. the other part I blame on laxadasical design practices

 

[solarburn]

Looks like these tubes are in the same boat then with the Winged =C=s being a more reliable tube due to the higher rating. No wonder it has the rep it has. The rest are pretty equal though as far as the screen voltage goes. Kinda a crap shoot depending on the amp used...

I'll run mine longer and see. Really I'm not hearing any more horror stories then other tubes I've followed and even used. Enough to have me keep a close watch on them though since Racko's drifted around the 3 month mark too. I only had one drift and had it warrantied so I will give them so more time and see.

Winged =C='s are the only tube I find with a great rep especially from those who gig regularly.

 

[Jonathan Wilder]

Looks like these tubes are in the same boat then with the Winged =C=s being a more reliable tube due to the higher rating. No wonder it has the rep it has. The rest are pretty equal though as far as the screen voltage goes. Kinda a crap shoot depending on the amp used...

I'll run mine longer and see. Really I'm not hearing any more horror stories then other tubes I've followed and even used. Enough to have me keep a close watch on them though since Racko's drifted around the 3 month mark too. I only had one drift and had it warrantied so I will give them so more time and see.

Winged =C='s are the only tube I find with a great rep especially from those who gig regualarly.

The interesting thing is that the Winged =C='s are claimed to be the closest Mullard copy on the market, and yet even the original Mullard data sheets spec a design maximum screen rating of 425V, yet the Winged =C=s are supposedly rated a bit higher. The one thing I do have to say is that the Winged =C='s do seem to have the best track record in regards to reliability in Marshalls new and old so I think the proof is in the pudding there.

However, Joey does seem to bring up some interesting points regarding blaming the misuse on the manufacturers. Also, laxadasical design practices on the part of the amp builders comes into play...they're all using the same decades old single rail RC/LC power supply, which from one standpoint yes this does give the vintage sound that lots lust after. But the drawback is that your screen voltage rating sets the limit as to how hot you can go on plate voltage, and as such the amount of obtainable output power per pair of valves is limited by this design practice.

This is why it's been said countless times that 6550s in Marshalls aren't being utilized to their fullest potential in Marshalls. They're a high transconductance valve that's designed to pull big power per pair into a decent plate-plate load figure. However, they simply cannot do this on a single rail supply, or with the low plate loads found in Marshalls for that matter. Hence, this is also why I've stated many times that when you bias 6550s in a Marshall you go by the max figure for EL34s, not 6550s. Their transconductance was designed for higher plate loads than what's found in a Marshall for the plate loads they run at (data sheets spec a 5K plate-plate load on a 600V plate/300V screen supply for 6550s, yet Marshalls run a 3.4K in a 50 watter and a 1.7K in a 100 watter at whatever voltage they wanna run the valves at while referencing the screens at a point that's only a few volts lower than the plate). Too low of a plate load on a high transconductance valve (especially one that's too low for the valve it's designed to work with in the first place) can sent it into overcurrent quicker than shit.

 

[MajorNut1967]

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...

Exactly Dooley,

They say they are like Philips version not even close! and we shouldn't have to be looking inside the tube make sure it what the fuck they say they are!

 

[solarburn]

Here's the Tube Store's review of actually using them:

At last something to replace the hole left since the EI 6CA7 went out of stock. These tubes sound every bit as good as the old EI tubes and are probably my favorite tubes for Hiwatt guitar amplifiers. It’s like they were made for each other. These tubes are like EL34’s on steroids. They handle high voltage and current without problems. As mentioned, there is nothing like a Hiwatt head powered by 6CA7 tubes and driving a 4x12 cab loaded with Fane speakers. This is classic rock tone at its finest. It’s the sound of Pete Townsend and The Who. The 6CA7 has more headroom than an EL34 and by the time you get it really crunchy sounding your pants are flapping in the breeze. Just the right amount of compression for great thick rock tone. Biasing levels can be dialed in from warm to hot without causing significant tonal changes. The EH is well constructed and should not pose any noise problems. If using these tubes in more reasonable systems the EH 6CA7 is capable of some really nice clean tones thanks to that extra headroom. If you want early Van Halen, these are not the tubes for you. If you want something loud and proud, you want to put the EH 6CA7 in your amp.

Here is their description of the tube:

Designed to the exacting specifications of the Philips 6CA7 tube, this classic tetrode returns to life in a big bottle design. The 6CA7-EH is built to withstand today's high-gain amps while still retaining the detailed tone and component integrity of the original. A direct replacement, with military reliability, for any EL34.

For me the tone delivers. I'm not comparing them to the NOS Philips at all. Reliability wise I don't know yet. Not looking good but I'll see. Description wise, a bit misleading to say the least...

 

[solarburn]

Here is the review of the KT-77 by the Tube Store so you can compare to the 6CA7:

The KT77 has been gone for many years and original NOS pieces are rare and expensive. JJ electronics has revived the design and released their own version. To my ears it sounds a lot like a 6L6 but has the heater current, max plate voltage and output rating of an EL34. The JJ KT77 has published specs that are identical to the original Genalex specs. The base has pin 1 included but there is no connection to any internal element. The sound is somewhere between an EL34 and a 6L6. Overall a nice balance of tone in bass, midrange and treble. The breakup is earlier than a 6L6 with more compression but not as compressed as the EL34. Unlike an EL34 this tube can be used in place of a 6L6 in Fender amps with minimal modifications. Vintage Fender amps usually use pins 1 and 6 as tie-off points for input grids and screen grids, mounting resistors on the socket. A standard EL34 can’t be plugged in because the input grid would be shorted to the suppressor grid via pin one. The KT77 avoids this. The only caveats are to ensure that your power transformer can supply an extra 500ma of heater current per tube and that the range of bias voltage adjustment is correct. If you have a Marshall amp and find EL34’s too compressed and 6550’s too crunchy the JJ KT77 may be just the ticket.

 

[big dooley]

The only caveats are to ensure that your power transformer can supply an extra 500ma of heater current per tube and that the range of bias voltage adjustment is correct.

ok... this rises another question...
this is the story;
the JVM is supposed to be biased at 60Vdc on both points at 480Vdc on the plates (marshall recommendation)
when i swapped those EL34's for KT77's in my JVM, when it came in i measured plate voltage... (never checked the plate voltage with the EL34's)
+/-435Vdc... nowhere near 480Vdc... bias was very cold at 45mV so turned it up, losing more voltage on pin 3... i've settled the bias at around 80mV @ 425Vdc

the main voltage in our country is a bit lower then in the UK (220 vs 230/240) so obviously, this takes some toll as well, but after reading the above quote... do the KT77's play a role in this as well?

not that i care too much about it, the amp sounds rocksolid and the valves are working flawlessly so far (1,5 year of service, they had their burn-in in a biasdrifting DSL100)

 

[Jonathan Wilder]

ok... this rises another question...
this is the story;
the JVM is supposed to be biased at 60Vdc on both points at 480Vdc on the plates (marshall recommendation)
when i swapped those EL34's for KT77's in my JVM, when it came in i measured plate voltage... (never checked the plate voltage with the EL34's)
+/-435Vdc... nowhere near 480Vdc... bias was very cold at 45mV so turned it up, losing more voltage on pin 3... i've settled the bias at around 80mV @ 425Vdc

the main voltage in our country is a bit lower then in the UK (220 vs 230/240) so obviously, this takes some toll as well, but after reading the above quote... do the KT77's play a role in this as well?

not that i care too much about it, the amp sounds rocksolid and the valves are working flawlessly so far (1,5 year of service, they had their burn-in in a biasdrifting DSL100)

When you bias valves, the more current they draw the more the plate voltage drops. This is because the valve in series with 1/2 the OT primary forms a voltage divider. As current draw through the OT primary increases, so does the voltage drop across it and as such the voltage drop across the valve does the opposite. Also the more current it draws the more of a load gets placed on the power supply. These power supplies are unregulated so this causes the supply voltage to drop some the more current you draw. The "stiffness" of the power supply controls how much it will drop.

Different valves drawing the same amount of current should end up roughly at the same plate voltage since the same load is being placed on the supply as well as that branch of the circuit.

However, I find it funny that Marshall can't even use PTs rated for their own mains voltage.

 

[RickyLee]

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.

Sorry fellas for digging up such an old post, but this EH6CA7 issue might possibly be part of a problem I started hearing yesterday.

First off, I need to ask you Mr. Wilder, if you could tell me which models of the Traynor amps are biased with a fixed negative voltage on Grid 3?

The other question is what models of guitar amps have this dual rail B+ supply - have the screens on a different rail than the plate supply?



As for my possible problem? I noticed yesterday that my '88 2205 that is running the EH6CA7's did not sound as good as it did this past weekend when I used it for two nights and quite a few hours each night. It went from sounding absolutely incredible to a bit harsh and lack of clarity, a bit distorted in the bass frequencies. In fact it was sounding so good (I did a few minor tweaks to it awhile back that made a huge impact for the better tonally), it made my Black Jubilee '89 2550 sound weak and boring. So now I am going to set it up here shortly and check it out and swap in a different brand of known good power tubes - maybe just put in a set of JJ E34L's. I know this 2205 runs at about 475V on it's B+.

I am starting to lean towards using my VariAC with my amps at all times as my service voltage at home is usually around 125VAC and the few times I have measured and checked service voltage at gigs, bars ect. I have seen some huge variances as well.

Anyway, I guess I better check out these EH6CA7's that are in my 2205 . . .

 

[MajorNut1967]

Sorry fellas for digging up such an old post, but this EH6CA7 issue might possibly be part of a problem I started hearing yesterday.

First off, I need to ask you Mr. Wilder, if you could tell me which models of the Traynor amps are biased with a fixed negative voltage on Grid 3?

The other question is what models of guitar amps have this dual rail B+ supply - have the screens on a different rail than the plate supply?



As for my possible problem? I noticed yesterday that my '88 2205 that is running the EH6CA7's did not sound as good as it did this past weekend when I used it for two nights and quite a few hours each night. It went from sounding absolutely incredible to a bit harsh and lack of clarity, a bit distorted in the bass frequencies. In fact it was sounding so good (I did a few minor tweaks to it awhile back that made a huge impact for the better tonally), it made my Black Jubilee '89 2550 sound weak and boring. So now I am going to set it up here shortly and check it out and swap in a different brand of known good power tubes - maybe just put in a set of JJ E34L's. I know this 2205 runs at about 475V on it's B+.

I am starting to lean towards using my VariAC with my amps at all times as my service voltage at home is usually around 125VAC and the few times I have measured and checked service voltage at gigs, bars ect. I have seen some huge variances as well.

Anyway, I guess I better check out these EH6CA7's that are in my 2205 . . .

I'm not Mr. Wilder, but tell you what Traynor amps have it. Most of the YBA series that use EL34’s have the negative grid supply. Some Sunn amps and a few others I can't quite remember, some of the orange amps had to.

Have you noticed your power transformer or your output transformer getting warmer than usual?

 

[Joey Voltage]

The other question is what models of guitar amps have this dual rail B+ supply - have the screens on a different rail than the plate supply?

Predominantly Bass amps. Take a look at some ampeg schemes andyou will them.

 

[RickyLee]

Have you noticed your power transformer or your output transformer getting warmer than usual?

I am going to set my 2205 up here in the living room and blast away for a bit. I did not notice my iron getting hotter than ususal as I basically just dug this amp back out a few weaks ago and took it out for a few gigs, being it was sounding so incredible after some work I had done on it. So you are saying that these EH6CA7's might be pullin' a bit more current than ususal? So I might see that my bias/idle current has drifted, or is it going to be under load/cranking I might see more draw? And the thing with the 2205, is that it uses the same power transformer as the 2204, but runs two extra preamp tubes more than the 2204. So I am thinking the 2205 power transformer should be quite warm under normal operating conditions to begin with? So that alone is something to be cautious about anyway regardless.

Anyone know what the spec's are on these 2204/2205 50 watt Drake JCM800 era power transformers? Either they are quite close on filament spec's for the 2205's (7 total tubes) or have plenty to spare for the 2204's (5 total tubes)?

 

[RickyLee]

OK. I have no issues going on with my EH6CA7's in my 2205, though I am running over the rated limits of this tube. These tubes just spank the crap out of the JJ E34L's, and these tubes are not the issue that I am hearing with my amp. But I left the E34L's in there for the time being.

Also, the 2205 transformers are not that hot at all to the touch after using the amp for awhile. But then I was not even cranked close to what this amp is capable of putting out.

 

[MajorNut1967]

I am going to set my 2205 up here in the living room and blast away for a bit. I did not notice my iron getting hotter than ususal as I basically just dug this amp back out a few weaks ago and took it out for a few gigs, being it was sounding so incredible after some work I had done on it. So you are saying that these EH6CA7's might be pullin' a bit more current than ususal? So I might see that my bias/idle current has drifted, or is it going to be under load/cranking I might see more draw? And the thing with the 2205, is that it uses the same power transformer as the 2204, but runs two extra preamp tubes more than the 2204. So I am thinking the 2205 power transformer should be quite warm under normal operating conditions to begin with? So that alone is something to be cautious about anyway regardless.

Anyone know what the spec's are on these 2204/2205 50 watt Drake JCM800 era power transformers? Either they are quite close on filament spec's for the 2205's (7 total tubes) or have plenty to spare for the 2204's (5 total tubes)?

The only reason I asked if your transformers were overheating is that that happened in my JCM 800 clone. And the reason it happened is because my JCM 800 clone as a native supply grid, which is not stock on a normal JCM 800. What was funny is that I had the tubes biased correctly well within spec and my power transformer got hot as hell. Normally all my transformers in my JCM 800 run exceptionally cool even after hour of loud plane can put your hand on top of the power transformer no problem.

The Drake Transformers for power should spec out as follows:

Input side should have taps for 120 V, 220 V, and 240 V

Secondary side should have 690 V with the center tap at 150 mA, the heater Tap should be 6.3 V at 5 amps.

 

[kenm]

Well I'm either the exception to the rule or just plain lucky but I've used the EH 6CA7's in a 1970 Traynor YBA-1 for years and have had no problems.

 

[johnfv]

Welcome to Marshall Forum Kenm. This actually is a rather old thread, if you search you can find details from a number of us using the EH 6CA7 with good results.